Patent ID: 12194571

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail below referring to the drawings. The present invention is not limited by the contents of description in the following embodiments. In addition, the components described below include those which can easily be conceived of by a person skilled in the art and those which are substantially the same. Further, the following configurations can be combined as required. Besides, various kinds of omission, replacement, or modification are possible within such ranges as not to depart from the gist of the present invention.

First Embodiment

A wafer forming method according to a first embodiment of the present invention will be described based on the drawings.FIG.1is a plan view of an ingot as an object of processing by the wafer forming method according to the first embodiment.FIG.2is a side view of the ingot depicted inFIG.1.FIG.3is a perspective view of a wafer produced by the wafer forming method according to the first embodiment.FIG.4is a flow chart depicting the flow of the wafer forming method according to the first embodiment.

The wafer forming method according to the first embodiment is a producing method for forming a wafer20depicted inFIG.3from a SiC ingot1which is the ingot depicted inFIGS.1and2.

(SiC Ingot)

In the first embodiment, the SiC ingot1illustrated inFIGS.1and2includes SiC (silicon carbide) and is formed in a cylindrical shape as a whole. In the first embodiment, the SiC ingot1is a hexagonal single-crystal SiC ingot.

As depicted inFIGS.1and2, the SiC ingot1has a first surface2which is a circular end face, a circular second surface3on the back side of the first surface2, and a circumferential surface4continuous with an outer edge of the first surface2and an outer edge of the second surface3. In addition, the SiC ingot1has, at the circumferential surface4, a first orientation flat5indicative of crystal orientation and a second orientation flat6orthogonal to the first orientation flat5. A length51of the first orientation flat5is longer than a length61of the second orientation flat6.

Besides, the SiC ingot1has a c-axis9inclined by an off angle α in an inclination direction8toward the second orientation flat6relative to a perpendicular7of the first surface2and a c-plane10orthogonal to the c-axis9. The c-plane10is inclined by the off angle α relative to the first surface2of the SiC ingot1. In the SiC ingot1, the inclination direction8of the c-axis9from the perpendicular7is orthogonal to the extending direction of the second orientation flat6and is parallel to the first orientation flat5. Numberless c-planes10are set in the SiC ingot1on a molecular level of the SiC ingot1. In the first embodiment, the off angle α is set to 1°, 3° or 6°, but in the present invention, the SiC ingot1can be produced with the off angle α set, for example, freely in a range of 1° to 6°. The SiC ingot1has the first surface2ground by a grinding apparatus and thereafter polished by a polishing apparatus, whereby the first surface2is formed to be a mirror surface.

The wafer20illustrated inFIG.3is produced by cutting a part of the SiC ingot1and subjecting a surface21cut from the SiC ingot1to grinding, polishing and the like. Devices are formed on a front surface of the wafer20after the wafer20is cut from the SiC ingot1. In the first embodiment, the device is a metal-oxide semiconductor field-effect transistor (MOSFET), micro electro mechanical systems (MEMS), or a Schottky barrier diode (SBD), but, in the present invention, the device is not limited to the MOSFET, MEMS, and SBD. Note that the same parts of the wafer20as those of the SiC ingot1are denoted by the same reference symbols and descriptions of them are omitted. As depicted inFIG.4, the wafer forming method according to the first embodiment includes a peeling layer forming step1001and a wafer forming step1002.

(Peeling Layer Forming Step)

FIG.5is a perspective view depicting the peeling layer forming step of the wafer forming method depicted inFIG.4.FIG.6is a plan view of the SiC ingot after the peeling layer forming step of the wafer forming method depicted inFIG.4.FIG.7is a sectional view of a major part of the SiC ingot after the peeling layer forming step of the wafer forming method depicted inFIG.4.

The peeling layer forming step1001is a step of applying, to the SiC ingot1, a laser beam31(depicted inFIG.5) of such a wavelength as to be transmitted through the SiC ingot1, with a focal point32(depicted inFIG.5) positioned at a depth corresponding to a thickness22(depicted inFIG.3) of the wafer20to be formed from the first surface2of the SiC ingot1. In addition, the peeling layer forming step1001is also a step of applying the laser beam31to the SiC ingot1to form a peeling layer23, depicted inFIGS.6and7, in the inside of the SiC ingot1.

