Seigo Makida

640 x 400 pixels a-Si:H TFT-driven 2D image sensor

A 640 X 400 pixels and 64 dots/mm2 2-dimensional (2-D) contact image sensor has been fabricated by integrating amorphous silicon (a-Si:H) photodiodes (PDs) and thin film transistors (TFTs). The sensor consists of 400 gate and 640 signal lines, and thus contains 256,000 pixels. The PD has a Cr/n-i-p a-Si:H/ITO structure. The TFT has an inverted staggered structure and Ta/TaMo is used for a gate electrode in order to suppress a gate pulse delay. When the TFTs of the n-th row are turned on, photo-generated charges stored in the PD capacitances in the n-th row are transferred to their respective signal line capacitances. After that, the charges are detected by an external voltage sensitive amplifier. The 640 parallel outputs are converted to the serial analog signal by a multiplexer for image processing. The sensor has achieved photoresponsivity of 7.2 V/lx-s, photoresponsivity non-uniformity of +/- 8% and the signal-to-noise (S/N) ratio of 50 dB at the operation of 30 ms/frame scanning speed. Crosstalk as the influence of adjacent lines both X and Y direction is less than 1.3%. This value is estimated to enable achievement of more than 64 levels of gray. The reproduced image quality regarding resolution was good for 8-point kanji characters. This technology will be applicable for multifunctional input/output device mounted on a system such as a pen computer and X-ray detector coupled to a scintillator.