Patent Publication Number: US-2023141560-A1

Title: Wafer processing method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer processing method by which a wafer is divided into individual device chips. 
     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 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 electrical equipment such as mobile phones and personal computers. 
     A laser processing apparatus substantially includes 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 relative to each other, and can process the wafer with high accuracy (for example, refer to Japanese Patent Laid-open No. 2015-085347). 
     There are the following types as types of the laser beam irradiation unit: 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 a numerical aperture (NA) of a light collector by a refractive index (N) of the wafer is in a range from 0.05 to 0.2, to form a shield tunnel including 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, when a wafer is divided into individual device chips through irradiation with a laser beam along planned dividing lines by the above-described laser processing apparatus, there is a problem that, due to a crystal structure of a material that configures the wafer, a crack develops to a region outside a planned dividing line through the irradiation with the laser beam, which gives damage to a device. 
     Thus, an object of the present invention is to provide a wafer processing method that can solve the problem that a crack develops to a region outside a planned dividing line and gives damage to a device when a wafer is processed through irradiation with a laser beam. 
     In accordance with an aspect of the present invention, there is provided a wafer processing method by which 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 is divided into individual device chips, the wafer processing method including a shield tunnel forming step of executing irradiation with a laser beam with a wavelength having transmissibility with respect to the wafer to form shield tunnels each including a fine pore and a modified tube that surrounds the fine pore, and a dividing step of applying an external force to the wafer to divide the wafer into the individual device chips after executing the shield tunnel forming step. The shield tunnel forming step includes a first shield tunnel forming step of successively forming the shield tunnels in one planned dividing line with interposition of at least intervals corresponding to one shield tunnel, and a second shield tunnel forming step of successively forming the shield tunnels in regions in which the intervals are provided in the planned dividing line. 
     Preferably, the shield tunnels formed in the first shield tunnel forming step and the shield tunnels formed in the second shield tunnel forming step are formed in such a manner that steps are alternately made in a thickness direction. Preferably, the shield tunnels are stacked in the thickness direction of the wafer in the shield tunnel forming step. Preferably, the shield tunnel forming step includes, when the shield tunnels are stacked, a third shield tunnel forming step of forming shield tunnels above the shield tunnels formed in the first shield tunnel forming step, and a fourth shield tunnel forming step of forming shield tunnels above the shield tunnels formed in the second shield tunnel forming step. 
     Preferably, in the shield tunnel forming step, when the shield tunnels are stacked in the thickness direction of the wafer, the shield tunnels on an upper side are stacked in such a manner as not to be in contact with the shield tunnels on a lower side. Preferably, the wavelength of the laser beam is 532 nm, power per pulse is 2.0 to 4.0·10 −3  J, and an interval of a spot is 10 to 15 μm. 
     According to the present invention, development of a crack to a region outside the planned dividing line can be suppressed. Moreover, an influence of a hot spot that would be generated when irradiation with the laser beam is executed to successively form shield tunnels adjacent to each other is avoided, and the problem that a crack develops to a region in which a device is formed and the device is damaged can be 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 wafer processing method according to the present invention; 
         FIG.  2    is a perspective view of a wafer unit; 
         FIG.  3 A  is a perspective view illustrating a shield tunnel forming step; 
         FIG.  3 B  is a perspective view of the wafer unit obtained after completion of the shield tunnel forming step; 
         FIG.  3 C  is a plan view in which part of a wafer for which the shield tunnel forming step has been executed is illustrated in an enlarged manner; 
         FIG.  3 D  is a sectional view in which part of the wafer obtained after completion of first and second shield tunnel forming steps is illustrated in an enlarged manner; 
         FIG.  3 E  is a schematic perspective view of a shield tunnel; 
         FIG.  4    is a sectional view illustrating a manner in which a dividing step is executed by a dividing apparatus giving an external force to the wafer; and 
         FIG.  5    is a sectional view in which part of the wafer in another embodiment of the shield tunnel forming step is enlarged. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A wafer processing method according to embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     In the following, a laser processing apparatus suitable to execute the processing method of an embodiment of the present invention will be described with reference to the accompanying drawings, and the wafer processing method according to 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 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, and an imaging unit  6  that images the wafer  10  held by the holding unit  3 . The laser processing apparatus  1  further includes 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, and a frame body  5  having 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 a horizontal direction. Although diagrammatic representation is omitted, optical systems that configure 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 in 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 defined based on X coordinates and Y coordinates as a holding surface, and can be rotated 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, 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 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  into 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  into 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 not illustrated. The controller is configured by a computer and includes a central processing unit (CPU) that executes calculation processing in accordance with 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 of 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 to be controlled. 
     By the wafer processing method based on the present invention, for example, the wafer  10  illustrated in  FIG.  2    is processed. The wafer  10  is a wafer made of SiC with a thickness of approximately 100 μm, for example, and a plurality of devices  12  are formed on a front surface  10   a  of the wafer  10  in such a manner as to be marked out by a plurality of planned dividing lines  14  that intersect. When being processed by the above-described laser processing apparatus  1 , as illustrated in the diagram, 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 . The wafer  10  is divided into individual device chips by execution of the wafer processing method according to the present embodiment to be described below. 
