Enhanced sensitivity achievement using advanced device simulation of multifinger photo gate active pixel sensors

Abstract

A 2-dimensional device simulation of Multi finger active pixel sensors is investigated for obtaining enhanced pixel sensitivity. Photo gate APS use a MOS capacitor that can capture incident illumination with a potential well created under the photo gate. The major drawback of such a technology is the absorption of shorter wavelength by the polysilicon gate resulting in a higher sensitivity in the red visible spectrum than in the blue range. In our previous work we implemented 0.18μm CMOS standard and multi fingered photo gate design where the enclosed detection area is divided by 3, 5 and 7 fingers. The experimental results showed that fringing field created potential wells for the 3 and 5 finger photo gate designs have 1.7 times higher collection of photo carriers over the standard photo gate. The device simulation showed that fringing fields from the edges of the poly gates created potential wells that fully covered the open silicon areas allowing light conversion without the optical absorption in the poly silicon gates. Extending simulations to 0.5 μm, 0.25 μm and 0.18 μm multifinger poly gates showed that the fringing fields stayed the same width as the gates shrunk, so that as the number of fingers increased the potential well in the open areas became more uniform. The device sensitivity based on the potential well locations, and previous experimental results, suggested peak efficiencies for the 0.5 μm design as 7 fingers, 0.25 μm at 9 fingers and 0.18 μm at 11 fingers. Peak efficiency was projected to be 2.2 times that of a standard photogate.