CLEANROOM NEWS
Process Spotlight: Liquid Crystal Alignment Layers, Part I
Liquid crystals possess interesting electro-optic properties, but LCDs would not have become what they are today if scientists had not come up with a way to anchor their alignment at a surface. Alignment layers are used to control orientation of the liquid crystal director. Finding a suitable alignment agent was not easy initially—surfactants could be used, but tended to lack long term stability. Polymer materials such as polyvinyl alcohol (PVA) were also used. Simply rubbing a glass plate repeatedly with a cotton swab seemed to work, but was obviously not the final solution. Obliquely evaporated thin films, such as
SiOx, were effective, but not well suited for production. Liquid crystal displays finally began to take off when polyimide materials were developed for alignment—they provided a durable, stable alignment that could stand up to environmental testing.
Polyimide materials generally require a buffing or rubbing step to define a particular alignment direction.
Rubbing will be covered in detail in a future process spotlight, but is generally accomplished by using a velvet cloth attached to a rotating drum to gently buff the surface of the polyimide in a single direction.
Most of the liquid crystal materials used in displays are nematic phase LCs, and are rod-like in shape.
The axis of the molecule, or director, tends to align parallel to the rub direction on most polyimide materials; this alignment can be referred to as planar or homogeneous alignment. In this case, the molecule aligns roughly parallel to the surface, although there is generally a tilt angle induced by the polyimide material, in the direction of the rub. Materials with low tilt (1-4 degrees) are generally used for twisted nematic (TN) devices, whereas super-twist displays (STNs) require higher tilt angles (4-10 degrees).
Polyimides are also available for homeotropic alignment, in which case the director aligns perpendicular to the surface. This type of alignment is useful for vertically-aligned (VA) displays, which are used for liquid crystal televisions.
Most polyimide materials are applied by spincoating or offset printing. Polyimides are not easily soluble, but can be dissolved in gamma-butyrolactone. However, this type of polyimide (called preimidized) does not spincoat well, as GBL is a less than ideal solvent for spincoating. More commonly, the polyimide material is provided to the user as a polyamic acid precursor solution, which is soluble in polar solvents
(such as NMP or butyl cellosolve) that are well suited to spincoating. These polyamic acid types require an imidization bake, generally 180-275C for 1 hour depending upon the material.
Polyimides today are generally mixtures of several components. Next month we will discuss different types of polyimides, and some of the engineering that goes into today’s materials.

The LCDRF has a number of materials available for use by IPP members, for both homeotropic and planar alignment. Please contact Doug Bryant if you are interested in learning more about the facility capabilities.
A few references:
1. Liquid Crystals: How a Useless Lab Anomaly Grew up to Become an Indispensable Part of Hundreds of Everyday Devices by Linda Hamilton http://www.americanheritage.com/articles/magazine/it/2002/4/2002_4_20.shtml
2. Janning, J.L., “Thin Film Surface Orientation for Liquid Crystals,” Applied Physics Letters 21:173-4,
1972.
3. http://www.nissanchem.co.jp/english/businesses/electronic_materials.html
4. http://www.brewerscience.com/products/sm/product-information/alignment-coatings/
Several tools in the LCDRF will be undergoing maintenance and upgrades, or have recently completed maintenance:
The MRC 603III Sputter Coater has undergone repairs in the past few weeks. The DC power supply failed in early January, but was recently repaired and is now fully functional. The user interface monitor also failed, and has been replaced with an external monitor until it can be properly repaired. The machine is fully functional at this time despite these issues.
The Tencor Alpha Step 200 surface profilometer was recently repaired. The printer unit on the machine had stopped functioning, but is now operational again. Calibration of the unit has been checked as well— measurements corresponded well with two calibration standards.
The Technics PlanarEtch II plasma etcher is currently down. No work is planned on this machine in the next month. If you have interest in using this machine, please contact Doug Bryant; if there is a need for it, we can bump this up in priority.
The Clean Air Systems Wet Bench has been gutted and is being rebuilt for maximum efficiency in working with 6” and 7” glass. Kevin Ballard, an undergraduate cleanroom assistant, has installed new process tanks, and is rebuilding the exhaust plenum before the unit will be ready to reinstall in the cleanroom. The bench should be ready for commissioning in late February or early March.
PHOTOLITHOGRAPHIC MASK DESIGN CAPABILITIES
Here at LCI, we use AutoCAD 2000 and AutoCAD R14 to design our photomasks.
We work with two mask production companies:
Infinite Graphics
4611 East Lake Street
Minneapolis, MN 55406
800-679-0676 http://www.igi.com/services/phototooling/photomasks/ and
Advanced Reproductions
100 Flagship Drive

