Patent Publication Number: US-10319593-B2

Title: Wafer thinning method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer thinning method for thinning a wafer composed of an SiC substrate and a plurality of devices formed on the front side of the SiC substrate. 
     Description of the Related Art 
     Various devices such as integrated circuits (ICs) and large-scale integrations (LSIs) are formed by forming a functional layer on the front side of a wafer formed from a silicon substrate and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines. The back side of the wafer is ground by a grinding apparatus to thereby reduce the thickness of the wafer to a predetermined thickness. Thereafter, 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 equipment such as mobile phones and personal computers. 
     Further, power devices or optical devices such as light-emitting diodes (LEDs) and laser diodes (LDs) are formed by forming a functional layer on the front side of a wafer formed from an SiC substrate and partitioning this functional layer into a plurality of regions along a plurality of crossing division lines. As similarly to the case of the silicon wafer mentioned above, the back side of the SiC wafer is ground by a grinding apparatus to thereby reduce the thickness of the SiC wafer to a predetermined thickness. Thereafter, the division lines of the SiC wafer are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus to thereby divide the SiC wafer into a plurality of individual device chips corresponding to the power devices or the optical devices. The device chips thus obtained are widely used in various electronic equipment. 
     As described above, the back side of the SiC wafer is ground to thin the SiC wafer to a predetermined thickness before cutting the SiC wafer along the division lines as in the case of the silicon wafer. To achieve a recent reduction in weight and size of electronic equipment, it has been required to further reduce the thickness of a wafer to a thickness of approximately 50 μm, for example. However, such a wafer thinned by grinding is hard to handle and there is accordingly a possibility of damage to the wafer during transfer or the like. The front side of the wafer has a device area where the devices are formed and a peripheral marginal area surrounding the device area. To solve the above problem, there has been proposed a grinding method of grinding the back side of the wafer in only a central area corresponding to the device area, thereby forming a circular recess on the back side of the wafer in this central area, whereby a ring-shaped reinforcing portion is formed on the back side of the wafer in a peripheral area corresponding to the peripheral marginal area (see Japanese Patent Laid-Open No. 2007-19379). 
     SUMMARY OF THE INVENTION 
     However, an SiC substrate has Mohs hardness much higher than that of a silicon substrate. Accordingly, in grinding the back side of a wafer formed from an SiC substrate by using a grinding wheel having abrasive members, there is a problem such that the abrasive members may wear in an amount of approximately 4 times to 5 times the grinding amount of the wafer, causing very poor economy. For example, when the grinding amount of a silicon substrate is 100 μm, the wear amount of the abrasive members becomes 0.1 μm. In contrast, when the grinding amount of an SiC substrate is 100 μm, the wear amount of the abrasive members becomes 400 μm to 500 μm. Accordingly, the wear amount of the abrasive members in grinding an SiC substrate is 4000 times to 5000 times that in grinding a silicon substrate. 
     It is therefore an object of the present invention to provide a wafer thinning method which can thin a wafer formed from an SiC substrate to a predetermined thickness without grinding the back side of the wafer, wherein a plurality of devices are previously formed on the front side of the SiC substrate. 
     In accordance with an aspect of the present invention, there is provided a wafer thinning method for thinning a wafer formed from an SiC substrate 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 first surface of the SiC substrate having a device area where a plurality of devices are formed and a peripheral marginal area surrounding the device area. The wafer thinning method includes an annular groove forming step of forming an annular groove on the second surface of the SiC substrate in an annular area corresponding to the boundary between the device area and the peripheral marginal area in the condition where a thickness corresponding to the finished thickness of the wafer after thinning is left. The wafer thinning method further includes a separation start point forming step of setting the focal point of a laser beam having a transmission wavelength to the SiC substrate inside the SiC substrate in a central area surrounded by the annular groove at a predetermined depth from the second surface, which depth corresponds to the finished thickness of the wafer after thinning, and next applying the laser beam to the second surface as relatively moving the focal point and the SiC substrate to thereby form a modified layer inside the SiC substrate at the predetermined depth in the central area and also form cracks extending from the modified layer along the c-plane, thus forming a separation start point. The wafer thinning method further includes a wafer thinning step of applying an external force to the wafer after performing the separation start point forming step, thereby separating the wafer into a first wafer having the first surface of the SiC substrate and a second wafer having the second surface of the SiC substrate at the separation start point, whereby the thickness of the wafer is reduced to the finished thickness of the wafer after thinning as the thickness of the first wafer having the first surface of the SiC substrate, and a ring-shaped reinforcing portion is formed on a back side of the first wafer in a peripheral area corresponding to the peripheral marginal area. 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 second surface and the off angle is formed between the second 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 wafer thinning method further includes a grinding step of grinding the back side of the first wafer having the first surface except the ring-shaped reinforcing portion after performing the wafer thinning step, thereby flattening the back side of the first wafer except said ring-shaped reinforcing portion. 
