by
Eric Jamesson Wilhelm
B.S. Mechanical Engineering
Massachusetts Institute of Technology, 1999
M.S.
Massachusetts Institute of Technology, 2001
Submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Mechanical Engineering at the Massachusetts Institute of Technology
D2004 "- L,---'--''-
June 2004
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Signature of Author:........
Certified
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Depa>*nent of Mechanical Engineering
May 7, 2004
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Joseph Jacobson
Associate Professor of Media Arts and Sciences
Ce'rtif ied
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Certified by:........ ...... ......
Certified by:........ .:.
.................................................................
George Barbastathis
Esther and Harold E. Edgerton Assistant Professor
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...............................................
)l Livermore
:ngineering
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H. Slocum
Professor of Mechanical Enaineerina. MacVicar Faculty Fellow
A cce pted by:........................................... .
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A. Sonin
Chairman, Department Committee on Graduate Students

Printed Electronics and Micro-electromechanical Systems
by Eric Jamesson Wilhelm
Submitted to the Department of Mechanical Engineering on May 7 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in
Mechanical Engineering.
Abstract
Current electronics and micro-electromechanical systems (MEMS) manufacture is optimized for the production of very high-volume parts on a limited range of substrates. These processes are long, consume large amounts of resources, and require expensive machines and facilities, but yield excellent products. Cheaper, faster printing processes are beginning to emerge with the ability to economically produce low or high-volume electronics and MEMS on flexible substrates.
This thesis describes the theoretical and practical design of a suite of printing processes including liquid embossing and offset liquid embossing (OLE). These printing techniques have created resistors, capacitors, and thin-film transistors without etching, vacuum deposition, or high temperatures. Here, the fabrication of all-printed electrostatic actuators is described
In liquid embossing a polydimethylsiloxane (PDMS) stamp with bas-relief features is brought into intimate contact with a thin liquid film such as a metal or semi-conductor nanoparticle colloid, spin-on-glass, or polymer to create patterns as small as 100 nm. A simulation of liquid embossing was developed by coupling fluid flow in a thin liquid film to the diffusion of solvent into a PDMS stamp. The model accurately predicts real aspects of the printing process including the time required to stamp and usable stamp geometries.
OLE was designed to address some of the limitations of liquid embossing. In
OLE the patterned liquid film is transferred to a different substrate, allowing finer control over geometry and material placement and leaving behind excess material trapped during stamping. All-printed electrostatic actuators were fabricated using
OLE by patterning gold on flexible polyimide and then under-etching with oxygen plasma. The polyimide acts as a sacrificial material, dielectric layer, and mechanical substrate. Square electrostatic actuators 50 microns on a side can modulate light up to approximately 1 kHz with fields of 1-2 volts per micron.
These actuators also show a sharp non-linear response to driving voltage that could be used as part of a passive row - column addressing scheme.
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In five years one makes a great many friends and receives a tremendous amount of help and support from those friends.
I especially thank my advisor, Joseph Jacobson, for putting together an amazing group of people, driving us hard to only do the best and newest work, and keeping the lab running.
Thanks to everyone in the lab for innumerable contributions including Vikrant,
Vikas, Brian C., Will, Dave K., Dave M., Mike, Mark, Chris, Peter, Kie-moon, Kim,
Brain H., Sawyer, Brent, and Colin.
Thanks to Saul for keeping my mind sharp and going on adventures with me.
Thanks to my thesis committee and especially to Alex for not only being my advisor, but a role model for my nine years at MIT.
Thanks to Brian N., Emily, Tim H., and Josh the outstanding UROPs that have helped with the project.
Thanks to some of the great people around MIT that have helped me out including Neil, Manu, Yael, John, Andy, Cagri, Alex Sp., Jeremy, Paul, and Kurt.
Thanks to Dan and Tim A. for taking me to the beach.
Thanks to my family: Laura, Mom, Dad, Grandma, and Grandpa,
Sometimes people ask where Christy and I went on our honeymoon, and I jokingly answer, "grad school". Essentially this is true; I started graduate school two days after being married. While graduate school has not exactly been a vacation, I have enjoyed it very much, which is in large part due to Christy.
Everyday with her is good day. So now that graduate school is coming to an end, when people ask where we went for our honeymoon I will need to tell them,
"we're still on it".
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1. Introduction .......................................................................................
2. W hy Print? .................................................
8
... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... . .
8
2.1. Production Gap in Electronics and MEMS manufacturing ........
8
2.1.1. Cost .....................................................................................
2.1.2. Speed ..................................................................................
9
9
2.1.3. Area................................................................................... 10
2.1.4. Use of resources ............................................................... 11
2.1.5. Products currently not feasible .......................................... 12
2.2. Printing Electronics.................................................................. 13
2.2.1. Standard Sem iconductor Processing ................................ 14
2.2.2. M icro contact printing ........................................................ 15
2.2.3. Im print Lithography............................................................. 16
2.2.4. Fluidic Self Assembly ........................................................ 18
2.2.5. Nano transfer printing ........................................................ 19
2.2.6. Ink-jet printing.................................................................... 19
2.2.7. O ptical disc m anufacturing ................................................. 20
2.2.8. Printing comparison........................................................... 20
3. Liquid em bossing techniques......................................................... 22
3.1. Active Inks ............................................................................. 22
3.1.1. Nanoparticles .................................................................... 22
3.1.2. Spin-on-glasses................................................................ 24
3.1.3. Polym ers ...........................................................................
3.2. Liquid Em bossing ....................................................................
24
25
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3.2.1. Theory of liquid embossing............................................... 25
3.2.1.1. Diffusion constant and saturation density ................... 27
3.2.1.2. Liquid embossing finite element analysis.................... 30
3.2.2. Liquid em bossing sim ulations............................................ 41
3.2.2.1. Solvent variations .......................................................
3.2.2.2. Vacuum backing ........................................................
41
42
3.2.2.3. Vacuum cleaning ........................................................ 43
3.2.2.4. Geom etry..................................................................... 48
3.3. Offset Liquid Em bossing ......................................................... 51
3.4. Other Investigated Printing techniques ................................... 61
3.4.1. M icro intaglio .................................................................... 62
3.4.2. M icro letterpress printing .................................................... 63
4. Exploration of the liquid embossing technologies .......................... 65
4.1. Equipm ent and practical issues of printing ............................ 66
4.1.1. Stam ps ..............................................................................
4.1.2. Offset plates .......................................................................
66
68
4.1.3. Plasm a treatm ent ............................................................. 68
4.1.4. Ink form ulation.................................................................. 69
4.1.5. Dispensing and coating ..................................................... 70
4.1.6. Em bossing.......................................................................... 71
4.1.7. Alignm ent ......................................................................... 72
4.1.8. Transfer............................................................................ 73
4.1.9. Curing................................................................................. 74
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4.1.10. Etching ........................................................................... 75
4.1.11. Im aging............................................................................ 75
4.1.12. Stam p cleaning................................................................ 75
4.2. All printed electrostatic actuator.............................................. 76
4.2.1. Polyim ide sacrificial m aterial ............................................. 77
4.2.2. Spin-on-glass sacrificial m aterial...........................................90
4.2.2.1. Conform al printing ..................................................... 94
4.2.2.2. Surface Tension........................................................... 96
4.2.2.3. Porosity of the nanoparticle m etals...............................100
5. Results and Conclusions .................................................................
101
5.1. Results......................................................................................101
5.2. Conclusions ..............................................................................
102
5.3. Future work...............................................................................103
5.3.1. Rewritable stam p.................................................................103
5.3.2. Alignm ent ............................................................................ 104
5.3.3. Active Inks...........................................................................106
5.3.4. Printing resists by O LE ........................................................
107
5.3.5. M odeling..............................................................................108
5.3.6. Other applications ...............................................................
109
6. Appendix : Alpha-terpineol diffusion constant ..................................
110
7. Appendix : Solvent diffusion in stam p ..............................................
113
8. Appendix : Print Experim ental Details..............................................117
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This thesis describes efforts to print electronics and micro-electromechanical systems (MEMS). The major contributions are development of a theoretical model of liquid embossing, invention of the offset liquid embossing process to address the limitations of liquid embossing, and fabrication of all-printed electrostatic actuators as a demonstration of the embossing techniques.
Section 2, Why Print?, argues for printing as a solution to the inherent production gaps in standard electronics and MEMS manufacturing. Section 3,
Liquid Embossing Techniques, describes the theoretical aspects of the embossing techniques. Section 4, Exploration of the Liquid Embossing
Techniques, illustrates the practical issues of printing and shows the all-printed electrostatic actuators. Section 5, Results and Conclusions, summarizes the work and then gives ideas that could build upon this work.
Most integrated-circuit electronics and MEMS are manufactured in semiconductor fabrication facilities, or "fabs". Fabs excel at optimizing very difficult processing to create many identical products; billions of identical products if individual transistors are considered. However, these abilities come at steep prices. Fabs are expensive, have high through-put times, are constrained to
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substrates of limited area, and consume large amounts of resources. The production gap between what fabs can currently offer, and what they might be able to offer is discussed in the following sections.
2.1.1. Cost
A new fab working with 300 mm diameter silicon wafers costs more than one billion dollars
1
, four billion for a 90 nm feature size, 300 mm diameter wafer fab.
2 With costs this high, only a small number of companies can afford to build new capacity, leaving others to rely on partnerships or grants and loans from governments. Some companies are looking towards a system of foundries, or shared manufacturing facilities, but these may not offer the tight integration required for state-of-the-art production.
With this situation, new technologies become more expensive and difficult to develop. Electronics and MEMS manufacturers are only able to focus on the biggest products and customers because that is the only way to sell enough identical units to justify the high initial costs. There exists a production gap for electronics and MEMS that require low unit counts, in addition to low cost.
2.1.2. Speed
While the output of a new 300 mm fab may be an impressive 500 wafers per day, the speed of a wafer within the fab is much more sluggish. The cycle time (time from start to finished product) of a wafer may be four weeks of
1 Chasey, A. D. and S. Merchant (2000). "Issues for construction of 300-mm fab." Journal of
Construction Engineering and Management-Asce 126(6): 451-457.
2 LaPedus, M. (2003). Leading-edge fab costs soar to $4 billion. EETimes.,
[http://www.eetimes.com/semi/news/0EG2003031 0S0067]
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continuous processing. 0.25 days per layer is considered a very aggressive cycle time.
3 Long cycle times indicate a large number of wafers within the fab at any one point waiting to be processed. This work in progress (WIP) is typically wafers queuing up for batch processing steps such as thermal treatment and surface preparation.
4 Large numbers of WIP are undesirable because it makes the fab unresponsive to changes in customer demands and imposes unduly long design cycles for micro-fabricated products. Simultaneously, the fab is vulnerable to large losses should an error go undetected, even for a short period of time. While reducing the cycle time of fabs by creating single wafer processing clusters is active research, 5 a day and a half to process a six layer device is not as fast as a fab should or could be.
2.1.3. Area
The excellent resolution afforded by standard processing allows fabs to offset a high per area cost by packing in large numbers of devices. This strategy works for device-dense microprocessors and memory, but fails for products that require larger area or more external connections. A 300 mm silicon wafer currently costs between $100 and $200 and is estimated to account for 8% of
3
Ikeda, S., K. Nemoto, M. Funabashi, T. Uchino, H. Yamamoto, N. Yabuoshi, Y. Sasaki, K.
Komori, N. Suzuki, S. Nishihara, S. Sasabe and A. Koike (2003). "Process integration of single wafer technology in a 300-mm fab, realizing drastic cycle time reduction with high yield and excellent reliability." leee Transactions on Semiconductor Manufacturing 16(2): 102-110.
4
Bonnin, 0., D. Mercier, D. Levy, M. Henry, I. Pouilloux and E. Mastromatteo (2003). "Singlewafer/mini-batch approach for fast cycle time in advanced 300-mm fab." leee Transactions on
Semiconductor Manufacturing 16(2): 111-120.
g Lopez, M. J. and S. C. Wood (2003). "Systems of multiple cluster tools: Configuration, rellability and performance." leee Transactions on Semiconductor Manufacturing 16(2): 170-178.
10

production costs.
6 Only a thin layer of silicon is actually processed and the rest acts merely as a handle and substrate. Current processing steps with high temperatures and caustic chemicals place a high demand on any possible alternative substrate. A change to plastic, for example, would necessitate drastic processing changes. Even if a new substrate were found, fabs would not be able to physically deal with a shape other than a wafer.
2.1.4. Use of resources
Fabs have voracious appetites for resources. A 2 gram 32 Mb DRAM microchip is estimated to require 41 MJ of energy to produce.
7
This is a stunning energy usage of 20 GJ / kg when compared to the 42 MJ / kg of energy used for production of an automobile,
8 or 96 to 140 MJ / kg for polystyrene.
9
Clearly, the mass of automobiles produced per year is far greater than the mass of microchips, however, mass comparison is still insightful because it highlights the drastic difference between fabs and other types of manufacturing.
6 Pfitzner, L., N. Benesch, R. Ochsner, C. Schmidt, C. Schneider, T. Tschaftary, R. Trunk and H.
M. Dudenhausen (2001). "Cost reduction strategies for wafer expenditure." Microelectronic
Engineering 56(1-2): 61-71.
Williams, E. D., R. U. Ayres and M. Heller (2002). "The 1.7 kilogram microchip: Energy and material use in the production of semiconductor devices." Environmental Science & Technology
36(24): 5504-5510.
8 E. Williams, R. Ayres, and M. Heller, "Energy and chemical use in the production chain for microchips", in 2002 IEEE International Symposium on Electronics and the Environment, IEEE:
Piscataway, New Jersey, 2002, 184-189
9
WTEC Panel Report on: Environmentally Benign Manufacturing (EBM) (2001) Timothy Gutowski,
Cynthia Murphy, David Allen, Diana Bauer, Bert Bras, Thomas Piwonka, Paul Sheng, John
Sutherland, Deborah Thurston, Egon Wolff, World Technology (WTEC) Division of International
Technology Research Institute, Loyola College, 4501 North Charles Street Baltimore, Maryland
21210-2699.
11

