Incorporation of a sensor, such as an optical or laser based sensor, into a wafer edge processing unit, such as a WEE unit or mechanism. This sensor enables the WEE unit to be referenced to the wafer edge. Specifically, the sensor can be used to place a WEE unit in a fixed but accurate location at the beginning of the wafer edge expose process. Another approach is to have the WEE drive controller actively follow the edge of the wafer as it rotates during the WEE process, which has the advantage of compensating for any wafer centering errors as well as diameter and placement errors. In yet another approach, the edge sensor is used to sense and track the edge of a previous layer WEE pattern. The sensor can also facilitate the measuring of a distance from a wafer edge to a WEE edge feature.