Patent Publication Number: US-2021162536-A1

Title: Method of adjusting laser processing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method of adjusting the position where a pulsed laser beam is applied to a workpiece in a laser processing apparatus. 
     Description of the Related Art 
     Wafers having a plurality of devices such as integrated circuits (ICs) and large scale integration (LSI) circuits formed in respective areas on a face side that are demarcated by a plurality of projected dicing lines are divided into individual device chips by a laser processing apparatus, and the divided device chips will be used in electric appliances such as mobile phones and personal computers. 
     The laser processing apparatus includes a chuck table having a flat surface in an XY plane as a holding surface for holding a workpiece, i.e., a wafer, thereon, a laser beam applying unit for applying a pulsed laser beam to the workpiece held on the chuck table, an image capturing unit for capturing an image of the workpiece held on the chuck table, an X-axis moving mechanism for moving the chuck table and the image capturing unit relatively to each other in X-axis direction, a Y-axis moving mechanism for moving the chuck table and the image capturing unit relatively to each other in Y-axis direction perpendicular to the X-axis direction, and a control unit. The laser processing apparatus is able to process the wafer to a nicety. 
     A laser processing apparatus that processes a workpiece by dispersing and applying a laser beam to a plurality of locations on the workpiece includes a laser oscillator for oscillating pulsed laser, an X-axis galvanometer scanner for swinging a pulsed laser beam emitted from the laser oscillator in X-axis direction, a Y-axis galvanometer scanner for swinging the pulsed laser beam emitted from the laser oscillator in Y-axis direction, and an fθ lens for converging the pulsed laser beam that has been swung in the X-axis direction and the Y-axis direction onto the workpiece held on the chuck table (see, for example, JP 2008-264805A). The laser processing apparatus may be used to apply the pulsed laser beam to a plurality of electrodes on the devices on a wafer to form through holes in the electrodes. 
     SUMMARY OF THE INVENTION 
     However, the fθ lens is defective in that the laser beam as it travels through the fθ lens tends to be distorted progressively to a greater extent from the center toward outer circumference thereof, and is problematic in that, even when the X-axis galvanometer scanner and the Y-axis galvanometer scanner are controlled highly accurately, the through holes formed in an outer circumferential region of the wafer are likely to be shifted from the electrodes. 
     It is therefore an object of the present invention to provide a method of adjusting a laser processing apparatus to process a workpiece with a pulsed laser beam accurately at positions where the pulsed laser beam is to be applied to electrodes or the like on devices on the workpiece. 
     In accordance with an aspect of the present invention, there is provided a method of adjusting a laser processing apparatus. The laser processing apparatus includes a chuck table having a flat surface in an XY plane as a holding surface for holding a workpiece thereon, a laser beam applying unit for applying a pulsed laser beam to the workpiece held on the chuck table, an image capturing unit for capturing an image of the workpiece held on the chuck table, an X-axis moving mechanism for moving the chuck table and the image capturing unit relatively to each other in an X-axis direction, a Y-axis moving mechanism for moving the chuck table and the image capturing unit relatively to each other in a Y-axis direction perpendicular to the X-axis direction, and a control unit having a coordinate recording section and a corrective value recording section. The laser beam applying unit includes a laser oscillator for oscillating pulsed laser, an X-axis galvanometer scanner for swinging a pulsed laser beam emitted from the laser oscillator in the X-axis direction, a Y-axis galvanometer scanner for swinging the pulsed laser beam emitted from the laser oscillator in the Y-axis direction, and an fθ lens for converging the pulsed laser beam that has been swung in the X-axis direction and the Y-axis direction onto the workpiece held on the chuck table. The method includes a holding step of holding the workpiece on the chuck table, a coordinate recording step of recording, in the coordinate recording section, X coordinates and Y coordinates of positions where the pulsed laser beam is to be applied on the workpiece held on the chuck table, a processing step of controlling the X-axis galvanometer scanner and the Y-axis galvanometer scanner on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section to apply the pulsed laser beam to the workpiece held on the chuck table, thereby processing the workpiece, a processed mark image capturing step of actuating the X-axis moving mechanism and the Y-axis moving mechanism on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section to position the image capturing unit in alignment with processed marks on the workpiece held on the chuck table and causing the image capturing unit to capture an image of the processed marks, and a corrective value recording step of detecting shifts between X coordinates and Y coordinates of the processed marks whose image has been captured in the processed mark image capturing step and the X coordinates and the Y coordinates recorded in the coordinate recording section and recording corrective values in the corrective value recording section. 