Resource consumption within a fab is often measured per unit area rather than mass, yielding 1-1.4 KWh / cm 2 of energy consumption and 8 - 10 liters / cm 2 net feed water consumption.
10 Despite predictions of reductions, these rates surprisingly have remained stable. While more transistors can be packed into the same area, for the industry as a whole the total area of silicon processed still increases.
2.1.5. Products currently not feasible
The production gap in electronics and MEMS manufacture leads to products and devices desired by consumers that are not yet feasible to produce.
A few of these ideas are discussed here. Radio frequency identification tags could be used to uniquely track every item in warehouses and stores but are currently too expensive to be used on all but the highest-value items for theft deterrence. Visions of the future often include large area newspaper-like displays showing video clips, animations, and targeted advertisements. Ignoring the issues of whether one would actual want to develop a technology that could be used primarily for more advertisements, large area displays are not held back today for ethical reasons. Fabs inherently cannot process large area substrates, and imagine the waste that would be generated were a fab to process the same area that is currently produced in a single day by newspapers. It is easy to imagine the parts required to build millimeter scale flying or swimming robots,"
10 2003 International Technology Roadmap for Semiconductors [http://public.itrs.net/]
" Lauder, G. V. (2001). "Aerodynamics - Flight of the robofly." Nature 412(6848): 688-689.
12

but building the parts and putting them together proves much more difficult.' 2
Silicon is the substrate of choice,' 3 and creating micro-machined parts involves demanding trade-offs including material limitations, restrictions on the number of layers, and the need to share process runs and wafer space with others due to cost constraints, which are all coupled with long cycle times. Even those fortunate enough to have direct access to a fab still have long processing and other time consuming steps that keep the possible number of iterations on a design very low.
The production gap has attracted a set of technologies, loosely termed
"printed electronics". While the scope of the included technologies runs the gambut from home production of micro devices to the dream of displacing fabs entirely with radically different manufacturing methods, the idea of printing is an excellent metaphor to describe the envisioned changes. No longer would electronics manufacture require a billion dollar factory staffed by engineers tending high vacuum chambers and complex process tools, but rather compact printers will take rolls of plastic and output neat spools of computers and displays, or so the vision goes.
The fundamental production of electronics involves the creation of patterns in conductors, insulators, and semiconductors. The production of MEMS can
12 Ho, S., H. Nassef, N. Pornsinsirirak, Y. C. Tai and C. M. Ho (2003). "Unsteady aerodynamics and flow control for flapping wing flyers." Progress in Aerospace Sciences 39(8): 635-681.
13 Petersen, K. E. (1982). "Silicon as a Mechanical Material." Proceedings of the leee 70(5): 420-
457.
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similarly be broken down into the creation of patterns in structural materials.
Going one step further leads to the basic processes of depositing the materials and then patterning them. While this may be a simplistic view, it forms the basis of tools useful for the comparison of the various printing technologies. In the following sections a variety of technologies will be presented, with particular attention paid to their methods of deposition and patterning. The electronics printing community is fairly large, and only a small cross-section of technologies can be presented here; the technologies included are chosen because of their similarity to liquid embossing techniques, or their ability to fill similar niches.
Where possible, quantitative comparisons are made.
2.2.1. Standard Semiconductor Processing
Deposition in standard processing is done by a variety of different methods. The substrate itself is often the material to be patterned and hence requires no deposition. Oxides can be thermally grown on the substrate. Wide ranges of materials can be deposited under vacuum by chemical vapor deposition, physical vapor deposition, and evaporation. There are also wet techniques such as electroplating. All of these techniques are highly developed and so all yield excellent materials with good purity and tight control over deposition rates. However, they are not typically rapid or low-cost. While the deposition under vacuum may be quick, pumping from atmospheric pressure to low pressure is not. Thermal processes are similarly time consuming, especially when done in batch as is typical for thermal oxide growth.
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Patterning in standard processing is done by lithography and etching.
Radiation exposes regions of a resist that may be converted to an insoluble state in the case of a negative resist, or to a soluble state as with positive resists. The resist is then developed and used to modify the underlying material by etching.
Micro-lithography is extremely well developed.
14 90 nm features are nearly routine, while the latest fabs will soon be making products with 65 nm features.
Production lithography such as photolithography is fast, as the resists are tuned or chemically amplified and do not require large amounts of energy to expose.
Throughputs of 100 wafers per hour are possible. The drawback comes from the cost of micro-lithography tools. 248 nm steppers, capable of 110 nm features with advanced masks, today cost four million dollars; 193 nm steppers cost 16 million.
With costs this high, very large numbers of parts must be sold to amortize the costs. Etching technologies are also well developed, and similar to deposition can be done wet or under vacuum.
2.2.2. Micro contact printing
Micro contact printing refers generally to the technique of printing a selfassembled monolayer onto a surface.
15 There are a large number of variations possible.
6
,
17 The most common technique involves transfer of a thiol from a
14
Thompson, L. F., C. G. Willson and M. J. Bowden (1994). Introduction to microlithography.
Washington, DC, American Chemical Society.
15
Kumar, A. and G. M. Whitesides (1993). "Features of Gold Having Micrometer to Centimeter
Dimensions Can Be Formed through a Combination of Stamping with an Elastomeric Stamp and an Alkanethiol Ink Followed by Chemical Etching." Applied Physics Letters 63(14): 2002-2004.
16
Michel, B., A. Bernard, A. Bietsch, E. Delamarche, M. Geissler, D. Juncker, H. Kind, J. P.
Renault, H. Rothuizen, H. Schmid, P. Schmidt-Winkel, R. Stutz and H. Wolf (2002). "Printing meets lithography: Soft approaches to high-resolution patterning." Chimia 56(10): 527-542.
15

polymer stamp with pattern in bas-relief to a noble metal surface. The thiol molecules self assemble into a single molecular layer and can be used as an etch mask for the underlying layer. Deposition of the noble metal is typically done by thermal evaporation under vacuum because a reasonably flat surface is required.
Resolutions better than 100 nm are achievable, 18 and the pattern transfer is fast.
What microcontact printing loses due to a lack of generality in printing only self assembling molecules on specific surfaces, it gains from its absolute simplicity.
High quality prints can be made by hand using simple laboratory materials.
2.2.3. Imprint Lithography
There are a number of imprint lithographic techniques, where a microstructured mold is used to pattern a thin film of material. Nano imprint lithography is an embossing technique where a rigid stamp is used to create a pattern in a material above its glass transition.
19 Polymeric materials can be embossed, back etched to expose the underlying material and then used as an etch mask.
Similarly, in laser assisted direct imprint, a laser pulse melts a thin surface layer of silicon, and a mold is embossed into the liquid layer.
20
This surface embossing is very fast, but further processing would be required to form useful devices. The technique should be applicable to thin films of metal or other inorganic materials,
'7
Xia, Y. N. and G. M. Whitesides (1998). "Soft lithography." Annual Review of Materials Science
28:153-184.
18
Hull, R., T. Chraska, Y. Liu and D. Longo (2002). "Microcontact printing: new mastering and transfer techniques for high throughput, resolution and depth of focus." Materials Science &
Engineering C-Biomimetic and Supramolecular Systems 19(1-2): 383-392.
1
Chou, S. Y., P. R. Krauss and P. J. Renstrom (1995). "Imprint of Sub-25 Nm Vias and Trenches in Polymers." Aplied Physics Letters 67(21): 3114-3116.
20
Chou, S. Y., C. Keimel and J. Gu (2002). "Ultrafast and direct imprint of nanostructures in silicon." Nature 417(6891): 835-837.
16

but results of this nature have yet to be published. In another technique, the imprinted material is a photopolymer that is cross-linked while in contact with the mold.
2 1 This method overcomes problems with elevated temperatures and pressures and allows for good overlay and alignment using transparent molds.
The imprint techniques have excellent resolution. While the embossing steps may be rapid, 2 2 to fully expose the underlying material, an anisotropic back etch is required and that underlying material, must be deposited by standard techniques. A potential drawback of imprint techniques is the cost effective production of high-resolution molds. Unlike projection lithography, where a reduction in feature size is possible between the mask and patterned resist, imprint techniques are inherently one-to-one, so high resolution molds would be very expensive. Additionally, imprint molds have a lifetime measured in hundreds of imprints or tens of wafers (with multiple imprints per wafer).
While other electronics printing techniques aim to fill gaps left by standard processing, the proponents of imprint lithography and similar processes have taken a different tack, pitching a seemingly head to head battle with next generation photolithography.
2 3 , 24 Standard processing has plenty of support, 2 5 and it will be very interesting to see if imprint techniques are successful.
21 Haisma, J., M. Verheijen, K. vandenHeuvel and J. vandenBerg (1996). "Mold-assisted nanolithography: A process for reliable pattern replication." Journal of Vacuum Science &
Technology B 14(6): 4124-4128.
0 Xia, Q. F., C. Keimel, H. X. Ge, Z. N. Yu, W. Wu and S. Y. Chou (2003). "Ultrafast patterning of nanostructures in polymers using laser assisted nanoimprint lithography." Applied Physics Letters
83(21): 4417-4419.
23 Zhang, W. and S. Y. Chou (2003). "Fabrication of 60-nm transistors on 4-in. wafer using nanoimprint at all lithography levels." Aplied Physics Letters 83(8): 1632-1634.
17

2.2.4. Fluidic Self Assembly
Fluidic self assembly is a technique that builds upon standard processing, but decreases the density to make larger area devices. Integrated circuits are produced on silicon substrates. The silicon is then processed by back etching to form small pyramidal shaped devices called "nanoblock ICs". The nanoblocks are dispersed into liquid and flowed over a substrate with pre-shaped receptor sites.
By control of the flow conditions and the nanoblock geometry, the nanoblocks can be rapidly and accurately positioned. The nanoblocks are then secured in position and connected by post-processing such as metalization, lithography, and via creation.
Fluidic self assembly relies on the advantages of standard processing, but allows for the cost effective production of devices with lower density and large area. Large area, low resolution displays and RFID tags are ideally suited products. The placement of the ICs is very rapid, but integration and external connections, for example to radio frequency antennas, remains a challenge. So far this has limited fluidic self assembly to production of higher cost RFID tags for case and pallet tagging, rather than individual product tagging.
24
Resnick, D. J., W. J. Dauksher, D. Mancini, K. J. Nordquist, T. C. Bailey, S. Johnson, N. Stacey,
J. G. Ekerdt, C. G. Willson, S. V. Sreenivasan and N. Schumaker (2003). "Imprint lithography for integrated circuit fabrication." Journal of Vacuum Science & Technology B 21(6): 2624-2631.
25
Brunner, T. A. (2003). "Why optical lithography will live forever." Journal of Vacuum Science &
Technology B 21(6): 2632-2637.
" Snyder, E. J., J. Chideme and G. S. W. Craig (2002). "Fluidic self-assembly of semiconductor devices: A promising new method of mass-producing flexible circuitry." Japanese Journal of
Applied Physics Part 1-Regular Papers Short Notes & Review Papers 41 (6B): 4366-4369.
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2.2.5. Nano transfer printing
Nano transfer printing overcomes some of the limitations of micro contact printing, by extending the range of materials that can be printed.
2 7 2 8
Oxide forming materials and noble metals with oxide forming adhesion layers are evaporated onto stamps. The evaporated materials are transferred to a substrate, and attached by a condensation reaction. Patterns of gold with features as small as 70 nm have been printed on PDMS and GaAs surfaces. The transfer itself takes place in seconds, while the actual deposition of material must be done by standard vacuum techniques. Multi-layer alignment with this technique may prove difficult as the stamps with deposited material are inherently opaque.
2.2.6. Ink-jet printing
In ink-jet printed electronics, a liquid solution is directly printed onto a substrate. The material range is broad; almost any liquid can be successfully printed including inorganic nanoparticle colloids, 2
9 polymers, 30 molten metals, 31 and inks. Ink-jet printing has resolutions in the tens of micrometers and
2
Loo, Y. L., R. L. Willett, K. W. Baldwin and J. A. Rogers (2002). "Additive, nanoscale patterning of metal films with a stamp and a surface chemistry mediated transfer process: Applications in
?lastic electronics." Aplied Physics Letters 81(3): 562-564.
Loo, Y. L., J. W. P. Hsu, R. L. Willett, K. W. Baldwin, K. W. West and J. A. Rogers (2002).
"High-resolution transfer printing on GaAs surfaces using alkane dithiol monolayers." Journal of
Vacuum Science & Technology B 20(6): 2853-2856.
29 Fuller, S. B., E. J. Wilhelm and J. A. Jacobson (2002). "Ink-jet printed nanoparticle microelectromechanical systems." Journal of Microelectromechanical Systems 11(1): 54-60.
30 de Gans, B. J., P. C. Duineveld and U. S. Schubert (2004). "Inkjet printing of polymers: State of the art and future developments." Advanced Materials 16(3): 203-213.
31
J. Priest, E. J., C. Smith, Jr., P. DuBois, B. Holt, and B. Hammer-schlag (1994). "Liquid metaljetting technology: Application issues for hybrid technology." Int. J. Microcircuits Electron. Packag.
17(3): 219-227.
19

comparatively slow deposition rates, but its real advantage is programmatic patterning. Without the need for masters, masks, or stamps computer designed parts can be printed with almost no set-up time. Also, since the process can be fully additive, three-dimensional parts can be directly printed.
2.2.7. Optical disc manufacturing
The manufacture of optical data discs may not be true printing, but it represents an excellent benchmark for the extremely rapid and inexpensive production of sub-micron features. Compact discs and DVD discs are replicated
by injection molding polycarbonate against a micro-patterned metal sub-master and then metalized by sputtering.
32 The master is fabricated through photoexposure of a polymer resist and etching, in much the same way as the stamps used in the imprint technologies. The process creates 400 nm pits in only a few seconds with the metalization step actually being even shorter. Only passive elements are created and only gross alignment between the two layers in a DVD is required. Master and sub-master creation is also fast, requiring only a few hours.
2.2.8. Printing comparison
Table 1 summarizes and compares the discussed electronics printing schemes. Where applicable comparisons are made for printing a conducting line.
32 Nakajima, H. D. and H. Ogawa (1992). Compact disc technology. Tokyo, Japan
Burke, Va., U.S.A., Ohmsha; Distributed by IOS Press.
20

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Engineering 20(1): 53-62.
21

This section describes the liquid embossing techniques. It begins with a discussion of active inks, followed by the theory and model of liquid embossing and offset liquid embossing, and concludes with a brief discussion of some similar printing techniques.
From Table 1 there is no clear winner for all electronics manufacturing situations. The patterning steps are for the most part all fast, leaving the deposition and etching steps as rate limiters. Active inks are a potential solution to this problem. Rather than incurring the process step required in patterning a resist and using it as an etch mask, the desired materials can be patterned directly. An active ink is then any liquid solution that can form a pattern of desired material. This can include molten metals, bio-molecules, and sol-gel precursors, but for the purposes of this work, three main active inks are considered: nanoparticle solutions, spin-on-glasses, and several polymers.
3.1.1. Nanoparticles
Nanoparticles are clusters of atoms several nanometers in diameter.
Unlike bulk materials, the surface area of a nanoparticle is nonnegligible. With characteristics somewhere between an atomic species and the bulk material, a nanoparticle's physical, optical, and electronic properties are determined by
22

size.
37
melting or sintering temperature. With very high surface area to volume ratios, it is possible to cap nanoparticles with molecules that allow for high solubility to form a nanoparticle colloid. Nanoparticles also exhibit significant melting point reduction compared with the bulk material.
3 8
,
39 These attributes allow for the processing of materials that would otherwise be unsuitable for low-cost substrates such as plastics.
Devices based on solution phase inorganic nanoparticles have exhibited some of the best performance characteristics of any printed device.
40 While maintaining low temperature and liquid deposition similar to organic materials, 41 inorganic nanoparticles allow for much higher performance. Field effect mobilities of 1 cm 2 / volt second using CdSe nanocrystals without process optimization are possible. These are on par with the suggested theoretical maximum for organic materials.
42 Gold nanoparticles have been ink-jet printed and sintered using a maximum temperature of 140 'C yielding conductivities 70% of bulk gold.
4 3 This work chiefly uses gold and silver nanoparticles. However, it is possible to liquid
37
Alivisatos, A. P. (1998). "Electrical studies of semiconductor-nanocrystal colloids." MRS Bulletin
23(2): 18-23.
3 Buffat, P. and J. P. Borel (1976). "Size Effect on Melting Temperature of Gold Particles."
Physical Review A 13(6): 2287-2298.
Goldstein, A. N., C. M. Echer and A. P. Alivisatos (1992). "Melting in Semiconductor
Nanocrystals." Science 256(5062): 1425-1427.
40
Ridley, B. A., B. Nivi and J. M. Jacobson (1999). "All-inorganic field effect transistors fabricated
4
by printing." Science 286(5440): 746-749.
Dimitrakopoulos, C. D. and P. R. L. Malenfant (2002). "Organic thin film transistors for large area electronics." Advanced Materials 14(2): 99-+.
42
Garnier, F., R. Hajlaoui and M. El Kassmi (1998). "Vertical device architecture by molding of organic-based thin film transistor." Applied Physics Letters 73(12): 1721-1723.
43
Huang, D., F. Liao, S. Molesa, D. Redinger and V. Subramanian (2003). "Plastic-compatible low resistance printable gold nanoparticle conductors for flexible electronics." Journal of the
Electrochemical Society 150(7): G412-G417.
23

emboss CdSe nanoparticles, and in principal any inorganic nanoparticle colloid should be patternable by the liquid embossing techniques.
3.1.2. Spin-on-glasses
Spin-on-glass (SOG) refers to a family of silicon and oxygen polymers that form inter-level dielectric materials in microchips. SOG is deposited, usually by spin coating, as a liquid that is baked to remove the solvent and then cured to fully cross-link the polymer. There are several different types of SOGs including silicates, phosphosilicates, and siloxanes that each offer different crack resistance, processing temperatures, and dopant concentrations. SOGs can be patterned by E-beam lithography, or once fully cured, can be etched by solutions of hydrofluoric acid. In this work, SOGs were used as sacrificial materials.
3.1.3. Polymers
An extremely wide range of polymers exist for use in micro-fabrication.
These polymers range from conducting and semi-conducting materials, to the resists used as etch masks, to the plastic substrates. The polymers used in this work were polyimides, due to their high glass transition temperature, both for printing and as flexible substrates. Other polymers, such as PMMA, can be liquid embossed, but tend to flow, distorting a multi-layer print, during the cure steps of subsequent SOG or nanoparticle layers.
24

