Abstract:
Disclosed herein is a beam processing machine comprising laser beam application device for applying a laser beam to a workpiece, which device includes a laser beam oscillator and a condenser for converging a laser beam to apply the converged laser beam. The condenser includes a first prism for splitting the laser beam oscillated from the laser beam oscillator into a first laser beam and a second laser beam both having a semicircular section and interchanging the first laser beam with the second laser beam, a second prism for correcting the optical paths of the first laser beam and the second laser beam split by the first prism to become parallel to each other, and an image forming lens for focusing the first laser beam and the second laser beam whose optical paths have been corrected by the second prism to become parallel to each other into spots having linear portions on the outer side and arcuate portions on the inner side. The image forming lens partially overlaps the arcuate portions of the spots of the first laser beam and the second laser beam with each other to form a rectangular spot.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a laser beam processing machine for forming a groove along a predetermined dividing line on a wafer such as an optical device wafer or the like. 
   DESCRIPTION OF THE PRIOR ART 
   An optical device wafer comprising optical devices, which are composed of a gallium nitride-based compound semiconductor layer or the like that is laminated in each of a plurality of areas sectioned by dividing lines formed in a lattice pattern on the front surface of a sapphire substrate and the like is divided along the dividing lines into individual optical devices such as light emitting diodes or laser diodes which are widely used in electric appliances. 
   Cutting along the dividing lines of a wafer such as the above optical device wafer is generally carried out by using a cutting machine for cutting it by rotating a cutting blade at a high speed. However, as the sapphire substrate has such a high Moh&#39;s hardness that it is difficult to be cut, the processing speed must be slowed down, there by reducing productivity. 
   Meanwhile, as a means of dividing a plate-like workpiece such as a wafer, JP-A 10-305420 discloses a method comprising applying a pulse laser beam along dividing lines formed on a workpiece to form grooves and dividing to cut the workpiece along the laser-processed grooves by a mechanical breaking apparatus. 
   JP-A 2004-9139 discloses a method comprising applying a pulse laser beam having absorptivity for a sapphire substrate to the substrate to form grooves. 
   The laser beam to be applied to form the above grooves goes straight even when it is hit against a substance which absorbs it. Therefore, even when a laser beam having absorptivity for a substrate constituting the wafer is applied to the substrate, all the energy of the laser beam is not absorbed by the substrate and the unabsorbed laser beam goes to the side opposite to the incident side of the wafer. When a groove is to be formed on an optical device wafer having a plurality of optical devices on the front surface of a sapphire substrate or the like, a laser beam is applied from the back surface side of the wafer so as to prevent damage caused by the adhesion of debris produced at the time of laser processing to an optical device formed on the front surface of the substrate. Meanwhile, the energy of the laser beam shows a Gaussian distribution that it is strong at the center and becomes weaker toward the outer sides. However, as described above, when the laser beam not absorbed by the substrate, especially the laser beam having relatively low energy which is on the outer sides of the Gaussian distribution and does not contribute to processing reaches the front surface of the substrate, it damages a device layer formed on the front surface of the substrate, thereby reducing the quality of the optical device. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a laser beam processing machine which can form a groove along dividing lines on the back surface of a wafer without damaging the front surface of the wafer by applying a laser beam to the back surface of the wafer along a predetermined dividing line. 
   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 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 and a condenser for converging a laser beam oscillated from the laser beam oscillation means to apply the converged laser beam, wherein 
   the condenser comprises a first prism for splitting the laser beam oscillated from the laser beam oscillation means into a first laser beam and a second laser beam both having a semicircular section and interchanging the first laser beam with the second laser beam, a second prism for correcting the optical paths of the first laser beam and the second laser beam split by the first prism to become parallel to each other, and an image forming lens for focusing the first laser beam and the second laser beam whose optical paths have been corrected by the second prism to become parallel to each other into spots having linear portions on the outer side and arcuate portions on the inner side; and 
   the image forming lens partially overlaps the arcuate portions of the spots of the first laser beam and the second laser beam with each other to form a rectangular spot. 
