September/October 2009
CLEANROOM NEWS
PROCESS NOTES: SPIN CURVES
LCI NEWS
TECHNOLOGY TRANSFER
FACULTY PROFILE
CLEANROOM NEWS
The purchase order has been issued for upgrades to the MRC sputter coater located on the 1st floor
(“MRC Room”). After upgrade, the tool will be capable of ITO deposition. A pulsed DC module and
third cathode will be to the machine. It is currently configured to deposit nickel via DC magnetron
sputtering and SiO2 via RF magnetron sputtering. The machine is currently down due to a vacuum leak,
but is expected to be operational late next week.
A new substrate platen for the Asymtek A403 XY Dispenser has been fabricated and anodized to make
vacuum hold down possible for various small size substrates. Installation should be complete by end of
October. Additionally, the camera illumination will be upgraded so that users can easily view fiducial
markings in ITO. Camera mods should be completed concurrently with the platen upgrade.
Training Update: Cleanroom assistants Bill Eckert, Matt Wayman, and Pat Toothaker are continuing
to produce process training videos for cleanroom users. Videos have been completed for glass scribing,
cleaning, and photolithography.
If there are particular pieces of equipment that are of interest to you, or if you would like to see
particular capabilities added to the room, please contact Doug Bryant.
PROCESS NOTES: SPIN CURVES
Many different materials in liquid crystal device fabrication are deposited using spin coating (see May
2008 Process Notes). Photoresist materials, barrier coatings, and polyimides are the most common
materials deposited using this method. Reliable thickness control is important, especially in production
settings where films are often index matched for optimal device contrast. In photolithography, control
of resist thickness is necessary to avoid incomplete or overexposure.
Material manufacturers usually supply materials in solutions using solvents that are appropriate for spin
coating. Appropriate solvents are low vapor pressure materials that will not readily evaporate during the
spin process (see Table 1). PGMEA is common for resists, while NMP is the predominant solvent in
polyimide blends.
Figure 1. Common spincoating solvents (from http://www.microchemicals.com/ )
Materials are also typically diluted to the appropriate concentration for the most common range of
thicknesses. Photoresists, for instance, may have concentrations of 6-20% depending upon desired end
thickness. Similarly, polyimides may be supplied at anywhere from 3 to 20 weight percent. The end
user is responsible for “dialing in” the process conditions to achieve the appropriate result, however. In
most cases, spin speeds between 1000 and 4000 rpm are most appropriate for LCD processing (toward
the lower end for larger substrates).
Spin curves are used for this final process calibration. If a particular concentration of material is spin
coated at a range of speeds (using same spin time, post bake, etc.), the resulting thickness vs. spin speed
can be plotted as in Figure 2 below. The resulting graph is referred to as a “spin curve”.
Figure 2. Spin curves for Rohm & Haas S1800 series photoresist. For S18xx, xx is the weight
percentage of resist (from Rohm & Haas data sheets).
Spin curves can be fitted to a function if desired (see reference 3 below), but are most often left as a
stand alone data set and interpreted directly.
Thicknesses can be measured using profilometry, which requires a “step” to accurately measure
thickness (usually produced using a razor blade on finished coating; taping off an edge or wiping
material clean before soft baking often generates an edge bead and gives inaccurate thickness
information). Very accurate curves can be obtained by spin coating the material on a silicon wafer using
the same process conditions, then measuring thickness with an ellipsometer. Since the silicon is
reflective, this becomes a simple single film system and can be quickly measured and fit (see results of
polyimide fitting in Table 1 and Figure 3).
When spin curves are often provided by manufacturers, why go to the trouble of generating them again?
It may be necessary to dilute materials further to achieve desired thicknesses, rendering original spin
curves useless. Also, in many cases (such as polyimides), material viscosity will change over time due to
polymerization or degradation of the polymer. This will cause a drift in the spin curves. This is a major
reason why polyimide manufacturers quote very short (often 2 month) shelf lives for their materials.