     According to the wafer thinning method of the present invention, the modified layers and the cracks are formed inside the wafer in the central area surrounded by the annular groove. Thereafter, an external force is applied to the wafer to thereby separate the wafer into two wafers, that is, the first wafer and the second wafer at the separation start point (along a separation plane) composed of the modified layers and the cracks. Accordingly, the thickness of the wafer can be easily reduced to the finished thickness of the wafer after thinning as the thickness of the first wafer having the first surface on which the plural devices are formed. Accordingly, the wafer formed from the SiC substrate can be thinned without grinding the back side of the wafer by using abrasive members, so that the problem of uneconomical wearing of the abrasive members can be solved. Furthermore, the first wafer having the first surface has the ring-shaped reinforcing portion on the back side of the first wafer along the outer circumference thereof, so that damage to the first wafer can be suppressed by this reinforcing portion. 
     In the case of grinding and flattening the back side of the first wafer obtained by the wafer thinning step mentioned above, it is only necessary to slightly grind the back side of the first wafer by an amount of approximately 1 μm to 5 μm, so that the wear amount of the abrasive members can be suppressed to approximately 4 μm to 25 μm. In addition, the second wafer separated from the first wafer can be reused as an SiC substrate, thereby achieving great economy. 
     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 thinning method of the present invention; 
         FIG. 2  is a block diagram of a laser beam generating unit; 
         FIG. 3A  is a perspective view of an SiC ingot; 
         FIG. 3B  is an elevational view of the SiC ingot shown in  FIG. 3A ; 
         FIG. 4  is a perspective view showing a step of attaching a protective tape to the front side of an SiC wafer having a plurality of devices on the front side; 
         FIG. 5A  is a perspective view showing a step of placing the wafer shown in  FIG. 4  through the protective tape on a chuck table; 
         FIG. 5B  is a perspective view showing a condition where the wafer shown in  FIG. 5A  is held on the chuck table under suction; 
         FIG. 6  is a perspective view showing an annular groove forming step; 
         FIG. 7A  is a perspective view showing a condition where an annular groove has been formed on the back side of the wafer by performing the annular groove forming step; 
         FIG. 7B  is a sectional view of an essential part of the wafer including the annular groove shown in  FIG. 7A ; 
         FIG. 8  is a perspective view for illustrating a separation start point forming step; 
         FIG. 9  is a plan view of the wafer shown in  FIG. 8  as viewed from the back side of the wafer; 
         FIG. 10  is a schematic sectional view for illustrating a modified layer forming step; 
         FIG. 11  is a schematic plan view for illustrating the modified layer forming step; 
         FIGS. 12A and 12B  are perspective views for illustrating a wafer thinning step; 
         FIG. 13  is a perspective view showing a condition where the wafer has been separated into two wafers by performing the wafer thinning step; 
         FIG. 14  is a perspective view showing a grinding step of grinding the back side of the thinned wafer to thereby flatten the back side thereof; and 
         FIG. 15  is a perspective view of the wafer flattened by the grinding step as viewed from the back side of the wafer. 
     
    
    
     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 thinning 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 chuck table  26  having a suction holding portion  26   a  is mounted on the second slide block  16 . The chuck 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  mounted 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  such as YAG laser and YVO4 laser for generating a pulsed laser beam, repetition frequency setting means  42  for setting the repetition frequency of the pulsed laser beam to be generated by the laser oscillator  40 , pulse width adjusting means  44  for adjusting the pulse width of the pulsed laser beam to be generated by the laser oscillator  40 , and power adjusting means  46  for adjusting the power of the pulsed laser beam generated by 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 an SiC wafer  31  (to be hereinafter described) as a workpiece held on the suction holding portion  26   a  of the chuck table  26 . 
