Abstract:
A laser oscillation mechanism includes a pulse laser oscillator configured to oscillate a pulse laser beam, and a branching unit which branches the pulse laser beam oscillated by the pulse laser oscillator. The branching unit includes a diffraction optical element and a volume Bragg grating. The diffraction optical element branches the pulse laser beam oscillated by the pulse laser oscillator into a plurality of laser beams in an effective region. The volume Bragg grating refracts, from among the pulse laser beams branched by the diffraction optical element, a particular pulse laser beam to be excluded from the effective region to exclude the particular pulse laser beam.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a laser oscillation mechanism incorporated in a laser processing apparatus which performs laser processing for a workpiece or a like apparatus. 
         [0003]    2. Description of the Related Art 
         [0004]    In a semiconductor device fabrication process, a plurality of regions are partitioned by crossing division lines arrayed on the surface of a semiconductor wafer having a substantially circular disk shape, and a device such as an IC or an LSI is formed in each of the partitioned regions. Then, by cutting the semiconductor wafer along the division lines, the regions in each of which a device is formed are divided to fabricate individual semiconductor chips. 
         [0005]    As a method for dividing a wafer as described above, a laser processing method is attempted wherein a pulse laser beam of a wavelength having permeability to a wafer is used and irradiated with the focal point thereof adjusted to the inside of a region to be divided. In a dividing method which uses the laser processing method, a pulse laser beam of a wavelength having permeability to a wafer is irradiated with the focal point thereof adjusted to the inside of the wafer from one face side of the wafer to form a modification layer continuously along a division line inside the workpiece, whereafter external force is applied along the division line along which the strength is dropped by the formation of the modification layer to divide the wafer. 
         [0006]    Further, as a method for dividing a wafer along a division line, a technology has been placed into practical use wherein a pulse laser beam of a wavelength having absorbability to a wafer is irradiated along a division line to perform ablation processing to form a laser processed groove and then external force is applied along the division line along which the laser processed groove which serves as a start point of breaking is formed to divide the wafer. 
         [0007]    A laser processing apparatus which carries out the laser processing described above includes workpiece holding means for holding a workpiece, laser beam irradiation means for laser-processing the workpiece held by the workpiece holding means, and moving means for moving the workpiece holding means and the laser beam irradiation means relative to each other. A method for branching a laser beam into a plurality of laser beams to form a plurality of focal points is attempted in order to improve the processing efficiency in laser processing described above using such a laser processing apparatus as just described (for example, refer to Japanese Patent Laid-Open No. 2006-95529 or Japanese Patent Laid-Open No. 2008-290086). 
       SUMMARY OF THE INVENTION 
       [0008]    However, if a polarizing beam splitter is used in order to branch a laser beam oscillated by a laser oscillator into a plurality of laser beams to form a plurality of focal points as in the case of the laser beam irradiation means disclosed in Japanese Patent Laid-Open No. 2006-95529 or Japanese Patent Laid-Open No. 2008-290086, then the laser beam is branched into p polarized light and s polarized light. Consequently, the power density per one pulse decreases to one half and the polarization planes become different, and there is a problem that the processing quality does not become stable. 
         [0009]    Further, if a laser beam is branched using a diffraction optical element (DOE), then the power density per one pulse is maintained and the laser beam is not branched into p polarized light and s polarized light. Therefore, the problem described above does not arise. However, since the branching angle of the DOE is small, laser beam absorption means must be disposed at the center at a point one to several meters ahead of the DOE in order to exclude zero-order light having passed through the DOE. Therefore, there is a problem that the apparatus size increases. 
         [0010]    It is therefore an object of the present invention to provide a laser oscillation mechanism which can branch a pulse laser beam oscillated by a pulse laser oscillator into a plurality of pulse laser beams by using a diffraction optical element without upsizing the apparatus. 
         [0011]    In accordance with an aspect of the present invention, there is provided a laser oscillation mechanism including a pulse laser oscillator configured to oscillate a pulse laser beam, and branching means for branching the pulse laser beam oscillated by the pulse laser oscillator, wherein the branching means includes a diffraction optical element configured to branch the pulse laser beam oscillated by the pulse laser oscillator into a plurality of laser beams in an effective region, and a volume Bragg grating configured to refract, from among the pulse laser beams branched by the diffraction optical element, a particular pulse laser beam to be excluded from the effective region to exclude the particular pulse laser beam. 
