| 主权项 |
1. A wafer thinning method for thinning a wafer formed from an SiC substrate having a first surface, a second surface opposite to said first surface, a c-axis extending from said first surface to said second surface, and a c-plane perpendicular to said c-axis, said first surface of said SiC substrate having a device area where a plurality of devices are formed and a peripheral marginal area surrounding said device area, said wafer thinning method comprising:
an annular groove forming step of forming an annular groove on said second surface of said SiC substrate in an annular area corresponding to the boundary between said device area and said peripheral marginal area in a condition where a thickness corresponding to a finished thickness of said wafer after thinning is left; a separation start point forming step of setting the focal point of a laser beam having a transmission wavelength to said SiC substrate inside said SiC substrate in a central area surrounded by said annular groove at a predetermined depth from said second surface, which depth corresponds to the finished thickness of said wafer after thinning, and next applying said laser beam to said second surface as relatively moving said focal point and said SiC substrate to thereby form a modified layer inside said SiC substrate at said predetermined depth in said central area and also form cracks extending from said modified layer along said c-plane, thus forming a separation start point; and a wafer thinning step of applying an external force to said wafer after performing said separation start point forming step, thereby separating said wafer into a first wafer having said first surface of said SiC substrate and a second wafer having said second surface of said SiC substrate at said separation start point, whereby the thickness of said wafer is reduced to the finished thickness of said wafer after thinning as the thickness of said first wafer having said first surface of said SiC substrate, and a ring-shaped reinforcing portion is formed on a back side of said first wafer in a peripheral area corresponding to said peripheral marginal area; said separation start point forming step including
a modified layer forming step of relatively moving the focal point of said laser beam in a first direction perpendicular to a second direction where said c-axis is inclined by an off angle with respect to a normal to said second surface and said off angle is formed between said second surface and said c-plane, thereby linearly forming said modified layer extending in said first direction, andan indexing step of relatively moving said focal point in said second direction to thereby index said focal point by a predetermined amount. |
