SiC wafer producing method

An SiC wafer is produced from a single crystal SiC ingot by a method that includes forming a plurality of breakable layers constituting a separation surface in the SiC ingot, each breakable layer including a modified layer and cracks extending from the modified layer along a c-plane, and separating part of the SiC ingot along the separation surface as an interface to thereby produce the SiC wafer. In forming the separation surface, the energy density of a pulsed laser beam is set to an energy density not causing the formation of an upper damage layer above the breakable layer previously formed due to the reflection of the pulsed laser beam from the breakable layer and not causing the formation of a lower damage layer below the breakable layer previously formed due to the transmission of the pulsed laser beam through the breakable layer.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an SiC wafer producing method for producing an SiC wafer from a single crystal SiC ingot.

Description of the Related Art

Various devices such as integrated circuits (ICs), large-scale integrations (LSIs), and light-emitting diodes (LEDs) are formed by forming a functional layer on the front side of a wafer formed of Si (silicon) or Al2O3(sapphire) and partitioning this functional layer into a plurality of separate regions along a plurality of division lines. Further, power devices or optical devices such as LEDs are formed by forming a functional layer on the front side of a wafer formed of single crystal SiC (silicon carbide) and partitioning this functional layer into a plurality of separate regions along a plurality of division lines. The division lines of such a wafer having these devices are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus to thereby divide the wafer into a plurality of individual device chips corresponding to the devices. The device chips thus obtained are used in various electrical equipment such as mobile phones and personal computers.

In general, the wafer on which the devices are to be formed is produced by slicing a cylindrical ingot with a wire saw. Both sides of the wafer sliced from the ingot are polished to a mirror finish (see Japanese Patent Laid-Open No. 2000-94221). However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, a large proportion (70% to 80%) of the ingot is discarded to cause a problem of poor economy. In particular, a single crystal SiC ingot has high hardness and it is therefore difficult to cut this ingot with the wire saw. Accordingly, considerable time is required for cutting of the ingot, causing a reduction in productivity. Furthermore, since this ingot is high in unit price, there is a problem in efficiently producing a wafer in this prior art.

A technique for solving this problem has been proposed (see Japanese Patent Laid-Open No. 2013-49161). This technique includes the steps of setting the focal point of a laser beam having a transmission wavelength to SiC inside an SiC ingot, next applying the laser beam to the SiC ingot as scanning the laser beam on the ingot to thereby form modified layers in a separation plane previously set inside the ingot, and next breaking the ingot along the separation plane where the modified layers are formed, thus separating an SiC wafer from the SiC ingot. However, in producing the SiC wafer from the SiC ingot by using this prior art technique, there is a problem such that the modified layers must be densely formed with a pitch of approximately 10 μm, causing a reduction in productivity.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an SiC wafer producing method which can improve the productivity of wafers.

In accordance with an aspect of the present invention, there is provided an SiC wafer producing method for producing an SiC wafer from a single crystal SiC ingot having a first surface, a second surface opposite to the first surface, a c-axis extending from the first surface to the second surface, and a c-plane perpendicular to the c-axis, the c-axis being inclined by an off angle with respect to a normal to the first surface, the off angle being formed between the c-plane and the first surface, the SiC wafer producing method including a breakable layer forming step of setting a focal point of a pulsed laser beam having a transmission wavelength to SiC inside the SiC ingot at a predetermined depth from the first surface, the predetermined depth corresponding to the thickness of the SiC wafer to be produced, and next applying the pulsed laser beam to the SiC ingot as relatively moving the SiC ingot and the focal point in a first direction perpendicular to a second direction where the off angle is formed, thereby forming a breakable layer inside the SiC ingot at the predetermined depth, the breakable layer including a modified layer extending in the first direction and cracks extending from the modified layer in opposite directions along the c-plane, the modified layer being formed in such a manner that SiC is decomposed into Si and C by the pulsed laser beam first applied, and the pulsed laser beam next applied is absorbed by C previously produced to continue the decomposition of SiC into Si and C in a chain reaction manner with the relative movement of the SiC ingot and the focal point in the first direction; a separation surface forming step of relatively indexing the SiC ingot and the focal point in the second direction and performing the breakable layer forming step plural times to thereby form a plurality of breakable layers constituting a separation surface; and a wafer producing step of separating part of the SiC ingot along the separation surface as an interface to thereby produce the SiC wafer; the separation surface forming step including the step of setting the energy density of the pulsed laser beam to an energy density not causing the formation of an upper damage layer above the breakable layer previously formed due to the reflection of the pulsed laser beam from the breakable layer or not causing the formation of a lower damage layer below the breakable layer previously formed due to the transmission of the pulsed laser beam through the breakable layer.

