Endpoint determination for recess etching to a precise depth

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. The model preferably takes into account each of a weakening of the reflectance signal as the recess or trench becomes deeper, any residual oxide layer breakthrough step performed prior to etching the recess or trench and preferably as well any etching of the mask or top reference layer. An etch stop time is determined based on the etch rate, the etch start time and the predetermined recess or trench depth. Preferably, a software timer will then trigger the endpoint such that etching is stopped at the determined etch stop time.