Back-Side Illuminated Photogate CMOS Active Pixel Sensor Structure With Improved Short Wavelength Response

When light of a short wavelength, typically blue light, is incident on the polysilicon gate of an image sensor, it tends to be absorbed significantly. This has hindered the extensive application of photogate (PG) CMOS image sensor. To overcome this difficulty, a new backside illuminated (BSI) photogate CMOS active pixel sensor structure is proposed. While maintaining other specifications at similar level as those of the front-side illuminated (FSI) structure, the proposed new BSI structure with inverted gate is shown to reduce the absorption effectively. Based on the 0.35 μm CMOS process, the new BSI CMOS image sensor structure is simulated with Synopsys' MEDICI two-dimensional device simulator, and the simulation results show that while maintaining the crosstalk at similar level, the collecting efficiency is improved by 14.89% in the new BSI structure.