Preferably, the energy density per pulse E (J/cm2) of the pulsed laser beam and the feed speed V (mm/second) of the SiC ingot satisfy the conditions of 0<V≤600 and 0.184≤E and the energy density per pulse E is set to −0.35+0.0042×(V−100)≤E≤0.737+0.0024×(V−100).

According to the SiC wafer producing method of the present invention, the separation surface forming step includes the step of setting the energy density of the pulsed laser beam to an energy density not causing the formation of an upper damage layer above the breakable layer previously formed due to the reflection of the pulsed laser beam from the breakable layer or not causing the formation of a lower damage layer below the breakable layer previously formed due to the transmission of the pulsed laser beam through the breakable layer. Accordingly, a good breakable layer can be formed without the formation of a damage layer above or below the breakable layer. In the SiC wafer producing method of the present invention, a damage layer is not formed above or below the breakable layer. Accordingly, there is no possibility that the quality of the SiC wafer may be reduced and that the amount of the ingot to be removed by grinding may be increased to cause a reduction in productivity. As a result, the amount of an ingot portion to be discarded can be reduced to thereby improve the productivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the SiC wafer producing method according to the present invention will now be described with reference to the drawings.FIG. 1is a perspective view of a laser processing apparatus2for performing the SiC wafer producing method according to this preferred embodiment. The laser processing apparatus2depicted inFIG. 1includes a base4, holding means6, moving means8for moving the holding means6, laser beam applying means10, imaging means12, display means14, and separating means16.

The holding means6includes a rectangular X movable plate18mounted on the base4so as to be movable in an X direction, a rectangular Y movable plate20mounted on the X movable plate18so as to be movable in a Y direction, and a cylindrical chuck table22rotatably mounted on the upper surface of the Y movable plate20. The X direction is defined as the direction depicted by an arrow X inFIG. 1, and the Y direction is defined as the direction depicted by an arrow Y inFIG. 1, which is perpendicular to the X direction in an XY plane. The XY plane defined by the X direction and the Y direction is a substantially horizontal plane.

The moving means8includes X moving means24, Y moving means26, and rotating means (not depicted). The X moving means24includes a ball screw28extending in the X direction on the base4and a motor30connected to one end of the ball screw28. The ball screw28has a nut portion (not depicted), which is fixed to the lower surface of the X movable plate18. The X moving means24is operated in such a manner that the rotary motion of the motor30is converted into a linear motion by the ball screw28and this linear motion is transmitted to the X movable plate18, so that the X movable plate18is moved in the X direction along a pair of guide rails4aprovided on the base4. Similarly, the Y moving means26includes a ball screw32extending in the Y direction on the X movable plate18and a motor34connected to one end of the ball screw32. The ball screw32has a nut portion (not depicted), which is fixed to the lower surface of the Y movable plate20. The Y moving means26is operated in such a manner that the rotary motion of the motor34is converted into a linear motion by the ball screw32and this linear motion is transmitted to the Y movable plate20, so that the Y movable plate20is moved in the Y direction along a pair of guide rails18aprovided on the X movable plate18. The rotating means has a motor (not depicted) built in the chuck table22to rotate the chuck table22with respect to the Y movable plate20.

The laser beam applying means10includes an L-shaped casing36provided on the base4at its rear end portion, oscillating means (not depicted) built in the casing36, focusing means38mounted on the lower surface of the casing36at its front end portion, and focal position adjusting means (not depicted). The L-shaped casing36includes a vertical portion extending upward from the upper surface of the base4and a horizontal portion extending from the upper end of the vertical portion in a substantially horizontal direction. Although not depicted, the oscillating means includes a laser oscillator for oscillating a pulsed laser beam LB, frequency setting means for setting the repetition frequency F of the pulsed laser beam LB to be oscillated from the laser oscillator, and power adjusting means for adjusting the power of the pulsed laser beam LB oscillated from the laser oscillator. The focusing means38has a focusing lens (not depicted) for focusing the pulsed laser beam LB oscillated from the laser oscillator. The imaging means12is provided on the lower surface of the front end portion of the casing36so as to be spaced from the focusing means38in the X direction. The display means14is mounted on the upper surface of the front end portion of the casing36to display an image obtained by the imaging means12.