     In the wafer processing method according to the present embodiment, first, a shield tunnel forming step of executing irradiation with a laser beam with a wavelength having transmissibility with respect to the wafer  10  to form shield tunnels each including a fine pore and a modified tube that surrounds the fine pore is executed. A procedure of executing the shield tunnel forming step of the present embodiment will be more specifically described below. 
     In execution of the shield tunnel forming step, after the above-described wafer  10  is prepared, the wafer  10  is placed on the suction adhesion chuck  36  of 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. Position information regarding the devices  12  and the planned dividing lines  14  of the wafer  10  is detected and is stored in the above-described controller. Moreover, based on the position information, the movement mechanism  4  and so forth are actuated, and a predetermined one of the planned dividing lines  14  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 stored in the controller, a region in which the planned dividing line  14  is formed is irradiated with a laser beam LB. As is understood from  FIG.  3 C  in which part of the planned dividing line  14  is illustrated in an enlarged manner, a dividing layer  100  including shield tunnels  102  and  104  is formed inside the wafer  10  along a central region of the planned dividing line  14 . 
     In the irradiation with the above-described laser beam LB, a numerical aperture (NA) of a light collecting lens included in 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 a refractive index (N) of the wafer  10  falls within a range from 0.05 to 0.2, for example. Further, a wavelength of the laser beam LB is set to, for example, 532 nm, which is a wavelength having transmissibility with respect to the wafer  10 . An average output power is set to 0.2 to 0.4 W, and a repetition frequency is set to 10 kHz. Energy per pulse is set to 2.0 to 4.0·10 −3  J, and an interval of a spot is set to 10 to 15 μm. By positioning a focal point of the laser beam LB to a position inside the wafer  10  and executing irradiation with the laser beam LB on the basis of such laser processing conditions, as illustrated in  FIG.  3 D , the shield tunnels  102  and  104  are formed, and the dividing layer  100  that becomes a division initiating point when applied with an external force to be described later is formed. As is understood from a perspective view illustrated in  FIG.  3 E  in an enlarged manner, the shield tunnels  102  and  104  each have a fine pore  130  and a modified tube  140  that surrounds the fine pore  130 . For example, a diameter of the fine pore  130  is approximately 1 μm, and a diameter of the modified tube  140  is approximately 10 μm. 
     In the above-described shield tunnel forming step, for example, a first shield tunnel forming step of successively forming the shield tunnels  102  first along the central region of the planned dividing line  14  with interposition of at least intervals (for example, 10 to 15 μm) corresponding to one shield tunnel is executed. Subsequently, a second shield tunnel forming step of successively forming the shield tunnels  104  for the regions in which the intervals are provided in the central region of the planned dividing line  14  is executed. That is, the shield tunnels  102  and the shield tunnels  104  are alternately formed along the X-axis direction to form the dividing layer  100 . By executing the first shield tunnel forming step and the second shield tunnel forming step in the formation of the dividing layer  100  in this manner, development of a crack to a region outside the planned dividing line  14  can be suppressed. Moreover, an influence of a hot spot that would be generated when irradiation with the laser beam LB is executed to successively form shield tunnels adjacent to each other is avoided, and the problem that a crack develops to a region in which a device  12  is formed and the device  12  is damaged can be avoided. 
     The purpose of executing the first shield tunnel forming step and the second shield tunnel forming step with 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 the first shield tunnel 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 a region in which a device  12  is formed. Preferably, the laser processing conditions are the same between the first shield tunnel forming step and the second shield tunnel forming step. However, different laser processing conditions may be employed. 
     After the dividing layer  100  including the shield tunnels  102  and  104  is formed along the predetermined planned dividing line  14  as described above, indexing feed of the wafer  10  is executed in the Y-axis direction, and an unprocessed planned dividing line  14  that is adjacent to the predetermined planned dividing line  14  in the Y-axis direction and extends in a first direction is positioned directly under the light collector  71 . Then, as is the case described above, the focal point of the laser beam LB is positioned inside the wafer  10  along the central region of the unprocessed planned dividing line  14 , and irradiation is executed to sequentially execute the above-described first shield tunnel forming step and second shield tunnel forming step. The shield tunnels  102  and  104  are thus formed to form another dividing layer  100 . Similarly, processing feed and indexing feed of the wafer  10  are executed in the X-axis direction and the Y-axis direction, and the dividing layers  100  similar to the above-described one are formed along all 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 1  in  FIG.  3 A , and planned dividing lines  14  extending in a second direction orthogonal to the planned dividing lines  14  along which the dividing layers  100  have already been formed are aligned with the X-axis direction. Then, as is the case described above, the focal point of the laser beam LB is positioned inside the wafer  10  along each of the central regions of the planned dividing lines  14  extending in the second direction, and irradiation is executed. Accordingly, as illustrated in  FIG.  3 B , the dividing layers  100  are formed along all the planned dividing lines  14  formed on the front surface  10   a  of the wafer  10 , so that the shield tunnel forming step of the present embodiment is completed. 