         [0012]    The volume Bragg grating (VBG) refracts zero-order light to exclude the zero-order light from the effective region. Preferably, a plurality of VBGs are disposed so as to refract zero-order light and secondary light to exclude the zero-order light and the secondary light from the effective region. 
         [0013]    Since the branching means which configures the laser oscillation mechanism according to the present invention includes a DOE which branches the pulse laser beam oscillated by the pulse laser oscillator into a plurality of laser beams in an effective region and a VBG which refracts, from among the pulse laser beams branched by the DOE, a particular pulse laser beam to be excluded from the effective region to exclude the particular pulse laser beam, the volume Bragg grating can be disposed in a neighboring relationship with the diffraction optical element. Consequently, the laser beam to be excluded from the effective region can be excluded with certainty and upsizing of the apparatus can be prevented. 
         [0014]    Further, since the branching means which configures the laser oscillation mechanism according to the present invention branches the pulse laser beam using the diffraction optical element, the power density per one pulse is maintained. Further, since the pulse laser beam is not branched into p polarized light and s polarized light, the processing quality is stabilized. 
         [0015]    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 
         [0016]      FIG. 1  is a perspective view of a laser processing apparatus including a laser oscillation mechanism configured in accordance with the present invention; 
           [0017]      FIG. 2  is a block diagram of laser beam irradiation means including a laser oscillation mechanism according to an embodiment of the present invention; 
           [0018]      FIG. 3  is a block diagram of another embodiment of the laser oscillation mechanism; and 
           [0019]      FIG. 4  is a schematic view depicting a processed state of a workpiece processed using the laser processing apparatus depicted in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In the following, preferred embodiments of a laser oscillation mechanism configured in accordance with the present invention are described in detail with reference to the accompanying drawings.  FIG. 1  depicts a perspective view of a laser processing apparatus  1  which includes a laser oscillation mechanism configured in accordance with the present invention. The laser processing apparatus  1  depicted in  FIG. 1  includes a stationary base  2 , a chuck table mechanism  3  disposed for movement in a processing feeding direction (X-axis direction) indicated by an arrow mark X on the stationary base  2  and configured to hold a workpiece thereon, and a laser beam irradiation unit  4  as laser beam irradiation means disposed on the stationary base  2 . 
         [0021]    The chuck table mechanism  3  includes a pair of guide rails  31  disposed in parallel to each other along the X-axis direction on the stationary base  2 , a first sliding block  32  disposed for movement in the X-axis direction on the pair of guide rails  31 , a second sliding block  33  disposed for movement in a Y-axis direction indicated by an arrow mark Y orthogonal to the X-axis direction on the first sliding block  32 , a support table  35  supported by a cylindrical member  34  on the second sliding block  33 , and a chuck table  36  as workpiece holding means. The chuck table  36  includes an absorption chuck  361  configured from a porous material, and, for example, a circular semiconductor wafer which is a workpiece is held by suction means not depicted on a holding face which is an upper face of the absorption chuck  361 . The chuck table  36  configured in such a manner as just described is rotated by a pulse motor not depicted disposed in the cylindrical member  34 . It is to be noted that a clump  362  for fixing an annular frame for supporting a workpiece such as a semiconductor wafer through a protective tape is disposed on the chuck table  36 . 
         [0022]    The first sliding block  32  includes a pair of guiding target grooves  321  provided on the lower face thereof for fitting with the pair of guide rails  31  and a pair of guide rails  322  formed in parallel to each other along the Y-axis direction and provided on the upper face thereof. The first sliding block  32  configured in such a manner as just described is configured for movement in the X-axis direction along the pair of guide rails  31  by fitting the guiding target grooves  321  with the pair of guide rails  31 . The chuck table mechanism  3  includes X-axis direction moving means  37  for moving the first sliding block  32  in the X-axis direction along the pair of guide rails  31 . The X-axis direction moving means  37  includes an external thread rod  371  disposed in parallel to and between the pair of guide rails  31  and a driving source such as a pulse motor  372  for driving the external thread rod  371  to rotate. The external thread rod  371  is supported at one end thereof for rotation on a bearing block  373  fixed to the stationary base  2  and transmission-coupled at the other end thereof to an output power shaft of the pulse motor  372 . It is to be noted that the external thread rod  371  is screwed into a penetrating internal thread hole formed on an internal thread block not depicted provided in a projecting manner on the lower face of a central portion of the first sliding block  32 . Accordingly, by driving the external thread rod  371  for forward rotation and reverse rotation by the pulse motor  372 , the first sliding block  32  is moved in the X-axis direction along the guide rails  31 . 
