Abstract:
Disclosed herein is a laser beam processing machine having a chuck table for holding a workpiece, a laser beam applicator for applying a laser beam capable of passing through the workpiece to the workpiece held on the chuck table, and a processing-feeder for moving the chuck table and the laser beam applicator relative to each other, the laser beam applicator having a laser beam oscillator, an optical transmitter for transmitting a laser beam oscillated from the laser beam oscillator and a condenser lens for converging a laser beam transmitted by the optical, transmitter. The optical transmitter includes a birefringence lens for separating the laser beam oscillated from the laser beam oscillator into normal light and abnormal light; and the condenser lens converges respectively the normal light and the abnormal light separated by the birefringence lens to form the focal point of the normal light and the focal point of the abnormal light.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a laser beam processing machine for applying a laser beam such as a pulse laser beam capable of passing through a workpiece such as a semiconductor wafer to form deteriorated layers in the inside of the workpiece. 
     DESCRIPTION OF THE PRIOR ART 
     In the production process of a semiconductor device, a plurality of areas are sectioned by dividing lines called “streets” formed in a lattice pattern on the front surface of a wafer comprising a suitable substrate such as a silicon substrate, sapphire substrate, silicon carbide substrate, lithium tantalate substrate, glass substrate or quartz substrate, and a device (functional device) such as IC or LSI is formed in each of the sectioned areas. Individual semiconductor devices are manufactured by cutting this wafer along the dividing lines to divide it into the areas on which a functional device has been formed. As a means of dividing the wafer, various methods making use of a laser beam are proposed. 
     As a means of dividing a plate-like workpiece such as a semiconductor wafer, a laser processing method for applying a pulse laser beam capable of passing through the workpiece with its focal point set to the inside of the area to be divided is also attempted, and disclosed by Japanese Patent No. 3408805, for example. In the dividing method making use of this laser processing technique, the workpiece is divided by applying a pulse laser beam capable of passing through the workpiece, for example, a wavelength of 1,064 nm from one side of the workpiece with its focal point set to the inside to continuously form a deteriorated layer in the inside of the workpiece along the dividing lines and exerting external force along the dividing lines whose strength has been reduced by the formation of the deteriorated layers. 
     To divide the wafer along the dividing lines accurately by applying external force to the wafer, the thickness of the deteriorated layer, that is, the size of the deteriorated layer in the thickness direction of the wafer must be made relatively large. Since the thickness of the deteriorated layer formed by the above laser processing method is 10 to 50 μm in the vicinity of the focal point of the pulse laser beam, the position of the focal point of the pulse laser beam must be displaced in the thickness direction of the wafer, and the pulse laser beam and the wafer must be moved relative to each other along the dividing line repeatedly in order to increase the thickness of the deteriorated layer. Therefore, when the thickness of the wafer is relatively large, it takes long time to form a deteriorated layer thick enough to divide the wafer accurately. 
     To solve the above problem, JP-A 2005-28438 discloses a laser beam processing machine for forming a plurality of deteriorated layers at the same time by forming a plurality of focal points of a pulse laser beam in the thickness direction of a workpiece. 
     With the laser beam processing machine disclosed by the above publication, deteriorated layers can be formed at a plurality of focal points formed in the thickness direction of the workpiece at the same time. However, this laser beam processing machine comprises a relatively complicated mechanism for forming a plurality of focal points and therefore, is not always satisfactory for practical use. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a laser beam processing machine capable of forming two deteriorated layers at the same time in the thickness direction of a workpiece with a simple structure. 
