Patent Publication Number: US-2023141691-A1

Title: Processing method

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The present invention relates to a processing method of a workpiece including a necessary region and an unnecessary region. 
     DESCRIPTION OF THE RELATED ART 
     A wafer in which a plurality of devices such as integrated circuits (ICs) and large-scale integration (LSI) circuits are formed on a front surface in such a manner as to be marked out by a plurality of planned dividing lines that intersect each other is divided into individual device chips by a dicing apparatus or a laser processing apparatus, and the device chips obtained by the dividing are used for pieces of electrical equipment such as mobile phones and personal computers. 
     The laser processing apparatus is composed substantially of a chuck table that holds a wafer, an imaging unit that images the wafer held by the chuck table and detects a region to be processed, a laser beam irradiation unit that irradiates the wafer held by the chuck table with a laser beam, and a processing feed mechanism that executes processing feed of the chuck table and the laser beam irradiation unit relatively, and can process the wafer with high accuracy (for example, refer to Japanese Patent Laid-open No. 2015-085347). 
     As types of the laser beam irradiation unit, there are a type that executes irradiation with a laser beam with a wavelength having absorbability with respect to a wafer, to execute ablation processing (for example, refer to Japanese Patent Laid-open No. 2004-188475); a type that executes irradiation with a laser beam with a wavelength having transmissibility with respect to a wafer, to execute internal processing by which a modified layer is formed inside the wafer (for example, refer to Japanese Patent No. 3408805); and a type that executes irradiation with a laser beam with a wavelength having transmissibility with respect to a wafer, in such a manner that a value obtained by dividing the numerical aperture (NA) of a light collector by the refractive index (N) of the wafer is in a range of 0.05 to 0.2 and forms shield tunnels composed of a fine pore and a modified tube that surrounds the fine pore inside the wafer (for example, refer to Japanese Patent Laid-open No. 2014-221483). 
     SUMMARY OF THE INVENTION 
     Incidentally, in the case in which a workpiece is, for example, a wafer in which a plurality of devices are formed on a front surface in such a manner as to be marked out by planned dividing lines and a metal layer that is referred to as a test elementary group (TEG) and that is for execution of evaluation and management of the devices is formed on the planned dividing lines, irradiation needs to be executed with the power of a laser beam set high when the planned dividing lines are irradiated with the laser beam and are processed. As a result, there is a problem that a crack attributed to the irradiation with the laser beam develops to a necessary region in which the device is formed outside the planned dividing line and gives damage to the device. 
     The situation in which the above-described problem occurs is not necessarily limited to the case in which the TEG is disposed on the planned dividing lines. Even when the TEG is not formed on the planned dividing lines, due to the crystal structure of the material that configures a wafer, a crack easily develops to a region outside the planned dividing line in some cases when the planned dividing line is irradiated with a laser beam. Further, the above-described crack problem possibly occurs also when a wafer is divided into individual device chips by cutting the wafer along the planned dividing lines by a cutting blade rotatably held. Moreover, the situation in which such a problem occurs is not limited to the case of executing laser processing along the planned dividing lines of a wafer in which a plurality of devices are formed on a front surface in such a manner as to be marked out by the planned dividing lines. Even in the case of dividing a necessary region with a certain shape from a plate-shaped workpiece by removing an unnecessary region, a problem that a crack develops from the unnecessary region side to the necessary region side and a desired processing result is not obtained possibly occurs. 
     Thus, an object of the present invention is to provide a processing method by which a desired processing result is obtained without causing a crack to develop to a certain necessary region side in the case of executing, for a workpiece, processing of removing an unnecessary region from the workpiece to obtain the necessary region. 
     In accordance with an aspect of the present invention, there is provided a processing method of a workpiece including a necessary region and an unnecessary region. The processing method includes a protective wall forming step of irradiating a region that defines a boundary between the necessary region and the unnecessary region with a laser beam having a wavelength that has transmissibility with respect to the workpiece and forming a plurality of shield tunnels composed of a fine pore and a modified tube that surrounds the fine pore, thereby forming a protective wall and an unnecessary region removal step of removing the unnecessary region after executing the protective wall forming step. 
