Many materials besides the liquid crystal are important to the operation of an LCD. As display technology advances, the role of these materials becomes more important. Some of the important materials are discussed below.
In every LCD the LC material must be anchored in some way to a surface associated with the display. In the case of a TN, STN, ECB, or FLC, the surface is the glass substrate. In the case of a PDLC, the surface is the droplet wall of the polymer. There are various ways the elongated liquid crystal molecule can be anchored to a wall: perpendicular, parallel, or at some angle (often called a pretilt). The strength of the anchoring is important, because it must compete favorably with the elastic energies of the liquid crystal in the presence and absence of applied fields.
Very little is known about surface anchoring, and it is usually treated as "black magic" with standards and techniques. Industry currently prefers the polyimides for the alignment layer. Each industry has its own proprietary way of preparing layers. Scientists at Merck-Japan say that customers for materials usually request nematic mixtures based on their own proprietary alignment layer. Some customers seek advice, and Merck-Japan and Dainippon Ink are doing some work on how their materials respond on various surface layers. At Dainippon Ink, an R&D program is working on controlling pretilt by mixing polyimide derivatives that give low pretilt with other derivatives that provide 90 degree anchoring to give a desired tilt angle.
The stability of high pretilt in the STN display is a problem, particularly for uniformity. There is research in Japan, such as at Dainippon Ink, not only to improve or strengthen high pretilt anchoring but also to better understand the anchoring mechanism. The subject presents several problems, some of which Professor Kobayashi's laboratory and Tokyo University is working on (see Table 2.6). Innovative work there uses Langmuir-Blodgett films to avoid rubbing and conductive alignment layers to solve "second-order" cross-talk or ghosting from charge buildup. Theoretical and experimental programs are studying the polar and torsional components of anchoring.
Japanese suppliers of alignment materials include Nisson Chemical, Japan Synthetic Rubber, Hitachi Chemical, Toray, and others.
Retardation film is a Japanese innovation to improve contrast on STN and TN cells and to provide for black and white and color on STN display cells. The principal operation of the film is to retard or shift one component of the light to convert the elliptically polarized light generated by the display cell into the linear polarized light required by the polarizing sheet, as Figure 2.1 illustrates. The film is sometimes referred to as a phase compensating sheet or compensator.
Figure 2.1. Illustration of a Retardation Film
(Courtesy of Nitto Denko)
An important feature of the retardation film is that it offers uniform retardation over the wavelength spread of the visible spectrum. Nitto Denko has performed research in this area using different polymeric materials and polymer film combinations (Kato et al., 1991).
Another interesting feature studied and reported by Nitto Denko is the use of retardation films to enhance the viewing angle of a TN or STN display. They have shown the importance of controlling the refraction index of the film in three directions, n(subscript x), n(subscript y), and n(subscript z). Figure 2.2 shows one of their plots, illustrating how the proper selection of indices can give the retardation a desired angular dependence by proper selection of indices.
Retardation film suppliers include Nitto Denko, Sumitomo Chemical Inc., San Ritz Corporation, Toray, Kayapolar, and others.
Figure 2.2. Illustration of the Change of Retardation Versus View Angle
Control by Adjustment of the Three-Dimensional Refractive Index
There are a variety of ways of making RGB color filters for full-color active and passive matrix displays. Four preferred methods are illustrated in Figure 2.3. Toppan Printing indicated a fifth, dichroic, method. For LCDs the dyeing type has been the predominant method, according to Toppan, but pigment-dispersed filters are expanding because when compared with the dye method they prove to be superior in light resistance for TV, automobile, and aircraft applications.
