Wafer producing method

A wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal ingot including a separation start point forming step of setting the focal point of a laser beam inside the ingot at a predetermined depth from the ingot's upper surface, which depth corresponds to the thickness of the wafer to be produced, and next applying the laser beam to the upper surface of the ingot while relatively moving the focal point and the ingot to thereby form: (i) a modified layer parallel to the ingot's upper surface, and (ii) cracks extending from the modified layer, thus forming a separation start point. Preferably, the laser beam includes a plurality of laser beams to be simultaneously applied to form a plurality of linear modified layers. The focal points of the laser beams are arranged with predetermined spacing in the direction of formation of an off angle.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wafer producing method for slicing a hexagonal single crystal ingot to produce a wafer.

Description of the Related Art

Various devices such as ICs and LSIs are formed by forming a functional layer on the front side of a wafer formed of silicon or the like and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines. The division lines of the wafer 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 widely used in various electronic apparatuses such as mobile phones and personal computers. Further, power devices or optical devices such as LEDs and LDs are formed by forming a functional layer on the front side of a wafer formed of a hexagonal single crystal such as SiC and GaN and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines.

In general, the wafer on which the devices are to be formed is produced by slicing an ingot with a wire saw. Both sides of the wafer obtained above are polished to a mirror finish (see Japanese Patent Laid-Open No. 2000-94221, for example). This wire saw is configured in such a manner that a single wire such as a piano wire having a diameter of approximately 100 to 300 μm is wound around many grooves formed on usually two to four guide rollers to form a plurality of cutting portions spaced in parallel with a given pitch. The wire is operated to run in one direction or opposite directions, thereby slicing the ingot into a plurality of wafers.

However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, 70 to 80% of the ingot is discarded to cause a problem of poor economy. In particular, a hexagonal single crystal ingot of SiC or GaN, for example, has high Mohs 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. That is, there is a problem in efficiently producing a wafer in this prior art.

A technique for solving this problem is described in 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 a hexagonal single crystal ingot, next applying the laser beam to the ingot as scanning the laser beam on the ingot to thereby form a modified layer and cracks in a separation plane inside the ingot, and next applying an external force to the ingot to thereby break the ingot along the separation plane where the modified layer and the cracks are formed, thus separating a wafer from the ingot. In this technique, the laser beam (pulsed laser beam) is scanned spirally or linearly along the separation plane so that a first application point of the laser beam and a second application point of the laser beam nearest to the first application point have a predetermined positional relation with each other. As a result, the modified layer and the cracks are formed at very high density in the separation plane of the ingot.

SUMMARY OF THE INVENTION

However, in the ingot cutting method described in Japanese Patent Laid-Open No. 2013-49161 mentioned above, the laser beam is scanned spirally or linearly on the ingot. In the case of linearly scanning the laser beam, the direction of scanning of the laser beam is not specified. In the ingot cutting method described in Japanese Patent Laid-Open No. 2013-49161, the pitch (spacing) between the first application point and the second application point of the laser beam as mentioned above is set to 1 μm to 10 μm. This pitch corresponds to the pitch of the cracks extending from the modified layer along a c-plane defined in the ingot.

In this manner, the pitch of the application points of the laser beam to be applied to the ingot is very small. Accordingly, regardless of whether the laser beam is scanned spirally or linearly, the laser beam must be applied with a very small pitch and the improvement in productivity is not yet sufficient.

It is therefore an object of the present invention to provide a wafer producing method which can efficiently produce a wafer from an ingot.

In accordance with an aspect of the present invention, there is provided a wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal 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 wafer producing method including a separation start point forming step of setting the focal point of a laser beam having a transmission wavelength to the ingot inside the ingot at a predetermined depth from the first surface, which depth corresponds to the thickness of the wafer to be produced, and next applying the laser beam to the first surface as relatively moving the focal point and the ingot to thereby form a modified layer parallel to the first surface and cracks extending from the modified layer along the c-plane, thus forming a separation start point; and a wafer separating step of separating plate-shaped member having a thickness corresponding to the thickness of the wafer from the ingot at the separation start point after performing the separation start point forming step, thus producing the wafer from the ingot; the separation start point forming step including a modified layer forming step of relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by an off angle with respect to a normal to the first surface and the off angle is formed between the first surface and the c-plane, thereby linearly forming the modified layer extending in the first direction; and an indexing step of relatively moving the focal point in the second direction to thereby index the focal point by a predetermined amount; wherein in the modified layer forming step, the laser beam includes a plurality of laser beams to be simultaneously applied to form a plurality of linear modified layers, the focal points of the laser beams being arranged in the second direction with a predetermined spacing.

