Method of processing monocrystalline silicon wafer

A method of processing a monocrystalline silicon wafer fabricated such that a particular crystal plane included in crystal planes {100} is exposed on each of face and reverse sides of the monocrystalline silicon wafer, having a plurality of devices formed on the face side, and having a beveled outer circumferential region, by applying a laser beam having a wavelength transmittable through the monocrystalline silicon wafer while positioning a focused spot of the laser beam within the monocrystalline silicon wafer, thereby forming a peel-off layer within the monocrystalline silicon wafer, and thereafter separating the monocrystalline silicon wafer along the peel-off layer that functions as separation initiating points, the method comprising:

• an affixing step of affixing the face side of the monocrystalline silicon wafer to a face side of a support wafer;
• a first peel-off layer forming step of applying the laser beam from the reverse side of the monocrystalline silicon wafer to an annular first region of the monocrystalline silicon wafer that is positioned radially inwardly of the outer circumferential region of the monocrystalline silicon wafer, thereby forming a first peel-off layer that functions as separation initiating points between a region of the monocrystalline silicon wafer where the devices are formed and the outer circumferential region thereof;
• a second peel-off layer forming step of applying the laser beam from the reverse side of the monocrystalline silicon wafer to a second region of the monocrystalline silicon wafer that is positioned radially inwardly of the outer circumferential region of the monocrystalline silicon wafer, thereby forming a second peel-off layer that functions as separation initiating points between a part of the monocrystalline silicon wafer that belongs to the face side thereof and a part of the monocrystalline silicon wafer that belongs to the reverse side thereof;
• and
• a separating step of, after the affixing step, the first peel-off layer forming step and the second peel-off layer forming step, exerting external forces on the monocrystalline silicon wafer, thereby separating the monocrystalline silicon wafer along the first peel-off layer and the second peel-off layer as the separation initiating points,
• wherein the second peel-off layer forming step includes
• a laser beam applying step of applying the laser beam to a straight region of the monocrystalline silicon wafer along a first direction included in the second region while moving the monocrystalline silicon wafer and the focused spot relatively to each other along the first direction that lies parallel to the particular crystal plane and is inclined to a particular crystal orientation included in crystal orientations <100> at an angle of 5° or less, and
• an indexing feed step of moving a position in the monocrystalline crystal wafer where the focused spot is formed by the applied laser beam along a second direction parallel to the particular crystal plane and perpendicular to the first direction, and
• the laser beam applying step and the indexing feed step are repeatedly carried out to form the second peel-off layer.