As illustrated inFIG.5, in the peeling layer forming step1001, a laser processing apparatus30holds the second surface3side of the SiC ingot1on a chuck table33, images the SiC ingot1by unillustrated imaging means, adjusts the orientation of the SiC ingot1to a predetermined orientation, and adjusts the relative positions of a laser beam applying unit34and the SiC ingot1in horizontal directions. In the peeling layer forming step1001of the first embodiment, the laser processing apparatus30sets the second orientation flat6parallel to an X-axis direction parallel to a horizontal direction and sets the inclination direction8and the first orientation flat5parallel to a Y-axis direction parallel to a horizontal direction and orthogonal to the X-axis direction.

In the peeling layer forming step1001, the laser processing apparatus30sets a focal point32of the laser beam31applied from the laser beam applying unit34, to a position of a desired depth35(depicted inFIG.7) from the first surface2of the SiC ingot1. Note that the desired depth35is a depth corresponding to the thickness22of the wafer20.

In the peeling layer forming step1001, the laser processing apparatus30applies a pulsed laser beam31of such a wavelength as to be transmitted through the SiC ingot1from the laser beam applying unit34to the SiC ingot1, while relatively moving the chuck table33and the laser beam applying unit34in the X-axis direction at a predetermined feeding speed. As a result, as depicted inFIG.6, by the application of the pulsed laser beam31, a modified section24in which SiC is separated into Si (silicon) and C (carbon), the pulsed laser beam31applied next is absorbed in previously formed C, and SiC is separated into Si and C in a chain reaction manner is formed in the SiC ingot1along the X-axis direction, and cracks25extending from the modified section24along the c-plane10are formed. In this way, in the peeling layer forming step1001, the peeling layer23including the modified section24and the cracks25formed along the c-plane10from the modified section24is formed.

In the peeling layer forming step1001, when the laser processing apparatus30has applied the laser beam31over the whole length of the SiC ingot1in the X-axis direction, the chuck table33and the laser beam applying unit34are put into relative indexing feeding in the Y-axis direction. In the peeling layer forming step1001, the laser processing apparatus30repeats an operation of relatively moving the chuck table33and the laser beam applying unit34in the X-axis direction while applying the laser beam31and an operation of putting the chuck table33and the laser beam applying unit34into relative indexing feeding in the Y-axis direction.

As a result, the peeling layers23which include the modified section24where SiC has been separated into Si and C and cracks25and which are lowered in strength than the other parts can be formed at the desired depth35corresponding to the thickness22of the wafer20from the first surface2of the SiC ingot1, on the basis of the moving distance in the indexing feeding in the Y-axis direction. When the peeling layers23have been formed at the desired depth35of the SiC ingot1over the whole length in the Y-axis direction on the basis of the moving distance in the indexing feeding, the control proceeds to the wafer forming step1002.

(Wafer Forming Step)

FIG.8is a sectional view schematically illustrating the wafer forming step of the wafer forming method depicted inFIG.4.FIG.9is a diagram depicting an example of a voltage which an ultrasonic power source applies to an ultrasonic vibrator in the wafer forming step of the wafer forming method depicted inFIG.4.FIG.10is a diagram depicting another example of the voltage which the ultrasonic power source applies to the ultrasonic vibrator in the wafer forming step of the wafer forming method depicted inFIG.4.FIG.11is a diagram depicting a further example of the voltage which the ultrasonic power source applies to the ultrasonic vibrator in the wafer forming step of the wafer forming method depicted inFIG.4.

The wafer forming step1002is a step of immersing the SiC ingot1in a liquid42in a liquid tank41of a peeling apparatus40depicted inFIG.8, and an ultrasonic wave is applied to the SiC ingot1through the liquid42, to peel off a part on the first surface2side of the SiC ingot1with the peeling layers23as an interface and form the peeled-off part as a wafer20.

In the wafer forming step1002of the first embodiment, the second surface3side of the SiC ingot1is mounted on a mount table43in the liquid tank41accommodating the liquid42of the peeling apparatus40depicted inFIG.8, and an ultrasonic wave applying unit46including an ultrasonic vibrator45including a piezoelectric ceramic to which a voltage is applied by the ultrasonic power source44is inserted into the liquid tank41from the position indicated by a broken line inFIG.8toward the position indicated by a solid line. In the wafer forming step1002of the first embodiment, the ultrasonic wave applying unit46is immersed in the liquid42in the liquid tank41and is made to face the first surface2of the SiC ingot1on the mount table43through the liquid42.