Preferably, in the modified layer forming step, the predetermined spacing between any adjacent ones of the focal points is set so that the upper limit of the predetermined spacing becomes nearly equal to a spacing defined when the front ends of the cracks extending from the adjacent modified layers in the second direction overlap each other.

Preferably, in the indexing step, the index amount L is given as L=H×M, where H is the predetermined spacing and M is the number of the focal points.

According to the wafer producing method of the present invention, the focal point of the laser beam is relatively moved in the first direction perpendicular to the second direction where the off angle is formed between the first surface and the c-plane of the ingot, thereby linearly forming the modified layer extending in the first direction. Thereafter, the focal point of the laser beam is indexed in the second direction by the predetermined amount. Thereafter, the focal point of the laser beam is relatively moved again in the first direction to thereby linearly form the modified layer extending in the first direction. Such a series of steps are repeated to form a plurality of modified layers extending in the first direction, wherein each modified layer is formed at the predetermined depth from the first surface of the ingot and the cracks are formed on both sides of each modified layer so as to propagate along the c-plane. Accordingly, any adjacent ones of the plural modified layers are connected together through the cracks formed therebetween, so that the plate-shaped member having the thickness corresponding to the thickness of the wafer can be easily separated from the ingot at the separation start point, thus producing the hexagonal single crystal wafer from the ingot.

Furthermore, the plural laser beams are simultaneously applied to form the plural linear modified layers parallel to each other in the condition where the cracks formed between the adjacent modified layers are connected together. As a result, the separation start point can be efficiently formed to thereby sufficiently improve the productivity. Further, the amount of the ingot to be discarded can be sufficiently reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described in detail with reference to the drawings. Referring toFIG. 1, there is shown a perspective view of a laser processing apparatus2suitable for use in performing the wafer producing method of the present invention. The laser processing apparatus2includes a stationary base4and a first slide block6mounted on the stationary base4so as to be movable in the X direction. The first slide block6is moved in a feeding direction, or in the X direction along a pair of guide rails14by a feeding mechanism12including a ball screw8and a pulse motor10.

A second slide block16is mounted on the first slide block6so as to be movable in the Y direction. The second slide block16is moved in an indexing direction, or in the Y direction along a pair of guide rails24by an indexing mechanism22including a ball screw18and a pulse motor20. A support table26is mounted on the second slide block16. The support table26is movable in the X direction and the Y direction by the feeding mechanism12and the indexing mechanism22and also rotatable by a motor stored in the second slide block16.

A column28is provided on the stationary base4so as to project upward therefrom. A laser beam applying mechanism (laser beam applying means)30is mounted on the column28. The laser beam applying mechanism30includes a casing32, a laser beam generating unit34(seeFIG. 2) stored in the casing32, and focusing means (laser head)36mounted on the front end of the casing32. An imaging unit38having a microscope and a camera is also mounted on the front end of the casing32so as to be aligned with the focusing means36in the X direction.

As shown inFIG. 2, the laser beam generating unit34includes a laser oscillator40such as YAG laser and YVO4 laser for generating a pulsed laser beam, repetition frequency setting means42for setting the repetition frequency of the pulsed laser beam to be generated by the laser oscillator40, pulse width adjusting means44for adjusting the pulse width of the pulsed laser beam to be generated by the laser oscillator40, and power adjusting means46for adjusting the power of the pulsed laser beam generated by the laser oscillator40. Although especially not shown, the laser oscillator40has a Brewster window, so that the laser beam generated from the laser oscillator40is a laser beam of linearly polarized light. After the power of the pulsed laser beam is adjusted to a predetermined power by the power adjusting means46of the laser beam generating unit34, the pulsed laser beam is reflected by a mirror48included in the focusing means36and next branched into three laser beams by a diffractive optical element (DOE)50included in the focusing means36. These three laser beams obtained by the DOE50are next focused by a focusing lens51included in the focusing means36. The focusing lens51is positioned so that the pulsed laser beam is focused inside a hexagonal single crystal ingot11as a workpiece fixed to the support table26.

