The application of silicon-on-insulator (SOI) technology for the fabrication of fully scanned active matrix flat panel displays

Abstract

SO1 technology is presently being used for the fabrication of both Liquid Crystal Displays (LCDs) and Electroluminescent (EL) displays. It has unique advantages for the fabrication of flat panel displays such as the production of devices with high performance and high breakdown voltage. It is the object of this paper to review the process technologies used in the fabrication of these displays and also to show the latest developments in these technologies.The processing sequence for the fabrication of high density SO1 LCDs is shown in figure 1. The novel aspect of this technology involves the, "transfer," of the thin silicon SO1 film onto a glass substrate. The process used to fabricate LCDs containing relatively large picture elements (pixels) is similar to standard SO1 processes. For the smaller pixels that are needed for high density projection displays, however, a more complex process is needed. This process includes the addition of a layer to act as an electrical shield to isolate the pixel electrode from the select and data lines. A typical, "shielded," silicon pixel is shown in figure 2 with the corresponding illuminated pattern shown in figure 3. Pixels as small as 12 pm X 12 pm have been made using this shielded structure. This is the smallest pixel structure ever reported and this is the first time that a shielded pixel structure has been used is SOI-LCD's. SO1 technology is particularly suited to the fabrication of scanned LCD's because the, "transferred," surface is completely flat eliminating any electric field distortions in the liquid crystal due to mechanical steps in the surface. These electrical field distortions can cause disclinations or bright spots in the pixel area.For the fabrication of EL displays a high voltage SO1 technology is used in conjunction with an EL technology as described in figure 4. SO1 technology is the only suitable technology for achieving high performance 200 V high density active matrix fully scanned EL displays (AMEL's). Pixels as small as 24 X 24 pm have been successfully fabricated in this technology as shown in figure 5. An optical pattern of a 128 X 128 array is shown in figure 6. This SO1 active matrix EL technology has the potential for achieving the highest efficiency of any flat panel display. This is a direct result of the fact that only pixels which are actually, "turned on," dissipate any power. Fully functional 128 X128 arrays have been fabricated haveing brightness levels of 800 fL (2400 cdlm2) and contract ratios of better than 1OOO:l. These are the highest values ever reported for an AMEL display.