Low-Temperature Polycrystalline Silicon Thin-Film Transist or Technologies for System-on-Glass Displays
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Low-Temperature
Polycrystalline Silicon Thin-Film Transistor Technologies for System-on-Glass Displays Shuichi Uchikoga
Abstract The elimination of conventional peripheral LSI (large-scale integration) drivers is considered essential to the development of future low-cost, energy-efficient, lightweight, and thin displays. System-on-glass (SOG) displays are a type of display with various functional circuits integrated on a glass substrate. Low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) make the integration of circuits possible because they can be assembled into complex, high-current driver circuits. Furthermore, LTPS TFTs are attracting attention for driving organic light-emitting devices (OLEDs). This article introduces present and future LTPS TFT technologies for SOG displays. Keywords: electronic materials, flat-panel displays, excimer-laser annealing, liquidcrystal displays (LCDs), organic light-emitting devices (OLEDs), polycrystalline silicon, system-on-glass displays, thin films.
Introduction The liquid-crystal display industry has shown rapid growth in five market areas, namely, notebook computers, monitors, mobile equipment, mobile telephones, and televisions. The continual growth in network infrastructures will drive the demand for displays in mobile applications and flat panels for computer monitors and TVs. The specifications of these applications will require high-quality displays that are inexpensive, energy-efficient, lightweight, and thin. Amorphous silicon (a-Si) thin-film transistors (TFTs) are widely used for flat-panel displays. However, the low field-effect
MRS BULLETIN/NOVEMBER 2002
mobility (ability to conduct current) of a-Si TFTs allows their application only as pixelswitching devices; they cannot be used for complex circuits. In contrast, the high driving ability of polycrystalline Si TFTs allows the integration of various circuits such as display drivers. Eliminating LSI (large-scale integration) chips for display drivers will decrease the cost and thickness of displays for various applications. There are high-temperature and lowtemperature poly-Si TFTs, defined by the maximum process temperature they can withstand. The process temperature for high-temperature poly-Si can be as high as
900C. Hence, expensive quartz substrates are required, and the profitable substrate size is limited to around 6 in. (diagonal). Typical applications are limited to small displays. The process temperature for low-temperature poly-Si (LTPS) TFTs, on the other hand, is 600C, which would allow the use of low-cost glass substrates. This makes possible direct-view largearea displays—for example, UXGA (ultraextended graphics array) monitors of up to 15.1 in. (diagonal) with a resolution of 1600 1200 pixels.1 System-on-glass (SOG) displays are value-added displays with various functional circuits, including static randomaccess memory (SRAM) in each pixel, integrated on the glass substrate; eventually, it may be possible to combine the keyboard, CPU, memory, and display into a s
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