     Preferably, the processed mark image capturing step includes the step of positioning the image capturing unit spirally toward the outer circumference of the workpiece from the processed mark corresponding to the center of the fθ lens as a starting point. 
     According to the present invention, the coordinates of target positions to which the pulsed laser beam is to be applied are corrected from the coordinates recorded in the coordinate recording section on the basis of the coordinates of the processed marks actually produced on the workpiece. Consequently, the workpiece can accurately be processed with the pulsed laser beam at positions, such as electrodes of devices on the workpieces, where the pulsed laser beam is to be applied. 
     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 of a laser processing apparatus to be adjusted by an adjusting method according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of some components of the laser processing apparatus illustrated in  FIG. 1 ; 
         FIG. 3A  is a perspective view of a wafer with devices formed thereon that is to be processed by the laser processing apparatus; 
         FIG. 3B  is a perspective view of a dummy wafer; 
         FIG. 4  is a table of X coordinates and Y coordinates recorded in a coordinate recording section of a control unit of the laser processing apparatus; 
         FIG. 5  is a schematic diagram of some of X coordinates and Y coordinates of a plurality of processed marks whose images are captured by an image capturing unit in a processed mark image capturing step and some of the X coordinates and the Y coordinates recorded in the coordinate recording section; and 
         FIG. 6  is a table of X coordinates and Y coordinates of processed marks recorded in a corrective value recording section of the control unit. 
     
    
    
     DETAILED DESCRIPTON OF THE PREFERRED EMBODIMENT 
     A method of adjusting a laser processing apparatus according to a preferred embodiment of the present invention will hereinafter be described below with reference to the accompanying drawings. The method of adjusting a laser processing apparatus will also be referred to as an adjusting method.  FIG. 1  illustrates, in perspective, a laser processing apparatus to be adjusted by the adjusting method according to the present embodiment. As illustrated in  FIG. 1 , the laser processing apparatus, denoted by  2 , includes a holding unit  4  having a flat surface in an XY plane as a holding surface for holding a workpiece, a laser beam applying unit  6  for applying a pulsed laser beam to the workpiece held on the holding unit  4 , an image capturing unit  8  for capturing an image of the workpiece held on the holding unit  4 , an X-axis moving mechanism  10  for moving the holding unit  4  and the image capturing unit  8  relatively to each other in an X-axis direction, a Y-axis moving mechanism  12  for moving the holding unit  4  and the image capturing unit  8  relatively to each other in a Y-axis direction, and a control unit  14  for controlling operation of the laser processing apparatus  2 . The X-axis direction represent a direction indicated by an arrow X in  FIG. 1  and an opposite direction thereto, and the Y-axis direction represent a direction indicated by an arrow Y in  FIG. 1  and an opposite direction thereto, the Y-axis direction being perpendicular to the X-axis direction. The XY plane is defined by the X-axis direction and the Y-axis direction and lies essentially horizontally. 
     The holding unit  4  includes an X-axis movable plate  18  movably disposed on a base  16  for movement in the X-axis direction, a Y-axis movable plate  20  movably disposed on the X-axis movable plate  18  for movement in the Y-axis direction, a support post  22  fixedly mounted on an upper surface of the Y-axis movable plate  20 , and a cover plate  24  fixed to an upper end of the support post  22 . The cover plate  24  has an oblong hole  24   a  defined therein and longitudinally extending in the Y-axis direction. The holding unit  4  also includes a chuck table  26  rotatably disposed on the upper end of the support post  22  and extending upwardly through the oblong hole  24   a . The chuck table  26  is rotatable by rotating means such as an electric motor, not illustrated, housed in the support post  22 . 