   The interval between the spots of the first laser beam and the second laser beam focused by the image forming lens is controlled by adjusting the interval between the first prism and the second prism. A cylindrical lens is arranged on the upstream side in the laser beam application direction of the first prism or on the downstream side in the laser beam application direction of the second prism. 
   In the laser beam processing machine according to the present invention, the linear portions of the image forming spots of the first laser beam and the second laser beam for forming a groove are located on the outer side and the arcuate portions of the spots are located on the inner side and partially overlapped with each other to form a rectangular spot. Therefore, a portion having relatively low energy on the outer sides of the Gaussian distribution of the laser beam is located on the inner side and a portion having high energy on the center portion of the Gaussian distribution of the laser beam is located on the outer side. Consequently, the groove can be formed with the outermost sides of the image forming spots of the first laser beam and the second laser beam, whereby the laser beam does not exceed the width of the groove. Therefore, a device formed on the front surface of the workpiece is not damaged by the energy of the laser beam when the groove is formed by applying the laser beam from the back surface side of the workpiece. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a laser beam processing machine constituted according to the present invention; 
       FIG. 2  is a block diagram schematically showing the constitution of laser beam application means provided in the laser beam processing machine shown in  FIG. 1 ; 
       FIG. 3  is an explanatory diagram showing a first embodiment of the condenser provided in the laser beam processing machine shown in  FIG. 1 ; 
       FIG. 4  is an enlarged view of configuration of a first laser beam spot and a second laser beam spot focused by the condenser shown in  FIG. 3 ; 
       FIG. 5  is an explanatory showing a state where the interval between the first laser beam spot and the second laser beam spot is changed by altering the interval between a first prism and a second prism constituting the condenser shown in  FIG. 3 ; 
       FIG. 6  is an explanatory of a second embodiment of the condenser provided in the laser beam processing machine shown in  FIG. 1 ; 
       FIG. 7  is an explanatory of a third embodiment of the condenser provided in the laser beam processing machine shown in  FIG. 1 ; 
       FIG. 8  is an explanatory of a fourth embodiment of the condenser provided in the laser beam processing machine shown in  FIG. 1 ; 
       FIG. 9  is a perspective view of an optical device wafer as a workpiece; 
       FIG. 10  is a partial enlarged sectional view of the optical device wafer shown in  FIG. 9 ; 
       FIG. 11  is a perspective view showing a state where aprotective tape is affixed to the optical device wafer shown in  FIG. 8 ; 
       FIGS. 12(   a ) and  12 ( b ) are explanatory diagrams showing the laser beam application step for forming a groove along a dividing line of the optical device wafer shown in  FIG. 9  by the laser beam processing machine shown in  FIG. 1 ; and 
       FIG. 13  is an enlarged sectional view of the groove formed in the optical device wafer by carrying out the laser beam application step shown in  FIG. 12 . 
   

   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 perspective view of a laser beam processing machine constituted according to the present invention. The laser beam processing machine shown in  FIG. 1  comprises a stationary base  2 , a chuck table mechanism  3  for holding a workpiece, which is mounted on the stationary base  2  in such a manner that it can move in a processing-feed direction indicated by an arrow X, a laser beam application unit support mechanism  4  mounted on the stationary base  2  in such a manner that it can move in an indexing-feed direction indicated by an arrow Y perpendicular to the direction indicated by the arrow X, and a laser beam application unit  5  mounted on the laser beam application unit support mechanism  4  in such a manner that it can move in a focal position adjustment direction indicated by an arrow Z. 