     To attain the above object, according to the present invention, there is provided a laser beam processing machine comprising a chuck table for holding a workpiece, a laser beam application means for applying a laser beam capable of passing through the workpiece to the workpiece held on the chuck table, and a processing-feed means for moving the chuck table and the laser beam application means relative to each other, the laser beam application means comprising a laser beam oscillation means, an optical transmission means for transmitting a laser beam oscillated from the laser beam oscillation means and a condenser lens for converging a laser beam transmitted by the optical transmission means, wherein 
     the optical transmission means comprises a birefringence lens for dividing the laser beam oscillated from the laser beam oscillation means into normal light and abnormal light; and 
     the condenser lens converges respectively the normal light and the abnormal light separated by the birefringence lens to form the focal point of the normal light and the focal point of the abnormal light. 
     The above optical transmission means comprises a wave plate which is interposed between the laser beam oscillation means and the birefringence lens. 
     The above birefringence lens is composed of a glass body having a convex surface with a predetermined curvature and a crystal body having a concave surface with a curvature corresponding to the convex surface of the glass body, and the convex surface of the glass body and the concave surface of the crystal body are united. 
     Preferably, a birefringence deflection plate for displacing the optical axis of the laser beam applied to the condenser lens in a feed direction is interposed between the birefringence lens and the condenser lens. The birefringence deflection plate is composed of a crystal body having an inclined surface with a predetermined inclination angle and a glass body having an inclined surface corresponding to the inclined surface of the crystal body, and the inclined surface of the crystal body and the inclined surface of the glass body are united. 
     Preferably, a focal point depth displacing means for displacing the depth position of the focal point of the laser beam formed by means of the condenser lens is interposed between the laser beam oscillation means and the birefringence lens. 
     In the laser beam processing machine of the present invention, a laser beam oscillated from the laser beam oscillation means is separated into normal light and abnormal light by the birefringence lens, and the normal light and the abnormal light are respectively converged by the condenser lens to form the focal point of the normal light and the focal point of the abnormal light. Therefore, two deteriorated layers can be formed at the same time. With the laser beam processing machine of the present invention, which has a simple structure and comprises the birefringence lens, two deteriorated layers can be thus formed in the thickness direction of the workpiece at the same time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic constituent diagram of a laser beam processing machine constituted according to the present invention; 
         FIG. 2  is a schematic diagram of another embodiment of laser beam application means provided in the laser beam processing machine shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing a state where two deteriorated layers are formed at the same time in the inside of the workpiece by the laser beam processing machine shown in  FIG. 1 ; 
         FIG. 4  is a schematic diagram showing another embodiment of optical transmission means constituting the laser beam application means provided in the laser beam processing machine shown in  FIG. 1 ; and 
         FIG. 5  is a schematic diagram showing still another embodiment of optical transmission means constituting the laser beam application means provided in the laser beam processing machine shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a laser beam processing machine constituted according to the present invention will be described in detail hereinunder with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of an embodiment of a laser beam processing machine constituted according to the present invention. The laser beam processing machine shown in  FIG. 1  comprises a chuck table  3  for holding a wafer  2  as a workpiece and a laser beam application means denoted by  4  as a whole. 
     The chuck table  3  comprises an adsorption chuck  31  made of a porous member or having a plurality of suction holes or grooves, and the adsorption chuck  31  is communicated with a suction means that is not shown. Therefore, when the circuit side, onto which a protective tape  21  is affixed, of the wafer  2  as a workpiece is placed on the adsorption chuck  31  and the suction means (not shown) is activated, the wafer  2  is suction-held on the chuck table  3 . The chuck table  3  constituted as described above is designed to move in a processing-feed direction indicated by an arrow X in  FIG. 1  by a processing-feed means that is not shown. Therefore, the chuck table  3  and the laser beam application means  4  can move relative to each other in the processing-feed direction indicated by the arrow X. 
     The laser beam application means  4  comprises a pulse laser beam oscillation means  5 , an optical transmission means  7  for transmitting a pulse laser beam oscillated by this pulse laser beam oscillation means  5 , and a condenser lens  8  for converging the laser beam transmitted by the optical transmission means  7 . The pulse laser beam oscillation means  5  oscillates a linearly polarized pulse laser beam  10  capable of passing through the wafer  2  as a workpiece. This pulse laser beam oscillation means  5  may be a YVO 4  pulse laser oscillator or YAG pulse laser oscillator for oscillating a pulse laser beam  10  having a wavelength of 1,064 nm when the wafer  2  is a wafer comprising a silicon substrate, silicon carbide substrate, lithium tantalate substrate, glass substrate or quartz substrate. 
