WAFER THINNING METHOD

A wafer thinning method for a wafer includes bonding a first support substrate to a first surface of the wafer, positioning a focused spot of a laser beam with a wavelength transmittable through the wafer from a second surface side of the wafer inside the wafer, and applying the laser beam while moving the focused spot and the wafer relative to each other in a direction parallel to the second surface, thereby forming separation start points each including a modified layer parallel to the second surface and cracks extending from the modified layer, applying an ultrasonic wave to the wafer from the second surface side of the wafer, bonding a second support substrate to the second surface of the wafer, and separating the wafer at the separation start points into a first wafer having the first surface and a second wafer having the second surface, thereby thinning the wafer.