In the wafer forming step1002of the first embodiment, a voltage is applied from the ultrasonic power source44to the ultrasonic vibrator45, to put the ultrasonic vibrator45into ultrasonic vibration. As a result, the ultrasonic vibration at a frequency according to the vibration of the ultrasonic vibrator45is propagated in the liquid42. In the wafer forming step1002of the first embodiment, the ultrasonic vibration generated in the liquid42is applied to the SiC ingot1.

In the wafer forming step1002, the ultrasonic vibration is applied to the SiC ingot1through the liquid42in the liquid tank41, whereby a part on the first surface2side as compared to the peeling layers23of the SiC ingot1is peeled off as the wafer20.

In the wafer forming step1002of the first embodiment, the ultrasonic vibration is applied to the SiC ingot1while a sweep treatment of regularly varying the frequency of the voltage applied to the ultrasonic vibrator that is the oscillation frequency of the ultrasonic vibrator45is performed, as depicted inFIGS.9,10, and11. Note that the axis of abscissas inFIGS.9,10, and11represents the time elapsed from the start of application of the voltage and the axis of ordinates represents the voltage applied to the ultrasonic vibrator45.

As depicted inFIGS.9,10, and11, in the wafer forming step1002of the first embodiment, repeating one cycle101of applying voltages of a plurality of different frequencies to the ultrasonic vibrator45is referred to as performing a sweep treatment of regularly varying the oscillation frequency of the ultrasonic vibrator. In the sweep treatment, the predetermined frequency at which the one cycle101is repeated is referred to as the frequency of the sweep treatment.

In the wafer forming step1002of the first embodiment, the frequency varied by the sweep treatment in the one cycle101of applying a plurality of different voltages to the ultrasonic vibrator45includes the resonance frequency46of the ultrasonic vibrator45, but, in the present invention, the resonance frequency46may not necessarily be included. In addition, in the wafer forming step1002, for example, in the case where the resonance frequency46of the ultrasonic vibrator45is 35 kHz, the frequency varied by the sweep treatment in the one cycle101of applying a plurality of different voltages to the ultrasonic vibrator45includes the resonance frequency46and different frequencies46′,46-1,46-1′,46-2,46-3,46-4,46-5,46-6and46-6′ such as ±1 kHz, ±5 kHz, ±10 kHz, and ±15 kHz.

Note thatFIG.9illustrates an example in which the frequency varied by the sweep treatment in the one cycle101, for example, in the case where the resonance frequency46of the ultrasonic vibrator45is 35 kHz, includes a frequency46-1of 34 kHz which is −1 kHz relative to the resonance frequency46, the resonance frequency46, and a frequency46-1′ of 36 kHz which is +1 kHz relative to the resonance frequency46. In the example depicted inFIG.9, the frequency in the one cycle101is set to the frequency46-1for a predetermined time period102, thereafter the frequency is set to the resonance frequency46for a predetermined time period103, and thereafter the frequency is set to a frequency46-1′ for a predetermined time period104.

FIG.10depicts an example in which the frequency varied by the sweep treatment in the one cycle101, for example, in the case where the resonance frequency46of the ultrasonic vibrator45is 35 kHz, includes the resonance frequency46and five frequencies46′,46-2,46-3,46-4, and46-5different from the resonance frequency46. In the example depicted inFIG.10, the frequency in the one cycle101is set to the resonance frequency46for a predetermined time period105, thereafter the frequency is set to a frequency46′ for a predetermined time period106, thereafter the frequency is set to a frequency46-2for a predetermined time period107, thereafter the frequency is set to a frequency46-3for a predetermined time period108, thereafter the frequency is set to a frequency46-4for a predetermined time period109, and thereafter the frequency is set to a frequency46-5for a predetermined time period110.

FIG.11illustrates an example in which the frequency varied by the sweep treatment in the one cycle101, for example, in the case where the resonance frequency46of the ultrasonic vibrator45is 35 kHz, includes a frequency46-6of 20 kHz which is −15 kHz relative to the resonance frequency46, the resonance frequency46, and a frequency46-6′ of 50 kHz which is +15 kHz relative to the resonance frequency46. In the example depicted inFIG.11, the frequency in the one cycle101is set to the frequency46-6for a predetermined time period111, thereafter the frequency is set to the resonance frequency46for a predetermined time period112, and thereafter the frequency is set to the frequency46-6′ for a predetermined time period113.