     A circular suction chuck  28  made of a porous material and connected to suction means, not illustrated, is disposed on an upper end surface of the chuck table  26 . The suction chuck  28  has an upper surface lying in the XY plane. In the chuck table  26 , when the suction means is actuated, it generates suction forces acting on the upper surface of the suction chuck  28  for thereby holding the workpiece placed thereon under suction. In the holding unit  4 , therefore, the upper surface of the suction chuck  28  lies in the XY plane for holding the workpiece therein. A plurality of circumferentially spaced clamps  30  are disposed around the outer circumferential edge of the chuck table  26  for clamping the workpiece on the chuck table  26 . 
     As illustrated in  FIG. 2 , the laser beam applying unit  6  includes a laser oscillator  32  for oscillating pulsed laser, an X-axis galvanometer scanner  34  for swinging a pulsed laser beam LB emitted from the laser oscillator  32  in the X-axis direction, a Y-axis galvanometer scanner  36  for swinging the pulsed laser beam LB emitted from the laser oscillator  32  in the Y-axis direction, and an fθ lens  38  for converging the pulsed laser beam LB that has been swung in the X-axis direction and the Y-axis direction onto the workpiece held on the holding unit  4 . 
     The X-axis galvanometer scanner  34  and the Y-axis galvanometer scanner  36  may each be of a known structure having a mirror, not illustrated, and an angle adjusting actuator, not illustrated, for adjusting the angle at which the mirror is disposed. The fθ lens  38  applies the pulsed laser beam LB that has been swung in the X-axis direction and the Y-axis direction, perpendicularly to the upper surface of the chuck table  26 . 
     As illustrated in  FIG. 1 , the laser beam applying unit  6  according to the present embodiment includes a housing  40  including a vertical portion extending upwardly from an upper surface of the base  16  and a horizontal portion extending substantially and horizontally from an upper portion of the vertical portion. The housing  40  houses therein the laser oscillator  32 , the X-axis galvanometer scanner  34 , the Y-axis galvanometer scanner  36 , and a mirror  42  (see  FIG. 2 ) for reflecting the pulsed laser beam LB that has been swung in the X-axis direction and the Y-axis direction toward the fθ lens  38 . As illustrated in  FIG. 1 , an fθ lens casing  44  that houses the fθ lens  38  therein is disposed on a lower surface of the distal end of the horizontal portion of the housing  40 . 
     The pulsed laser beam LB emitted from the laser oscillator  32  of the laser beam applying unit  6  is swung in the X-axis direction and the Y-axis direction by the X-axis galvanometer scanner  34  and the Y-axis galvanometer scanner  36 , reflected by the mirror  42  toward the fθ lens  38 , converged by the fθ lens  38 , and then dispersed and applied to a plurality of locations on the workpiece held on the holding unit  4 . 
     As illustrated in  FIG. 1 , the image capturing unit  8  is mounted on the lower surface of the distal end of the horizontal portion of the housing  40  at a position spaced from the fθ lens casing  44  in one of the X-axis direction. The image capturing unit  8  includes an image capturing device such as a charge-coupled device (CCD) for capturing an image of the workpiece with a visible light beam. The image capturing unit  8  is electrically connected to a display unit  46  disposed on an upper surface of the distal end of the horizontal portion of the housing  40 , so that the image captured of the workpiece by the image capturing unit  8  can be displayed on the display unit  46 . 
     As illustrated in  FIG. 1 , the X-axis moving mechanism  10  includes a ball screw  48  extending in the X-axis direction along the upper surface of the base  16 , and an electric motor  50  for rotating the ball screw  48  about its central axis. The ball screw  48  is operatively threaded through a nut, not illustrated, coupled to the X-axis movable plate  18 . When the electric motor  50  is energized, the ball screw  48  converts the rotary motion of the electric motor  50  into linear motion and transmits the linear motion to the X-axis movable plate  18 , thereby moving the X-axis movable plate  18  along guide rails  16   a  on the base  16  relatively to the image capturing unit  8  in the X-axis direction. 
