Patent ID: 8506356

Claim:
An apparatus for chemical mechanical polishing of a wafer, comprising: a rotatable platen to hold a polishing pad, the platen having a hole therein, wherein the hole extends entirely through the platen; a polishing head for holding the wafer against the polishing pad; an optical monitoring system including a light source to direct a light beam through the aperture toward the wafer from a side of the wafer contacting the polishing pad and a detector to receive reflections of the light beam from the wafer, wherein the light source is stationary, the detector is stationary and the hole intermittently provides a pathway for the light beam to impinge on the wafer; and a position sensor for sensing when the hole is adjacent the wafer such that the light beam generated by the light source can pass through the hole and impinge on the wafer, wherein the position sensor includes a flag and an optical interrupter-type sensor and the flag is attached along a portion of a periphery of the platen and the sensor is mounted to a chassis at the periphery of the platen.