In addition, in the first embodiment, the frequency of the sweep treatment, or the frequency of repeating the one cycle101depicted inFIGS.9,10, and11, is a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20to be formed from the SiC ingot1but equal to or less than the resonance frequency of the ultrasonic vibrator45; in the present invention, however, the frequency of the sweep treatment, or the frequency of repeating the one cycle101, is not limited to a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20but equal to or less than the resonance frequency of the ultrasonic vibrator45. Note that, in the present invention, the frequency approximate to the specific frequency of the wafer20is a frequency of 0.8 to 1.2 times the specific frequency of the wafer20.

For example, in the case where the thickness22of the wafer20is 370 μm, the specific frequency of the wafer20is 707 Hz. The frequency of the sweep treatment is, for example, 500 Hz, 625 Hz, 714 Hz, 833 Hz, 1.1 kHz, 1.7 kHz, or 3.3 kHz. Note that, in the present invention, when the frequency of the sweep treatment, or the frequency of repeating the one cycle101, is equal to or more than a frequency approximate to the specific frequency of the wafer20, the wafer20can be efficiently vibrated in the liquid42, and the required time until the wafer20is peeled off from the SiC ingot1can be reduced. In addition, when the frequency of the sweep treatment, or the frequency of repeating the one cycle101, is less than a frequency approximate to the specific frequency of the wafer20, the required time until the peeling-off cannot be reduced. Besides, when the frequency of the sweep treatment, or the frequency of repeating the one cycle101, is a frequency equal to or less than the resonance frequency of the ultrasonic vibrator45, the wafer20can be efficiently vibrated in the liquid42, and the required time until the wafer20is peeled off from the SiC ingot1can be reduced. In addition, in the present invention, when the frequency of the sweep treatment, or the frequency of repeating the one cycle101, is a frequency in excess of the resonance frequency of the ultrasonic vibrator45, the sweep treatment is physically impossible, and, therefore, the frequency of the sweep treatment is a frequency equal to or less than the resonance frequency of the ultrasonic vibrator45.

In the wafer forming step1002of the first embodiment, the ultrasonic vibration is applied to the SiC ingot1through the liquid42for a predetermined period of time to peel off a part of the SiC ingot1and form the wafer20, upon which the wafer forming step1002is completed.

As has been described above, in the wafer forming step1002of the wafer forming method according to the first embodiment, the ultrasonic vibration is applied to the SiC ingot1while the sweep treatment of regularly varying the frequency of the voltage applied to the ultrasonic vibrator45is performed, and thus, an effect that the wafer20can be stably and efficiently peeled off from the SiC ingot1formed with the peeling layers23, by, for example, movement of positions of a compressional wave, even if the resonance frequency of the ultrasonic vibrator45is varied due to load variation, is exhibited.

In addition, in the wafer forming method according to the first embodiment, by setting the frequency of the sweep treatment to a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20, the wafer20can be efficiently vibrated, and thus, an effect that the required time until the wafer20is peeled off from the SiC ingot1can be reduced and the wafer20can be stably and efficiently peeled off from the SiC ingot1is exhibited.

Besides, in the wafer forming method according to the first embodiment, the frequency in the one cycle101varied by the sweep treatment includes the resonance frequency of the ultrasonic vibrator45, and thus, the amplitude of the ultrasonic vibrator45can be enlarged. Accordingly, the ultrasonic vibration can be stably and efficiently applied to the whole of the SiC ingot1formed with the peeling layers23, or the whole of the SiC ingot1inclusive of the wafer20yet to be peeled off.

Second Embodiment

A wafer forming method according to a second embodiment of the present invention will be described based on the drawings.FIG.12is a diagram depicting an example of a voltage which an ultrasonic power source applies to an ultrasonic vibrator in a wafer forming step of the wafer forming method according to the second embodiment.

The wafer forming method according to the second embodiment is the same as that in the first embodiment, except that, in the wafer forming step1002, the ultrasonic vibration is applied to the SiC ingot1while the ultrasonic vibration from an ultrasonic vibrator45is intermittently oscillated by intermittent application of a voltage to the ultrasonic vibrator45, as depicted inFIG.12. Note that the axis of abscissas inFIG.12represents the time elapsed from the start of application of the voltage and the axis of ordinates represents the voltage applied to the ultrasonic vibrator45.

