A new interferometer system has been developed at Lam Research Corp.. It provides capability of in-situ etch depth measurement and can be used as an endpoint device when the traditional OES system fails. It consists of a light source and a CCD array spectrometer. A fiber optic cable delivers the light from the lamp to the center of a wafer being processed at normal incident angle. The CCD array spectrometer records the spectrum of the reflected light from the wafer surface. The optical window for light access is engineered to prevent polymer deposition. The analysis of the spectrum yields the thickness information of the films on the wafer. Frequently, the etch depth can be obtained by simply counting the interferometric fringes. With the use of a deep UV light source, an etch depth as shallow as 1000A can be accurately measured. The advantage of the interferometer is demonstrated for applications such as shallow trench isolation (STI) and recess etch. The traditional OES system cannot provide an endpoint signal since there is no stopping layer. Using the interferometer, the etch depth is monitored and etching process can be stopped when the desired depth is reached. In the case of poly gate etch with a very thin (10A-20A) gate oxide underlayer, the interferometer can be used to predict the remaining poly layer thickness so the process can be changed from the main etch process to the overetch process before the oxide layer is exposed. A typical overetch process has greater than 200:1 selectivity to underlayer, while the main etch selectivity typically is much less. Thus, punching-through of the oxide layer is prevented. The details of the interferometer and process results will be presented and discussed.