Patent Publication Number: US-2022236185-A1

Title: Method and apparatus for detecting facet region, wafer producing method, and laser processing apparatus

Description:
This is a divisional application of application Ser. No. 16/675,798 filed Nov. 6, 2019, which claims the benefit of Japanese Patent Application No. 2018-210679, filed on Nov. 8, 2018. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method and an apparatus for detecting the Facet region of a SiC ingot, a wafer producing method for producing a SiC wafer from the SiC ingot, and a laser processing apparatus for forming a peeling layer in the SiC ingot. 
     Description of the Related Art 
     Devices such as integrated circuits (ICs), large-scale integrations (LSIs), light emitting diodes (LEDs), or the like are formed by laminating a functional layer to the top surface of a wafer whose material is Si (silicon), Al 2 O 3  (sapphire), or the like, and demarcating the devices by a plurality of planned dividing lines intersecting the functional layer. In addition, power devices, LEDs, or the like are formed by laminating a functional layer to the top surface of a wafer whose material is hexagonal single crystal SiC (silicon carbide), and demarcating the power devices, the LEDs, or the like by a plurality of planned dividing lines intersecting the functional layer. The wafer on which the devices are formed is divided into individual device chips by processing the planned dividing lines by a cutting apparatus or a laser processing apparatus. Each of the divided device chips is used in an electric apparatus such as a mobile telephone, a personal computer, or the like. 
     The wafer on which the devices are formed is generally produced by thinly cutting an ingot in a cylindrical shape with a wire saw. The top surface and undersurface of the cut wafer are finished into a mirror surface by polishing (see Japanese Patent Laid-Open No. 2000-94221, for example). However, when the ingot is cut by a wire saw, and the top surface and undersurface of the cut wafer are polished, a large part (70% to 80%) of the ingot is discarded, which is uneconomical. The SiC ingot, in particular, has a high hardness, and is difficult to cut with a wire saw. A considerable time is therefore taken to cut the SiC ingot with a wire saw, thus resulting in poor productivity. In addition, the unit price of the ingot is high, and there is a problem in producing the wafer efficiently. 
     Accordingly, the present applicant has proposed a technology that forms a peeling layer in a planned cutting plane by irradiating a SiC ingot with a laser beam having a wavelength transmissible through hexagonal single crystal SiC while positioning the focusing point of the laser beam within the SiC ingot, and peeling off a SiC wafer from the SiC ingot along the planned cutting plane in which the peeling layer is formed (see Japanese Patent Laid-Open No. 2016-111143, for example). 
     SUMMARY OF THE INVENTION 
     However, a region having a different crystal structure which region is referred to as a Facet region may be present within the SiC ingot. The Facet region has a high index of refraction and a high energy absorption rate as compared with a non-Facet region. Thus, the position and finished quality of the peeling layer formed within the SiC ingot by the application of the laser beam become nonuniform, and a level difference occurs between the Facet region and the non-Facet region in the wafer. 
     Accordingly, an object of the present invention is to provide a method of detecting the Facet region of a SiC ingot which method can detect the Facet region and a non-Facet region. 
     Another object of the present invention is to provide an apparatus for detecting the Facet region of a SiC ingot which apparatus can detect the Facet region and a non-Facet region. 
     Yet another object of the present invention is to provide a wafer producing method that can produce a wafer without a level difference between a Facet region and a non-Facet region. 
     Yet another object of the present invention is to provide a laser processing apparatus that can produce a wafer without a level difference between a Facet region and a non-Facet region. 
     In accordance with an aspect of the present invention, there is provided a method of detecting a Facet region of a SiC ingot, the method including: a fluorescence luminance detecting step of detecting fluorescence luminance unique to SiC by irradiating the SiC ingot with exciting light having a predetermined wavelength from a top surface of the SiC ingot; and a coordinate setting step of setting a region in which the fluorescence luminance is equal to or higher than a predetermined value in the fluorescence luminance detecting step as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value in the fluorescence luminance detecting step as a Facet region, and setting coordinates of a boundary between the Facet region and the non-Facet region. 
     In accordance with another aspect of the present invention, there is provided a wafer producing method for producing a SiC wafer from a SiC ingot, the wafer producing method including: a flat surface forming step of forming a top surface of the SiC ingot into a flat surface by grinding the top surface of the SiC ingot; a fluorescence luminance detecting step of detecting fluorescence luminance unique to SiC by irradiating the SiC ingot with exciting light having a predetermined wavelength from the top surface of the SiC ingot; a coordinate setting step of setting, as an X-axis, a direction orthogonal to a direction in which a c-plane is inclined with respect to the top surface of the SiC ingot and an off angle is formed, setting a direction orthogonal to the X-axis as a Y-axis, setting a region in which the fluorescence luminance is equal to or higher than a predetermined value in the fluorescence luminance detecting step as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value in the fluorescence luminance detecting step as a Facet region, and setting X-coordinates and Y-coordinates of a boundary between the Facet region and the non-Facet region; a processing feed step of forming a band-shaped peeling layer in which SiC is separated into Si and C and a crack extends along the c-plane, by positioning a focusing point formed by condensing a laser beam having a wavelength transmissible through SiC by a condenser at a depth corresponding to thickness of a wafer to be produced from the top surface of the SiC ingot, and processing-feeding the SiC ingot and the focusing point relative to each other in an X-axis direction while irradiating the SiC ingot with the laser beam; an indexing feed step of arranging band-shaped peeling layers in a Y-axis direction in parallel with each other by indexing-feeding the SiC ingot and the focusing point relative to each other in the Y-axis direction; and a peeling step of peeling off the wafer to be produced from the peeling layers; the processing feed step increasing energy of the laser beam and raising a position of the condenser at a time of irradiating the Facet region with the laser beam with respect to the energy of the laser beam and the position of the condenser at a time of irradiating the non-Facet region with the laser beam on a basis of the X-coordinates and the Y-coordinates of the boundary between the Facet region and the non-Facet region, the X-coordinates and the Y-coordinates being set in the coordinate setting step. 
