Patent Publication Number: US-9899262-B2

Title: Wafer processing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer processing method for dividing a wafer into individual device chips, the wafer being 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 respective 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 respective power devices or optical devices. The device chips thus obtained are widely used in various electronic equipment. 
     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 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 wear processing method which can thin a wafer formed from an SiC substrate to a predetermined thickness and divide the wafer into individual device chips, 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 processing method for dividing a wafer into individual device chips, the wafer being 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 wafer having a plurality of devices formed on the first surface of the SiC substrate so as to be separated by a plurality of crossing division lines. The wafer processing method includes: a division start point forming step of forming a division start point having a depth corresponding to the finished thickness of each device chip along each division line formed on the first surface; a protective member providing step of providing a protective member on the first surface after performing the division start point forming step; 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 at a predetermined depth from the second surface, which depth corresponds to a vertical position near the division start point, after performing the protective member providing step, and then applying the laser beam to the second surface as relatively moving the focal point and the SiC substrate 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 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. 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 first wafer is divided into the individual device chips by separating the wafer into the first wafer and the second wafer in the wafer separating step. Preferably, the wafer processing method further includes a grinding step of grinding the back side of the first wafer after performing the wafer separating step, thereby flattening the back side of the first wafer and dividing the first wafer into the individual device chips. 
     According to the wafer processing method of the present invention, the separation start point forming step is performed after performing the division start point forming step, thereby forming the separation start point inside the wafer in the whole area thereof, wherein the separation start point is composed of the modified layers and the cracks extending from the modified layers along the c-plane. 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 wafer formed from the SiC substrate can be thinned and divided into the individual device chips without grinding the second surface of the SiC substrate, that is, the back side of the wafer by using abrasive members. As a result, the problem of uneconomical wearing of the abrasive members can be solved. 
     In the case of flattening the back side of the first wafer obtained by the wafer separating 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 some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a laser processing apparatus suitable for use in performing the wafer processing 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 of an SiC wafer as viewed from the front side thereof; 
         FIG. 5  is a perspective view showing a first preferred embodiment of a division start point forming step constituting the wafer processing method of the present invention; 
         FIG. 6  is a perspective view showing a second preferred embodiment of the division start point forming step; 
         FIG. 7  is a perspective view showing a step of attaching a protective tape to the front side of the SiC wafer after performing the division start point forming step; 
         FIG. 8A  is a perspective view showing a step of placing the SiC wafer through the protective tape on a chuck table; 
         FIG. 8B  is a perspective view showing a condition where the SiC wafer shown in  FIG. 8A  is held on the chuck table under suction; 
         FIG. 9  is a perspective view for illustrating a separation start point forming step; 
         FIG. 10  is a plan view of the SiC wafer shown in  FIG. 9  as viewed from the back side thereof; 
         FIG. 11  is a schematic sectional view for illustrating a modified layer forming step; 
         FIG. 12  is a schematic plan view for illustrating the modified layer forming step; 
         FIGS. 13A and 13B  are perspective views for illustrating a wafer separating step; 
         FIG. 14  is a perspective view showing a condition where the SiC wafer has been separated into first and second wafers by performing the wafer separating step; 
         FIG. 15  is a perspective view showing a grinding step of grinding the back side of the first wafer to thereby flatten the back side thereof; and 
         FIG. 16  is a perspective view of the first wafer flattened by the grinding step as viewed from the back side of the first wafer, wherein the first wafer has been divided into individual device chips at the division start point. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 the wafer processing 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  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 ha 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, 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 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 . 
     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 the front side  31   a  of the SiC wafer  31  in perspective. 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 respective plural devices  35  are formed. 
     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 . 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 . 