The cost of production is equally high for dye-method and pigment-dispersed filters. The pigment-dispersed filter is spun onto the substrate: an easier method of production than used for the dyeing type. Fuji-Hunt is the top vendor of pigment-based filter materials. Roller coating could reduce the amount of pigment-dispersed filter material. Low-reflectivity chrome is used for the black matrix, and it is patterned with a stepper. An overcoat is deposited on the gelatin filters (but not often for pigment filters) before the indium tin oxide (ITO) is deposited. Improvement in light transmission through pigment-dispersed filters is under investigation. If the pigment is made smaller, transmission increases, but generally speaking the light resistance has a tendency to gradually decrease.
Figure 2.3. Color Filter Formation Methods
(Courtesy of Sharp Corporation)
Companies have investigated producing color filters by electrodeposition, but this method can be used only with certain filter layouts (such as stripe) in which there is a continuous path from one side of the display to the other. Printing technologies offer the possibility of fabricating low-cost filters. According to Toppan, the main issue with printing is the maximum size of substrate. A comparison of the dye, printing, and pigment methods is shown in Table 2.7, supplied by Toppan.
Characteristics of Color Filters
(Information supplied by Toppan)
Color filter suppliers include Toppan Printing, Dainippon Printing, Fuji-Hunt Electronics, Hoya, Kyodo Printing, Nagase Sangyo, Nippon Sheet Glass, RODIC Shinto Chemitron Co., and Toray.
Major components of the display are the flat-glass substrates, which must meet demanding specifications that are challenging to today's glass manufacturers. The glass must incorporate a high degree of flatness over large areas as well as a high level of microscopic-scale flatness for low-temperature processing for TFT manufacturing. For AM displays the most popular glass is a borosilicate glass that Corning produces using a fusion draw process. Corning's 7059 glass has a strain- point temperature of 590 degrees centigrade and has about 90% of the AM market. Japanese companies such as Nippon Electric Glass (NEG) and Asahi Glass are developing competitive technologies. NEG produces an alkali- free sheet, OA-2, and is attempting to increase its market share by specifying OA-2 with a slightly higher strain-point temperature of 635 degrees centigrade and fewer defects. A recent price increase by Corning is expected to help NEG, which has sufficient manufacturing capacity to supply a greater market share. Asahi Glass is also working on a high-temperature flat glass, but its researchers are not attempting a strain-point temperature greater than 600 degrees centigrade because they believe lower temperature polysilicon processes will be developed. Asahi Glass claims its product improves on Corning's 7059 in flatness and etch resistance. A concern over a stable supply of glass for TFT LCDs was expressed during the visit to Asahi Glass. Where high-temperature processing is not required, as for technologies on a passive matrix, the lower cost conventional soda- lime glass can be used if a passivation layer is added to one surface to provide a barrier for ion migration. Companies such as Pilington Micronics in the United Kingdom continue development of lower cost soda-lime substrates. There is hope that in the near future TFT processes will allow use of these substrates, which can be made on high-quality float lines.
The area of polarizing sheets is a more mature technology, with several suppliers in Japan. Typical properties quoted for the optical properties of these films include a transmittance of 40-43%, with a degree of polarization closely approaching 100%. Concerns are the deterioration of the transmission and of the polarizing efficiency. Mr. Mochizuki of Fujitsu mentioned that problems with the polarizer can become apparent when STN displays are used on overhead projectors. With time the image loses its uniformity, an effect that is thought to be primarily due to bleaching of the polarizing sheets.
The charts (Figure 2.4) for a polarizing film under development by Toray can give an estimate of state-of-the-art film stability. Figure 2.4 is a reproduction of Toray's announcement for a HC type (high polarizing efficiency/high durability type) under a test condition: 60 deg., 90 RH.
Figure 2.4. Stability of a Polarizing Film Under Development
(Courtesy of Toray)
A useful product that is available only through Japanese suppliers is precision plastic spheres ranging from 3 to 4 microns in diameter. These spheres are particularly useful as spacers where soft (plastic) substrates are used, say for PDLC displays. One supplier of this product, under the name of micropearl SP, is Sehisai Fine Chemical Co., Ltd., in Osaka-shi.