Performance comparison of CMOS-based photodiodes for high-resolution and high-sensitivity digital mammography

Abstract

In order to develop a high-resolution and high-sensitivity digital mamographic detector, to use a commercially-available and well-developed CMOS image sensor (CIS) process can be a cost-effective way. However, in any commercial CIS process, several different types of n- or p-layers can be used so that various pn-junction structures could be formed depending on the choice of n- and p-layer combination. We performed a comparative analysis on the characteristics of three types of photodiodes formed on a high-resistivity p-type epitaxial wafer by applying three available n- layer processes in order to develop the high-sensitivity photodiode for a scintillator-based X-ray imaging detector. As a preliminar study, a small test-version CIS chip with an 80 x 80 pixel array of a 3-transistor active pixel sensor structure, 50 m m pitch and 80% fill factor was fabricated. The pixel area is subdivided into four 40 x 40 sub-arrays and 3 different types of photodides are designed for each sub-array by using n(+), n(-) and n- well layers. All other components are designed to be identical for impartial comparison of the photodiodes only. Among 3 types, the n(-)/p-epi photodiode exhibited high charge-to-voltage gain (0.86 mu V/e(-)), high quantum efficiency (49 % at 532 nm wavelength) and low dark current (294 pA/cm(2)). The test CIS chip was coupled to a phosphor screen, Lanex Fine or Lanex Regular, both composed of Gd2O2S:Tb, and was tested using X-rays in a mammography setting. Among 6 cases, n(-)/p-epi photodiode coupled with the Lanex Regular also showed the highest sensitivity of 30.5 mV/mR.