STATUS AND PROSPECTS FOR THE FUTURE

The JTEC team visited the major suppliers of LCDs in Japan and reviewed the published literature on the subject. It based its conclusions about the status and future prospects for each of the major types of LCDs on the visits and the literature review. In cases where there was not general agreement, I have tried to present all sides of the issue.

Twisted Nematic LCDs

Despite their limitations, TN LCDs are still the most widely used LCD type in use today. They are used extensively in watches, calculators, games, instrumentation, and "personal information products." They are the lowest cost LCD and are the lowest power (in reflective mode) flat-panel display ever developed. TN LCDs will continue to be used in applications where cost, size, and power are important, and especially in "direct- drive" (no multiplexing) applications, where the contrast and brightness can be quite high. TN LCDs will be gradually replaced by FSTN LCDs in applications where multiple lines of data are required.

The main research topics in Japan in the field of TN LCDs are materials improvements to widen the temperature range for automotive and outdoor applications, lower voltage switching to reduce the power and circuit cost, and, of course, lower overall cost.

Supertwisted Nematic LCDs

STN LCDs and, in particular, FSTN LCDs are the LCDs of choice for office automation applications. As their cost comes down, they are finding their way into an ever-increasing number of applications. The performance of the FSTN LCD is good in multiplexing ratios up to 240:1 and is adequate even at 480:1. Sizes up to a 17-inch diagonal have been demonstrated (Sanyo), with resolution up to 1024 lines. The contrast ratio has been extended to 20:1, and response time has decreased to the 100-150 msec range--a range that is more than adequate for mouse operation on portable computers. Viewing angle is still somewhat limited compared to active matrix LCDs, but improvements in retardation films have led to wider viewing angles. Response times as low as 50 msec have been demonstrated on small panels, so FSTN LCDs may be usable for limited video applications (e.g., slow-scan phone).

The main problems today are cross-talk, response time, and viewing angle. Cross-talk appears as a shadowing on the screen and gets worse as the multiplexing ratio increases. Response time, although it is adequate for portable computer use, must be improved for full video applications. Viewing angle is adequate for one-person viewing but not for viewing by larger groups.

The principal research topics for STN LCDs are understandably in the materials area. New materials with lower viscosity and higher ratios of bend/splay elastic constants are under development. The physics of surface anchoring is also under study to determine better ways to control surface tilt angle, which directly affects the uniformity of the LCD.

Several labs are working to develop electronic drive circuits to solve some of the optical problems. Asahi Glass reported a new drive scheme with which it has achieved 20:1 contrast with 50 msec response time on a 5.7-inch diagonal color FSTN LCD (method not disclosed). Sanyo uses double drive for the row drivers to reduce horizontal cross-talk.

LCD manufacturing methods are also being improved. For example, lower resistance transparent conductors are being developed to reduce cross-talk. More accurate control of cell spacing is also required to improve background uniformity, especially for thinner LCDs.

The addition of color filters to FSTN has created new markets but has caused more severe manufacturing problems. It was reported that nonuniform color filter flatness is a major problem in attaining high yield for color FSTN LCDs. At least one manufacturer (Sanyo) is now producing its own filters to control this problem. It is likely that other LCD manufacturers will begin to make their own filters as well.

The prospect for the future of FSTN is steady improvements and lowering costs. FSTN LCDs will dominate the LCD business in the next five years. Optical performance will improve to levels of 30:1 contrast, < 100 msec response times, and multiplexing ratios > 500. While optical performance will improve, FSTN will remain inferior to active matrix LCDs. Improvements in materials will lead to wider viewing angles and wider temperature ranges. Automotive applications will become commonplace as costs come down. Packaging of the LCD module will also improve, and thinner, lighter FSTN LCD modules with backlights will be produced. TAB packaging will replace PCBs in order to accomplish this size and weight reduction.

Vertically Aligned Nematic LCDs

VAN LCDs have been demonstrated by Toshiba and Stanley, but the largest effort is at Stanley Electric Company, which has achieved full-color VAN LCDs with excellent viewing angle in sizes up to a 14-inch diagonal. Gray scale was demonstrated using frame rate control. The advantages of VAN, according to Stanley Electric, are easier manufacturing (cell gap control is not so critical) and wider viewing angle in gray scale. Problem areas today include slow response time (250 msec achieved), low transmission (1.5-2.0% for color VAN), and limited temperature range (not yet sufficient for automotive applications). A basic problem that continues to delay progress is the lack of materials for VAN; since the market is limited for the special liquid crystal mixtures required for VAN, there is not as much research going on in this area as in the other areas. Stanley received funding for this development work through a grant from the Japan Research and Development Corporation; one of the requirements for the grant is to establish a pilot line. Therefore, Stanley will produce limited quantities of the VAN LCDs. Because of the low transmission efficiency, use will be limited to AC-powered monitors; therefore, primarily the larger sizes are of interest. The slow response time will further limit their use to nonvideo applications.

The general feeling among the Japanese LCD suppliers is that this technology will be used only in niche markets, if at all. Toshiba said they had reduced their effort in this area. Stanley is continuing and will have some production capacity.

Ferroelectric LCDs

For the past several years, ferroelectric LCDs have held much promise as "the next" LCD to be commercialized. So far, however, manufacturing problems have kept the technology in the lab. Most LCD suppliers have research programs in ferroelectric LCDs, with Canon, Inc., at the head of the list. Our group did not visit Canon, but during our trip they announced both monochrome and color 15-inch diagonal ferroelectric LCDs and demonstrated them at the Japan Data Show in October 1991. The quality was reportedly excellent. The cell spacing is 1.5 microns +/-0.05 microns, which is a very severe tolerance. The report is that Canon will start manufacturing in 1992, but other LCD manufacturers greeted this announcement with some skepticism.

The problems facing ferroelectric researchers are numerous. The most important issues are alignment defect control (sensitive to shock and vibration), cell spacing control, temperature range, response time, and gray scale. New fluorinated liquid crystal compounds are being developed to help decrease the response time and improve the contrast ratio (contrast is limited by defects). Sony is using a SiO evaporation for alignment layers to improve uniformity and contrast ratio; Sony is also developing gray scale techniques to address the video requirement.

Prospects for the future are mixed for this technology. While much research continues, it is unclear what market the ferroelectric LCD will serve. Certainly, if the problems can be solved, then the high contrast and wide viewing angle achieved with ferroelectric LCDs will put them in competition with active matrix LCDs. The biggest problem now is manufacturability; if Canon has solved this problem, then ferroelectric LCDs will be another viable LCD technology.


Published: June 1992; WTEC Hyper-Librarian