&.2.
Liquid embossing is a printing technique that uses a PDMS stamp to define features in a thin film of active ink." A wide range of materials have been patterned with good fidelity and resolution by liquid embossing.
45 A liquid embossing tool has also been designed that allowed for multi-layer alignment and automated patterning.
46 This section presents the theory of liquid embossing, a model of the process, and model predictions.
3.2.1. Theory of liquid embossing
Liquid embossing was discovered when a thin film of an active ink was used to micro contact print. A PDMS stamp with features in bas-relief was placed in contact with a film of active ink, in an attempt to ink the stamp, similar to letter press printing. The stamp did not accept and hold the ink as expected, like in micro contact printing, but rather patterns were formed in the ink pad. The process is so robust that even without knowledge of why or how it worked, it was possible to build a wide range of devices. Resistors, capacitors, photo detectors, thin-film transistors, inductors, and electrostatic actuators were all made from liquid inks by liquid embossing. Several different theories of how liquid embossing worked were presented, including the transfer of monolayers of PDMS
"4Bulthaup, C. A., E. J. Wilhelm, B. N. Hubert, B. A. Ridley and J. M. Jacobson (2001). "All- additive fabrication of inorganic logic elements by liquid embossing." Applied Physics Letters
79(10): 1525-1527.
4 Bulthaup, C. A. (2001). SM Thesis, Liquid Embossing: A Technique for Fabricating Sub-micron
Electrical, Mechanical, and Biological Structures. Department of Electrical Engineering and
Computer Science. Cambridge, MA, Massachusetts Institute of Technology.
Wilhelm, E. J. (2001). SM Thesis, Design of a Liquid Embossing Machine. Department of
Mechanical Engineering. Cambridge, MA, Massachusetts Institute of Technology.
25

to the substrate surface and differential surface tension forces between the stamp and substrate. Yet, with such strong experimental results the theory of liquid embossing was not fully explored until recently.
Figure 1 A schematic of liquid embossing that shows the thickness of the liquid layer as thinner than the raised features of the stamp
In liquid embossing, a thin film of liquid is placed in intimate contact with a stamp. The stamp has raised features that are 1 pm tall. The key to understanding the actual patterning mechanism is knowing the thickness of that liquid film. Measured by SEM and AFM, cured films of nanoparticle gold and silver average 100 nm thick. The solvent in these inks is alpha-terpineol, a heavy organic solvent with a very low vapor pressure (ink formulation will be discussed in full detail in section 4.1.4). Using the ratio of densities it is easy to find that the thickness of the liquid film must be greater than the height of the raised features.
Experimental evidence indicates that the raised features of the stamp touch down against the substrate during printing. So the idea of thicker films of liquid was at odds with the original thinking that the liquid films were only half the thickness of the raised features.
26

The diffusion of solvents 4 7 and vapors4s in PDMS is fairly well studied and understood. The soft lithographic technique known as micro molding in capillaries 49 actually relies on the diffusion of solvent into PDMS to achieve full patterning.
50 Due to these reasons the diffusion of solvents in the active inks was thought to play a dominate role in liquid embossing and further investigated.
3.2.1.1. Diffusion constant and saturation density
To setup a diffusion model two parameters were required: the diffusion constant of alpha-terpineol in PDMS and the saturation density. These parameters are always measured experimentally, but unfortunately do not appear in the literature. They were measured by soaking slabs of PDMS in alphaterpineol and measuring the increase in mass.
47
Zhao, Y. Q. and B. E. Eichinger (1992). "Study of Solvent Effects of the Dilation Modulus of
Poly(Dimethylsiloxane)." Macromolecules 25(25): 6988-6995.
48
Blume, I., P. J. F. Schwering, M. H. V. Mulder and C. A. Smolders (1991). "Vapor Sorption and
Permeation Properties of Poly(Dimethylsiloxane) Films." Journal of Membrane Science 61: 85-97.
49 Kim, E., Y. N. Xia and G. M. Whitesides (1995). "Polymer Microstructures Formed by Molding in
Capillaries." Nature 376(6541): 581-584.
'5 Duineveld, P. C., M. Lilja, T. Johansson and 0. Inganas (2002). "Diffusion of solvent in PDMS elastomer for micromolding in capillaries." Langmuir 18(24): 9554-9559.
27

Figure 2 Image of slabs of PDMS soaked in alpha-terpineol to determine the saturation density.
The saturation density is the maximum concentration of solvent in the polymer.
51 The interface between a polymer and a liquid solvent is assumed to be at the saturation density. The find the saturation density, slabs of PDMS several millimeters on a side, shown in Figure 2, were immersed in alphaterpineol until their mass stopped increasing. The percentage increase in mass was multiplied by the density of PDMS yielding the saturation density of 128 kg
/ m
3 . 120 hours were required for the mass of the slabs to stop increasing, and they were further left under alpha-terpineol for 144 hours to ensure equilibrium had been reached. This saturation density is comparable to those of other, similar solvents reported in the literature.
51
Crank, J. (1956). The mathematics of diffusion. Oxford,, Clarendon Press.
28

The diffusion constant was calculated by measuring the mass increase of
PDMS over time and comparing it to a diffusion model. PDMS was cast to a depth of 3.7 mm in a petri dish 34 mm in diameter. Alpha-terpineol was placed on top of the PDMS for timed intervals. To measure the mass of the PDMS the alpha-terpineol was removed by spraying the PDMS surface with deionized water and then blowing it dry with nitrogen. The strong hydrophobic surface of PDMS prevents absorption of any water.
52
3.5
x104
--
3
C,,
C
2.5
7g
2
C
1.5
- - -
- --,T-- -- ----------- - - --- -
-- 1 - - --- - -
Z0
0.5
-
I
------ -- ----
0 1000 2000 3000 4000 5000 6000 7000 8000
Time (minutes)
Figure 3 Mass of diffused alpha-terpineol in PDMS in a 34 mm petri dish. The points represent measured data, and the curve simulated data.
The rate of mass increase was compared against a simulation where the boundary conditions, including the saturation density, were fixed and the diffusion
52
Favre, E., P. Schaetzel, Q. T. Nguygen, R. Clement and J. Neel (1994). "Sorption, Diffusion and
Vapor Permeation of Various Penetrants through Dense Poly(Dimethylsiloxane) Membranes - a
Transport Analysis." Journal of Membrane Science 92(2): 169-184.
29

N r
..
constant varied to fit the data. The experimental and simulated data is shown in
Figure 3. The good match between the measured and simulated data indicates that the assumption of a fixed diffusion constant, i.e. one that is not a function of concentration, is valid. Measured data and simulation details are given in Section
6. The diffusion constant of alpha-terpineol in PDMS was found to be 1.2 * 10
-1 m 2 /s. This is approximately an order of magnitude smaller than comparable solvents. Solvents such as toluene and hexane diffuse so rapidly into PDMS that the PDMS can be seen visibly swelling.
3.2.1.2. Liquid embossing finite element analysis t t , ~I t i I I t I t T I t T t T T T T TTI t t I T T
Figure 4 Image of the liquid embossing model. The colors represent concentration of alpha-terpineol In the stamp and the arrows represent the fluid velocity in the thin film of liquid. The Image shows a representative part of the model, not its full extents.
30

Liquid embossing was simulated using Femlab 3 and Matlab.
54 The model consisted of a cross section of a PDMS stamp 50 pm wide and 500 pm thick in contact with a thin film of liquid 5 pm thick. In the physical system, features like those modeled may continue for many millimeters into the plane of the cross section, so a two dimensional model should be accurate. Figure 4 shows a small, but representative portion of the model and Figure 5 shows a schematic of the model.
stamp top surface insulation boundary condition
PDMSstam
Diffusion model
",saturation density symmetry symmetr flux boundary condition fluid film
Navier-Stokes model normal flow boundary condition
Figure 5 Schematic of the liquid embossing model
In the PDMS stamp region the standard diffusion equation,
*5
Femlab version 3.0, COMSOL, Inc., 8 New England Executive Park Suite 310 Burlington, MA
01803
'5 Matlab version 6.5, The MathWorks, 3 Apple Hill Drive, Natick, MA 01760-2098
31

6c
-+ V -(-DVc)= R o5t is valid. R, the reaction rate, is zero for this case, D is the diffusion constant, c is the concentration, and t is time. The boundary between the liquid and the stamp is held fixed at the saturation density. The boundaries forming the cross section are symmetric boundary conditions and the top surface of the stamp is an insulated boundary condition, both of which have the same symbolic form,
n -(-DVc) = 0, where n is a unit vector normal to the boundary surface.
In the fluid film region, the Navier-Stokes equation,
DV 1
-t=---Vp+
Dt p p g+--VV2 p if V. V = 0 55 is valid. V is the fluid vector velocity, p is the fluid density, g is the acceleration of gravity, and p is the fluid viscosity. The fluid flows in this system have Reynolds numbers less than 10-6 and the film is so thin that inertial and gravity effects are negligible. The boundaries forming the cross section are modeled as slip boundaries, n-V=O,
T -F= 0, where T is a unit tangential vector and F is the stress tensor. Modeling the motion of constant volumes of liquid and moving boundaries by finite element analysis is
55
Fay, J. A. (1994). Introduction to fluid mechanics. Cambridge, Mass., MIT Press.
32

very difficult because it involves non-linear geometries. To overcome this limitation in the liquid embossing model, the boundary between the liquid film and the substrate was modeled as a normal flow boundary to simulate the motion of the substrate towards the stamp. Symbolically this is
T-V =0 n-f=O
n- -l= 0.
This assumption is valid until the substrate is close to the raised features of the stamp and is discussed further in section 5.3.5. A continuity equation governs the interface between the stamp and the liquid film. The diffusive flux of solvent into the stamp is equal to the flux of solvent flowing out of the fluid film. This is modeled as an outflow boundary condition in the fluid film where nV= DVc
T-V=O.
Even though the diffusion constant is measured experimentally it has a theoretical value given by:
D = KB mp 5 6 f where KB is Boltzmann's constant, temp is the temperature, and f is a frictional factor. For spherical particles the frictional factor is: f = 67R,
56 Hiemenz, P. C. and R. Rajagopalan (1997). Principles of colloid and surface chemistry. New
York, Marcel Dekker.
33

where q is the viscosity of the material the particles are diffusing through and R. is the radius of the particles. This leads to the Stokes-Einstein relation:
D- KBtemp
The diffusion constants of solvents similar to alpha-terpineol in PDMS increase by approximately a factor of two for 1000 C temperature changes.
57 In practice, if the temperature goes above 600 C the thin liquid films begin to evaporate and liquid embossing is not possible. For this reason the temperature of the system was held constant and not included in the simulation.
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Figure 6 Mass of diffused solvent in the simulated stamp. The mass was found by integrating the concentration of the solvent over the diffusion region. Printing is considered complete when the mass of a 5 pm thick film of alpha terpineol (2.3 *
10-7 kg/m for the model geometry) has diffused into the stamp. As this is a two-dimensional model, the mass is per unit length into the plane.
57 Chandak, M. V., Y. S. Lin, et al. (1998). "Sorption and diffusion of volatile organic compounds in polydimethylsiloxane membranes." Journal of Applied Polymer Science 67(1): 165-175.
34
-

The model was compared against the physical system by its estimate of how long the stamp needs to be in contact with the thin liquid film. This was calculated in two different ways. First, the concentration of solvent in the simulated stamp was integrated and plotted versus stamping time as shown in
Figure 6. The print is considered complete when the mass of solvent in a 5 pm thick layer of alpha terpineol (per unit length into the plane of the simulation) has accumulated in the stamp. The second method calculated the average velocity of fluid moving through the normal flow boundary into the fluid film. Since this movement represents the movement of the stamp towards the substrate, it was integrated over time to give an approximate value of the position of the stamp with respect to the substrate as shown in Figure 7. The print was considered complete when the stamp had moved 5 pm towards the substrate.
35

FA , '- --- --- .1 .
5
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3 --- - -
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-
----- --- - - ---- ---- -
- ---
0.
0 10 20 30 40 50
Time (s)
60 70 80 90
Figure 7 Simulated distance between the stamp and the substrate, found by Integrating the average velocity on the normal flow boundary.
A time to stamp of 80-90 seconds agrees very well with the physical system. A stamp free of solvent used to pattern an alpha-terpineol based ink will take this period of time to create a good print. If the stamp is removed from the thin liquid film too soon, the ink will smudge and the pattern will not be fully defined. Longer contact has no effect for liquid embossing, though it can make the patterned film too dry to transfer for offset liquid embossing, discussed in
Section 3.3. In practice the stamps are left in contact with the film for 300 seconds. The extra patterning time accounts for slight variations in liquid film thickness, without imposing excessive waiting periods.
There is some subtlety about the actual process of setting up the simulation. The diffusion model in the stamp is fully defined and can be run
36

without the Navier-Stokes region giving accurate results. This fact is exploited to simulate multiple stampings, vacuum backing, and different geometries, as discussed in section 3.2.2, because the diffusion model is much less computationally intensive. Since the diffusion region is fully defined, the continuity boundary equation linking the two regions is modeled with non-ideal weak constraints. Reaction forces in this boundary only affect the fluid flow field, not the diffusion variable. The use of non-ideal weak constraints compared to symmetric ideal constraints does not change the predicted time required for a good print, but it does change the velocity field to give accurate results.
MO 3O
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Figure 8 Liquid embossing simulation image 0.5 seconds after the stamp first contacts the
liquid film. Concentration of solvent in the stamp is represented by color and the flow field of the liquid film represented by arrows. The exposed corners of the stamp quickly saturate with solvent and the fluid then tends to go around them.
37