     In accordance with a further aspect of the present invention, there is provided an apparatus for detecting a Facet region of a SiC ingot, the apparatus including: fluorescence luminance detecting means detecting fluorescence luminance unique to SiC by irradiating the SiC ingot with exciting light having a predetermined wavelength from a top surface of the SiC ingot; and coordinate setting means setting a region in which the fluorescence luminance detected by the fluorescence luminance detecting means is equal to or higher than a predetermined value as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value as a Facet region, and setting coordinates of a boundary between the Facet region and the non-Facet region. 
     In accordance with a still further aspect of the present invention, there is provided a laser processing apparatus for forming a peeling layer in a SiC ingot, the laser processing apparatus including: a holding table configured to hold the SiC ingot; fluorescence luminance detecting means detecting fluorescence luminance unique to SiC by irradiating the SiC ingot with exciting light having a predetermined wavelength from a top surface of the SiC ingot; coordinate setting means setting, as an X-axis, a direction orthogonal to a direction in which a c-plane is inclined with respect to the top surface of the SiC ingot and an off angle is formed, setting a direction orthogonal to the X-axis as a Y-axis, setting a region in which the fluorescence luminance detected by the fluorescence luminance detecting means is equal to or higher than a predetermined value as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value as a Facet region, and setting X-coordinates and Y-coordinates of a boundary between the Facet region and the non-Facet region; a laser beam irradiating unit including a condenser that forms a peeling layer in which SiC is separated into Si and C and a crack extends along the c-plane, by positioning a focusing point of a laser beam having a wavelength transmissible through SiC at a depth corresponding to thickness of a wafer to be produced from the top surface of the SiC ingot, and irradiating the SiC ingot with the laser beam; an X-axis feeding mechanism configured to processing-feed the holding table and the condenser relative to each other in an X-axis direction; a Y-axis feeding mechanism configured to indexing-feed the holding table and the condenser relative to each other in a Y-axis direction; and a control unit configured to increase energy of the laser beam and raise a position of the condenser at a time of irradiating the Facet region with the laser beam with respect to the energy of the laser beam and the position of the condenser at a time of irradiating the non-Facet region with the laser beam on a basis of the X-coordinates and Y-coordinates of the boundary between the Facet region and the non-Facet region. 
     According to the Facet region detecting method in accordance with the present invention, the boundary between the Facet region and the non-Facet region can be detected. Hence, on the basis of data on the detected Facet region and the detected non-Facet region, processing conditions for irradiating the SiC ingot with the laser beam can be controlled appropriately, so that the wafer without a level difference between the Facet region and the non-Facet region can be produced. 
     According to the wafer producing method in accordance with the present invention, the position and finished quality of the peeling layer formed in the Facet region and the non-Facet region become uniform, so that the wafer without a level difference between the Facet region and the non-Facet region can be produced. 
     According to the Facet region detecting apparatus in accordance with the present invention, processing conditions for irradiating the SiC ingot with the laser beam can be controlled appropriately on the basis of data on the detected Facet region and the detected non-Facet region, so that the wafer without a level difference between the Facet region and the non-Facet region can be produced. 
     According to the laser processing apparatus in accordance with the present invention, the position and finished quality of the peeling layer formed in the Facet region and the non-Facet region become uniform, so that the wafer without a level difference between the Facet region and the non-Facet region can be produced. 
     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 depicting some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a laser processing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of fluorescence luminance detecting means depicted in  FIG. 1 ; 
         FIG. 3  is a graph depicting relation between fluorescence wavelengths of a Facet region and a non-Facet region and luminance in cases where the wavelength of exciting light is 370 nm and 273 nm; 
         FIG. 4  is a perspective view depicting a state in which a flat surface forming step is being performed; 
         FIG. 5A  is a front view of a SiC ingot; 
         FIG. 5B  is a plan view of the SiC ingot; 
         FIG. 6  is a perspective view depicting a state in which a fluorescence luminance detecting step is being performed; 
         FIG. 7A  is a schematic diagram of an image of the SiC ingot imaged in the fluorescence luminance detecting step; 
         FIG. 7B  is a table of X-coordinates and Y-coordinates of a boundary between the Facet region and the non-Facet region, the X-coordinates and Y-coordinates being set in a coordinate setting step; 
         FIG. 8A  is a perspective view depicting a state in which a processing feed step is being performed; 
         FIG. 8B  is a sectional view depicting the state in which the processing feed step is being performed; 
         FIG. 9  is a sectional view depicting a peeling layer formed within the SiC ingot; and 
         FIG. 10  is a perspective view depicting a state in which a peeling step is being performed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a method and an apparatus for detecting a Facet region, a wafer producing method, and a laser processing apparatus according to the present invention will hereinafter be described with reference to the drawings. 
