Date Visited: October 7, 1991
Report Author: T. Credelle
Asahi Glass is a leading supplier of glass products to the LCD industry; they also are producing LCDs in their Optrex facility (60% Asahi, 40% Mitsubishi) and are working with Mitsubishi to commercialize their TFT work (Advanced Displays, Inc.--80% Mitsubishi, 20% Asahi).
The facility we visited is the Central R&D Center for Asahi Glass. The LCD team is actually from the Electronic Products R&D Center, and the TFT team is from the Central R&D Center. We discussed three areas: STN LCDs, Poly-Si TFTs, and polymer-dispersed LCDs. TFT efforts will support new business opportunities in active matrix LCDs.
Principal topics are fast response, wide temperature range, and color. They are also looking into basic physics of LC alignment.
Fast response: Asahi is looking at thinner cell gaps (6-7 microns now, 4-5 microns in the future) and high delta n, low-viscosity LC materials. They do not synthesize materials but do mix different compounds from the suppliers. A figure of merit described is viscosity/(delta n)2; conventional materials are 1000; the new materials are 100 (smaller is better).
One remaining problem with the fast response materials is called "frame response" by Asahi; because of the fast response, the LC does not respond just to rms applied voltage, but to peak voltage as well. This leads to flicker because of the relaxation during the frame time. One fix is to use higher frame frequencies; for example, at 400 Hz the contrast ratio of one sample was 50:1, and at 60 Hz only 2:1. Higher frequencies can cause cross- talk problems, so it is not necessarily a good solution. Asahi has therefore invented a new drive scheme (not disclosed) that gets around this problem. With it they achieved a 20:1 contrast ratio at 50 ms response on a 5.7-inch diagonal, 320 x 240 8-color LCD. They are not ready to commercialize the drive scheme yet because a new controller and new driver ICs are required. The method is claimed to have lower cross-talk and lower display power; they are aiming the development at the PC market.
Color STN: Asahi is working on color STN with Optrex. They now use Shintoh and Toppan for color filters (Shintoh uses a electrodeposition method). They believe that the cost may be lowered in the future with this technique.
Viewing Angle: Asahi is developing new retardation films with controllable indices or refraction (nx, ny, and nz). They have obtained higher contrast ratio and wider viewing angles without color shifts with these new films.
STN vs. TFT: The feeling of Asahi is that for color, TFT LCDs will dominate, but color STN will be suitable in the VGA and below resolution. Mono STN will dominate over mono TFT LCD because of the cost. Their prediction for 1995 is a 50/50 split between color STN and color TFT LCD for the PC market; color TFT LCD only for the workstation market; decreasing share for mono TFT LCD.
The TFT discussion centered on poly-Si R&D since the a-Si R&D is done primarily at Mitsubishi and Advanced Displays, Inc. The thrust is to develop low- temperature poly-Si processes using laser-assisted processes. Claimed advantages for poly-Si include higher mobility for integrated drivers, lower photoconductivity for projection applications (high luminance), and increased stability for higher reliability. The higher mobility also allows increased numbers of lines and therefore higher resolution.
Asahi's approach to recrystallization is to use a cw-Ar laser to recrystallize in the solid phase. The process is to deposit SiOx, a-Si, and SiON by PECVD, to dehydrogenate at 400-450 degrees centigrade, and then to recrystallize with a 500-nm wavelength cw- Ar laser drawing at 10 m/sec. Overlapping the beam paths can increase the stripe width without degradation. Each stripe is 30 microns wide. Average grain size is 30 nm, with a diffuse grain boundary. Lower leakage currents compared to other techniques was claimed; mobility is 40.
To form the contacts, an ion doping process is used; 5 Kev ions from a bucket-type source with PH3 source gas are used. Beam current is 4.4 microns A/cm2, and the substrate is at room temperature. After a 300 degrees centigrade anneal, a sheet resistance of 1 x 104 ohms/square is achieved.
Comparison of a-Si and poly-Si: Asahi scientists believe that a-Si is suitable for VGA resolution, but that poly-Si will be needed for 1000-line resolution (reduces parasitic capacitance, increases aperture ratio). Asahi's goal is to develop a compatible process (to the a-Si process) in 3 years and to make projection LCDs from poly-Si by 1994-95.
New glasses: Asahi is working on a new float glass that is compatible with Corning 7059. They are not actively working on high-temperature (>600 degrees centigrade) glasses.
Factory: Asahi and Mitsubishi are investing in a pilot factory for TFT LCDs, a-Si at first. The stated investment was 10 billion yen for the building and 10 billion yen for the equipment. The factory is scheduled to open in July 1992.
Asahi is one of several companies in Japan working on PDLCs. They are investigating a photopolymerization-induced phase separation process to make the PDLC. Advantages of PDLC include high transmission (>80%), large-area fabrication (>3m2), and high speed response (<1 ms). Applications include windows, direct-view LCDs, projection LCDs, and shutters. Limitations today include low multiplexing ratio (3:1 max.), weak backscattering, and high-voltage operation. At a recent SAE conference (1991) they described a direct-view LCD using a backlight with louvers to control the angle of the light. They achieved a 15:1 contrast ratio.
PDLC projectors: A principal application is for projectors, and Asahi has published several papers on the subject. Present and future state of the art are:
One of the most important problems to solve is the hysteresis, which is presently 0.5V (difference in voltage at 50% brightness). Recently they have achieved 0.03 V in the lab. Another problem is the resistivity, since TFTs are required to address the PDLC. New materials are needed to solve this problem. A third problem is addressing voltage; to reduce the voltage, better process control is required to improve the droplet size control.
Other companies working on PDLCs, according to Asahi, are Dainippon Ink, Seiko-Epson, Hitachi, Sharp, and Toshiba.