     Preferably, the shield tunnels formed in the protective wall forming step are formed in such a manner that the modified tubes of the shield tunnels adjacent are in contact with each other. Preferably, the protective wall forming step includes a first protective wall forming step of successively forming the shield tunnels in a planned dividing line with the interposition of at least intervals corresponding to one of the shield tunnels and a second protective wall forming step of successively forming the shield tunnels in regions in which the intervals are interposed in the planned dividing line. Preferably, the shield tunnels formed in the first protective wall forming step and the shield tunnels formed in the second protective wall forming step are formed in such a manner that steps are alternately made in the thickness direction of the workpiece. Preferably, the shield tunnels are stacked in the thickness direction in the protective wall forming step. 
     Preferably, the protective wall forming step includes a third protective wall forming step of forming shield tunnels above the shield tunnels formed in the first protective wall forming step and a fourth protective wall forming step of forming shield tunnels above the shield tunnels formed in the second protective wall forming step, when the shield tunnels are stacked in the thickness direction. Preferably, in the protective wall forming step, when the shield tunnels are stacked in the thickness direction, the shield tunnels of an upper part are stacked in such a manner as not to be in contact with the shield tunnels of a lower part. 
     Preferably, the workpiece is a wafer in which a plurality of devices are formed on a front surface in such a manner as to be marked out by a plurality of planned dividing lines that intersect each other, and the necessary region is a region in which the device is formed, and the unnecessary region is a region in which the planned dividing line is formed. Further, in the protective wall forming step, the protective wall is formed on each of opposite sides of the planned dividing line that define the width of the planned dividing line, and, in the unnecessary region removal step, planned dividing line removal processing to remove the planned dividing line that is the unnecessary region sandwiched by the pair of protective walls is executed. Preferably, the planned dividing line removal processing is either laser processing by irradiation with a laser beam or cutting processing executed by a cutting blade. Preferably, the wavelength of the laser beam with which the irradiation is executed in the protective wall forming step is 532 nm, energy per pulse is 2.0 to 4.0·10 -5  J, and the interval of a spot is 10 to 15 µm. 
     The necessary region in the present invention is a region used in another step as it is after being divided from the workpiece by laser processing, cutting processing, or the like, and the unnecessary region is a region that is not used as it is after the processing is executed. For example, the unnecessary region is a discarded region or a region subjected to recycling in some cases. 
     According to the present invention, even when the unnecessary region is broken to be removed through irradiation with the laser beam with high power or by the cutting blade, development of a crack to the necessary region is prevented by the protective wall and, a problem that damage is given to the necessary region to be divided is eliminated. 
     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 an overall perspective view of a laser processing apparatus suitable to execute a processing method according to an embodiment of the present invention; 
         FIG.  2    is a perspective view of a wafer that is a workpiece of the present embodiment; 
         FIG.  3 A  is a perspective view illustrating a protective wall forming step; 
         FIG.  3 B  is a perspective view of a wafer unit for which the protective wall forming step has been completed; 
         FIG.  3 C  is a plan view in which part of the wafer for which the protective wall forming step has been executed is illustrated in an enlarged manner; 
         FIG.  3 D  is a sectional view in which part of the wafer for which first and second protective wall forming steps have been completed is illustrated in an enlarged manner; 
         FIG.  3 E  is a schematic perspective view of a shield tunnel; 
         FIG.  4 A  is a perspective view of a form in which an unnecessary region removal step is executed by laser processing; 
         FIG.  4 B  is a perspective view of the wafer unit for which the unnecessary region removal step has been completed; 
         FIG.  5    is a perspective view illustrating a mode in which the unnecessary region removal step is executed by cutting processing; 
         FIG.  6    is a partially enlarged sectional view illustrating another embodiment of the protective wall forming step; and 
         FIG.  7    is a plan view illustrating still another embodiment of the workpiece. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A laser processing apparatus suitable to execute a processing method of an embodiment of the present invention will be described below with reference to the accompanying drawings, and thereafter, the processing method of the present embodiment will be described. 
