Flat Panel Imagers Based on Excimer Laser Annealed, poly-Si Thin Film Transistor Technology
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Flat Panel Imagers Based on Excimer Laser Annealed, poly-Si Thin Film Transistor Technology J.P. Lu, K. Van Schuylenbergh, R. T. Fulks, J. Ho, Y. Wang, R. Lau, P. Nylen, P. Mei, M. Mulato, J. B. Boyce, and R. A. Street Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA ABSTRACT Pulsed Excimer-Laser Annealing (ELA) has become an important technology to produce high performance, poly-Si Thin Film Transistors (TFTs) for large area electronics. The muchimproved performance of these poly-Si TFTs over the conventional hydrogenated amorphous Si TFTs enables the possibility of building next generation flat panel imagers with higher-level integration and better noise performance. Both the on-glass integration of peripheral driver electronics to reduce the cost of interconnection and the integration of a pixel level amplifier to improve the noise performance of large area imagers have been demonstrated and are discussed in this paper. INTRODUCTION Interest in polycrystalline-silicon (poly-Si) devices has been driven largely by the rapid growth of large area electronic systems, specifically, flat-panel active-matrix displays [1]. Higher performance, beyond that which can be achieved by using standard hydrogenated amorphous Si (a-Si:H) technology, is required to produce high resolution flat-panel displays and to integrate peripheral circuits, which include gate-line drivers, data-line multiplexers, and pixellevel electronics. Poly-Si TFTs can satisfy many of these requirements due to their higher carrier mobility (about 100 cm2/V-sec versus 1 cm2/V-sec for a-Si) and the availability of good p-type polysilicon TFTs, enabling high-performance CMOS circuits. The preferred method for creating these good-quality poly-Si devices on low-temperature substrates is excimer laser processing of amorphous silicon [2]. This is due to the fact that the excimer’s short wavelength, high intensity, and narrow temporal pulse width ensure that thin a-Si films are melted and solidified rapidly, producing high-quality poly-Si before thermal damage of the substrate occurs. In addition to the widely accepted application of poly-Si TFTs in active-matrix displays, this new technology provides a unique possibility of building the next generation flat panel imagers that have much improved noise performance and lower production cost [3]. Figure 1 shows the comparison of the structure of a currently commercially available, conventional flat panel imager versus the structure of a next generation flat panel imager that we envision and are exploring in our lab. Figure 1a shows a conventional flat panel imager, which is simply an active matrix photo diode array. Each pixel consists of a PIN photo diode and a TFT as pixel switch, both of which are made of a-Si:H. All the amplifiers and driver electronics are implemented externally, using standard single crystalline Si technology and connected to the flat panel imager through thousands of dense interconnects. This approach has two major drawbacks. First, the cost and difficulty of making these
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