In this paper, we report a direct-conversion-type X-ray sensor composed of trench-structured silicon photodiodes, which achieves a high X-ray-tocurrent conversion efficiency under side X-ray irradiation. The silicon X-ray sensor with a length of 22.6 mm and a trench depth of 300 µm was fabricated using a single-poly single-metal 0.35 µm process. X-rays with a tube voltage of 80 kV were irradiated along the trench photodiode from the side of the test chip. The theoretical limit of X-ray-to-current conversion efficiency of 83.8% was achieved at a low reverse bias voltage of 25 V. The X-ray-to-electrical signal conversion efficiency of conventional indirect-conversion-type X-ray sensors is about 10%. Therefore, the developed sensor has a conversion efficiency that is about eight times higher than that of conventional sensors. It is expected that the developed X-ray sensor will be able to markedly lower the radiation dose required for X-ray diagnoses.

A silicon X-ray sensor with trench-structured photodiodes was studied and the influence of Compton scattering was estimated. By irradiating the target pixel with X-rays and measuring the signal from adjacent pixels, X-ray scattering and pixel blur of the proposed sensor was determined. An X-ray sensor with a length of 22.6 mm was designed and fabricated, and its modulation transfer function (MTF) was obtained. A sensor structure to improve the MTF level to that of CdTe was proposed.

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