TSMC patent application US20130207218 "
Novel Condition Before TMAH Improved Device Performance" by En-Ting Lee, Kun-El Chen, Yu-Sheng Wang, Chien-Chung Chen, and Huai-Tei Yang proposes to improve remaining thickness uniformity in BSI sensors (TMAH is Tetramethylammonium Hydroxide etchant).
It's known that the backside etch stop in bulk BSI process is not that efficient as one in SOI. So, some systematic backside etch variations are inevaitable. TSMC proposes to learn these variation patterns and put an implant on the backside to compensate them:
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