The effect of oxygen flow rate on bi-level contact etch was studied by observing uv-visible emission from the plasma, during CHF3/CO/O-2 etching of di-electric layers consisting of SiO2 and SiNx. The emission intensity of CN at 387 nm drifted progressively from wafer to wafer during plasma etch. Such a phenomenon became more obvious when using low or high oxygen flow rate, whereas for intermediate flow rates, no significant drift of emission intensity was observed. The critical dimension (CD) bias of each wafer showed a strong correlation with CN emission intensity. Possible mechanisms for such an intensity drift phenomenon are proposed. The drift of emission intensity indicates that the contribution of chamber wall polymers in wafer etching is non-negligible. The CN emission intensity is an indication of the magnitude of etching rate. Our results suggest that the variation of plasma emission intensity might be used as an index for in-line monitoring of CD bias fluctuation.