In this paper, three low yield case studies in wafer fabrication are reviewed. These issues/problems include thicker gate oxide due to contamination from the wafer fab process, QBD failures due to silicon crystalline defects caused by charging during the BN+ implant process and memory failures relating to tunnel oxide defects in EEPROM devices. Chemical deprocessing techniques, 155 Wright Etch, Scanning Electron Microscope, Transmission electron microscopy & Secondary Ion Mass Spectroscopy were used to identify the root causes. Some new chemical deprocessing techniques in exposing the tunnel window & oxide for the memory cell failures were developed. Moreover, some new failure mechanisms relating to the low yield due to thicker gate oxide, silicon crystalline defects and QBD failure were also discussed.