Patent Publication Number: US-2023142363-A1

Title: Processing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a processing method for processing a single-crystal silicon wafer that has a first surface and a second surface located on the opposite side to the first surface that are formed in such a manner that a specific crystal plane included in the crystal plane {100} is exposed in each of the first surface and the second surface and has devices each formed in corresponding one of multiple regions marked out by multiple planned dividing lines set in a lattice manner in the first surface. 
     Description of the Related Art 
     To manufacture semiconductor device chips, for example, a wafer formed of single-crystal silicon is used. Specifically, first, multiple planned dividing lines are set in a lattice manner in the front surface of the wafer and devices such as an integrated circuit (IC) are formed in the respective regions that are marked out by the multiple planned dividing lines and have a rectangular shape. Subsequently, a dividing origin when the wafer is divided is formed along each planned dividing line by executing cutting processing or laser processing along each planned dividing line. Thereafter, the wafer is divided into multiple device chips by grinding the back surface side of the wafer (for example, refer to Japanese Patent Laid-open No. Hei 11-40520 and Japanese Patent Laid-open No. 2006-12902). 
     SUMMARY OF THE INVENTION 
     However, when the back surface side is ground, for example, a part with a thickness equal to or larger than half the thickness of the wafer before the grinding is removed, and therefore the amount of wear of grinding abrasive stones is comparatively large, which is uneconomic. In addition, a grinding apparatus is contaminated due to a lot of grinding dust generated in the grinding, and therefore a need to frequently clean the grinding apparatus arises. There is also a problem that the frequent cleaning is troublesome for the worker. The present invention is made in view of such a problem and intends to reduce the amount of grinding of a wafer after formation of a dividing origin when the wafer is divided into multiple device chips. 
     In accordance with an aspect of the present invention, there is provided a processing method for processing a single-crystal silicon wafer that has a first surface and a second surface located on the opposite side to the first surface that are formed in such a manner that a specific crystal plane included in a crystal plane {100} is exposed in each of the first surface and the second surface and has devices each formed in corresponding one of multiple regions marked out by multiple planned dividing lines set in a lattice manner in the first surface. The processing method includes a dividing origin forming step of forming dividing origins for dividing the single-crystal silicon wafer along each planned dividing line at least at a depth corresponding to a finished thickness of device chips, a separation layer forming step of forming a separation layer along the crystal plane of the second surface at a depth corresponding to a position on the side of the second surface relative to the dividing origins through positioning a focal point of a pulsed laser beam having such a wavelength as to be transmitted through the single-crystal silicon wafer to the inside of the single-crystal silicon wafer and relatively moving the focal point and the single-crystal silicon wafer along a first direction that is parallel to the crystal plane of the second surface and in which an acute angle formed between the first direction and a crystal orientation &lt;100&gt; is equal to or smaller than 5°, and a separation step of separating the single-crystal silicon wafer into a first-surface-side wafer including multiple devices formed on the side of the first surface and a second-surface-side wafer that is located on the side of the second surface and does not include the devices by using the separation layer as an origin after the dividing origin forming step and the separation layer forming step. The separation layer forming step has a modified region forming step of forming modified regions by relatively moving the focal point of the laser beam and the single-crystal silicon wafer along the first direction and an indexing feed step of executing indexing feed of the focal point and the single-crystal silicon wafer relatively in a second direction that is parallel to the crystal plane of the second surface and is orthogonal to the first direction. The separation layer includes the modified regions and cracks that extend with the modified regions being origins. 
     Preferably, the processing method further includes a grinding step of grinding the side of a third surface located on the opposite side to the first surface in the first-surface-side wafer and dividing the first-surface-side wafer into multiple device chips after the separation step. 
     In the processing method according to the aspect of the present invention, the dividing origins are formed at the depth corresponding to the finished thickness of the device chips in the dividing origin forming step. In addition, the separation layer is formed at the depth corresponding to a position on the second surface side relative to the dividing origins in the separation layer forming step. Moreover, in the separation step, the single-crystal silicon wafer is separated into the first-surface-side wafer including the multiple devices formed on the first surface side and the second-surface-side wafer that is located on the second surface side and does not include the devices by using the separation layer as the origin. Grinding the separation layer side of the first-surface-side wafer can divide the first-surface-side wafer into the multiple device chips. Therefore, the amount of grinding of the single-crystal silicon wafer can be reduced compared with the case of grinding the single-crystal silicon wafer from the second surface of the single-crystal silicon wafer and dividing the single-crystal silicon wafer into the multiple device chips. 
     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 wafer; 
         FIG.  2    is a plan view of the wafer; 
         FIG.  3    is a flowchart of a processing method; 
         FIG.  4    is a perspective view of a laser processing apparatus; 
         FIG.  5    is a schematic diagram of a laser beam irradiation unit; 
         FIG.  6    is a perspective view illustrating a dividing origin forming step; 
         FIG.  7    is a partially sectional side view illustrating the dividing origin forming step; 
         FIG.  8    is a perspective view illustrating a separation layer forming step; 
         FIG.  9    is a partially sectional side view illustrating the separation layer forming step; 
         FIG.  10    is a diagram illustrating a modified region forming step in the separation layer forming step; 
         FIG.  11    is a sectional view of part of the wafer illustrating a separation layer formed in a first round and a second round of the modified region forming step; 
         FIG.  12    is a diagram illustrating the state in which an external force is given to the wafer in a separation step; 
         FIG.  13    is a sectional view of part of the wafer illustrating the separation layer in the state in which cracks connect to each other; 
         FIG.  14 A  is a partially sectional side view illustrating the separation step; 
         FIG.  14 B  is a partially sectional side view illustrating a front-surface-side wafer and a back-surface-side wafer after the separation step; 
         FIG.  15    is a diagram illustrating a grinding step; 
         FIG.  16    is a perspective view illustrating multiple device chips; 
         FIG.  17    is a graph illustrating the width of the separation layer formed inside the wafer when linear regions that are each along a different crystal orientation are irradiated with a laser beam; 
         FIG.  18    is a partially sectional side view illustrating the state in which cutting processing of the front surface side is executed; 
         FIG.  19 A  is a flowchart of a processing method according to a second embodiment; 
         FIG.  19 B  is a flowchart of a processing method according to a third embodiment; 
         FIG.  20    is a partially sectional side view illustrating the state in which ablation processing is executed for the front surface side; 
         FIG.  21 A  is a flowchart of a processing method according to a fourth embodiment; and 
         FIG.  21 B  is a flowchart of a processing method according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the accompanying drawings.  FIG.  1    is a perspective view illustrating a wafer  11 .  FIG.  2    is a plan view illustrating the wafer  11 . The wafer  11  of the present specification means a single-crystal silicon wafer. In  FIG.  2   , crystal orientations of the wafer  11  are also illustrated. The wafer  11  has a circular plate shape and has a front surface (first surface)  11   a  and a back surface (second surface)  11   b  that have a substantially circular shape. The back surface  11   b  is located on the opposite side to the front surface  11   a  in the thickness direction of the wafer  11 . 
     The diameter of the wafer  11  is approximately 300 mm (12 inches), for example, and the thickness from the front surface  11   a  to the back surface  11   b  is approximately 775 μm. However, the diameter and the thickness of the wafer  11  are not limited to this example. The wafer  11  is formed in such a manner that a specific crystal plane included in the crystal plane {100} is exposed in each of the front surface  11   a  and the back surface  11   b . For example, as illustrated in  FIG.  2   , the crystal plane (100) is exposed in the back surface  11   b , and the crystal plane (100) is exposed also in the front surface  11   a . That is, each perpendicular line to the back surface  11   b  and the front surface  11   a  (crystal axis orthogonal to each surface) is along the crystal orientation [100]. 
     Each of the back surface  11   b  and the front surface  11   a  may be a surface slightly inclined from the specific crystal plane included in the crystal plane {100} due to a processing error in the manufacturing of the wafer  11 , or the like. Specifically, each of the back surface  11   b  and the front surface  11   a  may be a surface in which the acute angle formed between the surface and the crystal plane (100) is equal to or smaller than 1°. That is, the crystal axis orthogonal to the back surface  11   b  and the front surface  11   a  may be along a direction in which the acute angle formed between the direction and the crystal orientation [100] is equal to or smaller than 1°. 
     In the present specification, a state that the front surface  11   a  and the back surface  11   b  are formed in such a manner that a specific crystal plane included in the crystal plane {100} is exposed in each of the front surface  11   a  and the back surface  11   b  includes a case in which the front surface  11   a  and the back surface  11   b  are surfaces slightly inclined from the specific crystal plane in addition to a case in which the front surface  11   a  and the back surface  11   b  are the specific crystal plane included in the crystal plane {100}. A state of being slightly inclined from the specific crystal plane means that the acute angle formed between the front surface  11   a  (the back surface  11   b ) and the crystal plane (100) is equal to or smaller than 1°, for example. 
     Incidentally, a notch  13  indicating the crystal orientation of the wafer  11  is formed at the outer circumferential part of the wafer  11 . The direction that goes from the notch  13  to a center A of the back surface  11   b  (or the center of the front surface  11   a ) is a specific crystal orientation included in the crystal orientation &lt;110&gt;. In the present embodiment, as illustrated in  FIG.  2   , the direction that goes from the notch  13  to the center A is the crystal orientation [011]. As illustrated in  FIG.  1   , multiple planned dividing lines (streets)  15  are set in a lattice manner in the front surface  11   a.    