     A laser processing apparatus according to an embodiment of the present invention will first be described with reference to  FIG. 1 . The laser processing apparatus indicated in entirety by reference numeral  2  is constituted at least of: a holding unit  4  that holds a SiC ingot; fluorescence luminance detecting means  6  irradiating the SiC ingot with exciting light of a predetermined wavelength from the top surface of the SiC ingot and detecting fluorescence luminance unique to SiC; coordinate setting means  8  setting a region in which the fluorescence luminance detected by the fluorescence luminance detecting means  6  is equal to or higher than a predetermined value as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value as a Facet region, and setting the X-coordinates and Y-coordinates of a boundary between the Facet region and the non-Facet region; a laser beam irradiating unit  12  including a condenser  10  that forms a peeling layer in which SiC is separated into Si and C and cracks extend along a c-plane, by positioning a focusing point of a laser beam having a wavelength transmissible through SiC at a depth corresponding to the thickness of a wafer to be produced from the top surface of the SiC ingot, and irradiating the SiC ingot with a laser beam; an X-axis feeding mechanism  14  that processing-feeds the holding unit  4  and the condenser  10  relative to each other in an X-axis direction; a Y-axis feeding mechanism  16  that indexing-feeds the holding unit  4  and the condenser  10  relative to each other in a Y-axis direction; and a control unit  18  that controls the operation of the laser processing apparatus  2 . Incidentally, the X-axis direction is a direction indicated by an arrow X in  FIG. 1 , and the Y-axis direction is a direction indicated by an arrow Y in  FIG. 1  and is a direction orthogonal to the X-axis direction. In addition, a plane defined by the X-axis direction and the Y-axis direction is substantially horizontal. 
     As depicted in  FIG. 1 , the holding unit  4  includes: an X-axis movable plate  22  mounted on a base  20  so as to be movable in the X-axis direction; a Y-axis movable plate  24  mounted on the X-axis movable plate  22  so as to be movable in the Y-axis direction; a holding table  26  rotatably mounted on the top surface of the Y-axis movable plate  24 ; and a holding table motor (not depicted) that rotates the holding table  26 . 
     The fluorescence luminance detecting means  6  will be described with reference to  FIG. 1  and  FIG. 2 . The fluorescence luminance detecting means  6  according to the present embodiment is provided to a frame body  28  that extends upward from the top surface of the base  20  and next extends substantially horizontally. The fluorescence luminance detecting means  6  includes a case  30  fitted to an undersurface of an end of the frame body  28 . In addition, as depicted in  FIG. 2 , the fluorescence luminance detecting means  6  includes: a light source  32  that oscillates exciting light EL having a low power (for example, 0.1 W) at such a level that laser processing is not performed on the SiC ingot, and having a predetermined wavelength (for example, 370 nm); a dichroic mirror  34  that reflects the exciting light EL having the predetermined wavelength which exciting light is oscillated from the light source  32  and transmits light having a wavelength outside a first predetermined wavelength range (for example, 365 to 375 nm) including the above-described predetermined wavelength; a condensing lens  36  that condenses the exciting light EL reflected by the dichroic mirror  34  and irradiates the SiC ingot with the condensed exciting light EL; a band-pass filter  38  that transmits light in a second predetermined wavelength range (for example, 395 to 430 nm); and a photodetector  40  that detects the luminance of the light transmitted by the band-pass filter  38 . 
     In the present embodiment, as depicted in  FIG. 2 , the light source  32 , the dichroic mirror  34 , the condensing lens  36 , and the band-pass filter  38  are arranged within the case  30 . In addition, though not depicted, the fluorescence luminance detecting means  6  includes focusing point position adjusting means adjusting the vertical position of the focusing point of the exciting light EL by raising or lowering the case  30 . The focusing point position adjusting means can be constituted of a ball screw coupled to the case  30  and extending in a vertical direction and a motor for rotating the ball screw. 
     The exciting light EL emitted from the light source  32  is reflected by the dichroic mirror  34 , guided to the condensing lens  36 , condensed in the condensing lens  36 , and applied to the SiC ingot. When the exciting light EL is applied to the SiC ingot, fluorescence (radiated light) FL including a wavelength (for example, approximately 410 nm) different from the wavelength of the exciting light EL is emitted from the SiC ingot. The fluorescence FL passes through the condensing lens  36  and the dichroic mirror  34 . Only the fluorescence FL in the second predetermined wavelength range thereafter passes through the band-pass filter  38 . The luminance of the fluorescence FL passed through the band-pass filter  38  is detected by the photodetector  40 . The fluorescence luminance detecting means  6  detects the luminance of the fluorescence FL unique to SiC on the entire top surface of the SiC ingot by irradiating the SiC ingot with the exciting light EL having the predetermined wavelength from the top surface of the SiC ingot while the SiC ingot and the case  30  are moved relative to each other. 