For example, the DOE50is provided by a blazed DOE as shown inFIG. 3. The blazed DOE50includes a transparent plate62and a sawtooth sectional structure64finely formed on one side (upper surface as viewed inFIG. 3) of the transparent plate62. The size d1of each sawtooth in the structure64is set to several tens to several hundreds of micrometers. The laser beam as incident light to the blazed DOE50is branched into zeroth-order light, first-order light, and second-order light. The zeroth-order light, the first-order light, and the second-order light are next emerged as three separate laser beams from the blazed DOE50.

Referring toFIG. 4A, there is shown a perspective view of the hexagonal single crystal ingot11as a workpiece to be processed.FIG. 4Bis an elevational view of the hexagonal single crystal ingot11shown inFIG. 4A. The hexagonal single crystal ingot (which will be hereinafter referred to also simply as ingot)11is selected from an SiC single crystal ingot and a GaN single crystal ingot. The ingot11has a first surface (upper surface)11aand a second surface (lower surface)11bopposite to the first surface11a. The first surface11aof the ingot11is preliminarily polished to a mirror finish because the laser beam is applied to the first surface11a.

The ingot11has a first orientation flat13and a second orientation flat15perpendicular to the first orientation flat13. The length of the first orientation flat13is set longer than the length of the second orientation flat15. The ingot11has a c-axis19inclined by an off angle α toward the second orientation flat15with respect to a normal17to the upper surface11aand also has a c-plane21perpendicular to the c-axis19. The c-plane21is inclined by the off angle α with respect to the upper surface11a. In general, in the hexagonal single crystal ingot11, the direction perpendicular to the direction of extension of the shorter second orientation flat15is the direction of inclination of the c-axis. The c-plane21is set in the ingot11innumerably at the molecular level of the ingot11. In this preferred embodiment, the off angle α is set to 4°. However, the off angle α is not limited to 4° in the present invention. For example, the off angle α may be freely set in the range of 1° to 6° in manufacturing the ingot11.

Referring again toFIG. 1, a column52is fixed to the left side of the stationary base4. The column52is formed with a vertically elongated opening53, and a pressing mechanism54is vertically movably mounted to the column52so as to project from the opening53.

As shown inFIG. 5, the ingot11is fixed to the upper surface of the support table26by using a wax or adhesive in the condition where the second orientation flat15of the ingot11becomes parallel to the X direction. In other words, as shown inFIG. 6, the direction of formation of the off angle α is shown by an arrow Y1. That is, the direction of the arrow Y1is the direction where the intersection19abetween the c-axis19and the upper surface11aof the ingot11is present with respect to the normal17to the upper surface11a. Further, the direction perpendicular to the direction of the arrow Y1is shown by an arrow A. Then, the ingot11is fixed to the support table26in the condition where the direction of the arrow A becomes parallel to the X direction.

Accordingly, the laser beam is scanned in the direction of the arrow A perpendicular to the direction of the arrow Y1, or the direction of formation of the off angle α. In other words, the direction of the arrow A perpendicular to the direction of the arrow Y1where the off angle α is formed is defined as the feeding direction of the support table26.

In the wafer producing method of the present invention, it is important that the scanning direction of the laser beam to be applied from the focusing means36is set to the direction of the arrow A perpendicular to the direction of the arrow Y1where the off angle α of the ingot11is formed. That is, it was found that by setting the scanning direction of the laser beam to the direction of the arrow A as mentioned above in the wafer producing method of the present invention, cracks propagating from a modified layer formed inside the ingot11by the laser beam extend very long along the c-plane21.

In performing the wafer producing method according to this preferred embodiment, a separation start point forming step is performed in such a manner that the focal point of the laser beam having a transmission wavelength (e.g., 1064 nm) to the hexagonal single crystal ingot11fixed to the support table26is set inside the ingot11at a predetermined depth from the first surface (upper surface)11a, which depth corresponds to the thickness of a wafer to be produced, and the laser beam is next applied to the upper surface11aas relatively moving the focal point and the ingot11to thereby form a modified layer23parallel to the upper surface11aand cracks25propagating from the modified layer23along the c-plane21, thus forming a separation start point.

This separation start point forming step includes a modified layer forming step of relatively moving the focal point of the laser beam in the direction of the arrow A perpendicular to the direction of the arrow Y1where the c-axis19is inclined by the off angle α with respect to the normal17to the upper surface11aand the off angle α is formed between the c-plane21and the upper surface11a, thereby forming the modified layer23inside the ingot11and the cracks25propagating from the modified layer23along the c-plane21, and also includes an indexing step of relatively moving the focal point in the direction of formation of the off angle α, i.e., in the Y direction to thereby index the focal point by a predetermined amount as shown inFIGS. 8 and 9.