     A device  17  such as an IC is formed in each of multiple regions marked out by the multiple planned dividing lines  15 . Next, description will be made about a processing method of the wafer  11  in which the wafer  11  is divided into multiple device chips  35  by processing the wafer  11  along each planned dividing line  15  (that is, a manufacturing method of the device chips  35 ). 
       FIG.  3    is a flowchart of a processing method according to a first embodiment. The outline of the processing method is as follows. First, the wafer  11  is held by a chuck table  26  (see  FIG.  4   ) to be described later (holding step S 10 ). Thereafter, dividing origins  23  (see  FIG.  7   ) are formed on the side of the front surface  11   a  of the wafer  11  by using a laser processing apparatus  2  (see  FIG.  4   ) (dividing origin forming step S 20 ). Subsequently, a separation layer  25  (see  FIG.  9    to  FIG.  12   ) is formed inside the wafer  11  by using the laser processing apparatus  2  (separation layer forming step S 30 ). 
     Thereafter, the wafer  11  is separated into a front-surface-side wafer (first-surface-side wafer)  31  having the devices  17  and a back-surface-side wafer (second-surface-side wafer)  33  that does not have the devices  17  by using the separation layer  25  as the origin (see  FIG.  14 A  and  FIG.  14 B ) (separation step S 40 ). Subsequently, the side of a separation surface (third surface)  31   a  located on the opposite side to the front surface  11   a  in the front-surface-side wafer (first-surface-side wafer)  31  having the devices  17  is ground (see  FIG.  15   ), and thus the front-surface-side wafer  31  is divided into the multiple device chips  35  (grinding step S 50 ) (see  FIG.  16   ). 
       FIG.  4    is a perspective view of the laser processing apparatus  2 . An X-axis direction (left-right direction), a Y-axis direction (front-rear direction), and a Z-axis direction (upward-downward direction, vertical direction) each illustrated in  FIG.  4    are orthogonal to each other. The laser processing apparatus  2  has a base  4  that supports the respective constituent elements. An X-axis Y-axis movement mechanism  6  is disposed on the upper surface of the base  4 . The X-axis Y-axis movement mechanism  6  has a pair of Y-axis guide rails  8  that are fixed to the upper surface of the base  4  and are disposed along the Y-axis direction. 
     A Y-axis moving plate  10  is attached to the upper surface side of the pair of Y-axis guide rails  8  slidably along the pair of Y-axis guide rails  8 . A ball screw is disposed on the lower surface side of the Y-axis moving plate  10 . The ball screw has a nut part (not illustrated) fixed to the lower surface of the Y-axis moving plate  10 . A screw shaft  12  is coupled to the nut part in such a manner as to be capable of rotating by using balls (not illustrated). The screw shaft  12  is disposed between the pair of Y-axis guide rails  8  along the Y-axis direction. A motor  14  for rotating the screw shaft  12  is coupled to one end part of the screw shaft  12 . When the motor  14  is operated, the Y-axis moving plate  10  moves along the Y-axis direction. The pair of Y-axis guide rails  8 , the Y-axis moving plate  10 , the screw shaft  12 , the nut part, the motor  14 , and so forth configure a Y-axis movement mechanism. 
     A pair of X-axis guide rails  16  are fixed to the upper surface of the Y-axis moving plate  10 . The pair of X-axis guide rails  16  are disposed along the X-axis direction. An X-axis moving plate  18  is attached to the upper surface side of the pair of X-axis guide rails  16  slidably along the pair of X-axis guide rails  16 . A ball screw is disposed on the lower surface side of the X-axis moving plate  18 . The ball screw has a nut part (not illustrated) fixed to the lower surface of the X-axis moving plate  18 . A screw shaft  20  is coupled to the nut part in such a manner as to be capable of rotating by using balls (not illustrated). The screw shaft  20  is disposed between the pair of X-axis guide rails  16  along the X-axis direction. A motor  22  for rotating the screw shaft  20  is coupled to one end part of the screw shaft  20 . When the motor  22  is operated, the X-axis moving plate  18  moves along the X-axis direction. The pair of X-axis guide rails  16 , the X-axis moving plate  18 , the screw shaft  20 , the nut part, the motor  22 , and so forth configure an X-axis movement mechanism. 
     A table base  24  with a circular column shape is disposed on the upper surface side of the X-axis moving plate  18 . The table base  24  has a rotational drive source (not illustrated) such as a motor. The chuck table  26  with a circular plate shape is disposed on the top part of the table base  24 . The rotational drive source can rotate the chuck table  26  in a predetermined angle range with a straight line that passes through the center of a holding surface  26   a  of the chuck table  26  and is parallel to the Z-axis direction being the rotation axis. The chuck table  26  has a circular plate-shaped frame body formed of a non-porous metal. A recessed part (not illustrated) with a circular plate shape is formed at a central part of the frame body. A circular plate-shaped porous plate formed of a ceramic is fixed to this recessed part. A predetermined flow path (not illustrated) is formed in the frame body. A negative pressure is transmitted from a suction source (not illustrated) such as an ejector to the upper surface of the porous plate through the predetermined flow path. 
     The annular upper surface of the frame body and the circular upper surface of the porous plate are substantially flush with each other and function as the substantially flat holding surface  26   a  for sucking and holding the wafer  11 . The wafer  11  can move along both the X-axis and Y-axis directions by the X-axis Y-axis movement mechanism  6  in the state of being sucked and held by the holding surface  26   a . At the outer circumferential part of the chuck table  26 , multiple (in the present embodiment, four) clamp units  26   b  are disposed at substantially equal intervals along the circumferential direction of the chuck table  26 . Each clamp unit  26   b  clamps a frame  19   b  (see  FIG.  16   ) of a wafer unit  21  to be described later. 
     A support structure  30  is disposed on a predetermined region in the base  4  located on the rear side of the X-axis Y-axis movement mechanism  6 . A Z-axis movement mechanism  32  is disposed on one side surface along the Y-Z plane in the support structure  30 . The Z-axis movement mechanism  32  has a pair of Z-axis guide rails  34 . The pair of Z-axis guide rails  34  are fixed to the one side surface of the support structure  30  and are disposed along the Z-axis direction. A Z-axis moving plate  36  is attached to the pair of Z-axis guide rails  34  slidably along the pair of Z-axis guide rails  34 . A ball screw (not illustrated) is disposed on the back surface side of the Z-axis moving plate  36 . The ball screw has a nut part (not illustrated) fixed to the back surface of the Z-axis moving plate  36 . 
     A screw shaft (not illustrated) is coupled to the nut part in such a manner as to be capable of rotating by using balls. The screw shaft is disposed between the pair of Z-axis guide rails  34  along the Z-axis direction. A motor  38  for rotating the screw shaft is coupled to an upper end part of the screw shaft. When the motor  38  is operated, the Z-axis moving plate  36  moves along the Z-axis direction. A support implement  40  is fixed to the front surface side of the Z-axis moving plate  36 . The support implement  40  supports part of a laser beam irradiation unit  42 . 
       FIG.  5    is a schematic diagram of the laser beam irradiation unit  42 . In  FIG.  5   , part of constituent elements of the laser beam irradiation unit  42  is illustrated by functional blocks. The laser beam irradiation unit  42  has a laser oscillator  44  fixed to the base  4 . For example, the laser oscillator  44  has Nd:YVO 4  or the like as a laser medium and emits a pulsed laser beam L A  having such a wavelength (for example, 1342 nm) as to be transmitted through the wafer  11  (that is, a single-crystal silicon). The laser beam L A  travels to a splitting unit  48  after its output power is adjusted by an attenuator  46 . The splitting unit  48  of the present embodiment has a liquid crystal on silicon-spatial light modulator (LCOS-SLM). 
     The splitting unit  48  has a function of splitting the laser beam L A . For example, the splitting unit  48  splits the laser beam L A  in such a manner that the laser beam L A  emitted from an irradiation head  52  forms multiple focal points that line up at substantially equal intervals along the Y-axis direction. In  FIG.  5   , an example in which the laser beam L A  is split to form five focal points P 1  to P 5  by the splitting unit  48  is illustrated. However, the laser beam L A  may be split to form focal points in a predetermined number equal to or larger than two (more specifically at least two and at most 16). 
     The splitting unit  48  has also a function of allowing the laser beam L A  to be merely transmitted through the splitting unit  48  without splitting the laser beam L A . Whether or not to split the laser beam L A  can be selected by controlling operation of the splitting unit  48 . Incidentally, the splitting unit  48  may have a diffraction grating instead of the LCOS-SLM. The diffraction grating splits the laser beam L A  to form a predetermined number of focal points. Therefore, it is sufficient that the diffraction grating is removed from the optical path of the laser beam L A  in a case in which the laser beam L A  is not split. 
     The laser beam L A  that has passed through the splitting unit  48  is reflected by a mirror  50  and is guided to the irradiation head  52 . A collecting lens (not illustrated) that focuses the laser beam L A  and so forth are housed in the irradiation head  52 . At the time of laser processing, the irradiation head  52  is disposed to face the holding surface  26   a , and the laser beam L A  is emitted to the holding surface  26   a . The irradiation head  52  is disposed at a front end part of a circular columnar housing  54  having a longitudinal part disposed along the Y-axis direction (see  FIG.  4   ). 
     Part of the housing  54  on the rear end part side thereof is fixed by the support implement  40 . Moreover, an imaging unit  56  is fixed to the side surface of the housing  54  located near the irradiation head  52  in such a manner as to be capable of facing the holding surface  26   a . For example, the imaging unit  56  is a visible light camera unit having an objective lens, a light source such as a light emitting diode (LED), and an imaging element such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor. 
     In a case of the visible light camera unit, for example, a photodiode made of silicon (Si) is used as the imaging element. The imaging unit  56  may be an infrared camera unit having a light source such as an LED and an imaging element. In a case of the infrared camera unit, for example, a photodiode made of indium gallium arsenide (InGaAs) is used as the imaging element. When the infrared camera unit is used, even in a case in which the side of the front surface  11   a  is sucked and held by the holding surface  26   a , the planned dividing line  15  in the front surface  11   a  can be imaged with transmission through the wafer  11  from the side of the back surface  11   b.    
     The irradiation head  52 , the housing  54 , the imaging unit  56 , and so forth can integrally move along the Z-axis direction by the Z-axis movement mechanism  32 . A cover (not illustrated) that covers the above-described constituent elements is disposed over the base  4 . A touch panel  58  is disposed on the front surface of this cover. The touch panel  58  functions as an input device such as a touch sensor of the capacitive system and a display device such as a liquid crystal display. A worker can set a processing condition for the laser processing apparatus  2  through the touch panel  58  and can also view an image of the wafer  11  obtained by the imaging unit  56 . 