As illustrated inFIG.12, in the wafer forming step1002of the second embodiment, combining predetermined time periods114and115of not applying a voltage to the ultrasonic vibrator45and not oscillating an ultrasonic wave from the ultrasonic vibrator45with a predetermined time period116of oscillating an ultrasonic wave from the ultrasonic vibrator45by applying a voltage to the ultrasonic vibrator45to thereby configure one cycle101and repeat this one cycle101at a predetermined frequency is referred to as applying the ultrasonic vibration to the SiC ingot1while intermittently oscillating the ultrasonic wave.

In the wafer forming step1002of the second embodiment, the frequency in the one cycle101of applying the voltage to the ultrasonic vibrator45includes the resonance frequency46of the ultrasonic vibrator45, but, in the present invention, the frequency may not necessarily include the resonance frequency.

Note that the example depicted inFIG.12is an example in which, in one cycle101, a voltage is not applied to the ultrasonic vibrator45for a predetermined time period114, thereafter the frequency of the voltage applied to the ultrasonic vibrator45is the resonance frequency46for a predetermined time period116, and thereafter a voltage is not applied to the ultrasonic vibrator45for a predetermined time period115.

In addition, in the second embodiment, the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, or the frequency of repeating the one cycle101depicted inFIG.12, is a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20to be formed from the SiC ingot1but equal to or less than the resonance frequency of the ultrasonic vibrator45. In the present invention, however, the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, or the frequency of repeating the one cycle101, is not limited to the frequency equal to or more than the frequency approximate to the specific frequency of the wafer20but equal to or less than the resonance frequency of the ultrasonic vibrator45.

Note that, in the present invention, when the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, or the frequency of repeating the one cycle101, is equal to or more than a frequency approximate to the specific frequency of the wafer20, the wafer20can be efficiently vibrated in the liquid42, and the required time until the wafer20is peeled off from the SiC ingot1can be reduced. In addition, when the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation is less than the frequency approximate to the specific frequency of the wafer20, the required time until the peeling-off cannot be reduced. Besides, when the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, or the frequency of repeating the one cycle101, is equal to or less than the resonance frequency of the ultrasonic vibrator45, the wafer20can be efficiently vibrated in the liquid42, and the required time until the wafer20is peeled off from the SiC ingot1can be reduced. In addition, in the present invention, the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, or the frequency of repeating the one cycle101, is a frequency equal to or less than the resonance frequency of the ultrasonic vibrator45, since it is physically impossible to intermittently oscillate the ultrasonic wave at a frequency in excess of the resonance frequency of the ultrasonic vibrator45.

As has been described above, in the wafer forming step1002of the wafer forming method according to the second embodiment, the ultrasonic vibration is applied to the SiC ingot1while the ultrasonic wave from the ultrasonic vibrator45is intermittently oscillated by intermittent application of a voltage to the ultrasonic vibrator45. Therefore, the ultrasonic vibration can be efficiently applied to the SiC ingot1by, for example, movement of the positions of a compressional wave, even if the resonance frequency of the ultrasonic vibrator45is varied due to load variation. As a result, the wafer forming method according to the second embodiment exhibits an effect that the wafer20can be stably and efficiently peeled off from the SiC ingot1formed with the peeling layers23.

In addition, in the wafer forming method according to the second embodiment, the frequency of a process ranging from oscillation of the ultrasonic vibration of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation is set to a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20, whereby the wafer20can be vibrated efficiently. Therefore, the required time until the wafer20is peeled off from the SiC ingot1can be reduced, and an effect that the wafer20can be stably and efficiently peeled off from the SiC ingot1formed with the peeling layers23is exhibited.

Besides, in the wafer forming method according to the second embodiment, the frequency in the one cycle101includes the resonance frequency of the ultrasonic vibrator45, and, therefore, the amplitude of the ultrasonic vibrator45can be enlarged, so that the ultrasonic vibration can be stably and efficiently applied to the whole of the SiC ingot1formed with the peeling layers23.

Next, the inventors of the present invention checked the effects of the wafer forming methods according to the first embodiment and the second embodiment. The results are set forth in Table 1 below.