     In  FIG.  1   , an overall perspective view of a laser processing apparatus  1  is illustrated. The laser processing apparatus  1  includes a holding unit  3  that is disposed over a base  2  and that holds a wafer  10  (see  FIG.  2   ) to be described later, a movement mechanism  4  that moves the holding unit  3  in an X-axis direction and a Y-axis direction, an imaging unit  6  that images the wafer  10  held by the holding unit  3 , and a laser beam irradiation unit  7  that irradiates the wafer  10  held by the holding unit  3  with a laser beam, to execute desired processing. Further, the laser processing apparatus  1  includes a frame body  5  composed of a vertical wall part  5   a  disposed upright on a lateral side of the movement mechanism  4  and a horizontal wall part  5   b  extending from an upper end part of the vertical wall part  5   a  in the horizontal direction. The imaging unit  6  and the laser beam irradiation unit  7  are housed and held inside the horizontal wall part  5   b . 
     As illustrated in  FIG.  1   , the holding unit  3  includes a rectangular X-axis direction movable plate  31  mounted over the base  2  movably in the X-axis direction, a rectangular Y-axis direction movable plate  32  mounted over the X-axis direction movable plate  31  movably in the Y-axis direction, a circular cylindrical support column  33  fixed to an upper surface of the Y-axis direction movable plate  32 , and a rectangular cover plate  34  fixed to an upper end of the support column  33 . A chuck table  35  that passes through a long hole formed on the cover plate  34  and extends upward is disposed over the cover plate  34 . The chuck table  35  is means that holds the wafer  10  by using an XY plane identified based on an X coordinate and a Y coordinate as a holding surface, and is configured rotatably by rotational drive means that is housed in the support column  33  and is not illustrated. At an upper surface of the chuck table  35 , a suction adhesion chuck  36  that configures the holding surface formed of a porous material having gas permeability is disposed. The suction adhesion chuck  36  is connected to suction means that is not illustrated, by a flow path passing through the support column  33 . Around the suction adhesion chuck  36 , four clamps  37  used when the wafer  10  to be described later is held on the chuck table  35  are disposed at equal intervals. The wafer  10  can be held under suction by the suction adhesion chuck  36  by actuating the suction means. 
     The movement mechanism  4  includes an X-axis movement mechanism  4   a  that moves the above-described chuck table  35  in the X-axis direction and a Y-axis movement mechanism  4   b  that moves the chuck table  35  in the Y-axis direction. The X-axis movement mechanism  4   a  converts rotational motion of a motor  42   a  to linear motion through a ball screw  42   b  and transmits the linear motion to the X-axis direction movable plate  31  to move the X-axis direction movable plate  31  in the X-axis direction along a pair of guide rails  2 A disposed along the X-axis direction on the base  2 . The Y-axis movement mechanism  4   b  converts rotational motion of a motor  44   a  to linear motion through a ball screw  44   b  and transmits the linear motion to the Y-axis direction movable plate  32  to move the Y-axis direction movable plate  32  in the Y-axis direction along a pair of guide rails  31   a  disposed along the Y-axis direction on the X-axis direction movable plate  31 . 
     The above-described laser processing apparatus  1  is controlled by a controller that is not illustrated. The controller is configured by a computer and includes a central processing unit (CPU) that executes calculation processing according to a control program, a read only memory (ROM) that stores the control program and so forth, a readable-writable random access memory (RAM) for temporarily storing a detection value obtained by detection, a calculation result, and so forth, an input interface, and an output interface (diagrammatic representation about details is omitted). The imaging unit  6 , the laser beam irradiation unit  7 , the X-axis movement mechanism  4   a  and the Y-axis movement mechanism  4   b  that configure the movement mechanism  4 , and so forth are connected to the controller and are controlled. 
     A workpiece processed by the processing method configured based on the present invention is, for example, the wafer  10  illustrated in  FIG.  2   . The wafer  10  has a thickness of approximately 100 µm, for example, and is a SiC wafer in which a plurality of devices  12  are formed on a front surface  10   a  in such a manner as to be marked out by a plurality of planned dividing lines  14  that intersect each other. In processing by the above-described laser processing apparatus  1 , the wafer  10  is supported, through an adhesive tape T, by an annular frame F having an opening part Fa that can house the wafer  10 , and is loaded into the laser processing apparatus  1  as a wafer unit  13 . 
