Substrate dividing method

A semiconductor chip manufacturing method wherein a silicon substrate having a plurality of circuit devices formed in a matrix on a front face of the silicon substrate is divided into the circuit devices to obtain semiconductor chips, the method comprising:nirradiating the silicon substrate with laser light, thereby forming a plurality of modified regions embedded within the silicon substrate along each line of a plurality of cutting lines arranged in a grid running between the circuit devices which are adjacent to one another;reducing a thickness of the silicon substrate after forming the modified regions so that the silicon substrate is thinned so as to have a thickness not larger than 50 μm; andcutting the silicon substrate by reducing the thickness of the silicon substrate, or by fracture in the silicon substrate of which the thickness is reduced.