TABLE 1Peeling-Required timeoffuntil peeling-propertyoffComparative ExampleFairLongInvention Product 1GoodFairly shortInvention Product 2GoodShortInvention Product 3GoodShortInvention Product 4GoodShortInvention Product 5GoodShortInvention Product 6GoodShortInvention Product 7GoodShortInvention Product 8GoodFairly shortInvention Product 9GoodShortInvention Product 10GoodShort

In Table 1, after the peeling layer forming step1001was carried out, ultrasonic vibration was applied to the SiC ingot1by use of a peeling apparatus40including an ultrasonic vibrator45having a resonance frequency of 35 kHz depicted inFIG.8. In addition, in Table 1, the thickness22of the wafer20was set to 370 μm, the specific frequency of the wafer20was set to 707 Hz, an acoustic emission (AE) sensor was attached to the first surface2of the SiC ingot1from which the wafer20is to be peeled off, the amplitude of vibration of the first surface2(in practice, the voltage value which increases as the amplitude as an output value of the AE sensor increases) was measured, and the required time until the wafer20is peeled off from the SiC ingot1was measured.

In the Comparative Example, a voltage of the resonance frequency was applied to the ultrasonic vibrator continuously for a predetermined period of time. In Invention Product 1, the wafer forming step1002of the first embodiment was carried out for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 500 Hz. In Invention Product 2, the wafer forming step1002of the first embodiment was conducted for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 625 Hz. In Invention Product 3, the wafer forming step1002of the first embodiment was carried out for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 714 Hz. In Invention Product 4, the wafer forming step1002of the first embodiment was performed for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 833 Hz.

In Invention Product 5, the wafer forming step1002of the first embodiment was carried out for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 1.1 kHz. In Invention Product 6, the wafer forming step1002of the first embodiment was conducted for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 1.7 kHz. In Invention Product 7, the wafer forming step1002of the first embodiment was carried out for a predetermined period of time, with the frequency of the sweep treatment in the wafer forming step1002of the first embodiment set to 3.3 kHz.

In Invention Product 8, the wafer forming step1002of the second embodiment was carried out for a predetermined period of time, with the frequency of a process ranging from oscillation of the ultrasonic wave through the rest of the oscillation to the re-oscillation in the wafer forming step1002of the second embodiment set to 500 Hz. In Invention Product 9, the wafer forming step1002of the second embodiment was conducted for a predetermined period of time, with the frequency of a process ranging from oscillation of the ultrasonic wave through the rest of the oscillation to the re-oscillation in the wafer forming step1002of the second embodiment set to 625 Hz. In Invention Product 10, the wafer forming step1002of the second embodiment was carried out for a predetermined period of time, with the frequency of a process ranging from oscillation of the ultrasonic wave through the rest of the oscillation to the re-oscillation in the wafer forming step1002of the second embodiment set to 3.3 kHz.

In the Comparative Example, the required time until the wafer20is peeled off from the SiC ingot1within a predetermined period of time was very long, but, in Invention Product 1 to Invention Product 10, the wafer20can be peeled off from the SiC ingot1within a predetermined period of time. Therefore, according to Table 1, it has been made clear that, by applying an ultrasonic vibration to the SiC ingot1, while performing the sweep treatment of regularly varying the frequency of the voltage applied to the ultrasonic vibrator45, or while intermittently oscillating the ultrasonic wave from the ultrasonic vibrator45, in the wafer forming step1002, an effect that the wafer20can be stably and efficiently peeled off from the SiC ingot1formed with the peeling layers23can be exhibited.

In addition, while the output value of the AE sensor in Invention Product 1 was 2.5 mV, the output value of the AE sensor in Invention Product 2 was 22 mV, and the output value of the AE sensor in Invention Product 3 was 16 mV, and the required time until the wafer20is peeled off from the SiC ingot1in Invention Product 2 to Invention Product 7, Invention Product 9, and Invention Product 10 was by far shorter than that in Invention Product 1 and Invention Product 8. Therefore, according to Table 1, it has been made clear that, by setting the frequency of the sweep treatment, or the frequency of a process ranging from oscillation of the ultrasonic wave of the ultrasonic vibrator45through the rest of the oscillation to the re-oscillation, to be a frequency equal to or more than a frequency approximate to the specific frequency of the wafer20, in the wafer forming step1002, a part of the SiC ingot1can be efficiently vibrated, and an effect that the wafer20can be stably and efficiently peeled off from the SiC ingot1formed with the peeling layers23is exhibited.

The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.