The separating means16includes a columnar casing40extending upward from the upper surface of the base4at a position near the left ends of the guide rails4aas viewed inFIG. 1. The separating means16further includes an arm42having a base end vertically movably supported to the casing40and a front end projecting from the casing40so as to extend in the X direction. Although not depicted, elevating means for vertically moving the arm42is built in the casing40. A motor44is provided at the front end of the arm42. A disk-shaped suction member46is connected to the lower surface of the motor44so as to be rotatable about a vertical axis. The lower surface of the suction member46is formed with a plurality of suction holes (not depicted), which are connected through a suction passage to suction means (not depicted). Further, ultrasonic vibration applying means (not depicted) is built in the suction member46to apply ultrasonic vibration to the lower surface of the suction member46.

FIGS. 2A and 2Bdepict a generally cylindrical hexagonal single crystal SiC ingot50(which will be hereinafter referred to simply as “ingot50”) as a workpiece to be processed. The ingot50has a substantially circular first surface52, a substantially circular second surface54opposite to the first surface52, a substantially cylindrical surface56formed so as to connect the first surface52and the second surface54, a c-axis (<0001> direction) extending from the first surface52to the second surface54, and a c-plane ({0001} plane) perpendicular to the c-axis. In the ingot50, the c-axis is inclined by an off angle α with respect to a normal58to the first surface52. The off angle α (e.g., α=4 degrees) is formed between the c-plane and the first surface52(the direction of formation of the off angle α is depicted by an arrow A inFIGS. 2A and 2B). Further, the cylindrical surface56of the ingot50is formed with a first orientation flat60and a second orientation flat62, which are rectangular in side elevation and function to indicate crystal orientation. The first orientation flat60is parallel to the direction A of formation of the off angle α, and the second orientation flat62is perpendicular to the direction A of formation of the off angle α. As depicted inFIG. 2A, which is a plan view taken in the direction of extension of the normal58, the length L2of the second orientation flat62is set shorter than the length L1of the first orientation flat60(L2<L1).

There will now be described an SiC wafer producing method using the laser processing apparatus2. First, as depicted inFIG. 1, the ingot50is fixed to the chuck table22in the condition where an adhesive (e.g., epoxy resin adhesive) is interposed between the second surface54of the ingot50and the upper surface of the chuck table22. As a modification, the upper surface of the chuck table22may be formed with a plurality of suction holes, whereby a suction force may be produced on the upper surface of the chuck table22to thereby hold the ingot50under suction. Thereafter, the moving means8is operated to move the chuck table22to a position below the imaging means12, and the imaging means12is next operated to image the ingot50.

Thereafter, a separation surface forming step is performed. In the separation surface forming step, the moving means8is first operated to move and rotate the chuck table22according to the image of the ingot50detected by the imaging means12, thereby adjusting the orientation of the ingot50to a predetermined orientation and also adjusting the positional relation between the ingot50and the focusing means38in the XY plane. In adjusting the orientation of the ingot50to a predetermined orientation, the first orientation flat60is made parallel to the Y direction and the second orientation flat62is made parallel to the X direction as depicted inFIG. 3B. Accordingly, the direction A of formation of the off angle α is made parallel to the Y direction, and the direction perpendicular to the direction A of formation of the off angle α is made parallel to the X direction. Thereafter, the focal position adjusting means is operated to vertically move the focusing means38, thereby setting a focal point FP inside the ingot50at a predetermined depth from the first surface52, wherein this predetermined depth corresponds to the thickness of a wafer to be produced. Thereafter, as depicted inFIGS. 3A and 3B, a pulsed laser beam LB having a transmission wavelength to SiC is applied from the focusing means38to the ingot50as moving the chuck table22relative to the focal point FP at a predetermined feed speed V in the X direction (i.e., in the direction perpendicular to the direction A of formation of the off angle α) by operating the X moving means24. As a result, a breakable layer64is formed inside the ingot50along a line in the X direction (breakable layer forming step).

