The HDP-CVD oxide deposition process has been fully investigated with the change of gas flow and RF power up to +/- 20%. The film quality is very stable with a wet etch rate ratio (WERR) of 1.5 relative to thermal oxide. The uniformity for both sputter and deposition are all below 3%, and the D/S ratio is only sensitively affected by bias RF power and side SiH4 flow. Three important factors, namely bias-RF, backside He flow and O2/SiH4 ratio, which affect strongly on the film quality, are studied in detail by measurement of film refractive index, stress, water absorption, WERR, and by pressure cook test (PCT). Lower backside He pressure as well as higher bias-RF power result in higher wafer temperature and better film quality. After PCT, the films do not show any increase in the silanol content and WERR. A liner process with lower bias-RF power is discussed to protect corner sputtering and plasma charging.