Device modeling and advanced 2-d TCAD simulation of multifinger photogate APS for enhanced sensitvity

Multifinger photogate Active Pixel Sensor (APS) design employs a finger-like photogate pattern for better collection of light entering through the gaps between gate fingers. This design feature compensates for the significant light absorption caused by the polysilicon gate of the present day CMOS image sensors and has been experimentally verified. In order to optimize the multifinger design, a systematic theoretical analysis is required.In this study, advanced TCAD device simulations were used to analyse and predict the performance of multifinger photogate APS fabricated through CMOS 0.18um technology with a device physics perspective. The formation of the potential well under the photogate and its spreading in the open spaces between adjacent gate fingers is observed. Sensitivities of various designs with reducing gate widths of 0.5µm, 0.25µm and 0.18µm and increasing the number of gate fingers were estimated to address the trade off between open spacing between the fingers and the strength of potential well created in the open area. Pixel response of all the designs were measured over visible spectrum using optical simulations to predict the optimum design with maximum sensitivity gain.