In the breakable layer forming step, the pulsed laser beam LB is initially applied to the ingot50to thereby decompose SiC into Si (silicon) and C (carbon). Thereafter, the pulsed laser beam LB is next applied to the ingot50and absorbed by C previously produced. Thus, SiC is decomposed into Si and C in a chain reaction manner with the movement of the chuck table22in the X direction to thereby for a modified layer66extending in the X direction. At the same time, cracks68(seeFIG. 4B) are also formed so as to extend from the modified layer66in opposite directions along the c-plane. In this manner, the modified layer66and the cracks68constitute the breakable layer64. As depicted inFIG. 4B, the modified layer66extending in the X direction (i.e., in the direction perpendicular to the direction A of formation of the off angle α) is present in the same c-plane and flattened along the c-plane. Further, the modified layer66has void. Further, the length Lc of each crack68extending from the modified layer66in one direction is approximately 250 μm. In the breakable layer forming step, the chuck table22is fed in the X direction so that the adjacent spots of the pulsed laser beam LB applied to the ingot50are overlapped with each other at the depth where the modified layer66is formed. Accordingly, the pulsed laser beam LB is applied again to the modified layer66where SiC has been decomposed into Si and C. To ensure that the adjacent spots of the pulsed laser beam LB are overlapped with each other in the breakable layer forming step, the relation of G=(V/F)−D<0 must hold, where F is the repetition frequency (kHz) of the pulsed laser beam LB, V is the feed speed (mm/second) of the chuck table22, and D is the diameter (μm) of each spot. Further, the overlap rate of the adjacent spots is defined as |G|/D.

After performing the breakable layer forming step along a line in the X direction, indexing is performed in such a manner that the chuck table22is moved relative to the focal point FP by a predetermined index amount Li (e.g., 250 μm) in the Y direction (i.e., in the direction A of formation of the off angle α) by operating the Y moving means26. Thereafter, the breakable layer forming step is similarly performed along the next line in the X direction. Thereafter, the indexing and the breakable layer forming step are repeated plural times to thereby form a plurality of similar breakable layers64as depicted inFIGS. 4A and 4B. Thus, a separation surface70is formed by these plural breakable layers64.

In the separation surface forming step, the energy density of the pulsed laser beam LB must be set to an energy density not causing the formation of an upper damage layer above the breakable layer64previously formed due to the reflection of the pulsed laser beam LB from the breakable layer64or not causing the formation of a lower damage layer below the breakable layer64previously formed due to the transmission of the pulsed laser beam LB through the breakable layer64. In this preferred embodiment, the energy density per pulse E (J/cm2) and the feed speed V (mm/second) satisfy the following conditions:
0<V≤600  (Eq. 1); and
0.184≤E(Eq. 2).
On the above conditions, the energy density per pulse E is set as follows:
−0.35+0.0042×(V−100)≤E≤0.737+0.0024×(V−100)  (Eq. 3).
The energy density E (J/cm2) is defined by the average power P (W), the area S=ΠD2/4 (cm2) of the spot at the position where the modified layer66is formed, and the repetition frequency F (kHz) to give E=P/(S·F).

The reason for setting the relation between the energy density per pulse E (J/cm2) and the feed speed V (mm/second) to the range specified by Eqs. 1 to 3 will now be described with reference toFIG. 5, which depicts the results of tests performed by the present inventor under the following test conditions.

Wavelength of the pulsed laser beam: 1064 nm

Average power: 0.2 to 5.0 W

Diameter of the focal point: 1 μm

Numerical aperture (NA) of the focusing lens: 0.65

Position of the focal point: Position obtained by lowering the focusing means by 120 μm from the condition where the focal point was set on the upper surface of the ingot.

Refractive index of SiC: 2.65

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 100 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 1)

(1) When the average power was 0.2 W, no breakable layer was formed.

(2) When the average power was in the range of 0.4 to 1.6 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was in the range of 1.8 to 2.8 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed.

(4) When the average power was in the range of 3.0 to 5.0 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed. Further, a damage layer was also formed below the breakable layer due to the transmission of the laser beam through the breakable layer previously formed.

InFIG. 5, the symbol “●” indicates the result that a good breakable layer was formed and no damage layer was formed above or below the breakable layer, the symbol “x” indicates the result that no breakable layer was formed, the symbol “Δ” indicates the result that a damage layer was formed only above the breakable layer, the symbol “□” indicates the result that damage layers were formed above and below the breakable layer, the symbol “⋄” indicates the result that a damage layer was formed only below the breakable layer, and the symbol “+” indicates the result that no breakable layer was formed and only a damage layer was formed. Also inFIG. 8, these symbols “●,” “x,” “Δ,” “□,” “⋄,” and “+” are similarly used.

