The drift of TEOS etch rate has been observed during MERIE oxide etch for the damascene process. The etch rate typically fluctuates between 5300 Angstrom/min and 6000 Angstrom/min. Studies using fluorocarbon-based chemistry show a normal TEOS etch rate when the chamber wall is heavily coated with polymer deposition. On the other hand, a lower etch rate appears when the chamber has less deposition. Hysteresis behavior has been observed during the etch rate of TEOS, as well as emission intensity trends of F, CFx(x = 1 similar to 3), and SiF. From the observed emission intensity variation of F, CFx, and SiF, a model is proposed to explain the impact of chamber wall polymer deposition on the etch rate of TEOS. This model includes a mechanism of etch rate enhancement by embedding oxygen in the chamber wall polymer. From the correlation between etch rate and emission intensity, it clearly shows that F is directly responsible for the etch of TEOS. Compared to F, CFx plasma chemistry has a closer link to chamber wall polymer formation, but contributes less＇ in the etch of TEOS.