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- , - , , -.% :-- -I ".. - - - -- - - - --
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The velocity field of the thin fluid film is one of the most interesting parts of the liquid embossing simulation. The flow field around a raised feature is shown in Figure 8. With more exposed area, the exposed corners of the raised features saturate with solvent faster than the rest of the stamp. The diffusive flux into these corners is much reduced and the fluid tends to go around them diffusing into the stamp at the base of the raised feature and into the recessed parts of the stamp. This phenomenon becomes even more pronounced for thinner raised features as shown in Figure 9. It is these fluid flows that drive liquid embossing.
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Solids in the active inks cannot diffuse into the PDMS. So, these nanoparticles, polymer chains, or other printed materials are swept into the space
38

between the raised features of the stamp by the diffusive flows. Once enough of the solvent has diffused, the stamp can be removed leaving behind a visco-elastic layer or even a dry film. The lack of solvent prevents the material from reflowing and distorting the pattern. The solids can then be cured by sintering or crosslinking. A schematic of this process is shown in Figure 10.
39

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Solids in the ink collect between the features, and remain once the solvent and stamp are removed.
//////////////////////////////////
Figure 10 Schematic of liquid embossing
40

3.2.2. Liquid embossing simulations
Several printing situations were simulated with the model to gain insight into the bounds of liquid embossing. These included printing inks with different solvents, using vacuum to change to the solvent's diffusive behavior and to clean the stamps, and stamps of different geometries.
3.2.2.1. Solvent variations
A key parameter of any patterning technique is its speed. Although liquid embossing combines the patterning and deposition steps, 100 seconds can still be considered a somewhat lengthy patterning time. Since diffusion of solvent into the stamp is the rate limiting step, patterning times for other possible solvents were calculated. While different solvents would have different densities and saturation densities, these parameters are kept constant in this simulation to clarify the effect of the diffusion constant. This is a reasonable approximation because the diffusion constants of various solvents in PDMS span more than two orders of magnitude 58
,
5 9 while the saturation and physical densities are at most
50% different from alpha-terpineol. Also, the densities only appear as linear factors in the governing equations. The time for a 5 pm layer of fluid to diffuse into a stamp for various diffusion constants is shown in Figure 11. If the active inks were to use 1 -propanol, which has a diffusion constant in PDMS of 5*10-10
58 Buraphacheep, V., D. E. Wurster and D. E. Wurster (1994). "The Use of Fourier-Transform
Infrared (Ft-Ir) Spectroscopy to Determine the Diffusion-Coefficients of Alcohols in
Polydimethylsiloxane." Pharmaceutical Research 11(4): 561-565.
59 Muzzalupo, R., G. A. Ranieri, G. Golemme and E. Drioli (1999). "Self-diffusion measurements of organic molecules in PDMS and water in sodium alginate membranes." Journal of AD1lied
Polymer Science 74(5): 1119-1128.
41

m 2 /s, it would take only two seconds to pattern. Perhaps a more realistic solvent variation would be 3-octanol, which has a diffusion constant in PDMS of approximately 10-10 m 2 /s. Although 3-octanol diffuses slower in PDMS, unlike 1- propanol, it has no polar compatibility problems with current nanoparticle active inks, and has a vapor pressure allowing a thin film to remain wet after the spin coating step.
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Figure 11 Simulated time required for the mass in a 5 pm fluid layer of a solvent with the same density and saturation density of alpha-terpineol to diffuse into PDMS for a range of diffusion constants. Solvents with these properties may not exist, however this shows the sensitivity of liquid embossing's patterning time to the diffusion constant of the solvent.
3.2.2.2. Vacuum backing
The application of vacuum to the back (non-featured side) of the stamp was modeled to determine if the speed of diffusion could be increased. The back side was held at zero concentration to simulate vacuum evaporating solvent on
42

the surface. For single patterns over the complete range of diffusion constants, vacuum backing had no effect on the solvent diffusion time. Figure 12 explains this effect best. The solvent from a single pattern does not diffuse very far and the bulk of the stamp is already at zero concentration regardless of vacuum. The simulated stamp is 500 pm thick, which is near the lower limit for physical stamps, so vacuum backed simulations with even thinner stamps were not performed.
0.
Ox
Figure 12 Image of the liquid embossing simulation showing a stamp slice with all of the mass from a 5 pm layer of solvent diffused into it. Due to the large relative thickness of the
stamp, all of the diffusion is limited to the side in contact with the solvent.
Vacuum may make little difference for a single pattern, but it has a large effect for multiple prints. If a stamp is used to print again, immediately following the previous print, the mass of solvent already in the stamp must be accounted for. Figure 13 shows the time for multiple prints as measured by solvent mass diffusion. Matlab code for this simulation is given in Section 7.
43

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Figure 13 Time required for multiple, successive prints using an alpha-terpineol based ink in a 5 pm thick layer of fluid. As the stamp saturates with solvent, more time is required for each print. The curve has the same shape for higher diffusion constant solvents.
A more realistic situation is shown in Figure 14. In this simulation, after the solvent has diffused into the stamp, some time is allowed to elapse. Since it is difficult to measure the exact time when all the solvent has diffused, the stamps are generally left in contact longer than required. During this extra time the solvent further diffuses within the stamp, but it does not have a significant effect on the print time.
44

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Number of prints
5 6 7
Figure 14 Time required for multiple, successive prints using an alpha-terpineol based ink in a 5 pm thick layer of fluid, with a 200 second waiting period between each print.
A stamp used for multiple prints also requires a cleaning step. Material left on the surface can be cleaned by solvent wash or by application of tape, as discussed in section 4.1.12, but solvent diffused into the stamp was removed by vacuum. Figure 15 shows that the print time for an alpha-terpineol based ink stabilizes around 110 seconds with 200 seconds of vacuum cleaning.
45

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Figure 15 Time required for multiple, successive prints using an aipha-terpineol based ink in a 5 pm thick layer of fluid, with a 200 second vacuum cleaning period between each print.
The print time for a faster diffusing 1 -propanol based ink stabilizes under 3 seconds with 5 seconds of vacuum for a total print time under 10 seconds. A 3- octanol based ink stabilizes under 20 seconds with a 15 second vacuum clean for a 35 second print time. This is shown in Figure 16 and Figure 17. The quantization in these graphs is the result of constraints placed on the simulation so that is could be solved in a timely fashion and does not negatively affect the trend. Physically, this type of printing could be accomplished by patterning in a chamber under vacuum. The vacuum cleaning time could be combined with any required positioning or alignment procedures. For such short printing times, other factors may dominate including absorption and desorption of solvent from the
46

PDMS surface and these factors would warrant further exploration to push to speed limits of liquid embossing.
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Figure 16 Time required for multiple, successive prints using an ink based on a solvent with a 5*1010 m
2
/s diffusion constant (such as 1-propanol) in a 5 Pm thick layer of fluid, with a 5 second vacuum cleaning period between each print.
47

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Figure 17 TIme required for multiple, successive prints using an ink based on a solvent with a 10.1 m
2
/s diffusion constant (such as 3-octanol) in a 5 pm thick layer of fluid, with a
15 second vacuum cleaning period between each print.
3.2.2.4. Geometry
This section explores the geometry of the stamp's raised features in simulation. Three geometries are presented that better explain the role of the diffusive flux into the stamp.
A great advantage of simulated experiments is the ability to work with physically unrealizable systems as test cases. Figure 18 shows a stamp that only has material above the raised features. In such a stamp, the regions between the features saturates very quickly and the solvent then flows around these regions.
A stamp like this would very likely not make good prints, but rather it is a
48

.
............ demonstration of how the solvent flow into the PDMS dominates liquid embossing and could be controlled by judicious placement of PDMS.
14
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Figure 18 Simulation image from a stamp of drastically different geometry. With only
PDMS above the raised features for solvent to diffuse into, the solvent flows into the raised features and around the space between them.
A practical rule of thumb for liquid embossing is that patterns with greater than 50% clearing cannot be printed. For example, a pattern of 1 pm lines with 5 pm spaces might be attempted with a stamp such as the one simulated in Figure
19. The greatest flow is still between the raised features, but this stamp would likely result in a surface emboss. A pattern could be created from an active ink, but it is unlikely that material would be fully displaced from under the raised features.
49

09
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Figure 19 Simulation of a stamp with 5 pm raised featues and 1 pm spaces. The greatest
flow Is between the features, as with other stamps, but not enough of the solvent Is
diverted around the features. Note that the flow around the fully saturated edges Is not significantly reduced as in other geometries.
Another rule of thumb for liquid embossing is that spaces wider than 5 pm cannot be cleanly cleared. Using alpha-terpineol based active inks, 5 pm is the widest raised feature that can make good prints. Figure 20 shows a simulation image of a stamp with 10 pm raised features. The corners of the features still saturate with solvent, but are spaced far enough apart that flow in the center of the feature is nearly unaffected. A pattern from this stamp would likely have a nice break in the printed material at the corner of the feature, but would still have material left in the space, or channel. This break is very important for the related embossing technique, offset liquid embossing.
50

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Figure 20 Simulation image of a stamp with 10 pm lines and spaces. With such wide raised features, saturation at the corners does not affect the flow as strongly as thinner features.
Optical images, such as Figure 21, indicated that 5 pm wide features printed by liquid embossing were devoid of material. This finding was corroborated by electrical tests where printed resistors showed no conductivity across the cleared channels. Unwanted printed material under the raised features, affectionately named the "crud" in the channels was thought to only be a problem for features wider than 5 pm, until a scanning electron microscopy (SEM) study was conducted.
51

Figure 21 Optical image of test patterns liquid embossed in a nanoparticle colloid of silver.
As It is a transmission mode image, the bright regions are where the silver ink has been cleared by the stamp.
SEM images, such as Figure 22 indicated that there was always crud in the channels, expect for a small region right around the outside edges of the stamp's features. This unwanted layer was often thin enough to see through, as in the optical images, and likely also too thin to be conductive. Via connections between multiple layers were possible either due to the edge break, or because the crud layer was thin and porous enough. While the crud did not really affect electrical devices, it can be a major problem for MEMS devices because a sacrificial layer with only small edge breaks or a thin porous region would lift-off all the layers printed on top of it.
52

.. .
... ...................
Figure 22 SEM image of a nanoparticle colloid of gold printed by liquid embossing. The desired pattern is the series of bars, which are surrounded by a small black, break in the material and a thin layer of unwanted material in the channels.
The problem of unwanted printed material in the channels was overcome
by the new technique offset liquid embossing (OLE). Reflection on other printing techniques,
60 ,
64 particularly offset printing
65 ' 66 and careful design led to a technique where liquid embossed active inks could be transferred from substrates of low wettability to substrates of high wettability.
Figure 23 shows a cross-sectional schematic of the offset liquid embossing process. A PDMS stamp with relief approximately 1 pm is used to pattern a thin film of liquid on an offset substrate by liquid embossing. The offset substrate is a layer of PDMS approximately 50 um thick, typically on glass or flexible plastic.
The offset substrate's wettability is tuned by exposure to argon or oxygen plasma
6
Ross, J., C. Romano, T. Ross and Roundtable Press. (1990). The complete printmaker: techniques, traditions, innovations. New York, Free Press; Collier Macmillan Publishers.
61
Sturge, J. M., V. Walworth, A. Shepp and C. B. Neblette (1989). Imaging rocesses and materials. New York, Van Nostrand Reinhold.
62Newman, T. R. (1977). Innovative rintmaking the making of two- and three-dimensional Drints and multiles. New York, Crown Publishers.
6 Gross, A. (1973). Etching, engraving, and intaglio grinting. London, New York,, Oxford
University Press.
6 Jaffe, E. (1954). The science of physics in lithography. New York,, Lithographic Technical
Foundation.
r Latimer, H. C. (1980). Production lanning and reoro mechanicals for offset rintin: the first updated user's manual. including phototpsettina [sici and new press production planning. New
York, McGraw-Hill.
6 Peterdi, G. (1980). Printmaking : methods old and new. New York, Macmillan;Collier Macmillan.
53

to between that of the stamp and the final substrate. The final substrate is brought into contact with the patterned film and 50 300 kPa of pressure is applied for 5 - 10 seconds. The final and offset substrates are separated, transferring the desired material to the final substrate. Undesired material trapped under the raised features of the stamp remains on the offset substrate. The patterned material on the final substrate is then cured or sintered as desired. If another final substrate is brought into contact with remaining material on the offset substrate it can also be transferred off, leaving the offset substrate clean. The stamp is reusable, as is the offset substrate due to the hydrophobic recovery of plasma treated PDMS. Once cured, all layers pass a tape adhesion test.
The process relies on careful selection of the surface properties of the stamp, offset substrate, and final substrate. Plasma treatment of PDMS results in a more hydrophilic surface.
7 ' 68 Current research indicates that plasma treatment causes the formation of a SiOx layer several nanometers thick on the surface of the PDMS, and that this layer controls the surface properties.
6 9 Both the stamp and the offset substrate are PDMS, but the wettability of the offset substrate is tuned by argon or oxygen plasma (discussed in section 4.1.3) to between that of the stamp and the final substrate, which can be a silicon wafer, glass, plastic, or the like. The advancing contact angles of a nanoparticle colloid of gold and spin-
67
Morra, M., E. Occhiello, R. Marola, F. Garbassi, P. Humphrey and D. Johnson (1990). "On the
Aging of Oxygen Plasma-Treated Polydimethylsiloxane Surfaces." Journal of Colloid and Interface
Science 137(1): 11-24.
* Owen, M. J. and P. J. Smith (1994). "Plasma Treatment of Polydimethylsiloxane." Journal of
Adhesion Science and Technology 8(10): 1063-1075.
69
Eon, D., L. de Poucques, M. C. Peignon, C. Cardinaud, G. Turban, A. Tserepi, G. Cordoyiannis,
E. S. Valamontes, I. Raptis and E. Gogolides (2002). "Surface modification of Si-containing polymers during etching for bilayer lithography." Microelectronic Engineering 61-2: 901-906.
54

on-glass on several substrates measured by the captive drop technique 70 are shown in Table 2.
Substrate Contact angle (degrees) nanoparticle gold
Untreated PDMS 42
Plasma treated PDMS 7.5 (oxygen plasma)
Silicon wafer 0 (complete spreading)
Contact angle (degrees)
SOG
42
9.1 (argon plasma)
0 (complete spreading)
Table 2 Advancing contact angles of a nanoparticle colloid of gold (20% by mass in alphaterpineol) and SOG (50% by mass in alpha-terpineol) on untreated PDMS, oxygen and argon plasma treated PDMS, and silicon wafer. Measurements were made using the captive drop technique and all values are the average of measurements on both sides of three drops.
70 Ulman, A. (1991). An introduction to ultrathin organic films
: assembly. Boston, Academic Press.
from Langmuir-Blodgett to self-
55

Fabrication of stamp and creation of thin liquid film stamp thin liquid film offset substrate
Pattern thin film on offset substrate by
Liquid Embossing offset substrate
Bring final substrate Into contact with patterned film final substrate offset substrate
Transfer desired material to final substrate and separate final substrate offset substrate
Figure 23 Cross-sectional schematic of Offset Liquid Embossing.
The SiOx layer acts both to increase the wettability of the offset substrate compared to the stamp and to reduce the rate of solvent absorption by the PDMS.
A several millimeter thick slab of PDMS will swell visibly when a drop of a nonpolar solvent is placed on it. A plasma treated PDMS slab will swell a similar amount, but the solvent is absorbed more slowly than on the untreated slab.
56

