Patent Publication Number: US-2022238379-A1

Title: Wafer dividing method and dividing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer dividing method for dividing, into individual device chips, a wafer formed on a front surface thereof with a plurality of devices partitioned by a plurality of intersecting streets and a dividing apparatus suitable for the dividing method. 
     Description of the Related Art 
     A wafer formed on a front surface thereof with a plurality of devices such as integrated circuits (ICs) and large scale integrated circuits (LSIs) partitioned by a plurality of intersecting streets is divided into individual device chips by a laser beam of such a wavelength as to be transmitted through the wafer being applied to the wafer with a condensing point of the laser beam positioned to the inside of the wafer corresponding to the streets, to form modified layers which will become starting points of division, and an external force being exerted on the wafer. The thus divided device chips are used for electric apparatuses such as mobile phones and personal computers (see, for example, Japanese Patent No. 3408805). 
     Incidentally, when the external force is exerted on the wafer to divide the wafer into the individual device chips, upon division, fine particles (particles) are scattered from the modified layers serving as the starting points of division, and are dropped and deposited on the front surface of the wafer, lowering the quality of the device chips. In view of this, a technology by which the wafer is divided in a downwardly directed state such that the scattered particles are prevented from being deposited on the device chips has been proposed by the present applicant (see Japanese Patent Laid-open No. 2020-096177). 
     SUMMARY OF THE INVENTION 
     Here, in the case where an external force is exerted on the wafer to divide the wafer into individual device chips, using the technology described in Japanese Patent No. 3408805, whether or not the modified layers have been properly formed can be determined by checking the number and state of the particles dropped on the front surface of the wafer. On the other hand, in the case of the technology described in Japanese Patent Laid-open No. 2020-096177, since the wafer is divided in a downwardly directed state, particles are not dropped and deposited on the front surface of the wafer, so that whether or not the modified layers have been properly formed in the wafer cannot be checked after the division. 
     Accordingly, it is an object of the present invention to provide a wafer dividing method and a dividing apparatus by which whether or not modified layers have been properly formed in the wafer can be checked after division of the wafer, even in the case where the wafer is divided in such a manner that scattered particles are not deposited on the front surface of the wafer. 
     In accordance with an aspect of the present invention, there is provided a wafer dividing method for dividing, into individual device chips, a wafer formed on a front surface thereof with a plurality of devices partitioned by a plurality of intersecting streets, the wafer dividing method including a modified layer forming step of applying a laser beam of such a wavelength as to be transmitted through the wafer to the wafer while positioning a condensing point of the laser beam to an inside of the wafer corresponding to the streets, to form modified layers that will be starting points of division, a frame disposing step of positioning the wafer at an opening of a frame provided in a center thereof with the opening for accommodating the wafer, and integrally attaching the frame and the wafer together through a dicing tape, before or after the modified layer forming step, a dividing step of directing the wafer downward, and expanding the dicing tape to divide, into individual device chips, the wafer along the modified layers formed in the inside of the wafer corresponding to the streets, and a determining step of counting particles scattered at the time of division of the wafer by a particle counter disposed in a dust collection path set directly below the wafer, and determining, on the basis of the number of the particles, whether or not the modified layers have been properly formed, at the time of carrying out the dividing step. 
     In accordance with another aspect of the present invention, there is provided a wafer dividing apparatus for positioning a wafer at an opening of a frame provided in a center thereof with the opening for accommodating the wafer, integrally attaching the frame and the wafer together through a dicing tape, and dividing, into individual device chips, the wafer formed with starting points of division along a plurality of intersecting streets partitioning a plurality of devices, the dividing apparatus including frame holding means that holds the frame with the wafer directed downward, dividing means for expanding the dicing tape present between the frame and the wafer, to divide the wafer into the individual device chips, and a particle counter that is disposed in a dust collection path set directly below the wafer and counts particles scattered at the time of dividing the wafer. 
     Preferably, the dividing apparatus further includes a determining section that is connected to the particle counter and determines, on the basis of the number of particles counted by the particle counter, whether or not modified layers have been properly formed. 
     According to the wafer dividing method of the present invention, whether or not the wafer is divided after the modified layers have been properly formed in the modified layer forming step can be determined, even in the case where the wafer is divided by such a dividing method that particles are not deposited on the front surface of the wafer. 
     According to the dividing apparatus of the present invention, whether or not the wafer is divided after the modified layers have been properly formed in the modified layer forming step can be determined, even in the case where the wafer is divided in such a manner that particles do not deposit on the front surface of the wafer. 