     The Y-axis moving mechanism  12  includes a ball screw  52  extending in the Y-axis direction along an upper surface of the X-axis movable plate  18 , and an electric motor  54  for rotating the ball screw  52  about its central axis. The ball screw  52  is operatively threaded through a nut, not illustrated, coupled to the Y-axis movable plate  20 . When the electric motor  54  is energized, the ball screw  52  converts the rotary motion of the electric motor  54  into linear motion and transmits the linear motion to the Y-axis movable plate  20 , thereby moving the Y-axis movable plate  20  along guide rails  18   a  on the X-axis movable plate  18  relatively to the image capturing unit  8  in the Y-axis direction. 
     The control unit  14  is constructed as a computer including a central processing unit (CPU), not illustrated, for performing arithmetic processing operations according to control programs, a read only memory (ROM), not illustrated, for storing the control programs, etc., and a read/write random access memory (RAM), not illustrated, for storing the results of the arithmetic processing operations, etc. As illustrated in  FIG. 1 , the random access memory implements a coordinate recording section  56  for recording the X coordinates and the Y coordinates of positions where the pulsed laser beam LB is to be applied to the workpiece held on the holding unit  4 , and a corrective value recording section  58  for recording corrective values on the basis of shifts of the X coordinates and the Y coordinates of processed marks whose image has been captured by the image capturing unit  8  from the X coordinates and the Y coordinates recorded in the coordinate recording section  56 . 
     The control unit  14  that controls overall operation of the laser processing apparatus  2  controls the laser beam applying unit  6  for dispersing and applying the pulsed laser beam LB to a plurality of locations on the workpiece held on the holding unit  4 , on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section  56 , for example. Further, the control unit  14  controls the X-axis moving mechanism  10  and the Y-axis moving mechanism  12  for positioning the image capturing unit  8  in alignment with processed marks on the workpiece held on the holding unit  4  to allow the image capturing unit  8  to capture an image of the processed marks, on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section  56 . 
       FIG. 3A  illustrates, in perspective, a wafer  60  shaped as a circular plate as an example of the workpiece. The wafer  60  may be made of silicon or the like, for example. The wafer  60  has a plurality of rectangular areas on a face side  60   a  thereof that are demarcated by a grid of projected dicing lines  62 , and a plurality of devices  64  such as ICs and LSI circuits formed in the respective rectangular areas. Each of the devices  64  has a plurality of electrodes  66 . The wafer  60  has a notch (recess)  68  defined in a peripheral edge thereof as an indicator of the crystal orientation of the wafer  60 . The wafer  60  according to the present embodiment has a reverse side  60   b  affixed to an adhesive tape  72  whose peripheral edge portion is secured to an annular frame  70 . 
       FIG. 3B  illustrates, in perspective, a dummy wafer  74  shaped as a circular plate as another example of the workpiece. The dummy wafer  74  that is supported on an annular frame  70  by an adhesive tape  72  is free of projected dicing lines and devices. The dummy wafer  74  has dimensions, i.e., a diameter and a thickness, identical to those of the wafer  60 , and is made of a material identical to that of the wafer  60 . The dummy wafer  74  according to the present embodiment has a notch (recess)  76  defined in a peripheral edge thereof as an indicator of the crystal orientation of the dummy wafer  74 . The notch  76  is of the same shape as the notch  68  of the wafer  60 . 
     The method of adjusting the laser processing apparatus  2 , that is, adjusting the position where the pulsed laser beam LB is applied to the workpiece, i.e., the wafer  60 , will be described below. In the adjusting method according to the present embodiment, a holding step of holding the workpiece on the holding unit  4  is carried out at first. In the holding step, the dummy wafer  74  that is supported on the annular frame  70  by the adhesive tape  72  is placed on the suction chuck  28  on the upper surface of the chuck table  26  of the holding unit  4 . Then, the suction means connected to the suction chuck  28  is actuated to develop suction forces on the upper surface of the suction chuck  28 , thereby holding the dummy wafer  74  under suction on the suction chuck  28 . Further, the clamps  30  are turned to clamp the annular frame  70  in place. 
