Diamond substrate producing method

A diamond substrate producing method for producing a diamond substrate from a diamond ingot having a flat surface formed on a (001)-plane as a crystal plane, said diamond substrate producing method comprising:

• a focal point setting step of setting a focal point of a laser beam having a transmission wavelength to diamond inside said diamond ingot at a predetermined depth from said flat surface of said diamond ingot, said predetermined depth corresponding to a thickness of said diamond substrate to be produced;
• a belt-shaped separation layer forming step of applying said laser beam to said diamond ingot as relatively moving said diamond ingot and said focal point in a [110]-direction perpendicular to a (110)-plane as another crystal plane, after performing said focal point setting step, thereby forming a belt-shaped separation layer extending in said [110]-direction inside said diamond ingot at said predetermined depth;
• an indexing step of relatively moving said diamond ingot and said focal point in an indexing direction parallel to said (001)-plane and perpendicular to said [110]-direction, after performing said belt-shaped separation layer forming step;
• a planar separation layer forming step of repeating said belt-shaped separation layer forming step and said indexing step to thereby form a planar separation layer parallel to said (001)-plane inside said diamond ingot, said planar separation layer being composed of a plurality of belt-shaped separation layers arranged side by side in said indexing direction;
• and
• a separating step of separating said diamond substrate from said diamond ingot along said planar separation layer after performing said planar separation layer forming step.