     After the above-described shield tunnel forming step is executed, a dividing step of applying an external force to the wafer  10  to divide the wafer  10  into individual device chips  12 ′ is executed with use of a dividing apparatus  50  illustrated in  FIG.  4   , for example. 
     The illustrated dividing apparatus  50  includes a frame holding component  51  that holds the annular frame F holding the wafer  10 , a plurality of clamps  52  as fixing means disposed at an outer circumference of the frame holding component  51 , and an expanding drum  55  disposed inside the frame holding component  51 . An outer diameter of the expanding drum  55  is set smaller than an inner diameter of the annular frame F, and an inner diameter of the expanding drum  55  is set larger than an outer diameter of the wafer  10 . Further, a plurality of air cylinders  53  that cause the frame holding component  51  to advance and retreat in an upward-downward direction are disposed outside the expanding drum  55 , and piston rods  54  caused to advance and retreat in the upward-downward direction by the air cylinders  53  are coupled to a lower surface of the frame holding component  51 . The plurality of air cylinders  53  and the piston rods  54  thus constitute support means, which is configured to allow the annular frame holding component  51  to selectively move between a reference position at which the frame holding component  51  is at substantially the same height as an upper end of the expanding drum  55  as illustrated by solid lines in  FIG.  4    and an expanding position at which the frame holding component  51  is separated downward from the upper end of the expanding drum  55  by a predetermined amount as illustrated by two-dot chain lines in  FIG.  4   . 
     Operation of the above-described dividing apparatus  50  will be described. The annular frame F that supports the wafer  10  in which the dividing layers  100  have been formed along the planned dividing lines  14  is placed on a placement surface of the frame holding component  51  and is fixed to the frame holding component  51  by the clamps  52 . At this time, the piston rods  54  of the air cylinders  53  are in an extended state, and the frame holding component  51  is positioned at the reference position as illustrated by the solid lines in  FIG.  4   . 
     The frame holding component  51  positioned at the reference position as illustrated by the solid lines in the diagram is lowered through actuation of the plurality of air cylinders  53  that configure external force applying means, and the annular frame F also lowers. As a result, the adhesive tape T attached to the annular frame F abuts against the upper end edge of the expanding drum  55  that relatively rises, and is expanded as illustrated by the two-dot chain lines in the diagram. Consequently, a tensile external force radially acts on the wafer  10  attached to the adhesive tape T, and the wafer  10  is divided into the individual device chips  12 ′ in such a manner that the planned dividing lines  14  having been made fragile by the dividing layers  100  become division initiating points. Through the above, the dividing step is completed. 
     The present invention is not limited to the above-described embodiment. In the formation of the shield tunnels in the above-described shield tunnel 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 is executed in a thickness direction of the wafer  10 . For example, in the shield tunnel forming step executed for a wafer  10  with a thickness of 500 μm, the following processing may be executed as is understood from a sectional view of the wafer  10  illustrated in  FIG.  5   . The first shield tunnel forming step of successively forming shield tunnels  111  in the planned dividing line  14  aligned with the X-axis direction in the wafer  10  with interposition of at least intervals corresponding to one shield tunnel is executed, and the second shield tunnel forming step of successively forming shield tunnels  112  in the regions in which the intervals are provided in the planned dividing line  14  is executed. Subsequently, a third shield tunnel forming step of forming shield tunnels  113  above the formed shield tunnels  111  is executed, and a fourth shield tunnel forming step of forming shield tunnels  114  above the shield tunnels  112  formed in the second shield tunnel forming step is executed. 
     In the embodiment illustrated in  FIG.  5   , fifth and sixth shield tunnel forming steps of forming shield tunnels  115  and  116  in such a manner as to further stack the shield tunnels  115  and  116  above the shield tunnels  113  and  114  are executed in addition to the above-described first to fourth shield tunnel forming steps, to thereby form a dividing layer  110 . Further, the focal points used when irradiation with the laser beam LB is executed to form the shield tunnels  111  to  116  are positioned in such a manner as to be shifted from each other in the upward-downward direction, and the shield tunnels  111  formed in the first shield tunnel forming step and the shield tunnels  112  formed in the second shield tunnel forming step are formed in such a manner that steps are alternately made in the thickness direction of the wafer  10 . 
     Accordingly, when the shield tunnel forming step is executed for a thick workpiece, the influence attributable to a hot spot is avoided more effectively, and development of a crack to a region in which a device  12  is formed is prevented by the dividing layers  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 shield tunnel forming steps, and a similar effect is provided. In the above-described first to sixth shield tunnel forming steps, when the shield tunnels are stacked in the thickness direction, the shield tunnels on the upper side are stacked in such a manner as not to be in contact with the shield tunnels on the lower side. This can suppress occurrence of a crack compared with the case in which the shield tunnels on the upper side are formed to be in contact with the shield tunnels on the lower side. 
     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.