         [0023]    The second sliding block  33  includes a pair of guiding target grooves  331  provided on the lower face thereof for fitting with the pair of guide rails  322  provided on the upper face of the first sliding block  32 , and is configured for movement in the Y-axis direction by fitting the guiding target grooves  331  with the pair of guide rails  322 . The chuck table mechanism  3  includes Y-axis direction moving means  38  for moving the second sliding block  33  in the Y-axis direction along the pair of guide rails  322  provided on the first sliding block  32 . The Y-axis direction moving means  38  includes an external thread rod  381  disposed in parallel to and between the pair of guide rails  322  and a driving source such as a pulse motor  382  for driving the external thread rod  381  to rotate. The external thread rod  381  is supported at one end thereof for rotation on a bearing block  383  fixed to the upper face of the first sliding block  32  and transmission-coupled at the other end thereof to an output power shaft of the pulse motor  382 . It is to be noted that the external thread rod  381  is screwed in a penetrating internal thread hole formed on an internal thread block not depicted provided in a projecting manner on the lower face of a central portion of the second sliding block  33 . Accordingly, by driving the external thread rod  381  for forward rotation and reverse rotation by the pulse motor  382 , the second sliding block  33  is moved in the Y-axis direction along the guide rails  322 . 
         [0024]    The laser beam irradiation unit  4  includes a support member  41  disposed on the stationary base  2 , a casing  42  supported by the support member  41  and extending substantially in a horizontal direction, laser beam irradiation means  5  disposed on the casing  42 , and image pickup means  6  disposed at a front end portion of the casing  42  for detecting a processing region for which laser processing is to be performed. It is to be noted that the image pickup means  6  includes illumination means for illuminating a workpiece, an optical system for capturing a region illuminated by the illumination means, an image pickup device (CCD) for picking up an image captured by the optical system, and so forth. 
         [0025]    The laser beam irradiation means  5  described above is described with reference to  FIG. 2 . The laser beam irradiation means  5  includes a laser oscillation mechanism  50  and a condenser  55 . The laser oscillation mechanism  50  is configured from a pulse laser oscillator  51  for oscillating a pulse laser beam, and branching means  52  for branching the pulse laser beam oscillated by the pulse laser oscillator  51 . In the embodiment depicted, the pulse laser oscillator  51  oscillates a pulse laser beam LB of a wavelength (for example, 355 nm) having absorbability to a workpiece formed, for example, from a silicon wafer. 
         [0026]    The branching means  52  which configures the laser beam irradiation means  5  is configured from a DOE  521  which branches the pulse laser beam LB oscillated by the pulse laser oscillator  51  into a plurality of laser beams in an effective region, and a VBG  522  which refracts a pulse laser beam to be excluded from among the pulse laser beams branched by the DOE  521  from the effective region to exclude the pulse laser beam. The DOE  521  branches the pulse laser beam LB into zero-order light LB 0  on the optical axis and primary light LB 1   a  and primary light LB 1   b  branched at angles equal to each other with respect to the zero-order light LB 0 . It is to be noted that the branching angle (θ) between the primary light LB 1   a  and the primary light LB 1   b  is 0.1 to 0.2 degrees. 
         [0027]    The VBG  522  which configures the branching means  52  refracts, in the present embodiment, the zero-order light LB 0  from among the zero-order light LB 0 , primary light LB 1   a  and primary light LB 1   b  branched by the DOE  521  toward laser beam absorption means  523  disposed at a position displaced from the effective region as indicated by a broken line. Then, the VBG  522  introduces the primary light LB 1   a  and the primary light LB 1   b  to the condenser  55 . Since the zero-order light LB 0  to be excluded is refracted toward the laser beam absorption means  523  disposed at a position displaced from the effective region by the VBG  522 , the VBG  522  can be disposed in a neighboring relationship with the DOE  521 . Therefore, upsizing of the apparatus can be prevented. 
         [0028]    The condenser  55  is configured from a direction conversion mirror  551  for converting the direction of the primary light LB 1   a  and the primary light LB 1   b  introduced thereto by the VBG  522  to a downward direction, and a condensing lens  552  configured to converge the primary light LB 1   a  and the primary light LB 1   b , whose direction has been converted by the direction conversion mirror  551 , to irradiate them upon a workpiece W held on the chuck table  36 . The primary light LB 1   a  and the primary light LB 1   b  converged by the condensing lens  552  are converged to positions spaced by a predetermined distance (L) in the Y-axis direction as depicted in  FIG. 2 . 
