Method for forming a groove on a surface of flat plate formed of a nitride semiconductor crystal

A method for forming a groove on a surface of a flat plate formed of a nitride semiconductor crystal having an A-axis, a C-axis, and an M-axis, the method comprising:

• (a) moving a disk-shaped dicing blade along a direction of the A-axis to form a first inclined surface and a second inclined surface on the surface of the flat plate, wherein
• the flat plate has a principal surface;
• the disk-shaped dicing blade has an edge along the circumference thereof;
• the edge is in contact with the flat plate;
• the following mathematical formulae (I), (II), and (III) are satisfied:
• 45 degrees≦θb−a≦60 degrees (I)
• 45 degrees≦θb+a≦60 degrees (II)
• 0 degrees≦|a|≦7.5 degrees (III)
• where
• the angle θb represents an angle formed between a surface of the edge and a radial direction of the disk-shaped dicing blade in a cross-section which includes the M-axis and the C-axis;
• the angle a represents an angle formed between the principal surface and the M-axis;
• and
• each of the first inclined surface and the second inclined surface has an arithmetical mean deviation Ra of not less than 0.046 and not more than 0.131.