     Referring to  FIG. 3A , there is shown a perspective view of an SiC ingot (which will be hereinafter referred to also simply as ingot)  11 .  FIG. 3B  is an elevational view of the SiC ingot  11  shown in  FIG. 3A . 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 longer 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  of the ingot  11 . In general, in a hexagonal single crystal ingot including the SiC 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 the preferred embodiment, the off angle α is set to 4 degrees. However, the off angle α is not limited to 4 degrees in the present invention. For example, the off angle α may be freely set in the range of 1 degree to 6 degrees 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 . 
     Referring to  FIG. 4 , there is shown a perspective view of an SiC wafer  31  (SiC substrate) having a front side  31   a  (first surface) and a back side  31   b  (second surface).  FIG. 4  shows a step of attaching a protective tape  41  to the front side  31   a  of the SiC wafer  31 . The SiC wafer (which will be hereinafter referred to also simply as wafer)  31  is obtained by slicing the SiC ingot  11  shown in  FIGS. 3A and 3B  with a wire saw. For example, the SiC wafer  31  has a thickness of approximately 700 μm. After polishing the front side  31   a  of the wafer  31  to a mirror finish, a plurality of devices  35  such as power devices are formed on the front side  31   a  of the wafer  31  by photolithography. A plurality of crossing division lines  33  are formed on the front side  31   a  of the wafer  31  to thereby define a plurality of separate regions where the plural devices  35  are individually formed. The front side  31   a  of the wafer  31  has a device area  35   a  where the plural devices  35  are formed and a peripheral marginal area  31   c  surrounding the device area  35   a.    
     The SiC wafer  31  has a first orientation flat  37  and a second orientation flat  39  perpendicular to the first orientation flat  37 . The length of the first orientation flat  37  is set longer than the length of the second orientation flat  39 . Since the SiC wafer  31  is obtained by slicing the SiC ingot  11  shown in  FIGS. 3A and 3B  with a wire saw, the first orientation flat  37  corresponds to the first orientation flat  13  of the ingot  11 , and the second orientation flat  39  corresponds to the second orientation flat  15  of the ingot  11 . 
     The wafer  31  has a c-axis  19  inclined by an off angle α toward the second orientation flat  39  with respect to a normal  17  to the front side  31   a  and also has a c-plane  21  perpendicular to the c-axis  19  (see  FIGS. 3A and 3B ). The c-plane  21  is inclined by the off angle α with respect to the front side  31   a  of the wafer  31 . In the SiC wafer  31 , the direction perpendicular to the direction of extension of the shorter second orientation flat  39  is the direction of inclination of the c-axis  19 . 
     After attaching the protective tape  41  to the front side  31   a  of the wafer  31 , the wafer  31  is placed on a chuck table  60  included in a cutting apparatus (not shown) in the condition where the protective tape  41  is oriented downward as shown in  FIG. 5A . The chuck table  60  has a suction holding portion  60   a  for holding the wafer  31  under suction. The wafer  31  is placed on the chuck table  60  in the condition where the protective tape  41  is in contact with the suction holding portion  60   a  of the chuck table  60 . In this condition, a vacuum is applied to the suction holding portion  60   a  of the chuck table  60 , thereby holding the wafer  31  through the protective tape  41  on the chuck table  60  under suction in the condition where the back side  31   b  of the wafer  31  is exposed upward as shown in  FIG. 5B . 
     Thereafter, as shown in  FIG. 6 , the back side  31   b  of the wafer  31  held on the chuck table  60  is cut in an annular area corresponding to the boundary between the device area  35   a  and the peripheral marginal area  31   c  of the wafer  31  by using a cutting blade  64  of a cutting unit  62  included in the cutting apparatus. More specifically, the cutting blade  64  is rotated at a high speed in the direction shown by an arrow A in  FIG. 6 , and at the same time the chuck table  60  is slowly rotated in the direction shown by an arrow B in  FIG. 6 . The cutting blade  64  being rotated is lowered to cut in the back side  31   b  of the wafer  31  held on the chuck table  60  being rotated, so that an annular groove  47  is formed on the back side  31   b  of the wafer  31  as shown in  FIG. 7A  in the condition where a thickness corresponding to the finished thickness of the wafer  31  after thinning is left (annular groove forming step). As shown in  FIG. 7B  which is a sectional view of an essential part of the wafer  31  shown in  FIG. 7A , the thickness t 1  of the wafer  31  is 700 μm, and the finished thickness t 2  of the wafer  31  after thinning is 50 μm, so that the depth of the annular groove  47  is 650 μm. 
