Large Area Electronics for Flat Panel Imagers Progress and Challenges
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LARGE AREA ELECTRONICS FOR FLAT PANEL IMAGERS PROGRESS AND CHALLENGES J.P. Lu, K. Van Schuylenbergh, J. Ho, Y. Wang, J. B. Boyce, and R. A. Street Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA
ABSTRACT The technology of large area electronics has made significant progress in recent years because of the fast maturing excimer laser annealing process. The new thin film transistors based on laser processed poly silicon provide unprecedented performance over the traditional thin film transistors using amorphous silicon. They open up the possibility of building flat panel displays and imagers with higher integration and performance. In this paper, we will review the progress of poly-Si thin film transistor technology with emphasis on imager applications. We also discuss the challenges of future improvement of flat panel imagers based on this technology. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) based Thin Film Transistors (TFTs) are the dominant active devices used in current large area electronics, including flat pannel displays and imagers [1]. Limited by their performance, a-Si:H based TFTs have only been popularly used in active matrix arrays. Most higher level electronic circuits are currently implemented in external circuits, which are typically composed by traditional single crystalline silicon chips. As an example, Fig. 1a shows the block diagram of a commercially available, conventional flat pannel 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. The amplifiers and driver electronics are implemented externally and connected to the flat panel imager through thousands of dense interconnects. Note the similarity of this configuration to the Active Matrix Liquid Crystal Displays (AMLCDs), which have a liquid-crystal cell instead of a PIN photo diode in each pixel, and external analog data-line drivers instead of amplifiers. The configuration of Fig. 1a has two major drawbacks. First, the cost and difficulty of making these dense interconnects is significant, which is the same problem of typical AMLCDs. Second, unique and even more important to imagers, the noise performance of this type of imager is limited by the data-line capacitance, which scales with the size of imager panel size, and becomes important for large size imagers [2]. The need for better material for building high performance TFTs to solve these drawbacks is quickly realized in the large area electronics society. Poly-Si TFTs are the leading candidate for next generation large area electronics due to their higher carrier mobility (about 100 cm2/V·s vs. 1 cm2/V·s for a-Si:H) and the availability of good p-type polysilicon TFTs, enabling high-performance CMOS circuits [3]. Excimer laser processing of amorphous silicon is the preferred method to create these high quality poly-Si devices on low-temperature substrates. The excimer’s short wavelength, high intensity, and narrow temporal pulse width ensure that thin a
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