         [0029]    By irradiating the primary light LB 1   a  and the primary light LB 1   b  converged by the condensing lens  552  at positions on the workpiece W spaced by the predetermined distance (L) from each other in the Y-axis direction as described above and processing-feeding the chuck table  36  at a predetermined processing speed in the X-axis direction in  FIG. 1 , two laser processed grooves Wa and Wb are formed on the workpiece W as depicted in  FIG. 4 . It is to be noted that, since the branching means  52  of the laser oscillation mechanism  50  in the present embodiment branches a pulse laser beam using the DOE  521 , the power density per one pulse is maintained, and since the laser beam is not branched into p polarized light and s polarized light, the processing quality is stabilized. 
         [0030]    Now, another embodiment of the laser oscillation mechanism according to the present invention is described with reference to  FIG. 3 . A laser oscillation mechanism  50   a  depicted in  FIG. 3  is configured from a pulse laser oscillator  51   a  which oscillates a pulse laser beam, and branching means  52   a  which branches the pulse laser beam oscillated by the pulse laser oscillator  51   a . The pulse laser oscillator  51   a  may be same as the pulse laser oscillator  51  described hereinabove with reference to  FIG. 2 . 
         [0031]    The branching means  52   a  which configures the laser beam irradiation means  5  is configured from a DOE  521   a  which branches a pulse laser beam LB oscillated by the pulse laser oscillator  51   a  into a plurality of laser beams in an effective region, and a first VBG  522   a  and a second VBG  522   b  which refract pulse laser beams to be excluded from among the pulse laser beams branched by the DOE  521   a  from within the effective region to exclude the pulse laser beams. The DOE  521   a  branches the pulse laser beam LB into zero-order light LB 0  on the optical axis, primary light LB 1   a  and primary light LB 1   b , and secondary light LB 2   a  and secondary light LB 2   b  as depicted in  FIG. 3 . 
         [0032]    The first VBG  522   a  which configures the branching means  52   a  refracts, in the present embodiment, the secondary light LB 2   a  and the secondary light LB 2   b  from among the zero-order light LB 0 , primary light LB 1   a  and primary light LB 1   b , and secondary light LB 2   a  and secondary light LB 2   b  branched by the DOE  521   a  toward laser beam absorption means  523   a  disposed at positions displaced from the effective region as indicated by alternate long and short dashes lines. Then, the first VBG  522   a  introduces the zero-order light LB 0  and the primary light LB 1   a  and primary light LB 1   b  to the second VBG  522   b.    
         [0033]    The second VBG  522   b  which configures the branching means  52   a  refracts, in the present embodiment, the zero-order light LB 0  from among the zero-order light LB 0 , primary light LB 1   a  and primary light LB 1   b  branched by the first VBG  522   a  toward laser beam absorption means  523   a  disposed at a position displaced from the effective region as indicated by a broken line. Then, the second VBG  522   b  introduces the primary light LB 1   a  and the primary light LB 1   b  to the condenser  55  similarly to the laser beam irradiation means  5  described hereinabove with reference to  FIG. 2 . 
         [0034]    As described above, since the secondary light LB 2   a  and the secondary light LB 2   b  to be excluded by the first VBG  522   a  and the zero-order light LB 0  to be excluded by the second VBG  522   b  are refracted toward the laser beam absorption means  523   a  disposed at the positions displaced from the effective region, the first VBG  522   a  and the second VBG  522   b  can be disposed in a neighboring relationship with the DOE  521   a . Consequently, upsizing of the apparatus can be prevented. 
         [0035]    Although the present invention has been described in connection with the embodiments depicted in the drawings, the present invention is not limited to the embodiments but can be modified in various manners in accordance with the subject matter of the present invention. For example, in the embodiments described hereinabove, an example is described wherein the laser oscillation mechanism according to the present invention is mounted on the laser processing apparatus and irradiates a pulse laser beam of a wavelength having absorbability to a wafer to form two laser processed grooves. However, two modification layers can be formed in the inside of a workpiece by irradiating a pulse laser beam of a wavelength having permeability to a wafer with a focal point thereof positioned in the inside of the workpiece. 
         [0036]    Further, the laser oscillation mechanism according to the present invention can be applied also to laser equipment other than a laser processing apparatus. 
         [0037]    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.