The STI Fill Post Clean is a critical wet processing step positioned immediately after the deposition of the STI conformal CVD oxide and prior to the high-temperature densification anneal P3.The upstream conformal CVD process, which may utilize TEOS-O3 or flowable CVD (FCVD) techniques, leaves the trench filled with an initially low-density, metastable dielectric network P1, P3.If airborne contaminants, metallic traces, or loosely bound particles from the deposition chamber remain on this surfac