     Next, the processing method of the wafer  11  will be described along the respective steps illustrated in  FIG.  3   . First, prior to the holding step S 10 , the wafer unit  21  in which the wafer  11  is supported by the annular frame  19   b  through a protective tape  19   a  with a larger diameter than the wafer  11  is formed (see  FIG.  7   ). More specifically, the wafer unit  21  is formed by sticking the side of the front surface  11   a  of the wafer  11  to a central part of the protective tape  19   a  and sticking one surface of the frame  19   b  that has an opening with a larger diameter than the wafer  11  and that is made of a metal and has an annular shape to an outer circumferential part of the protective tape  19   a.    
     In the present embodiment, the wafer  11  is processed in the form of the wafer unit  21 . However, the steps from the holding step S 10  to the grinding step S 50  may be executed in a state in which the protective tape  19   a  with substantially the same diameter as the wafer  11  is stuck to the side of the front surface  11   a  without using the frame  19   b . In the holding step S 10 , the side of the front surface  11   a  is sucked and held by the holding surface  26   a , and the frame  19   b  is clamped by the respective clamp units  26   b . At this time, the side of the back surface  11   b  is exposed upward. 
     Subsequently, modified regions  23   a  that function as the dividing origins  23  are formed along each planned dividing line  15  by executing laser processing of the wafer  11  (dividing origin forming step S 20 ). The modified region  23   a  is a region in which the crystallinity of the wafer  11  is disturbed due to the occurrence of multiphoton absorption, and the mechanical strength is lowered therein compared with the region that is not irradiated with the laser beam L A . 
     At the time of the formation of the modified regions  23   a , cracks  23   b  extending from the modified regions  23   a  to the side of the front surface  11   a  and the side of the back surface  11   b  are collaterally formed. However, in the present specification, the dividing origin  23  formed by using the laser beam L A  refers to the modified region  23   a . In the dividing origin forming step S 20 , first, the deviation between one planned dividing line  15  and the X-axis direction is detected by using the imaging unit  56 . Thereafter, the chuck table  26  is rotated around the predetermined rotation axis to cause the one planned dividing line  15  to become substantially parallel to the X-axis direction. 
     Then, without splitting the laser beam L A , one focal point of the laser beam L A  is positioned between a depth  11   d  from the back surface  11   b  corresponding to a finished thickness  11   c  of the device chips  35  and the back surface  11   b . For example, when the finished thickness  11   c  is 50 μm, the focal point is set to a position at 70 μm from the front surface  11   a . Thereafter, by moving the chuck table  26  in the X-axis direction, the modified regions  23   a  are formed from one end to the other end of the one planned dividing line  15  at least at the depth  11   d.    
       FIG.  6    is a perspective view illustrating the dividing origin forming step S 20 .  FIG.  7    is a partially sectional side view illustrating the dividing origin forming step S 20 . In  FIG.  7   , the distance between the adjacent modified regions  23   a  is exaggerated and is illustrated as a large distance for convenience of explanation. In  FIG.  6   , the protective tape  19   a  and the frame  19   b  are omitted for convenience. In  FIG.  7   , the frame  19   b  is omitted for convenience. Laser processing conditions are set as follows, for example. 
     Wavelength of laser beam: 1342 nm 
     Average output power: predetermined value of at least 0.5 W and at most 1 W 
     Repetition frequency of pulse: predetermined value of at least 60 kHz and at most 90 kHz 
     Processing feed rate: predetermined value of at least 600 mm/s and at most 800 mm/s 
     The number of passes: predetermined number of at least one and at most three 
     The number of passes means the number of times of irradiation of the wafer  11  with the laser beam L A  from one end to the other end of one planned dividing line  15 . When the number of passes is set to two or more, the modified regions  23   a  are further formed at a different depth position from the depth  11   d . In the dividing origin forming step S 20 , laser processing may be executed in a state in which the laser beam L A  is split to be focused on different positions in the depth direction of the wafer  11 , or laser processing may be executed in a state in which the laser beam L A  is split to cause focal points to line up along the Y-axis direction. 
     After the irradiation with the laser beam L A  is executed from one end to the other end of the one planned dividing line  15 , indexing feed of the chuck table  26  is executed in the Y-axis direction by a predetermined distance (index). Thereafter, the modified regions  23   a  are formed from one end to the other end of another planned dividing line  15  adjacent to the already-processed planned dividing line  15  in the Y-axis direction. After the modified regions  23   a  are formed along all planned dividing lines  15  along the one direction in this manner, the chuck table  26  is rotated by 90°. Then, the modified regions  23   a  are formed along all planned dividing lines  15  along the other direction orthogonal to the one direction. In this manner, between at least the depth  11   d  and the back surface  11   b , the dividing origins  23  (that is, the modified regions  23   a ) for dividing the wafer  11  are formed along each planned dividing line  15 . 
     After the dividing origin forming step S 20 , by using the laser processing apparatus  2  continuously, the separation layer  25  along the crystal plane (100) of the front surface  11   a  and the back surface  11   b  is formed at a predetermined depth corresponding to a position on the side of the back surface  11   b  relative to the dividing origins  23  (separation layer forming step S 30 ).  FIG.  8    is a perspective view illustrating the separation layer forming step S 30 .  FIG.  9    is a partially sectional side view illustrating the separation layer forming step S 30 . In  FIG.  8   , the protective tape  19   a  and the frame  19   b  are omitted for convenience. In  FIG.  9   , the frame  19   b  is omitted for convenience. Further, in  FIG.  9   , in the thickness direction of the wafer  11 , the depth range in which the modified regions  23   a  and the cracks  23   b  are formed is given dots. 
     In the separation layer forming step S 30 , first, the direction that goes from the notch  13  to the center A of the back surface  11   b  (in the present embodiment, the crystal orientation [011]) is detected by using the imaging unit  56 . Subsequently, as illustrated in  FIG.  8   , the chuck table  26  is rotated around the predetermined rotation axis to cause the acute angle formed by the direction that goes from the notch  13  to the center A of the back surface  11   b  (crystal orientation) [011]) and the X-axis direction to become 45°. Thereby, the orientation of the wafer  11  is adjusted in such a manner that the crystal orientation [010], which is parallel to the crystal plane (100) and is one of the crystal orientations &lt;100&gt; of the wafer  11 , becomes parallel to the X-axis direction. 
     Then, the focal points P 1  to P 5  of the laser beam L A  are positioned to the predetermined depth on the side of the back surface  11   b  relative to the dividing origins  23  inside the wafer  11 . For example, the focal points P 1  to P 5  are set at a predetermined position in a range from the position of the half thickness of the wafer  11  to a position of a predetermined thickness when the back surface  11   b  is regarded as the start point (when the total thickness is 775 μm, predetermined position in a range of 387.5 μm to 600 μm when the back surface  11   b  is regarded as the start point). In the present embodiment, scattering of the laser beam L A  by the dividing origins  23  can be prevented by positioning the focal points P 1  to P 5  on the side of the back surface  11   b  relative to the dividing origins  23  (modified regions  23   a ). 
     Thereafter, near one end part of the wafer  11  in the Y-axis direction, irradiation with the laser beam L A  is executed along the crystal orientation [010] (first direction) by relatively moving the focal points P 1  to P 5  and the wafer  11  from one end to the other end of the outer circumferential part of the wafer  11  along the X-axis direction (see the line  25   c   1  in  FIG.  8   ). Laser processing conditions are set as follows, for example. 
     Wavelength of laser beam: 1342 nm 
     Average output power at one focal point: 0.5 W 
     Repetition frequency of pulse: 60 kHz 
     Processing feed rate: 360 mm/s 
     The number of passes: predetermined number of at least one and at most three 
     Thereby, as illustrated in  FIG.  10   , modified regions  25   a  are formed along the movement direction of each of the focal points P 1  to P 5  (modified region forming step S 32 ). That is, multiple modified regions  25   a  that line up along the Y-axis direction and extend along the X-axis direction are formed at substantially the same depth position in the Z-axis direction.  FIG.  10    is a diagram illustrating the modified region forming step S 32  in the separation layer forming step S 30 . In  FIG.  10   , width B and thickness C of the separation layer  25  formed in the modified region forming step S 32  that is performed one time are illustrated. In the modified region forming step S 32 , cracks  25   b  extend along a predetermined crystal plane from each of the multiple modified regions  25   a . As a result, the separation layer  25  including the multiple modified regions  25   a  and the cracks  25   b  that extend with each of the multiple modified regions  25   a  being the origin is formed inside the wafer  11 . 
     Here, explanation will be made about formation of the cracks  25   b  in single-crystal silicon. In general, the single-crystal silicon is cleaved along the crystal plane {111} most easily and is cleaved along the crystal plane {110} second most easily. Thus, when the modified region  25   a  is formed along the crystal orientation &lt;110&gt; of the wafer  11 , there occur many cracks  25   b  that extend along the crystal plane {111} from the modified region  25   a . For example, if the modified region  25   a  is formed along the direction from the notch  13  toward the center A (crystal orientation [011]), there occur many cracks  25   b  that extend along the crystal plane {111} from the modified region  25   a.    
     On the other hand, when multiple modified regions  25   a  are formed in a linear region along the crystal orientation &lt;100&gt; in such a manner as to line up along the direction orthogonal to the direction in which this linear region extends in plan view, there occur many cracks  25   b  that extend along a crystal plane parallel to the direction in which the linear modified regions  25   a  extend in crystal planes {N10} (N is a natural number equal to or smaller than 10 excluding 0) from each of the multiple modified regions  25   a . For example, when multiple modified regions  25   a  are formed in a linear region along the crystal orientation [010] (X-axis direction) in such a manner as to line up along the direction (Y-axis direction) orthogonal to the direction in which this linear region extends in plan view as described above, there occur many cracks  25   b  that extend along a crystal plane parallel to the crystal orientation 
     in crystal planes {N10} (N is a natural number equal to or smaller than 10 excluding 0) from each of the multiple modified regions  25   a.    
     Specifically, when the multiple modified regions  25   a  are formed as above, the cracks  25   b  easily extend in crystal planes indicated by the following expression 1 and expression 2. 
     
