Method of processing a wafer

A method of processing a wafer comprising:

• a bonded wafer assembly forming step of forming a bonded wafer assembly by bonding one of opposite surfaces of a first wafer to one of opposite surfaces of a second wafer, the first wafer having a device region where a plurality of devices are formed in the one of the opposite surfaces, an outer circumferential excessive region surrounding the device region, and a beveled outer circumferential edge;
• a separation layer forming step of applying a laser beam to the outer circumferential excessive region of the first wafer while positioning a focused spot of the laser beam in a region spaced a predetermined distance radially inwardly from the outer circumferential edge, on an inclined plane that is progressively closer to the one of the opposite surfaces of the first wafer toward the outer circumferential edge, thereby forming a separation layer shaped as a side surface of a truncated cone that is inclined from the one of the opposite surfaces of the first wafer to an other one of the opposite surfaces of the first wafer;
• a grinding step of, after the separation layer forming step has been carried out, grinding the first wafer of the bonded wafer assembly from the other one of the opposite surfaces of the first wafer to thin down the first wafer to a predetermined thickness;
• and
• a detecting step, performed by a controller including a processor, of, while the grinding step is being carried out or after the grinding step has been carried out, applying measurement light with a light source to the outer circumferential excess region, measuring the intensity of reflected measurement light with a light detecting unit, and using an electrical signal converted, by the light detecting unit, from the measured intensity of the reflected measurement light to determine whether the outer circumferential excessive region that extends from the separation layer to the outer circumferential edge has been removed from the first wafer.