Wafer manufacturing method
A wafer manufacturing method includes a separation layer forming step of forming a separation layer including a plurality of modified regions within a workpiece by moving the workpiece and a condensing point of a pulsed laser beam having a wavelength transmittable through the workpiece relative to each other at a predetermined speed along a processing feed direction in a state in which the workpiece is irradiated with the laser beam, and the condensing point of the laser beam is located at a predetermined depth position of the workpiece, and a separating step of separating a wafer from the workpiece with the separation layer as a starting point, in the separation layer forming step, a repetition frequency of the laser beam is set according to pulse energy and the relative predetermined speed between the workpiece and the condensing point such that the separation layer can be formed.
1 . A wafer manufacturing method for manufacturing, from a workpiece as an ingot of gallium nitride or a single crystal substrate of gallium nitride, a wafer of gallium nitride having a thickness smaller than a thickness of the workpiece, the wafer manufacturing method comprising:
a holding step of holding one surface side of the workpiece by a holding table;
a separation layer forming step of forming a separation layer including a plurality of modified regions within the workpiece by moving the workpiece and a condensing point of a pulsed laser beam having a wavelength transmittable through the workpiece relative to each other at a predetermined speed along a processing feed direction in a state in which the workpiece is irradiated with the pulsed laser beam from another surface side of the workpiece, the other surface side being located on an opposite side from the one surface of the workpiece, and the condensing point of the pulsed laser beam is located at a predetermined depth position of the workpiece; and
a separating step of separating the wafer from the workpiece with the separation layer as a starting point, wherein,
in the separation layer forming step, a repetition frequency of the pulsed laser beam is set according to pulse energy as energy of the pulsed laser beam per pulse and the relative predetermined speed between the workpiece and the condensing point such that the separation layer is formed;
wherein the repetition frequency is a frequency defined by a reciprocal of a cycle of a plurality of pulse groups in the laser beam in a burst mode,
in the separation layer forming step, the laser beam being applied in a burst mode.
2 . The wafer manufacturing method according to claim 1 , wherein,
in the separation layer forming step, the repetition frequency of the pulsed laser beam is set to be equal to or higher than 25 KHz.
3 . The wafer manufacturing method according to claim 1 , wherein,
in the separation layer forming step, the repetition frequency of the pulsed laser beam is set to be equal to or higher than 50 kHz.
4 . The wafer manufacturing method according to claim 1 , wherein,
in the separation layer forming step, the predetermined speed is set such that a distance between the plurality of modified regions adjacent to each other along the processing feed direction is equal to or less than 17.5 μm.
5 . The wafer manufacturing method according to claim 4 , wherein,
in the separation layer forming step, a lower limit value of the pulse energy of the pulsed laser beam per pulse is set to be equal to or higher than 1.60 μJ and equal to or lower than 4.00 μJ.
6 . The wafer manufacturing method according to claim 1 , wherein
an angle formed between the processing feed direction in the separation layer forming step and crystal directions represented by
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in a (0001) plane of the workpiece is equal to or less than 10°;
wherein the processing feed direction is linear.
7 . The wafer manufacturing method according to claim 5 , wherein even in a case where the pulse energy is relatively decreased and the relative predetermined speed between the workpiece and the condensing point is relatively raised (that is, throughput is increased), the separation layer is formed by setting the repetition frequency above a predetermined threshold, wherein
the lower limit value of the pulse energy of the pulsed laser beam per pulse is set to be equal to or higher than 1.60 μJ and equal to or lower than 4.00 μJ,
the predetermined speed is set between 250 mm/s and 1312.5 mm/s; and
the repetition frequency of the pulsed laser beam is set to be equal to or higher than 25 kHz.
8 . The wafer manufacturing method according to claim 1 , wherein the ingot has a diameter of approximately 100 mm and a thickness of 500 μm.
9 . The wafer manufacturing method according to claim 1 , wherein,
in the separation layer forming step, the repetition frequency of the pulsed laser beam is set equal to or higher than 1 kHz and equal to or lower than 100 KHz.