     In  FIG.  2   , a perspective view in which part of the wafer  10  is enlarged is illustrated. As is understood from the enlarged perspective view, a TEG  16  that is a metal layer for execution of evaluation and management of the devices  12  is disposed on the planned dividing lines  14  formed in the front surface  10   a  of the wafer  10  of the present embodiment. 
     In the processing method of the present embodiment, by executing processing for the wafer  10  as described below, the planned dividing lines  14  that are unnecessary regions in the wafer  10  are removed, and the regions in which the devices  12  are formed as necessary regions are divided into individual chips. 
     In the processing method of the present embodiment, first, a protective wall forming step of irradiating regions that define the boundary between the region in which the device  12  is formed and which is the necessary region and the planned dividing line  14  that is the unnecessary region with a laser beam with a wavelength that has transmissibility with respect to the wafer  10  and forming a plurality of shield tunnels composed of a fine pore and a modified tube that surrounds the fine pore, thereby forming protective walls is executed. A procedure of executing the protective wall forming step of the present embodiment will be described more specifically. 
     In execution of the protective wall forming step, after the above-described wafer  10  is prepared, the wafer  10  is placed over the chuck table  35  of the laser processing apparatus  1  illustrated in  FIG.  1    and is held under suction, and the frame F is clamped and fixed by the clamps  37 . Subsequently, the above-described movement mechanism  4  is actuated, and the wafer  10  is positioned directly under the imaging unit  6  and is imaged to detect position information regarding the devices  12  and the planned dividing lines  14  of the wafer  10 . Moreover, based on the position information, the movement mechanism  4  and so forth are actuated, and the predetermined planned dividing line  14  extending in a first direction of the wafer  10  is aligned with the X-axis direction. 
     Next, as illustrated in  FIG.  3 A , the wafer  10  is positioned directly under a light collector  71  of the laser beam irradiation unit  7 . Then, while the X-axis movement mechanism  4   a  is actuated to execute processing feed of the wafer  10  in the X-axis direction on the basis of the position information detected by the imaging unit  6 , the regions that define the boundary between the device  12  and the planned dividing line  14  are irradiated with a laser beam LB 1 . As a result, two streaks of protective walls  100  are formed as is understood from  FIG.  3 C  in which part of the planned dividing line  14  is illustrated in an enlarged manner. In the present embodiment, the regions that define the boundary between the device  12  and the planned dividing line  14  are regions on both sides of the planned dividing line  14  that define the width of the planned dividing line  14  that is the unnecessary region, as is understood from  FIG.  3 C . 
     In the irradiation with the above-described laser beam LB 1 , the numerical aperture (NA) of a collecting lens that configures the light collector  71  of the laser beam irradiation unit  7  of the present embodiment is set in such a manner that a value obtained by dividing the NA by the refractive index (N) of the wafer  10  falls within a range of 0.05 to 0.2, for example. Further, the wavelength of the laser beam LB 1  is set to 532 nm, which is a wavelength having transmissibility with respect to the wafer  10 . The average output power is set to 0.2 to 0.4 W, and the repetition frequency is set to 10 kHz. The energy per pulse is set to 2.0 to 4.0·10 -5  J, and the interval of the spot is set to 10 to 15 µm. By positioning the focal points to the inside of the wafer  10  and executing irradiation with the laser beam LB 1  on the basis of such a laser processing condition, shield tunnels  102  and  104  are formed as illustrated in  FIG.  3 D . As illustrated in  FIG.  3 E , the shield tunnels  102  and  104  are composed of a fine pore  130  and a modified tube  140  that surrounds the fine pore  130 . For example, the diameter of the fine pore  130  is approximately 1 µm, and the diameter of the modified tube  140  is approximately 10 µm. The above-described protective walls  100  are formed along the planned dividing line  14  by successively forming the shield tunnels  102  and  104  adjacent to each other. The energy per pulse regarding the laser beam LB 1  with which irradiation is executed when the protective walls  100  are formed is set to a value at such a degree that the protective walls  100  do not become starting points of dividing when the wafer  10  is divided along the planned dividing lines  14 . 
