Mechanical Property Characterization of Metal Nano-particle Films for MEMS Devices by Eric Wing-Jing Lam S.M. Electrical Engineering & Computer Science, Massachusetts Institute of Technology (2008) B.S. Bioengineering, B.S. Electrical Engineering, University of Washington (2005) SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING & COMPUTER SCIENCE IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING & COMPUTER SCIENCE AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEPTEMBER TUTE - 2012 @ 2012 Massachusetts Institute of Technology. All rights reserved. Signature of Author: Department of Electrical Engineering & Computer Science August 31, 2012 III r?nfr(~11 Certified by: Martin A. Schmidt Professor of Electrical Engineering & Computer Science & Associate Provost Thesis Supervisor Accepted by: Lalie A. Kolodziejski Professor of Electrical Engineering & Computer Science Chair, Department Committee on Graduate Students 1 2 Mechanical Property Characterization of Metal Nano-particle Films for MEMS Devices by Eric Wing-Jing Lam Submitted to the Department of Electrical Engineering & Computer Science on August 31, 2012 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering & Computer Science ABSTRACT Printing-based fabrication methods have emerged as a promising alternative to conventional lithographic processes in a number of applications. These methods are being exploited in display manufacturing, flexible electronics, and more recently MEMS. Unlike applications in printed electronics, MEMS devices require comparatively thick layers, typically on the order of microns. In the micronthickness regime, nanoparticle-based inks are the preferred means for material delivery, in large part due to the ability to deliver high solids content. However, sintered nanoparticle films possess unique morphology and hence different properties when compared to bulk material or materials deposited through other methods and are dependent on the processing conditions. As such, careful characterization of the film's properties is critical to successful adoption of this technology. A detailed methodology to identify the process-mechanical property of metal nanoparticle-based films was developed using silver nanoparticles as the case study. Silver nanoparticle-based cantilevers and films were fabricated through inkjet printing and conventional microfabrication techniques. These structures were mechanically characterized by beam deflection analysis and nanoindentation to map Young's moduli versus the processing conditions. The results were coupled with sintering and powder metallurgy models to explain the data. For silver-based nanoparticle films, it was determined that the process and mechanical property have a power law relationship with the ratio of the sintering temperature and the melting point of bulk silver. This relationship enables prediction of mechanical properties and provides guidance for optimization of sintering conditions towards a desired mechanical property. The specific results reported include: i) process flows to fabricate nanoparticle-based microstructures, ii) detailed methodology to map film features and properties versus processing conditions, and iii) an empirical model explaining the data and enabling prediction of the resultant properties. While this methodology was shown to determine the process-mechanical property relationship for silver nanoparticle-based films, it should be generally applicable to other metal nanoparticle-based films and lays the groundwork for characterizing this class of materials. Thesis Supervisor: Martin A. Schmidt Title: Professor of Electrical Engineering & Computer Science & Associate Provost 3 4 Acknowledgements I would like to thank my thesis advisor, Professor Martin A. Schmidt, for all of his knowledge, expertise, and experience throughout my Ph.D. career. His assistance was instrumental in the formation of this thesis and the breath of information it has managed to cover. I would like to also thank my thesis committee members, Professors Vladimir Bulovic and Sang-gook Kim, for their insights, advice, and different perspectives which helped increase the scope of this thesis beyond its original goal of mapping the process-property relationship. This project could not have been done alone: Dr. Jianglong Chen originally introduced me into this project and did much of the initial work of characterizing the TIPS controller and identifying potential materials, Dr. Hanqing Li helped me troubleshoot and navigate past the traditional pitfalls and mistakes of microfabrication, Eric Newton would listen to my ideas and provide generous feedback and suggest improvements, and Dennis Ward helped me use the Resonetics laser ablation tool to greatly decrease the feature sizes of my surface micromaching processes. I thank you all for your great assistance, inspiration, and motivation that you have given me throughout the years. Finally, I also must thank my collaborator, Dr. Steven Bathurst: not only did Steve constantly modify and improve the digital fabrication system, but he was also great to bounce ideas off of, commiserate when our projects weren't working, and was an awesome drinking buddy. The work presented in this thesis was performed in several different laboratories throughout MIT's campus. The conventional microfabrication, laser-trimming, contact profilometry, and SEM imaging was performed primarily in the Microsystems Technology laboratories (MTL). Pre-printing sample preparation was originally done in the Organic & Nanostructured Electronics (ONE) Laboratory. The digital fabrication tool is located in the Micro & Nano Systems Laboratory (MNSL). Characterization of the nanoparticle microstructures was performed in the Department of Material Science & Engineering's (DMSE) Nanolab. I am grateful to all the respective laboratory staff members and managers for their help, training, and access to their tools and resources. I also must thank Hewlett-Packard for providing the TIPS system and especially would like to thank Dr. Paul Benning, Dr. Peter Madilovich, Dr. Murali Chapalara, Dr. Luanne Rolly, Dr. Tom Etheridge, and the Imaging and Printing Group (IPG) in Corvallis, Oregon. I want to thank my friends and family for their love and support. The countless colleagues and timeless friendships that I have made throughout my time at MIT is undoubtedly my most treasured experience. I have to thank my friends and family back at home for not giving me too much grief for not visiting as often as I should have and for providing words of encouragement instead. Lastly, I especially want to thank my wife, Dr. Grace Chen, whom I met on basically my first day at MIT. Your love and support kept me motivated and undeterred throughout my Ph.D. experience. Without you this entire experience would not have been as enjoyable. To everyone I have mentioned: thank you all from the bottom of my heart; this thesis would not have been possible without you. 5 6 Table of Contents Acknowledgem ents.......................................................................................................................................5 Ta ble of Contents .......................................................................................................................................... 7 List of Figures...............................................................................................................................................10 List of Tables................................................................................................................................................12 List of Equations .......................................................................................................................................... 13 Chapter I Introduction.................................................................................................................................15 Chapter I Background.................................................................................................................................17 11.1 Overview ........................................................................................................................................... 17 11.2 Conventional m icrofabrication...................................................................................................... 17 11.3 Direct-write m icrofabication ............................................................................................................. 18 11.3.1 Laser-writing...............................................................................................................................18 11.3 .2 n kje t...........................................................................................................................................1 9 11.3.3 Extrusion.....................................................................................................................................22 11.3.4 Electrochem ical .......................................................................................................................... 23 Chapter III M aterial Choices ........................................................................................................................ 25 111.1 Overview .......................................................................................................................................... 25 111.2 Substrate .......................................................................................................................................... 25 111.3 Structural Layer ................................................................................................................................ 25 111.3.1 Overview of Silver Deposition M ethods............................................................................... 26 111.3.2 Chem ical Deposition..................................................................................................................26 111.3.3 Physical Deposition ................................................................................................................... 28 111.3.4 Sum m ary of Silver Deposition M ethods............................................................................... 30 111.4 Sacrificial Layer ................................................................................................................................. 31 111.4.1 Poly(m ethyl m ethacrylate) (PM MA) ..................................................................................... 32 111.4.2 Positive photoresist...................................................................................................................32 111.4.3 Amorphous silicon (a-Si)............................................................................................................32 Chapter IV Fabrication Process Characterization and Optim ization...................................................... 33 IV.1 Overview .......................................................................................................................................... 33 IV.2 Structural Layer ................................................................................................................................ 34 7 IV.2.1 Printing apparatus overview ................................................................................................. 34 IV.2.2 Inkjet Printing Param eters ................................................................................................... 36 IV.2.3 Film deposition optim ization ................................................................................................ 39 IV.2.4 Depositing nanoparticle silver process of record ................................................................ 44 IV.3 Sacrificial Layer.................................................................................................................................45 IV.3.1 Poly(m ethyl m ethacrylate) (PM M A)..................................................................................... 45 IV.3.2 Positive Photoresist...................................................................................................................47 IV.3.3 Am orphous silicon (a-Si) ....................................................................................................... 49 IV.4 Sacrificial barriers to prevent uneven ink spreading ................................................................... 51 IV.5 Fabrication sum m ary ....................................................................................................................... 52 Chapter V Experim ental M ethods and Results ....................................................................................... 55 V.1 Film thickness versus peak sintering tem perature ....................................................................... 55 V.2 Nanostructure versus peak sintering tem perature ..................................................................... 57 V.3 Film m echanical properties versus sintering param eters............................................................. 59 V.3.1 Characterization m ethod #1: Equally-spaced beam deflection ............................................. 59 V.3.2 Characterization m ethod #2: Nanoindentation...................................................................... 61 V.3.3 M odulus versus peak sintering tem perature........................................................................ 62 V.3.4 M odulus versus dw ell tim e at peak sintering tem perature.................................................... 63 V.4 Sum m a .ry ........................................................................................................................................... Chapter VI M odeling of Sintered Nanoparticle Film s............................................................................. VI1. Overview .......................................................................................................................................... VI.1.1 Nanoparticle M elting Point Depression............................................................................... 64 65 65 65 VI.1.2 Electrical m odel.........................................................................................................................67 VI.1.3 M etal pow der sintering.............................................................................................................67 VI.2 M echanical m odeling of sintered m etal nanoparticle film s ........................................................ 68 VI.2.1 Sintering process-film m orphology m odel............................................................................. 68 VI.2.2 Nanostructure-m aterial property m odel............................................................................... 69 VI.3 Applying m odel to N2-sintered nanoparticle film s...................................................................... 71 VI.4 Sum m ary .......................................................................................................................................... 74 Chapter VII Conclusions and Future W ork .............................................................................................. VII.1 Suggestions for future w ork............................................................................................................77 References...................................................................................................................................................79 8 77 Appendix A Photolithography m asks ..................................................................................................... 85 Appendix B M ATLAB Scripts ........................................................................................................................ 89 B.1 Printing pattern generation scripts ............................................................................................... 89 B.1.1 Control m odule .......................................................................................................................... 89 B.1.2 Pattern generation m odule................................................................................................... 93 B.1.3 Graphing m odule......................................................................................................................100 B.2 Young's m odulus extrapolation via beam deflection m easurem ents ............................................ 101 Appendix C Ink preparation.......................................................................................................................105 Appendix D Printing troubleshooting........................................................................................................107 9 List of Figures Figure Il-1. Illustrations of bulk and surface micromachining processes.............................................. 17 Figure 11-2. Illustrations showing the principle of LDI and LITI .............................................................. 19 Figure 11-3. Illustration of continuous inkjet printing. ............................................................................. 20 Figure 11-4. DoD inkjet print head designs.............................................................................................. 21 Figure 11-5. Examples of materials and devices deposited using inkjet deposition.................................22 Figure 11-6. Silver m icroelectrodes using extrusion deposition.............................................................. 22 Figure 11-7. Illustrations of electrochemical deposition and etching......................................................23 Figure 111-1. Melting Temperature vs. Particle Diameter for the thermodynamic models.....................30 Figure IV-1. Fully-printed surface m icrom achining process................................................................... 34 Figure IV-2. Picture of Thermal inkjet Pico-fluidic System (TIPS)............................................................ 35 Figure IV-3. Replaceable TIJ printhead & reservoir................................................................................. 35 Figure IV-4. Digital fabrication system with a TIPS controller connected to an X-Y motion controller......36 Figure IV-5. Illustration of inkjet printing.............................................................................................. 37 Figure IV-6. Simplistic representation of electrical pulse applied to TIJ printhead. ............................... 38 Figure IV-7. Example of printing pattern and the parameters used to generate the pattern...............41 Figure IV-8. Ink spreads when significant volumes of ink are printed. .................................................. 42 Figure IV-9. Film quality comparison between standard and individual layer printing methods...........43 Figure IV-10. Elastic moduli versus sintering method/environment. .................................................... 43 Figure IV-11. Effect of slow heating and cooling rates on film cracking. ................................................ 44 Figure IV-12. Standard temperature profile for sintering process........................................................ 44 Figure IV-13. Contact profilometry scans of 2 PMMA molds.................................................................46 Figure IV-14. Nanoparticle films before and after 10 minute 02 plasma exposure...............................48 Figure IV-15. Laser-trimmed nanoparticle films on photoresist............................................................48 Figure IV-16. Nanoparticle film on photoresist before and after 3 hours in 80 0 C acetone bath............49 Figure IV-17. a-Si delamination after developing photoresist photomask............................................50 Figure IV-18. a-Si after developing photoresist photomask.................................................................... 50 Figure IV-19. Illustrations of uneven film spreading and design to prevent uneven spreading.............52 Figure IV-20. Nanoparticle-only sam ple fabrication process................................................................. 52 Figure IV-21. Nanoparticle and a-Si surface micromachining process...................................................53 Figure IV-22. Exam ple cantilever using a-Si sacrificial layer................................................................... 10 53 Figure V-1. Sum m ary of film thickness measurem ents.......................................................................... 56 Figure V-2. Film thickness versus peak sintering temperature. .............................................................. 56 Figure V-3. Illustration of sam ple cleaving .............................................................................................. 57 Figure V-4. Top-view and cross-sectional electron microscopy of air-sintered nanoparticle films........58 59 Figure V-5. Top-view electron microscopy of N2-sintered nanoparticle films. ....................................... Figure V-6. Determining Young's modulus via multiple point-force deflections....................................60 62 Figure V-7. M odulus versus peak sintering tem perature. ...................................................................... Figure V-8. Modulus versus peak sintering temperature in air- and N2-environments..................63 Figure V-9. Young's modulus versus dwell time at peak sintering temperature....................................64 Figure VI-1. Diagram of 2-sphere sintering model ................................................................................. 65 Figure VI-2. Silver nanoparticle melting temperature vs. nanoparticle diameter. ................................. 66 Figure VI-3. Part I of Greer and Street electrical sintering model..........................................................67 H Figure VI-4. -ve rsus Sintering Tempe rature..................................................................69 Figure VI-5. Relationship betw een p and L............................................................................................ 70 Figure VI-6. Young's modulus versus peak sintering temperature. ....................................................... 71 Figure VI-7. Melting temperature vs. particle diameter comparison. .................................................. 72 H Figure VI-8. - versus sintering temperature in N 2 -------------------------------- 73 . .. . .73 . .. . . .. . . Figure VI-9. Elastic modulus versus peak sintering temperature when sintering in N 2 ---- .. . .. H Figure VI-10. Sintering environment effects on - ve rsus sintering temperature ... .............. 74 Figure VI-11. Sintering environment effects on Young's modulus versus sintering temperature........75 Figure A-1. Die design for generation 1 photolithography mask.......................................................... 85 Figure A-2. Generation 1 m ask design. .................................................................................................. 