(Conclusion Based on Test 1)

In the case that the feed speed is 100 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 0.4 to 1.6 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 200 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 2)

(1) When the average power was in the range of 0.2 to 0.4 W, no breakable layer was formed.

(2) When the average power was in the range of 0.6 to 2.0 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was in the range of 2.2 to 2.8 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed.

(4) When the average power was in the range of 3.0 to 5.0 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed. Further, a damage layer was also formed below the breakable layer due to the transmission of the laser beam through the breakable layer previously formed. In some case, a damage layer was formed only below the breakable layer.

(Conclusion Based on Test 2)

In the case that the feed speed is 200 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 0.6 to 2.0 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 300 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 3)

(1) When the average power was in the range of 0.2 to 0.8 W, no breakable layer was formed.

(2) When the average power was in the range of 1.0 to 2.8 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was 3.0 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed.

(4) When the average power was in the range of 3.2 to 4.8 W, a breakable layer was formed and a damage layer was formed above the breakable layer due to the reflection of the laser beam from the breakable layer previously formed. Further, a damage layer was also formed below the breakable layer due to the transmission of the laser beam through the breakable layer previously formed.

(5) When the average power was 5.0 W, a breakable layer was formed and a damage layer was formed below the breakable layer due to the transmission of the laser beam through the breakable layer previously formed.

(Conclusion Based on Test 3)

In the case that the feed speed is 300 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 1.0 to 2.8 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 400 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 4)

(1) When the average power was in the range of 0.2 to 1.8 W, no breakable layer was formed.

(2) When the average power was in the range of 2.0 to 3.2 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was in the range of 3.4 to 5.0 W, a breakable layer was formed and a damage layer was formed below the breakable layer due to the transmission of the laser beam from the breakable layer previously formed.

(Conclusion Based on Test 4)

In the case that the feed speed is 400 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 2.0 to 3.2 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 500 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 5)

(1) When the average power was in the range of 0.2 to 2.8 W, no breakable layer was formed.

(2) When the average power was in the range of 3.0 to 3.6 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was in the range of 3.8 to 5.0 W, a breakable layer was formed and a damage layer was formed below the breakable layer due to the transmission of the laser beam from the breakable layer previously formed.

(Conclusion Based on Test 5)

In the case that the feed speed is 500 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 3.0 to 3.6 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 600 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 6)

(1) When the average power was in the range of 0.2 to 3.6 W, no breakable layer was formed.

(2) When the average power was in the range of 3.8 to 4.0 W, a good breakable layer was formed and no damage layer was formed above or below the breakable layer.

(3) When the average power was in the range of 4.2 to 5.0 W, a breakable layer was formed and a damage layer was formed below the breakable layer due to the transmission of the laser beam from the breakable layer previously formed.

(Conclusion Based on Test 6)

In the case that the feed speed is 600 mm/second under the above test conditions, the average power of the pulsed laser beam is set to the range of 3.8 to 4.0 W, so as to form a good breakable layer.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 700 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 7)

(1) When the average power was in the range of 0.2 to 1.0 W, no breakable layer was formed.

(2) When the average power was in the range of 1.2 to 5.0 W, no breakable layer was formed and only a damage layer was formed, wherein the damage layer becomes larger with an increase in average power.

(Conclusion Based on Test 7)

In the case that the feed speed is 700 mm/second under the above test conditions, no breakable layer is formed.

A single crystal SiC ingot having a thickness of 1 mm was held on the chuck table22and the pulsed laser beam was applied to the ingot as moving the chuck table22at a feed speed of 800 mm/second in the direction perpendicular to the direction of formation of the off angle, wherein the average power of the pulsed laser beam was changed from 0.2 W to 5.0 W at intervals of 0.2 W.

(Result of Test 8)

(1) When the average power was in the range of 0.2 to 1.0 W, no breakable layer was formed.

(2) When the average power was in the range of 1.2 to 5.0 W, no breakable layer was formed and only a damage layer was formed, wherein the damage layer becomes larger with an increase in average power.

(Conclusion Based on Test 8)

In the case that the feed speed is 800 mm/second under the above test conditions, no breakable layer is formed.

