Date Visited: October 3, 1991
Report Author: T. Credelle
Our visit was to the R&D Center, which employs 1450 people (1000 scientists). NEC spends 1% of sales ($23B in 1990) at the R&D Center; 8% overall. LCD research is part of Functional Devices Research Lab. NEC produces color LCDs, CRT, VFD, and plasma (are exiting plasma business). Most important R&D topic now is color LCD for office automation (OA). NEC PC is designed around a color screen, so color LCD is essential. NEC believes that color STN is not sufficient for market acceptance. They have developed MIM for OA, but have no plans yet to produce. NEC stopped working on ferroelectric LCDs two years ago because of manufacturing problems. Recently they started investigating PDLC for projection TV.
The main area of R&D is in a-Si TFT, but NEC is also developing MIM and p-Si TFT active matrices. NEC is also researching color plasma for large screen sizes (surface discharge AC plasma); 100 cd/m2 brightness in color has been achieved.
NEC has developed a 4.3-inch 720 x 480 mono AMLCD for projection; pitch is 0.13 mm. A contrast ratio of 200:1 has been achieved with a black matrix. NEC is investigating the relationship between surface roughness and a-Si TFT mobility. Through the use of an atomic force microscope they have optimized the deposition conditions to achieve higher mobility (1.0 vs. 0.3 for rough surfaces). The deposition speed is slow, but the critical region is only near the interface. This process would probably be used only for high- resolution projection LCDs because of the need for small devices.
Another development topic is self-aligned TFT to reduce nonuniformities caused by source--drain capacitance. A back exposure is used in a staggered bottom gate structure; an ion shower is used to dope the contact regions. Chromium silicide is formed in the contact region to achieve self-alignment. They may use this development on high- resolution LCDs.
A 9.3-inch MIM LCD was described (Cr/SiNx/Cr) next. Resolution is 640 x 400, with a 0.312-mm pitch. Contrast ratio of 30:1 and 20-ms response was achieved. Recently they produced a color version with 16 gray levels and 100:1 contrast ratio (black matrix included). They claim to have solved the drift voltage problem with time and showed data of essentially no drift in 1000 hours of operation. Even though they have achieved this level of performance, they still believe that gray scale uniformity is a problem, especially in 16-level mode.
NEC has an active program in p-Si using an excimer laser remelting method. They are planning to use recrystallization. They are developing processes that work on large substrates at temperatures below 600 degrees centigrade. The p-Si is deposited by LPCVD and then converted in p-Si through excimer laser anneal. To reduce reverse current leakage, they are planning to use a staggered structure with an offset field to reduce high field near the channel. NEC has achieved 20 MHz shift register operation at <20V. For the future, they will try to develop gas-phase doping to eliminate the ion implant step.
NEC estimates that the cost breakdown of OA-size LCDs is 1/3 TFT and 1/3 panel assembly. This is the main reason for the p-Si program. A main application at first is projection TV. They have built a 4.3-inch a-Si TFT LCD but plan to switch to p-Si "in the near future." The aperture ratio for 50-śm pitch (HDTV) is approximately 30%.
NEC now mass-produces about 10,000 a-Si TFT LCDs/month at a plant on Kyushu Island. Newspaper reports are that the yield is >60%; they did not confirm or deny this report. The factory is highly automated with robot carts to transfer cassettes from one machine to the next (operator transfers substrates manually); mother glass size is 300 mm x 350 mm.
NEC feels that there are no major research issues in a-Si. Driving methods are still being developed, and, of course, manufacturing yield improvements are being studied.