     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 a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view depicting the manner in which a protective tape is attached to a wafer as a workpiece; 
         FIG. 2  is a perspective view depicting the manner in which the wafer of  FIG. 1  is mounted on a chuck table of a laser processing apparatus; 
         FIG. 3A  is a perspective view depicting the manner in which a modified layer forming step is carried out; 
         FIG. 3B  is a partially enlarged sectional view of the wafer in the modified layer forming step; 
         FIG. 4A  is a perspective view depicting an embodiment of a frame disposing step; 
         FIG. 4B  is a perspective view depicting the manner in which the protective tape is peeled off from the wafer depicted in  FIG. 4A ; 
         FIG. 5A  is a perspective view depicting another embodiment of the frame disposing step; 
         FIG. 5B  is a perspective view depicting an embodiment in a case where the modified layer forming step is carried out after the frame disposing step depicted in  FIG. 5A  is performed; 
         FIG. 5C  is a partially enlarged sectional view of the modified layer forming step; 
         FIG. 6  is a perspective view, as viewed from an oblique lower side, of a dividing apparatus in the present embodiment; and 
         FIG. 7  is a partially enlarged sectional view depicting an embodiment of a dividing step and a determining step. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wafer dividing method according to an embodiment of the present invention and a dividing apparatus to be used in the wafer dividing method will be described in detail below referring to the attached drawings. 
     A workpiece to which the wafer dividing method of the present embodiment is to be applied is, for example, a wafer  10  as depicted in  FIG. 1 . The wafer  10  is a wafer of silicon (Si), and a wafer provided on a front surface  10   a  thereof with a plurality of devices  12  partitioned by a plurality of intersecting streets  14 . A modified layer forming step of applying a laser beam of such a wavelength as to be transmitted through the wafer  10  is applied to the wafer  10  while positioning a condensing point of the laser beam to the inside of the wafer  10  corresponding to the streets  14 , to form modified layers which will be starting points of division, will be described below. 
     In carrying out the modified layer forming step, first, as depicted in  FIG. 1 , a protective tape T 1  is attached to the front surface  10   a  of the wafer  10  to unite them. The wafer  10  united with the protective tape T 1  is fed to a laser processing apparatus  2  (only a part thereof is illustrated) depicted in  FIG. 2 , and is mounted on a chuck table  21 , with the protective tape T 1  side directed downward and with a back surface  10   b  side directed upward. The chuck table  21  includes a holding surface  21   a  on an upper side, and the holding surface  21   a  includes a gas-transmitting suction chuck. The holding surface  21   a  is connected to suction means which is omitted in illustration, and, by operating the suction means, the wafer  10  is held under suction onto the chuck table  21 . 
     When the wafer  10  is held under suction on the chuck table  21 , carried out as required is alignment in which position information concerning the streets  14  formed on the front surface  10   a  side of the wafer  10  is detected by use of an imaging unit (omitted in illustration) including an infrared charge coupled device (CCD) disposed in the laser processing apparatus  2 . Next, a moving mechanism, which is omitted in illustration, is operated in reference to the position information concerning the streets  14 , to position the chuck table  21  holding the wafer  10  directly below a beam condenser  23  of a laser beam applying unit  22 , as depicted in  FIG. 3 . The laser beam applying unit  22  is a unit that includes an optical system which is omitted in illustration, emits a laser beam LB of such a wavelength as to be transmitted through the wafer  10 , and applies the laser beam LB while adjusting the output to a predetermined output and condensing the laser beam LB. Note that the abovementioned moving mechanism includes a mechanism for moving the chuck table  21  in an X direction and in a Y direction orthogonal to the X direction, a mechanism for rotationally driving the chuck table  21 , and a mechanism for moving the laser beam applying unit  22  in a Z direction orthogonal to the X direction and the Y direction. 
     When the wafer  10  is positioned directly below the beam condenser  23 , as depicted in  FIG. 3A , the laser beam applying unit  22  is operated, and the laser beam LB is applied to the wafer  10  with a condensing point S of the laser beam LB being positioned to the inside of the wafer  10  corresponding to the street  14  and the moving mechanism that relatively moves the beam condenser  23  and the chuck table  21  in the X direction (see  FIG. 3B ) being operated, to form a modified layer  100  which will be a starting point of division. By operating the moving mechanism to move the chuck table  21  in a processing feeding direction (X direction), an indexing feeding direction (Y direction) and a rotating direction, as required, the laser beam LB is applied along all the streets  14 , whereby the modified layers  100  are formed along all the streets  14 , and the modified layer forming step is completed. 
     The processing conditions for the laser processing carried out by the laser processing apparatus  2  are, for example, set as follows. 
     Wavelength: 1,064 nm or 1,342 nm 
     Average output: 0.5 to 2.0 W 
     Repetition frequency: 60 to 90 kHz 
     Processing feeding speed: 200 to 1,000 mm/s 
     When the modified layer forming step is carried out, as depicted in  FIG. 4A , performed is a frame disposing step in which the wafer  10  is positioned at an opening Fa of a frame F provided in the center thereof with the opening Fa for accommodating the wafer  10  and the frame F and the wafer  10  are integrally attached together through a dicing tape T 2 . More specifically, a peripheral part of the dicing tape T 2  having a pressure sensitive adhesive layer on a surface thereof is attached to a back surface of the frame F, the back surface  10   b  side of the wafer  10  is positioned at the center of the opening Fa of the frame F, and the frame F and the wafer  10  are integrally attached together in a state in which the protective tape T 1  is exposed on the upper side. In the present embodiment, after the frame disposing step is carried out, as depicted in  FIG. 4B , the protective tape T 1  is peeled off and removed from the front surface  10   a  of the wafer  10 . 