     As depicted in  FIG. 2 , the coordinate setting means  8  is electrically connected to the photodetector  40  of the fluorescence luminance detecting means  6 . Data on the fluorescence luminance of each part of the SiC ingot which fluorescence luminance is detected by the photodetector  40  is input to the coordinate setting means  8 . Then, the coordinate setting means  8  sets, as an X-axis, a direction orthogonal to a direction in which the c-plane is inclined with respect to the top surface of the SiC ingot and an off angle is formed, sets a direction orthogonal to the X-axis as a Y-axis, sets, as a non-Facet region, a region in which the fluorescence luminance detected by the fluorescence luminance detecting means  6  is equal to or higher than the predetermined value, sets a region in which the fluorescence luminance is lower than the predetermined value as a Facet region, and sets the X-coordinates and Y-coordinates of a boundary between the Facet region and the non-Facet region. Incidentally, the X-axis and the Y-axis used by the coordinate setting means  8  are substantially identical to the above-described X-axis direction and the above-described Y-axis direction depicted in  FIG. 1 . 
     Here, description will be made of the predetermined value of the luminance as a determination criterion for the coordinate setting means  8  to distinguish between the Facet region and the non-Facet region. When the SiC ingot is irradiated with light having a wavelength of 370 nm or a wavelength of 273 nm as the exciting light EL, a luminance peak value appears in the vicinity of 410 nm in the wavelengths of the fluorescence FL emitted from the SiC ingot at either wavelength, as depicted in  FIG. 3 . On the other hand, as is understood by reference to  FIG. 3 , luminance peak values of the Facet region and the non-Facet region in the case where the wavelength of the exciting light EL is 370 nm are different from luminance peak values of the Facet region and the non-Facet region in the case where the wavelength of the exciting light EL is 273 nm. Hence, the predetermined value of the luminance as a determination criterion for distinguishing between the Facet region and the non-Facet region is set so as to be between the luminance peak value of the Facet region and the luminance peak value of the non-Facet region according to the wavelength of the exciting light EL. It suffices for the predetermined value to be about an intermediate value between the luminance peak value of the Facet region and the luminance peak value of the non-Facet region. In the case where the wavelength of the exciting light EL is 370 nm, for example, the luminance peak value of the Facet region is approximately 48 A. U. (see a thin solid line in  FIG. 3 ), and the luminance peak value of the non-Facet region is approximately 65 A. U. (see a thin dotted line in  FIG. 3 ). Thus, the predetermined value can be set to approximately 55 to 58 A. U. In addition, in the case where the wavelength of the exciting light EL is 273 nm, the luminance peak value of the Facet region is approximately 28 A. U. (see a thick solid line in  FIG. 3 ), and the luminance peak value of the non-Facet region is approximately 40 A. U. (see a thick dotted line in  FIG. 3 ). Thus, the predetermined value can be set to approximately 33 to 35 A. U. 
     As depicted in  FIG. 1 , the condenser  10  of the laser beam irradiating unit  12  is fitted to the undersurface of the end of the frame body  28  at an interval in the X-axis direction from the case  30  of the fluorescence luminance detecting means  6 . In addition, though not depicted, the laser beam irradiating unit  12  includes: a laser oscillator that oscillates a pulsed laser having a wavelength transmissible through SiC; an attenuator that adjusts the power of the pulsed laser beam emitted from the laser oscillator; and focusing point position adjusting means adjusting the vertical position of the focusing point of the pulsed laser beam by raising or lowering the condenser  10 . It suffices for the focusing point position adjusting means to have a configuration including a ball screw coupled to the condenser  10  and extending in the vertical direction and a motor that rotates the ball screw. 
     In the laser beam irradiating unit  12 , the condenser  10  is raised or lowered by the focusing point position adjusting means to position the focusing point of the pulsed laser beam having the wavelength transmissible through SiC at a depth corresponding to the thickness of a wafer to be produced from the top surface of the SiC ingot held by the holding unit  4 , and then the pulsed laser beam emitted from the laser oscillator and adjusted in power by the attenuator is condensed by the condenser  10  and applied to the SiC ingot. A peeling layer decreased in strength is thereby formed within the SiC ingot. 
     As depicted in  FIG. 1 , the X-axis feeding mechanism  14  includes a ball screw  42  coupled to the X-axis movable plate  22  and extending in the X-axis direction and a motor  44  coupled to one end portion of the ball screw  42 . The X-axis feeding mechanism  14  converts a rotary motion of the motor  44  into a rectilinear motion by the ball screw  42  and transmits the rectilinear motion to the X-axis movable plate  22 , and thereby advances or retreats the X-axis movable plate  22  relative to the condenser  10  in the X-axis direction along guide rails  20   a  on the base  20 . 
