Seiko Epson Corporation
Electronic Devices
Panels for projectors are exposed to high illumination intensities of more than 50,000lx, therefore protection of the TFT from light damage is essential. For this purpose, a lightproof layer placed in matrix pattern over the pixel transistors, called a black matrix, blocks the light at the incident aperture. In addition, another lightproof layer is mounted on the backside of the TFT to block the reflected light.
To cool down the LCD panels that can reach temperatures of over 70 degrees C, a cooling fans circulates air around them. The dust-proof glass covering on both sides of the panel also acts to defocus the image of any dust present on the screen.
To achieve higher luminous efficacy, some panels have a micro lens array (MLA) on the outside of the panel. The MLA consists of a tiny convex lens for each pixel on the LCD panel (Fig. 2). Incident light to the LCD panel is first concentrated by the micro lenses (Fig. 2a), and then passes through the openings without being blocked by the black matrix. In solving the trade-off problem between high luminance and high definition within a small body, the MLA technology plays a considerable role. Depending on the size of panels, it achieves an effect equivalent to about a 30% increase of the open area ratio.
Fig.1 : The cross section structure of HTPS light valve.
Fig.2 : Micro Lens Array (MLA)
The data projector market continued growing steadily, and in 1999, the cumulative production of HTPS topped one million. Around that time, Epson started to develop a new element configuration and new process so as to improve the productivity of HTPS.
Major technical developments have been: technology for planarizing the TFT surface of the LCD, micro wiring to minimize the electrode contact area, internal shading to prevent leakage from the TFT, stacking structure technology to increase the accumulation capacity, and so on. For instance, planarization technology reduced alignment disarray and increased the contrast, while combining the miniaturization and shading technology helped realize an improvement in luminance efficacy equivalent to about a 30% increase of the open area ratio.
We called this first generation technology "D1", and we have continued to upgrade the performance and quality, and expand our production capabilities every year since. Thanks to these technological developments, even as the trend toward higher definition through the reduction of pixel pitch continues, we have been able to keep the open area ratio almost constant (Fig. 3). They also contribute to the improving luminance of 3LCD projectors. (Fig. 4)
Fig.3 Trend of higher definition in minimum pixel pitch and aperture ratio of HTPS.
Fig.4 : Improvement of Light out put for 3LCD projector with HTPS.
HTPS continues to evolve. The latest High Temperature Polysilicon Panel technology is explained below in order of (i) driving technology, (ii) element configuration, and (iii) alignment technology.
(i) In the beginning, HTPS TFT liquid crystal was driven in each line by reversing the polarity; but this method creates a horizontal electromagnetic field between lines, and disarray of the alignment, called disclination, arises. As a result, light leakage occurs between the line electrodes, which lowers the contrast when displaying black (Fig. 7a). Therefore, the black matrix covered more area in order to prevent light leakage, but this remedy hampered efforts to make the products more compact with higher definition. Reverse frame driving, which involves driving the whole screen with the same polarity, can restrain the disclination, but it could not be used because of the major side effects such as flicker, discoloration at the top and bottom of the screen, and cross talk, etc.
To solve these problems, a new driving method was created. It scan-drives alternately in the horizontal zone fields. Reducing disarray of the alignment by making the polarity the same in lines corrected the deficiencies of reverse frame driving (Fig 7c). This technology keeps the horizontal electromagnetic field at 0V, and improves disclination (Fig. 7b). Epson first introduced this driving technology in the 5th generation "D5" technology. Thanks to this technology, for 0.7" (1.8cm diagonal) panels, contrast was improved 1.5-fold.
Fig.7 : Driving display technology of HTPS
a) Conventional driving system: Cross section structure of liquid crystal of HTPS (left) and Photograph of Pixels (right)
b) New driving system: Cross section structure of liquid crystal of HTPS (left) and Photograph of Pixels (right)
c) Driving sequence of area scanning drive system.
(ii) Development of new element structures and process technologies continue, so as to achieve higher definition in smaller panels. Reducing the size of panels not only impacts the cost of panels, it also contributes to savings in the cost of parts for optical engines, and helps to make projectors themselves more compact.
Regarding development up to now, in order to attain both higher definition and miniaturization, the contact structure of the drain electrode and conductive electrode has been simplified. Up to this point, they were attached with a connecting layer in between, but in the new structure in the "D6" process they were attached directly to each other by making an etching protection layer. This helped realize a 8.5袖m-pixel pitch.
Moreover, the "D6" technology adopted a hybrid driving method in which the same table mounts both the external driver and the liquid crystal driving controller. This method reduced variation in the writing characteristics of each pixel, and produced smoother image quality as well as enabling smaller circuit boards by making the mounting of a panel driving controller unnecessary.
(iii) Both the inorganic alignment layer and vertical alignment technologies were introduced around the same time as the "D6" technology. These became named collectively as "C2FINE" technology. (The details of these technologies are introduced in Technical Description -> C2FINETM)
This technology is ideally suited for use in home projectors or projection TVs, etc. which require high-definition (HD) and much higher image quality. Vertical alignment is an effective technology for obtaining higher contrast. By combining it with normally-black, we realized "Pure Black". And inorganic alignment, which removed the need for rubbing when creating the surface, achieved a smooth image by reducing unevenness of alignment.
Reprinted from "Nikkei Microdevice" June 2008 issue, p73-81.
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