     Note that, while the frame disposing step is carried out after the modified layer forming step is performed in the above embodiment, the present invention is not limited to this; the frame disposing step may be carried out before the modified layer forming step. For example, as depicted in  FIG. 5A , an unprocessed wafer  10  (a wafer  10  in the same state as the wafer  10  depicted in  FIG. 1 ) for which the modified layer forming step has not yet been carried out and an annular frame F provided in its center with an opening Fa for accommodating the wafer  10  are prepared, the periphery of the dicing tape T 2  having a pressure sensitive adhesive layer on its surface is attached to the back surface of the frame F, the back surface  10   b  side of the wafer  10  is positioned at the center of the opening Fa of the frame F, and the frame F and the wafer  10  are integrally attached in a state in which the front surface  10   a  of the wafer  10  is exposed on the upper side (frame disposing step). 
     In the case where the frame disposing step is carried out before the modified layer forming step is conducted, after the frame disposing step is carried out, the wafer  10  held by the frame F is fed to the laser processing apparatus  2 . Then, as depicted in  FIG. 5B , the frame F is inverted upside down to position the wafer  10  side on the lower side, and the frame F is mounted on the chuck table  21 , which is omitted in illustration, with the dicing tape T 2  side directed upward, and is held under suction on the chuck table  21 . Next, alignment is performed from the back surface  10   b  side of the wafer  10  in a manner similar to the one described above, the position of the street  14  of the wafer  10  is detected, and, while the moving mechanism is being operated, the laser beam LB of such a wavelength as to be transmitted through the wafer  10  is applied from the dicing tape T 2  side along the street  14  with the condensing point S of the laser beam LB being positioned to the inside of the wafer  10  corresponding to the street  14 , to form the modified layer (see  FIG. 5C ). In this way, the modified layers  100  which will be starting points of division are formed in the inside of the wafer  10  corresponding to all the streets  14  (see  FIG. 5C ), whereby the modified layer forming step is completed. 
     Even in the case where the frame disposing step is carried out before the modified layer forming step is conducted, it is preferable to attach the protective tape T 1  to the front surface  10   a  of the wafer  10 . Note that it is favorable that the laser processing conditions for the modified layer forming step to be carried out in the case where the frame disposing step is carried out before the modified layer forming step be set similar to the processing conditions for the modified layer forming step described referring to  FIG. 3 . In addition, in the above embodiment, the laser beam LB has been applied from the back surface  10   b  side of the wafer  10  in carrying out the modified layer forming step, but, in the case where test element group (TEG) and the like are not formed on the streets  14 , the laser beam LB may be applied from the front surface  10   a  side of the wafer  10  to form the modified layers  100  in the inside of the wafer  10  corresponding to the streets  14 . 
     When the modified layer forming step and the frame disposing step described above are carried out, conducted is a dividing step in which the wafer  10  is directed downward, the dicing tape T 2  is expanded, and the wafer  10  is divided along the modified layers  100  formed along the streets  14  into individual device chips. 
     Referring to  FIG. 6 , a dividing apparatus  3  suitable for carrying out the dividing step of the present embodiment will be described. As depicted in the figure, the dividing apparatus  3  is a dividing apparatus that positions the wafer  10  at the opening Fa of the frame F provided in its center with the opening Fa for accommodating the wafer  10 , integrally attaches the frame F and the wafer  10  through the dicing tape T 2 , and divides, into individual device chips, the wafer  10  formed with the modified layers  100  as starting points of division in the inside of the wafer  10  corresponding to the plurality of streets  14  partitioning the plurality of devices  12 . A more specific description will be given referring to  FIG. 6 . 
     The dividing apparatus  3  depicted in  FIG. 6  includes a frame holding section  31  that holds the frame F supporting the wafer  10  side directed downward; a dividing section  32  that expands the dicing tape T 2  attached to the frame F held by the frame holding section  31 , to divide the wafer  10  into individual device chips; and a dust collection section  35  provided directly below the frame holding section  31 , with a particle counter  37  for counting particles scattered from the wafer  10  divided. 
     The frame holding section  31  includes a frame holding member  31   a  formed in an annular shape for holding the annular frame F, and a plurality of (in the embodiment depicted, four) clamps  31   b  as fixing means disposed at regular intervals in the outer circumference of the frame holding member  31   a . A lower surface of the frame holding member  31   a  is formed to be flat, and the frame F is mounted thereto. To prevent the frame F mounted to the lower surface of the frame holding member  31   a  from being dropped, the frame F is fixed to the lower surface of the frame holding member  31   a  by the clamps  31   b.    