     The Y-axis feeding mechanism  16  includes a ball screw  46  coupled to the Y-axis movable plate  24  and extending in the Y-axis direction and a motor  48  coupled to one end portion of the ball screw  46 . The Y-axis feeding mechanism  16  converts a rotary motion of the motor  48  into a rectilinear motion by the ball screw  46  and transmits the rectilinear motion to the Y-axis movable plate  24 , and thereby advances or retreats the Y-axis movable plate  24  relative to the condenser  10  in the Y-axis direction along guide rails  22   a  on the X-axis movable plate  22 . 
     The control unit  18  is electrically connected to the coordinate setting means  8 . The X-coordinates and Y-coordinates of the boundary between the Facet region and the non-Facet region which coordinates are set by the coordinate setting means  8  are input to the control unit  18 . The control unit  18  increases the energy of the laser beam and raises the position of the condenser  10  at a time of irradiating the Facet region with the laser beam with respect to the energy of the laser beam and the position of the condenser  10  at a time of irradiating the non-Facet region with the laser beam on the basis of the X-coordinates and Y-coordinates of the boundary between the Facet region and the non-Facet region. Incidentally, while the control unit  18  and the coordinate setting means  8  may be constituted by respective separate computers, the control unit  18  and the coordinate setting means  8  may be constituted by a single computer. 
     In the present embodiment, as depicted in  FIG. 1 , the laser processing apparatus  2  further includes: an imaging unit  50  that images the SiC ingot held by the holding unit  4 ; a display unit  52  that displays an image imaged by the imaging unit  50 ; a grinding unit  54  that grinds the top surface of the SiC ingot held by the holding unit  4 ; and a peeling mechanism  56  that peels off the wafer to be produced from the peeling layer of the SiC ingot held by the holding unit  4 . 
     The imaging unit  50  is fitted to the undersurface of the end of the frame body  28 , and is disposed between the case  30  of the fluorescence luminance detecting means  6  and the condenser  10  of the laser beam irradiating unit  12 . In addition, the display unit  52  is disposed on the top surface of the frame body  28 . 
     The grinding unit  54  includes: a casing  58  fitted to a side surface of the frame body  28  so as to be movable in the Y-axis direction; a casing moving mechanism  60  that moves the casing  58  in the Y-axis direction; an arm  62  extending in the Y-axis direction from a base end supported by the casing  58  so as to be raisable and lowerable; arm raising and lowering means (not depicted) for raising and lowering the arm  62 ; and a spindle housing  64  fitted to an end of the arm  62 . 
     The spindle housing  64  rotatably supports a spindle  66  extending in the vertical direction, and includes a spindle motor (not depicted) that rotates the spindle  66 . Making description with reference to  FIG. 4 , a disk-shaped wheel mount  68  is fixed to a lower end of the spindle  66 , and an annular grinding wheel  72  is fixed by a bolt  70  to an undersurface of the wheel mount  68 . A plurality of grinding stones  74  annularly arranged at intervals in a circumferential direction are fixed to an outer circumferential edge portion of an undersurface of the grinding wheel  72 . 
     As depicted in  FIG. 1 , the peeling mechanism  56  includes: a casing  76  disposed at terminal portions of the guide rails  20   a  on the base  20 ; an arm  78  extending in the X-axis direction from a base end supported by the casing  76  so as to be raisable and lowerable; and arm raising and lowering means (not depicted) for raising and lowering the arm  78 . A motor  80  is attached to an end of the arm  78 . A suction piece  82  is coupled to an undersurface of the motor  80  so as to be rotatable about an axis extending in the vertical direction. A plurality of suction holes (not depicted) are formed in an undersurface of the suction piece  82 . The suction piece  82  is connected to suction means (not depicted). In addition, the suction piece  82  includes ultrasonic vibration applying means (not depicted) for applying ultrasonic vibration to the undersurface of the suction piece  82 . 
       FIG. 5A  and  FIG. 5B  show a SiC ingot  84  formed of SiC. The SiC ingot  84  as a whole is formed in a cylindrical shape. The SiC ingot  84  includes: a circular first end surface  86 , a circular second end surface  88  on an opposite side from the first end surface  86 ; a peripheral surface  90  located between the first end surface  86  and the second end surface  88 ; a c-axis (&lt;0001&gt; direction) extending from the first end surface  86  to the second end surface  88 ; and a c-plane ({0001} plane) orthogonal to the c-axis. 
     In the SiC ingot  84 , the c-axis is inclined with respect to a normal  92  to the first end surface  86 , and an off angle α (for example, α=1, 3, 6 degrees) is formed between the c-plane and the first end surface  86 . A direction in which the off angle α is formed is indicated by an arrow A in  FIG. 5 . In addition, a rectangular first orientation flat  94  and a rectangular second orientation flat  96  indicating a crystal orientation are formed on the peripheral surface  90  of the SiC ingot  84 . The first orientation flat  94  is parallel with the direction A in which the off angle α is formed. The second orientation flat  96  is orthogonal to the direction A in which the off angle α is formed. As depicted in  FIG. 5B , as viewed from above, a length L 2  of the second orientation flat  96  is shorter than a length L 1  of the first orientation flat  94  (L 2 &lt;L 1 ). 