86 Figure A-3. Die design for generation 2 photolithography mask.......................................................... 87 Figure A-4. Generation 2 m ask design. .................................................................................................. 88 Figure D-1. Printing troubleshooting flow chart. ....................................................................................... 11 107 List of Tables Table 111-1. Example of nanoparticle synthesis process.......................................................................... 29 Table IV-1. Standard CCI-300 printing and sintering process................................................................. 45 Table IV-2. Photoresist after 3 hour, 150*C exposure versus etchant ................................................... 47 Table IV-3. Standard a-Si deposition and patterning process................................................................ 51 Table VI-1. Silver numerical values..............................................................................................................66 Table C-1. Standard ink dilution process...................................................................................................105 12 List of Equations 4T I(Ps~2 / 3 0 Fd T(d) = piP ....... 'To 1(OSV 2 Uv 3dTcr 1- (i- 30 )... ............................................................................................ Ps Fd ds .................................................................................................... 59..3 ............................................................................. dkx................. F 3 F = k x ...................................................................................... 59 EWH 3 40 3 . WH E 3 . . .. . . .L]....... [L + ...... ...................................... 60 4 -WH3SS 4 WH 3S 3 . . E 1-v 2 r- 2 . .. . .. . .. . .. . .. . .. . .. .. . . .. . .. . .. . .. . .. . .. . .. . .. . . .. .. . .. . . .. .. . . .. .. . . .. .. . .. . .. . .. . Ap (he 2 1-vfilm +1-vti..................61 Er Efilm T = To Etip 1 - -" H0 H1P, = pom T T (L ( _j 2 ) )..................................................................................... ...... T .m...................................................................................... > 66 6 68 To m -Tm.. ............................................................................................................. H 1 = fl2 HO H0 - [Usv ........................................................................ HO p 2j T 0-T O H0 L H 2.099--_ 1.099 ; -2.099--_ E 2 E 2.103 1.099..................................................................... ~T-T 2 0 9 9 T-Tmn T-Tmin 0.0 68 71 71 0.5498...................................................71 13 14 Chapter I Introduction Standard microelectronics and microfabrication uses the semiconductor manufacturing paradigm. This paradigm is based on using standardized processes and packaging to leverage the economy-of-scale, thus outweighing and spreading the large capital cost over many diverse products. Because microelectronics are ubiquitous in modern technology, this high-volume business model remains economically viable by distributing the large capital costs over many diverse products. Unfortunately, the microelectromechanical systems (MEMS) industry, while using several of the same tools and processes as their microelectronic counterparts, has not been as successful in using this manufacturing paradigm. This is partially because most MEMS products require specialized processes and packaging for each product, consequently requiring dedicated capital equipment, and ultimately the smaller MEMS market size cannot offset the capital costs to become economically viable. Unfortunately, despite potentially having better performance metrics, this increase in price results in many MEMS devices being economically non-viable, unable to compete with cheaper, non-MEMS alternatives. As such, only MEMS products that follow the semiconductor manufacturing paradigm of high-volume have made significant impact in present day markets [1, 2]. Because of this, novel fabrication techniques need to be explored to lower the price of MEMS devices. Historically, commercially-successful MEMS devices have followed the semiconductor manufacturing paradigm. These devices include inkjet printheads, pressure sensors, microphones, and inertial sensors (accelerometers, gyroscopes, and digital compasses). To date, these products still account for 67% of the MEMS market [2]. While low-volume MEMS products and applications do exist, such as oil/gas exploration [3], most low-volume MEMS solutions are too expensive and do not match the price point to meet market demands. Ideally, if MEMS fabrication was to be decoupled from the semiconductor manufacturing paradigm, it might be possible to reduce the large overhead costs that are required for developing and manufacturing new MEMS devices. This would consequently reduce the price of the MEMS and would allow additional niche MEMS products to become economically-viable. Reducing the price of MEMS can be separated into two categories: low-volume and large-area microfabrication. While the former depends on new small-batch tools to be developed and arguably mainly benefits MEMS fabrication, the latter relies on new tools that are not limited to the standard silicon wafer and is applicable to all areas of microelectronics. Because of this, there has substantial effort in developing large-area microfabrication including roll-to-roll manufacturing and direct-write patterning processes. It is important to note, however, that these two categories are not mutually exclusive: one of the direct-write patterning processes, drop-on-demand (DoD) inkjet deposition, is mutually beneficial to both low-volume and large-area microfabrication. DoD inkjet deposition is capable of large-area microfabrication because the inkjet printhead is not reliant on the underlying substrate and thus can be used on any substrate ranging from the standard silicon wafers to the 2.88m x 3.13m panes of glass used in the latest generation of display manufacturing. However, the inkjet deposition apparatus can also be scaled down, enabling benchtop microfabrication units capable of lowvolume microfabrication. 15 Because of the potential impact of DoD inkjet deposition, the author of this thesis previously developed and demonstrated a direct-write surface micromachining process using Thermal Inkjet (TIJ) printing technology by fabricating silver nanoparticle cantilevers [4]. These cantilevers were mechanically characterized to have a Young's modulus of 22 ± 1.5 GPa. While these cantilevers were self-supporting, it was noted at the time that the silver nanoparticle cantilevers exhibited a significantly lower modulus than the 83 GPa modulus of bulk silver. Subsequent investigations into this lower Young's modulus suggested that the silver films created from this surface micromachining process are process-dependent and are particularly sensitive to the process used to sinter the silver nanoparticles into films [5, 6]. Because careful mechanical characterization is critical for successful application of the technology, determining the process-mechanical property relationship is important for the adoption of this class of procedures into a variety of applications. This thesis aims to deliver a methodology for developing process-property models for metal nanoparticle films using silver nanoparticles as a case study. The surface micromachining process will be modified and optimized for studying how sintering affects the nanoparticle film's mechanical properties. Finally, the methodology for characterizing and optimizing metal nanoparticle microstructures will be discussed. To study a larger range of temperatures for the thermal sintering process, a surface micromachining process will be developed using a robust sacrificial layer that is thermally stable at high temperatures. Using this robust sacrificial layer, silver nanoparticles will be printed, sintered under varying conditions, and mechanically characterized to determine how the sintering parameters affect the resulting mechanical properties. Using this data, a process-mechanical property prediction model will be developed accordingly. In addition to developing the model, several new learnings in characterizing and optimizing printed nanoparticle film will be introduced. This thesis will start with a general overview of surface microfabrication techniques, primarily focusing on prior art in direct-write microfabrication and metal MEMS devices. The following chapter highlights the materials chosen for the surface micromachining process, including several potential alternative materials that were also considered. Afterwards the surface micromachining process development, characterization, and optimization will be presented, followed by the methodologies used to characterize the metal nanoparticle films and their respective results. Finally, the development of the process-mechanical property model will be discussed and a process-mechanical property model for silver metal nanoparticle films will be presented. At the closing, a summary of the work highlighted in this thesis, as well as a few final remarks and suggestions on future work will be made. 16 Chapter II Background II.1 Overview There have been significant efforts to lower the cost of microfabrication. These efforts range from reducing material waste to nonconventional fabrication techniques. One notable area isthe development of tools and techniques to reduce the need for conventional photolithography. Implications of reduced reliance on photolithography is four-fold: fewer fabrication steps, lower costs, fewer design-to-fabrication delays because the devices would no longer require etch masks for patterning, and less material waste because the entire substrate would no longer need to be covered prior to patterning. In order to replace photolithography, the capability of depositing pre-patterned layers is currently being explored and is generally known as direct-write microfabrication. Despite already deciding to use DoD thermal inkjet technology for this body of work, a general review on direct-write microfabrication will be presented in this chapter. The purpose of this chapter is to describe the two conventional micromachining techniques and highlight some of the existing alternative technologies that exist and summarize the advantages and disadvantages of direct-write technologies. II.2 Conventional microfabrication Microelectromechanical systems (MEMS) fabrication primarily fits into two categories: bulk and surface micromachining. The former consists of removing significant portions of the underlying substrate to define the desired features, whilst the latter consists of addition and subsequent removal of materials on top of the base substrate [7]. Figure 1I-1 summarizes the general processes for both micromachining techniques. Both of these categories rely heavily on semiconductor industry manufacturing methods, especially lithography, a technique which relies on mask layers to selectively protect the covered portions while etching the exposed regions. Surface Micromachining 1) Deposit and pattern sacrificial layer Bulk Micromachining 1) De osit and pattern mask layer Eiciallaer Etch mask - 2) Deposit and pattern structural layer Substrate 2) Etch substrate to release structure 3)Remove sacrificial layer to release structure Figure 11-1. Illustrations of bulk and surface micromachining processes. 17 While there were many reasons that led to focusing on surface micromachining instead of bulk micromachining, the main reason was to decouple microfabrication from the substrate restrictions of the semiconductor manufacturing paradigm. Because bulk micromachining requires removing sections of the substrate to create microstructures, new techniques and processes need to be developed for every new substrate material. Surface micromachining, on the other hand, ideally only needs one process and technique to be developed for surface all substrate materials. Taking this into consideration, we chose to focus on developing a direct-write surface micromachining process. 11.3 Direct-write microfabication After deciding to focus on surface micromachining, several technologies were taken into consideration to replace photolithography. These technologies fall into a category known as direct-write microfabrication. Direct-write microfabrication encompasses a wide variety of technologies including laser-writing, inkjet deposition, extrusion deposition, and electrochemical deposition [8]. These technologies have advantages and disadvantages that make them great for specific applications and poor in others. 11.3.1 Laser-writing Laser-writing encompasses a class of deposition techniques that utilizes laser excitation mechanisms to pattern micro- or nano-features onto a substrate. This excitation causes a change in the materials, resulting in deposition, ablation, sensitization, or chemical activation. Laser direct imaging (LDI) has demonstrated microfluidic channels formed by physically ablating glass [9-11]. Laser induced thermal imaging (LITI) uses a donor film and selectively deposits bioactive components for biosensors [12] and OLED displays [13] by thermally evaporating them onto the adjacent substrate. Other forms of laserwriting include using the thermal effects of the laser to drive localized chemical reactions [14, 15], and modifying the substrate surface to seed metal depositions [16]. 18 a) Mirror LBam<_ z objective Dital Pattern y 6, j/w Substrate x. e a substrate Figure 11-2. Illustrations showing the principle of LDI and LITI. [8, 12] a) LDI etches the pattern into the substrate by digitally feeding it to the system which controls the optics as well as the location of the substrate. The laser ablates the substrate to form the microstructures. b) LITI uses a donor layer to selectively deposit material onto the substrate; this process wastes a lot of material because the remaining material on the donor layer is usually discarded after the deposition. One of the most notable examples of laser-writing for MEMS fabrication is the fabrication of magneticpolymer microstructures [17]. This work used lasers to electrostatically charge polymer bead embedded with ferromagnetic particles so that they adhere to an oppositely-charged substrate. The beads were fused together to form contiguous films and then released by removing the underlying substrate. While this work successfully demonstrated a bulk micromachining process, the materials are limited to polymers, thus narrowing it potential applications. While laser-writing is capable of a wide variety of micropatterning techniques, it has several limitations for MEMS fabrication. Firstly, lasers typically operate in line-of-sight configurations, making the complex structures required for MEMS difficult to fabricate. Secondly, lasers are typically used for their subtractive process and require extra materials and complexities to provide additive capabilities. Thirdly, because many additive laser technologies such as LITI require a transfer layer, the technology still wastes substantial amounts of the donor material. Fourthly, the excitation from the multiple laser pulses subjects the samples to repetitive thermal cycling which is potentially problematic if there is a large thermal expansion coefficient mismatch between the samples' materials. Finally, because the laser spot size is small and most lasers are Gaussian, the intensity of the laser spot is ultimately not uniform, potentially resulting in poor feature resolution as well as overheating issues. 11.3.2 Inkjet Having been used and optimized for decades in the printing industry, this mature technology has been adapted for use in microfabrication [8, 18]. Unlike laser-writing, inkjet deposition can be done at low temperatures, greatly expanding the materials to choose from. Inkjet printing relies on the Plateau19 Rayleigh instability which states that because of surface tension, a falling stream of liquid will reduce its surface area by breaking up into smaller volumes known as droplets. inkjet deposition falls roughly into two categories: continuous and Drop-on-Demand (DoD). Continuous deposition is when the printhead generates a continuous flow that breaks apart into many individual droplets as it is extruded from the nozzle. As the droplets are produced, they are deflected to either deposit onto the substrate or into a gutter to be collected and reused. An illustration of continuous inkjet is shown in Figure 11-3. Ultimately, because of evaporation, the recycled ink is more concentrated than the original ink and needs to be rediluted with extra solvents to maintain the proper concentration and viscosity. Because of this, additional controls are required to constantly monitor and modify the ink in the reservoir to ensure consistency. A subset of continuous deposition, electrospraying deposition (ESD), utilizes electric fields to charge the droplets and deflect them towards the substrate. ESD is currently being explored in several applications, including producing drug delivery agents, biofilm coatings, and battery electrodes [19, 20]. Unfortunately, the large spray area of ESD currently eliminates it as a possible direct-write microfabrication technique. Drop Generator Charge electrode -=- High voltag deflection plate Guner PMpe Figure 11-3. illustration of continuous inkjet printing. This illustration uses electrostatic charging to deflect the droplets however other methods such as thermal gradients can be used. Not shown in this illustration are the additional controls that are required if the ink collected from the gutter is returned to the ink reservoir and recycled. [211 Unlike continuous deposition, which continually generates a steady flow, Drop-on-Demand (DoD) deposition generates droplets only when needed. This drastically reduces the amount of ink used and eliminates the need for extra components to monitor the ink properties. DoD printheads primarily use either thermal inkjet or piezoelectric inkjet technologies. In both cases, the ink within the firing chamber is rapidly displaced, generating a small liquid column that is ejected out of the printhead's nozzle. Due to surface tension effects, this liquid column rapidly coalesces into a single spherical droplet and subsequently impacts the substrate. Thermal inkjet (TIJ) printheads rapidly vaporize a small amount of the ink, creating the sudden expansion needed to jettison a droplet. Piezoelectric printheads utilize piezoelectric components, such as a ceramic perovskite that changes dimensions under electric fields called lead zirconate titanate (PZT), to generate the sudden displacement. There are several types of piezoelectric printheads, however they all share the same basic principle of creating a sudden displacement to generate the droplet. Figure 11-4 displays illustrations of TIJ and various piezoelectric inkjet configurations. Regardless of the mechanism, both TIJ and PZT printheads are capable of producing smaller spot sizes (<25ptm) in than the state-of-the-art Cli printheads (40pm). 20 Push-mode piezoelectric inkjet Thermal Inkjet Heer OriTransducer foot Presse chamber Piezo Diaphragm Ink Bend-mode piezoelectric inkjet iz Diaphragm Sheer-mode piezoelectric inkjet let s Piezo Piezo4 ceramic Figure 11-4. DoD inkjet print head designs. [21] While DoD inkjet printing has many advantages, it also has limitations. Several parameters such as ink formulation, jetting, material compatibility, and drying need to be optimized in order for to produce repeatable and reliable results. Another limitation is the inks need to be non-viscous (0.1-100cP) in order to be printable which subsequently limits the spatial resolution of the technology because the printed spot size is correlated with the ink viscosity. As such, the non-viscous inks have limited spatial resolution of roughly twice the diameter of droplet [8]. As the state-of-the-art printhead can deliver 0.52pL (9.8-15.6pm in droplet diameters), the spatial resolution of the technology is currently limited to 2031pm. These non-viscous inks also have another limitation that they are unable to deliver extremely high material loads in a single droplet, which will be described in detail in a following section. Another challenge with inkjet printing is reliability of successfully jetting a droplet. Unsuccessful jetting, or misfires, is a fairly regular occurrence even when the printing parameters are carefully optimized. As such, print jobs where droplet placement is critical require additional monitoring systems to ensure a droplet is successfully jetted. Despite all of the prerequisites and the limited spatial resolution, DoD inkjet printing has demonstrated several microfabication successes including printing etch masks, active and passive electrical components [22, 23], cantilevers [4, 22, 24], piezoelectric materials [25], organic light emitting diodes, and organic solar photovoltaic heterojunctions [18]. Figure 11-5 shows some materials deposited using DoD inkjet deposition. 21 b) c) Figure 11-5. Examples of materials and devices deposited using inkjet deposition. a) A silver nanoparticle resonant inductive coil [22], b) Alq 3, a material commonly used in small molecular organic light emitting diodes [18], c) A silver nanoparticle MEMS switch with a PVP gate dielectric [24], and d) a surface micromachined silver nanoparticle cantilever [4]. 11.3.3 Extrusion Arguably an extension of continuous inkjet printing, extrusion deposition is relatively new to microfabrication. Unlike continuous inkjet printing, the inks used in extrusion deposition are much more viscous (100cP-3000cP). As such, the Plateau-Rayleigh instability no longer applies and a continuous "string" of material is deposited onto the substrate. Extrusion deposition has been used to fabricate 3dimensional electrical traces for antennae and wire-bonding [26]. Because the inks' high viscosities, the inks do not spread as much when deposited and the resulting spatial resolution an order of magnitude higher than standard DoD inkjet printheads, and is capable of achieving 1:1 aspect ratios, thus delivering large amounts of material in a short amount of time. An example of extrusion deposition is shown in Figure 11-6 Figure 11-6. Silver microelectrodes using extrusion deposition. [26] The microelectrodes were patterned using 5- (top left), 10- (bottom left), and 30-pm (right) nozzles. Regardless of nozzle size, the deposition is capable of near 1:1 aspect ratios, allowing for high spatial resolution while delivery large amounts of material. 22 Unfortunately, while the viscous inks can deliver large material loads, depositing such viscous materials requires high pressures (70-700kPa) to extrude through small openings. As such, the printhead needs to be made out of robust materials and require industrial-grade piston pumps to drive the system. Similar to inkjet, many of the ink and drying parameters also need to be optimized before repeatable and reliable result can be achieved. 