     After performing the annular groove forming step, the wafer  31  is held under suction on the chuck table  26  of the laser processing apparatus  2 , and the chuck table  26  holding the wafer  31  is rotated so that the second orientation flat  39  of the wafer  31  becomes parallel to the X direction as shown in  FIGS. 8 and 9 . In other words, as shown in  FIG. 9 , 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 an intersection  19   a  between the c-axis  19  and the back side  31   b  is present with respect to the normal  17  to the back side  31   b  of the wafer  31 . Further, the direction perpendicular to the direction of the arrow Y 1  is shown by an arrow A. Then, the chuck table  26  holding the wafer  31  is rotated so that the direction of the arrow A becomes parallel to the X direction, that is, the direction of the arrow A parallel to the second orientation flat  39  coincides with 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 chuck table  26 . 
     In the wafer thinning 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 wafer  31  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 thinning method of the present invention, cracks propagating from a modified layer formed inside the wafer  31  by the laser beam extend very long along the c-plane  21 . 
     In performing the wafer thinning method according to the 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 wafer  31  (SiC substrate) held on the chuck table  26  is set inside the wafer  31  in a central area surrounded by the annular groove  47  at a predetermined depth from the back side  31   b  (second surface), which depth corresponds to the finished thickness of the wafer  31  after thinning, and the laser beam is applied to the back side  31   b  as relatively moving the focal point and the wafer  31  to thereby form a modified layer  43  parallel to the front side  31   a  and cracks  45  propagating from the modified layer  43  along the c-plane  21 , thus forming a separation start point (see  FIG. 10 ). 
     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 back side  31   b  and the off angle α is formed between the c-plane  21  and the back side  31   b  as shown in  FIG. 9 , thereby forming the modified layer  43  inside the wafer  31  in a central area surrounded by the annular groove  47  and also forming the cracks  45  propagating from the modified layer  43  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  FIGS. 10 and 11 . 
     As shown in  FIGS. 10 and 11 , the modified layer  43  is linearly formed so as to extend in the X direction, so that the cracks  45  propagate from the modified layer  43  in opposite directions along the c-plane  21 . In the wafer thinning method according to the preferred embodiment, the separation start point forming step further includes an index amount setting step of measuring the width of the cracks  45  formed on one side of the modified layer  43  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  45  formed on one side of the modified layer  43  so as to propagate from the modified layer  43  along the c-plane  21 , an index amount W 2  of the focal point is set in the range of W 1  to 2W 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 focusing lens: 0.45 
     Index amount: 400 μm 
     In the laser processing conditions mentioned above, the width W 1  of the cracks  45  propagating from the modified layer  43  along the c-plane  21  in one direction as viewed in  FIG. 10  is set to approximately 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 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 W 1  of the cracks  45  was approximately 100 μm. In the case that the average power was set to 4.5 W, the width W 1  of the cracks  45  was approximately 350 μm. 
     In the case that the average power is less than 2 W or greater than 4.5 W, the modified layer  43  cannot be well formed inside the wafer  31 . 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 wafer  31  was set to 3.2 W in the preferred embodiment. As shown in  FIG. 10 , the depth D 1  of the focal point from the back side  31   b  in forming the modified layer  43  was set to 650 μm. 
     In this manner, the focal point of the laser beam is sequentially indexed to form a plurality of modified layers  43  at the depth D 1  from the back side  31   b  of the wafer  31  in the central area surrounded by the annular groove  47  and also to form the cracks  45  extending from each modified layer  43  along the c-plane  21  as shown in  FIG. 11 . Thereafter, a wafer thinning step is performed in such a manner that an external force is applied to the wafer  31  to thereby separate the wafer  31  into two wafers at the separation start point composed of the modified layers  43  and the cracks  45 , thus reducing the thickness of the wafer  31  (having a plural devices  35 ) to a finished thickness of approximately 50 μm. 