Polyimide - 1211
1500
2250
3000
4000
1:1
746.64
403.07
307.89
250.17
1:2
455.74
317.65
213.79
162.38
1:3
317.08
217.36
167.19
129.67
800
Thickness (angstroms)
700
600
500
1:01
400
1:02
300
1:03
200
100
0
1500
2250
3000
4000
Spin Speed (RPM)
Table 1 and Figure 3. Thickness vs. spin speed and corresponding spin curves for Nissan polyimide SE1211, obtained by coating on silicon wafers (ref. 1).
References
1. Wayman, Matthew. “Spin Curves for LCI Polyimides,” LCI internal training document, August
2009.
2. http://www.brewerscience.com/research/processing-theory/spin-coater-theory/
3. Patent: Gurer, Emir (Scotvalley, CA), Savage, Richard (Livermore, CA) 2001
“Method and apparatus for adaptive process control of critical dimensions during spin coating
process” United States Silicon Valley Group, Inc. (San Jose, CA) 6177133
http://www.freepatentsonline.com/6177133.html
LCI NEWS
Professor L.C. Chien’s Late-News paper, “Electrically-Switched LC color for Electronic Papers” has
been accepted for invited presentation at the 16th International Display Workshops, December 9-11,
2009, in Miyazaki, Japan.
Recent Seminars
September 16, Prof. Martin Bier, Department of Physics, East Carolina University, NC, "The
Biological Significance of the Lipid Bilayer's Melting Transition"
September 23, Prof. Slobodan Zumer, Department of Mathematics and Physics, University of
Ljubljana and Jozef Stefan Institute, Slovenia, "Colloidal Superstructures in Achiral and Chiral Nematic
Phases"
September 30, Prof. Scott Milner, Department of Chemical Engineering, Penn State University, PA,
"Crystal-Melt Interfaces, Rotator Phases, and Nucleation in Polyethylene"
October 8, Prof. Stephen Morris, Department of Physics, University of Toronto, "Icicles, Washboard
Road and Meandering Syrup"
October 21, Prof. Thein Kyu, Department of Polymer Engineering, University of Akron, Akron, OH,
"Photopolymerization Induced Phase Transitions in Holographic Polymer Dispersed Liquid Crystals
and Photonic Crystals"
October 28, Dr. Gareth Alexander, University of Pennsylvania, "Periodic Structures in Chiral Liquid
Crystals"
Upcoming Seminars
November 17, Dr. Kyoungweon Park, Air Force Research Lab, Nanostructured and Biological
Materials Branch, Wright-Patterson Air Force Base, Dayton, OH, "Colloidal strategies to synthesize
architecturally and functionally complex nanoparticles"
December 9, Prof. Hartmut Lowen, Institute for Theoretical Physics II - Soft Matter, Heinrich-HeineUniversitat Dusseldorf, Title: T.B.A.
TECHNOLOGY TRANSFER
KSU.321 TRANSPARENT CONDUCTING ELECTRODES AND METHOD
Inventors: Dr. John L. West (faculty)
This KSU.321 invention is advantageous for flexible liquid crystal displays (LCDs) and other LC
devices. Applications beyond displays include electronics and battery technologies, radio frequency ID
tags or technologies, and photovoltaics, for example. It is available for licensing for all applications
other than cholesteric liquid crystal displays and signs.
This technology provides a flexible conducting electrode pattern/arrangement useful in many
applications. This technology can conform to 3D configurations and also be sufficiently rugged for
repeated flexing when incorporated on thin plastic sheets, paper, or fabric, for example. An electrode
arrangement is formed on a substrate, which can be flexible or drapable, to include small islands or
zones of highly conductive material which are then connected in a predetermined pattern. The
conducting layer may be formed by multiple techniques, such as ink jet printing or coatings. Other
advantages of this development include that it simplifies manufacturing and lowers the cost of
production. It is useful for devices with substrates including polymers, paper, fabric, and textiles, etc.