Referring toFIGS. 6 and 7, there are depicted the depth Z of the focal point FP of the pulsed laser beam LB applied to the ingot50, the modified layer66formed in the ingot50, and damage layers72and74formed in the ingot50above and below the modified layer66. The numerical aperture NA of the focusing lens is expressed as NA=n·sin θ, where n is the refractive index of the medium where the focusing lens is located, and θ is the angular aperture. Under the above test conditions, the numerical aperture NA of the focusing lens is 0.65. Accordingly, the angular aperture θ of the pulsed laser beam LB transmitted through the focusing lens in the air (refractive index n≈1) becomes as follows:

θ=⁢sin-1⁡(0.65)=⁢40.5.
In the case that the focal point FP is lowered by 120 μm from the upper surface (e.g., the first surface52) of the ingot50, the radius r of a spot of the laser beam LB formed on the upper surface of the ingot50becomes as follows:

r=⁢120×tan⁡(40.5)=⁢120×0.854=⁢102.5⁢⁢(µm).
Further, the refractive index of SiC is 2.65. Accordingly, the aperture angle θ′ of the pulsed laser beam LB in the ingot50becomes as follows:

θ′=⁢sin-1⁡(0.65⁢/⁢2.65)=⁢sin-1⁡(0.245)=⁢14.2.
Accordingly, the depth Z of the focal point FP is given by the relation with the radius r of the spot of the pulsed laser beam LB formed on the upper surface of the ingot50and the angular aperture θ′ in the ingot50as follows:

Z=⁢r⁢/⁢tan⁢⁢θ′=⁢102.5⁢/⁢tan⁡(14.2)=⁢102.5⁢/⁢0.253=⁢406⁢(µm)
The position of the modified layer66formed in Tests 1 to 6 was actually measured. As the result of this measurement, the depth of the modified layer66from the upper surface of the ingot50was 216 μm, and this depth was higher than the depth of the focal point FP by 190 μm. Due to the fact that the breakable layer64including the modified layer66and the cracks68is formed at the depth different from the depth of the focal point FP, there is a case that the damage layer72and/or the damage layer74are/is formed in the ingot50. More specifically, when the pulsed laser beam LB is applied to the ingot50so as to overlap the breakable layer64previously formed, the pulsed laser beam LB may be reflected from the breakable layer64, and the reflected pulsed laser beam LB1may form the damage layer72at the depth higher than the breakable layer64by approximately 190 μm. Further, the pulsed laser beam LB may be transmitted through the breakable layer64, and the transmitted pulsed laser beam LB2may form the damage layer74at the depth lower than the breakable layer64by approximately 190 μm (this depth being substantially the same depth of the focal point FP). When the damage layer72or74is formed in the ingot50, the quality of a wafer to be produced is decreased and the amount of the ingot50to be removed by grinding is increased to cause a reduction in productivity.

As apparent from the results of Tests 1 to 8 depicted inFIG. 5, the maximum average power for forming a good breakable layer64according to the feed speed V (mm/second) is 1.6 W for V=100 mm/second, 2.0 W for V=200 mm/second, 2.8 W for V=300 mm/second, 3.2 W for V=400 mm/second, 3.6 W for V=500 mm/second, 4.0 W for V=600 mm/second, unclear for V=700 mm/second, and unclear for V=800 mm/second. The energy density per pulse E (J/cm2) converted from this maximum average power is 0.737 J/cm2for V=100 mm/second, 0.922 J/cm2for V=200 mm/second, 1.29 J/cm2for V=300 mm/second, 1.474 J/cm2for V=400 mm/second, 1.65 J/cm2for V=500 mm/second, and 1.84 J/cm2for V=600 mm/second. As described above, the energy density E (J/cm2) is defined by the average power P (W), the area S=ΠD2/4 (cm2) of the spot at the position where the modified layer66is formed, and the repetition frequency F (kHz) to give E=P/(S·F). In the Tests 1 to 8, the diameter D of the spot at the position where the modified layer66is formed is given by the relation among the radius r (102.5 μm) of the spot formed on the upper surface of the ingot50, the depth Z (406 μm) of the focal point FP, and the height (190 μm) of the modified layer66from the focal point FP as follows:

Further, as apparent from the results of Tests 1 to 8 depicted inFIG. 5, the minimum average power for forming a good breakable layer64according to the feed speed V (mm/second) is 0.4 W for V=100 mm/second, 0.6 W for V=200 mm/second, 1.0 W for V=300 mm/second, 2.0 W for V=400 mm/second, 3.0 W for V=500 mm/second, 3.8 W for V=600 mm/second, unclear for V=700 mm/second, and unclear for V=800 mm/second. The energy density per pulse E (J/cm2) converted from this minimum average power is 0.184 J/cm2for V=100 mm/second, 0.276 J/cm2for V=200 mm/second, 0.461 J/cm2for V=300 mm/second, 0.921 J/cm2for V=400 mm/second, 1.382 J/cm2for V=500 mm/second, and 1.75 J/cm2for V=600 mm/second.

