In chemical mechanical polishing apparatus, a wafer carrier plate is provided with a cavity for reception of a sensor positioned very close to a wafer to be polished. Energy resulting from contact between a polishing pad and an exposed surface of the wafer is transmitted only a very short distance to the sensor and is sensed by the sensor, providing data as to the nature of properties of the exposed surface of the wafer, and of transitions of those properties. Correlation methods provide graphs relating sensed energy to the surface properties, and to the transitions. The correlation graphs provide process status data for process control.