10 . The wafer manufacturing method according to claim 1 , wherein,
in the separation layer forming step, a plurality of condensing points aligned with each other along a direction orthogonal the processing feed direction are formed to make corresponding orthogonally spaced modified regions, wherein
intervals between the plurality of condensing points aligned with each other along the direction orthogonal the processing feed direction to are set at a predetermined value equal to or more than 5 μm and equal to or less than 20 μm.
11 . The wafer manufacturing method according to claim 10 , wherein, the intervals between the plurality of condensing points aligned with each other along the direction orthogonal the processing feed direction to are set to 12.5 μm.
12 . The wafer manufacturing method according to claim 1 , wherein, in the separating step, the another surface side of the workpiece is sucked and held by a suction head and the one surface side of the workpiece is held by a chuck table and the suction head and the chuck table are moved relatively apart from each other.
13 . The wafer manufacturing method according to claim 12 , wherein, in the separating step, prior to the suction head and the chuck table being moved relatively apart from each other, an external force is applied to the workpiece.
14 . The wafer manufacturing method according to claim 1 , wherein the wafer has a thickness of approximately 50 μm to 60 μm.
15 . The wafer manufacturing method according to claim 1 , wherein a pitch between a plurality of modified regions adjacent to each other along the processing feed direction is a value obtained by dividing the predetermined speed by the repetition frequency,
wherein the pitch is set such a degree that the heat of the laser beam used for the formation of an immediately preceding modified region can be utilized for the formation of a next modified region.
16 . The wafer manufacturing method according to claim 15 , wherein
the pitch is set to 10 μm,
the pulse energy is set to at least 2 μJ when the repetition frequency is set to 10 kHz, and the predetermined speed is set to 100 mm/s;
the pulse energy is set to at least 1.6 μJ when the repetition frequency is set to 25 kHz, and the predetermined speed is set to 250 mm/s;
the pulse energy is set to at least 1.6 μJ when the repetition frequency is set to 50 kHz, and the predetermined speed is set to 500 mm/s;
the pulse energy is set to at least 1.6 μJ when the repetition frequency is set to 75 kHz, and the predetermined speed is set to 750 mm/s; and
the pulse energy is set to at least 1.6 μJ when the repetition frequency is set to 100 kHz, and the predetermined speed is set to 100 mm/s.
17 . The wafer manufacturing method according to claim 16 , wherein
the pitch is set to 12.5 μm,
the pulse energy is set to at least 2.5 μJ when the repetition frequency is set to 10 kHz, and the predetermined speed is set to 125 mm/s;
the pulse energy is set to at least 2 μJ when the repetition frequency is set to 25 kHz, and the predetermined speed is set to 312.5 mm/s;
the pulse energy is set to at least 1.8 μJ when the repetition frequency is set to 50 kHz, and the predetermined speed is set to 625 mm/s;
the pulse energy is set to at least 1.73 μJ when the repetition frequency is set to 75 kHz, and the predetermined speed is set to 937.5 mm/s; and
the pulse energy is set to at least 1.7 μJ when the repetition frequency is set to 100 kHz, and the predetermined speed is set to 1250 mm/s.
18 . The wafer manufacturing method according to claim 15 , wherein
the pitch is set to 15 μm,
the pulse energy is set to at least 3.33 μJ when the repetition frequency is set to 3 kHz, and the predetermined speed is set to 45 mm/s;
the pulse energy is set to at least 3 μJ when the repetition frequency is set to 10 kHz, and the predetermined speed is set to 150 mm/s;
the pulse energy is set to at least 2.4 μJ when the repetition frequency is set to 25 kHz, and the predetermined speed is set to 375 mm/s;
the pulse energy is set to at least 2 μJ when the repetition frequency is set to 50 kHz, and the predetermined speed is set to 750 mm/s;
the pulse energy is set to at least 1.87 μJ when the repetition frequency is set to 75 kHz, and the predetermined speed is set to 1125 mm/s.
19 . The wafer manufacturing method according to claim 1 , wherein heat of the laser beam used for the formation of the immediately preceding modified region is utilized for the formation of a next modified region by setting the repetition frequency relatively high according to the predetermined pulse energy and the predetermined speed, and thereby bringing the modified regions into proximity to each other.