     In the wafer processing method of the present invention, a division start point forming step is first performed in such a manner that a division start point having a depth corresponding to the finished thickness of each device chip is formed along each division line  33  formed on the first surface (front side)  31   a . Referring to  FIG. 5 , a first preferred embodiment of this division start point forming step is shown. The first preferred embodiment shown in  FIG. 5  is performed by using a cutting apparatus including a chuck table  60  for holding the wafer  31  and a cutting unit  62  for cutting the wafer  31  held on the chuck table  60 . The cutting unit  62  has a cutting blade  64  adapted to be rotated in the direction shown by an arrow A in  FIG. 5 . The wafer  31  is held on the chuck table  60  in the condition where the front side  31   a  is oriented upward. The cutting blade  64  of the cutting unit  62  is rotated at a high speed in the direction of the arrow A and then lowered to cut in the wafer  31  to a predetermined depth (e.g., approximately 50 μm) corresponding to the finished thickness of each device chip in an area corresponding to a predetermined one of the division lines  33  extending in a first direction. Thereafter, the chuck table  60  is fed in the X direction to thereby form a groove  41  as the division start point on the front side  31   a  along this predetermined division line  33 . 
     Thereafter, the cutting unit  62  is indexed in the Y direction to similarly form a plurality of grooves  41  along all of the other division lines  33  extending in the first direction. Thereafter, the chuck table  60  is rotated 90° to similarly form a plurality of grooves  41  along all of the division lines  33  extending in a second direction perpendicular to the first direction. 
     For example, the first preferred embodiment of the division start point forming step is performed under the following processing conditions. 
     Thickness of the cutting blade  64 : 30 μm 
     Diameter of the cutting blade  64 : 50 mm 
     Rotational speed of the cutting blade  64 : 20000 rpm 
     Work feed speed: 10 mm/second 
     Referring to  FIG. 6 , there is shown a perspective view for illustrating a second preferred embodiment of the division start point forming step according to the present invention. The second preferred embodiment shown in  FIG. 6  is performed by using the laser processing apparatus  2  shown in  FIG. 1 . The SiC wafer  31  is held on the chuck table  26  in the condition where the front side  31   a  is oriented upward. A laser beam having an absorption wavelength (e.g., 355 nm) to the SiC wafer  31  is applied from the focusing means  36  to the front side  31   a  along a predetermined one of the division lines  33  extending in a first direction as feeding the chuck table  26  in the X direction, thereby performing ablation to form a groove  41  as the division start point on the front side  31   a  along this predetermined division line  33 . 
     Thereafter, the chuck table  26  is indexed in the Y direction to similarly form a plurality of grooves  41  along all of the other division lines  33  extending in the first direction. Thereafter, the chuck table  26  is rotated 90° to similarly form a plurality of grooves  41  along all of the division lines  33  extending in a second direction perpendicular to the first direction. 
     For example, the second preferred embodiment of the division start point forming step is performed under the following processing conditions. 
     Light source: Nd:YAG pulsed laser 
     Wavelength of the laser beam: 355 nm 
     Repetition frequency: 50 kHz 
     Spot diameter: 10 μm 
     Average power: 2 W 
     Work feed speed: 100 mm/second 
     Although not shown, a third preferred embodiment of the division start point forming step may be performed by using the laser processing apparatus  2  shown in  FIG. 1 . In the third preferred embodiment, a laser beam having a transmission wavelength (e.g., 1064 nm) to the SiC wafer  31  is applied from the focusing means  36  to the front side  31   a  or the back side  31   b  of the SiC wafer  31  along a predetermined one of the division lines  33  extending in a first direction as feeding the chuck table  26  in the X direction, thereby forming a modified layer as the division start point near the front side  31   a  (at a depth of approximately 50 μm from the front side  31   a ) along this predetermined division line  33 . 
     Thereafter, the chuck table  26  is indexed in the Y direction to similarly form a plurality of modified layers along all of the other division lines  33  extending in the first direction. Thereafter, the chuck table  26  is rotated 90° to similarly form a plurality of modified layers along all of the division lines  33  extending in a second direction perpendicular to the first direction. 
     For example, the third preferred embodiment of the division start point forming step is performed under the following processing conditions. 
