Comparative Study of Various Pixel Photodiodes for Digital Radiography: Junction Structure, Corner Shape and Noble Window Opening

다국어 초록 (Multilingual Abstract)

Recently large-size 3-Tr active pixel CMOS image sensors have been being used for medium-size digital X-ray radiography such as dental CT, mammography and NDT for consumer products. We designed and fabricated 50 um x 50 um 3-Tr test pixels having a pixel photodiode with various structures and shapes using TSMC 0.25 um standard CMOS process to compare their optical characteristics. The pixel photodiode output was continuously sampled while a test pixel was continuously illuminated by 550 nm light at a constant intensity. The measurement was repeated 300 times for each test pixel to obtain reliable results on the pixel output mean and variance at each sampling time. The sampling rate was 50 kHz and the reset period was 200 msec. To estimate the conversion gain, the mean-variance method was used. From the measured results, the n-well/p-substrate photodiode showed the best performance at a low illumination condition equivalent to typical X-ray signal range among 3 photodiode structures available in a standard CMOS process. Quantum efficiencies of n+/p-well, n-well/p-substrate, and n+/p-substrate were 18.5 %, 62.1 %, and 51.5 %, respectively. From the comparison of pixels with rounded and rectangular corners, we found that rounded corner structure could reduce the dark current in large-size pixels. A pixel with four rounded corners showed the reduced dark current of about 200 fA compared to a pixel with four rectangular corners in our pixel sample size. Photodiodes with round p-implant openings showed about 5 % higher dark current, but about 34 % higher sensitivities than the conventional photodiodes.

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