Please contact us below if you wish to discuss a license for this or any other Kent State technologies.
From the Office of Technology Transfer and Economic Development, Kent State University
www.techtrans.kent.edu (Please visit the “For Industry” section)
Licensing Information Contacts:
Gregory B. Wilson, Associate Vice President,
Charmaine Streharsky, Ed.D.
Economic Development and Strategic Partnerships
Licensing Coordinator for Technology Transfer
greg.wilson@kent.edu 330-672-0704
cstrehar@kent.edu 330-672-3553
FACULTY PROFILE
Jonathan Selinger
Professor, Chemical Physics
Ohio Eminent Professor
330-672-4875
jselinger@kent.edu
http://www.lci.kent.edu/PI/SelingerJ.htm
Long-Term Interests
• Statistical mechanics of soft materials, including liquid crystals, polymers, Langmuir
monolayers, and lipid membranes.
• Relationship of geometry with long-range order and topological defects.
• Connections of fundamental theoretical physics with experimental research and technological
applications.
Current Research Projects
• Effects of ferroelectric nanoparticles on nematic liquid crystals. With graduate student Lena
Lopatina. Collaboration with experimental group of Prof. John West.
• Anomalous flexoelectric effect in bent-core liquid crystals. With graduate student Subas
Dhakal. Collaboration with experimental group of Prof. Antal Jakli.
• Chirality and biaxiality in cholesteric liquid crystals. With graduate student Subas Dhakal.
Collaboration with experimental group of Prof. Deng-Ke Yang.
• Modeling smectic liquid crystals with complex anchoring conditions. With co-investigator
Prof. Robin Selinger, and graduate students Lena Lopatina and Mikhail Pevnyi. Collaboration
with experimental group of Prof. Phil Bos.
• Diffusion of bent-core colloidal particles. With graduate student Lena Lopatina. Collaboration
with experimental group of Prof. Qi-Huo Wei.
• Defects and buckling of graphene sheets. With graduate student Jun Geng. Collaboration
with Center for Materials Informatics Prof. Laura Bartolo and postdoc Lan Li.
• Shape selection in self-assembled lipid membranes. With co-investigator Prof. Robin
Selinger, former postdoc Zhao Lu, and graduate student Jun Geng.
• Elasticity, geometry, and defects in liquid-crystalline elastomers. With co-investigator Prof.
Robin Selinger and graduate student Jun Geng.
• Helical shapes of chiral liquid crystalline elastomer films. With co-investigator Prof. Robin
Selinger, former postdoc Fangfu Ye, and graduate student Vianney Gimenez.
Interests in Industrial Collaboration:
• Modeling liquid crystal director configurations, alignment, and defects in complex geometries.
• Enhancing properties of liquid crystals by doping with colloidal particles.
• Acoustic alignment of liquid crystals.
Recent Publications
• C. M. Spillmann, J. Naciri, B. R. Ratna, R. L. B. Selinger, and J. V. Selinger, “Electrically
Induced Twist in Smectic Liquid-Crystalline Elastomers,” preprint.
• J. Geng and J. V. Selinger, “Theory and Simulation of Two-Dimensional Nematic and Tetratic
Phases,” Phys. Rev. E 80, 011707 (2009).
• L. M. Lopatina and J. V. Selinger, “Theory of Ferroelectric Nanoparticles in Nematic Liquid
Crystals,” Phys. Rev. Lett. 102, 197802 (2009). Also included in the Virtual Journal of
Nanoscale Science & Technology.
•
•
•
R. L. B. Selinger, B. L. Mbanga, J. V. Selinger, “Modeling Liquid Crystal Elastomers:
Actuators, Pumps, and Robots,” SPIE Proceedings 6911 (2008).
R. K. Gupta, K. A. Suresh, S. Kumar, L. M. Lopatina, R. L. B. Selinger, and J. V. Selinger,
“Spatiotemporal Patterns in a Langmuir Monolayer Due to Driven Molecular Precession,” Phys.