11.3.4 Electrochemical The final direct-write deposition discussed is electrochemical deposition. This process leverages electrochemistry to deposit or etch material with the aid of electricity. Direct-write capability is achieved by using small, localized electrodes to generate the electric fields such as the methods illustrated in . Unfortunately,:his type of deposition requires an electrolyte carrier liquid to facilitate the chemical reaction, and as such the area which the deposition/etching is occurring needs to be immersed in the electrolyte. Because of this added complexity, electrochemical deposition was never seriously considered as a potential solution for this MEMS fabrication process. b) Electrochemical Etching a) Electrochemical Deposition a Anode cathode Nozzle d Electrolyte .- Power Supply Met Anode Cathode Figure 11-7. illustrations of electrochemical deposition and etching. [8] Both deposition and etching require localized electrodes to selectively deposit/etch material. a) when depositing material, a nozzle is used to control the regions of the substrate that are exposed to the electrolyte, thus also controlling the deposition regions. b) A small, localized cathode is accurately placed above the anode; by carefully controlling the potential across the electrodes, the regions where the anode etches can also be isolated. 23 24 Because this body of work focuses on characterizing the process-mechanical property relationship of these metal nanoparticle films, no alternative materials were considered. However, it must be acknowledged that other methods of silver deposition via printing exist and the following section will highlight some of the alternative methods to direct-write silver films. 111.3.1 Overview of Silver Deposition Methods Depositing silver via inkjet printing can be classified into two categories: chemical and physical deposition. In chemical deposition, silver precursors are printed onto the substrate and subsequent reactions cause silver to precipitate and deposit onto the substrates. Physical deposition involves printing elemental silver directly onto the substrates. Because chemical depositions rely on both precipitation and reacting with the silver already deposited on the substrate, the porosity of resulting silver film is lower than films deposited using physical methods. This porosity leads to reduced electrical and, potentially, mechanical properties. However, chemical deposition also has its disadvantages: the chemical reactions have many byproducts and additional processing steps must be taken to remove the byproducts and avoid contamination. As such, chemical depositions are procedurally more complex than physical depositions. 111.3.2 Chemical Deposition The three forms of depositing silver chemically are electroplating, electroless plating, and pyrolysis. In all of these forms, silver precursors are deposited onto substrates and subsequently reduced to precipitate silver onto the substrate. The difference between each form is how the silver precursors are reduced. 111.3.2.1 Electroplating Simplistically, electroplating is an anode-cathode reaction where aqueous silver acts as an electrolyte and subsequently deposits onto the cathode (the desired substrate). Electroplating has two requirements: an electric current to facilitate depositing the metal ions and a seed layer that the metal ions are deposited on. The main challenge with electroplating is satisfying these two conditions while maintaining selective deposition. One electroplating method is to deposit and pattern the seed layer while ensuring each seed layer remains electrically connected so that they can be electroplated. Obviously maintaining electrical contact with every seed layer stipulates very unreasonable design restrictions and thus is not ideal for direct-write patterning. Another method requires depositing a seed layer over the entire substrate and subsequently depositing and patterning an insulator to be used as an electroplating mask. This ensures that the only locations where electroplating occurs is at the regions that are not covered by the electrical insulator. After electroplating, the insulator and unwanted seed layers would have to be stripped from the substrate. Unfortunately, this second method cannot be considered a direct-write technique because the seed layer has to uniformly coat the substrate. Ultimately, with the above considerations as well as the necessary compatibility between the electrolytic bath, substrate, and sacrificial layer, it was reasoned that electroplating silver was a poor solution for silver deposition. 111.3.2.2 Electroless Plating While electroplating has many restrictions, namely its inability to selectively deposit material and requiring the entire substrate be submerged in an electrolytic bath, many of these detractions can be 26 removed by using electroless plating. Electroless plating consists of reducing silver inorganic salts to precipitate elemental silver onto the substrate. The process is commonly used in the silver mirroring of laboratory glassware. Electroless plating is typically achieved by complexing silver nitrate with ammonium hydroxide (aqueous ammonia) in the following chemical reactions [27]: 2AgNO 3 + 2NH4 0H -> A920 + 2NH 4 NO3 + H2 0 A920 + 4NH4 0H -> 2[Ag(NH 3 ) 2 ]OH + 3H20 [Ag(NH 3 ) 2]OH + NH 4 NO3 <-+ [Ag(NH 3 ) 2]NO 3 + NH 4 0H The complex is then reduced by adding an aldehyde (R - CHO) such as glucose/dextrose or formic acid, resulting in the following chemical reaction: 2[Ag(NH 3 ) 2 ]NO3 + R - CHO -+ 2Ag + 4NH4 + R - COOH + H20 While silver nitrate is readily soluble in water and reasonable material loading is possible (a 2M ammoniacal silver solution made from 2M silver nitrate and 5M ammonium hydroxide is ~14% silver by weight), several challenges need to be taken into consideration before using electroless plating. Firstly, at high concentrations, aqueous silver nitrate is highly corrosive and should be handled with care. Secondly, ammoniacal silver is very unstable and batches need to be mixed on a daily basis. Careful consideration needs to be taken when disposing of old batches of ammoniacal silver as one of the main byproducts is silver nitride which is highly explosive. Another consideration is the disposal or remove of the chemical by products in electroless plating. There are three unwanted byproducts: ammonium hydroxide, carboxylic acid, and water. While the ammonium hydroxide and water can be removed by evaporation, ammonia gas is an irritant and should be disposed of properly. The carboxylic acid, on the other hand is usually removed by bathing the silver films in an oxidizing acid to remove all unwanted organic compounds. Ideally, if formic acid is used as the reducing agent, the resulting carboxylic acid oxidizes to water and carbon dioxide without assistance. However, because precursors are typically not completely balanced, it is assumed that some precursors will remain after the chemical reaction completes. This implies that there should be a mandatory cleaning step post-deposition. One final challenge is that the electroless plating requires a 2-step printing process. Because the chemical reactions readily occur at room temperatures, premixing the ammoniacal silver and reducing agent would result in silver being deposited onto the printhead while the solution is being printed. This ultimately would clog the print nozzle and little-to-no silver actually deposits onto the substrate. To rectify this, the ammoniacal silver and reducing agent would need to be printed separately, introducing several additional challenges including preventing the deposited films from drying before the other reactant is printed, alignment issues, and added complexity of swapping inks mid-printing. Despite all the challenges, electroless printing has recently been demonstrated for printing flexible electrodes [28]. In this demonstration a silver ink formed from silver acetate, ammonium, and formic acid was airbrushed onto a plastic film to create flexible electrodes that became conductive at 27 removed by using electroless plating. Electroless plating consists of reducing silver inorganic salts to precipitate elemental silver onto the substrate. The process is commonly used in the silver mirroring of laboratory glassware. Electroless plating is typically achieved by complexing silver nitrate with ammonium hydroxide (aqueous ammonia) in the following chemical reactions [27]: 2AgNO 3 + 2NH 4 0H -+ A920 + 2NH 4 NO3 + H2 0 A920 + 4NH4 0H -+ 2[Ag(NH 3 ) 2 ]OH + 3H 2 0 [Ag(NH 3 ) 2 ]OH + NH 4 NO 3 ++ [Ag(NH 3 ) 2 ]NO3 + NH4 0H The complex is then reduced by adding an aldehyde (R - CHO) such as glucose/dextrose or formic acid, resulting in the following chemical reaction: 2[Ag(NH 3 ) 2 ]NO3 + R - CHO -> 2Ag + 4NH4 + R - COOH + H2 0 While silver nitrate is readily soluble in water and reasonable material loading is possible (a 2M ammoniacal silver solution made from 2M silver nitrate and SM ammonium hydroxide is ~14% silver by weight), several challenges need to be taken into consideration before using electroless plating. Firstly, at high concentrations, aqueous silver nitrate is highly corrosive and should be handled with care. Secondly, ammoniacal silver is very unstable and batches need to be mixed on a daily basis. Careful consideration needs to be taken when disposing of old batches of ammoniacal silver as one of the main byproducts is silver nitride which is highly explosive. Another consideration is the disposal or remove of the chemical by products in electroless plating. There are three unwanted byproducts: ammonium hydroxide, carboxylic acid, and water. While the ammonium hydroxide and water can be removed by evaporation, ammonia gas is an irritant and should be disposed of properly. The carboxylic acid, on the other hand is usually removed by bathing the silver films in an oxidizing acid to remove all unwanted organic compounds. Ideally, if formic acid is used as the reducing agent, the resulting carboxylic acid oxidizes to water and carbon dioxide without assistance. However, because precursors are typically not completely balanced, it is assumed that some precursors will remain after the chemical reaction completes. This implies that there should be a mandatory cleaning step post-deposition. One final challenge is that the electroless plating requires a 2-step printing process. Because the chemical reactions readily occur at room temperatures, premixing the ammoniacal silver and reducing agent would result in silver being deposited onto the printhead while the solution is being printed. This ultimately would clog the print nozzle and little-to-no silver actually deposits onto the substrate. To rectify this, the ammoniacal silver and reducing agent would need to be printed separately, introducing several additional challenges including preventing the deposited films from drying before the other reactant is printed, alignment issues, and added complexity of swapping inks mid-printing. Despite all the challenges, electroless printing has recently been demonstrated for printing flexible electrodes [28]. In this demonstration a silver ink formed from silver acetate, ammonium, and formic acid was airbrushed onto a plastic film to create flexible electrodes that became conductive at 27 temperatures as low as 90*C. Unfortunately, like many of its predecessors, this silver ink was developed primarily for conductive traces, meaning that at this time there has been little to no characterization on film thickness and mechanical properties. In summary, when considering all the complexity required for electroless such as ink instability, additional cleaning steps, extra safety precautions, and 2-step printing processes, it was concluded that electroless plating was not the best technique to deposit silver via inkjet printing. 111.3.2.3 Pyrolysis Silver can also be deposited onto substrates by pyrolyzing silver organometallic resins. Unfortunately, such organometallic resins are typically extremely unstable and many of the compounds decompose at temperatures lower than O'C [29]. Additionally, not all organometallic silver compounds decompose down to their elemental form. One report [30] regarding a proprietary compound is stable at room temperatures, however, that resin requires temperatures greater than 4000 C to begin pyrolyzing, which is greatly out of the desired thermal limit for this process. Given the unstable nature of organometallic silver, such materials are not suited for depositing silver using inkjet printing. 111.3.3 Physical Deposition There are several physical methods for depositing silver including evaporation, sputtering, and even printing elemental silver. Because evaporation and sputtering require high vacuum environments, only one physical method was considered: printing nanoparticle silver. Nanoparticle silver consists of silver that has been precipitated using chemical reactions similar to the one mentioned above in electroless plating [26, 31-34]. To prevent excess aggregation and crystal growth, a surface capping agent or ligand shell is added to stabilize the nanoparticles. Typically the nanoparticles are then resuspended in a liquid medium, forming a nanoparticle ink. III.3.3.1 Nanoparticle Synthesis Synthesizing the nanoparticles is achieved by reducing silver salts, typically silver nitrate, to form elemental silver. Initially, the nanoparticles precipitated are 5-6nm in diameter; however recipes usually include exposing the particles to elevated temperatures (<50*C) to increase the stability and longevity of the nanoparticles by allowing them to aggregate and grow into larger particles. To prevent the nanoparticles from growing too large, ligands such as poly(vinyl pyrrolidone) (PVP) or poly(acrylic acid) (PAA) are added to the solution to stabilize the nanoparticles. The particles are then concentrated and reconstituted into another carrier liquid to form nanoparticle inks. This process is summarized in Table Ill-1. 28 Table 111-1. Example of nanoparticle synthesis process Other silver nanoparticle precursors, stabilizers, and reductants are possible [26, 31, 32, 34]. Description Step - Silver nanoparticle precursors Source: Silver nitrate (AgNO3), Stabilizer: PAA, Reductant: Diethoanolamine 1 2 3 Precipitate silver Grow nanoparticles Concentrate nanoparticles Homogenize and resuspend Dissolve precursors in water Sonicate particles for 2 hours in heated water bath Centrifuge particles at 9000rmp for 20 minutes, supernatant is decanted to recover the nanoparticle precipitate Dilute nanoparticles in ethylene glycol to desired concentration 4 nanoparticles Formulating a nanoparticle ink requires several considerations including particle size repeatability, ink viscosity variation, shelf-life. To avoid these challenges, a commercially available ink was chosen so that the ink recipe and compilation would not need to be optimized. The ink used in this work was Cabot Corporations' CCI-300. CCI-300 consists of 30-40nm-dia metered silver nanoparticles suspended in ethylene glycol and ethanol. The material loading of the ink was 30% by weight or 3.5% by volume. 111.3.3.2 Sintering nanoparticles Nanoparticles inks are an extremely effective method for delivering high material loads. However, in order to use such inks for surface micromachining processes, the nanoparticles need to be fused together to form a contiguous film. Fusing, or sintering, the nanoparticles typically entail thermally treating the nanoparticles, allowing the atoms within the particles to diffuse into neighboring particles and coalesce. Because of their extremely high surface:volume ratio, nanoparticles exhibit very interesting thermodynamic properties, most importantly nanoparticles have melting temperatures significantly lower than their bulk counterparts. While this phenomenon, known as melting point depression, has been studied extensively, the mechanisms governing the depression are not currently agreed upon. Several mechanisms have been proposed, ranging from classical thermodynamic analyses to atomistic approaches such as atomic diffusion and bond-order-length-strength (BOLS) models. Atomistic approaches suggest different mechanisms for nanoparticle sintering such as atomic diffusion [35] and BOLS [36]. Atomic diffusion is based on the concept of interstitial mechanisms that allow the metal atoms to migrate from point vacancies within the nanoparticles. Because the generation of point vacancies is correlated with the Arrhenius equation, the rate of atomic diffusion increases with temperature. In short, the atomic diffusion assumes that the individual atoms within the nanoparticles actually move between crystal lattices rather than physical melting. As such, atomic diffusion describes the sintering mechanism as both time-dependent and temperature-dependent. BOLS, on the other hand, focuses on the cohesive energy of the individual atoms to determine the nanoparticle's melting point. Because BOLS does not rely on point vacancies and diffusion for its derivation, it is not correlated to the Arrhenius equation and as such does not factor time-dependency into its calculations. Classic thermodynamic analyses have suggested three possible mechanisms for nanoparticle melting/sintering: the liquid drop model (LDM), liquid shell nucleation (LSN) model, and the liquid nucleation and growth (LNG) model [37]. While LDM predicts that the nanoparticle melts 29 instantaneously once it reaches temperature, the other models predict that the melting initiates at the particle surface and quickly propagate to the core. Even though each model is governed by different sets of equations, there is little difference in the predicted melting temperature between the models as shown in Figure 1ll-1. These melting-pointparticle-size curves were derived using the thermodynamic relationships as defined by the LDM, LSN, and LNG models and are shown in Equations (1)-(3): T(d) = LDM - 0 PS 2/3 T~l)vk'I Fd LSN (1) I (2) 1 Ps T ="0 (d) 6SV- 07yllp, Fd Ps,,( - 3 0 T (d) = (os LNG where T is the melting point of the nanoparticle, To isthe bulk material's melting point, d is the diameter of the nanoparticle, do is the atomic diameter of the material, o is the surface energy/tension, and p is the density the material. The subscripts represent the solid (s), liquid (1), and vapor (v) states of the material. Because the difference between predicted melting temperatures is insignificant, any of the models can be used to develop the semi-empirical relationship. According to the technical literature provided for CCI-300, the nanoparticle films become electrically conductive after greater than 100 0 C. Assuming that sintering is the cause for making the film electrical conductivity, models suggest that the nanoparticle should be 34-40nm. This is consistent with the technical literature, which states that the nanoparticles should be 30-40nm in diameter. 1050 900 750 600 450 300 150 0 -150 -3nn 50 100 150 I 200 I-LDM 250 -LSN 300 -LNG Particle Diameter [nm] Figure Ill-1. Melting Temperature vs. Particle Diameter for the thermodynamic models. III.3.4 Summary of Silver Deposition Methods While there are many methods to deposit metallic silver, many of these solutions have significant disadvantages when incorporating into surface micromachining processes. Of all the methods, physically depositing nanoparticles was determined to be the best solution for depositing silver and its use as a 30 structural layer was successfully demonstrated in a surface micromachining process [4, 5]. To reiterate nanoparticle inks, more specifically the CCI-300 ink, had several advantages which made it very promising for the project: * e * Commercially available As a commercial product, the silver ink was readily available at the start of the project, and experiments could start immediately. Lower fusing/sintering temperature The silver-based conductive ink primarily consists of 30-40nm diameter nanocrystals suspended in an ethylene glycol/ethanol ink vehicle. As a frame of reference, the product description states that the film becomes conductive after sintering at 100 0C sintering. This low temperature processing is very appealing because it allows for a wide variety of substrates, including plastic and flexible substrates. ink-Jet compatible Originally designed for printed electrodes and antennae, Cabot Corp.'s conductive ink was designed to be used with ink-jet technology. Very little modification was needed to make the ink readily printable with the TIJ system. " Previous experience and studies The final advantages with using this silver-based conductive ink were the previous studies and experiments to determine its compatibility with the TIJ system. Additionally, many of the parameters for printing, drying, and sintering were previously optimized but will be explained in further detail in a following chapter. Finally, it was successfully demonstrated in an all-printed surface micromachining process by fabricating free-standing cantilevers. III.4 Sacrificial Layer With the structural material already identified, the main challenge was to identify compatible and convenient materials that could be used as the CCI-300's sacrificial layer. While there are many potential materials that are compatible with the silver ink, the following guidelines were set to determine the ideal sacrificial material: * e * Inert to silver ink components Compatibility with the silver ink requires that the sacrificial material be nonreactive to the silver nanoparticles and its ink vehicle. In addition to being nonreactive, the sacrificial material should be extremely insoluble in ethylene glycol and ethanol to avoid the sacrificial layer being dissolved, subsequently destroying the features while jetting the silver ink. Able to withstand the silver nanoparticle's sintering temperatures Requiring sintering temperatures greater than 120*C to obtain a structurally sound silver film, the silver ink's post-processing step creates a steep requirement. Several organic polymers such as polystyrene, which chemically are promising due to their resistance to the silver ink, become unstable at elevated temperatures and thus cannot be considered. This temperature restriction also provides another obstacle concerning removal as will be discuss next. Easily removed without harming the silver film After the sintering step, the sacrificial material must be removed without affecting the silver film. This becomes increasingly difficult to do with higher sintering temperatures because some sacrificial layers densify and become harder to etch via chemical processing. Ideally, the sacrificial layer should be removed with dry release techniques such as pyrolysis, where the 31 sacrificial material decomposes at high temperatures, or sublimination where the material transitions from a solid to a gas. Dry release techniques are preferred over wet release techniques to avoid stiction, where the structural layer sticks to the substrate after release. e Produce reliable and repeatable sacrificial features Because the purpose of this investigation isto characterize the process-mechanical property relationship for sintered metal nanoparticle films, the sacrificial features ideally should be identical throughout the study. Identical sacrificial features would reduce the structural variation between samples, thus eliminating the uncertainty that the change in the mechanical properties are from inconsistent microstructures. Given these guidelines defining the ideal sacrificial layer, several materials were experimented on and studied to identify what materials are compatible with the silver ink structural layer and what advantages/disadvantages they had when being used in a surface micromachining process. 