       
         
           
             
               
                 
                   
                     ( 
                     101 
                     ) 
                   
                   , 
                   
                     ( 
                     201 
                     ) 
                   
                   , 
                   
                     ( 
                     301 
                     ) 
                   
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                     ( 
                     401 
                     ) 
                   
                   , 
                   
                     ( 
                     501 
                     ) 
                   
                   , 
                   
 
                   
                     ( 
                     601 
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                     ( 
                     701 
                     ) 
                   
                   , 
                   
                     ( 
                     801 
                     ) 
                   
                   , 
                   
                     ( 
                     901 
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         10 
                         _ 
                       
                       ⁢ 
                       01 
                     
                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     1 
                   
                   ] 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         1 
                         _ 
                       
                       ⁢ 
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                         _ 
                       
                       ⁢ 
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                     ) 
                   
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                         6 
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                         _ 
                       
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                   [ 
                   
                     Math 
                     . 
                         
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Incidentally, the acute angle formed by the crystal plane (100) and the crystal plane parallel to the crystal orientation [010] in crystal planes {N10} is larger than 0° and is equal to or smaller than 45°. In contrast, the acute angle formed by the crystal plane (100) and the crystal plane {111} is approximately 54.7°. Thus, in a case in which irradiation with the laser beam L A  is executed along the crystal orientation [010] (former case), the separation layer  25  tends to have a wide width and be thin compared with a case in which irradiation with the laser beam L A  is executed along the crystal orientation [011] (latter case). Therefore, the value of the ratio of width B and thickness C (B/C) in the separation layer  25  (including the modified regions  25   a  and the cracks  25   b ) illustrated in  FIG.  10    becomes larger in the former case than that in the latter case. 
     After the irradiation with the laser beam L A  is executed from one end to the other end of the outer circumferential part of the wafer  11  along the crystal orientation [010] (first direction), indexing feed of the focal points P 1  to P 5  and the wafer  11  is executed relatively (indexing feed step S 34 ). In the indexing feed step S 34 , indexing feed of the chuck table  26  is executed by a predetermined feed amount along the crystal orientation [001] (second direction, Y-axis direction), which is parallel to the crystal plane (100) and is orthogonal to the crystal orientation [010] (first direction, X-axis direction). The feed amount is set equal to or larger than the above-described width B of the separation layer  25 , for example. When width B of the separation layer  25  is a predetermined length of at least 250 μm and at most 280 μm, the feed amount is set to a predetermined value of at least 520 μm and at most 530 μm, for example. 
     Subsequently, the modified region forming step S 32  is executed again (see the line  25   c   2  in  FIG.  8   ).  FIG.  11    is a sectional view of part of the wafer  11  illustrating the separation layer  25  formed in the first round and the second round of the modified region forming step S 32 . In the second round of the modified region forming step S 32 , the separation layer  25  (separation layer  25 - 2 ) is formed that is substantially parallel to the separation layer  25  (separation layer  25 - 1 ) formed in the first round of the modified region forming step S 32 , that is at substantially the same depth position as the separation layer  25 - 1 , and that is separate from the separation layer  25 - 1  in the Y-axis direction. In this manner, the modified region forming step S 32  and the indexing feed step S 34  are repeated to form multiple separation layers  25 - 1 ,  25 - 2 , and so forth each extending from one end to the other end in the X-axis direction from one end part to the other end part in the Y-axis direction. This forms the separation layer  25  across substantially the whole surface of the wafer  11  at the predetermined depth. 
     After the separation layer  25  is formed from one end part to the other end part in the Y-axis direction, the wafer  11  is separated into the front-surface-side wafer  31  including the multiple devices  17  and the back-surface-side wafer  33  that is located on the side of the back surface  11   b  and does not include the devices  17  by using the separation layer  25  as the origin (separation step S 40 ). In the separation step S 40 , a separating apparatus  60  is used (see  FIG.  12   ). The separating apparatus  60  has a chuck table  62  with a circular plate shape. An annular groove (not illustrated) with a predetermined depth is formed in the upper surface of the chuck table  62 , and an opening is formed in the bottom surface of this groove. 
     The opening located in the bottom surface of the groove communicates with a suction source (not illustrated) such as a vacuum pump through a predetermined flow path. Thus, when the suction source is operated, a negative pressure is transmitted to the upper surface of the chuck table  62  through the opening and the groove. Therefore, the upper surface of the chuck table  62  functions as a holding surface  62   a  that sucks and holds the wafer  11  with the interposition of the protective tape  19   a . A rotational drive source (not illustrated) such as a motor for rotating the chuck table  62  around a rotation axis  62   b  that passes through the center of the holding surface  62   a  and is substantially parallel to the vertical direction is disposed at a lower part of the chuck table  62 . 
     A wedge part  64  is disposed near the chuck table  62 . The wedge part  64  has one end part  64   a  that is comparatively sharp. The one end part  64   a  is disposed to be oriented inward in the radial direction of the holding surface  62   a . A sharp pointed body such as a needle or pin may be used instead of the wedge part  64 . Further, as illustrated in  FIG.  14 A , a suction unit  66  is disposed over the holding surface  62   a . The suction unit  66  has a casing  68  with a circular column shape. For example, a raising-lowering mechanism (not illustrated) of a ball screw system is coupled to an upper part of the casing  68 . The suction unit  66  is raised and lowered by operating this raising-lowering mechanism. 
     A circular plate-shaped suction part  70  with a larger diameter than the wafer  11  is fixed to a lower end part of the casing  68 . Multiple suction ports (not illustrated) each having a circular shape are formed in the lower surface of the suction part  70 . Each suction port communicates with a suction source (not illustrated) such as a vacuum pump through a flow path formed inside the suction part  70 . A negative pressure is transmitted to the suction ports when the suction source is operated. This causes the lower surface of the suction part  70  to function as a holding surface  70   a  that sucks and holds the wafer  11 . 
     Next, the separation step S 40  will be described with reference to  FIG.  12    to  FIG.  14 B . First, the wafer unit  21  is placed on the holding surface  62   a  with the back surface  11   b  exposed upward. Subsequently, the side of the front surface  11   a  is sucked and held by the holding surface  62   a  with the interposition of the protective tape  19   a . Then, the one end part  64   a  of the wedge part  64  is positioned to a height position corresponding to the separation layer  25 . However, it suffices that the height position of the one end part  64   a  is near the separation layer  25 , and the height position does not necessarily need to be completely the same height position as the separation layer  25 . The orientation of the wedge part  64  is adjusted to cause application of an external force substantially perpendicular to the side surface of the wafer  11 . 
     Thereafter, while the chuck table  62  and the wedge part  64  are relatively rotated along the circumferential direction of the chuck table  62 , the worker pushes the one end part  64   a  of the wedge part  64  into the outer circumferential side surface of the wafer  11  to give an external force to the wafer  11 .  FIG.  12    is a diagram illustrating the state in which the external force is given to the wafer  11  in the separation step S 40 . In  FIG.  12   , the protective tape  19   a  and the frame  19   b  are omitted for convenience. Due to the giving of the external force, the cracks  25   b  of the separation layer  25  extend, and the cracks  25   b  connect to each other (see  FIG.  13   ).  FIG.  13    is a sectional view of part of the wafer  11  illustrating the separation layer  25  in a state in which the cracks  25   b  connect to each other. 
     Subsequently, the suction unit  66  is lowered, and the holding surface  70   a  is brought into contact with the back surface  11   b . Then, the side of the back surface  11   b  is sucked and held by the holding surface  70   a , and thereafter, the suction unit  66  is raised (see  FIG.  14 A ). The wafer  11  separates into the front-surface-side wafer  31  and the back-surface-side wafer  33  with the separation layer  25  being the origin (separation step S 40 ) (see  FIG.  14 B ).  FIG.  14 A  is a partially sectional side view illustrating the separation step S 40 .  FIG.  14 B  is a partially sectional side view illustrating the front-surface-side wafer  31  and the back-surface-side wafer  33  after the separation step S 40 . In  FIG.  14 A  and  FIG.  14 B , the protective tape  19   a  and the frame  19   b  are omitted for convenience. 
     Incidentally, in the separation step S 40 , the cracks  25   b  may be connected to each other along the front surface  11   a  or the back surface  11   b  by giving ultrasonic vibrations to the wafer  11  instead of giving an external force to the wafer  11  by using the wedge part  64  or the like. For example, for the wafer  11  in which the side of the front surface  11   a  is sucked and held, a circular plate-shaped vibrating component (not illustrated) having an ultrasonic transducer is brought close to the side of the back surface  11   b . Then, a liquid such as purified water is supplied from a nozzle (not illustrated) to the side of the back surface  11   b  at a predetermined flow rate and ultrasonic vibrations are given from the vibrating component to the wafer  11  through the liquid. 
