Spectral response improvement of CMOS APS pixel through lateral collection

A photodiode (PD) type complementary metal oxide semiconductor (CMOS) active pixel sensor (APS) composes a reverse biased PN-junction for photon conversion and charge storage. Peripheral junction of photodiode plays an important role on collecting photogenerated electrons in the vicinity of photodiode edges through lateral collection. Thus, lateral collection contribution would be improved by increasing total peripheral junction length of the photodiode. In this paper, photodiode peripheral utilization effect on the pixel´s spectral response was reported. Spectral response of an 18mum times 18mum photodiode CMOS APS pixel was improved by opening multiple circular openings on the photodiode. A CMOS APS imager test chip was designed, fabricated and tested in 0.5mum, 5V, 2P3M CMOS process containing 424 times 424 pixel arrays with smaller sub arrays for multiple pixel designs. Four pixels with 7, 11, 14, and 17 circular, 1.6mum diameter openings were designed and placed on the same imager chip along with one reference pixel with no openings. Spectral response of the pixels were measured and compared. It was found that increased photodiode peripheral improves spectral response of APS pixel by more than 10% in blue spectrum with the expanse of increased dark current