Substrate manufacturing method

A substrate manufacturing method of manufacturing a substrate from a workpiece having a first surface and a second surface on a side opposite to the first surface, comprising:

• a separation layer forming step of splitting and condensing a laser beam of a wavelength that transmits through a material that makes up the workpiece, to form a plurality of focal points aligned side by side along a first direction parallel to the first surface, and with the plurality of focal points positioned inside the workpiece, moving the plurality of focal points and the workpiece relative to each other along a second direction orthogonal to the first direction and parallel to the first surface such that, inside the workpiece, a separation layer including a plurality of modified regions formed in a manner centering around the respective plurality of focal points and cracks propagating from the plurality of modified regions is formed;
• an index feeding step of moving an area in which the plurality of focal points are formed and the workpiece relative to each other along the first direction;
• and
• a separating step of, after performing the separation layer forming step and the index feeding step along the first and second directions, separating the substrate from the workpiece with use of the separation layers as starting points,
• wherein, in the separation layer forming step, the laser beam is split and condensed such that, in association with the formation of the plurality of modified regions, a first volume expansion in a vicinity of at least one focal point that is among the plurality of focal points and that is formed on a side of a center in the second direction is smaller than a second volume expansion in a vicinity of another focal point that is among the plurality of focal points and that is formed on a side of an end in the second direction simultaneously with the formation of the at least one focal point.