     After the separation step S 40 , the side of the separation surface (third surface)  31   a  located on the opposite side to the front surface  11   a  in the front-surface-side wafer  31  is ground (grinding step S 50 ). In the grinding step S 50 , a grinding apparatus  72  is used (see  FIG.  15   ). The grinding apparatus  72  has a chuck table  74  with a circular plate shape. The chuck table  74  has a circular plate-shaped frame body formed of a non-porous ceramic. A recessed part (not illustrated) with a circular plate shape is formed at a central part of the frame body. A circular plate-shaped porous plate formed of a ceramic is fixed to this recessed part. 
     A predetermined flow path (not illustrated) is formed in the frame body. A negative pressure is transmitted from a suction source (not illustrated) such as an ejector to the upper surface of the porous plate through the predetermined flow path. The upper surface of the porous plate has a circular cone shape in which the central part slightly protrudes compared with the outer circumferential part. The circular upper surface of the porous plate and the annular upper surface of the frame body are substantially flush with each other and function as a substantially flat holding surface for sucking and holding the wafer  11 . A table base (not illustrated) that rotatably supports the chuck table  74  and has a circular annular flat plate shape is disposed at a lower part of the chuck table  74 . Further, an inclination adjustment mechanism (not illustrated) that adjusts the inclination of the chuck table  74  is disposed at a lower part of the table base. 
     Moreover, a rotating shaft  74   a  (in  FIG.  15   , illustrated by a one-dot chain line) is coupled to the lower part of the chuck table  74 . A rotational drive source (not illustrated) such as a motor is coupled to the rotating shaft  74   a  through a pulley, a belt, and so forth. The chuck table  74  rotates around the rotating shaft  74   a  when the rotational drive source is operated. 
     A grinding unit  76  is disposed over the chuck table  74 . The grinding unit  76  has a circular cylindrical spindle housing (not illustrated) having a longitudinal part disposed in substantially parallel to the vertical direction. A processing feed mechanism (not illustrated) of a ball screw system that moves the grinding unit  76  along a predetermined direction (for example, vertical direction) is coupled to the spindle housing. Further, part of a spindle  78  with a circular column shape is rotatably housed in the spindle housing. A rotational drive source such as a motor is disposed near an upper end part of the spindle  78 . A mount  80  with a circular plate shape is fixed to a lower end part of the spindle  78 . A circular annular grinding wheel  82  is mounted on the lower surface side of the mount  80 . 
     The grinding wheel  82  has a base  84  formed of an aluminum alloy. The upper surface side of the base  84  is disposed to be in contact with the mount  80 . On the lower surface side of the base  84 , multiple grinding abrasive stones  86  are disposed at substantially equal intervals along the circumferential direction of the base  84 . Each grinding abrasive stone  86  has a bond of a metal, ceramic, resin, or the like and abrasive grains of diamond, cubic boron nitride (cBN), or the like, for example. Abrasive grains whose average grain size is comparatively large are used for rough grinding abrasive stones and abrasive grains whose average grain size is comparatively small are used for finish grinding abrasive stones. When the spindle  78  is rotated, a circular annular grinding surface is formed by the locus of the lower surfaces of the multiple grinding abrasive stones  86 . The grinding surface is a plane orthogonal to the longitudinal direction of the spindle  78 . 
       FIG.  15    is a diagram illustrating the grinding step S 50 . In  FIG.  15   , the protective tape  19   a  and the frame  19   b  are omitted for convenience. In the grinding step S 50 , first, the side of the front surface  11   a  of the front-surface-side wafer  31  is sucked and held by the holding surface of the chuck table  74  with the interposition of the protective tape  19   a . Subsequently, the table base is inclined to cause part of the holding surface of the chuck table  74  to become substantially parallel to the grinding surface of the grinding wheel  82 . In this state, the chuck table  74  is rotated around the rotating shaft  74   a  at a predetermined rotation speed (for example, 200 rpm) and the grinding wheel  82  is rotated at a predetermined rotation speed (for example, 3000 rpm). 
     Moreover, while a grinding liquid such as purified water is supplied from a grinding liquid supply nozzle (not illustrated) to the contact region between the grinding surface and the separation surface  31   a , the grinding unit  76  is moved downward (that is, processing feed) at a predetermined processing feed rate (for example, 1.0 μm/s). The grinding surface gets contact with the side of the separation surface  31   a , and thereby the side of the separation surface  31   a  is ground. The side of the separation surface  31   a  is ground and planarized until the grinding surface reaches the above-described dividing origins  23 . In addition, the front-surface-side wafer  31  is divided into the multiple device chips  35  (see  FIG.  16   ).  FIG.  16    is a perspective view illustrating the multiple device chips  35 . 
     In the grinding step S 50  of the present embodiment, rough grinding of the side of the separation surface  31   a  is executed by one grinding unit  76  having rough grinding abrasive stones as the grinding abrasive stones  86  (that is, a rough grinding unit), and thereafter finish grinding of the side of the separation surface  31   a  is executed by another grinding unit  76  having finish grinding abrasive stones as the grinding abrasive stones  86  (that is, a finish grinding unit). Grinding the side of the separation layer  25  (that is, the side of the separation surface  31   a ) of the front-surface-side wafer  31  can divide the front-surface-side wafer  31  into the multiple device chips  35 . 
     Moreover, after the finish grinding, the side of the separation surface  31   a  may be polished by using a polishing unit (not illustrated). The polishing unit has the spindle  78  and a polishing pad mounted on the lower end part of the spindle  78 . By executing the polishing in addition to the rough grinding and the finish grinding, the flexural strength of the device chips  35  can be improved compared with a case in which the polishing is not executed. 
     In the present embodiment, the separation layer  25  is formed on the side of the front surface  11   a  relative to the back surface  11   b  and the side of the separation surface  31   a  of the front-surface-side wafer  31  is ground after the separation step S 40 . Therefore, the amount of grinding of the wafer  11  can be reduced compared with the case of grinding the wafer  11  from the back surface  11   b . In addition, the back-surface-side wafer  33  can be reused as a new single-crystal silicon wafer. 
     Incidentally, if the dividing origin forming step S 20  is executed after the separation layer forming step S 30 , the dividing origins  23  are formed on the side of the front surface  11   a  after the separation layer  25  is formed on the side of the back surface  11   b . In this case, in order to avoid scattering of the laser beam L A  due to the separation layer  25  on the side of the back surface  11   b , first, the side of the front surface  11   a  is sucked and held by the holding surface  26   a , and the separation layer  25  is formed on the side of the back surface  11   b , in the separation layer forming step S 30 . Then, in the subsequent dividing origin forming step S 20 , the dividing origins  23  need to be formed on the side of the front surface  11   a  relative to the separation layer  25  with the side of the back surface  11   b  sucked and held by the holding surface  26   a . For this purpose, the protective tape  19   a  needs to be removed from the side of the back surface  11   b , and the wafer  11  needs to be inverted, after the separation layer forming step S 30  and before the dividing origin forming step S 20 . 
     In contrast, in the present embodiment, the separation layer forming step S 30  is executed after the dividing origin forming step S 20 . Therefore, there is an advantage that the processing can be advanced to the grinding step S 50  with use of the protective tape  19   a  without any change therein. 
     Incidentally, in the above-described processing method, by irradiating a linear region along the crystal orientation [010] with the split laser beam L A , the multiple modified regions  25   a  are formed to line up along the direction orthogonal to the direction in which this linear region extends in plan view. In this case, there occur many cracks  25   b  that extend along a crystal plane parallel to the crystal orientation [010] of single-crystal silicon in crystal planes {N10} (N is a natural number equal to or smaller than 10 excluding 0) from each of the multiple modified regions  25   a . Due to this, in the above-described processing method, the separation layer  25  can be allowed to have a wide width and be thin compared with a case in which irradiation with the laser beam L A  is executed along the crystal orientation [011] of the wafer  11 . As a result, in manufacturing of the device chips  35  from the wafer  11 , shortening of the time necessary for laser processing and reduction in the amount of single-crystal silicon removed due to grinding or the like are obtained. 
     The above-described processing method of the wafer  11  is one aspect of the present invention, and the present invention is not limited to the above-described method. For example, the wafer  11  is not limited to that illustrated in  FIG.  1   ,  FIG.  2   , and so forth. Specifically, in the present invention, a single-crystal silicon wafer in which an orientation flat is formed at an outer circumferential part may be processed. A single-crystal silicon wafer in which neither the notch  13  nor an orientation flat is formed at an outer circumferential part may be processed. 
     Further, the structure of the laser processing apparatus used in the present invention is not limited to the structure of the above-described laser processing apparatus  2 . For example, the present invention may be carried out by using a laser processing apparatus equipped with a horizontal movement mechanism that moves the irradiation head  52  and so forth of the laser beam irradiation unit  42  along the X-axis direction and/or the Y-axis direction. That is, in the present invention, it suffices that the chuck table  26  that holds the wafer  11  and the irradiation head  52  that emits the laser beam L A  can relatively move along each of the X-axis direction and the Y-axis direction, and there is no restriction on the structure for this purpose. 
     Moreover, in the present invention, the linear region inside the wafer  11  irradiated with the laser beam L A  in the separation layer forming step S 30  is not limited to the linear region along the crystal orientation [010]. For example, in the present invention, a linear region along the along the crystal orientation [001] may be irradiated with the laser beam L A . In this case, the cracks  25   b  easily extend in crystal planes indicated by the following expression 3 and expression 4. 
     