     Light source: Nd:YAG pulsed laser 
     Wavelength of the laser beam: 1064 nm 
     Repetition frequency: 50 kHz 
     Spot diameter: 10 μm 
     Average power: 1 W 
     Work feed speed: 300 mm/second 
     After performing the division start point forming step, a protective tape attaching step (protective member providing step) is performed as shown in  FIG. 7  in such a manner that a protective tape  47  (protective member) is attached to the front side  31   a  of the wafer  31 , in which the grooves  41  as the division start point have been formed on the front side  31   a  along the division lines  33 . After attaching the protective tape  47  to the front side  31   a  of the wafer  31 , the wafer  31  with the protective tape  47  is placed on the chuck table  26  in the condition where the protective tape  47  comes into contact with the upper surface of the chuck table  26  as shown in  FIG. 8A . Then, a vacuum is applied to the suction holding portion  26   a  of the chuck table  26  to hold the wafer  31  through the protective tape  47  on the chuck table  26  under suction as shown in  FIG. 8B . In this condition, the back side  31   b  of the wafer  31  held on the chuck table  26  is exposed upward. 
     Thereafter, 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. 9 and 10 . In other words, as shown in  FIG. 10 , 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 back side  31   b  of the wafer  31  is present with respect to the normal  17  to the back side  31   b . 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 perpendicular to 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 processing 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 processing 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 processing 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 wafer  31  (SiC substrate) held on the chuck table  26  is set inside the wafer  31  at a predetermined depth from the back side  31   b  (second surface), which depth corresponds to a vertical position near each groove  41  as the division start point, 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. 11 ). 
     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. 10 , thereby forming the modified layer  43  inside the wafer  31  and also forming the cracks  45  propagating from the modified layer  43  along the c-plane  21 . The separation start point forming step further 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. 11 and 12 . 
     As shown in  FIGS. 11 and 12 , 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 processing 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  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 , 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 
     Work feed speed: 500 mm/second 
     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. 11  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 this preferred embodiment. As shown in  FIG. 11 , 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 because the depth of each groove  41  corresponding to the finished thickness was set to approximately 50 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 whole area of the wafer  31  and also to form the cracks  45  extending from each modified layer  43  along the c-plane  21  as shown in  FIG. 12 . Thereafter, a wafer separating 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  to a finished thickness of approximately 50 μm. 
     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. 13A and 13B . 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. 13B  with respect to the head  56 . As shown in  FIG. 13A , the pressing mechanism  54  is relatively positioned above the wafer  31  held on the chuck table  26 . Thereafter, as shown in  FIG. 13B , 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 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. 14 , 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. 14 , 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 this grinding step, the wafer  31 A is held under suction through the protective tape  47  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. 15 . In  FIG. 15 , 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. 15 . 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. 15 . 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. 16 . Further, the grooves  41  are exposed to the flat surface (back side) of the wafer  31 A, thereby dividing the wafer  31 A into individual device chips. 
     In the case of forming the modified layers as the division start point in the third preferred embodiment of the division start point forming step as described above, the modified layers are broken by a pressing force applied from the grinding wheel  76  to the wafer  31 A, thereby dividing the wafer  31 A into individual device chips. In the case of flattening the back side of the wafer  31 A obtained by the wafer separating 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. Further, the wafer  31 B separated from the wafer  31 A in  FIG. 14  can be reused as an SiC substrate, thereby achieving great economy. 
     As another preferred embodiment of the separation start point forming step, the separation start point composed of the modified layers  43  and the cracks  45  may be formed so as to be superimposed on the grooves  41 . In this case, the wafer  31 A held on the chuck table  26  can be divided into individual device chips by performing the wafer separating step shown in  FIGS. 13A and 13B . Also in this case, the back side of the wafer  31 A (each device chip) is preferably ground to remove the modified layers  43  and the cracks  45  left on the back side of the wafer  31 A, thereby flattening the back side of the wafer  31 A. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.