Method for determining thickness of a semiconductor substrate at the floor of a trench

Apparatus and method for exposing a selected feature of an integrated circuit device such as a selected portion of the metallization layer, from the backside of the integrated circuit substrate without disturbing adjacent features of the device such as the active semiconductor regions. This is performed using an FIB (focused ion beam) etching process in conjunction with observation by an optical microscope to form a trench through the substrate. The process includes a precise optical endpointing technique to monitor the remaining thickness of the semiconductor substrate at the floor of the trench. It is important to terminate etching of the trench so that the trench floor extends as close to the active semiconductor structures as desired and yet is not detrimental to device operation. This is done without introducing a need for any additional tool. This is carried out using an infra-red optical technique which observes the interference fringes generated by the reflections from the silicon substrate surface and from semiconductor device circuitry layers to quantify the remaining semiconductor substrate thickness in the trench.