We describe the design of a device for on-line endpoint monitoring of oxide thickness in silicon wafer etching. The endpoint is sensed by continuous in situ determination the Si02 layer thickness. The design utilizes an air-cooled Ar-laser whose plane polarized light passes through a rotating half-wave plate which alternately switches the polarization from transverse electric to transverse magnetic. The thus modulated beam is specularly reflected from the silicon wafer. AC detection instrumentation measures the normalized reflectance intensity (ITE - ITM)/(ITE + ITM ) which is a measure of the SiO2 film thickness. The normalization compensates for light intensity fluctuations.