Etch depth into bulk Si wafers was determined in real time using spectroscopic ellipsometry (SE). The mechanism that allows this is lateral interference, produced by patterning. For a pattern period much larger than an optical wavelength, diffraction effects can be ignored. Here, the Si was patterned with photoresist and etched in a reactive ion etching system. During etching SE data were analyzed in real time for etch depth, and etching was stopped when the fitted depth reached its target value of 500 nm. In addition to demonstrating accurate control of etch depth, etch rates were rapidly determined under varying etching conditions. (C) 2004 Elsevier B.V. All rights reserved.