     In the formation of the protective walls  100  illustrated in  FIGS.  3 C and  3 D , for example, a first protective wall forming step of successively forming the shield tunnels  102  with the interposition of at least intervals (approximately  10  to 13 µm) corresponding to one shield tunnel along the above-described regions that define the boundary between the device  12  and the planned dividing line  14  is first executed. Subsequently, a second protective wall forming step of successively forming the shield tunnels  104  for the regions in which the intervals are interposed is executed. That is, the shield tunnels  102  and the shield tunnels  104  are alternately formed along the X-axis direction to form the protective walls  100 . By executing the first protective wall forming step and the second protective wall forming step with the interposition of an interval in terms of time in the formation of the protective walls  100  as above, the influence of a hot spot caused when irradiation with the laser beam LB 1  is executed to successively form shield tunnels adjacent to each other is avoided and, development of a crack to the necessary region (region in which the device  12  is formed) caused when the shield tunnels  102  and  104  are formed can be avoided. 
     The purpose of executing the first protective wall forming step and the second protective wall forming step with the interposition of an interval in terms of time in the above-described embodiment is to avoid the influence of a hot spot as described above. This is because, when only the first protective wall forming step is executed with the interval of the shield tunnels  102  shortened, diffusion of heat generated when the shield tunnels  102  are formed is not sufficient and it is impossible to avoid development of a crack to the necessary region (region in which the device  12  is formed). Preferably, the laser processing condition is the same between the first protective wall forming step and the second protective wall forming step. However, different laser processing conditions may be employed. 
     After two streaks of the protective walls  100  composed of the shield tunnels  102  and  104  are formed along the predetermined planned dividing line  14  in the first direction as described above, indexing feed of the wafer  10  is executed in the Y-axis direction, and the planned dividing line  14  that is adjacent in the Y-axis direction, that has not yet been processed, and that extends in the first direction is positioned directly under the light collector  71 . Then, the focal points of the laser beam LB 1  are positioned to the inside of the regions that define the width of the planned dividing line  14  of the wafer  10 , and irradiation is executed similarly to the above description to sequentially execute the above-described first protective wall forming step and second protective wall forming step. 
     As a result, the shield tunnels  102  and  104  are formed to form two streaks of the protective walls  100 . Similarly, processing feed and indexing feed of the wafer  10  are executed in the X-axis direction and the Y-axis direction, and two streaks of the protective walls  100  are formed along all planned dividing lines  14  extending in the first direction. Subsequently, the wafer  10  is rotated by 90 degrees in a direction indicated by an arrow R 1 , and the planned dividing lines  14  extending in a second direction orthogonal to the planned dividing lines  14  along which the protective walls  100  have already been formed are aligned with the X-axis direction. Then, the focal points of the laser beam LB 1  are positioned and irradiation is executed similarly to the above description also for the inside of the regions that define the boundaries between the remaining devices  12  and the planned dividing lines  14  and, as illustrated in  FIG.  3 B , the protective walls  100  are formed corresponding to all planned dividing lines  14   formed in the front surface  10   a  of the wafer  10 . This completes the protective wall forming step of the present embodiment. 
     After the above-described protective wall forming step is executed, an unnecessary region removal step of removing the planned dividing lines  14  that are the unnecessary regions is executed. More specifically, the wafer  10  in which the above-described protective walls  100  have been formed corresponding to all planned dividing lines  14  is conveyed to a laser processing apparatus  20  illustrated in  FIG.  4 A  (only part thereof is illustrated). The laser processing apparatus  20  includes a holding unit that is not illustrated, a laser beam irradiation unit  21  that irradiates the wafer  10  held by the holding unit with a laser beam LB 2 , X-axis feed means that executes processing feed of the holding unit and the laser beam irradiation unit  21  relatively in the X-axis direction, Y-axis feed means that executes indexing feed of the holding unit and the laser beam irradiation unit  21  relatively in the Y-axis direction orthogonal to the X-axis direction, and rotational drive means that rotates the holding unit (none is illustrated). 