     The optical transmission means  7  comprises a direction changing mirror  71  for changing the direction of the pulse laser beam  10  oscillated from the pulse laser beam oscillation means  5  at 90° downward in  FIG. 1  and a birefringence lens  72  for separating the pulse laser beam  10  whose direction has been changed by the direction changing mirror  71 , into normal light and abnormal light. The birefringence lens  72  is composed of a LASF 35  glass body  721  and a YVO 4  crystal body  722  in the illustrated embodiment. The LASF 35  glass body  721  has a convex surface  721   a  with a predetermined curvature (for example, curvature radius of 58 mm) and the YVO 4  crystal body  722  has a concave surface  722   a  with a curvature corresponding to the convex surface  721   a  of the glass body  721 , and the convex surface  721   a  of the glass body  721  and the concave surface  722   a  of the crystal body  722  are united to constitute the birefringence lens  72 . The birefringence lens  72  constituted as described above separates the pulse laser beam  10  into normal light  10   a  indicated by a solid line and abnormal light  10   b  indicated by a broken line in FIG.  1  when the pulse laser beam  10  whose direction has been changed by the direction changing mirror  71  is caused to enter at a predetermined angle to the optical axis of the YVO 4  crystal body  722 . That is, the birefringence lens  72  allows the normal light  10   a  to pass through without refracting it and refracts the abnormal light  10   b  outward by means of the crystal body  722  having the concave surface  722   a . To cause the pulse laser beam  10  to be entered at a predetermined angle from the optical axis of the YVO 4  crystal body  722 , the birefringence lens  72  may be turned around the optical axis of the incident pulse laser beam  10  as the center to make adjustment, or the pulse laser beam oscillation means  5  may be turned around the optical axis of the pulse laser beam as the center to make adjustment. 
     A description will be subsequently given of another embodiment in which the pulse laser beam  10  is caused to enter at a predetermined angle with respect to the optical axis of the YVO 4  crystal body  722  with reference to  FIG. 2 . 
     In the embodiment shown in  FIG. 2 , a half-wave plate  75  is interposed between the direction changing mirror  71  and the birefringence lens  72  in the embodiment shown in  FIG. 1 . This half-wave plate  75  moves in the polarization plane to change the incidence angle of the pulse laser beam  10  with respect to the optical axis of the YVO 4  crystal body  722 . By setting the incidence angle of the pulse laser beam  10  with respect to the optical axis of the YVO 4  crystal body  722  to 45°, the ratio of the normal light  10   a  to the abnormal light  10   b  separated by the birefringence lens  72  can be set to 50:50. 
     The above condenser lens  8  converges the normal light  10   a  and the abnormal light  10   b  separated by the birefringence lens  72 . That is, the condenser lens  8  converges the normal light  10   a  at the focal point Pa in the inside of the wafer  2  as the workpiece and the abnormal light  10   b  at the focal point Pb in the inside of the wafer  2  as the workpiece. Since the abnormal light  10   b  is refracted outward by the birefringence lens  72  as described above, this focal point Pb is at a position deeper than the focal point Pa of the normal light  10   a  (lower position in  FIG. 1  and  FIG. 2 ), that is, at a position away from the condenser lens  8  in the optical axis direction. 
     In the embodiment shown in  FIG. 1  and  FIG. 2 , the birefringence lens  72  is composed of the glass body  721  having a convex surface  721   a  and the crystal body  722  having a concave surface  722   a . The glass body may have a concave surface and the crystal body may have a convex surface. In this case, the focal point of the abnormal light is at a position shallower than the focal point of the normal light (an upper position in  FIG. 1  and  FIG. 2 ), that is, a position close to the condenser lens  8  in the optical axis direction. 