Since solvent diffusion is an important factor in the liquid embossing step of OLE, preventing the offset substrate from drying out the thin liquid film too quickly is important.
After liquid embossing, the patterned film still has enough solvent so that it wets the final substrate, but does not re-flow across the patterned channels. In practice this is achieved through careful control of the wet film thickness by spincoating, the amount of time the film is allowed to be in contact with the stamp, and
by immediate transfer of the patterned film to the final substrate after removing the stamp. Inks containing more volatile solvents and higher mass percentages of solids yield sharper patterned features.
Figure 24 shows various structures printed from a nanoparticle colloid of gold 7 1 by OLE. The channels in OLE prints are totally clear, even for relatively wide geometries. Investigation by SEM reveals that the patterned channels are devoid of printed material, and that the edge roughness of patterned lines is extremely good. Since the patterned thin film is still liquid-like during the transfer step, multiple layers of material can be printed directly on top of each other without planarization. Cohesion in the printed film is stronger than adhesion to the offset plate. After sintering, the average conductivity of 16 resistors printed in gold by OLE on polyimide plastic was found to be 9 x 106 / ohm m with a standard deviation of 4 x 106 / ohm m, 19% of bulk gold's conductivity. The length, width,
71
Hayashi, C. (1987). "Ultrafine Particles." Journal of Vacuum Science & Technology a-Vacuum
Surfaces and Films 5(4): 1375-1384.
57

and thickness of the resistors varied from 1 - 9 mm, 5 - 10 pm, and 50
200 nm respectively and were measured optically and by atomic force microscope (AFM).
10 um
Figure 24 Images of gold printed from a nanoparticle colloid by OLE. a) Optical image of
40 gm wide channels printed on a glass slide. b) SEM image of the edge of a gold feature printed on a silicon wafer by OLE. Note that the sintered nanoparticles are just visible. c)
SEM image of two layers of gold printed on a silicon wafer without planarization. Enough solvent was maintained in the second layer that during the transfer step it conformally coated the first, fully sintered layer. d) Gold printed on flexible polyimide plastic.
Figure 25 shows structures printed in SOG
7 2
by OLE. The SOG contains volatile solvents and is intended to be dry after spin-coating and form a conformal
72
500FX and 20B from Filmtronics, Butler PA.
58

layer. To make the material printable by OLE, it was mixed 50% - 75% by mass with alpha-terpineol. The higher solid content and more volatile solvents, compared to the gold ink, resulted in structures with sharper corners and the ability to bridge gaps. The cured layers were measured by AFM to be 300 nm thick. After curing the first layer, the next layer of SOG was liquid embossed on the offset substrate, optically aligned looking through the clear PDMS offset substrate, and then transferred. Although OLE's multilayer registration has not yet been quantified, it is expected to be similar to the registration of comparable techniques, particularly those involving rigid stamps.
73 As with the gold structures, printing multiple layers of SOG required no planarization steps and the bridge-like structures were made without etching.
7
Zhang, W. and S. Y. Chou (2001). "Multilevel nanoimprint lithography with submicron alignment over 4 in. Si wafers." Applied Physics Letters 79(6): 845-847.
59

Figure 25 SEM images of structures printed in SOG by OLE. a) Single layer, b) two layers, and c) three layers forming a bridge-like structure all on Si wafer. d) Gold lines printed conformally over the darker SOG lines on a gold substrate. The gold ink had a higher solid content and the SOG ink a lower solid content than the other images yielding sharper features in the gold and smoother SOG features. The geometry of the SOG pattern is 2 pm lines with 8 pm spaces. The structures were fabricated under ambient conditions outside of a clean room and some dust is visible.
It is clear from Section 3.2.2.4 that during the liquid embossing step of
OLE, the fluid is not totally flowing around the wide features. Rather, the ink under the features, if not cleared, is preferentially dried by the stamp. Unlike liquid embossing, stamping time is a critical factor of OLE. If a stamp is left in contact with the liquid film for too long, there is not enough solvent left for the transfer step. Large raised features dry out regions of ink rather than displacing them, and the true breaks at the edges allow only the desired material to be
60

transferred to the final substrate. This preferential drying also allows OLE to create features with greater than 50% clearing, such as the gold lines shown in
Figure 26.
Figure 26 Optical image of a nanoparticle colloid of gold printed on glass by OLE. The geometry is I pm lines with 9 pm spaces.
In addition to OLE, two other PDMS based techniques to print active inks were investigated: micro intaglio and micro letterpress printing.
61

3.4.1. Micro intaglio
IMPRFSSION
PAPER
DOCTOR
BLADE
CED
INK
Fig. 1-9. Gravure prints from sunken images on the printing surface of a copper cylinder. The process is also referred to as intaglio.
(National Association of Printing Ink Manufacturers)
Figure 27 Schematic of gravure or intaglio printing.
74
Figure 27 shows a schematic of intaglio printing, where depressions in a stamp or plate are filled with ink that is then transferred to the substrate. An important part of the process is the doctor blade that wipes ink off of the high parts of the plate. Micro intaglio works in essentially the same way, but diffusion into the stamp acts as the doctor blade. Ink is dispensed, by spin-coating or similar method, onto an untreated PDMS stamp. The stamp, with ink layer, is placed under vacuum for several minutes. It is then placed in intimate contact with the substrate. Patterns like the ones shown in Figure 28 can be produced by this method. While the channels are clear and much greater than 50% clearing is possible, these results were difficult to duplicate. Spin coating an active ink onto a PDMS stamp is very sensitive to variations. Slight differences in the ink's solvent concentration, the spin conditions, the amount of ink, or even the method
74 Hird, K. F. (2000). Offset lithographic technology. Tinley Park, Ill., Goodheart-Willcox Co.
62

of ink deposition onto the stamp all have large effects on the liquid film thickness and the ability of this technique to create a good pattern. At the time of this investigation OLE showed more promise, so it received greater focus.
Ix
Figure 28 SEM images of a nanoparticle colloid of gold printed by micro intaglio on a silicon wafer.
3.4.2. Micro letterpress printing
Figure 29 shows a schematic of letterpress printing. Raised features in a stamp or plate, which can be a hard material such as etched copper, or a compliant material such as rubber, are inked and pressed into a substrate. Micro contact printing (discussed in section 2.2.2) is a form of letterpress printing that uses a very specific ink. An attempt was made to adapt letterpress printing to
63

active inks, without success, but is included here for completeness. PDMS stamps were treated with argon and oxygen plasma to form an SiOx layer on their surface. The stamps were then inked by spin coating and contact with a thin film of ink. The SiOx layer prevented the solvent in the ink from rapidly diffusing into the stamp. The inked stamps were then placed in intimate contact with a substrate. The pattern transfer had very little fidelity and was not pursued beyond initial tests.
Using the body of knowledge from OLE techniques it may be possible to adapt letterpress printing to active inks. An ink pad could be formed on an offset plate with a featureless stamp. A stamp could then be inked from the semi-dry film and printed. The degree of wettability control over the various surfaces would need to be greater than that of OLE because the ink contacts four different materials. There may also be cohesion and adhesion problems. If the ink on the ink pad has greater cohesion than adhesion to the pad, it may not be possible to ink a stamp. The ink could form bridges across features in the stamps, compromising fidelity. OLE solves these potential problems during the liquid embossing step by creating the small break between the desired pattern and the crud in the channels.
64

A B C
Fig. 1-8. Letterpress prints from a raised or relief printing surface. Three types of letterpress printng presses include
A-paten. B-cylinder, and C-web. (Eastman Kodak Co.)
Figure 29 Schematic of letterpress printing. Raised features on a stamp or plate are inked and then pressed into a substrate transferring the ink.
4
The bulk of the work reported here is experimental. The theory gave a good description of the processes involved and suggested important parameters but finding the actual printing conditions required many experiments. Over 1000 separate OLE experiments were performed along with similar numbers of liquid embossing prints. This section documents the best techniques, conditions, and recipes. The printing equipment and techniques are described first, followed by a discussion of all-printed electrostatic devices.
A key finding was that the liquid embossing processes are repeatable if all the important variables are held constant. While this is perhaps plainly obvious, identifying those variables was a significant, if not the most significant, part of the experimental exploration. Important variables, which are each discussed in turn, include liquid layer thickness, which is controlled by spin coating parameters and liquid deposition technique, stamping time, and number of previous stamps as a measure of stamp solvent saturation. The other parameters discussed in this
65

section can tolerate, in some cases, wide variations while still acting to make the embossing processes work.
This section documents the technique, procedures, and equipment used in each step of making a print by offset liquid embossing.
4.1.1. Stamps
Stamps are cast in PDMS from a master that has the appropriate surface topography. In addition to standard techniques for stamp formation,
7 5 stamps in this work were cast directly on masks and on masters made by focused ion beam
(FIB). Another method is to have a mask shop create a chrome on glass mask but not strip the photoresist. A typical mask used for photolithography consists of a pattern of chrome on quartz or glass. The chrome itself is patterned by etching through an etch mask that can be created by programmatic techniques, for example e-beam exposure of a polymer resist. Rather than etch the chrome and strip the polymer resist, it is possible to use the etch-mask as a master to cast
PDMS stamps. The chrome also serves a purpose acting as a good release layer for the PDMS and preventing the raised features from adhering to the glass or quartz substrate. If the chrome mask would otherwise have been used for photolithography to create masters this procedure removes that step and can save time in the design iteration process.
75
Biebuyck, H. A., N. B. Larsen, E. Delamarche and B. Michel (1997). "Lithography beyond light:
Microcontact printing with monolayer resists." Ibm Journal of Research and Development 41(1-2):
159-170.
66

.
.........
Figure 30 SEM image of a silicon wafer cut by FIB to form a master for casting PDMS stamps.
If features smaller than are practical with optical techniques are desired, masters can be created by FIB. Figure 30 shows a silicon master with trenches cut by FIB. PDMS forms a bond to silicon, but adhesion between the stamp and the silicon master can be prevented by proper surface treatment, for example by exposure to vapors of perfluoro-1, 1,2,2-tetrahydrooctyltrichlorosilane.
76
76
Jackman, R. J., D. C. Duffy, 0. Cherniavskaya and G. M. Whitesides (1999). "Using elastomeric membranes as dry resists and for dry lift-off." Lanqmuir 15(8): 2973-2984.
67

4.1.2. Offset plates
Offset plates were made by spin coating uncured PDMS onto glass slides or polyimide plastic sheets. The PDMS was mixed in a 1:10 ratio of curing agent to pre-polymer by mass. It was thoroughly mixed usually with an electric hand drill and degassed until bubble free. The uncured PDMS was dynamically dispensed onto substrates spinning at 1000-2000 RPM for 30 seconds. The offset plates were then cured in an oven at 60 *C or on a hotplate at 80 "C overnight. Offset plates were rinsed with isopropanol before vacuum treatment to remove any dust or debris.
Glass slide offset plates were used for flexible final substrates, and flexible offset plates were used for rigid final substrates. A rigid offset plate can be used with a rigid final substrate, but good contact and transfer is not consistent. Due to the hydrophobic recovery of PDMS, the offset plates can be reused. However, in this study to reduce the number of possible parameters they were not reused.
4.1.3. Plasma treatment
An Anatech SP100 barrel type plasma cleaner was used to prepare substrates, treat offset plates, and etch polyimide sacrificial material (which is discussed in section 4.2.1). Substrates of polyimide or gold were cleaned under oxygen plasma at approximately 1 Torr pressure, 25 watts RF power, for 1-10 minutes to remove any organic contaminants that might prevent good adhesion with the printed materials. If printing on both sides of a polyimide substrate was required, oxygen plasma treatment was often used to "rough up" any surface
68

exposed to the curing temperatures. Without this step, the liquid inks would not wet the polyimide.
Offset plates were treated with oxygen or argon plasma to increase wettability and decrease solvent diffusion into the PDMS. Plates used with nanoparticle inks were treated at 400 mTorr oxygen, 25 watts RF, for 1 minute.
Plates to be used with SOG inks were treated at 200 mTorr oxygen or argon, 25 watts RF, for 1 minute. 10% variations in any of these parameters did not negatively affect printing. However a patterned liquid layer would not transfer onto the final substrate if the offset plate was plasma treated too long, for example
5 minutes of plasma treatment.
4.1.4. Ink formulation
Ink formulation is very important to good prints. The ink formulation affects the liquid film thickness, the diffusion speed into the stamp, and the cured film thickness. The inks in this study were based largely on alpha-terpineol, and their formulation was the result of a great deal of trial and error. The formulations listed here were found to work the best for OLE. These formulations were choosen for their ability to be patterned by the stamp, ability not to evaporate when as a thin liquid film, and conductivity of the films after sintering.
69

Nanoparticle gold ink
12% gold by mass"
48% alpha-terpineol
40% hexane
Nanoparticle silver ink
8% silver by mass78
32% alpha-terpineol
60% hexane
Spin-on-glass ink
50-75% 500FX or 20Bm
50-25% alpha-terpineol
Table 3 OLE Ink formulation
The hexane increases the volume of the ink and decreases its viscosity, making it easier to coat larger areas while quickly evaporating before the stamping step. A small amount of the hexane diffuses into the offset plate, and with concentrations of hexane higher than 60% the surface becomes cracked, presumably from
PDMS solvent swelling. It is possible to use lower concentrations of hexane, but these are difficult to spin coat over reasonable areas without large volumes of the material. Spin-on-glasses required only mixing with alpha-terpineol to form a good ink. Only small volumes (less than 20 mL) of the inks were mixed and stored. Some of the solvents, particularly the hexane, are volatile and tend to evaporate, changing the concentration of inks left unused for several weeks.
4.1.5. Dispensing and coating
The method of dispensing the active inks and forming a thin liquid film has a strong effect on the film's thickness. Consistent results are possible if the same technique is used each time to create thin films. Standard techniques from photoresist deposition in micro-lithography were not used because of the high cost of the nanoparticle inks. The cost of the nanoparticle material in an embossing print is not very high, but only if little material is lost during the liquid
77 from 30% gold by mass Perfect Gold dispersed nanoparticle paste manufactured by Vacuum
Metallurgical Co., LTD, No.516 Yokota, Sambu, Chiba 289-1297, Japan
78 from 40% silver by mass Kovio ink. Kovio, Inc., 1145 Sonora Court, Sunnyvale, CA 94086
79 Filmtronics, Butler PA
70

deposition. Even the best spin-coating techniques offer 40% efficiencies80 and so are not suitable. It is possible to reduce the nanoparticle concentration to increase the usuable volumes of ink. However, reducing the nanoparticle concentration also reduces the cured film thickness and so is only a partial solution.
A technique involving a two step liquid dispense was developed. After plasma treatment, 1 mL of alpha-terpineol was dynamically spun-on onto a 2 inch
by 3 inch offset plate at 3000 RPM and held for 30 seconds. Without stopping the spinning substrate, 20 pL of the nanoparticle ink was dynamically dispensed onto the center of the substrate, which was held at 3000 RPM for another 30 seconds.
The SOG ink was dispensed similarly, but in a single step. A volume of 0.5 mL of the SOG ink was dynamically dispensed onto the center of the substrate spinning at 1500 RPM and held for 20 seconds. These conditions are good for approximately 25-27 *C, but must be adjusted with further changes in temperature because alpha-terpineol's viscosity has a sharp dependence on temperature.
4.1.6. Embossing
After spin-coating, the fluid film on the offset plate is immediately embossed either by hand or by machine. Good contact between the stamp and fluid film should be achieved without entraining bubbles and without excessive pressure. After sufficient time has elapsed to allow diffusion into the stamp, the
80 Han, S. J., J. Derksen and J. H. Chun (2004). "Extrusion spin coating: An efficient and deterministic photoresist coating method in microlithography." leee Transactions on
Semiconductor Manufacturing 17(1): 12-21.
Lide, D. R. (1995). Handbook of organic solvents. Boca Raton, CRC Press.
71

stamp and substrate are quickly separated in a peeling motion. For the conditions given in Section 4.1.5, 300 seconds is a sufficient amount of time. If thinner cured layers are desired, the spin-coating parameters can adjusted accordingly.
4.1.7. Alignment
Optical alignment between layers is possible by looking through the offset plate at a previously patterned final substrate. Since both substrates can be held flat and brought close together, alignment is very similar to proximity or contact photolithography. The patterning step and alignment step have been decoupled in OLE compared to liquid embossing, which leads to some advantages. Liquid embossing requires a slight bow to the stamp to prevent air entrapment. This bow makes aligning features at the edge of the stamp with previously patterned features somewhat difficult. In OLE the liquid embossing step is on a featureless offset plate and no alignment is required. After printing, the features on the offset plate and those on the final substrate are aligned before transfer. In this way rotation misalignment is easier to correct. There is also no chance an aligned stamp will slip slightly once in contact with the fluid film, as with liquid embossing.
72