North Andover, MA 01845
978-685-2911 http://www.advancerepro.com/optical_photo_masks.html
Each mask company has similar engineering drawing requirements, but either company will turn an
AutoCAD engineering file and turn into a file they can plot and write as a photomask. Check with each company for specific details.
Our design capability for photomasks is extensive. We most often design masks for patterning ITO and sometimes for metal films such as Chrome and Nickel. Our typical feature sizes are on the scale of mm and up to cm. For some applications, we have pushed the limits of our capabilities to as small as 3 um feature sizes and 3 um gaps and work with mask arrays that cover 7” by 7” plates of chrome-coated glass.
Most masks are designed to be used singly and are single layer, meaning one write operation per mask when they are manufactured. But masks can be multiple layer too when the need arises. In the case of a typical single-layer mask, a closed polyline shape in AutoCAD would be defined as ‘filled’ meaning that it would be a solid chrome feature in the final mask and the area surrounding the closed polyline shape would be chrome-free, or clear. However, the mask writing equipment can perform an opposite contrast write operation, meaning that a closed polyline feature can be ‘written’ clear while the surrounding areas remain chrome plated. If a mask contains both types of closed polyline features, filled chrome and filled clear, the mask must be done in layers. Many layers are possible, but in the simplest case one layer would contain all the chrome-filled closed polylines and a separate layer would contain all the clear-filled closed polyline features. It does matter in which order the layers are written because practically during a write operation, chrome is always removed. To date, our most complicated mask set was just completed for a special project that required the use of a complementary four-mask set, where two of the four masks are two-layer masks. The minimum feature size of these masks varied for each but the tightest specifications were 3 um inter-electrode gaps and 5 um wide address lines.
Please let us know if you require any assistance in designing a photomask for your application needs.
Professor Wei’s article in Nanotechnology a popular download
February 11, 2008
CPIP Professor Qi-Huo Wei recently published a paper, “Scaling laws for nanoFET sensors,” in
Nanotechnology , Vol 19, pp015504 (2008) that has already been downloaded over 250 times in 2008.
His group studied for the first time the scaling laws of nanoscale field effect transistor (nanoFET) sensors by using random resistor networks. Nanosensors with high sensitivity and low detection limit promise various practical applications in biological and chemical analysis ranging from health care to homeland security. Fundamental understanding on how their performances vary with their geometrical parameters is critical to designing devices with ultrahigh sensitivity.
NanoFET sensors are based on the sensitive conductance change of semiconductor nanowires and carbon nanotubes in response to binding of charged molecules. Computer simulations show that the field effect strength exerted by the binding molecules has significant impact on the scaling behaviors. When the field effect strength is small, nanoFETs have little size and shape dependence. In contrast, when the field-effect strength becomes stronger, there exists a lower detection threshold for charge accumulation FETs and an upper detection threshold for charge depletion FET sensors. At these thresholds, the nanoFET devices undergo a transition between low and large sensitivities. The discovery of detection thresholds intrinsic to

the devices has significant impact on nanoFET sensor design. In collaborating with his colleagues, Dr.
Wei’s group is trying to experimentally observe these scaling behaviors.
Spring 2008 LCI Seminar Schedule
Feb 4. (4:00 PM, Monday) Prof. B. Zalar , Department of Solid State Physics, J. Stefan Institute,
Ljubljana, Slovenia, "Some current topics in liquid crystal elastomer research: deuteron NMR study of paranematic-nematic transition criticality and piezoresistivity of nanoparticle-reprocessed networks"
Feb. 6, Prof. Richard J. Spontak , Department of Chemical and Biomolecular Engineering, North
Carolina State University, “Molecular Design of Nanostructured Block Copolymers as a Route to High-
Performance Electroelastomers”
Feb. 20, Prof. Sasha Govorov , Department of Physics, Ohio University, “Exciton-plasmon interaction in coupled semiconductor and metal nanocrystals"
March 5, Prof. Linda Hirst , Department of Physics, Florida State University, Title: TBA.
March 7 (12:00 PM, Friday), Prof. Kenji Urayama , Department of Material Chemistry, Kyoto
University, "Stimulus Response of Swollen Nematic Elastomers"
March 19, Prof. Mark Warner , Cavendish Laboratory, University of Cambridge, UK, Title: TBA
April 2, Prof. Vladmir M. Agranovich , The NanoTech Institute, The University of Texas at Dallas, and
Institute of Spectroscopy, Russian Academy of Science, "Negative refraction, polaritons and negative group velocity"
April 16 Prof. Matthew A Glaser , Department of Physics, University of Colorado, “Nanophase segregation and frustration: chirality, splay, and curvature in bent-core smectics”
April 23 Prof. Robert Austin , Department of Physics, Princeton University, Title: TBA
April 30 Prof. Paul Goldbart , Department of Physics, University of Illinois at Urbana-Champaign, Title:
TBA
New Faces at LCI
Ji Ma , postdoctoral fellow, is working in Prof. Yang's lab.
Jae Heung Jo , postdoctoral fellow, is working in Prof. Palffy-Muhoray's lab.
Hyun Wuk Kim is a Samsung Engineer in Residence.
Please let us know if you would like more detailed information about any activities at the LCI.
Phil Bos pbos@lci.kent.edu
330-672-2511