       
         
           
             
               
                 
                   
                     ( 
                     110 
                     ) 
                   
                   , 
                   
                     ( 
                     210 
                     ) 
                   
                   , 
                   
                     ( 
                     310 
                     ) 
                   
                   , 
                   
                     ( 
                     410 
                     ) 
                   
                   , 
                   
                     ( 
                     510 
                     ) 
                   
                   , 
                   
                     ( 
                     610 
                     ) 
                   
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                     ( 
                     710 
                     ) 
                   
                   , 
                   
                     ( 
                     810 
                     ) 
                   
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                     ( 
                     910 
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         10 
                         _ 
                       
                       ⁢ 
                       10 
                     
                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     3 
                   
                   ] 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         1 
                         _ 
                       
                       ⁢ 
                       10 
                     
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         2 
                         _ 
                       
                       ⁢ 
                       10 
                     
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         3 
                         _ 
                       
                       ⁢ 
                       10 
                     
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         4 
                         _ 
                       
                       ⁢ 
                       10 
                     
                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         5 
                         _ 
                       
                       ⁢ 
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                     ) 
                   
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                         6 
                         _ 
                       
                       ⁢ 
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                         7 
                         _ 
                       
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                     ) 
                   
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                         8 
                         _ 
                       
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                     ( 
                     
                       
                         9 
                         _ 
                       
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                     ) 
                   
                   , 
                   
                     ( 
                     
                       
                         