     On the inside of the annular frame holding member  31   a , an expansion drum  33  having an upper end fixed to a base  34  and being fixed to the base  34  in a hanging manner is disposed. The base  34 , for example, has an upper surface side fixed to a top wall of a housing (omitted in illustration) constituting the dividing apparatus  3 . In plan view, the expansion drum  33  is smaller than the inside diameter of the opening Fa of the frame F but larger than the outside diameter of the wafer  10  supported by the dicing tape T 2  mounted to the frame F. The dividing section  32  in the present embodiment includes air cylinders  32   a  that are disposed in plural number (for example, four) around the expansion drum  33  and that have upper ends fixed to the base  34 ; and piston rods  32   b  extending downward from the air cylinders  32   a  and having lower ends connected to an upper surface of the frame holding member  31   a . The air cylinders  32   a  are supplied with control air, and, by the action of the air cylinders  32   a , the piston rods  32   b  are advanced and retracted in the vertical direction, whereby the frame holding member  31   a  is advanced and retracted in the vertical direction. 
     The dust collection section  35  includes a dust collection path  36  and the particle counter  37 . The dust collection path  36  includes an inverted conical dust collection cover  36   a  positioned directly below the frame holding member  31   a ; and a flexible pipe  36   b  connected to a bottom portion of the dust collection cover  36   a . Suction means, which is omitted in illustration, is connected to the flexible pipe  36   b , and, by operating the suction means, a suction negative pressure V is generated in the dust collection path  36 . A dust collection box, which is omitted in illustration, is disposed at a final end portion of the dust collection path  36 , and the particles having passed through the dust collection path  36  are accommodated in the dust collection box and are periodically discarded. The particle counter  37  is disposed on the flexible pipe  36   b , and counts the particles passing through the dust collection path  36 ; for example, a laser beam is applied to the dust collection path  36 , and the intensity of scattered light generated by collision of the laser beam on the particles (light intensity) is measured. 
     In the present embodiment, a determining section  40  is electrically connected to the particle counter  37 . The determining section  40  includes, for example, a computer, to which an electrical signal indicative of the light intensity is inputted from the particle counter  37 , whereby the size of the particles passing through the dust collection path  36  is measured in reference to the magnitude of the electrical signal (electric pulse), and, in reference to the number of the electrical signals, the particles passing through the dust collection path  36  can be counted. In other words, according to the particle counter  37  and the determining section  40  of the present embodiment, the number of the particles passing through the dust collection path  36  can be counted on a size basis. The information concerning the particles counted by the particle counter  37  and the result of determination made by the determining section  40  can be displayed on a display unit  42  connected to the determining section  40 . 
     The dividing apparatus  3  of the present embodiment generally has the configuration described above, and, referring to  FIG. 7  in addition to  FIG. 6 , the dividing step of dividing the wafer  10  into individual device chips  12 ′ and the determining step in the present embodiment will be described. 
     In carrying out the dividing step of the present embodiment, as depicted in  FIG. 6 , the frame F is mounted to the lower surface of the frame holding member  31   a , with the front surface  10   a  side of the wafer  10  formed with the modified layers  100  along all the streets  14  directed downward, and the frame F is fixed by the clamps  31   b . In this instance, as depicted in  FIG. 7 , the lower end of the expansion drum  33  is positioned at a reference position (indicated by a solid line) which is at substantially the same height as the lower surface of the frame holding member  31   a . Next, the air cylinders  32   a  of the dividing section  32  are operated, to retract the piston rods  32   b  into the air cylinders  32   a , whereby the frame holding member  31   a  is raised in the direction indicated by an arrow R 1 . As a result, the frame F is raised together with the frame holding member  31   a , and the dicing tape T 2  held by the frame F is expanded by the expansion drum  33  lowered relative to the frame holding member  31   a  (indicated by alternate long and two dashes line), whereby radial pulling forces act on the dicing tape T 2 . As a result, as depicted in  FIG. 7 , the wafer  10  is divided along the streets  14 , and the device chips  12 ′ are formed (dividing step). 
     In carrying out the dividing step in which the dividing section  32  is operated, the suction means, which is omitted in illustration, is operated, whereby a suction negative pressure V is generated in the dust collection path  36 . As a result, the particles P scattered due to breakage of the modified layers  100  are dropped into the dust collection cover  36   a  of the dust collection path  36  positioned directly below the wafer  10  and are sucked. Here, as described above, the particle counter  37  is disposed on the flexible pipe  36   b  of the dust collection path  36 , and, by the determining section  40  to which electrical signals are sent from the particle counter  37 , the size of the particles P passing through the particle counter  37  and the number of the particles P are measured or counted. 
     Acceptance (OK) conditions for the number of particles, in the case where the dividing step is carried out in a state in which the modified layers  100  have been favorably formed along the streets  14  of the wafer  10 , are stored in the determining section  40  in the modified layer forming step. The acceptance conditions can be determined by experiments. The acceptance conditions for the number of particles refer to, for example, the case where the following conditions (1) and (2) are satisfied, where N 1  is the number of small particles (0.01 to 1.00 μm) and N 2  is the number of large particles (1.00 to 10.00 μm). 
     