111.4.1 Poly(methyl methacrylate) (PMMA) A transparent thermoplastic, PMMA is often used in microfabrication as a resist layer for electron beam lithography. PMMA has several characteristics that make it a promising sacrificial material for surface micromachining including chemical robustness and can withstand temperatures of at least 250*C [4]. PMMA also boasts several methods of removal, including 02 plasma, pyrolysis, and a wide variety of organic solvents. One final note about PMMA as a sacrificial material is that it has already been demonstrated in the author's fully-printed surface micromachining process. Because of these considerations, PMMA was identified as a strong candidate to use as a sacrificial material to study the process-mechanical property relationship of the CCI-300 films. The detailed investigation in the optimization of PMMA will be presented in the following chapter. 111.4.2 Positive photoresist Ubiquitous in cleanrooms, these photosensitive polymers are used as mask layers in photolithography procedures. Because photoresist deposition and patterning has been highly characterized, obtaining repeatable and reliable sacrificial features is a non-issue. In theory, photoresist is easily removed via exposure to photodeveloper; however, long exposures to greater than 100*C heat treatments make removal via photodeveloper challenging. Several other removal methods were investigated, including 02 plasma and 80 0C acetone. Detailed results of using photoresist as a sacrificial layer will be presented in a following chapter. 111.4.3 Amorphous silicon (a-Si) A non-crystalline form of silicon, a-Si has been explored for several low-cost, low-temperature electronic devices such as solar photovoltaics [38]. Capable of being deposited in thin films and at low temperatures onto a variety of substrates, a-Si is a promising candidate as a sacrificial layer. a-Si shares many of the same material properties as conventional silicon, including a very robust temperature budget and can be patterned by traditional microfabrication techniques. a-Si's robust temperature budget and reliable patterning makes it an excellent candidate to study the nanoparticle sintering process. Conventionally, a-Si is deposited by plasma-enhanced chemical vapor deposition and patterned by traditional photolithography and dry-etched with XeF 2, however it is possible in the future to migrate from conventionally patterned a-Si to printable a-Si through the formulation of a-Si inks [39]. 32 Chapter IV Fabrication Process Characterization and Optimization IV.1 Overview There are two distinct layers in a simplistic example of surface micromachining: the structural and sacrificial layers. The structural layer is comprised of materials that will form the final structures and features while the sacrificial layer holds the structural layer in place during the micromachining process. After the process is finished, and the structural layer is stable, the sacrificial layer is removed, or sacrificed, leaving the structural layer conformed to features found in the sacrificial layer. Realistically, surface micromachining is much more complex. The sacrificial and structural layers need to be deposited and patterned, both of these processes require the optimization of several parameters before the desired microstructure features such as thickness and thickness uniformity can be accurately and repeatedly obtained. In conventional microfabrication, many of these parameters and/or methodologies have already been determined by manufacturers, and as such standard processes do not require extensive optimization. However, non-standard processes, such as printing, have not been preoptimized by manufacturers and therefore require extensive characterization and optimization. As such, this chapter entails the development, characterization, and optimization of the microfabrication processes used to investigate the process-property relationship of nanoparticle silver films. As mentioned in the previous chapters, the author of this thesis previously developed a fully-printed surface micromachining process [4]. This process uses a digital fabrication system to print a PMMA sacrificial step, followed by a process to smooth the printed PMMA film known as solvent vapor annealing. A PMMA barrier is printed around the step to contain the silver nanoparticles as they sinter. After sintering, the silver is laser ablated or laser-trimmed to remove unwanted features. Finally, the structures are released by using organic solvents to dissolve the PMMA. Figure IV-1 summarizes the surface micromachining process. This process will be used as a template for developing the surface micromachining for studying the process property relationship. 33 1) Print PMMA Step Inkjet Nozzle 4) Smooth PMMA Barrier 6) Sinter silver 0 2) Smooth PMMA Spacer 7) Sinter silver Lase 5) Pattern silver 3) Print PMMA Barrier o0 .8) Release structure Slver ME NE M Figure IV-1. Fully-printed surface micromachining process. The process will be used as the basis for the surface micromachining process to study the process-property relationship. IV.2 Structural Layer Because the structural layer was already determined to be CCl-300, a commercially available silver nanoparticle ink, very little needed to be done in optimizing the physical material. However, the material composition comprises only a small amount of the actual process optimization necessary when depositing the ink onto substrates. This is especially pertinent to this body of work because the CCI-300 was printed onto non-porous substrates instead of conventional deposition methods such spin-coating or printing onto porous substrates. Because of this, several additional parameters needed to be characterized and optimized, including a variety of printing and drying parameters such as ink viscosity, ink reservoir back pressure, jetting pulse waveform, droplet density, and printing/drying substrate temperatures. Prior to addressing the optimization of depositing and drying CCI-300 onto borosilicate/glass substrates, an overview of the printing apparatus and printing and drying parameters will be presented. Aside from the specifications and capabilities of the apparatus, this overview will include the theory behind inkjet printing, the printing pattern generation, and the mechanisms that drive material transport during drying. After presenting the theory and the methodology to optimize the various parameters, the optimization of the CCI-300 ink will be used as an example. While the optimization of parameters will focus CCI-300, the methodology should be transferrable to other inks. At the end of this section a summary of all the optimized parameters and the process of record for printing and drying CCI-300 onto borosilicate/glass substrates will be presented. IV.2.1 Printing apparatus overview The printing technology used for this work is a custom system designed by HP. This digital digital fabrication system was originally designed as a testing apparatus for optimizing HP's printing parameters but its capability of high resolution placement and fully-customizable movement makes it the ideal system for printed microfabrication. The system consists of thermal inkjet pico-fluidic system (TIPS), a handheld TIJ controller that uses disposable 0.5mLTIJ printheads. The TIPS controller and the disposable TIJ printheads are respectively shown in Figure IV-2 and Figure IV-3. 34 Figure IV-2. Picture of Thermal Inkjet EIco-fluidic lystem (TIPS). The majority of the system is the TIPS controller. The controller is used to drive the TIJ die located on the Tip (lower left and Figure IV-3), and all the parameters needed to print with the device can be configured using the device's menus or the computer user interface. - Electrical interconnect TIJ Die -0.5mL reservoir Figure IV-3. Replaceable TIJ printhead & reservoir. A disposable TIJ printhead attached to a 0.5mL reservoir. This attaches to the TIPS controller shown in Figure IV-2. The TIPS is mounted to a 1im resolution Galil X-Y motion controller stage for precise droplet placement. Also mounted to the motion controller are top- and side-view cameras to visualize the droplets. Figure 35 IV-4 shows a picture of the assembled digital fabrication system. The digital fabrication system is assembled over a programmable heated stage for in-situ substrate heating during printing as well as providing post-printing thermal processing capabilities. Microcontroller arms Top-view camera Heated stage ' TIPS controller Back-pressure controller Figure IV-4. Digital fabrication system with a TIPS controller connected to an X-Y motion controller. The side-view camera is not mounted in this picture. The digital fabrication system is controlled through custom software. The primary advantage of the software over the stock HP software is its ability to x-, y-, and 6-offset the printed pattern, enabling precise alignment between the printed pattern and the substrate. The software supports additional functionality, such as multiple printing patterns functionality and automated delays between layers, that wasn't used in this work but will be useful for future printed microfabrication studies. IV.2.2 Inkjet Printing Parameters As mentioned earlier, inkjet exploits the Plateau-Rayleigh instability in order to break a column of ink into picoliter-sized droplets. Inkjet achieves this by causing a sudden volume change within the firing chamber, which in turn squeezes ink within through a small outlet to form the ink column. Then, by virtue of surface tension, the ink column separates from the remaining ink and begins to coalesce into 36 spherical droplets. Finally, as the column is turning into the spherical droplet, the droplet isjettisoned towards the substrate using the velocity generated by the sudden displacement. The process is summarized in Figure IV-5. b) a) c) I e) d) Figure IV-5. Illustration of inkjet printing. a) An inkjet nozzle primed for firing. b) Asudden displacement, depicted by the black arrow, forces a portion of the ink out of the nozzle as a column. c) The ink's surface tension causes the column to coalesce into a sphere, narrowing the column at the nozzle's opening. d) The column continues narrowing at the nozzle's opening until the column completely separates from the main ink body. e) The column finishes coalescing into a spherical droplet as it travels towards the substrate. Unfortunately, as mentioned earlier, there is only a small range of viscosities (0.1-10cP for TIJ, 0.1-100cP for PZT printheads) compatible with inkjets. This consequently creates a trade-off when using inkjets for surface micromachining and MEMS between high material loading and printability. In MEMS, many designs require layers on the order of several micrometers and as such, the inks should be loaded with as much material as possible so fewer drops need to be jetted. High material loading, however, typically raises the viscosity of the ink to the point where it can no longer be inkjetted. Thus there must be a balance between the amount of material in the ink and the viscosity where it is still reliably printable. Because the digital fabrication system uses TIJ technology, this work will not provide detailed methodology to optimize the parameters for PZT inkjet printing and will instead focus on the methodology for optimizing TIJ printing parameters. 37 IV.2.2.1 Nozzle size The TIPS controller uses disposable 0.5mL reservoirs that have a series of TIJ nozzles at the end. These nozzles differ by the geometry and firing chamber, resulting in droplet sizes ranging from 0.5pL to 300pL. The firing chamber volume ultimately determines droplet diameter, volume, and the resulting spot size. It can be argued that larger nozzles jet larger volumes of ink and thus deliver more material, however because the spot size is also dependent on nozzle size, using larger nozzles result in losing resolution. It can be quickly seen that a balance between printing speed and resolution needs to be determined. In the case of the surface micromachining process, high resolution was important, thus only the smaller nozzle sizes (<100pL) were considered. In addition to the droplet diameter and volume, nozzle size also determines the requisite parameters needed to successfully jet a droplet. Because nozzle sizes influences many of the other printing parameters ranging from the firing parameters to generating the appropriate printing pattern, it can be argued that nozzle size selection is the most important printing parameter. In the case for CCI-300, 35pL nozzles were used in the previously published process, however as the process became further optimized, printing switched to primarily 50pL nozzles. This was primarily because the 35pL nozzles would misfire and clog one out of every 3 dies printed while switching to the 50pL nozzles decreased the rate of misfire and clogging to one out of 10-15 dies printed, subsequently improving the reliability of the deposition. IV.2.2.2 TIJ firing parameters TIJ has four basic firing parameters: main pulse voltage, main pulse width, precursor pulse voltage, and gap time. These parameters describe the electric pulse waveform applied to the TIJ's resistive heaters. Pulse voltage and pulse width are the peak voltage of the waveform (in volts) and the duration of the waveform (in nanoseconds). Occasionally, especially if the ink is near the viscosity limits of TIJ technology, a precursor pulse is applied to partially heat the ink to assist the jetting event. A simplistic representation of the waveform is depicted in Figure IV-6. ---- Main Pulse Precursor Pulse time Figure IV-6. Simplistic representation of electrical pulse applied to TIJ printhead. The precursor pulse's amplitude (voltage) is adjustable but the duration of the precursor pulse is fixed. The delay between the precursor pulse and main pulse is known as the gap time. Both the main pulse's amplitude (voltage) and duration (width) are adjustable. 38 These printing parameters, while dependent on the ink, are primarily determined by the geometry and volume of the TIJ firing chamber. When using TIPS and digital fabrication system, nozzle geometry and volume is determined by the printhead model at the tip of the 0.5mL reservoir. Fortunately, HP has already optimized the pulse printing parameters for jetting isopropanol and water (no pre-pulse is required). Adjusting for other ink compositions requires steadily printing the ink and adjusting the printing parameters while determining if a droplet was jetted or misfired (not successfully jetted). Successful droplet jetting was determined by monitoring the droplet impacts onto a substrate using the side-view camera. IV.2.2.2.1 Pulse width and voltage Pulse width is essentially fixed. This is because HP optimized the pulse width such that the vapor bubble has enough time to properly form and it is not recommended varying the pulse width. While it is possible to vary pulse width instead of pulse voltage, more droplet misfires were observed when varying pulse width than when varying pulse voltage. Pulse voltage is determined by starting at the recommended voltage and slowly decreasing the voltage until the TIJ printhead starts misfiring. The pulse voltage at which droplets misfire is known as the pulse threshold voltage and the optimal pulse voltage is generally 110% of the pulse threshold voltage. The pulse voltage is reduced by 72% if using a precursor pulse which will be described in detail below. IV.2.2.2.2 Precursor pulse gap and voltage Determining whether the waveform requires a pre-pulse is achieved by printing a series of individual jetting events. The jetting events should be adequately spaced to prevent the droplets from overlapping and conglomerating. Post-jetting, the resulting spots are observed using the top-view camera to determine whether each pulse corresponds to a single spot or multiple spots. Multiple spots indicate the parameters are not optimized, causing the ink column to break up into multiple droplets known as satellite drops. Most satellite drops occur because the jetting energy provided by the single pulse is not sufficient to successfully jet the droplet and additional energy is needed. This energy is provided in the form of the pre-pulse. As a general guide, HP states that the pre-pulse should be 40% of the original pulse voltage and the gap between pulses should be 1ps. As mentioned above, if a pre-pulse is used, the pulse voltage should be reduced to 72% to avoid overheating and damaging the printhead. IV.2.2.2.3 Firing parameters for CCI-300 Because the majority of CCI-300's ink vehicle is ethanol, the firing parameters needed to successfully jet droplets was very close to HP's recommended values. For the 5OpL nozzles, the pulse voltage was 25.OOV, the pulse width was 1.Ops, and no pre-pulse was necessary. This leads to spot diameters of 80pm as measured by the top-view camera. IV.2.3 Film deposition optimization Aside from selecting the nozzle size and optimizing the firing parameters, it is also critical to optimize the actual nanoparticle film deposition processes. Failure to optimize these processes result in film defects such as non-continuity or non-uniformity. There two main categories to optimizing deposition: film thickness and film uniformity. Unfortunately, these two categories are somewhat influenced by each other and as such further complicate the optimization as will be explained below. 39 IV.2.3.1 Film thickness Simplistically, film thickness is determined by the amount of material that is delivered to the substrate. Because of this, by knowing the ink composition and the droplet volume, it is possible to roughly calculate the number of droplets required to obtain the desired film thickness. For example, CCI-300 originally contains 3.5% silver by volume. After diluting 2:1 ethanol:CCI-300, the modified ink is 1.17% silver by volume. Knowing this ink composition, and assuming that a 100pm x 100pm x 1pm film is desired and a 50pL average droplet volume, approximately 17 droplets would be needed. IV.2.3.2 Printing patterning Unfortunately, film printing is much more complicated than merely calculating the ink necessary to obtain film thickness. Because there is a 2-orders of magnitude difference in thickness between the wet ink and dried film, challenges concerning ink spreading become significant. While it is possible to control spreading via surface modification [40], the most common method to control the printed film dimensions is the printing pattern [41]. The printing patterns have three parameters: pitch, border spacing, and droplet layers as shown in Figure IV-7. Increasing the number of drop layers, or drops jetted on top of each other, leads to linearly increase the amount of material loading. Changing the drop pitch, or the spacing between adjacent drops, the amount of overlap between drops is affected and amount of material in a given area changes. By using different pitch and layer parameters, it is possible to obtain the amount of deposition that is needed for any surface micromachining process. 40 Figure IV-7. Example of printing pattern and the parameters used to generate the pattern. The three primary parameters used to generate the pattern are droplet pitch, border spacing, and layers (not shown). Droplet pitch is partially determined by the spot radius to ensure significant coverage. Layered printing is achieved by repeatedly printing the pattern. The last parameter, border spacing, is extremely important for pattern fidelity by incorporating nonprinted regions within the patterns to account for ink spreading. Ink spreading occurs when the wet ink film obtains more volume due to printing and eventually the film overcomes its contact angle and pinning effects and slowly spreads out as shown in Figure IV-8. In general, the larger the printed ink volume, the larger the border space that is necessary to accommodate the ink. For the CCI-300, it was discovered that 80parm borders were needed to properly account for ink spreading. 41 b% a0, / // 7/777 Figure IV-8. Ink spreads when significant volumes of ink are printed. a) Initially, ink's contact angle, 6, is below the critical angle, 6, thus the ink does not spread. b) As more ink is printed, 6 increases past 6c and overcomes the pinning forces and ink starts to spread. c) The ink spreads until 6 is equal to 6, with the ink film's in-plane dimensions greater than the original printing pattern. Accommodating for ink spreading is important when attempting to print accurate feature geometries. One final consideration when generating the printing pattern is the ink's volatility. Volatile inks evaporate and dry promptly after jetting, resulting in thickness non-uniformities known as scalloping [4, 41]. To suppress these non-uniform thicknesses, extremely dilute inks can be printed with a randomly generated "noise" factor, thus preventing material build up due to droplets being regularly jetted at the same location [25]. However, because CCI-300 contains ethylene glycol, a nonvolatile solvent, the film does not dry without an additional drying step and the consideration of in-situ drying is not applicable. As such, the detailed techniques required to ensure thickness uniformity when using volatile inks is not addressed in this thesis. IV.2.3.3 Drying conditions The ink drying conditions ultimately determine the surface uniformity. This is due to a combination of evaporation and capillary flows known as the coffee-ring and Marangoni effects [42-45]. The coffee-ring effect assumes that the drop area stays consistent during drying and subsequently more material is carried from the drop center to the edge, creating a ring of material. The Marangoni effect, on the other hand, assumes that the latent heat of evaporation produces a temperature gradient and consequently a surface tension gradient, thus recirculating the material to deposit preferentially in the center. These two effects essentially counteract each other and, depending on which effect is stronger, determines whether the dried film profile is thicker at the edge or the center. These effects can be controlled by changing the drying dynamics, more specifically the rate of drying. While this can be done in many ways, such as humidity, substrate temperature, and air circulation, due to the tools available, temperature was used to dictate drying. To increase the repeatability of the drying conditions, the digital fabrication system was situated in a glove box to nominally fix the humidity at 16%, maintain ambient temperatures between 20 0 -25 0 C, and reduce air circulation. Because CCI-300 contains ethylene glycol, its low vapor pressure prevents the films from drying without assistance. To dry the CCI-300 films, the films were heated to 80*C. This elevated the temperature enough to dry the films without negating the Marangoni effect. While the samples needed only 2-3 hours to dry, the samples were heated for 12 hours to ensure all the ethylene glycol was removed. Another technique that was investigated was drying the nanoparticles between printing individual layers. Adding a drying step in between layers theoretically would rectify many problems including ink spreading and film uniformity. However, initial investigations into drying after individual layer resulted in film thickness non-uniformity that was specific to the process. Figure IV-9 displays optical micrographs of these film micrographs. While the cause of these artifacts was not determined, it is believed to have 42 been caused by imperfections in the original layer that are further amplified as additional layers are printed and dried. Smooth sllver film Rough silver film Figure IV-9. Film quality comparison between standard and individual layer printing methods. The standard (a) and individual layer (b)printing methods demonstrated on a glass substrate and photoresist sacrificial step. As can be seen, the standard method has challenges with ink containment and spreading. While the individual layer method suppresses the ink spreading, the method also produces undesirable surface artifacts. IV.2.3.4 Optimizing Sintering After drying the silver nanoparticle films, the films needed to be sintered. Previously, sintering occurred in a N2-capped tube furnace at 250 0 Cand skipped the drying step [6]. However, as Figure IV-10 shows, initial studies of heating environment suggested that hotplate sintering improved the mechanical properties of the nanoparticles films. As such, the process-property relationship studies in this body of work were all conducted on hotplates instead of tube furnaces. To alleviate film cracking, the samples were heated and cooled at 3'C-min-, thereby reducing the thermal shock the nanoparticle film experienced and preventing cracking as shown in Figure IV-11. 