                           10 
                           _ 
                         
                         _ 
                       
                       ⁢ 
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                     ) 
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                         
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Moreover, in the present invention, a linear region along a direction slightly inclined from the crystal orientation [010] or the crystal orientation [001] in plan view may be irradiated with the laser beam L A . Regarding this point, description will be made with reference to  FIG.  17   .  FIG.  17    is a graph illustrating width B (see  FIG.  10   ) of the separation layer  25  formed inside the wafer  11  when linear regions that are each along a different crystal orientation are irradiated with the laser beam L A . 
     The abscissa axis of the graph indicates the angle formed by the direction in which a linear region orthogonal to the crystal orientation [011] (reference region) extends and the direction in which a linear region that becomes a measurement subject (measurement region) extends in plan view of the wafer  11 . That is, when the linear region along the crystal orientation [001] is the measurement subject, the value of the abscissa axis of this graph is 45° (see 45° in  FIG.  2   ). Similarly, when the linear region along the crystal orientation [010] is the measurement subject, the value of the abscissa axis of this graph is 135° (see 135° in  FIG.  2   ). 
     The ordinate axis of the graph illustrated in  FIG.  17    indicates the value obtained when width B of the separation layer  25  formed in the measurement region by irradiating the measurement region with the laser beam L A  is divided by width B of the separation layer  25  formed in the reference region by irradiating the reference region with the laser beam L A . As illustrated in  FIG.  17   , width B of the separation layer  25  in the measurement region becomes comparatively wide when the angle formed by the direction in which the reference region extends and the direction in which the measurement region extends is at least 40° and at most 50° or at least 130° and at most 140°. That is, width B of the separation layer  25  becomes comparatively wide not only when the linear region along the crystal orientation [001] or the crystal orientation [010] is irradiated with the laser beam L A  but also when the linear region along a direction in which the acute angle formed between the direction and either of these crystal orientations is equal to or smaller than 5° is irradiated with the laser beam L A . Thus, in the present invention, the linear region along a direction that is parallel to the crystal plane (100) and is inclined from the crystal orientation [001] or the crystal orientation [010] by at most 5° may be irradiated with the laser beam L A . 
     Incidentally, in the separation layer forming step S 30 , after the separation layer  25  is formed from one end part to the other end part of the inside of the wafer  11  in the Y-axis direction through repeating the modified region forming step S 32  and the indexing feed step S 34 , the separation layer  25  may be formed from one end part to the other end part in the Y-axis direction at substantially the same depth position inside the wafer  11  again. Through executing multiple times of the separation layer forming step S 30  in this manner, the density of each of the modified regions  25   a  and the cracks  25   b  included in the separation layer  25  increases compared with the case of executing the separation layer forming step S 30  one time. Therefore, the separation between the front-surface-side wafer  31  and the back-surface-side wafer  33  becomes easy in the separation step S 40 . 
     Further, in a case of executing multiple times of the separation layer forming step S 30 , the cracks  25   b  included in the separation layer  25  further extend, and width B (see  FIG.  10   ) of the separation layer  25  becomes wide compared with the case of executing the separation layer forming step S 30  one time. Therefore, in the case of executing multiple times of the separation layer forming step S 30 , the relative movement distance (index) of the irradiation head  52  and the chuck table  26  in the Y-axis direction in the indexing feed step S 34  can be set long compared with the case of executing the separation layer forming step S 30  one time. 
     Furthermore, in the separation layer forming step S 30 , after the modified region forming step S 32  and before the indexing feed step S 34 , the modified region forming step S 32  may be executed again. That is, the linear region in which the separation layer  25  has been already formed may be irradiated with the laser beam L A  to form the separation layer  25  again. Also in this case, the separation between the front-surface-side wafer  31  and the back-surface-side wafer  33  in the separation step S 40  becomes easy, and the index in the indexing feed step S 34  can be set longer. 
     Next, second and third embodiments in which the dividing origin forming step S 20  is executed by cutting processing will be described. For the cutting processing, a cutting apparatus  90  illustrated in  FIG.  18    is used. The cutting apparatus  90  has a chuck table  92  with a circular plate shape. The chuck table  92  has a holding surface  92   a  that sucks and holds the wafer  11 . The shape, structure, functions, and so forth of the chuck table  92  are the same as the chuck table  26  illustrated in  FIG.  4   , and thus, detailed description thereof is omitted. 
     A rotational drive source (not illustrated) such as a motor is coupled to a bottom part of the chuck table  92 . The chuck table  92  and the rotational drive source can move along the X-axis direction by an X-axis movement mechanism (not illustrated) of a ball screw system in a state in which they are supported by an X-axis moving plate (not illustrated). A cutting unit  94  is disposed over the chuck table  92 . The cutting unit  94  has a cylindrical spindle housing  96  having a longitudinal part disposed along the Y-axis direction. The spindle housing  96  can move along the Y-axis and Z-axis directions by a Y-axis movement mechanism and a Z-axis movement mechanism (neither is illustrated) each based on a ball screw system. Further, an imaging unit (not illustrated) similar to the imaging unit  56  in  FIG.  4    is disposed at a side part of the spindle housing  96 . Part of a spindle  98  with a circular column shape is rotatably housed in the spindle housing  96 . 
     A longitudinal part of the spindle  98  is disposed along the Y-axis direction. A rotational drive source (not illustrated) such as a motor is disposed near one end part of the spindle  98 . The other end part of the spindle  98  protrudes from the spindle housing  96 , and a cutting blade  100  is mounted on a tip part that protrudes. For example, the dividing origin forming step S 20  by the cutting processing is executed after the holding step S 10  and before the separation layer forming step S 30  (second embodiment). In a case of executing the cutting processing, first, a wafer unit  41  in which the side of the back surface  11   b  is supported by the frame  19   b  (not illustrated in  FIG.  18   ) through the protective tape  19   a  is formed. Then, the side of the back surface  11   b  is sucked and held by the holding surface  92   a  with the interposition of the protective tape  19   a  with the side of the front surface  11   a  exposed upward. 
     Subsequently, the orientation of the chuck table  92  is adjusted to cause one planned dividing line  15  to become substantially parallel to the X-axis direction. Then, outside the holding surface  92   a , the cutting blade  100  is rotated at a predetermined rotation speed, and a bottom part of the cutting blade  100  is disposed at a depth position corresponding to the finished thickness  11   c  from the front surface  11   a . By executing processing feed of the chuck table  92  along the X-axis direction at a predetermined processing feed rate in this state, a cut groove  11   e  having a depth corresponding to the finished thickness  11   c  from the front surface  11   a  is formed along the one planned dividing line  15 . The cut groove  11   e  functions as the dividing origin  23 . 
       FIG.  18    is a partially sectional side view illustrating the state in which the cutting processing of the side of the front surface  11   a  is executed. The cut groove  11   e  is referred to also as a half-cut groove but does not necessarily have the depth that is just half the thickness of the wafer  11 . The cut groove  11   e  of the present embodiment is a groove that has a depth that does not reach the back surface  11   b  and is shallow compared with the thickness of the wafer  11 . In the cutting, cutting water (not illustrated) such as purified water is supplied to the vicinity of the processing point at a predetermined flow rate. Processing conditions in the cutting are set as follows, for example. 
     Spindle rotation speed: 30,000 rpm 
     Processing feed rate: predetermined value of at least 1.0 mm/s and at most 20 mm/s 
     Flow rate of cutting water: predetermined value of at least 0.5 L/min and at most 1.5 L/min. 
