Precision etching of semiconductor wafers without continuous monitoring is not possible due to the variability of etch rate with time during etching, and due to the dependence of etch rate upon temperature, age, and rate of stirring of the etchant. Therefore, a method has been developed whereby the thickness of semiconductor wafers can be continuously measured to submicron precision during etching. Accuratein situmonitoring of wafer thickness is accomplished through the use of internal reflection interferometry. Experimental results indicate that a precision of 0.1 &mgr; is routinely achieved in gallium arsenide and silicon wafers with the interferometer source a 3.39 &mgr; wavelength helium‐neon laser. Infrared absorption data show that most semiconductors of current interest are sufficiently transparent at this wavelength to allow a wide variety of application of the technique.