To address the question whether it is possible to performinsituend point detection during etching using ellipsometry on patterned wafers, a number of experiments were performed on wafers that were covered with a photoresist line pattern on an oxide‐covered silicon substrate. Two situations were investigated, one in which the probe beam was directed parallel to the line pattern, and one in which the probe beam was perpendicular to the lines. For each situation, a separate ellipsometric model was developed. Employing photoresist and SiO2etch rates measured with blanket wafers, a good fit between the experimental curves and the simulations was obtained. Though the agreement between modeling and experiment is good, it is shown that the applicability of ellipsometry for end point detection on patterned wafers depends on the aspect ratio of etched holes, the selectivity of the etching process, and the pattern factor (the unmasked area fraction of the wafer). For comparison, experiments and simulations were also performed with a laser interferometer at normal incidence. In this case, only the selectivity and the pattern factor are important. It has been shown that the selectivity of the process becomes less important if the derivative of the reflectivity is used as an end point signal instead of the reflectivity itself. To optimize the end point signal for this method the wavelength can be used to maximize the change in the reflectivity slope at the end point of the process.