100 F IIA am0 80 60 ----- - m m m- m m 40- II 20 0- Tube Furnace (N2) (Air) Vacuum Oven Hotplate (Air) Evaporated (Control) Figure IV-10. Elastic moduli versus sintering method/environment. Samples were sintered by heating the samples at 3'C-min 1 to 2500 C,isothermally held for 3 hours and cooled to room at 3*C-min1 [5]. 43 b) a) 50 pm Figure IV-11. Effect of slow heating and cooling rates on film cracking. a) Sample was placed on a pre-heated 150* hotplate and removed immediately after 3 hours. b) Sample sintered at 150*C for 3 hours but was heated and cooled slowly (3*-minl). It can be seen that slowly heating and cooling the samples greatly reduces the formation of cracks [6]. To summarize, sintering the nanoparticle films consist of three parameters: ramp rate, peak sintering temperature, and dwell time. The ramp rate is the rate at which the hotplate heats and cools, and is kept at a low rate of 3*C-minl to prevent the film from cracking due to thermal shock. The peak sintering temperature is the maximum temperature that the sintering process reaches and is a parameter investigated to determine the process-mechanical property relationship. The dwell time is the amount of time the sample is isothermally held at the peak sintering temperature and is another parameter that was investigated for the process-mechanical property relationship. Figure IV-12 displays an example of the sintering temperature profile. Peak sintering temperature C-1 Dwell time 3*C-min T Timem Figure IV-12. Standard temperature profile for sintering process. The hotplate ramps up to the peak sintering temperature where it is isothermally held for the duration of the dwell time. While it is possible to have separate heating and cooling rates, the rates were kept at 3*C-min 1 to prevent the film from cracking. IV.2.4 Depositing nanoparticle silver process of record With all the printing, drying, and sintering parameters characterized, silver nanoparticles could be deposited both reliably and repeatedly. Table IV-1 summarizes the processes used to print the CCI-300 ink and turn it into a stable, contiguous metal film. Peak sintering temperature, Tsintering,max, and dwell 44 time at peak sintering temperature, tdwell, were not defined in the standard process because they were the primary parameters varied in the process-property investigation. Table IV-1. Standard CCI-300 printing and sintering process Step Description 1 Set print parameters Vpuise = 25V and tpulse = 1.0ps with no pre-pulse 2 Generate print pattern pitch = 120pm, rspot = 40pm, and border = 80pm 3 Print CCI-300 4 Dry CCI-300 5 Sinter CCI-300 Tsubstrate = 10 min 500 C 12 hrs ) 800 - Tsubstrate = 500 800 20 min > 200 dT=3 OC Tsubstrate = 2 0 o0 dt min tdwell > Tsinteringmax OC dT -=3- Tsintering,max 2 0* IV.3 Sacrificial Layer Three materials were considered for use as the sacrificial material: PMMA, positive photoresist, and amorphous silicon (a-Si). While PMMA was deposited via TIJ printing, the other materials used conventional microfabrication techniques for deposition and patterning. As stated previously, the purpose of this investigation is to study the property-mechanical process relationship for silver nanoparticle films. As such, the sacrificial layer ideally should be a non-issue so that the focus of the work is on characterizing the relationship rather than optimizing the sacrificial layer. IV.3.1 Poly(methyl methacrylate) (PMMA) The author's previously reported process uses PMMA as the sacrificial layer. The PMMA used was a lowmolecular-weight polymer to facilitate removal after exposure to elevated temperatures. The PMMA ink was created by dissolving PMMA powder in toluene. The PMMA ink was kept less than 4% w/w to ensure the ink's viscosity was low enough to be printable using TIJ. Removal of the PMMA is achieved by dissolving the PMMA in 15-, 10-, and 5-minute soaks in chloroform. Other solvents have been shown to remove PMMA such as acetone and toluene; however the solubility of PMMA in chloroform isgreater and facilitates quicker PMMA removal. The PMMA ink was formed by dissolving 2%by weight PMMA into toluene and 80pL TIJ nozzle were used. The PMMA was printed onto 400 Csubstrates to facilitate solvent evaporation. The droplets were determined to have a spot diameter of 120pm. To circumvent thickness non-uniformity, the PMMA films were solvent vapor annealed by suspending the sample above 10mL of acetone in a sealed 200mL container. The acetone vapors in the vessel lower the PMMA film's glass-transition temperature and cause the film to reflow and redistribute, eliminating the film's non-uniformity by normalizing the thickness. There are several obstacles in using PMMA as a sacrificial layer. These challenges include nozzle clogging, surface uniformity, and material reflow during sintering. While nozzle clogging is an engineering problem and the ink and firing parameters could be optimized for more reliable printing, optimization of process to improve the surface wettability and material reflow requires precise control of several environmental parameters. These environmental include relative humidity, partial vapor 45 pressures, and ambient and substrate temperatures which are difficult to control and makes optimizing these processes nontrivial and would require several systematic changes. To highlight surface wettability challenges, a single glass substrate was cleaned in a 4:1 H2 SO 4 :H2 0 2 solution for 10 minutes and two PMMA molds were immediately printed. Figure IV-13 shows contact profilometry scans from the two PMMA molds. As can be seen, despite being printed onto the same, clean substrate, the sacrificial step is non-uniform and not consistent. It is believed that subtle nonuniformities in the surface energy of the glass substrate directly affect the PMMA ink's contact angle and wettability, thereby affecting the surface uniformity. 12 10 8 E' :L I- 6 4 2 0 0 500 1000 1500 2000 Scan Length (pm) - Mold1 - Mold 2 Figure IV-13. Contact profilometry scans of 2 PMMA molds. The PMMA barriers were used to contain the silver nanoparticles as they sinter and as alignment to compare the PMMA steps. As can be seen, the coverage of the PMMA steps between mold 1 and 2 are different despite the samples being printed simultaneously onto the same substrate. The difference in step morphology is attributed to non-uniform surface energy affecting the surface wettability. While the surface energy was normalized immediately prior to printing by cleaning the substrate, the efforts were not enough to prevent variation between samples. PMMA also suffers from material reflow issues. This stems from using a low-molecular-weight PM MA, which has a lower glass transition temperature compared to high-molecular-weight PMMA formulations. This ultimately results in the sacrificial layer reflowing and deforming when the silver nanoparticles are sintering. Because the mechanics governing this deformation require precise control 46 of the parameters mentioned above, such deformations are not well-controlled and not repeatable. As such, for the purpose of studying nanoparticle sintering, PMMA inks were poor candidates. IV.3.2 Positive Photoresist Conventionally deposited and patterned photoresist was a very promising candidate as the sacrificial material. Unlike printed PMMA, photoresist leveraged one of the most important traits of conventional microfabrication: because deposition and patterning have been highly characterized by the manufacturers and occurs in a well-controlled environment, obtaining reliable and consistent sacrificial features is trivial. To fabricate the sacrificial features, OCG 825-20 was spun-coated onto 100mm borosilicate wafers. Patterns were transferred into the photoresist by UV exposure with a transparency photomask followed by developing the photoresist in Fujitsu photodeveloper. Before testing compatibility between OCG 825-20 and the CCI-300, the photoresist was tested for thermal compatibility. Photoresist is usually not exposed to high (>150 0 C) temperatures for prolonged (3 hours) periods of time. However, because the photoresist was to be used as the sacrificial layer for CCI300, it had to be tested to see whether it could withstand thermal conditions similar to the levels used to sinter CCI-300. In addition to see whether the photoresist could withstand the thermal conditions, confirmation on removal capability also needed to be tested. The post-sintered photoresist was exposed to several etchants to determine if it was possible to remove after the thermal treatments. Table IV-2 summarizes the findings. Table IV-2. Photoresist after 3 hour, 150*C exposure versus etchant Etchant 80*C Acetone N-Methylpyrrolidonebased stripper 1-Methoxy-2-Propanol Acetate Observations and Results e Photoresist is removed after >3 hours. e Silver films remain metallic and optically reflective. 0 Photoresist is removed 1-2 hours. Silver films become rough, black, and start delaminating. e Photoresist remains intact after 8 hour soak. e 02 Plasma 0 Photoresist is removed after 10 minute exposure to 02 plasma Silver films blacken and tarnish As Table IV-2 highlights, the post-sinter photoresist was removable in 800 Cacetone, NMethylpyrrolidone-based stripper, and 02 plasma. Unfortunately, it was determined that the NMethylpyrrolidone-based stripper and 02 plasma are not compatible with the silver nanoparticles as shown in Figure IV-14, effectively leaving only the heated acetone as the possible method to remove the photoresist. 47 Figure IV-14. Nanoparticle films before and after 10 minute 02 plasma exposure. The silver film on the right was printed, dried, and subjected to 10 minutes of 02 plasma. The silver film on the left was subsequently printed and dried to demonstrate how 02 plasma affects the silver films. As can be seen, the silver films blacken and tarnish when exposed to 02 plasma. Subsequently, CCI-300 was printed onto the photoresist. The silver was dried for 12 hours at 800 C and laser trimmed. Laser trimming the silver proved ineffective as shown in Figure IV-15. The laser pulses seem to crack the underlying photoresist and the cracks ultimately propagate through the silver film. Photoresist Sacrificial Step Silver anchor Figure IV-15. Laser-trimmed nanoparticle films on photoresist. Silver cantilever After sintering the silver films, photoresist removal was attempted using 500 Cacetone. While the photoresist was ultimately removed, most of the silver films delaminate from the substrate. Figure IV-16 shows an optical micrograph of a silver cantilever midway through the etch process. As can be seen, the photoresist is partially removed, but not before the cantilever's anchor has partially delaminated. It is hypothesized that removal of the extreme baked photoresist requires the photoresist to swell prior to dissolving. This volume change ultimately causes the silver to delaminate from the substrate while the photoresist is being removed. Because of the incompatibility with laser trimming, slow etch rates, and delaminating the silver films during removal, it was concluded that photoresist could not be used as the sacrificial layer for the nanoparticle silver studies. 48 Delaminated silver anchor 100 gm Figure IV-16. Nanoparticle film on photoresist before and after 3 hours In 80*C acetone bath. Before acetone bath (a), the silver film is in contact with the glass substrate. After the bath, the silver is clearly delaminated from the substrate as indicated by the changes in reflectivity. IV.3.3 Amorphous silicon (a-Si) Amorphous silicon, the only inorganic material considered, was a very strong candidate for the sacrificial layer. Unlike the polymer-based sacrificial materials, a-Si offered a significantly wider temperature budget, remaining stable at temperatures greater than 450*C. Because of previous efforts to use a-Si for low-cost microelectronics, many microfabrication techniques have already been developed and very little optimization was required to deposit and pattern a-Si. IV.3.3.1 Depositing a-Si a-Si is deposited using plasma-enhanced chemical vapor deposition (PECVD). PECVD is a process used to deposit thin films onto a substrate directly from a vapor state to a solid state via chemical reactions. Unlike regular CVD, PECVD utilizes RF excitation to strike a plasma inside the vapor, significantly decreasing the temperatures required to CVD silicon from 600-1050*C (depending on gas composition and quality of silicon desired) to the 200-300*C [46]. a-Si is deposited from silane (SiH4 ) using the following reaction: SilH4 -+ Si + 2H 2 PECVD a-Si deposition was accomplished using a ST System CVD machine. Prior to depositing onto freshly-cleaned borosilicate substrates, a test deposition of 15 minutes was performed to determine the a-Si deposition rate. Typical deposition rates for the ST System CVD was 23-25nm-min', resulting in a-Si film thicknesses of 1.1pm being deposited in 45 minutes. The standard a-Si recipe for the machine uses showerhead temperatures of 300*C, substrate temperatures of 250*C, and a gas pressure of 400mTorr. However, subsequent deposition and patterning of an OCG photomask on the a-Si resulted in the delamination of the a-Si as shown in Figure IV-17. 49 Figure IV-17. a-Si delamination after developing photoresist photomask. As can be seen in Figure IV-17, the delamination of a-Si was primarily limited to the regions with the developed photomask and is believed to be caused partially by the weak adhesion between the borosilicate substrate and a-Si and partially from the stress mismatch between the deposited a-Si and developed photoresist. To address this issue, the showerhead and substrate temperatures were reduced to 2000 C. The change in deposition temperatures reduced the stress within the a-Si film and subsequently alleviated the delamination issues as shown in Figure IV-18. a-Si on glass Photoresist etch mask 100 pm Figure IV-18. a-Si after developing photoresist photomask. Decreasing the showerhead and substrate temperatures during deposition results in the a-Si layer not delaminating when developing the photomask. IV.3.3.2 Patterning a-Si Because a-Si shares many of the same material compatibilities to crystalline silicon, a-Si can be patterned using the same techniques used to pattern crystalline silicon. In addition to KOH etching, it is also possible to etch silicon using a XeF 2 dry-etching. XeF 2 dry-etching is an isotropic silicon etch where gaseous XeF 2 removes silicon using the following chemical reaction: 2XeF 2 (g) + Si(s) -> 2Xe(g) + SiF4 (g) 50 Because this reaction occurs at the exposed silicon surfaces, non-permeable materials, such as a photoresist mask, act as an etch barrier and can be used to transfer patterns into the a-Si. Unfortunately, because there is no known silicon etch stops for XeF 2, care must be taken to avoid completely removing the thin a-Si film by over-etching. XeF 2, while solid at atmospheric pressures, sublimates at pressures less than 3.8 Torr. Because of this material property, many XeF 2 etch processes utilize the "pulse method" which exposes samples to alternating cycles of XeF 2 gas and vacuum to remove the byproducts and allow for more XeF2 to sublimate. To determine the necessary number of etch cycles to pattern/completely remove the a-Si, the samples were examined under optical microscopy after each successive 60s XeF 2 exposure until the desired amount of a-Si was removed. It was determined that five 60s XeF 2 cycles were necessary to pattern the a-Si. To completely remove the patterned a-Si, ten additional 60s cycles were needed. IV.3.3.3 a-Si deposition and patterning process of record While a-Si has been reasonably explored for conventional microfabrication and most of the parameters were already determined, some subtle optimizations were needed in order to make the process compatible with the borosilicate substrates and the OCG 825-20 photoresist etch masks. Table IV-3 summarizes the process of record for depositing and patterning the a-Si sacrificial layer. Table IV-3. Standard a-Si deposition and patterning process. 1 2 3 4 5 6 Step Clean substrate a-Si deposition Photolithography Pattern a-Si Strip photoresist Remove a-Si (release structures) Description 10 minutes in 4:1 H 2 SO4 : H2 0 2 bath 200C showerhead & substrate, 400mT SiH4 , 45 minutes 1pm OCG 825-20 for XeF 2 etch mask Five 60s cycles of XeF 2 10 minutes in 4:1 H2SO4: H2 0 2 bath Ten 60s cycles of XeF 2 IV.4 Sacrificial barriers to prevent uneven ink spreading Another repeatability challenge was uneven ink spreading. This occurs because the sacrificial step and substrate have different wetting characteristics as observed in Figure IV-9a and illustrated in Figure IV-19. Because the ink wets the substrate and sacrificial step differently, the ink spreads out more on the substrate than the sacrificial material and consequently the in-plane dimensions and film thickness are uneven. This uneven ink spreading was prevented by incorporating a sacrificial barrier that surrounded the anchor regions of the silver microstructures. Because the sacrificial barriers nominally have identical wetting characteristics to the sacrificial step, the ink wetting is symmetric and consequently results in even spreading. 51 Structural material Sacrificial material Uneven spreading due to surface wetting differences Anchor Figure IV-19. Illustrations of uneven film spreading and design to prevent uneven spreading. Originally, only the sacrificial step was patterned (on left), but the difference between the substrate and sacrificial step's wetting properties resulted in uneven film spreading as observed in Figure IV-9a. By patterning sacrificial barriers with the step (on right), the structural film is surrounded by the sacrificial material, resulting in uniform wetting properties and even film spreading. IV.5 Fabrication summary Two distinct designs were used when investigating nanoparticle sintering: the nanoparticle-only deposition and the surface micromachining process. The nanoparticle-only design entails printing CCl300 onto borosilicate or glass substrates that were die-sawed 75% of their thickness in a grid-like pattern. The die-sawing's purpose was two-fold: to create an alignment pattern on the glass to align the printing patterns and to create cleaving sites so that the films could be easily cleaved for scanning electron microscopy (SEM). This design was used primarily to optimize the printing, drying, and sintering parameters of the silver nanoparticles and to study the relationship between film morphology and sintering temperature. Figure IV-20 summarizes the fabrication process. 1) Die-saw substrate 2) Print silver 3) Dry silver ie-saw traces Figure IV-20. Nanoparticle-only sample fabrication process. The second fabrication design was the surface micromachining process. This design used a-Si as the sacrificial layer to create microstructures with the sintered nanoparticle films. The main purpose of this design was to fabricate cantilevers to mechanically characterize via beam deflections and to characterize the mechanical properties-sintering temperature relationship. Figure IV-21 summarizes the surface micromachining process. Using this process, 50pm wide and 500pm long cantilevers were fabricated to investigate the process-mechanical property relationship. Figure IV-22 shows an example cantilever fabricated with this process. 52 2) Deposit & pattern photaresist 3) thS 6) Dry and sinter silver 4) Strip photoresist 1) PECVD aSI 7) Laser-trim silver 5) Pattern silver inkjet Nozzle - Laser * U 0 8)Release structure Figure IV-21. Nanoparticle and a-Si surface micromachining process. SEM HV: 5.0 kV WD 16,00 mm View field: 155 pm Det: SE VG3TSA 20 pm Figure IV-22. Example cantilever using a-Si sacrificial layer. The cantilever was fabricated using the process summarized in Figure IV-21. It is important to note that the laser trimming did not fully ablate the surrounding silver, leaving a thin layer that cleaved from the cantilever upon release. This cleaving results in the appearance of heterogeneity in the cantilever's nanostructure, however other electron micrographs of film cross-sections (Figure V-4) do not have this heterogeneity and suggests that this is just an artifact of the cleaving. [47] 53 54 Chapter V Experimental Methods and Results The experimental methods are separated into two categories: film morphology and mechanical properties. Film morphology studies involve characterizing the film densification in terms of macroscale properties such as thickness and in-plane features, as well as the film nanostructure such as porosity, grain size, and grain growth. Film morphology studies were primarily done using the nanoparticle-only fabrication process described above. The mechanical properties of the film were characterized by beam deflection measurements and nanoindentation. Nanoindentation measurements occurred on both the nanoparticle-only samples and the surface micromachined samples. Beam deflection measurements required self-supporting structures and as such could only be on the surface micromachined samples. In case for all the samples, the processing differences focuses on three parameters: peak sintering temperature, time at peak sintering temperature, and sintering ambient environment. V. Film thickness versus peak sintering temperature Nanoparticle sintering is a process of densification and can be described by the reduction of nanoscopic spacing between individual nanoparticles as the particle fuse together. While this densification is ideally 3-dimensional, because the in-plane features are orders of magnitude larger than film thickness and are fixed through adhesion to the substrate, it has been suggested that in-plane densification is negligible and densification primarily occurs in film thickness [35]. To measure film thickness, the dried nanoparticle films are measured using contact profilometry. The films are sintered for three hours at the peak sintering temperature and the thicknesses are remeasured. To ensure the same locations are measured, the die saw line traces on the underside of the substrate are used for alignment. The curves are aligned and the ratio between thickness the sintered and dried films are calculated. Figure V-1 summarizes the experimental methodology and the results are displayed in Figure V-2. As can be seen in Figure V-1, sharp peaks are measured at the edge of the silver films. It is hypothesized that these peaks are caused by the film slightly delaminating from glass. To ensure that the delamination does not factor into the measurements, 10% of the measurements on both sides of the film are discarded. 