The variation over time of the etch rates in <0.5 mu m diameter contacts is a serious manufacturing concern. This etching variation originates in a deposition variation resulting from changes in the chamber’s temperature and carbon species coverage. This article reports on a real time optical emission method for monitoring this varying deposition, which is directly related to the contact etch rate. Etch rate variation has been deliberately induced by varying the amount of the oxygen postetch treatment. With insufficient O-2 cleaning, the optical emission indicates that the photoresist residue is not completely removed from the chamber. This leads to decreasing contact etch rate as the cassette of wafers is etched. This can be detected by noting the variation in the time dependence of either the Ct or SIF optical emission signal. The work reported here was done in an Applied Materials HDP 5300, but the authors have confirmed that a similar optical emission technique can be used to determine contact etch rate variation in the LAM TCP.