. .........
4.1.8. Transfer
Figure 31 A rubber roller is used to ensure good contact between the substrate and offset substrate before applying pressure.
Once the final and offset substrates are aligned, the printed layer is transferred by application of pressure. First, good contact is ensured with a rolling motion to displace any possible trapped air as shown in Figure 31. 50 - 300 kPa of pressure is then applied to the substrates. The bladder system shown in
Figure 32 was constructed to do this with air pressure. It is possible to apply the transferring pressure by hand, but this has less uniform results. The transfer requires less than 5 seconds, and the substrates can be immediately peeled apart.
73

Figure 32 The bladder system used to applied constant air pressure between the offset and final substrates. The bladder itself is visible in the bottom frame.
4.1.9. Curing
Nanoparticle inks were cured at 300
0
C for at least 10 minutes. Longer cure times and higher temperatures increased conductivity. The silver ink was cured in a nitrogen atmosphere to prevent oxidation. SOG films were cured at
425 *C for 60 minutes under nitrogen. If printed on a polyimide plastic substrate, the SOG curing temperature was reduced to 300 0C for 60 minutes under nitrogen. Shorter SOG cure times reduced the material's etch rate in buffered oxide etch (BOE).
74

4.1.10. Etching
Sacrificial material is etched using standard techniques including wet etches, dry plasma etches, and vapor etches. Specific etch recipes are given in
Sections 4.2.1 and 4.2.2. Care must be taken when etching as the nanoparticle materials are often porous to the etchants, so the substrate may be attacked through a printed metal layer.
4.1.11. Imaging
Prints were imaged optically and with SEM. Sample preparation is especially important for SEM imaging. There must be a conductive path to ground to prevent charging and to allow high resolution images. Often printing on a conductive substrate such as a silicon wafer is not enough. The best images in this work were obtained by placing the conductive side of copper tape in contact with the print as close as possible to the area of interest and then forming a conductive path directly to a metal mounting stud. The conductive path from a printed structure to the stud was then checked with an ohmmeter. Prints on plastic were imaged under water vapor in environmental SEM mode. Ensuring a good conductive path exists is even more important for environmental images.
Although the water vapor helps to prevent charging, it also reduces resolution, so the best images are taken with the least possible water vapor.
4.1.12. Stamp cleaning
After liquid embossing the stamps are usually clean and can be immediately used for another print. If any ink was left on the stamps after printing
75

the stamps were cleaned either by tape or solvent rinse. Tape intended for use on paper has a thin layer of a very viscous liquid that acts as an adhesive. This liquid does not wet PDMS but will stick to dried out ink and dust on the stamps.
Tape is a very effective way of cleaning the stamps without subjecting them to solvents such as isopropanol and methanol.
Accumulated solvent in the stamps was removed by vacuum. Stamps were placed under 100 mTorr of vacuum for 20 minutes. A vacuum hotplate with higher pressures was also used, but found to be less effective. This time is much longer than the theoretical analysis indicates and is likely due to both modeling errors in the assumption that vacuum creates surfaces at zero concentration and excessive, unnecessary vacuum time. A systematic study of the required vacuum time was not conducted but would likely indicate that less time was required.
As a demonstration of the liquid embossing techniques, all printed electrostatic actuators were fabricated. This section describes the fabrication process and operation of the actuators.
Electrostatic actuators were chosen for a number of reasons. They require only conducting, insulating, and sacrificial materials. Their operation is relatively simple, requiring only moderate voltage (100 - 200 volts), and their output can be detected optically. There is also some historical significance to printing
76

electromechanical systems intended to modulate light, as some of the very first silicon MEMS did just that.
Electrostatic actuators of two different types were created by OLE. Their key difference was the sacrificial material. The first type of actuator relied on the polyimide substrate as the sacrificial material which was removed with a dry oxygen plasma. The second type of actuator described uses SOG as the sacrificial material, which was etched by a variety of methods.
4.2.1. Polyimide sacrificial material
Figure 33 Top down schematic of a polyimide sacrificial material actuator. The black lines represent regions where the liquid ink has been patterned and removed. The dashed line marks the cross-section shown in Figure 34.
Polyimide sacrificial material electrostatic actuators were fabricated by printing a metal layer on a polyimide substrate and then under-etching the polyimide. Oxygen plasma was used, which is capable of either an anisotropic etch or an isotropic etch depending on conditions.
83
,'8 In this work the plasma
8
Petersen, K. E. (1977). "Micromechanical Light-Modulator Array Fabricated on Silicon." Applied
Physics Letters 31(8): 521-523.
Das, N. C. (2002). "Release of multi-layer metal structure in MEMS devices by dry etching technique." Solid-State Electronics 46(4): 501-504.
77

etching conditions were 100 - 1000 mTorr pressure, 20 - 50 watts RF power, for
20 minutes to 10 hours. Higher powers result in higher substrate temperatures that change the etch conditions, and may actually damage the gold film or the substrate. A film of nanoparticle gold appears to dewet the substrate if heated above 600 'C or if exposed to high RF powers.
etch
E__V
Figure 34 Cross-sectional schematic of a polyimide sacrificial material actuator. Here the polyimide substrate is 80 pm thick. The top printed layer of metal acts as an etch mask during the oxygen plasma etch. The isotropic etch is allowed to under etch approximately
5 pm. Once freed, the metal beam can then be electrostatically attracted to the bottom, printed metal layer.
A doubly-clamped beam 140 pm long, 5 pm, and 100 nm thick was printed on polyimide by OLE as shown in Figure 36. After etching the freed metal beam was electrostatically attracted towards the bottom printed conductor. The gold is very reflective, and this deflection was detected optically. As the beam bent it would appear to go dark as it no longer reflected along the same optical path when undeflected. A field of 0.25 volts / pm (20 volts across the 80 pm thick
8
Bagolini, A., L. Pakula, T. L. M. Scholtes, H. T. M. Pham, P. J. French and P. M. Sarro (2002).
"Polyimide sacrificial layer and novel materials for post-processing surface micromachining."
Journal of Micromechanics and Microenqineering 12(4): 385-389.
78

polyimide substrate) was enough to cause optical deflection. This was in good agreement with a finite element analysis of the beam, which predicted optical deflection should be detectable at fields just under 0.5 volts / pm, shown in Figure
37.
Figure 35 SEM image of a polyimide sacrificial material actuator printed in nanoparticle gold on print 1001. This image is at an image tilt that best shows the under-etched polyimide. Wrinkles in the gold film's edges are clearly seen in the optical images where they appear as dark scalloping.
Figure 36 SEM image of a 140 pm beam printed in gold on polyimide and release by oxygen plasma etching on print 1001.
79

1.5
E
U
1 -
0
0 5
Field (V/m)
10 15 x 105
Figure 37 Finite element analysis of the center deflection of a 140 pm long, 5 pm wide, nm thick doubly-clamped beam subjected to an electrostatic force.
100
In addition to bridge-type actuators, cantilevers actuators were also made
by this method. The nanoparticle films of metal are thin and act more as ribbons than plates of material. Slight residual stress from the curing step will often cause these ribbons to roll up away from the substrate. This effect is taken advantage of to build electrostatic actuators with very large displacements, known as a zipping actuator.
85,86,87
A voltage applied between the top and bottom electrodes will tend to unroll the cantilever, "zipping" it down against the substrate. The electrostatic force between two flat plates is
F=
C V
2x 2
85
Ohnstein, T., T. Fukiura, J. Ridley and U. Bonne (1990). Micromachined silicon microvalve.
Proc. IEEE Ann. Int. Conf. Micro Electro Mechanical Syst.
86
Shikida, M., K. Sato and T. Harada (1997). "Fabrication of an S-shaped microactuator." Journal of Microelectromechanical Systems 6(1): 18-24.
87
Legtenberg, R., J. Gilbert, S. D. Senturia and M. Elwenspoek (1997). "Electrostatic curved electrode actuators." Journal of Microelectromechanical Systems 6(3): 257-265.
80

where F is the force, Eo the permittivity of free space, A is the surface area of the plates, and x is their separation. For the bridges or cantilevers discussed here, the equation becomes a differential equation along the length of the beam. This force only acts over short distances so a zipping actuator can undergo large displacements because it can continuously bring more area into the short attraction zone. Figure 39 and Figure 40 show a zipping actuator before and after being electrostatically actuated by the e-beam during the imaging process. Figure
42 and Figure 43 show optical images of the same size actuator used to reflect light as a demonstration pixel element. Not only do the etch holes in the metal films allow the oxygen plasma to under-etch, but they create a dimpled pattern on the polyimide. These dimples reduced the total contact area between the release metal and the polyimide, helping to prevent stiction.88
8 Tas, N., T. Sonnenberg, H. Jansen, R. Legtenberg and M. Elwenspoek (1996). "Stiction in surface micromachining." Journal of Micromechanics and Microengineering 6(4): 385-397.
81

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Figure 38 SEM Images of print 1017 showing the progression of the oxygen plasma etch on an array of 50 pm on a side actuators. Images were taken every 10 minutes during the etch. The top left image is 0 minutes and the bottom right 50 minutes. Once released, the actuators are attracted against the substrate by the e-beam during Imaging.
82

Figure 39 SEM image of print 1015 after the polyimide plasma etch. The open flap is a film of nanoparticle gold pinned down on the left side of the image. A series of 5 pm etch holes on 10 pm spaces allow the oxygen plasma to under etch and release the flap.
Figure 40 SEM image of print 1015 after the e-beam was used to selectively charge the flap and attract it down against the surface. The other flaps in the field were electrostatically attracted down while focusing before images could be recorded. The bright dots above the actuators are defects in the gold film.
83

Figure 41 SEM image of print 1019. Some of the 50 pm square actuators in this image are still in their neutral position, rolled away from the surface. The electron beam used during imaging electrostatically attracted the others towards the substrate.
Figure 42 Optical image of a zipping actuator on print 1016 with no voltage. This is the same size actuator shown in Figure
40, 300 pmn on a side.
Figure 43 Optical image of the same actuator shown in Figure 42 with 200 volts applied. As the rolled up gold film Is attracted against the substrate it begins to reflect light.
The output of these electrostatic actuators was measured optically by a silicon photodector
89 in the reflected light path of a microscope. Both white light and 650 nm laser light were modulated by the actuators. The sample was orientated at a 45' angle to the microscope objective so that when the actuators were electrostatically attracted to the surface they would reflect the laser beam into the microscope and photodetector. This setup was used to take the data shown in the graphed figures. Except for Figure 47, the actuators' output is shown in normalized arbitrary units because there was often unwanted reflected
89
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84

light from other parts of the substrate. In Figure 47 the measured output of the photo-detector was compared against the response when the same size laser spot was focused on an evaporated gold surface. Zipping actuators 50 pm on a side have been run for greater than 108 cycles with no degradation in the amplitude of modulated light.
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The light intensity was measured by a photo-detector and compared against a mirror.
There is a sharp non-linearity in the amplitude versus driving voltage. This is a result of the differences between the electrostatic and elastic forces. The electrostatic force has squared terms while the elastic restoring force is linear.
The crossing point where the two forces are equal is unstable and the actuator will snap past this point. This nonlinearity yields two advantages. The actuator
85

can be controlled with a static DC offset voltage and a relatively small switching voltage, simplifying any control electronics. Alternatively, an array of these zipping actuators could be addressed without transistors or other active elements in a row-column addressing scheme using their nonlinearity.
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86

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The larger zipping actuators have similar, but slower responses. Figure 48 and Figure 49 show optical images of 100 pm on a side actuators.
87

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Figure 48 Optical Image of a zipping actuator on print 1016. It is 100 pm on a side.
Figure 49 Optical image of the same actuator shown in Figure 48 with 200 volts applied.
Since the return force in these zipping actuators is from residual stress in the film they sometimes get stuck against the substrate. To overcome this limitation, floating membranes were printed and under-etched. These are very similar to the zipping actuators, but are constrained on all sides. The electrostatic forces pull them towards the substrate, and the metal film's elasticity acts as the
88

restoring force. By etching far enough into the polyimide, the membrane can never touch it and is prevented from getting stuck. However, this results in significantly reduced displacement. Some actuators of this type are shown in
Figure 50.
Figure 50 SEM Image of a membrane type actuator on print 1015. The etch holes allow the oxygen plasma to under-etch and free the membranes that are attached on all four corners.
During Imaging the electron-beam has charged the membranes and attacted them towards the substrate.
89

4.2.2. Spin-on-glass sacrificial material
Print first gold layer by offset liquid embossing
Print SOG layer by offset liquid embossing
Print second gold layer by liquid embossing
Etch out SOG
Figure 51 Cross-sectional schematic of the fabrication process for an SOG sacrificial material based electrostatic actuator.
Figure 51 shows the process for building an electrostatic actuator using
SOG as the sacrificial material. The bottom conductor is printed by OLE on a substrate impervious to the SOG etch, such as polyimide. The SOG layer is then aligned and printed by OLE, followed by the final top conductor, which is printed
90

by liquid embossing. The SOG is finally etched out forming a bridge. SEM images of the metal bridges are shown in Figure 52 and Figure 53.
Figure 52 SEM image of print 806 after the SOG etch. The top metal layer was patterned by liquid embossing.
91

.
: T_ .
Figure 53 SEM Image of print 806 after the SOG etch. The top metal layer was patterned by liquid embossing.
Figure 54 shows an SEM image of another set of bridges where lift-off was used to allow further control over the placement of the final metal layer. Since the final layer is printed by liquid embossing, the nanoparticle ink is printed over the entire substrate. This poses a problem for wiring up and probing the devices as the top metal layer makes unwanted connections to the bottom layer. By patterning on a small area of the SOG layer, most of the top metal layer is printed only on SOG. Without a physical connection through the SOG these regions liftoff during a wet etch allowing the effective removal of materials over a wider area.
92

Figure 54 SEM image of print 869 after SOG etch and top metal layer lift-off. The metal bars running across the field of view were printed everywhere, but only anchored where the
SOG was patterned.
Actuators of this size are typically measured by setups including lasers,90 electronic methods,
91 and even more involved techniques.
92 Bridges spanning distances wider than 15 pm proved extremely difficult to fabricate due to conformal printing problems, surface tension, and the porosity of the nanoparticle metal films. Bridges of this size have calculated natural frequencies ranging from
100 kHz to 10 MHz depending on the thickness, and deflections of only a few nanometers for reasonable voltages. In practice, more than 30 volts between the top and bottom conductor leads to a spark that destroys the device. The following sections describe these problems, some solutions, and trade-offs.
90 Gaspar, J., V. Chu and J. P. Conde (2003). "Electrostatic actuation of thin-film microelectromechanical structures." Journal of Applied Physics 93(12): 10018-10029.
91
Gritz, M. A., M. Metzler, J. Moser, D. Spencer and G. D. Boreman (2003). "Fabrication of air bridges using electron beam lithography." Journal of Vacuum Science & Technology B 21(1): 332-
334.
92 Nakaoka, T., T. Kakitsuka, T. Saito and Y. Arakawa (2004). "Manipulation of electronic states in single quantum dots by micromachined air-bridge." Applied Physics Letters 84(8): 1392-1394.
93