55 a) 1) Measure silver thickness 35000 Dried -Sintered 23 000 E 30000 2 1U 25000 .. - 2000 2) Sinter silver 10000- - 5000 3) Remeasure silver thickness = 0 2000 Micrometers 0 4000 Figure V-1. Summary of film thickness measurements. a) The film thickness measurement process entails contact profilometry on the dried films, sintering the films, and measuring the thickness of the sintered film. b) Example film thickness H measurement. To find w-, >300 points are sampled. The sharp peaks at both ends of the silver film are hypothesized to be the film slightly delaminating at the edges. As can be seen, there is monotonic relationship between film thickness and peak sintering temperature where the film thickness decreases as the temperature increases. This was to be expected because higher temperatures suggest more nanoparticle fusion, subsequently causing more densification and decreasing the film thickness. To test sintering ambient environment's effects on film thickness, the experiments were repeated by flushing out the air surrounding the hotplate with N2 and the results are also summarized in Figure V-2. While the reduction of film thickness is less strong with the N2 samples, the monotonic relationship between film thickness and sintering temperature is still observable. 1 *Air 0.95 AN2 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 - 0 100 200 300 400 500 Sintering Temperature Figure V-2. Film thickness versus peak sintering temperature. The film thickness ratio is the average of >300 samples. The error bars represent 1 standard deviation. As the sintering temperature increases, the film surface becomes rougher which consequently influences the film thickness ratio measurements and increases error. 56 V.2 Nanostructure versus peak sintering temperature After measuring the effect that sinter temperature has on film thickness, the samples were cleaved as summarized in Figure V-3 and imaged using SEM. Figure V-4 showcase top-view and cross-sectional micrographs of multiple samples. As can be seen, as the peak sintering temperature increase, the degree of particle fusion and growth also increases, which is consistent with the film thickness results. It is interesting to note that at lower temperatures the particles densify by fusing together but at temperatures higher than 250'C, particles exhibit more grain growth instead of fusing. SEM -- +nanoindentation Figure V-3. Illustration of sample cleaving. Sample is cleaved in half along first die saw trace. Half of the sample is mounted and prepped for electron microscopy. The remaining sample half is cleaved along remaining die saw trace to obtain sample quarters. One of the sample quarters is mounted to a magnetic substrate with epoxy and prepped for nanoindentation. 57 Sintering Temperature Vantage Point Cross-sectional View Top-View -- 200nm - SOOnm 125'C 150*C 177*C 250*C 363*C Figure V-4. Top-view and cross-sectional electron microscopy of air-sintered nanoparticle films. Electron microscopy of N2-sintered samples were also performed. These images are shown in Figure V-5. Much like the air-sintered samples, the degree of particle fusion and grow increases as the peak sintering temperature increases. However, it must be noted that N2-sintered samples have different nanostructure than their air-sintered counterparts. This is especially noticeable in the samples sintered at 250OC: the N2-sintered sample is comprised of large nanoparticles surrounded by small nanoparticles while the air-sintered sample only has large nanoparticles and no small nanoparticles. This suggests that sintering environment affects the sintering process. 58 Arqn*c i rlner 200nm ammm Figure V-5. Top-view electron microscopy of N2-sintered nanoparticle films. V.3 Film mechanical properties versus sintering parameters There are many methods to extrapolate Young's modulus of elasticity of a material. Many of these methods involve material deformation measurements such as tension, microbeam bending, bulge, and resonance tests [48]. Two of the methods used to characterize structures fabricated using the previously reported surface micromachining process, equally-spaced beam deflection measurements and nanoindentation, remain ideal for characterizing the sintered nanoparticle films fabricated for this investigation [4]. V.3.1 Characterization method #1: Equally-spaced beam deflection Equally-spaced beam deflection uses a nanoindentor to perform a series of beam deflection measurements to determine the material's elastic modulus. Typically used for microhardness testing, nanoindentors are equipped with micrometer-resolution x-y stages and pN-resolution force transducers, making them ideal for deflecting cantilevers. Normally, cantilevers are depicted by a simple spring model: F = kx, (4) where F is the applied force, k is the system's spring constant, and x is the amount of displacement. Equation (4) implies that in typical cantilever operation, the amount of force used on a cantilever is linearly proportional to the amount of displacement the cantilever experiences. Assuming ideal conditions where a rectangular beam is perfectly anchored to an infinite plane, the relationship between the spring constant to the geometry of the beam and its material properties can be ideally defined as: 59 k (5) EWH3 where E is the material's Young's modulus, and W, H, and L are the beam's physical dimensions.To account for the point-force location inaccuracy, it is possible to make additional force-deflection measurements at different locations along the beam. The main difficulty in using a nanoindentor to measure the cantilever's beam deflection is its inability to accurately identify where the deflection point is relative to the cantilever's anchor. While it is possible to measure the force-displacement values anywhere on the cantilever, it is not reliable because the spring constant has a third-order dependency on the distance from the cantilever's anchor as shown in Equation (5). Therefore, any inaccuracy positioning the point force along the length of the beam is magnified and leads to large uncertainties when determining the elastic modulus. To account for the point-force location inaccuracy, it is possible to make additional force-deflection measurements at different locations along the beam. This is possible because while it is difficult to precisely place the probe on specific locations of the cantilever, it is easy to accurately position the probe relative to previous point-force locations. Rearranging Equation (5), the following relationship between known deviations in length and the perceived spring constant is obtained: EWH 3 [L + 6L],( Equation (6) indicates a linear relationship between the inverse-cube-root of the spring constant and the distance away from the cantilever's anchor. This suggests the elastic modulus can be determined without knowing the point-force's absolute position by instead using multiple point-force measurements that are offset by known quantities along the length of the beam as described in Figure V-6. I "I U.- I - (a) XM Displacement (b) L-61L L I 'X L+6L (c) Figure V-6. Determining Young's modulus via multiple point-force deflections. (a) Cantilever is deflected multiple times, each load-point offset by a known amount. (b) Forcedisplacement trends for each load-point; as can be seen, the beam appears stiffer when loading closer to the anchor. (c) Using the slopes found from (b), the apparent spring constant is plotted with respect to its relative load-point; the slope (S)from the resulting regression is used to determine the elastic modulus, given that the width and height of the cantilever are known. Using the method described in Figure V-6 and Equation (6), Young's modulus can be obtained: E = _4 3 3 WH S ' where S is the slope of the linear regression described in Figure V-6. 60 (7) It is important to note that, depending on the cantilever's length:width ratio, Equation (7) may need to be modified to account for the cantilever's anticlastic response. This is because at low length:width ratios the cantilever responds more like a plate rather than a beam and needs to be compensated accordingly by incorporating the material's Poisson ratio. As such, Equation (7) to obtain the following relationship: E 1-v 4 WH 3 S 3 ' 2 (8) where v is the material's Poisson ratio. However, Equation (8) is more applicable with cantilevers with length:width ratios less than 5:1. With the cantilevers in this study having a length:width greater than 5:1, Equation (7) is more accurate and was used to mechanically characterizing these cantilevers. V.3.2 Characterization method #2: Nanoindentation Another method of determining the elastic modulus is by using the Hysitron nanoindentor to perform nanoindentation or microhardness testing. This technique, also known as the Oliver-Pharr method [49], works on the principle that when materials are plastically deformed, a certain amount of the exerted energy is elastically stored in the material and will undergo elastic rebound when the material is unloaded. The entire indentation process is monitored and a force-displacement curve is generated. By analyzing this curve, particularly the initial, linear regime of the unloading cycle, it is possible to extract material properties such as hardness and reduced modulus of elasticity. Since the Hysitron nanoindentor was designed for this particular test, the software is capable of analyzing the curve and calculating these material properties. By analyzing the unloading curve, it is possible to extract the reduced Young's modulus, Er: Er = i-1- (9) S ,8 2 A(he) where S is the stiffness of the material (determined by the slope of the unloading curve), Ap(he) is the projected area of the tip's contact depth, he, and fl is a geometrical constant. The Oliver-Pharr method measures Er, as opposed to the actual Young's modulus, because of the tool's physical limitations. Because the nanoindentor's tip has finite hardness and Young's modulus, there is some deformation in the tip itself during the measurements, thus the force-displacement curve generated incorporates both the deformation of the film as well as the slight deformation of the tip. However, by knowing some other material properties of the material in question and the tip, the material's Young's modulus can be calculated via the following equation: 1 =1-Vum Er Efim + 1vij, (10) Eti, where vfilm = 0.37, the Poisson ratio of silver, and because the nanoindentor tip is made out of diamond, vet = 0.07, and Etip = 1140GPa. There are some concerns with using this method. Firstly, it is assumed that the nanoparticle's Poisson ratio is the same as bulk silver. If this assumption is incorrect, then the resulting Young's modulus will also be incorrect. However, it is generally accepted that vfilm = vAg and this assumption has been used 61 in previous studies that used nanoindentation to measure the mechanical properties of nanoparticle films [501. Another major concern is that the sintered silver films are porous and there are reports suggesting that porous materials densify and subsequently exhibit higher elastic moduli when the nanoindented [51]. As such, the measurement may report different material properties due workhardening. Finally, the measurements were restricted to less than 1/10 of the film thickness to prevent the substrate from influencing the nanoindentation measurements [48-50]. V.3.3 Modulus versus peak sintering temperature Using the samples mentioned above for the film thickness measurements, the samples are nanoindented to determine the films' elastic moduli. To confirm that the nanoparticle-only sample exhibit similar properties to their surface micromachined counterparts, cantilevers were also fabricated and mechanically characterized via beam deflection measurements and all the results are summarized in Figure V-7. While these nanoindentations were performed on the silver films attached to glass substrates, there was no observable difference between nanoindents performed on silver films attached to glass and silver films attached to a-Si. 90 80 70 - 60 50 .a 40 -30 20 - A Nanoindentation 10 - *Beam 0 0 50 100 200 150 250 300 Deflection 1 1 350 400 T (*C) Figure V-7. Modulus versus peak sintering temperature. Error bars for the beam deflection measurements represent one standard deviation from 210 different samples. Nanoindentation error bars represent one standard deviation from 236 0 nanoindentation measurements on a single sample with the exception of the 150 C sample which was consisted of 134 measurements over three samples. As expected, the moduli measurements are consistent with each other and for the purposes of this work, the two characterization methods are treated as interchangeable. Because characterizing the nanoparticle films with nanoindentation measurements require fewer assumptions than characterizing with the beam deflection measurements, nanoindentation measurements were primarily used to 62 characterize the nanoparticle films. Another observation is that the data suggests there is a monotonic relationship between the elastic modulus and peak sintering temperature. It must also be noted that the error in the nanoindentation measurements increases at higher sintering temperatures. This is believed to be caused by variation in the film's nanostructure post-sintering: as shown in Figure V-4, the film's nanofeatures increase in size the sintering temperature increases. As these features increase, the film's topography ultimately begins to influence the nanoindentation, thereby increasing the variation in the measurements. The modulus versus sintering temperature relationship was repeated with samples sintered in a N2 environment and summarized in Figure V-8. The modulus measurements were made by nanoindentation. As can be seen, while the monotonic relationship is still observable, the moduli are slightly different than their air-sintered counterparts. 90 - 4 80 70 - 60 050 - 44 LLI40 9 30 20 - A Nanoindentation (air) 10 0 Nanoindentation (N2) 0 0 50 100 200 150 250 I I I 300 350 400 T (C) Figure V-8. Modulus versus peak sintering temperature in air- and N2-environments. Measurements were obtained by 236 nanoindentation measurements on a single sample. As can be seen, the average moduli for the air- and N2-sintered samples are slightly different. V.3.4 Modulus versus dwell time at peak sintering temperature Aside from sintering temperature, Greer and Street have suggested that dwell time also influences the resulting material properties [35, 50]. This is based on the assumption that atomic diffusion is the primary densification mechanism for metal nanoparticles and thus film densification is not only dependent on temperature, but is dependent on time as well. To test this assumption a series of samples were sintered at a peak temperature of 250*C for 0, 1, 2, 3, and 6 hours and nanoindented to determine their elastic moduli. Figure V-9 summarizes the results of the nanoindentation. As can be seen, while there is some variation, there does not appear to be a significance difference between samples. This is most likely attributed to the slow heating and cooling rates (3*C-min") that the samples required to prevent film cracking. Because the films were ostensibly sintered for a minimum of 150 minutes and Greer and Street's time-dependency was on the order of 63 minutes, it is believed that at the time scales used for this work were too large to observe any dependency on time. Therefore, because of the longer time scales, it is believed that the processes highlighted in this work are essentially independent of time. 100 . 80 aT 60 'U 40 20 0 0 1 2 3 4 5 6 7 8 Sintering Time (Hrs) Figure V-9. Young's modulus versus dwell time at peak sintering temperature. Samples were heated and cooled at 30C-min- and peak sintering temperature was 250 0C. Measurements were obtained via nanoindentation. Dashed line indicates average Young's modulus of all samples. V.4 Summary Using the microfabrication processes mentioned in the previous chapter, the process-mechanical property relationship for metal nanoparticles was investigated. Two sintering parameters were the primary focus of the investigation: peak temperature and dwell time. While monotonic relationships could be established with peak temperature, it was concluded that dwell time did not affect the resulting film properties. In addition to the two primary parameters, the ambient sintering environment was preliminarily investigated by sintering samples in N2 instead of air. While similar monotonic relationships could be established, it is also observed that the relationships were less pronounced and required higher temperatures to be observable. 64 Chapter VI Modeling of Sintered Nanoparticle Films VI.1 Overview Because of their primary commercial application as a printed thin film conductor, metal nanoparticlebased materials have been optimized for their electrical properties while for the large part ignoring their mechanical characteristics. As such, previous work has investigated the effect sintering has on the films' resulting electrical properties and models correlating electrical properties to sintering have been proposed [35]. Most models assume that the electrical properties can be explained by the physical geometry of the films' nanostructures and subsequently simplified representations, such as Frenkel's 2sphere model, are used to predict the resulting electrical properties. This representation, in conjunction with observed sintering process-film nanostructure relationships, is the basis for the previous electrical models for sintered materials. Because these models have shown strong correlation with experimentally-measured electrical values, it is worth considering whether the nanostructuremechanical property can be explained with a similar representation. Figure VI-1. Diagram of 2-sphere sintering model. Sintering is assumed to be dictated by the geometry of the individual nanoparticles as they fuse together and thus can be simplified to the interaction between two particles. The model assumes that as the particles sinter, the centers of both nanoparticles approach each other until the two spheres become a single sphere. As the particles fuse, the "neck" between the particles deviates from the original sphere boundaries (shown in dashed lines) to observe the conservation of volume. VI.1.1 Nanoparticle Melting Point Depression As mentioned in 0, the high surface-to-volume ratio of nanoparticles lowers or depresses the material's melting po int. The seminal study performed by Buffat et o/ [52] on particle size versus melting temperature of gold particles provided extensive insight into this phenomenon. Buffat assumed that as soon as the nanoparticles are heated to a threshold temperature, the entire particle would melt. This model is now known as the liquid drop model (LDM) and provides the following relationship between particle size and melting temperature: 65 T = To x (1 - Ps r p a0S - ( (11) (1)) s)], where p, Hf, and a are physical constants of bulk metal near the melting point, To is the bulk metal's melting point and r is the nanoparticle radius. The subscripts s, 1, and v denote whether the parameter is for the metal's solid, liquid, or vapor state. While Buffat's work was performed on gold, by using the material properties of silver (Table VI-1), Buffat's equations can be recalibrated for silver nanoparticles as shown in Figure VI-2. Table VI-1. Silver numerical values. The values stated are the physical constants of bulk silver near the Solid Specifig gravity, p (g-cm-) 9.98 Bulk heat of fusion, Hf (J-kg1 ) Melting temperature of bulk (*K) Surface energy/tension, a (J-m2 ) 7.2 [53] melting point. Liquid 9.32 111 1234.93 0.600 [54] 1000 900 U 800 700 0. 600 I- - CL 500 400 300 Z' a, 200 100 0 0 50 100 150 200 250 300 Particle Diameter (nm) Figure VI-2. Silver nanoparticle melting temperature vs. nanoparticle diameter. The solid line represents the required temperature to start melting the respectively sized nanoparticle. As the particles increase in size, the temperature required to melt the particle drastically increases and approaches the bulk melting temperature (depicted by the dashed line). According to the technical specifications and other literature [35, 50], Cabot CCI-300 silver nanoparticle diameters are 30-40nm, implying their melting temperature is 42-2720 C, which is consistent with the manufacturer's literature stating the ink becomes electrically conductive at 100 0C. According to Figure VI-2, silver nanoparticles with a diameter of 34nm will start melting at 1000 C. This is consistent with Cabot's technical literature which state their nanoparticles are 30-40nm in diameter and become electrically conductive at 100 0 C, assuming electrical conductivity is caused by the nanoparticles beginning to melt and sinter. 