     When the normal light  10   a  of the pulse laser beam  10  is converged at the focal point Pa as described above, a deteriorated layer W 1  is formed in the wafer  2  as the workpiece in the vicinity of the focal point Pa, generally in an area having a thickness T 1  in an upward direction from the focal point Pa. Meanwhile, when the abnormal light  10   b  of the pulse laser beam  10  is converged at the focal point Pb, a deteriorated layer W 2  is formed in the wafer  2  as the workpiece in the vicinity of the focal point Pb, generally in an area having a thickness T 2  in an upward direction from the focal point Pb. The deteriorated layers formed in the wafer  2  as the workpiece are generally molten and re-solidified voids or cracks (that is, voids or cracks which are formed by being molten when the normal light  10   a  and the abnormal light  10   b  of the pulse laser beam  10  are converged and then solidified after the convergence of the pulse laser beam  10  ends) though this depends on the material of the wafer  2  or the intensities of the converged normal light  10   a  and abnormal light  10   b  of the pulse laser beam  10 . 
     The laser beam processing machine in the illustrated embodiment moves the chuck table  3  (therefore, the wafer  2  as the workpiece held on the chuck table  3 ), for example, in a left direction in  FIG. 1  and  FIG. 2  while applying the pulse laser beam as described above. As a result, two deteriorated layers W 1  and W 2  having thicknesses T 1  and T 2  are formed at the same time along a predetermined dividing line in the inside of the wafer  2  as shown in  FIG. 3 , respectively. Thus, according to the laser beam processing machine in the illustrated embodiment, with a simple structure having the birefringence lens  72 , two deteriorated layers W 1  and W 2  having thicknesses T 1  and T 2  can be formed at the same time in two areas displaced in the thickness direction of the wafer  2  as the workpiece, respectively. 
     The processing conditions in the above laser processing are set as follows, for example.
         Light source: LD excited Q switch Nd:YAG pulse laser   Wavelength: 1,064 nm   Pulse output: 2.5 μJ   Focal spot diameter: 1 μm   Pulse width: 40 ns   Repetition frequency: 100 kHz   Processing-feed rate: 100 mm/sec       

     When the wafer  2  as the workpiece is thick and the deteriorated layers W 1  and W 2  having thicknesses T 1  and T 2  are not satisfactory for dividing the wafer  2  along the dividing lines accurately, the laser beam application means  4  and the chuck table  3  are moved relative to each other by a predetermined distance in the optical axis direction, that is, in the vertical direction indicated by an arrow Z in  FIG. 1  and  FIG. 2  to displace the focal point Pa and the focal point Pb in the optical axis direction, that is, in the thickness direction of the wafer  2  as the workpiece. Then, the chuck table  3  is moved in the processing-feed direction indicated by the arrow X in  FIG. 1  and  FIG. 2  while the pulse laser beam is applied from the laser beam application means  4 . As a result, the deteriorated layers W 1  and W 2  having thicknesses T 1  and T 2  can be formed in areas displaced in the thickness direction in the wafer  2  as the workpiece, respectively, in addition to the above deteriorated layers W 1  and W 2 . 
     A description is subsequently given of another embodiment of the optical transmission means  7  constituting the laser beam application means  4  with reference to  FIG. 4 . 
     The optical transmission means  7  shown in  FIG. 4  comprises a birefringence deflection plate  73  for displacing the optical axes of the normal light  10   a  and the abnormal light  10   b  of the laser beam  10  applied to the condenser lens  8  in the processing-feed direction X, the birefringence deflection plate  73  being interposed between the birefringence lens  72  and the condenser lens  8  in the embodiment shown in  FIG. 1 . Since the constituent elements of the laser beam application means  4  in the embodiment shown in  FIG. 4  are identical to those of the laser beam application means  4  shown in  FIG. 1  except for the birefringence deflection plate  73 , the same members are given the same reference symbols, and their descriptions are omitted. 