     After the cut groove  11   e  is formed along the one planned dividing line  15 , indexing feed of the cutting unit  94  is executed, and the cut groove  11   e  is similarly formed along another planned dividing line  15  adjacent to the planned dividing line  15  along which the cut groove  11   e  has been formed. In this manner, the cut grooves  11   e  are formed along all planned dividing lines  15  along the one direction. Thereafter, the chuck table  92  is rotated by 90°. Then, the cut grooves  11   e  are similarly formed along all planned dividing lines  15  along the other direction orthogonal to the one direction. After the dividing origin forming step S 20 , the wafer unit  21  in which the protective tape  19   a  is stuck to the side of the front surface  11   a  is formed. 
     Then, in the separation layer forming step S 30 , the focal point of the laser beam L A  is positioned to the side of the back surface  11   b  relative to the bottom parts of the cut grooves  11   e , and the separation layer  25  is formed. In the separation layer forming step S 30 , the wafer unit  21  in which the protective tape  19   a  is stuck to the side of the front surface  11   a  is formed. Then, the focal point of the laser beam L A  is positioned to the side of the back surface  11   b  relative to the bottom parts of the cut grooves  11   e , and the separation layer  25  is formed. After the separation layer forming step S 30 , the separation step S 40  and the grinding step S 50  are sequentially executed.  FIG.  19 A  is a flowchart of the processing method according to the second embodiment. 
     In contrast, in the third embodiment, a dividing origin forming step S 35  by cutting processing is executed after the holding step S 10  and the separation layer forming step S 30 .  FIG.  19 B  is a flowchart of the processing method according to the third embodiment. In the dividing origin forming step S 35  of the third embodiment, the cut grooves  11   e  are formed in such a manner that the bottom parts of the cut grooves  11   e  are located on the side of the front surface  11   a  relative to the separation layer  25 . That is, the separation layer  25  is formed at a depth position on the side of the back surface  11   b  relative to the dividing origins  23 . 
     Also in the processing methods according to the second and third embodiments, the front-surface-side wafer  31  can be divided into multiple device chips  35  when the side of the separation layer  25  in the front-surface-side wafer  31  is ground in the grinding step S 50  after the separation step S 40 . Therefore, the amount of grinding of the wafer  11  can be reduced compared with the case of grinding the wafer  11  from the back surface  11   b  of the wafer  11 . 
     Next, fourth and fifth embodiments in which the dividing origin forming step S 20  is executed by ablation processing will be described. For the ablation processing, a laser processing apparatus  102  illustrated in  FIG.  20    is used. Compared with the laser processing apparatus  2  illustrated in  FIG.  4   , the laser processing apparatus  102  has a laser beam irradiation unit  104  different from the laser beam irradiation unit  42  but is substantially the same as the laser processing apparatus  2  except for the laser beam irradiation unit  104 . The laser beam irradiation unit  104  has the laser oscillator  44  fixed to the base  4 . 
     The laser oscillator  44  has a rod-shaped laser medium formed of Nd:YAG or Nd:YVO 4 . A pulsed laser beam emitted from the laser oscillator  44  passes through a wavelength converting part (not illustrated), the attenuator  46 , and so forth and is output from the irradiation head  52  toward the holding surface  26   a . The wavelength converting part has a nonlinear optical crystal that generates harmonics of the laser beam, for example. The wavelength converting part converts the fundamental wavelength emitted from the laser oscillator  44  to a pulsed laser beam L B  having such a wavelength as to be absorbed by the wafer  11 . For example, the wavelength converting part converts a fundamental wavelength 1064 nm to the third harmonic (for example, 355 nm). Processing conditions in the ablation processing are set as follows, for example. 
     Wavelength of laser beam: 355 nm 
     Average output power: predetermined value of at least 0.3 W and at most 4.0 W 
     Repetition frequency of pulse: predetermined value of at least 10 kHz and at most 200 kHz 
     Processing feed rate: predetermined value of at least 1.0 mm/s and at most 1000 mm/s 
     The number of passes: predetermined number of at least one and at most 10 
     The laser beam irradiation unit  104  does not have the splitting unit  48 . The laser beam L B  is not split, and the number of focal points is one. For example, the dividing origin forming step S 20  by the ablation processing is executed after the holding step S 10  and before the separation layer forming step S 30  (fourth embodiment). In a case of executing the ablation processing, first, the wafer unit  41  in which the side of the back surface  11   b  is supported by the frame  19   b  (not illustrated in  FIG.  20   ) through the protective tape  19   a  is formed. Then, a water-soluble resin film (not illustrated) with a substantially uniform thickness is formed on the side of the front surface  11   a.    
     Subsequently, the side of the back surface  11   b  is sucked and held by the holding surface  26   a  with the interposition of the protective tape  19   a  with the side of the front surface  11   a  exposed upward. Next, the orientation of the chuck table  26  is adjusted to cause one planned dividing line  15  to become substantially parallel to the X-axis direction. Then, the focal point of the laser beam L B  is positioned to substantially the same height as the front surface  11   a , and processing feed of the chuck table  26  is executed along the X-axis direction at a predetermined processing feed rate. Thereby, a processed groove  11   f  having a depth corresponding to the finished thickness  11   c  from the front surface  11   a  is formed along the one planned dividing line  15 . The processed groove  11   f  functions as the dividing origin  23 . 
       FIG.  20    is a partially sectional side view illustrating the state in which the ablation processing is executed for the side of the front surface  11   a . After the processed groove  11   f  is formed along the one planned dividing line  15 , indexing feed of the chuck table  26  is executed and the processed groove  11   f  is similarly formed along another planned dividing line  15  adjacent to the planned dividing line  15  along which the processed groove  11   f  has been formed. In this manner, the processed grooves  11   f  are formed along all planned dividing lines  15  along the one direction. 
     Thereafter, the chuck table  92  is rotated by 90°. Then, the processed grooves  11   f  are similarly formed along all planned dividing lines  15  along the other direction orthogonal to the one direction. Subsequently, by removing the water-soluble resin film on the side of the front surface  11   a  by spin cleaning, processing dust (debris) that has adhered to the water-soluble resin film in the ablation processing is removed. Thereafter, through drying the side of the front surface  11   a , the dividing origin forming step S 20  is ended. 
     After the dividing origin forming step S 20 , the wafer unit  21  in which the protective tape  19   a  is stuck to the side of the front surface  11   a  is formed. Then, in the separation layer forming step S 30 , the focal point of the laser beam L A  is positioned to the side of the back surface  11   b  relative to the bottom parts of the processed grooves  11   f , and the separation layer  25  is formed. Thereafter, the separation step S 40  and the grinding step S 50  are sequentially executed.  FIG.  21 A  is a flowchart of the processing method according to the fourth embodiment. In contrast, in the fifth embodiment, the dividing origin forming step S 35  by ablation processing is executed after the holding step S 10  and the separation layer forming step S 30 . 
     In the dividing origin forming step S 35  of the fifth embodiment, the processed grooves  11   f  are formed in such a manner that the bottom parts of the processed grooves  11   f  are located on the side of the front surface  11   a  relative to the separation layer  25 . That is, the separation layer  25  is formed at a depth position on the side of the back surface  11   b  relative to the dividing origins  23 .  FIG.  21 B  is a flowchart of the processing method according to the fifth embodiment. Also in the processing methods according to the fourth and fifth embodiments, the front-surface-side wafer  31  can be divided into multiple device chips  35  when the side of the separation layer  25  in the front-surface-side wafer  31  is ground in the grinding step S 50  after the separation step S 40 . Therefore, the amount of grinding of the wafer  11  can be reduced compared with the case of grinding the wafer  11  from the back surface  11   b  of the wafer  11 . 
     Besides, structures, methods, and so forth according to the above-described embodiments can be carried out with appropriate changes without departing from the range of the object of the present invention. 
     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.