Rev. E 78, 041703 (2008).
E. V. Timofeeva, A. N. Gavrilov, J. M. McCloskey, Y. V. Tolmachev, S. Sprunt, L. M.
Lopatina, and J. V. Selinger, “Thermal Conductivity and Particle Agglomeration in Alumina
Nanofluids: Experiment and Theory,” Phys. Rev. E 76, 061203 (2007).
Lena Lopatina
Ph.D. Candidate
Liquid Crystal Institute &
Chemical Physics Program
330-672-1574
Current research project
We are studying the effect of ferroelectric nanoparticles on properties of nematic liquid crystals. Many
recent experiments have reported that low concentrations of such particles increase the isotropic-nematic
transition temperature by over 10 C, and greatly increase the sensitivity of the nematic phase to applied
electric fields. To understand these effects, we develop a theory for the statistical mechanics of
ferroelectric nanoparticles in liquid crystals. In this theory, the key issue is the distribution of
orientations for the electrostatic dipole moments of the nanoparticles. This distribution is characterized
by an orientational order parameter, which interacts with the orientational order of the liquid crystals and
stabilizes the nematic phase. We estimate the coupling strength and calculate the resulting enhancement
in the transition temperature, in good agreement with experiments. We also predict the response to
applied electric fields, showing that the Kerr effect is enhanced above the isotropic-nematic transition.
These predictions apply even when the electrostatic interactions are partially screened by moderate
concentrations of ions.
Long term interests
After I graduate I will be looking for postdoctoral position in major university or national lab, and then
permanent position that will allow me to do research, and also to teach and supervise students.
Interest in short term projects
I would like to have experience in collaborative work on one of the industrial projects. I believe it will
be very beneficial for both sides. As a theoretical physicist I can offer analytical and computational
approach to the practical problems, companies face every day. On the other had, it will be very useful
for me to learn practical approaches companies use every day.
Subas Dhakal
Ph.D. Candidate
Department of Physics
Kent State University
330 -672 -1524
Research Interest
Statistical mechanics in soft matter, Monte Carlo simulation, molecular dynamics, mean field theory,
phenomenological theory, molecular structure and property, flexo electricity, chirality , biaxiality in
bent core liquid crystals, parallel programming and algorithms for molecular modeling.
Research Activities
• Modeled electric field induced phase transition in SmCP phases of bent-core liquid crystals.
• Statistical mechanics of splay flexoelectricity in nematic liquid crystals of pear shaped
molecules.
• Giant flexoelectricity in bent core nematic liquid crystals.
• Interplay between chirality and biaxiality in cholesteric phase.
• Molecular structure and thermotropic biaxial nematic phase.
• Chiral symmetry breaking in bent core liquid crystals.
Vianney Gimenez
Ph.D. Candidate
Liquid Crystal Institute &
Chemical Physics Program
330-672-1529
Long term and Short term projects
I would like to work as a postdoctoral fellow or research scientist in a research center/institute or a
company. The main areas in which I have experience and interest in doing further work are
o
o
o
o
Device interface development
Colorimetric measurements and characterization
LC Displays technology
Simulations of Liquid Crystal elastomers
o
o
Monte Carlo simulations, Molecular dynamic simulations.
Elasticity theory
Current Research Project
Model defect texture evolution in droplets of cholesteric liquid crystals used in a novel bistable display
with applications in e-books, signage, and other low- or zero-power displays. Bistability is achieved by
switching each droplet between planar and focal conic textures. We model the dynamics of this
transition as a function of droplet geometry, encapsulation, and voltage pulse duration, in order to
optimize device performance and minimize power usage.
In a second project, we carry out finite element simulation studies of shape evolution in nematic
elastomers whose initial director configuration is non-uniform. We model the system using a novel 3-d
finite element elastodynamics approach that explicitly couples strain and nematic order. Finite element
simulation results are compared to both experimental and theoretical predictions.