     After the holding step has been carried out, a coordinate recording step is carried out to record, in the coordinate recording section  56 , the X coordinates and the Y coordinates of positions where the pulsed laser beam LB is to be applied on the workpiece held on the holding unit  4 . The X coordinates and the Y coordinates to be recorded in the coordinate recording section  56  in the coordinate recording step represent the X coordinates and the Y coordinates of a plurality of positions where the workpiece is to be processed by the pulsed laser beam LB, e.g., the X coordinates and the Y coordinates that indicate the positions of the electrodes  66  on the wafer  60 .  FIG. 4  illustrates, as an example of the X coordinates and the Y coordinates that indicate the positions of the electrodes  66 , the X coordinates and the Y coordinates (Xn, Yn) of a total of 2601 (51×51) electrodes  66  that are arranged in a matrix of 51 electrodes spaced in the X-axis direction and 51 electrodes spaced in the Y-axis direction. The X coordinates and the Y coordinates to be recorded in the coordinate recording section  56  can be established in any way by using the notch  68  of the wafer  60  or the like as a reference. 
     After the coordinate recording step has been carried out, a processing step is carried out to control the X-axis galvanometer scanner  34  and the Y-axis galvanometer scanner  36  on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section  56  to apply the pulsed laser beam LB to the workpiece held on the holding unit  4 , thereby processing the workpiece. 
     In the processing step, the X-axis moving mechanism  10  and the Y-axis moving mechanism  12  move the chuck table  26  to position the dummy wafer  74  held on the holding unit  4  below the fθ lens casing  44 , and the electric motor for the chuck table  26  rotates the chuck table  26  to adjust the orientation of the dummy wafer  74  to a predetermined orientation. The coordinates of target positions to which the pulsed laser beam LB is to be applied are set to the X coordinates and the Y coordinates recorded in the coordinate recording section  56 , and then, the pulsed laser beam LB is dispersed and applied to the dummy wafer  74  to process a plurality of locations on the dummy wafer  74  that are spaced in the X-axis direction and the Y-axis direction, leaving processed marks on the locations. In the processing step according to the present embodiment, a total of 2601 (51×51) processed marks are formed on the dummy wafer  74  on the basis of the X coordinates and the Y coordinates (Xn, Yn) that indicate the positions of the electrodes  66 . 
     After the processing step has been carried out, a processed mark image capturing step is carried out to actuate the X-axis moving mechanism  10  and the Y-axis moving mechanism  12  on the basis of the X coordinates and the Y coordinates recorded in the coordinate recording section  56  to position the image capturing unit  8  in alignment with the processed marks on the workpiece held on the holding unit  4  and to cause the image capturing unit  8  to capture an image of the processed marks. 
       FIG. 5  schematically illustrates some of the X coordinates and the Y coordinates of a plurality of processed marks whose images are captured by the image capturing unit  8 . Specifically, the locations indicated by a symbol x in  FIG. 5  represent the positions of processed marks formed on the dummy wafer  74  by the pulsed laser beam LB applied after the coordinates of the target positions to which the pulsed laser beam LB is to be applied have been set to the X coordinates and the Y coordinates (Xn, Yn) recorded in the coordinate recording section  56 . Moreover, in  FIG. 5 , the locations indicated by the symbol x are accompanied by X coordinates and Y coordinates (αn, βn), and the locations represented by some of the X coordinates and the Y coordinates recorded in the coordinate recording section  56  are indicated by a symbol ● and accompanied by X coordinates and Y coordinates (Xn, Yn). 
     In the processed mark image capturing step, it is preferable to position the image capturing unit  8  spirally toward the outer circumference of the dummy wafer  74  from the processed mark (α 1300 , β 1300 ) corresponding to the center of the fθ lens  38  as a starting point. In this manner, the image capturing unit  8  can capture each processed mark reliably even in case the field of vision of the image capturing unit  8  is smaller than the shifts between the coordinates (αn, βn) of the processed marks and the coordinates (Xn, Yn) recorded in the coordinate recording section  56 . In the example illustrated in  FIG. 5 , the coordinates (α 1300 , β 1300 ) of the processed mark corresponding to the center of the fθ lens  38  agree with the coordinates (X 1300 , Y 1300 ) recorded in the coordinate recording section  56 , whereas the coordinates of the processed marks other than the coordinates (α 1300 , β 1300 ) are shifted from the coordinates recorded in the coordinate recording section  56 , i.e., the coordinates of target positions to which the pulsed laser beam LB is to be applied. 
