Laser processing method for wafer

A laser processing method for a wafer in which an epitaxial layer is laminated to a top surface of a substrate, and a device is formed in each of regions demarcated by a plurality of intersecting planned dividing lines on a top surface of the epitaxial layer, the laser processing method comprising:

• a pulse thinning-out step of thinning out pulses of a pulsed laser beam of a wavelength transmissible through the wafer, the pulsed laser beam being oscillated from a laser oscillator, by first thinning-out means at first predetermined intervals;
• an amplifying step of amplifying the pulsed laser beam after the pulse thinning-out step is performed;
• a burst pulse laser beam generating step of generating a burst pulse laser beam by continuously and intermittently thinning out the pulsed laser beam amplified in the amplifying step by second thinning-out means at second predetermined intervals;
• a test substrate preparing step of preparing a test substrate having a material and a thickness identical to a material and a thickness of the substrate of the wafer;
• a shield tunnel forming step of linearly forming a plurality of shield tunnels each constituted of a fine hole and an amorphous region surrounding the fine hole at predetermined intervals within the test substrate by holding the test substrate by a chuck table of a laser processing apparatus, irradiating the test substrate with the burst pulse laser beam while positioning a condensed region of the burst pulse laser beam within the test substrate, and processing-feeding the chuck table, while changing time intervals of a plurality of pulses constituting the burst pulse laser beam;
• a rupture strength measuring step of measuring a rupture strength when the test substrate is ruptured along the plurality of shield tunnels after the shield tunnel forming step is performed;
• a minimum value calculating step of calculating the time intervals of the pulses that minimize the rupture strength of the test substrate after the rupture strength measuring step is performed a plurality of times while changing the time intervals of the plurality of pulses constituting the burst pulse laser beam;
• and
• a laser processing step of linearly forming a plurality of shield tunnels each constituted of a fine hole and an amorphous region surrounding the fine hole at predetermined intervals in an inner part of the wafer, the inner part corresponding to a planned dividing line, by holding the wafer by the chuck table of the laser processing apparatus, irradiating the wafer with the burst pulse laser beam while positioning the condensed region of the burst pulse laser beam having the calculated time intervals of the pulses, the calculated time intervals being calculated in the minimum value calculating step, in the inner part of the wafer, the inner part corresponding to the planned dividing line, and processing-feeding the chuck table, after the minimum value calculating step is performed.