     For the wafer  10  that is conveyed to the laser processing apparatus  20  and is held by the holding unit, an alignment step is executed by using alignment means (not illustrated) disposed in the laser processing apparatus  20 , and the position of the planned dividing lines  14  formed in the front surface  10   a  is detected. In addition, the wafer  10  is rotated by the rotational drive means, and the planned dividing lines  14  in the first direction are aligned with the X-axis direction. Information regarding the detected position of the planned dividing lines  14  is stored in a controller that is not illustrated. 
     Based on the position information regarding the planned dividing lines  14  detected by the above-described alignment step, a light collector  22  of the laser beam irradiation unit  21  is positioned to a processing start position of the predetermined planned dividing line  14  extending in the first direction. Then, the focal point of the laser beam LB 2  is positioned to the front surface  10   a  of the wafer  10 , irradiation is executed, and processing feed of the wafer  10  together with the holding unit is executed in the X-axis direction, to execute ablation processing along the predetermined planned dividing line  14  extending in the first direction. As a result, a removal groove  200  that divides the wafer  10  along the planned dividing line  14   is formed. As illustrated on the right side of  FIG.  4 A  on which part of the wafer  10  is illustrated in an enlarged manner, the removal groove  200  is formed to remove the unnecessary region sandwiched by the protective walls  100  formed on both sides that define the width of the planned dividing line  14 . The laser beam LB 2  emitted from the laser beam irradiation unit  21  is a laser beam with a wavelength (for example, 355 nm) having absorbability with respect to, for example, SiC that configures the wafer  10 . 
     After the removal groove  200  is formed along the predetermined planned dividing line  14  extending in the first direction as described above, indexing feed of the wafer  10  is executed in the Y-axis direction by the interval of the planned dividing lines  14 , and the planned dividing line  14  that is adjacent to the predetermined planned dividing line  14  in the Y-axis direction and that has not yet been processed is positioned directly under the light collector  22 . Then, the focal point of the laser beam LB 2  is positioned to a front surface of the planned dividing line  14  of the wafer  10 , and irradiation is executed similarly to the above description, and processing feed of the wafer  10  is executed in the X-axis direction to form the removal groove  200 . Similarly, processing feed and indexing feed of the wafer  10  are executed in the X-axis direction and the Y-axis direction, and the removal grooves  200  are formed along the planned dividing lines  14  extending in the first direction. 
     Subsequently, the wafer  10  is rotated by  90  degrees in a direction indicated by an arrow R 2 , and the planned dividing lines  14  extending in a second direction that is a direction orthogonal to the planned dividing lines  14  in the first direction in which the removal grooves  200  have already been formed and in which the removal grooves  200  have not yet been formed are aligned with the X-axis direction. Then, the focal point of the laser beam LB 2  is positioned, and irradiation is executed similarly to the above description also for the remaining planned dividing lines  14 . As a result, planned dividing line removal processing to form the removal grooves  200  along all planned dividing lines  14  formed in the wafer  10 , as illustrated in  FIG.  4 B , is executed. Through the above, the devices  12  are divided from the wafer  10 , and the unnecessary region removal step is completed. 
     The unnecessary region removal step executed in the present invention is not limited to the laser processing by irradiation with the above-described laser beam LB 2 . For example, the wafer  10  in which the protective walls  100  are formed in all planned dividing lines  14  may be conveyed to a cutting apparatus  50  illustrated in  FIG.  5    (only part thereof is illustrated), and the unnecessary region removal step may be executed by the cutting apparatus  50 . 
     The cutting apparatus  50  includes a chuck table (not illustrated) that holds the wafer  10  under suction and a cutting unit  52  that cuts the wafer  10  held under suction by the chuck table. The chuck table is rotatably configured and includes a movement mechanism (not illustrated) that executes processing feed of the chuck table in a direction indicated by an arrow X in  FIG.  5   . Further, the cutting unit  52  includes a spindle  54  rotatably held by a spindle housing  53  disposed in the Y-axis direction indicated by an arrow Y in  FIG.  5    and an annular cutting blade  56  held by the tip of the spindle  54 , and includes a Y-axis movement mechanism (not illustrated) that executes indexing feed of the cutting blade  56  in the Y-axis direction. The spindle  54  is rotationally driven by a spindle motor that is not illustrated. 