     The birefringence deflection plate  73  is composed of a YVO 4  crystal body  731  and a LASF 35  glass body  732  in the illustrated embodiment. The YVO 4  crystal body has a surface with a predetermined inclination angle (for example, 3.5°) and the LASF 35  glass body  732  has an inclined surface  732   a  corresponding to the inclined surface  731   a  of the YVO 4  crystal body  731 , and the inclined surface  731   a  of the YVO 4  crystal body  731  and the inclined surface  732   a  of the LASF 35  glass body  732  are united to constitute the birefringence deflection plate  73 . The birefringence deflection plate  73  constituted as described above allows the normal light  10   a  of the pulse laser beam  10  separated by the birefringence lens  72  to pass through without refracting it and refracts the abnormal light  10   b  in a left direction in  FIG. 4 . As a result, the condenser lens  8  converges the normal light  10   a  at the focal point Pa in the inside of the wafer  2  as the workpiece and the abnormal light  10   b  at the focal point Pb displaced in the left direction from the focal point Pa of the normal light  10   a  in the inside of the wafer  2  as the workpiece by a distance S in the processing-feed direction indicated by the arrow X in  FIG. 4 . Since the abnormal light  10   b  is refracted outward by the birefringence lens  72  as described above, the focal point Pb is at a position deeper than the focal point Pa of the normal light  10   a  (lower position in  FIG. 4 ), that is, a position away from the condenser lens  8  in the optical axis direction. 
     When the normal light  10   a  of the pulse laser beam  10  is converged at the focal point Pa as described above, the deteriorated layer W 1  is formed in the wafer  2  as the workpiece in the vicinity of the focal point Pa, generally in the area having a thickness T 1  in an upward direction from the focal point Pa. Meanwhile, when the abnormal light  10   b  of the pulse laser beam  10  is converged at the focal point Pb, the deteriorated layer W 2  is formed in the wafer  2  as the workpiece  2  in the vicinity of the focal point Pb, generally in the area having a thickness T 2  in an upward direction from the focal point Pb. Since the focal point Pb of the abnormal light  10   b  is displaced from the focal point Pa of the normal light  10   a  by the distance S in the processing-feed direction indicated by the arrow X in  FIG. 4 , the focal point Pa of the normal light  10   a  and the focal point Pb of the abnormal light  10   b  do not interfere with each other, and the abnormal light  10   b  having a deep focal point is not blocked by the normal light  10   a  having a shallow focal point. Therefore, the deteriorated layers W 1  and W 2  having desired depths can be formed in the vicinity of the focal point Pa of the normal light  10   a  and in the vicinity of the focal point Pb of the abnormal light  10   b , respectively. 
     A description will be subsequently given of still another embodiment of the optical transmission means  7  in the laser beam application means  4  with reference to  FIG. 5 . 
     The optical transmission means  7  shown in  FIG. 5  comprises a focal point depth displacing means  74  which is interposed between the above pulse laser beam oscillation means  5  and the direction changing mirror  71 . Since the constituent elements of the laser beam application means  4  in the embodiment shown in  FIG. 5  are identical to those of the embodiment shown in  FIG. 4  except for the focal point depth displacing means  74 , the same members are given the same reference symbols, and their descriptions are omitted. 
