US Patent 7892863 Measuring apparatus

Measuring apparatus for monitoring the position of the center of mass of a semiconductor wafer is disclosed. The apparatus includes a wafer support (14) with a ledge for supporting an edge of a wafer (2) when it is lifted at a detection point by a probe (16). The probe (16) is connected to a force sensor (18) which senses a force due to a moment of the wafer about a fulcrum (4) on the wafer support (14). Moment measurements are taken at a plurality of detection points and a processing unit calculates the position of the center of mass from the moment measurements. Changes in wafer mass distribution (e.g. due to faulty treatment steps) which cause movement of the center of mass can be detected.