     In addition, while the illustrated SiC ingot  84  is formed mainly of a hexagonal single crystal SiC, a Facet region  98  having a different crystal structure is locally present. The Facet region  98  is formed in a columnar shape from the first end surface  86  to the second end surface  88  of the SiC ingot  84 , and is in a same shape in a thickness direction (vertical direction) of the SiC ingot  84  as in a Kintaro candy. Incidentally, a non-Facet region other than the Facet region  98  is indicated by reference numeral  100 . 
     An embodiment of a wafer producing method according to the present invention will next be described. However, a wafer producing method using the above-described laser processing apparatus  2  will be described in the following. In the present embodiment, first, the SiC ingot  84  is fixed on the top surface of the holding table  26  via an appropriate adhesive (for example, an epoxy resin-based adhesive). Incidentally, a plurality of suction holes may be formed in the top surface of the holding table  26 , and the SiC ingot  84  may be sucked and held by generating a suction force in the top surface of the holding table  26 . 
     After the SiC ingot  84  is fixed on the holding table  26 , a flat surface forming step is performed in which the top surface of the SiC ingot  84  is ground and formed into a flat surface, except for a case where a flat top surface of the SiC ingot  84  is already formed. 
     In the flat surface forming step, first, the holding table  26  is positioned below the grinding wheel  72  of the grinding unit  54 . Next, as depicted in  FIG. 4 , the holding table  26  is rotated counterclockwise as viewed from above at a predetermined rotational speed (for example, 300 rpm). In addition, the spindle  66  is rotated counterclockwise as viewed from above at a predetermined rotational speed (for example, 6000 rpm). Next, the grinding stones  74  are brought into contact with the top surface of the SiC ingot  84  (first end surface  86  in the present embodiment) by lowering the arm  62  by the arm raising and lowering means. The arm  62  is thereafter lowered at a predetermined grinding feed speed (for example, 0.1 μm/s). Consequently, the top surface of the SiC ingot  84  can be ground and formed into such a flat surface as not to hinder the incidence of the laser beam. 
     After the flat surface forming step is performed, a fluorescence luminance detecting step is performed which detects fluorescence luminance unique to SiC by irradiating the SiC ingot  84  with the exciting light EL having the predetermined wavelength from the top surface of the SiC ingot  84 . 
     In the fluorescence luminance detecting step, first, the holding table  26  is positioned below the imaging unit  50 , and the imaging unit  50  images the SiC ingot  84  from the top surface thereof. Next, the orientation of the SiC ingot  84  is adjusted to a predetermined orientation and the positions in the XY plane of the SiC ingot  84  and the case  30  of the fluorescence luminance detecting means  6  are adjusted by moving and rotating the holding table  26  by the X-axis feeding mechanism  14 , the Y-axis feeding mechanism  16 , and the holding table motor on the basis of an image of the SiC ingot  84  imaged by the imaging unit  50 . When the orientation of the SiC ingot  84  is adjusted to a predetermined orientation, a direction orthogonal to the direction A in which the off angle α is formed is aligned with the X-axis direction and the direction A in which the off angle α is formed is aligned with the Y-axis direction by aligning the second orientation flat  96  with the X-axis direction, as depicted in  FIG. 6 . 
     Next, the focusing point of the exciting light EL is positioned at an appropriate position (for example, the first end surface  86 ) of the SiC ingot  84  by raising or lowering the case  30  by the focusing point position adjusting means. Next, the SiC ingot  84  is irradiated with the exciting light EL having a low power (for example, 0.1 W) at such a level that laser processing is not performed on the SiC ingot  84 , and having the predetermined wavelength (for example, 370 nm), while the X-axis feeding mechanism  14  moves the holding table  26  in the X-axis direction aligned with the direction orthogonal to the direction A in which the off angle α is formed. Then, as depicted in  FIG. 2 , fluorescence (radiated light) FL including a wavelength (for example, approximately 410 nm) different from the wavelength of the exciting light EL is emitted from the SiC ingot  84 . The fluorescence FL passes through the condensing lens  36  and the dichroic mirror  34 . Only the fluorescence FL in the second predetermined wavelength range (for example, 395 to 430 nm) thereafter passes through the band-pass filter  38 . The luminance of the fluorescence FL passed through the band-pass filter  38  is detected by the photodetector  40 . 
     Next, the SiC ingot  84  is indexing-fed relative to the focusing point of the exciting light EL in the Y-axis direction aligned with the direction A in which the off angle α is formed, by moving the holding table  26  by the Y-axis feeding mechanism  16 . Then, the irradiation with the exciting light EL and the indexing feed are alternately repeated to detect, in association with an X-coordinate and a Y-coordinate, the luminance of the fluorescence FL in each of minute regions obtained by dividing the whole of the first end surface  86  of the SiC ingot  84  into meshes of an appropriate size in the X-axis direction and the Y-axis direction. Data on the luminance of the fluorescence FL detected by the photodetector  40  is sent to the coordinate setting means  8  in association with the X-coordinates and the Y-coordinates. 