     In execution of the unnecessary region removal step, first, the wafer  10  is placed over the chuck table of the cutting apparatus  50  and is held under suction with the front surface  10   a  of the wafer  10  oriented upward, and the planned dividing lines  14  extending in the first direction of the wafer  10  are aligned with the X-axis direction. In addition, position adjustment with the cutting blade  56  is executed. Subsequently, the cutting blade  56  rotated at high speed is positioned, in the planned dividing lines  14  aligned with the X-axis direction, to the unnecessary region sandwiched by the protective walls  100  formed on both sides that define the width of the planned dividing line  14 , and is caused to cut into the wafer  10  from the side of the front surface  10   a . In addition, processing feed of the chuck table is executed in the X-axis direction to form a removal groove  220  that divides the wafer  10 . Moreover, indexing feed of the cutting blade  56  of the cutting unit  52  is executed on the planned dividing line  14  that is adjacent in the Y-axis direction to the planned dividing line  14  in which the removal groove  220  has been formed and that does not have the removal groove  220  formed therein, and cutting processing to form the removal groove  220  similarly to the above description is executed. By repeating them, the removal grooves  220  are formed along all planned dividing lines  14  along the X-axis direction. 
     Subsequently, the chuck table is rotated by 90 degrees in a direction indicated by an arrow R3, and the second direction orthogonal to the first direction in which the removal grooves  220  have been formed first is aligned with the X-axis direction. Then, the above-described cutting processing is executed for all planned dividing lines  14  newly aligned with the X-axis direction, to form the removal grooves  220  along all planned dividing lines  14  formed in the wafer  10 . The cutting step is executed in this manner, and the planned dividing line removal processing to divide the wafer  10  into device chips of each device  12  along the planned dividing lines  14  is executed, and the unnecessary region removal step is completed, so that the devices  12  that are the necessary regions are divided similarly to the wafer  10  illustrated in  FIG.  4 B . 
     As described above, in the present embodiment, before execution of the unnecessary region removal step of removing the planned dividing lines  14  that are the unnecessary regions, the regions that define the boundary between the necessary region in which the device  12  is formed and the unnecessary region in which the planned dividing line  14  is formed are irradiated with the laser beam with a wavelength having transmissibility with respect to the wafer  10 , and the shield tunnels  102  and  104  composed of the fine pore and the modified tube that surrounds the fine pore are formed to form the protective walls  100 . Thus, even when the planned dividing lines  14  are broken to be removed through irradiation with the laser beam with high power or by the cutting blade, development of a crack to the region in which the device  12  is formed and which is the necessary region is prevented by the protective walls  100 , and the problem that damage is given to the devices  12  to be individually divided is eliminated. 
     The present invention is not limited to the above-described embodiment. In the formation of the shield tunnels in the above-described protective wall forming step, the shield tunnels may be formed to be stacked by shifting the position of the focal point in the upward-downward direction when irradiation with the laser beam LB 1  is executed in the thickness direction of the wafer  10 . For example, in the protective wall forming step executed for the wafer  10  with a thickness of 500 µm, as is understood from a sectional view of the wafer  10  illustrated in  FIG.  6   , the first protective wall forming step of successively forming shield tunnels  111   in the planned dividing line  14  aligned with the X-axis direction of the wafer  10  with the interposition of at least intervals corresponding to one shield tunnel is executed, and the second protective wall forming step of successively forming shield tunnels  112  in the regions in which the intervals are interposed in the planned dividing line  14  is executed. Subsequently, a third protective wall forming step of forming shield tunnels  113  above the formed shield tunnels  111  may be executed, and a fourth protective wall forming step of forming shield tunnels  114  above the shield tunnels  112  formed in the second protective wall forming step may be executed. In the embodiment illustrated in  FIG.  6   , a protective wall  110  is formed through execution of fifth and sixth protective wall forming steps of forming shield tunnels  115  and  116  further stacked above the shield tunnels  113  and  114 , in addition to the above-described first to fourth protective wall forming steps. Further, the focal point when irradiation with the laser beam LB 1  to form the shield tunnels  111  to  116  is executed is positioned in such a manner as to be shifted in the upward-downward direction, and the shield tunnels  111  to  116  are stacked in the thickness direction. This makes it possible to effectively form the protective walls  110  that prevent development of a crack even for a thick workpiece. 
