Hexagonal silicon oxide defects induced by an ex-situ remote plasma dry etch pre-clean process (SiCoNiTM) were observed at the NPN emitter step of a 40 nm BiCMOS device, leading to high defect density and device yield loss. In this work, a systematic investigation was performed to identify the root cause and formation mechanism of these defects. Comprehensive characterization and experiments reveal that fluorosilicate byproducts trapped in the frontside silicon nitride layer interact with moisture absorbed in the backside TEOS-based silicon oxide during FOUP storage, leading to localized silicon oxide crystallization. Although aggressive annealing can eliminate the defects, it deteriorates the doping profile and device performance. In contrast, removal of the wafer backside TEOS layer effectively suppresses defect formation without compromising device performance. This study provides practical insights into SiCoNi-induced defect formation and proposes compatible mitigation strategies for remote plasma-assisted dry clean processes in advanced technologies.