     The focal point depth displacing means  74  shown in  FIG. 5  comprises a first convex lens  741  and a second convex lens  742  which are spaced apart from each other, a first pair of mirrors  743  and a second pair of mirrors  744  which are interposed between the first convex lens  741  and the second convex lens  742 . The first pair of mirrors  743  consist of a first mirror  743   a  and a second mirror  743   b  which are parallel to each other and fixed to a mirror holding member (not shown) while the spacing between them is kept. The second pair of mirrors  744  consist of a first mirror  744   a  and a second mirror  744   b  which are parallel to each other and fixed to a mirror holding member (not shown) while the spacing between them is kept. In the state shown in  FIG. 5 , the focal point (f 1 ) of the first convex lens  741  and the focal point (f 2 ) of the second convex lens  742  are constituted to meet at a convergence point D between the second mirror  743   b  of the first pair of mirrors  743  and the first mirror  744   a  of the second pair of mirrors  744 . In this state, the pulse laser beam  10  applied toward the direction changing mirror  71  from the second convex lens  742  becomes parallel. 
     In the thus constituted focal point depth displacing means  74  shown in  FIG. 5 , the pulse laser beam  10  oscillated from the pulse laser beam oscillation means  5  is introduced into the direction changing mirror  71  through the first convex lens  741 , the first mirror  743   a  and the second mirror  743   b  of the first pair of mirrors  743 , the first mirror  744   a  and the second mirror  744   b  of the second pair of mirrors  744 , and the second convex lens  742 . The focal point depth displacing means  74  shown in  FIG. 5  turns the mirror holding members (not shown) respectively holding the first pair of mirrors  743  and the second pair of mirrors  744  around points Q and Q, as the centers, where the first mirror  743   a  and the first mirror  744   a  are each point-symmetrical to the second mirror  743   b  and the second mirror  744   b  to change the installation angles of the mirrors, thereby making it possible to respectively displace the focal point (f 1 ) of the first convex lens  741  and the focal point (f 2 ) of the second convex lens  742  in the right-and-left direction in  FIG. 5 . In the focal point depth displacing means  74  constituted as described above, in the state shown in  FIG. 5 , the focal point (f 1 ) of the first convex lens  741  and the focal point (f 2 ) of the second convex lens  742  meet at the convergence point D as described above to make the pulse laser beam  10  applied toward the direction changing mirror  71  from the second convex lens  742  parallel. Meanwhile, when the first pair of mirrors  743  and the second pair of mirrors  744  are turned around the points Q and Q as the centers in one direction, the focal point (f 1 ) of the first convex lens  741  is displaced from the convergence point D in the left direction in  FIG. 5 , and the focal point (f 2 ) of the second convex lens  742  is displaced from the above convergence point D in the right direction in  FIG. 5 , the pulse laser beam  10  applied toward the direction changing mirror  71  from the second convex lens  742  is broaden toward the end. As a result, the pulse laser beam  10  applied to the above birefringence lens  72  through the direction changing mirror  71  is also broaden toward the end, whereby the focal point Pa of the normal light  10   a  and the focal point Pb of the abnormal light  10   b  separated by the birefringence lens  72  and converged by the condenser lens  8  are displaced in a downward direction from the illustrated state. On the other hand, when the first pair of mirrors  743  and the second pair of mirror  744  are turned around the points Q and Q as the centers in the other direction, the focal point (f 1 ) of the first convex lens  741  is displaced from the above convergence point D in the right direction in  FIG. 5 , and the focal point (f 2 ) of the second convex lens  742  is displaced from the convergence point D in the left direction in  FIG. 5 , the pulse laser beam  10  applied toward the direction changing mirror  71  from the second convex lens  742  becomes narrower toward the end. As a result, the pulse laser beam  10  applied to the above birefringence lens  72  through the direction changing mirror  71  becomes also narrower toward the end, whereby the focal point Pa of the normal light  10   a  and the focal point Pb of the abnormal light  10   b  separated by the birefringence lens  72  and converged by the condenser lens  8  are displaced in an upward direction from the illustrated state. 
     In the optical transmission means  7 , the distance between two focal points can be properly changed in the Z direction and the X direction by preparing a plurality of birefringence lenses having different curvature radii or different crystal bodies and a plurality of birefringence deflection plates having different inclination angles or different crystal bodies and exchanging these birefringence lenses and birefringence deflection plates properly.