-----------------
4.2.2.1. Conformal printing
Figure 55 SEM Image of print 570, a single layer of SOG on nitride.
OLE yields SOG patterns with excellent sharp sidewalls. While great for
SEM images, such as Figure 55, printing a continuous layer over a sharp corner is very difficult. The printed ink becomes very thin at these corners and breaks, as shown in Figure 56.
4A
94

Figure 56 SEM Image of print 721. Cracks in the OLE printed top metal layer are visible at the sharp corners of the darker SOG bars.
Printed lines that did not break at the sharp SOG corners were made by liquid embossing. By dispensing ink everywhere and then patterning, the continuous patterns shown in Figure 57 were possible. The trade-off was that the bridges were connected in the areas between SOG. With the SOG pattern approximately 300 nm thick, the raised features of the stamp were unable to clear the liquid ink between them.
95

Figure 57 SEM Image of print 730 after etching. The top metal layer was patterned by liquid embossing, which results In thicker sidewalls, but leaves material between the SOG patterns.
4.2.2.2. Surface Tension
Surface tension during or after the SOG etch was a major problem. As liquid droplets dried or pulled out from under the bridges, the bridges would collapse, like those shown in Figure 58. It was possible to observe this collapse
by letting a sample dry under an optical microscope after a wet etch. Switching the devices to a lower surface tension liquid before drying had the best results.
Sublimation drying, reactive ion etching, and HF vapor etching are also described.
96

Figure 58 Sample 692. All layers are printed by OLE on a polylmide substrate. This is one of the few devices where the top OLE printed layer of metal was thick enough to etch.
However, surface tension collapsed the bridges.
Methanol has a lower surface tension than water and can be used to dry the devices. After a wet etch step, the etchant can de diluted with water to stop the reaction, and the samples transferred, while still under liquid, into methanol.
The methanol is stirred to ensure sufficient mixing on the substrate surface. The devices are then air dried. This is the simplest way to avoid surface tension defects, and was found to be the most reliable.
Sublimation drying is a common way to release delicate structures from a liquid.
9 Here, prints were transferred from the diluted etching solution to both water and dimethylsulfoxide (DMSO). The liquid, with prints, was frozen and then sublimed under vacuum at room temperature. Even though the sublimation took several hours, the water and DMSO did not melt while under vacuum. Once the water and DMSO were fully removed, the structures were investigated. The
9
Kim, C. J., J. Y. Kim and B. Sridharan (1998). "Comparative evaluation of drying techniques for surface micromachining." Sensors and Actuators a-Physical 64(1): 17-26.
97

bridges in both liquids did not collapse, but rather were destroyed or otherwise broken, indicating that either the freezing or sublimation processes were not gentle enough. Neither of these techniques were investigated beyond preliminary experiments. Supercritical drying with C02 is reported to yield much better results than sublimation, but unfortunately it was not possible to test it in this work.
Figure 59 SEM image of print 261 after RIE etching. The conditions required to etch the
SOG film attacked the underlying silicon layer at a faster rate. The high regions of this print are where the SOG was not patterned.
The SOG layer was also etched by reactive ion etching (RIE). RIE is a dry etching process and is free of any surface tension. The 500 FX SOG has a high polymer content and is essentially impervious to HF unless the polymer chains are broken by exposure to e-beam, in much the same way as PMMA e-beam resist works. It was thought that conditions could be found that would allow for an isotropic etch similar to the isotropic polyimide etch by oxygen plasma. A
Plasmatherm PECVD-RIE 740/730 was used to etch the films. However, the
SOG was so tenacious that it only etched under very aggressive conditions that attacked the underlying layers at even faster rates. Figure 59 shows a silicon
98

wafer where the SOG pattern has been etched away. Instead of a sacrificial material, the SOG acted almost as a resist or etch mask. Other SOG inks were not etched by RIE because it was deemed very unlikely that isotropic conditions would be found.
Figure 60 Image container's lid and of the HF vapor etching setup. A sample is taped to the inside of the then positioned beneath the light, which controls the temperature. The light can be moved up and down as needed.
It is also possible to etch SOG under an atmosphere of hydrofluoric acid vapor.94' 95 Complicated setups are not required and this work built upon a technique where the temperature of the substrate was used to control the HF vapor reaction.
96 The substrate was taped to the inside of the cap of a small
9 Lee, Y. I., K. H. Park, J. Lee, C. S. Lee, H. J. Yoo, C. J. Kim and Y. S. Yoon (1997). "Dry release for surface micromachining with HF vapor-phase etching." Journal of
Microelectromechanical Systems 6(3): 226-233.
9 Williams, K. R. and R. S. Muller (1996). "Etch rates for micromachining processing." Journal of
Microelectromechanical Systems 5(4): 256-269.
0 Fukuta, Y., H. Fujita and H. Toshiyoshi (2003). "Vapor hydrofluoric acid sacrificial release technique for micro electro mechanical systems using labware." Japanese Journal of Applied
Physics Part 1-Regular Papers Short Notes & Review Papers 42(6A): 3690-3694.
99

container filled with HF acid (48% with water). A light bulb was positioned over the container and used to heat the back side of the sample through a small hole cut in the container's lid. The setup is shown in Figure 60. While the HF vapor is able to etch out the SOG and create some small span bridges, shown in Figure
61, wider bridges still collapse. There is no evidence of water on the samples, as discussed in the literature, and the substrate itself is quite hot so even the small amount of water generated by the etching reaction should remain in gas phase.
Instead it is thought that the etch goes through the nanoparticle metal, and the etch process itself is responsible for collapsing the bridges.
Figure 61 SEM image of sample 917, after the SOG layer was etched by HF vapor. The white lumps are products of the reaction and are a result of the specific conditions in this sample (others vapor etched at different conditions had no residual material).
4.2.2.3. Porosity of the nanoparticle metals
A drawback of working with nanoparticle metals is that the printed films are porous. This was determined experimentally by printing layers of a nanoparticle colloid of gold on a glass substrate and then etching the glass in 7:1 BOE. If fewer than 6 layers of metal were printed the BOE etch would immediately
100

delaminate and lift-off the metal. Control samples of gold evaporated on glass only showed lift-off at the edges. Considering that the particles are 5-10 nm in diameter and are only sintered, not melted, this porosity is not surprising. It is believed that this porosity is to blame for the collapse of under-etched structures in surface tension free etches.
Two techniques were tried to counteract the porosity, both without success.
Gold patterns on a thermally grown silicon dioxide substrate were immersed in a
10 mMolar solution of C,
0
H
4
F
17
S with ethanol. This fluorinated alkane thiol forms a self assembling monolayer on gold surfaces and is hydrophobic so it should repel the etching solution. This technique did not work either because the pores were still big enough to allow the etchant to pass through, or because the nanoparticle gold surface was not amenable to the monolayer. The gold patterns were also thermally reflowed at 550 'C for an hour. Films exposed to this temperature showed a slowed rate of delamination, but not an arrested rate.
Additionally, this temperature is unsuitable for plastic substrates. Higher temperatures caused the gold films to dewet and agglomerate into islands, destroying the pattern.
This section summaries the major result of the work, draws conclusions, and then gives recommendations for future work.
The major results of this work include:
101

1. Development of a finite element analysis of liquid embossing.
The analysis was first compared against the experimental system and the predicted time required for solvent to diffuse into a stamp was found to be in excellent agreement the measured time. The simulated velocity field in a thin layer of solvent in contact with a stamp lead to insight and a better understanding of liquid embossing. The simulation was used to predict the time required to stamp was a range of different solvents and stamp geometries.
2. Design of offset liquid embossing.
Offset liquid embossing was designed to overcome some of the limitations of liquid embossing and allows the printing of a wider range of geometries.
3. Electrostatic actuators
Electrostatic actuators were printed using the liquid embossing techniques. The nanoparticles films were found to be porous to HF based etches. Actuators were printed using polyimide as the mechanical handle, sacrificial material, and dielectric layer. Once released the nanoparticle films bent away from the surface due to residual stress. This deformation was exploited to form zipping electrostatic actuators that modulated light.
Liquid embossing relies on solvent diffusion in a PDMS stamp to create fine features in thin liquid films of active inks. By printing on an offset plate and transferring it is possible to obtain greater control over material placement. The
102

liquid embossing techniques are ideally suited to create patterns in single layers of nanoparticle materials. Multiple layer structures are possible and suggest some interesting challenges concerning porosity of the nanoparticle films and conformal coverage when printing over steps. More work is required to determine if liquid embossing is appropriate for true electronics and MEMS manufacturing.
However, single layer, high-resolution nanoparticle prints could have immediate application in making flexible connectors and RFID antennae. The ability to print and release thin metal films could have further application in bio-medical applications and electrodes for fuel cells. In these applications the porosity of the material may actually be advantageous to increase surface area or to allow diffusion through a metal film.
This section gives a few ideas for future work specific to printing active inks.
5.3.1. Rewritable stamp
Section 3.2.1 makes it clear that the primary function of the raised features in the stamp is to saturate with solvent and act as impediments to diffusive flow into the stamp. A rewritable stamp could be created if selective areas of a stamp could be changed from allowing diffusive flow to preventing it. The formation of
SiOx groups on the surface of the offset plates appear to do this. Creating patterns of SiOx with plasma would require a mask, but for fine features this might
103

be just as costly as making a master. SiOx groups can also be created on PDMS
by UV ozone treatment.
97
,9
8 In an ozone atmosphere it may be possible to selectively change the surface properties of PDMS by scanning or projecting UV light. A laser scanning system or digital micro-mirror array could be used to programmably create a stamp. This surface modification also increases the wettability of PDMS, which could have a negative effect on such a scheme.
The complexity of microfluidic devices is rapidly increasing.
99 It may be possible to create a stamp with individually addressable regions that can come into contact with a thin liquid film. Such a stamp could be brought close to a film of active ink, but not in contact. Internal microfluidic channels could then be swelled to bring small areas of the stamp into contact, acting very much as raised features.
5.3.2. Alignment
Alignment is a critical issue for any high resolution multi-layer process. In addition to the simple optical alignment used in this work, the embossing techniques may be amenable to physical alignment and an offset self alignment technique. Since liquid embossing involves physical contact, it may be possible for a previous pattern to help lock-in the next pattern. Figure 62 and Figure 63
9
Park, H. B., D. W. Han and Y. M. Lee (2003). "Effect of a UV/ozone treatment on siloxanecontaining copolyimides: Surface modification and gas transport characteristics." Chemistry of
Materials 15(12): 2346-2353.
SEfimenko, K., W. E. Wallace and J. Genzer (2002). "Surface modification of Sylgard-184 poly(dimethyl siloxane) networks by ultraviolet and ultraviolet/ozone treatment." Journal of Colloid and Interface Science 254(2): 306-315.
W Groisman, A., M. Enzelberger and S. R. Quake (2003). "Microfluidic memory and control devices." Science 300(5621): 955-958.
104

show a pattern that could allow this lock-in. Once the first layer is cured, a thin film of ink is applied on top of that layer. The second layer stamp is grossly aligned to the first layer. Once in contact with the thin liquid film the raised features slide into the grooves of the first layer. Elastic averaging could then be used to obtain a high degree of overlay alignment on features around the repeated structures, such as the x and cross shown.
Figure 62 Schematic of the first layer of the Figure 63 Schematic of the second layer of a physically aligned pattern. Black represents physically aligned pattern.
the raised features of the stamp, where material would be removed.
Using OLE it is possible to print self aligned structures. The surface topography of a previously patterned layer can be used to transfer material off of an offset plate. Typically the liquid embossing step of OLE creates high and low regions where only the high material is transferred. Instead, a previous pattern can be used, transferring material only onto the high regions of that pattern.
Figure 64 shown an SEM image of two layers of SOG printed in this fashion.
105

Figure 64 SEM image of print 680. A first layer of SOG was printed and cured. A layer of
SOG on an offset plate that was in contact with a featureless stamp was then brought into contact with the first layer. SOG transferred only directly on top of the previous features.
The second layer transfer was not especially uniform, but showed promise as a self aligning technique.
5.3.3. Active Inks
The inks used in this work were not specifically designed for OLE. Two notable problems were encountered that were specifically related to the ink formulation. The SOG ink tended to dry out very quickly and if left more than a few minutes in air would be too dry to transfer. This made alignment of a SOG layer somewhat difficult because it needed to be done very rapidly. Higher percentages of alpha-terpineol made for an ink that did not pattern well on the offset plates by liquid embossing. The nanoparticles in some of the active inks prevented printing very high resolution features. Figure 65 shows a 50 nm dot in a field of nanoparticles. Very small raised features in a PDMS stamp cannot be the normal height, so the liquid film must be correspondingly thin. The thin films require dilute solutions of nanoparticles and the printed features become more like clearings in a field of particles then true features. Inks design specifically for
106

OLE could address both of these problems and extend the range of printed features.
Figure 65 An SEM image of a 50 nm dot printed in a nanoparticle colloid of gold by liquid embossing. The size of the nanoparticies ultimately limited the highest resolution.
5.3.4. Printing resists by OLE
It is possible to pattern resists by OLE. If OLE were used to pattern an etch resist, many of the advantages of printed active materials would be lost; however it still may be of some use. A demonstration of this ability was done by
OLE printing SOG on an evaporated gold surface. The gold was then etched'00 to define patterns. A timed etch was required because the gold was easily underetched beneath the SOG patterns. OLE prints with a PMMA based ink were attempted, but it was difficult to obtain a liquid film of the proper thickness that would still transfer after liquid embossing.
100
Etched by Transene TFA Gold Etchant. Transene Company, Inc. Danvers, MA 01923
107

5.3.5. Modeling
The finite element analysis of liquid embossing does a good job of describing the process and making predictions, but could be improved. In particular there are three parts of the model that need refinement: fluid flow as the stamp touches down, modeling the motion of the nanoparticles in the solvent carrier, and a mechanical model of the stamp.
Due to the limitation of the modeling software the liquid embossing model used a quasi-static approximation. The flow at the liquid substrate interface was modeled as a normal boundary flow to approximate the motion of the substrate towards the stamp. The approximation makes for a good snap-shot of the stamp / solvent interaction but prevents modeling the flow as the raised features in the stamp touch down against the substrate. Such a model would require modeling the motion of a fixed volume of liquid and would require dynamic geometry to capture the stamp's motion. A model such as this would allow for quantitative predictions regarding the maximum and minimum feature sizes for given fluid film layers.
The model described here treats the liquid film as homogenous. In reality the motion of the nanoparticles within the liquid is what is truly of interest. Also, as the solvent diffuses into the stamp, the active inks become more viscous until they will no longer reflow into the cleared channels. This is another effect the current model misses.
Including a mechanical model of the stamp may also help in generating better quantitative information about the maximum and minimum printable
108

features. The current model assumes the stamp is rigid. A better model would capture the deformations of the stamp during the printing process.
5.3.6. Other applications
The porosity of the nanoparticles films was considered to be a drawback in this work. However, high surface-area, porous metal films from liquid precursors may find applications in other fields including catalysts' 1 0 2 and fuel cells 1 03
-'
0 4
101
102
Cho, A. Connecting the dots to custom catalysts. Science 299, 1684-1685 (2003).
Aiken, J. D. & Finke, R. G. A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis. Journal of Molecular Catalysis a-Chemical 145, 1-
44 (1999).
103 Wagner, N., Schulze, M. & Gulzow, E. Long term investigations of silver cathodes for alkaline fuel cells. Journal of Power Sources 127, 264-272 (2004).
'0 Uemiya, S. Brief review of steam reforming using a metal membrane reactor. Topics in
Catalysis 29, 79-84 (2004).
109