     In such a fluorescence luminance detecting step, the coordinate setting means  8  performs a coordinate setting step of setting, as the X-axis, the direction orthogonal to the direction A in which the c-plane is inclined with respect to the top surface of the SiC ingot  84  (first end surface  86  in the present embodiment) and the off angle α is formed, setting the direction orthogonal to the X-axis as the Y-axis, setting, as the non-Facet region  100 , a region in which the luminance of the fluorescence FL is equal to or higher than the predetermined value (for example, approximately 55 to 58 A. U. in the case where the wavelength of the exciting light EL is 370 nm), setting a region in which the luminance of the fluorescence FL is lower than the predetermined value as the Facet region  98 , and setting the X-coordinates and Y-coordinates of the boundary between the Facet region  98  and the non-Facet region  100 . In the coordinate setting step in the present embodiment, the coordinate setting means  8  sets a plurality of X-coordinates and Y-coordinates (of 24 points from point a to point x, for example) of the boundary between the Facet region  98  and the non-Facet region  100 , as depicted in  FIG. 7 , on the basis of the data on the luminance of the fluorescence FL which data is sent from the photodetector  40  of the fluorescence luminance detecting means  6 . Data on the plurality of X-coordinates and Y-coordinates of the boundary between the Facet region  98  and the non-Facet region  100  which coordinates are set by the coordinate setting means  8  is sent to the control unit  18 . Incidentally, the coordinate setting step may set the X-coordinates and Y-coordinates of the entire Facet region  98  and set the X-coordinates and Y-coordinates of the entire non-Facet region  100 , and send the X-coordinates and Y-coordinates of the entire Facet region  98  and the X-coordinates and Y-coordinates of the entire non-Facet region  100  to the control unit  18 . 
     After the coordinate setting step is performed, a processing feed step is performed which positions the focusing point formed by condensing the laser beam having the wavelength transmissible through SiC by the condenser  10  at a depth corresponding to the thickness of the wafer to be produced from the top surface of the SiC ingot  84 , processing-feeds the SiC ingot  84  and the focusing point relative to each other in the X-axis direction while irradiating the SiC ingot  84  with the laser beam, and thereby forms a band-shaped peeling layer in which SiC is separated into Si and C and cracks extend along the c-plane. 
     In the processing feed step, first, the positions in the XY plane of the SiC ingot  84  and the condenser  10  are adjusted by moving the holding table  26  in the X-axis direction and the Y-axis direction on the basis of the image of the SiC ingot  84  imaged by the imaging unit  50  in the fluorescence luminance detecting step. Next, the condenser  10  is raised or lowered by the focusing point position adjusting means to position the focusing point FP (see  FIG. 8B ) at the depth corresponding to the thickness of the wafer to be produced from the top surface of the SiC ingot  84  in the non-Facet region  100 . Next, as depicted in  FIG. 8A , the pulsed laser beam LB having the wavelength transmissible through SiC is applied from the condenser  10  to the SiC ingot  84  while the holding table  26  is moved at a predetermined feed speed in the X-axis direction aligned with the direction orthogonal to the direction A in which the off angle α is formed. Then, as depicted in  FIG. 9 , a band-shaped peeling layer  106  is formed along the X-axis direction in which peeling layer SiC is separated into Si (silicon) and C (carbon) by the application of the pulsed laser beam LB, the pulsed laser beam LB applied next is absorbed by C formed previously, and SiC is separated into Si and C in a chained manner, and also cracks  104  extend isotropically along the c-plane from a part  102  in which SiC is separated into Si and C. 
     In such a processing feed step, the control unit  18  controls the laser beam irradiating unit  12  so as to increase the energy of the pulsed laser beam LB and raise the position of the condenser  10  at a time of irradiating the Facet region  98  with the pulsed laser beam LB with respect to the energy of the pulsed laser beam LB and the position of the condenser  10  at a time of irradiating the non-Facet region  100  with the pulsed laser beam LB on the basis of the X-coordinates and Y-coordinates of the Facet region  98  and the non-Facet region  100  which coordinates are set in the coordinate setting step. The index of refraction of the Facet region  98  is higher than the index of refraction of the non-Facet region  100 . However, by performing control as described above, it is possible to make the depth of the focusing point FP substantially the same in the Facet region  98  and the non-Facet region  100 , and make the depth of the peeling layer  106  formed in the Facet region  98  and the non-Facet region  100  substantially uniform, as depicted in  FIG. 8B . In addition, the Facet region  98  has a higher energy absorption rate than the non-Facet region  100 . However, finished quality of the peeling layer  106  formed in the Facet region  98  and the non-Facet region  100  can be made uniform by increasing the energy of the pulsed laser beam LB applied to the Facet region  98  as compared with the energy of the pulsed laser beam LB applied to the non-Facet region  100 . 
