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

1. 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≦θ b−a ≦60 degrees  (I)

45 degrees≦θ b+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.

2. The method according to claim 1 , wherein

the nitride semiconductor crystal is monocrystalline.

3. The method according to claim 1 , wherein

the dicing blade is symmetrical with respect to a plane which includes the normal direction P of the principal surface, the A-axis, and a gravity center of the dicing blade.

4. The method according to claim 1 , further comprising:

(b) moving a disk-shaped dicing blade along a direction of the C-axis to form a third inclined surface and a fourth inclined surface on the surface of the flat plate, wherein

each of the third inclined surface and the fourth inclined surface has an arithmetical mean deviation Ra of not less than 0.230.