Abstract: A method for plasma etching a shallow recess or shallow trench at a predetermined depth by illuminating a wafer with a light source and using a spectrometer to receive the light reflected from the wafer begins with a step of detecting an etch start time, either by detecting a time of plasma ignition, as extracted from reflectance data, or a time extracted from the reflectance data when a wafer reflectance signal is observed to begin to change after a residual layer is etched away prior to beginning a recess or trench etch. The next step is measuring a reflectance intensity of light reflected from the wafer. Preferably, a plasma background signal is removed from this measurement and an array detector is used wherein the wavelength is determined using the reflectance model. Next, an etch rate is determined by fitting data representing the collected reflectance signal to the wafer reflectance model as a function of time, and extracting the etch rate from the model.