     Such a processing feed step can be performed under the following processing conditions, for example. Incidentally, defocus in the following is an amount of movement when the condenser  10  is moved toward the top surface of the SiC ingot  84  from a state in which the focusing point FP of the pulsed laser beam LB is positioned at the top surface of the SiC ingot  84 . (Non-Facet region: an index of refraction of 2.65) Wavelength of the pulsed laser beam: 1064 nm 
     Average power: 7 W 
     Repetition frequency: 30 kHz 
     Pulse width: 3 ns 
     Feed speed: 165 mm/s 
     Defocus: 188 μm 
     Position of the peeling layer from the top surface of the SiC ingot: 500 μm (Facet region: an index of refraction of 2.79) 
     Wavelength of the pulsed laser beam: 1064 nm 
     Average power: 9.1 W 
     Repetition frequency: 30 kHz 
     Pulse width: 3 ns 
     Feed speed: 165 mm/s 
     Defocus: 179 μm 
     Position of the peeling layer from the top surface of the SiC ingot: 500 μm 
     In addition, an indexing feed step is performed which arranges band-shaped peeling layers  106  in the Y-axis direction in parallel with each other by indexing-feeding the SiC ingot  84  and the focusing point FP relative to each other in the Y-axis direction. In the present embodiment, the above-described processing feed step is repeated while the SiC ingot  84  is indexing-fed relative to the focusing point FP in the Y-axis direction by a predetermined indexing feed amount Li (see  FIG. 8A  and  FIG. 9 ). Consequently, band-shaped peeling layers  106  extending in the X-axis direction can be arranged in the Y-axis direction in parallel with each other within the SiC ingot  84 . In addition, the peeling off of the wafer in the following peeling step is facilitated by setting the indexing feed amount Li in a range not exceeding the width of the cracks  104 , and making the cracks  104  of peeling layers  106  adjacent to each other in the Y-axis direction overlap each other as viewed in the vertical direction. 
     After a plurality of band-shaped peeling layers  106  are formed in the SiC ingot  84  by performing the processing feed step and the indexing feed step, a peeling step is performed which peels off the wafer to be produced from the peeling layer  106 . In the peeling step, first, the holding table  26  is moved to a position below the suction piece  82  of the peeling mechanism  56 . Next, the arm  78  is lowered by the arm raising and lowering means to bring the undersurface of the suction piece  82  into close contact with the first end surface  86  of the SiC ingot  84 , as depicted in  FIG. 10 . Next, the undersurface of the suction piece  82  is stuck to the first end surface  86  of the SiC ingot  84  by actuating the suction means. Next, ultrasonic vibration is applied to the undersurface of the suction piece  82  by actuating the ultrasonic vibration applying means, and the suction piece  82  is rotated by the motor  80 . A SiC wafer  108  to be produced can be thereby peeled off from the peeling layer  106 . 
     In addition, by flattening a peeling surface by subjecting the SiC ingot  84  from which the SiC wafer  108  is peeled off to the above-described flat surface forming step, and thereafter repeating the processing feed step, the indexing feed step, and the peeling step, it is possible to produce a plurality of SiC wafers  108  from the SiC ingot  84 . As for the fluorescence luminance detecting step and the coordinate setting step, because the Facet region  98  is formed in a columnar shape from the top surface to the undersurface of the SiC ingot  84  and has a same shape as in a Kintaro candy in the thickness direction, it suffices to perform the fluorescence luminance detecting step and the coordinate setting step when the first SiC wafer  108  is produced from the SiC ingot  84 , and the fluorescence luminance detecting step and the coordinate setting step do not have to be performed when a second and subsequent SiC wafers  108  are produced. 
     As described above, in the present embodiment, the position and finished quality of the peeling layers  106  formed in the Facet region  98  and the non-Facet region  100  can be made uniform. It is therefore possible to produce the SiC wafer  108  without a level difference between the Facet region  98  and the non-Facet region  100 . Hence, it is not necessary to allow for the level difference between the Facet region  98  and the non-Facet region  100  and peel off a thick SiC wafer  108 . An improvement in efficiency can therefore be achieved. 
     Incidentally, while description has been made of an example in which the fluorescence luminance detecting means  6  and the coordinate setting means  8  are incorporated in the laser processing apparatus  2  in the foregoing present embodiment, the fluorescence luminance detecting means  6  and the coordinate setting means  8  may not be incorporated in the laser processing apparatus  2 . That is, the fluorescence luminance detecting means  6  and the coordinate setting means  8  may be constituent elements of a Facet region detecting apparatus including at least the fluorescence luminance detecting means  6  and the coordinate setting means  8 . Then, using the Facet region detecting apparatus including at least the fluorescence luminance detecting means  6  and the coordinate setting means  8 , a Facet region detecting method including at least the fluorescence luminance detecting step and the coordinate setting step described above may be performed. It is thereby possible to detect the Facet region  98  and the non-Facet region  100  of the SiC ingot  84 . Thus, on the basis of data on the detected Facet region  98  and the detected non-Facet region  100 , processing conditions for irradiating the SiC ingot  84  with the pulsed laser beam LB can be controlled appropriately, so that the SiC wafer  108  without a level difference between the Facet region  98  and the non-Facet region  100  can be produced. 
     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.