     After the processed mark image capturing step has been carried out, a corrective value recording step is carried out to detect shifts between the X and Y coordinates (αn, βn) of the processed marks whose image has been captured in the processed mark image capturing step and the X and Y coordinates (Xn, Yn) recorded in the coordinate recording section  56  of the control unit  14 , and to record corrective values in the corrective value recording section  58 . 
     In the corrective value recording step, the coordinates (αn, βn) of the processed marks are determined from the image of the processed marks whose image has been captured in the processed mark image capturing step (see  FIG. 6 ). Then, shifts, i.e., shifts in the X-axis direction and the Y-axis direction, between the coordinates (αn, βn) of the processed marks and the coordinates (Xn, Yn) recorded in the coordinate recording section  56  are detected. Then, corrective values for correcting the coordinates of the target positions to which the pulsed laser beam LB is to be applied from the coordinates (Xn, Yn) recorded in the coordinate recording section  56  are calculated on the basis of the detected shifts. The calculated corrective values are recorded in the corrective value recording section  58  of the control unit  14 . Further, the coordinates of the target positions to which the pulsed laser beam LB is to be applied are corrected from the coordinates (Xn, Yn) recorded in the coordinate recording section  56 , by using the calculated corrective values, and the corrected coordinates of the target positions to which the pulsed laser beam LB is to be applied are recorded in the corrective value recording section  58 . 
     In the corrective value recording step, in order to deal with various processing conditions, the corrective values referred to above may be replaced with a function, and such a function may be recorded in the corrective value recording section  58 , and then, by using the function replaced from the corrective values, the coordinates of the target positions to which the pulsed laser beam LB is to be applied may be corrected from the coordinates (Xn, Yn) recorded in the coordinate recording section  56 . Further, a region to which the pulsed laser beam LB can be applied in the laser processing apparatus  2  by the X-axis galvanometer scanner  34  and the Y-axis galvanometer scanner  36  as they operate may be divided into a plurality of regions, and a function for correcting the coordinates of the target positions to which the pulsed laser beam LB is to be applied from the coordinates (Xn, Yn) recorded in the coordinate recording section  56  may be calculated with respect to each of the divided regions. When each of the regions is to be processed by the pulsed laser beam LB, the coordinates of the target positions to which the pulsed laser beam LB is to be applied may be corrected from the coordinates (Xn, Yn) recorded in the coordinate recording section  56  according to the function calculated with respect to the region. 
     After the corrective value recording step has been carried out, the suction means connected to the suction chuck  28  of the holding unit  4  is inactivated to release the dummy wafer  74  from the suction chuck  28 , and the clamps  30  are turned back to unclamp the annular frame  70 . Then, the dummy wafer  74  used as the workpiece for calculating the corrective values is removed from the holding unit  4 . Thereafter, the wafer  60  as a workpiece to be processed with the pulsed laser beam LB, i.e., a workpiece where through holes are to be formed in the respective electrodes  66  according to the present embodiment, is held under suction on the holding unit  4 . The X-axis galvanometer scanner  34  and the Y-axis galvanometer scanner  36  are controlled to disperse and apply the pulsed laser beam LB to the wafer  60  on the basis of the corrected coordinates of the target positions to which the pulsed laser beam LB is to be applied, forming through holes in the electrodes  66  on the wafer  60 . 
     In the method of adjusting the laser processing apparatus  2  according to the present embodiment, as described above, the coordinates of the target positions to which the pulsed laser beam LB is to be applied are corrected from the coordinates (Xn, Yn) recorded in the coordinate recording section  56  on the basis of the X and Y coordinates (αn, βn) of the processed marks actually produced on the dummy wafer  74 . Consequently, the pulsed laser beam LB can be applied to the electrodes  66  of the devices  64  on the wafer  60  to form through holes accurately in the respective electrodes  66 . 
     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.