Patent Publication Number: US-9884390-B2

Title: Wafer producing method

Description:
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 respective devices. The device chips thus obtained are widely used in various equipment 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 about 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 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. 2013-9161 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 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. 
     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 a 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 a 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 a 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 includes: 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. 
     Preferably, the separation start point forming step further includes an index amount setting step of measuring a width of the cracks formed on one side of the modified layer so as to propagate along the c-plane and then setting the index amount of the focal point according to the width measured above. Preferably, the index amount of the focal point is set in a range of W to 2 W where W is the width of the cracks formed on one side of the modified layer so as to propagate along the c-plane. 
     Preferably, the index amount of the focal point is set to W or less until the modified layer is first formed after setting the focal point inside the ingot. Preferably, the separation start point forming step is performed on a forward path and a backward path in such a manner that the modified layer is formed in the ingot on the forward path, the focal point is next indexed by the predetermined amount, and the modified layer is next formed again in the ingot on the backward path. 
     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. 
     The scanning direction of the laser beam is set to the first direction perpendicular to the second direction where the off angle is formed. Accordingly, the cracks formed on both sides of each modified layer so as to propagate along the c-plane extend very long, so that the index amount of the focal point can be increased to thereby sufficiently improve the productivity. Further, the amount of the ingot to be discarded can be sufficiently reduced to about 30%. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a laser processing apparatus suitable for use in performing a wafer producing method of the present invention; 
         FIG. 2  is a block diagram of a laser beam generating unit; 
         FIG. 3A  is a perspective view of a hexagonal single crystal ingot; 
         FIG. 3B  is an elevational view of the ingot shown in  FIG. 3A ; 
         FIG. 4  is a perspective view for illustrating a separation start point forming step; 
         FIG. 5  is a plan view of the ingot shown in  FIG. 3A ; 
         FIG. 6  is a schematic sectional view for illustrating a modified layer forming step; 
         FIG. 7  is a schematic plan view for illustrating the modified layer forming step; 
         FIG. 8A  is a schematic plan view for illustrating an indexing step; 
         FIG. 8B  is a schematic plan view for illustrating an index amount; 
         FIGS. 9A and 9B  are perspective views for illustrating a wafer separating step; and 
         FIG. 10  is a perspective view of a hexagonal single crystal wafer produced from the ingot. 
     
    
    
     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 to  FIG. 1 , there is shown a perspective view of a laser processing apparatus  2  suitable for use in performing a wafer producing method of the present invention. The laser processing apparatus  2  includes a stationary base  4  and a first slide block  6  mounted on the stationary base  4  so as to be movable in the X direction. The first slide block  6  is moved in a feeding direction, or in the X direction along a pair of guide rails  14  by a feeding mechanism  12  composed of a ball screw  8  and a pulse motor  10 . 
     A second slide block  16  is mounted on the first slide block  6  so as to be movable in the Y direction. The second slide block  16  is moved in an indexing direction, or in the Y direction along a pair of guide rails  24  by an indexing mechanism  22  composed of a ball screw  18  and a pulse motor  20 . A support table  26  is mounted on the second slide block  16 . The support table  26  is movable in the X direction and the Y direction by the feeding mechanism  12  and the indexing mechanism  22  and also rotatable by a motor stored in the second slide block  16 . 
     A column  28  is provided on the stationary base  4  so as to project upward therefrom. A laser beam applying mechanism (laser beam applying means)  30  is mounted on the column  28 . The laser beam applying mechanism  30  is composed of a casing  32 , a laser beam generating unit  34  (see  FIG. 2 ) stored in the casing  32 , and focusing means (laser head)  36  movably mounted in the Z direction on the front end of the casing  32 . An imaging unit  38  having a microscope and a camera is also mounted on the front end of the casing  32  so as to be aligned with the focusing means  36  in the X direction. 
     As shown in  FIG. 2 , the laser beam generating unit  34  includes a laser oscillator  40  for generating a pulsed laser beam such as YAG laser and YVO4 laser, repetition frequency setting means  42  for setting the repetition frequency of the pulsed laser beam to be generated from the laser oscillator  40 , pulse width adjusting means  44  for adjusting the pulse width of the pulsed laser beam to be generated from the laser oscillator  40 , and power adjusting means  46  for adjusting the power of the pulsed laser generated from the laser oscillator  40 . Although especially not shown, the laser oscillator  40  has a Brewster window, so that the laser beam generated from the laser oscillator  40  is 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 means  46  of the laser beam generating unit  34 , the pulsed laser beam is reflected by a mirror  48  included in the focusing means  36  and next focused by a focusing lens  50  included in the focusing means  36 . The focusing lens  50  is positioned so that the pulsed laser beam is focused inside a hexagonal single crystal ingot  11  as a workpiece fixed to the support table  26 . 