In Tests 1 to 8, the overlap rate of the adjacent spots at the position where the breakable layer64is formed changes according to the feed speed V (mm/second). That is, the overlap rate decreases with an increase in the feed speed V (mm/second) in such a manner that the overlap rate is 96% for V=100 mm/second, 93% for V=200 mm/second, 89% for V=300 mm/second, 86% for V=400 mm/second, 83% for V=500 mm/second, 80% for V=600 mm/second, 76% for V=700 mm/second, and 73% for V=800 mm/second. Accordingly, the maximum energy density E for forming a good breakable layer64changes according to the feed speed V. However, when the overlap rate is less than 80%, the breakable layer64is not formed.

FIG. 8depicts the correlation between the feed speed V (mm/second) and the energy density per pulse E (J/cm2). InFIG. 8, the horizontal axis represents the feed speed V (mm/second) and the vertical axis represents the energy density per pulse E (J/cm2). As depicted inFIG. 8, the maximum energy density per pulse E for forming a good breakable layer64is plotted according to the feed speed V to perform linear approximation, thereby deriving E=0.737+0.0024×(V−100). Further, the minimum energy density per pulse E for forming a good breakable layer64is plotted according to the feed speed V to perform linear approximation, thereby deriving E=−0.35+0.0042×(V−100). Further, when the feed speed V is in the range of 100 to 600 mm/second, a good breakable layer64is formed, whereas when the feed speed V is 700 mm/second and 800 mm/second, a good breakable layer64is not formed. Accordingly, the condition of 0<V≤600 is set. Further, the minimum energy density per pulse E for forming a good breakable layer64is 0.184 J/cm2(average power of 0.2 W for V=100 mm/second) as apparent from the results of Tests 1 to 8. Accordingly, the condition of 0.184≤E is set. In conclusion, to form a good breakable layer64and prevent the formation of the damage layer72above the breakable layer64and/or the formation of the damage layer74below the breakable layer64, the energy density per pulse E (J/cm2) and the feed speed V (mm/second) satisfy the following conditions:
0<V≤600  (Eq. 1); and
0.184≤E(Eq. 2).
On the above conditions, the energy density per pulse E is set as follows:
−0.35+0.0042×(V−100)≤E≤0.737+0.0024×(V−100)  (Eq. 3).

InFIG. 8, the hatched area is the area specified by Eqs. 1 to 3. By setting the relation between the energy density per pulse E (J/cm2) and the feed speed V (mm/second) so that Eqs. 1 to 3 hold, the damage layer72or74is not formed above or below the breakable layer64. Accordingly, there is no possibility that the quality of a wafer to be produced may be reduced and that the amount of the ingot50to be removed by grinding may be increased to cause a reduction in productivity. As a result, the amount of an ingot portion to be discarded can be reduced to thereby improve the productivity.

After performing the separation surface forming step, a wafer producing step is performed to separate part of the ingot50along the separation surface70as an interface, thereby producing a wafer. In the wafer producing step, the moving means8is first operated to move the chuck table22to a position below the suction member46. Thereafter, the elevating means provided in the casing40is operated to lower the arm42until the lower surface of the suction member46comes into close contact with the first surface52of the ingot50as depicted inFIG. 9. Thereafter, the suction means connected to the suction member46is operated to hold the first surface52of the ingot50to the lower surface of the suction member46under suction. Thereafter, the ultrasonic vibration applying means built in the suction member46is operated to apply ultrasonic vibration to the lower surface of the suction member46. At the same time, the motor44is operated to rotate the suction member46. As a result, part of the ingot50can be separated along the separation surface70as an interface to thereby efficiently produce a wafer76having a desired thickness as depicted inFIG. 9. After producing the wafer76, the separation surface70of the ingot50remaining is polished by using polishing means (not depicted) provided on the base4. Thereafter, the separation surface forming step and the wafer producing step are sequentially performed in a similar manner to produce a plurality of wafers from the ingot50.