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GOI (Gate Oxide Integrity) improvement in 65nm SAC OX (Sacrificial Oxide) free process integration was studied in this paper. After studying the effects of various processes in STI (Shallow Trench Isolation) module with SAC OX free scheme on GOI, it was found that the dominant process that could degrade GOI was the STI HDP (High Density Plasma) gap fill process. STI HDP gap fill process could cause Si damage on AA corners, also named as AA clipping, which will result in irregular corners and hence a thinner gate oxide formation at imperfect AA corners. This was observed by a TEM (transmission electron microscopy) study. In SAC OX free scheme, the exposed AA Si corners that were damaged during the HDP process did not get consumed and repaired while they would have gotten in the scheme with SAC OX and hence degraded GOI. By optimizing the STI gap fill processes and hardware configuration, the AA clipping can be greatly reduced and GOI can be improved to the required level.