     Referring to  FIG. 3A , there is shown a perspective view of the hexagonal single crystal ingot  11  as a workpiece to be processed.  FIG. 3B  is an elevational view of the hexagonal single crystal ingot  11  shown in  FIG. 3A . The hexagonal single crystal ingot (which will be hereinafter referred to also simply as ingot)  11  is selected from a SiC single crystal ingot or a GaN single crystal ingot. The ingot  11  has a first surface (upper surface)  11   a  and a second surface (lower surface)  11   b  opposite to the first surface  11   a . The first surface  11   a  of the ingot  11  is preliminarily polished to a mirror finish because the laser beam is applied to the first surface  11   a.    
     The ingot  11  has a first orientation flat  13  and a second orientation flat  15  perpendicular to the first orientation flat  13 . The length of the first orientation flat  13  is set greater than the length of the second orientation flat  15 . The ingot  11  has a c-axis  19  inclined by an off angle α toward the second orientation flat  15  with respect to a normal  17  to the upper surface  11   a  and also has a c-plane  21  perpendicular to the c-axis  19 . The c-plane  21  is inclined by the off angle α with respect to the upper surface  11   a . In general, in the hexagonal single crystal ingot  11 , the direction perpendicular to the direction of extension of the shorter second orientation flat  15  is the direction of inclination of the c-axis  19 . The c-plane  21  is set in the ingot  11  innumerably at the molecular level of the ingot  11 . 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 ingot  11 . 
     Referring again to  FIG. 1 , a column  52  is fixed to the left side of the stationary base  4 . The column  52  is formed with a vertically elongated opening  53 , and a pressing mechanism  54  is vertically movably mounted to the column  52  so as to project from the opening  53 . 
     As shown in  FIG. 4 , the ingot  11  is fixed to the upper surface of the support table  26  by using a wax or adhesive in the condition where the second orientation flat  15  of the ingot  11  becomes parallel to the X direction. In other words, as shown in  FIG. 5 , the direction of formation of the off angle α is shown by an arrow Y 1 . That is, the direction of the arrow Y 1  is the direction where the intersection  19   a  between the c-axis  19  and the upper surface  11   a  of the ingot  11  is present with respect to the normal  17  to the upper surface  11   a . Further, the direction perpendicular to the direction of the arrow Y 1  is shown by an arrow A. Then, the ingot  11  is fixed to the support table  26  in 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 Y 1 , 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 Y 1  where the off angle α is formed is defined as the feeding direction of the support table  26 . 
     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 means  36  is set to the direction of the arrow A perpendicular to the direction of the arrow Y 1  where the off angle α of the ingot  11  is 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 ingot  11  by the laser beam extend very long along the c-plane  21 . 
     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 ingot  11  fixed to the support table  26  is set inside the ingot  11  at a predetermined depth from the first surface (upper surface)  11   a , which depth corresponds to the thickness of a wafer to be produced, and the laser beam is next applied to the upper surface  11   a  as relatively moving the focal point and the ingot  11  to thereby form a modified layer  23  parallel to the upper surface  11   a  and cracks  25  propagating from the modified layer  23  along the c-plane  21 , thus forming a separation start point (separation plane) where the modified layer  23  and the cracks  25  are formed. 
     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 Y 1  where the c-axis  19  is inclined by the off angle α with respect to the normal  17  to the upper surface  11   a  and the off angle α is formed between the c-plane  21  and the upper surface  11   a , thereby forming the modified layer  23  inside the ingot  11  and the cracks  25  propagating from the modified layer  23  along the c-plane  21 , 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 in  FIG. 7  and  FIGS. 8A and 8B . 
     As shown in  FIGS. 6 and 7 , the modified layer  23  is linearly formed so as to extend in the X direction, so that the cracks  25  propagate from the modified layer  23  in opposite directions along the c-plane  21 . 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 cracks  25  formed on one side of the modified layer  23  along the c-plane  21  and then setting the index amount of the focal point according to the width measured above. More specifically, letting W 1  denote the width of the cracks  25  formed on one side of the modified layer  23  so as to propagate from the modified layer  23  along the c-plane  21 , the index amount W 2  of the focal point is set in the range of W 1  to 2 W 1 . 
     For example, the separation start point forming step is performed under the following laser processing conditions. 