66 VI.1.2 Electrical model Previously, Greer and Street proposed a two-part model to explain the electrical conductivity observed in their sintered films. The first part of the model related film density and resistivity and was based on one physical parameter, the ratio between film thickness and original film thickness, and suggested a monotonic relationship between increasing electrical conductivity and decreasing film thickness. Conductivity of the film was predicted using a 2-sphere model where it was assumed that the "neck" formed between the sintered nanoparticles serve as an electrical bottleneck, effectively limiting the particle-particle electrical conduction and thus the overall electrical conductivity of the film. 40 b) a) 30 - - 20 0.8 x () 1 0.9 Remaining Film Thickness * L(t)/L(O) Figure VI-3. Part I of Greer and Street electrical sintering model. a) 2-sphere representation relating the radius of neck between the particles, z, to the distance between particles' centers, L. b) Resistivity vs. Remaining Film Thickness. The model assumes the film's conductivity is limited by the neck between the fusing particles. The resistivity is calculated using the film thickness by using the 2-sphere model shown in Figure 2a to relate the neck's cross-sectional area to the distance between the particles' centers. The second part of the model relates the sintering process to the film's physical densification. This process takes into consideration sintering studies performed by Kingery and Berg to determine the degree of film densification [55]. Using several material and physical factors such as surface energy, lattice diffusion energy, and particle radius, Kingery and Berg developed a model to describe several sintering mechanisms. Using this model, Greer and Street related the geometry of the neck growth to two parameters: sintering temperature and dwell time. By using both parts of the model, it is possible to generate a model relating sintering temperature and dwell time to the resulting electrical properties. However, it is important to note that additional factors such as sintering temperature ramp rates, ambient environment, and laser exposure can possibly affect the film sintering; thus the second part of Greer and Street's model does not adequately apply to the microstructures fabricated in this work. However, since their work was primarily based on fundamental material properties, it is very likely that a facsimile of their model will explain the process-property relationship observed in the process reported in this body of work. VI.1.3 Metal powder sintering While metal nanoparticle inks were primarily developed and optimized for printed electronic applications, historically metal particulates have been used in several manufacturing schemes such as metal injection molding. Encompassed in the field of powder metallurgy, these processes are used in a 67 wide variety of products due to the ability to manufacture products which other metallurgical processes cannot achieve. As these products are used in a wide variety of industries, including automotive and aerospace, these parts manufactured from sintered powdered metal have been mechanically characterized. Similar to their ceramic counterparts, sintered metals typically display the following mechanical relationship: E = E0 (1 f.EV (12) ~p0/ ' where EO is the elastic modulus of the bulk material, p is the film's current pore concentration, po is the initial pore concentration, and a and f are constants dependent on the material and sintering process used [56, 57]. Current and initial pore concentrations, p and po, are defined as the fraction of void volume to total volume such that p = and po = V"'*O. Because metal nanoparticles can be seen Vtotal V0,total as extremely fine metal powders, it is reasonable to assume that the process-property relationship for our particles should have similar mechanical relationships to those found in traditional powder metallurgy. VI.2 Mechanical modeling of sintered metal nanoparticle films As stated earlier, the semi-empirical model for the Cabot silver nanoparticles mechanical properties follows the methodology used to determine its electrical properties: the first part of the model will relate the sintering process to the film's densification and subsequently its nanostructure, and the second part will relate the film's nanostructure to the resulting mechanical properties. The physical structure-mechanical property relationship will be defined similarly to the relationships defined in powder metallurgy while the sintering process-physical structure will be based on fundamental principles and a few assumptions to simplify the physical particle fusion representation. VI.2.1 Sintering process-film morphology model This part of the model represents how the sintering process affects the film's morphology, namely film thickness. As shown in 0, a monotonic relationship between film thickness and sintering temperature has been observed. This relationship can be fitted mathematically to the linear equation: H = 1 - #l TTmin T > Tmin (13) where T is the sintering temperature, Tmin is particle's melting point, To is the melting point of the bulk metal, and fl is a unit-less fitting parameter. As can be seen in Figure VI-4, the data exhibits a slight nonlinearity and thus is not fully described by Equation (13). To further improve the fit, a non-linearity is introduced. This non-linearity is achieved by modifying the denominator in Equation (13) from To to To - T, which introduces an asymptote and effectively creates an upper temperature limit when the sintering temperature reaches the bulk silver melting temperature: H 2 T-Tmin HO TO-T 68 (14) The film thickness predicted by Equation (14) is also shown in Figure VI-4. As can be seen, Equation (14)'s prediction is also accurate at lower sintering temperatures and starts to deviate at higher temperatures. The deviation from the relationship is most likely caused by the drastic change in the film's nanostructure when sintering at higher temperatures. Analyzing the electron micrographs shown in V.2, the silver grain size and structure changes significantly when the sintering at temperatures greater than 250C. This suggests the sintering mechanism changes at higher sintering temperatures, reducing the accuracy of the relationship defined in Equation (14). For the purpose of developing the process-mechanical property model, Equation (13) will be disregarded and only Equation (14) will be evaluated. 10.95 0.9 0.85 0.8 %Z0.75 0.7 0.65 0.6 ----- 0.55 11 0 0.5 Figure VI-4. - HO Measured Linear Model Non-linear Model 1 50 1 100 1 1 1 250 200 150 Sintering Temperature 1 300 350 400 versus Sintering Temperature. The lines represent the fractional film thickness as predicted by Equation (13) and Equation (14) by fitting P1 = 1. 35 and #2 = 1. As can be seen, the measure values deviate from the predicted model as the film reaches higher densities. VI.2.2 Nanostructure-material property model As stated in Equation (12), the mechanical relationship used in powder metallurgy is dependent on the fractional porosity of the sintered metal film. Assuming the nanoparticle film is uniformly packed in a primitive cubic structure and p is only dependent on L because the film is pinned to the substrate in the in-plane dimensions and effectively only densifies in thickness, it can be established that p oc L. Establishing this relationship relies on unit cells, repetitive structures within the film, to represent the entire film, vastly simplifying the analysis of the film's nanostructure. For primitive cubic structures, a unit cell is represented by 18 of eight nanoparticles stacked 2x2x2 and the void volume between the particles as shown in Figure VI-5a. When the particles sinter, the unit cell shrinks and the excess material from the particles fusing together fills in the void volume, thereby reducing the amount of void volume 69 within the unit cell. Because of this, if p is defined as the fraction between void and total volumes of the unit cell, it can be established that p oc L as shown in Figure VI-5. Lo L c) 1 0.8 00.6 0.4 0.2 0 0.5 0.6 0.7 0.8 0.9 1 L/LO Figure VI-5. Relationship between p and L. a)The unsintered film can be represented by the particles organized in a primitive cubic crystal structure and a corresponding unit cell (outlined in light blue) can be drawn from the centers of the eight surrounding nanoparticles. The fractional void volume in the unit cell is considered the porosity, p, while po is the fractional porosity of the unsintered film. b) As the film densifies, the particles are pinned in two dimensions and effectively only densifies in one dimension by reducing the void volume. Because there is ample void volume in the unit cell, the increasing neck width between the sintered nanoparticles fills in this void and the linearity between p to L is maintained. c) From this representation, it is possible to analytically correlate p to L. The maximum densification possible is ~52.4% With this relationship, it is possible to approximate the fractional porosity of the sintered material to the distance between particles using the 2-sphere sintering model. From these assumptions and - HO possible equate 70 ~L- it is LO -- : 2.099- LO PO - 1.099 ~ 2.099 H- - 1.099, (15) HO where Lo is the initial distance between the two particle centers. Substituting Equations (14) and (15) into Equation (12), a relationship between film thickness (i-) and elastic modulus is established as shown below: = (1 - a [1 - 2 .0 9 9 TMI (16) )f This model assumes that sintering is complete when the centers of both particles are the same, or when L = 0.524. Using the measured data, it is possible to fit the data to the elastic modulus relationship for powder metallurgy and we obtain the following relationship: S 0.5498 T-Tmn (2~L.103 T 0.002) -- (17) where Lo = 2a ~ 34nm, the distance between the centers of two unsintered particles. Figure VI-6 compares the Young's modulus versus peak sintering temperature as predicted by Equation (17) to the measured data and shows that it is an accurate prediction for samples that are sintered at temperatures 5275*C. 90 80 70 60 (U 50 40 30 20 10 0 0 50 A 100 Nanoindentation 150 * 200 T (*C) 250 Beam Deflection 300 - 350 400 Predicted Figure VI-6. Young's modulus versus peak sintering temperature. The dashed line represents the elastic modulus of bulk silver. VI.3 Applying model to Nz-sintered nanoparticle films Now that a sintering model has been determined for air-sintered samples, the model is applied to N2 sintered samples to further validate the model's underlying mechanisms and assumptions. Ozawa et al. established that the 02 acts as a surfactant to molten silver and the surface tension, c7,, of molten silver reduced in the presence of gaseous 02 [54, 58]. This effectively implies that the q,, of molten silver 71 increases from 0.6001m- 2 to 0.966Jm- 2 and suggests the silver nanoparticles are more stable in N2 environments and, according to Equation (11), require higher temperatures to begin sintering as shown in Figure VI-7. 400 - 350 0 3UV (U C 250 200 150 1M 30 40 35 45 50 Particle Diameter [nm] Figure VI-7. Melting temperature vs. particle diameter comparison. Because silver particles are more stable in N2 , the predicted melting temperature (green) is higher than the predicted melting temperature of silver nanoparticles in air (blue). According to above, the minimum sintering temperature, Tmin, for the nanoparticle in N2 is 150*C compared to the 100"C required to begin sintering in air environments. With this change in predicted melting temperature, it is possible to repeat the -H-vs. sintering temperature experiments as shown in HO Figure VI-8. As can be seen, the results agree with the predicted values but start to deviate at higher temperatures, much like the results shown in Figure VI-4. However, unlike the previous results, the N2 sintered films also deviate at the extreme low temperatures. 72 1 - C 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 - - 50 0 100 150 250 200 300 350 400 450 Sintering Temperature 0 Figure VI-8. HO versus N2 - Predicted (N2) sintering temperature in N2 - The purple lines represent the fractional film thickness as predicted by the N 2 -sintering model. The measurements deviate from the predicted values at low and high sintering temperatures. Subsequent nanoindentation of the N2 -sintered films to determine elastic moduli are summarized in Figure VI-9. The predicted moduli curve was determined by Equation (17) with Tmin = 150*C. This shows that the model accommodates changes in sintering environment reasonably well and provides further validation for the semi-empirical model's underlying assumptions and mechanisms. 90 80 . . 704 mmmm.mmm.mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm . 60 50 0 wi 40 30 20 10 0 0 50 100 150 200 250 300 350 T ('C) 0 Nanoindentation (N2) - Predicted (N2) Figure VI-9. Elastic modulus versus peak sintering temperature when sintering in N 2 The dashed line represents the modulus of bulk silver. 73 400 VI.4 Summary A semi-empirical model relating sintering temperature to the resulting Young's modulus for silver nanoparticle films has been developed for temperatures less than 300*C. The model was developed by using film thickness as a proxy to estimate film densification as a function of peak sintering temperature, relating film densification to the film's fraction porosity, and finally predicting the Young's modulus using the classic Young's modulus-porosity relationship established in powdered metallurgy. Finally, as shown in Figure VI-10 and Figure VI-11, by factoring the change in sintering temperature predicted from the thermodynamic equations used to describe melting point depression for metal nanoparticles, the model is able to predict how sintering in N2 and air environments affects the film's densification and mechanical properties. While this analysis only used on silver nanoparticle films as a case study, it is believed to be applicable to the characterization of other metal nanoparticle films. 1 0.95 0.9 0.85 0.8 0 X 0.75 0.7 0.65 0.6 0.55 0.5 1 50 0 100 150 250 200 300 350 Sintering Temperature * Air - Predicted (Air) 0 N2 - Predicted (N2) versus sintering temperature. Figure VI-10. Sintering environment effects on 74 400 450 90 80 70 - 60 0 50 - " 40 - 30 20 10 - 0 0 50 200 150 100 250 300 350 400 T (*C) * A Nanoindentation (air) - Predicted (air) - Beam Deflection (air) 0 Nanoindentation (N2) Predicted (N2) Figure VI-11. Sintering environment effects on Young's modulus versus sintering temperature. The dashed line represents the modulus of bulk silver. 75 76 Chapter VII Conclusions and Future Work In closing, this thesis presented a hybrid surface micromachining process, utilizing a conventionally microfabricated sacrificial layer and a printed metal nanoparticle structural layer. This process was characterized and optimized to fabricate silver nanoparticle films for the purpose of investigating the effects of sintering on the resulting mechanical properties. Using a series of metrology measurements, including SEM, contact profilometry, beam deflection, and nanoindentation measurements, these sintered metal nanoparticle films were characterized both for their film morphology as well as their mechanical properties. These results were compiled and utilized to develop the following processmechanical property model for silver nanoparticle films: E - E0 / T -Tmin059 = 2.103 T - 0.002) To - T where Tmin is defined by the thermodynamic equations governing metal nanoparticle melting point depression. It is important to note that this model is accurate at lower temperatures but starts deviating at higher sintering temperatures due to the sintering mechanism changing at higher temperatures. While it is possible to further characterize the process-mechanical property model at higher sintering temperatures, this was seen as a lower priority. This is because this surface micromachining process was intended to be compatible with a wide variety of substrates, including several plastic substrates which have lower temperature budgets. Even though silver metal nanoparticle films was the only case study used in this investigation, because the mechanisms involved in this thermal sintering process are firmly based on the thermodynamics and metal's material properties, it is believed that this methodology can be applied to other metal nanoparticle materials. By measuring the film thickness and using it as a proxy for film densification, it is possible to directly determine a process-mechanical property model, avoiding the characterization of the underlying sintering mechanism and material properties all together. VII.1 Suggestions for future work' This process-mechanical property model only touches on the potential of metal nanoparticle films and many more studies are necessary before this class of materials can be readily adopted. Firstly, the affect sintering has on the nanoparticle film's residual stress needs to be characterized so that the full mechanical response of the film can be predicted. Secondly, this methodology should be carried out on different sized silver nanoparticle to validate the model's applicability to other silver metal nanoparticle inks. Finally, as an extension of the previously mentioned study, this methodology should also be carried out on other metal nanoparticles such as gold, iron, or tungsten to similarly validate the model's applicability to different material choices. Procedurally, several additional studies should also be conducted. It is possible that changing the heating and cooling rates during sintering will affect the nanoparticle's densification equilibrium, thus changing the resulting film morphology and mechanical properties. Also, while there was no 77 dependence on time when sintering at 250*, the nanoparticle films may display a time dependence when sintering at lower temperatures. Finally, because the silver nanoparticle films sensitive to their sintering environments, additional ambient environments such as forming gas and 40:60 0 2 :N2 should also be investigated. In addition to characterizing the process-mechanical properties of various metal nanoparticle films and investigation additional sintering parameters, the surface micromachining process can be further improved through better pattern design. Depending on the pattern's in-plane dimensions and the amount of ink printed, the film's profile can change drastically and as such every unique pattern requires a different drying temperature and process to obtain the ideal profile. Therefore, developing general methodology for characterizing the film profile as a function of the film's in-plane dimensions and amount of ink printed will greatly improve future designs of printed nanoparticle films. This will also significantly reduce the time spent to dry the films. As mentioned in IV.2.3.3, while the films nominally take 2-3 hours to dry, a full 12 hours was utilized to ensure the films all the ethylene glycol was removed. By optimizing the pattern generation and drying process, this excessive drying step would no longer be necessary. Lastly, it is important to improve the fabrication speed by modifying the digital fabrication system. In addition to the drying process, there are several improvements to the digital fabrication system that would facilitate faster manufacturing. A key example is the control software used to control the system: currently the system operates on a stop & drop method, which entails the system to move the TIPS controller, waits for the controller to stabilize, jet a droplet, and then move the next coordinate to repeat this process. While the wait time is minimal (on the order of 10ms), when printing hundreds of thousands of droplets, this delay becomes significant. This wait time can be reduced by changing the printing method to velocity printing. In velocity printing, the printer spools up to pre-designated velocity and subsequently prints all the droplets in its path without stopping, taking into account the offset due to the printhead's travel velocity. Because the system does not need to stop and stabilize the controller after each droplet, this method is significantly faster than the current printing method and would shorten the actual print time by at least an order of magnitude. In addition to velocity printing, there are several other improvements that would expedite the manufacturing process, such as incorporating active cooling into the heated stage to enable faster cooling of the samples, however, these will only be useful if major changes to the process, such as layer-by-layer drying/sintering, were implemented. 78 References [1] B. Bhushan, ed., Springer Handbook of Nanotechnology, Berlin, Heidelberg: Springer Berlin Heidelberg, 2007. [2] J. Eloy, E. Mounier, and L. Robin, Status of the MEMS Industry 2012, Lyon, France: 2012. [3] "HP Sensing Solutions" Available: http://www.hp.com/hpinfo/newsroom/press-kits/2011/sensingsolutions/. [4] E.W. 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Ozawa, "Surface tension of liquid metals and alloys - Recent developments," Advances in Colloid and Interface Science, vol. 159, Sep. 2010, pp. 198-212. 83 84 Appendix A Photolithography masks The photolithography masks were used to pattern photoresist for the surface micromachining process. Initially, the photoresist was used as the sacrificial layer; however, when it was determined that the thermal budget for photoresist was too low for the desired processing conditions, the photoresist was used as an etch masks for the a-Si sacrificial material. There were two generations for the photolithography masks: without sacrificial barriers and with sacrificial barriers. As mentioned in IV.4, the addition of sacrificial barriers provided features to control ink spreading and improved the repeatability of the process. Alignment marks Sacrificial Steps - Die 1 mm Figure A-1. Die design for generation 1 photolithography mask. The die consisted of five 500pim- and five 1mm-wide sacrificial steps. The alignment masks were for aligning the die in the digital fabrication system. The die ID was a pattern to identify the location the die on the wafer. 85 DDODUUUBaDo a Fiur Geeain1msA-2 ua a a dun D nU UU h t ein 10m mm ms Eu I Figure A-2. Generation 1 mask design. The mask was designed for 4-inch borosilicate wafer and produced 17 dies. Ultimately this design was discarded because of the uneven ink spreading that occurred when printing CCI-300. 86 Alignment marks _ I-I1 L_ LCI=i LFi== ii---J CL CIZ= CzJ LC=zi= CL=J CE=IIJ Lz=II= L__Ii L -4. Die ID 1 mm Figure A-3. Die design for generation 2 photolithography mask. This die design consisted of ten SOm- and ten 100pm-wide cantilever molds, ten 50pm- and ten 100pm-wide bridge beam molds, one variable-width cantilever mold, and one variable-width bridge mold. The design incorporated sacrificial barriers to encourage uniform ink spreading. The bridge molds were not used in this study but could be potentially used to measure the residual stress of the nanoparticle structures. The widths of the structures in the variable-width molds are determined by laser-trimming. 87 Test str C=1=111111 nroi LJWJJJ IE £JFlf 10 mm Figure A-4. Generation 2 mask design. The mask was design for a 4-inch borosilicate wafer and consists of 32 dies and 12 test structures. The test structures are were used to optimize the printing pattern parameters prior to using the dies. 88 Appendix B MATLAB Scripts This section consists of the scripts used to generate the printing patterns and the script used to extrapolate the Young's modulus from the beam deflection data. B.1 Printing pattern generation scripts This script is separated into three modules: the pattern generation, graphing, and control modules. The control module gave the pattern generation module the appropriate parameters (coordinates, droplet diameter, boundary spacing, etc.) so that the pattern could be generated. The graphing module was used to confirm the proper pattern was generated. B.1.1 Control module clear all; close all; newFile = 1; restore = 0; diameter = 80; border = 80; shifting = 0; shifting_.x = 0; shifting.y = 0; shiftByY = 550; offstageX = 40000; offstageY = 0; waltTime = 20; dropDelay = 50; %0 = alignment marks; 1= corners %in minutes %in ms if(newFile) delete('data.txt'); delete('boundary.txt'); end % Prints coupon number ID if(shifting) shiftingy = shiftByY; end sample =.