This is the Matlab and FEMIab code and measured data used to determine the diffusion constant of alpha-terpineol in PDMS.
% FEMLAB Model M-file
% Generated by FEMLAB 3.0 LCS 2 (FEMLAB 3.0.0.145,
2003/12/10 19:04:24 $)
$Date: flclear fem
% Femlab version clear vrsn vrsn.name = 'FEM LAB 3.0'; vrsn.ext =
'
LCS 2'; vrsn.major = 0; vrsn.build = 145 vrsn.rcs = '$Nam e: $'; vrsn.date ='$Da fem.version = vr te: 2003/ sn;
12/10 19:04:24
$';
% Geometry gl=rect2('0.017','0.00374
rot', '0') ; clear s s.objs={gl}; s .
name={ 'Ri' }; fem.draw=struct('s',s); fem.geom=geomcsg(fem);
% Constants fem.const = {'DO',1.2E-11 ,'satrho',131.0};
% Initialize mesh fem.mesh=meshinit(fem);
% Application mode 1 clear appl appl.mode.class = 'FlDiffusion'; appl.mode.type = 'axi'; appl.assignsuffix = '_di'; clear bnd bnd.cO = {0,'satrho',0}; bnd.type = {'NO','C','ax'}; bnd.ind = [3,1,2,1]; appl.bnd = bnd;
110

clear equ equ.D = 'DO'; equ.ind = [1]; appl.equ = equ; fem.appl{1} = appl; fem.sdim = {'r','z'};
% Multiphysics fem=multiphysics (fem);
% Extend mesh fem.xmesh=meshextend(fem);
% Solve problem fem.sol=femtime(fem,'solcomp', {'c'
'tlist', [0:100*60:
},'outcomp',{'c'},
...
10000*60],'tout','tlist');
5
6
7
2
3
4 timeData =
0
1
8
9
10
32
65
107
136
186
266
332
370
428
2946
4310
4700
5600
7370
1;
%9050 massData =
0
0.0055
0.0066
0.0079
111

0.0101
0 011
0.0129
0.0131
0.0139
0.0153
0.0269
0 . 0352
0 . 0428
0 0483
0.0575
0 0675
0.076
0.0786
0 . 0844
0 .2017
0 .2364
0 .2461
0 .2606
0.2928]; clf plot (timeData, massData./1000, '*k'); hold on massCalc = postint (fem, 'c*2*pi*r',
[0:100*60:8000*60]); timeCalc = [0 :100*60 : 8000*60]; plot (timeCalc/60, massCalc, 'k') grid on ylabel xlabel
('Mass of diffused alpha-terpineol in PDMS (kg)');
('Time (minutes)')
112

This section gives the Matlab and FEMIlab code used to generate the plots of Section 3.2.2. The code for each plot is not given because there are only minor variations between them.
% FEMLAB Model M-file
% Generated by FEMLAB 3.0 (FEMLAB 3.0.0.181, $Date:
2004/01/29 19:04:14 $)
%set up problem stamps=10; %number of stamps to simulate wait=0; %seconds to wait between each stamp to let the solvent move around maxtime=1000; % maximum number of seconds to wait for solvent to transfer into stamp timestep = 1; % number of seconds in each time step solventmass = 50e-6 * 5e-6 * 933; % the mass per unit length into the page of solvent in the thin film time-data = 0; mass afterwait=0; flclear fem
% Femlab version clear vrsn vrsn.name = 'FEMLAB 3.0'; vrsn.ext =
''; vrsn.major = 0; vrsn.build = 181; vrsn.rcs = '$Name: $'; vrsn.date = '$Date: 2004/01/29 19:04:14 $'; fem.version = vrsn;
% Geometry
6', '0.0005','base','corner','pos',{'0','0'},'rot','0');
% Constants fem.const = {'DO','1.2e-11','sat_rho','130'};
% Geometry clear s s.objs={gl}; s.name={'R1'1; s.tags={'g1}; fem.draw=struct('s',s);
113

fem.geom=geomcsg(fem);
% Initialize mesh fem.mesh=meshinit(fem);
% Refine mesh
%fem.mesh=meshrefine(fem,'rmethod','regular');
% Refine mesh
%fem.mesh=meshrefine(fem,'rmethod','regular');
%loop to simulate multiple stampings for i = 1:stamps
% stamp
% Application mode 1 clear appl appl.mode.class = 'FlDiffusion'; appl.assignsuffix = '_di'; clear bnd bnd.cO = {O,'satrho'}; % stamp is in contact with solvent bnd.type = {'NO','C'}; bnd.ind= [1,2,1,1]; appl.bnd = bnd; clear equ equ.D = 'DO'; equ.ind = [1]; appl.equ = equ; fem.appl{11 = appl; fem.outform = 'general';
% Multiphysics fem=multiphysics(fem);
% Extend mesh fem.xmesh=meshextend(fem); if i > 1
% Mapping stored solution to extended mesh init = asseminit(fem,'init',feml.sol,'solnum',1); end if i ==1
% Solve problem with no initial conditions fem.sol=femtime(fem, 'solcomp',{'c'},'outcomp',{'c'},
'tlist', [O:time-step:maxtime],'tout','tlist'); end
114

if i > 1
% Solve problem with initial conditions fem.sol=femtime(fem,
'init', init,
'solcomp', {'c'},'outcomp',{'c'}, ...
'tlist', [O:timestep:maxtime],'tout','tlist'); end
% Save current fem structure for restart purposes femO=fem;
% determine how much time it took to suck up the solvent in the thin film massdata = postint (fem, 'c', 'T', [O:time-step:maxtime]); poo=size(mass-data); nodata=O; for j = 1:poo(2) if massdata(j) > solventmass + massafterwait timedata(i) = (j-1)*time_step;
% Store solution feml.sol = asseminit(femO,'init',femO.sol,'T',[(j-
1)*timestep]); break elseif j ==poo(2) nodata =1; end end if nodata==1 break end massafterwait= postint( fem, 'c', 'T', timedata(i));
% model the time the stamp is not in contact with the solvent
% all boundaries are insulating to model stamp just sitting out in air if wait > 0
% Application mode 1 clear appl appl.mode.class = 'FlDiffusion'; appl.assignsuffix = '_di'; clear equ equ.D = 'DO'; equ.ind = [1]; appl.equ = equ; fem.appl{l} = appl; fem.outform = 'general';
115

% Multiphysics fem=multiphysics(fem);
% Extend mesh fem.xmesh=meshextend(fem);
% Mapping stored solution to extended mesh init = asseminit(fem,'init',feml.sol,'solnum',l);
% Solve problem fem.sol=femtime(fem,'init',init,'solcomp',{'c'},
'outcomp',{'c'},'tlist',[O:timestep:wait],
'tout','tlist');
% Save current fem structure for restart purposes femO=fem;
% Store solution feml.sol = asseminit(femO,'init',femO.sol
,'T',[wait]);
% store the mass of solvent in the stamp massafterwait = postint (fem, 'c', 'T',
[wait]); end end poo = size(time data); for j=l:poo(2) tickmarks(l, j) =j; end clf hold on plot([1:1:poo(2)], timedata, Ik*I) plot([1:1:poo(2)], timedata, 'k') ylabel ('Time for solvent to diffuse into stamp (s)') xlabel ('Number of prints') grid on set (gca, 'xtick', tick_marks);
116

This appendix gives the experimental recipes used for the prints discussed in the text.
OLE recipes use the following format:
Print number
RF power Gas and Ink and Spin speed
(watts) dispense and time time (minutes) (mTorr) type (kRPM and seconds)
Temperature Stamp time substrate Transfer Cure
(eC) (seconds)
This is followed by any notes specific to that sample.
pressure temperature
_(psi)
(eC)
Gold ink refers the ink described in section 4.1.4. Turbo Au VG ink refers to:
Turbo Au Vg ink:
8% gold by mass1
32% alpha-terpineol
60% hexane
A material with a percentage indicates that the listed material (nanoparticle colloid or SOG) was diluted to the stated percentage by mass with alpha-terpineol. A volume of alpha-terpineol listed before the ink indicates that alpha-terpineol was spin-coated on the substrate at the same listed speed and time as the active ink to pre-coat before dispensing the smaller amount of active ink.
Print 261
Layer 1
261 25 W, 1 min Ar
2
200 mTorr
25 *C
Etched by of 02 for
300 s Silicon wafer
RIE at 300 W, 320 mTorr,
20 minutes.
0. 5 mL of
500FX 50%,
6 kRPM,
40 s static
30 psi 425 eC
32 sccm of SF ,
I and 2 sccm
10 from 30% gold by mass Perfect Gold dispersed nanoparticle paste manufactured by Vacuum
Metallurgical Co., LTD, No.516 Yokota, Sambu, Chiba 289-1297, Japan
117

Print 570
Layer 1
570 25 W, 1 min Ar
2
200 mTorr
22.5 *C 300 s 02 plasma cleaned silicon nitride
0.5 mL of 20B 6 kRPM,
62%, static
20 psi
20 s
425 eC
Print 692
Layer 1
661
26.5 9C
25 W, 1 min 02, 400 mTorr 4 drops of
Turbo gold
120 s
VG, dynamic
30 psi 02 plasma cleaned polyimide
Layer 2
689
23 *C
25 W, 1 min 02,200 mTorr 0.5 mL 70%
20B SOG, static
360 s 1661 20 psi
2 kRPM,
30 s
300 eC
3 kRPM,
30 s
300 eC
Layer 3
692 25 W, 1 min 02,400 mTorr 2 drops of
Turbo gold
3 kRPM, 30 s
VG, dynamic
23.75 *C
300 s 689 30 psi 300
Wet etched with 7:1 BOE diluted 1:10 with distilled water for 10 seconds. Dump rinsed, transferred to methanol, and dried.
Print 721
Layer 1
704
26.5 *C
25 W, 1 min 02, 400 mTorr 3 drops of
Turbo gold
300 s 02 plasma cleaned polyimide
VG, dynamic
30 psi
Layer 2
716
26.75 9C
25 W, 1 min 02,200 mTorr 0.5 mL 70%
20B SOG,
300 s 704 static
20 psi
Layer 3
3 kRPM,
30 s
300 *C
3 kRPM,
30 s
300
*C
118

721
27 eC
25 W, 1 min 02, 400 mTorr 3 drops of
Turbo gold
300 s 716
VG, dynamic
30 psi
3 kRPM, 30 s
300 eC
Print 730
Layer 1
728 25 W, 1 min 02, 200 mTorr 0.5 mL 70%
20B SOG, static
25.5 *C 300 s polyimide 20 psi
3 kRPM,
20 s
300 oC
Layer 2
730 N/A LE N/A LE 20 *L Turbo
Au VG,
3 kRPM, 20
S
26 -C 300 s 728 dynamic
N/A LE 300 -C
Wet etched with 7:1 BOE diluted 1:10 with distilled water for 10 seconds. Dump rinsed, transferred to methanol, and dried.
Print 806
Layer 1
794 N/A LE N/A LE
26 -C 300 s 02 plasma cleaned
_Polyimide
20 9L Turbo
Au VG, dynamic
N/A LE
_
3 kRPM,
30 s
300 .C
Layer 2
804
27 eC
25 W, 1 min 02,200 mTorr 0.5 mL 70%
20B SOG,
240 s 794
1 static
20 psi
3.75
kRPM, 20 s
300 .C
Layer 3
806 N/A LE N/A -
LE 20 eL Kovio
Turbo Ag,
3 kRPM, 30 s
27 C 300 s
______
804
_dynamic
N/A - LE 300 *C
Wet etched with 7:1 BOE diluted 1:10 with distilled water for 10 seconds. Dump rinsed, transferred to methanol, and dried.
Print 869
Layer 1
855 25 W, 1 min 02, 400 mTorr 20 eL Turbo
Au VG, dynamic
3 kRPM,
30 s
119

27 eC
Layer 2
868
300 s polyimide 30 psi 300 -C
27 *C
25 W, 1 min 02, 200 mTorr 0.5 mL 60%
20B SOG,
300 s 855
-dynamic
20 psi
1.625
kRPM, 20 s
300 *C
Layer 3
869 N/A LE N/A LE 20 *L Kovio 3 kRPM, 30
Turbo Ag, s dynamic
27 *C 300 s 868 N/A LE 300 *C
Wet etched with 7:1 BOE diluted 1:10 with distilled water for 10 seconds. Dump rinsed, transferred to methanol, and dried.
Print 917
Layer 1
907
27 *C
25 W, 1 min 02, 400 mTorr 1 mL a-t,
*L 20% Au
20
600 s polyimide dynamic
30 psi
Layer 2
913
26.5 *C
25 W, 1 min 02, 200 mTorr 0.5 mL 60%
20B SOG,
300 s 907
.dynamic
30 psi
3.5 kRPM,
30 s
300
300
*C
1.5 kRPM,
20 s
300
*C
Layer 3
917 N/A LE N/A LE 20 eL Kovio 3 kRPM, 30
Turbo Ag, s
27 *C 300 s 913 dynamic
N/A LE 300 eC
Vapor etched over 50 mL of 48% HF with a 50 W light bulb 40 mm away from the sample for 10 minutes.
Print 1001
Layer 1
1001 25 W, 1 min 02, 400 mTorr 1 mL a-t, 20
3 kRPM,
*L gold ink 30 s
26.5 *C 300 s polyimide dynamic
40 psi
Oxygen plasma etched at 400 mTorr and 35 W for 120 minutes.
300
*C
Print 1009
Layer 1
120

1009 25 W, 1 min 02, 400 mTorr 1 mL a-t, 20 3 kRPM, eL gold ink 30 s dynamic
27 *C 300 s polyimide 40 psi
Oxygen plasma etched at 200 mTorr and 20 W for 80 minutes.
300 *C
Print 1015
Layer 1
1015
27 .C
25 W, 1 min 02, 400 mTorr 1 mL a-t, 20 3 kRPM, eL gold ink 30 s dynamic
300 s 02 plasma 40 psi 300 9C cleaned polyimide
Oxygen plasma etched at 1 Torr and 35 W for 40 minutes.
Print 1016
Layer 1
1016 25 W, 1 min 02, 400 mTorr 1 mL a-t, 20 3 kRPM,
*L gold ink 30 s dynamic
27 eC 300 s 02 plasma cleaned polyimide
40 psi 300 .C
Oxygen plasma etched at 1 Torr and 25 W for 40 minutes.
Print 1017
Layer 1
1017 25 W, 1 min 02, 400 mTorr 1 mL a-t, 20 3 kRPM,
*L gold ink 30 s dynamic
27 *C 300 s 02 plasma cleaned
40 psi 300 eC polyimide
Oxygen plasma etched at 1 Torr and 25 W for 80 minutes total. The substrate was rotated in the plasma chamber ever 5 minutes.
121