Performance Evaluation of Pinning Potential Adjustment in Two-Dimensional/Three-Dimensional Image Sensor

Standard pinned photodiode pixels with various pinning potentials are fully evaluated in two different operation modes: color image capture and depth map extraction. The diode pinning potential affects the quality of both color and depth images. On the one hand, it has an impact on the full well capacity and maximum signal-to-noise ratio (SNR) which determine the color pixel performance. On the other hand, it influences the charge transfer speed. Higher speed enables the use of higher modulation frequency which is beneficial to improve the depth accuracy in time-of-flight (TOF) measurement. Prototype pixel arrays with three types of pinning potentials have been fabricated in a 0.11-μm CMOS imaging technology by controlling the energy level of a buried implant layer. The pixels with low and high pinning potentials provide color images with 22- and 36-dB SNRs and 28 and 16 MHz of modulation frequencies in TOF operation with 40% demodulation contrast, respectively.