     Light source: Nd:YAG pulsed laser 
     Wavelength: 1064 nm 
     Repetition frequency: 80 kHz 
     Average power: 3.2 W 
     Pulse width: 4 ns 
     Spot diameter: 10 μm 
     Numerical aperture (NA) of the focusing lens: 0.45 
     Index amount: 400 μm 
     In the laser processing conditions mentioned above, the width W 1  of the cracks  25  propagating from the modified layer  23  along the C-plane  21  in one direction as viewed in  FIG. 6  is set to about 250 μm, and the index amount W 2  is set to 400 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 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 W 1  of the cracks  25  was about 100 μm. In the case that the average power was set to 4.5 W, the width W 1  of the cracks  25  was about 350 μm. 
     In the case that the average power is less than 2 W or greater than 4.5 W, the modified layer  23  cannot be well formed inside the ingot  11 . Accordingly, the average power of the laser beam to be applied is preferably set in the range of 2 to 4.5 W. For example, the average power of the laser beam to be applied to the ingot  11  was set to 3.2 W in this preferred embodiment. As shown in  FIG. 6 , the depth D 1  of the focal point from the upper surface  11   a  in forming the modified layer  23  was set to 500 μm. 
     Referring to  FIG. 8A , there is shown a schematic plan view for illustrating the scanning direction of the laser beam. The separation start point forming step is performed on a forward path X 1  and a backward path X 2  as shown in  FIG. 8A . That is, the modified layer  23  is formed in the hexagonal single crystal ingot  11  on the forward path X 1 . Thereafter, the focal point of the laser beam is indexed by the predetermined amount. Thereafter, the modified layer  23  is formed again in the ingot  11  on the backward path X 2 . 
     Further, in the case that the index amount of the focal point of the laser beam is set in the range of W to 2 W where W is the width of the cracks  25  formed on one side of the modified layer  23  along the c-plane  21 , the index amount of the focal point is preferably set to W or less until the modified layer  23  is first formed after setting the focal point of the laser beam inside the ingot  11 . 
     For example, in the case that the index amount of the focal point of the laser beam is 400 μm, the index amount is set to 200 μm until the modified layer  23  is first formed inside the ingot  11 , and the laser beam is scanned plural times with this index amount of 200 μm as shown in  FIG. 8B . That is, a first part of the plural scanning paths of the laser beam is idle, and when it is determined that the modified layer  23  has been first formed inside the ingot  11 , the index amount is set to 400 μm and the modified layer  23  is then formed inside the ingot  11 . 
     In this manner, the focal point of the laser beam is sequentially indexed to form a plurality of modified layers  23  at the depth D 1  in the whole area of the ingot  11  and also form the cracks  25  extending from each modified layer  23  along the c-plane  21 . Thereafter, a wafer separating step is performed in such a manner that an external force is applied to the ingot  11  to thereby separate a plate-shaped member having a thickness corresponding to the thickness of the wafer to be formed from the ingot  11  at the separation start point composed of the modified layers  23  and the cracks  25 , thus producing a hexagonal single crystal wafer  27  shown in  FIG. 10 . 
     This wafer separating step is performed by using the pressing mechanism  54  shown in  FIG. 1 . The configuration of the pressing mechanism  54  is shown in  FIGS. 9A and 9B . The pressing mechanism  54  includes a head  56  vertically movable by a moving mechanism (not shown) incorporated in the column  52  shown in  FIG. 1  and a pressing member  58  rotatable in the direction shown by an arrow R in  FIG. 9B  with respect to the head  56 . As shown in  FIG. 9A , the pressing mechanism  54  is relatively positioned above the ingot  11  fixed to the support table  26 . Thereafter, as shown in  FIG. 9B , the head  56  is lowered until the pressing member  58  comes into pressure contact with the upper surface  11   a  of the ingot  11 . 
     In the condition where the pressing member  58  is in pressure contact with the upper surface  11   a  of the ingot  11 , the pressing member  58  is rotated in the direction of the arrow R to thereby generate a torsional stress in the ingot  11 . As a result, the ingot  11  is broken at the separation start point where the modified layers  23  and the cracks  25  are formed. Accordingly, the hexagonal single crystal wafer  27  shown in  FIG. 10  can be separated from the hexagonal single crystal ingot  11 . After separating the wafer  27  from the ingot  11 , the separation surface of the wafer  27  and the separation surface of the ingot  11  are preferably polished to a mirror finish. 
     The present invention is not limited to the details of the above described preferred embodiment. 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.