[01 010101010101010101... 01010101010101010101... 01010101010101010101 ... 01010101010101010101 ... 1101]; run = 1; while (max(sample) > 0) direction = mod(run,8); % Prints coupon number ID if(sample(43) > 0) [boundary, dropCoordinates] = dropRectGen([-450, -10750+shiftingy],[450, 10050+shifting_y],diameter,border, 'hexHor',direction); end 89 if (run < 17) if (run < 9) [boundary, dropCoordinates] = dropRectGen([-5500+(run-1)*600, -10250+shiftingy],[-5200+(run-1)*600, 9950+shiftingy],diameter,75, 'hexHor',direction); else [boundary, dropCoordinates] = dropRectGen([-5200+(run-9)*600, -10750+shiftingy],[-4900+(run-9)*600, 10450+shiftingy],diameter,75, 'hexHor',direction); end else if (run < 25) [boundary, dropCoordinates] = dropRectGen([700+(run-17)*600, -10250+shiftingy],[1000+(run-17)*600, 9950+shiftingy],diameter,75, 'hexHor',direction); else [boundary, dropCoordinates] = dropRectGen([1000+(run-25)*600, -10750+shiftingy],[1300+(run-25)*600, 10450+shiftingy],diameter,75, 'hexHor',direction); end end startingx = -5350; endingx = starting x + 1600; if(sample(1) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -1250+shiftingy], [ending x,550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(2) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -2150+shifting_y],[ending x,1550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(3) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -3150+shiftingy],[endingx,2450+shiftingy],diameter, border, 'hexHor',direction); end if(sample(4) > 0) [boundary, dropCoordinates] = dropRectGen([starting_ x, -4050+shifting y],[endingx,3450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(5) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -5050+shiftingy], [ending x,4350+shiftingy],diameter,border, 'hexHor',direction); end if(sample(6) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -5950+shiftingy], [ending x,5350+shifting_yl,diameter, border, 'hexHor',direction); end if(sample(7) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -6950+shifting_y],[ending x,6250+shifting-y],diameter,border, 'hexHor',direction); end if(sample(8) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -7850+shifting y], [endingx,7250+shifting-y],diameter,border, 'hexHor',direction); end if(sample(9) > 0) 90 [boundary, dropCoordinates] = dropRectGen([startingx, -8850+shiftingy],[endingx,8150+shifting_y],diameter, border, 'hexHor',direction); end if(sample(10) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -9750+shiftingy],[endingx,9150+shiftingy],diameter,border, 'hexHor',direction); end startingx = -3450; endingx = startingx + 2100; if(sample(11) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -1250+shifting_y],[endingx,550+shiftingy],diameter, border, 'hexHor',direction); end if(sample(12) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -2150+shifting_y],[endingx,1550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(13) > 0) [boundary, dropCoordinates] = dropRectGen([starting_x, -3150+shifting-yl,[endingx,2450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(14) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 3450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(15) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 4350+shiftingy],diameter,border, 'hexHor',direction); end if(sample(16) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 5350+shifting-y],diameter,border, 'hexHor',direction); end if(sample(17) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 6250+shiftingy],diameter,border, 'hexHor',direction); end -4050+shifting_y], [ending x,- -5050+shiftingy], [ending x,- -5950+shiftingy], [ending x,- -6950+shiftingy],[endingx,- if(sample(18) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -7850+shiftingy],[endingx,7250+shifting-y],diameter,border, 'hexHor',direction); end if(sample(19) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -8850+shiftingy], [ending x,8150+shiftingy],diameter,border, 'hexHor',direction); end if(sample(20) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -9750+shiftingy], [ending x,9150+shiftingy],diameter,border, 'hexHor',direction); end startingx = 1350; endingx = startingx + 2100; if(sample(21) > 0) 91 [boundary, dropCoordinates] = dropRectGen([startingx, 550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(22) > 0) [boundary, dropCoordinates] = drop RectGen([startingx, 1550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(23) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 2450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(24) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 3450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(25) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 4350+shiftingy],diameter,border, 'hexHor',direction); end if(sample(26) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 5350+shiftingy],diameter,border, 'hexHor',direction); end if(sample(27) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 6250+shiftingy],diameter,border, 'hexHor',direction); end if(sample(28) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 7250+shiftingy],diameter,border, 'hexHor',direction); end if(sample(29) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 8150+shifting_y],diameter,border, 'hexHor',direction); end if(sample(30) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 9150+shifting_y],diameter,border, 'hexHor',direction); end startingx = 3750; endingx = startingx + 1600; if(sample(31) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 550+shifting-y],diameter,border, 'hexHor',direction); end if(sample(32) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 1550+shiftingy],diameter,border, 'hexHor',direction); end if(sample(33) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 2450+shifting_y],diameter,border, 'hexHor',direction); end 92 -1250+shifting_y],[endingx,- -2150+shiftingy],[ending x,- -3150+shiftingy],[endingx,- -4050+shifting-y],[endingx,- -5050+shiftingy], [endingx,- -5950+shifting_y], [ending x,- -6950+shiftingy],[endingx,- -7850+shiftingy], [endingx,- -8850+shiftingy],[ending x,- -9750+shifting_y], [endingx,- -1250+shiftingy], [endingx,- -2150+shiftingy], [ending x,- -3150+shiftingy], [endingx,- if(sample(34) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -4050+shiftingy],[endingx,3450+shiftingy],diameter,border, 'hexHor',direction); end if(sample(35) > 0) [boundary, dropCoordinates] = dropRectGen([starting.x, -5050+shifting_y], [endingx,4350+shiftingy],diameter,border, 'hexHor',direction); end if(sample(36) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -5950+shiftingy],[endingx,5350+shifting-y],diameter,border, 'hexHor',direction); end if(sample(37) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -6950+shiftingy], [endingx,6250+shifting_y],diameter,border, 'hexHor',direction); end if(sample(38) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, -7850+shifting_y], [endingx,7250+shiftingy],diameter,border, 'hexHor',direction); end if(sample(39) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 8150+shifting_y],diameter,border, 'hexHor',direction); end if(sample(40) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 9150+shifting-y],diameter,border, 'hexHor',direction); end starting-x = -1050; ending-x = startingx + 2100; if(sample(41) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 550+shIftng-y],diameter,border, 'hexHor',direction); end if(sample(42) > 0) [boundary, dropCoordinates] = dropRectGen([startingx, 5350+shiftingy],diameter,border, 'hexHor',direction); end sample = sample - 1; run a run+1; end -8850+shiftingy],[endingx,- -9750+shiftingy],[ending x,- -5050+shiftingy],[endingx,- -9750+shifting_y],[ending x,- plotDrops(diameter); B.1.2 Pattern generation module (dropRectGen.m) function [boundary, dropCoordinates] = dropRectGen(... corner1, corner2, diameter, border, fillType, printDirection) %dropRectGen creates a drop coordinate file based on rectangular %coordinates. In addition to the MATLab matrices outputs, the function also %creates two data files: "boundary.txt" and "data.txt" which are text file %outputs of the returned matrices. 93 % Outputs - provides lower-left coordinate, width & % [1x4 matrix]boundary height (in um) of the intended rectangle % % [Nx2 matrix]dropCoordinates - x-y coordinates (in mm) for drops to be % printed % Inputs % [1x2 matrix]corner1 - coordinates (in um) for lower-left corner % [1x2 matrix]corner2 - coordinates (in um) for upper-right corner - assumed droplet diameter (in um) % diameter - distance away (in um) from intended rectangle's % border boundary % - Type of fill: (default is 'hexHor') % fillType hexHor = hexagonal packing w/ horizontal align % hexVer = hexagonal packing w/ vertical align % ccpHor = close-circle packing w/ horizontal align % ccpVer = close-circle packing w/ vertical align % % printDirection - Direction of printing 1 = up->down & left->right % 2 = down->up & right->left % 3 = left->right & up->down % 4 = right->left & down->up % 5 = up->down & right->left % 6 = down->up & left->right % 7 = right->left & up->down % 8 = left->right & down->up % %Calculations bCorner = corner1; bWidth = (corner2(1)-corner1(1)); bHeight = (corner2(2)-cornerl(2)); %initial pitch switch fillType %hexagonal filling, vertical alignment case 'hexVer' corner1 = corner1 + border + diameter/2; corner2 = corner2 - border - diameter/2; width = abs(corner2(1) - cornerl(1)); height = abs(corner2(2) - cornerl(2)); xDrops = floor(width/(3/2*diameter)); yDrops = floor(height/(sin(pi/3)*diameter)); %close-circle filling w/ horizontal align case 'ccpHor' cornerl(1) = cornerl(1) + border + diameter/2; corner2(1) = corner2(1) - border - diameter/2; hexSide = 1/(2*sin(pi/3))*diameter; cornerl(2) = cornerl(2) + border + hexSide; corner2(2) = corner2(2) - border - hexSide; width = abs(corner2(1) - corner1(1)); height = abs(corner2(2) - cornerl(2)); xDrops = floor(width/diameter); yDrops = floor(height/(3*hexSide)); %close-circle filling w/ horizontal align case 'ccpVer' corner1(1) = cornerl(1) + border + diameter/2; 94 corner2(1) = corner2(1) - border - diameter/2; hexSide = 1/(2*sin(pi/3))*diameter; cornerl(2) = cornerl(2) + border + hexSide; corner2(2) = corner2(2) - border - hexSide; width = abs(corner2(1) - cornerl(1)); height = abs(corner2(2) - cornerl(2)); xDrops = floor(width/(3*hexSide)); yDrops = floor(height/dia meter); %case 'hexHor': hexagonal filling, horizontal alignment otherwise corner1 = corner1 + border + diameter/2; corner2 = corner2 - border - diameter/2; width = abs(corner2(1) - corner1(1)); height = abs(corner2(2) - cornerl(2)); xDrops = floor(width/(sin(pi/3)*diameter)); yDrops = floor(height/(3/2*diameter)); end if (xDrops > 0) xPitch = (corner2(1)-corner1(1))/xDrops; else xPitch = 0; cornerl(1) = (corner1(1)+corner2(1))/2; corner2(1) = cornerl(1); end if (yDrops > 0) yPitch = (corner2(2)-corner1(2))/yDrops; else yPitch = 0; cornerl(2) = (corner1(2)+corner2(2))/2; corner2(2) = cornerl(2); end xOffset = xPitch/2; yOffset = yPitch/2; %Pre-allocate matrix totalDrops = xDrops*yDrops + (xDrops-1)*(yDrops-1); if (xDrops < 1) totalDrops = yDrops; end if (yDrops < 1) totalDrops = xDrops; end dropCoordinates = zeros(totalDrops,2); drop = 1; direction = 1; if (printDirection == 2) %Makes drop coordinates (outside drops) - down->up & right->left for x = xDrops:-1:0 for y = yDrops:-1:0 if (direction) %determine of drop progression is up/down dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; 95 else dropCoordinates(drop,:) = [corner1(1)+x*xPitch,corner2(2)-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - down->up & right->left for x = (xDrops-1):-1:0 for y = (yDrops-1):-1:0 %determine of drop progression is up/down if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+xOffset+x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+xOffset+x*xPitch,corner2(2)-yOffset-y*yPitch]; drop = drop + 1; end end direction end = mod(direction+1,2); elseif (printDirection == 3) %Makes drop coordinates (outside drops) - left->right & up->down for y = 0:yDrops for x = O:xDrops %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - left->right & up->down for y = 0:(yDrops-1) for x = 0:(xDrops-1) %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+xOffset+x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-xOffset-x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); 96 end elseif (printDirection == 4) %Makes drop coordinates (outside drops) - right->left & down->up for y = yDrops:-1:0 for x = xDrops:-1:O %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [corner1(1)+x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - right->left & down->up for y = (yDrops-1):-1:O for x = (xDrops-1):-1:0 %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+xOffset+x*xPitch,cornerl(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-xOffset-x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end elseif (printDirection == 5) %Makes drop coordinates (outside drops) - up->down & right->left for x = xDrops:-1:0 for y = O:yDrops %determine of drop progression is up/down if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+x*xPitch,corner2(2)-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - up->down & right->left for x = (xDrops-1):-1:O for y = 0:(yDrops-1) %determine of drop progression is up/down if (direction) dropCoordinates(drop,:) = ... 97 [cornerl(1)+xOffset+x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+xOffset+x*xPitch,corner2(2)-yOffset-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end elseif (printDirection == 6) %Makes drop coordinates (outside drops) - down->up & left->right for x = O:xDrops for y = yDrops:-1:0 if (direction) %determine of drop progression is up/down dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+x*xPitch,corner2(2)-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - down->up & left->right for x = 0:(xDrops-1) for y = (yDrops-1):-1:0 if (direction) %determine of drop progression is up/down dropCoordinates(drop,:) = ... [corner1(1)+xOffset+x*xPitch,cornerl(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+xOffset+x*xPitch,corner2(2)-yOffset-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end elseif (printDirection == 7) %Makes drop coordinates (outside drops) - right->left & up->down for y = O:yDrops for x = xDrops:-1:0 if (direction) %determine of drop progression is left/right dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; 98 end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - right->left & up->down for y = 0:(yDrops-1) for x = (xDrops-1):-1:0 %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [corner1(1)+xOffset+x*xPitch,cornerl(2)+yOffset+y*yPitch); drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-xOffset-x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end elseif (printDirection == 8) %Makes drop coordinates (outside drops) - left->right & down->up for y = yDrops:-1:O for x = O:xDrops %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-x*xPitch,corner1(2)+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - left->right & down->up for y = (yDrops-1):-1:0 for x = 0:(xDrops-1) %determine of drop progression is left/right if (direction) dropCoordinates(drop,:) = ... [cornerl(1)+xOffset+x*xPitch,cornerl(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner2(1)-xOffset-x*xPitch,corner1(2)+yOffset+y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end else %Makes drop coordinates (outside drops) - up->down & left->right for x = O:xDrops 99 for y = O:yDrops if (direction) %determine of drop progression is up/down dropCoordinates(drop,:) = ... [cornerl(1)+x*xPitch,cornerl(2)+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = {corner1(1)+x*xPitch,corner2(2)-y*y Pitch]; drop = drop + 1; end end direction = mod(direction+1,2); end %Makes drop coordinates (inside drops) - up->down & left->right for x = 0:(xDrops-1) for y = 0:(yDrops-1) if (direction) %determine of drop progression is up/down dropCoordinates(drop,:) = ... [cornerl(1)+xOffset+x*xPitch,cornerl(2)+yOffset+y*yPitch]; drop = drop + 1; else dropCoordinates(drop,:) = [corner1(1)+xOffset+x*xPitch,corner2(2)-yOffset-y*yPitch]; drop = drop + 1; end end direction = mod(direction+1,2); end end %Rounds to nearest urn and converts to mm dropCoordinates = round(dropCoordinates)./1000; %makes file(s) fid = fopen('data.txt', 'a'); [lastDrop, discardedDrop] = size(dropCoordinates); for drop = 1:lastDrop fprintf(fid, '%g,%g\n', dropCoordinates(drop,1),dropCoordinates(drop,2)); end fclose(fid); boundary = [bCorner(1), bCorner(2), bWidth, bHeight]; fid = fopen('boundary.txt', 'a'); fprintf(fid, '%g,%g,%g,%g\n', bCorner(1),bCorner(2),bWidth,bHeight); fclose(fid); end B.1.3 Graphing module (plotDrops.m) function plotDrops(diameter) % Plots the droplet coordinates, droplet boundaries, & intended boundary %(assumes rectangles) for pattern analysis 100 % Inputs: % diameter - assumed diameter (in um) of the droplets %plots dropCoordintates file drops = importdata('data.txt plot(drops(:,1),drops(:,2),'*') %makes plot to 1:1 x-y scale daspect([ 11]); %plots intended boundaries boundary = importdata('boundary.txt'); boundary = boundary./1000; (, i] = size(boundary); for i = 1:j rectangle('Position', boundary(i,:)', 'edgeColor','r'); end %plots drop boundaries diameter = diameter/1000; shift = diameter/2; for i = 1:length(drops) rectangle('Position', [drops(i,1)-shift,drops(i,2)-shift,diameter,diameter],... 'Curvature', [1 1], 'edgeColor','g'); end end B.2 Young's modulus extrapolation via beam deflection measurements clear all; close all; baseFileName = '23beamDeflect_04_09r'; recordSummary = 0; recordEAnalysis = 0; recordE all = 0; noOfFiles = 6; noOfSlopes = 6; %removes 1st slope (tends to be worst) removelstSlope = 1; %(s) timePerSlope = 5; %proportion of slope to disregard timePadding = 0.05; %fitthreshold r2Threshold =0.50; %spacing between deflections (um) L= [0;-20;-40;-60;-80;-100]; %width (um) W = 100.0; %thickness (um) H = 2.80967; %selectively removes specific k-values kUse = logical([0 0 1111]); 123456 % %%Preallocates matrices k = zeros(noOfSlopes,1); r2 = zeros(noOfSlopes,1); k_ave = zeros(noOfFiles,1); if (recordSummary) 101 fid = fopen(strcat(baseFileName,' Summary.csv'),'w'); end %% Analyzing Files for file = 1:noOfFiles %% Opens file fileName = strcat(baseFileName,' 000',int2str(file-1),'.txt'); forcedisplacement = importdata(fileName); plot(forcedisplacement.data(:,1),forcedisplacement.data(:,2)); [dispStart(file),forceStart(file)] = ginput(1); [dispEnd(file),forceEnd(file)] = ginput(1); clear force; clear displacement; %% Separates Slopes for i = 1:noOfSlopes lower = find(force_displacement.data(:,3)>(i-1+timePadding)*timePerSlope,1,'first'); upper = find(forcedisplacement.data(:,3)<(i-timePadding)*timePerSlope,1,'last'); if (i>1) if length(force(:,i-1)) ~= (upper-lower+1) upper = lower+length(force(:,i-1))-1; end end force(:,i) = force_displacement.data(lower:upper,2); displacement(:,i) = forcedisplacement.data(lower:upper,1); end %% Determine slope spring constants & average (k & k-ave) if (removelstSlope) for i = 1:noOfSlopes indx = find(displacement(:,i)>=dispStart(file)); indx = find(displacement(indx,i)<=dispEnd(file)); p = polyfit(displacement(indx,i),force(indx,i),1); forceFit = polyval(p,displacement(indx,i)); forceResid = force(indx,i)-forceFit; SSresid = sum(forceResid.A2); SStotal = (length(force(indx,i))-1)*var(force(indx,)); r2(i) = 1 - SSresid/SStotal; k(i) = p(1); end else for i = 1:noOfSlopes indx = find(displacement(:,i)>=dispStart(file)); indx = find(displacement(indx,i)<=dispEnd(file)); p = polyfit(displacement(indx,i),force(indx,i),1); forceFit = polyval(p,displacement(indx,i)); forceResid = force(indx,i)-forceFit; SSresid = sum(forceResid.A2); SStotal = (length(force(indx,i))-1)*var(force(indx,i)); r2(i) = 1 - SSresid/SStotal; k(i) =p(); end end kThresh = find(r2>r2Threshold); 102 k_ave(file) = mean(k(kThresh)); %% Record results summary (if recordSummary==1) if (recordSummary) fprintf(fid, '%s summary:\n,k (uN/nm),r2\n',fileName); for i = 1:noOfSlopes fprintf(fid, 'Slope %d,%g,%g\n',i,k(i),r2(i)); end fprintf(fid, 'Average,%g,%g\n\n',k_ave(file),mean(r2(kThresh))); end end if (recordSummary) fclose(fid); end %% Determine Young's Modulus (E) using beam dimensions (W & H) if(exist('kUse','var')) else kUse = 1:length(L); end k_negl3 = (kave(kUse)).A(-1/3); k_negl_30rig = (kave).A(-1/3); p = polyfit(L(kUse),k neg1_3,1); k_negl_3Fit = polyval(p,L(kUse)); k_neg1_3Resid = knegl_3-knegl_3Fit; SSresid = sum(kneg1_3Resid.A2); SStotal = (length(kneg1_3)-1)*var(k_neg1_3); k_negl3R2 = 1 - SSresid/SStotal; E = p(1)A(-3)*4/(W*HA3); plot(L,k_neg1_3Orig,L(kUse),k_neg1_3Fit,L(kUse),k_neg1_3); %% Records Analysis in baseFileName Summary if (recordEAnalysis) fid = fopen(strcat(baseFileName,' Summary.csv'),'a'); fprintf(fid, 'Young"s Modulus (E) Analysis:\nRelative L (um),'); fprintf(fid, '%g,',L); fprintf(fid, '\nk (uN/nm),'); fprintf(fid, '%g,',kave(kUse)); fprintf(fid, '\nW:, %g,um\nH:, %g,um\nE:, %g,GPa',W,H,E); fclose(fid); end %% Records Analysis in baseFileName Summary if (recordEall) if (exist('summary.csv','file')==2) fid = fopen('summary.csv','a'); else fid = fopen('summary.csv','a'); fprintf(fid, 'Beam,E (GPa)\n'); end fprintf(fid, '%s,%g\n',baseFileName,E); fclose(fid); end 103 104 Appendix C Ink preparation CCI-300 silver nanoparticle ink's viscosity at stock concentration is 12.8cP at 25T. Because TIJ printheads require a viscosity range between 0.1-10cP for reliable printing, CCI-300 needs to be diluted to increase its printing reliability. Prior to diluting, the ink is rocked back and forth for 30 seconds, providing subtle agitation to ensure that the nanoparticles are evenly distributed within the ink. An aliquot of the ink isthen taken and diluted 1:2 volumetrically with ethanol to lower the viscosity using a micropipette. Lastly, the diluted aliquot is rocked back and forth for another 30 seconds to ensure even mixing before being filtered and loaded into the TIPS ink reservoir. Table C-1. Standard ink dilution process. 1 2 3 4 5 Step Homogenize CCI-300 Remove aliquot Dilute aliquot Homogenize aliquot Filter aliquot into TIPS reservoir Description Rock ink bottle back and forth for 30 seconds 25pL of ink is micropipetted into a vial for mixing 50jiL ethanol is micropipetted into mixing vial Rock mixing vial back and forth for 30 seconds Aliquot is passed through a 0.3pm syringe filter to remove large particulates, filtered aliquot is loaded into reservoir 105 106 Appendix D Printing troubleshooting While this thesis may have implied that setting up the digital fabrication system was reasonably straightforward, many considerations were taken to improve printing reliability. Assuming due diligence was performed when optimizing the jetting parameters, Figure D-1 summarizes the standard process used to prime the digital fabrication system for reliable printing. Yes t $$$0 4NoNo Yes No Appl R90lyve w Cent nuilni jt wipe printheadl with wb No swab Rostgr# aggetive sra ~on bage wipe 0i0thead with swalb No Yes Nos No NNo YN Yes Figure D-1. Printing troubleshooting flowchart. 107