In the Shallow Trench Isolation (STI) module, the SiN hard mask deposition follows pre-cleaning and serves as the primary structural boundary for the active area P1.This silicon nitride layer is typically deposited over a thin thermal pad oxide to buffer interfacial stress between the nitride and the underlying silicon lattice P2.Unlike the subsequent SiO hard mask—which acts as a disposable pattern-transfer or anti-reflective layer—this SiN layer persists through the deep trench etch and acts a