       
         
           
             
               
                 
                   500 
                   &lt; 
                   
                     N 
                     ⁢ 
                     1 
                   
                   &lt; 
                   2,000 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   0 
                   &lt; 
                   
                     N 
                     ⁢ 
                     2 
                   
                   &lt; 
                   50 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Note that the abovementioned acceptance conditions are to be modified according to the material, size, and thickness of the wafer, laser processing conditions, and the like, and the abovementioned conditions (1) and (2) are merely an example. In addition, depending on the quality demanded, further, the particle size condition may be further finely set, and three or more conditions may be set. 
     As described above, the size of the particles P passing through the dust collection path  36  and the number of the particles P are measured and counted by the particle counter  37 , and are then determined by the determining section  40 . Then, in the case where the particle number satisfies the acceptance conditions (1) and (2) and where it is determined that the modified layers have been properly formed, the determination result (in the present embodiment, “OK”) is displayed together with the size and number of the particles on the display unit  42 , as depicted in  FIG. 7 . 
     According to the wafer dividing method and the dividing apparatus of the above embodiment, even in the case where the wafer is divided to form the device chips in a state in which the particles are not accumulated on the surface of the wafer, it can be determined whether or not the modified layers have been properly formed in the modified layer forming step and the wafer has been divided. 
     The present invention is not limited to the details of the above described preferred embodiment. 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.