     In the embodiment illustrated in  FIG.  6   , in the execution of the second protective wall forming step after the execution of the first protective wall forming step, the shield tunnels  111  formed in the first protective wall forming step and the shield tunnels  112  formed in the second protective wall forming step are formed in such a manner that steps are alternately made in the thickness direction of the wafer  10 . Owing to this, when processing to remove the unnecessary region is executed for a thick workpiece, influence attributed to a hot spot is avoided more effectively, and development of a crack to the necessary region is prevented by the protective walls  110 . In the present embodiment, steps are alternately made in the thickness direction also when the shield tunnels  113  to  116  are formed in the third to sixth protective wall forming steps. In the above-described first to sixth protective wall forming steps, when the shield tunnels are stacked in the thickness direction, the shield tunnels of the upper part are stacked in such a manner as not to be in contact with the shield tunnels of the lower part. This can suppress the occurrence of a crack in a case in which the shield tunnels of the upper part are formed to be in contact with the shield tunnels of the lower part. Further, when the shield tunnels are formed in such a manner that the modified tubes of adjacent shield tunnels are in contact with each other in the protective wall forming step, it is possible to effectively prevent development of a crack from the unnecessary region to the necessary region when the unnecessary region removal step is executed. 
     Moreover, in the above-described embodiment, description has been made about the case in which the workpiece is the wafer  10  in which the plurality of devices  12  are formed on the front surface  10   a  in such a manner as to be marked out by the plurality of planned dividing lines  14  that intersect each other. However, the present invention is not limited thereto. For example, the workpiece may be a circular plate-shaped member  60  of SiC formed of a necessary region  62  marked out with a substantially rectangular shape at the center illustrated on the left side of  FIG.  7    and an unnecessary region  64  on the outer circumferential side that surrounds the necessary region  62 . When the plate-shaped member  60  is processed by the processing method of the present invention, the plate-shaped member  60  is held by an annular frame that is not illustrated through an adhesive tape and is conveyed to the above-described laser processing apparatus  1 . Subsequently, executed is the protective wall forming step in which the focal point of a laser beam with a wavelength, for example, 532 nm, having transmissibility with respect to the plate-shaped member  60  is positioned to the inside of a region that defines the boundary between the necessary region  62  and the unnecessary region  64 , irradiation is executed, and the shield tunnels composed of the fine pore and the modified tube that surrounds the fine pore are formed to form a protective wall  120  along the outer circumferential of the necessary region  62 . Detailed description of this protective wall forming step is omitted because it is a step executed under a condition similar to that of the protective walls  100  and  110  of the above-described protective wall forming steps. 
     After the protective wall  120  is formed as described above, the unnecessary region removal step of removing the unnecessary region  64  surrounding the necessary region  62  along the protective wall  120  is executed. The unnecessary region removal step is executed by laser processing by the above-described laser processing apparatus  20 , for example. As illustrated on the right side of  FIG.  7    on which part of the plate-shaped member  60  is illustrated in an enlarged manner, irradiation with the above-described laser beam LB 2  with a wavelength having absorbability with respect to the plate-shaped member  60  is executed along the outside of the protective wall  120 , and a first removal groove  130  that divides the plate-shaped member  60  is formed. Further, a plurality of second removal grooves  132  radially extending from the above-described first removal groove  130  to the outer circumferential edge of the plate-shaped member  60  are formed. By forming the first removal groove  130  and the second removal grooves  132  in this manner, the unnecessary region  64  of the plate-shaped member  60  is removed, and only the necessary region  62  can be obtained. At this time, a problem that a crack develops from the unnecessary region  64  to the necessary region  62  and the necessary region  62  is damaged does not occur. 
     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.