As shown inFIGS. 7 and 8, the modified layer23is linearly formed so as to extend in the X direction, so that the cracks25propagate from the modified layer23in opposite directions along the c-plane21. In the wafer producing method according to this preferred embodiment, the separation start point forming step further includes an index amount setting step of measuring the width of the cracks25formed on one side of the modified layer23along the c-plane21and then setting the index amount of the focal point according to the width mentioned above. More specifically, letting W1denote the width of the cracks25formed on one side of the modified layer23so as to propagate from the modified layer23along the c-plane21, the predetermined spacing H between any adjacent ones of the two or more focal points is set so that the upper limit of the spacing H becomes nearly equal to 2W1 defined when the front ends of the cracks25extending from the adjacent modified layers23in the Y direction overlap each other.

The index amount L set in the case of simultaneously applying the plural laser beams is given as L=H×M, where M is the number of focal points. In this preferred embodiment, the number of focal points is three, so that L=3H. When H=400 μm, L=1200 μm.

For example, the separation start point forming step is performed under the following laser processing conditions.

Numerical aperture (NA) of the focusing lens: 0.43

Index amount: (250 to 400 μm)×(the number of focal points)

In the laser processing conditions mentioned above, the width W1of the cracks25propagating from the modified layer23along the c-plane21on one side as viewed inFIG. 7is set to approximately 250 μm, and the index amount L is set to 1200 μm. However, the average power of the laser beam is not limited to 3.2 W. When the average power of the laser beam was set to 2 W to 4.5 W, good results were obtained in the preferred embodiment. In the case that the average power was set to 2 W, the width W1of the cracks25was approximately 100 μm. In the case that the average power was set to 4.5 W, the width W1of the cracks25was approximately 350 μm.

In the case that the average power is less than 2 W or greater than 4.5 W, the modified layer23cannot be well formed inside the ingot11. Accordingly, the average power of the laser beam to be applied is preferably set in the range of 2 W to 4.5 W. For example, the average power of the laser beam to be applied to the ingot11was set to 3.2 W in this preferred embodiment. As shown inFIG. 7, the depth D1of each focal point from the upper surface11ain forming the modified layer23was set to 500 μm.

Referring toFIG. 9, there is shown a schematic plan view for illustrating the scanning direction of the laser beams. The separation start point forming step is performed on a forward path X1and a backward path X2as shown inFIG. 9. That is, the modified layers23are formed in the hexagonal single crystal ingot11on the forward path X1. Thereafter, the focal points of the laser beams are indexed by the predetermined amount. Thereafter, the modified layers23are formed again in the ingot11on the backward path X2.

In this manner, the focal points of the laser beams are sequentially indexed to form a plurality of modified layers23at the depth D1in the whole area of the ingot11and the cracks25extending from each modified layer23along the c-plane21. Thereafter, a wafer separating step is performed in such a manner that an external force is applied to the ingot11to thereby separate a plate-shaped member having a thickness corresponding to the thickness of the wafer to be produced, from the ingot11at the separation start point including the modified layers23and the cracks25, thus producing a hexagonal single crystal wafer27shown inFIG. 11.

This wafer separating step is performed by using the pressing mechanism54shown inFIG. 1. The configuration of the pressing mechanism54is shown inFIGS. 10A and 10B. The pressing mechanism54includes a head56vertically movable by a moving mechanism (not shown) incorporated in the column52shown inFIG. 1and a pressing member58rotatable in the direction shown by an arrow R inFIG. 10Bwith respect to the head56. As shown inFIG. 10A, the pressing mechanism54is relatively positioned above the ingot11fixed to the support table26. Thereafter, as shown inFIG. 10B, the head56is lowered until the pressing member58comes into pressure contact with the upper surface11aof the ingot11.

In the condition where the pressing member58is in pressure contact with the upper surface11aof the ingot11, the pressing member58is rotated in the direction of the arrow R to thereby generate a torsional stress in the ingot11. As a result, the ingot11is broken at the separation start point where the modified layers23and the cracks25are formed. Accordingly, the hexagonal single crystal wafer27shown inFIG. 11can be separated from the hexagonal single crystal ingot11. After separating the wafer27from the ingot11, the separation surface of the wafer27and the separation surface of the ingot11are preferably polished to a mirror finish.