Workpiece cutting method

An object cutting method, comprising:

• a first step of preparing an object to be processed including a single crystal silicon substrate and a functional device layer provided on a first main surface side;
• a second step of, after the first step, irradiating the object with a laser light to form at least one row of modified regions in the single crystal silicon substrate along each of a plurality of lines to cut and to form a fracture in the object so as to extend between the at least one row of modified regions and a second main surface of the object along each of the plurality of lines to cut;
• and
• a third step of, after the second step, performing reactive ion etching on the object from the second main surface side to form a groove opening to the second main surface, in the object along each of the plurality of lines to cut,
• wherein in the third step, a black silicon layer is formed on the second main surface of the object and an inner surface of the groove while the reactive ion etching is performed.