Fabrication of electrical contacts in semiconductor device technology can be hampered by the presence of thin oxide layers that may form at the bottom of the contact or via holes as a consequence of the reactive ion etching process. In order to achieve low contact resistances after metallization this oxide thickness must be below some critical value. This article describes a method based upon Auger electron spectroscopy for the thickness determination of such thin oxide films at the bottom of micron-sized contact holes prior to metallization. In this technique, the different Si KLL line shapes of elemental and oxidized silicon are utilized for fitting the measured spectrum with a superposition of Si and SiO2 spectra; the oxide thickness follows from the fitting coefficients. The method is applicable to oxide thicknesses up to similar to 10 nm and to contact holes with aspect ratios below similar to 1. Systematic errors due to scattering and shadowing effects, and to electron-beam induced damaging are discussed. The method has proven to be extremely important in optimizing reactive ion etching processes.