     This wafer thinning step is performed by using the pressing mechanism  54  shown in  FIG. 12A . The configuration of the pressing mechanism  54  is shown in  FIGS. 12A and 12B . 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. 12B  with respect to the head  56 . As shown in  FIG. 12A , the pressing mechanism  54  is relatively positioned above the wafer  31  held on the chuck table  26 . Thereafter, as shown in  FIG. 12B , the head  56  is lowered until the pressing member  58  comes into pressure contact with the back side  31   b  of the wafer  31  in the central area surrounded by the annular groove  47 . 
     In the condition where the pressing member  58  is in pressure contact with the back side  31   b  of the wafer  31 , the pressing member  58  is rotated in the direction of the arrow R to thereby generate a torsional stress in the wafer  31 . As a result, the wafer  31  is broken at the separation start point where the modified layers  43  and the cracks  45  are formed. Accordingly, as shown in  FIG. 13 , the wafer  31  can be separated into a first wafer  31 A held on the chuck table  26  and a second wafer  31 B, wherein the first wafer  31 A has the front side  31   a  (first surface) and the second wafer  31 B has the back side  31   b  (second surface). 
     As shown in  FIG. 13 , the wafer  31 A held on the chuck table  26  has a separation surface  49  as the back side. The separation surface  49  is a slightly rough surface where the modified layers  43  and the cracks  45  are partially left. That is, microscopic asperities are formed on the separation surface  49 . Accordingly, it is preferable to perform a grinding step of grinding the separation surface  49  as the back side of the wafer  31 A to thereby flatten the separation surface  49 . 
     In performing the grinding step, the wafer  31 A is held under suction through the protective tape  41  on a chuck table  68  included in a grinding apparatus (not shown) in the condition where the separation surface  49  is exposed upward as shown in  FIG. 14 . In  FIG. 14 , reference numeral  70  denotes a grinding unit included in the grinding apparatus. The grinding unit  70  includes a spindle  72  adapted to be rotationally driven by a motor (not shown), a wheel mount  74  fixed to the lower end of the spindle  72 , and a grinding wheel  76  detachably mounted to the lower surface of the wheel mount  74  by a plurality of screws  78 . The grinding wheel  76  is composed of an annular wheel base  80  and a plurality of abrasive members  82  fixed to the lower surface of the wheel base  80  so as to be arranged along the outer circumference thereof. 
     In the grinding step, the chuck table  68  is rotated at 300 rpm, for example, in the direction shown by an arrow a in  FIG. 14 . At the same time, the grinding wheel  76  is rotated at 6000 rpm, for example, in the direction shown by an arrow b in  FIG. 14 . Further, a grinding unit feeding mechanism (not shown) is driven to lower the grinding unit  70  until the abrasive members  82  of the grinding wheel  76  come into contact with the separation surface  49  of the wafer  31 A held on the chuck table  68 . Then, the grinding wheel  76  is fed downward by a predetermined amount at a predetermined feed speed (e.g., 0.1 μm/second), thereby grinding the separation surface  49  of the wafer  31 A to flatten the separation surface  49 . As a result, the modified layers  43  and the cracks  45  left on the separation surface  49  of the wafer  31 A can be removed to obtain a flat surface as shown in  FIG. 15 . 
     In the case of grinding and flattening the back side of the wafer  31 A obtained by the wafer thinning step mentioned above, it is only necessary to slightly grind the back side of the wafer  31 A by an amount of approximately 1 μm to 5 μm, so that the wear amount of the abrasive members  82  can be suppressed to approximately 4 μm to 25 μm. As shown in  FIG. 15 , a circular recess  51  is formed on the back side of the wafer  31 A in the central area corresponding to the device area  35   a  by performing the wafer thinning step, and the circular recess  51  has a bottom surface  51   a  ground to be flattened by the grinding step. 
     The back side of the wafer  31 A in a peripheral area corresponding to the peripheral marginal area  31   c  is left to form a ring-shaped reinforcing portion  53  as shown in  FIG. 15 , thereby preventing damage to the wafer  31 A and facilitating the handling of the wafer  31 A. Further, the wafer  31 B separated from the wafer  31 A in  FIG. 13  can be reused as an SiC substrate, thereby achieving great economy. 
     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.