   The above chuck table mechanism  3  comprises a pair of guide rails  31  and  31 , which are mounted on the stationary base  2  and arranged parallel to each other in the direction indicated by the arrow X, a first sliding block  32  mounted on the guide rails  31  and  31  in such a manner that it can move in the direction indicated by the arrow X, a second sliding block  33  mounted on the first sliding block  32  in such a manner that it can move in the direction indicated by the arrow Y, a support table  35  supported on the second sliding block  33  by a cylindrical member  34 , and a chuck table  36  as workpiece holding means. This chuck table  36  is made of a porous material and has a workpiece holding surface  361 , and a plate-like workpiece, for example, disk-like semiconductor wafer is held on the chuck table  36  by a suction means that is not shown. The chuck table  36  is rotated by a pulse motor (not shown) installed in the cylindrical member  34 . The chuck table  36  is provided with clamps  362  for fixing an annular frame for supporting a semiconductor wafer which will be described later. 
   The above first sliding block  32  has, on its undersurface, a pair of to-be-guided grooves  321  and  321  to be fitted to the above pair of guide rails  31  and  31  and, on its top surface, a pair of guide rails  322  and  322  formed parallel to each other in the direction indicated by the arrow Y. The first sliding block  32  constituted as described above can move in the direction indicated by the arrow X along the pair of guide rails  31  and  31  by fitting the to-be-guided grooves  321  and  321  to the pair of guide rails  31  and  31 , respectively. The chuck table mechanism  3  in the illustrated embodiment comprises a processing-feed means  37  for moving the first sliding block  32  along the pair of guide rails  31  and  31  in the direction indicated by the arrow X. The processing-feed means  37  comprises a male screw rod  371  arranged between the above pair of guide rails  31  and  31  parallel thereto, and a drive source such as a pulse motor  372  for rotary-driving the male screw rod  371 . The male screw rod  371  is, at its one end, rotatably supported to a bearing block  373  fixed on the above stationary base  2  and is, at the other end, transmission-coupled to the output shaft of the above pulse motor  372  via a speed reducer that is not shown. The male screw rod  371  is screwed into a threaded through-hole formed in a female screw block (not shown) projecting from the undersurface of the center portion of the first sliding block  32 . Therefore, by driving the male screw rod  371  in a normal direction or reverse direction with the pulse motor  372 , the first sliding block  32  is moved along the guide rails  31  and  31  in the processing-feed direction indicated by the arrow X. 
   The above second sliding block  33  has, on its undersurface, a pair of to-be-guided grooves  331  and  331  to be fitted to the pair of guide rails  322  and  322  on the top surface of the above first sliding block  32  and can move in the direction indicated by the arrow Y by fitting the to-be-guided grooves  331  and  331  to the pair of guide rails  322  and  322 , respectively. The chuck table mechanism  3  in the illustrated embodiment comprises a first indexing-feed means  38  for moving the second sliding block  33  in the direction indicated by the arrow Y along the pair of guide rails  322  and  322  on the first sliding block  32 . The first indexing-feed means  38  comprises a male screw rod  381  which is arranged between the above pair of guide rails  322  and  322  parallel thereto, and a drive source such as a pulse motor  382  for rotary-driving the male screw rod  381 . The male screw rod  381  is, at its one end, rotatably supported to a bearing block  383  fixed on the top surface of the above first sliding block  32  and is, at the other end, transmission-coupled to the output shaft of the above pulse motor  382  via a speed reducer that is not shown. The male screw rod  381  is screwed into a threaded through-hole formed in a female screw block (not shown) projecting from the undersurface of the center portion of the second sliding block  33 . Therefore, by driving the male screw rod  381  in a normal direction or reverse direction with the pulse motor  382 , the second sliding block  33  is moved along the guide rails  322  and  322  in the indexing-feed direction indicated by the arrow Y. 
   The above laser beam application unit support mechanism  4  comprises a pair of guide rails  41  and  41 , which are mounted on the stationary base  2  and arranged parallel to each other in the direction indicated by the arrow Y and a moveable support base  42  mounted on the guide rails  41  and  41  in such a manner that it can move in the direction indicated by the arrow Y. This movable support base  42  comprises a movable support portion  421  movably mounted on the guide rails  41  and  41  and a mounting portion  422  mounted on the movable support portion  421 . The mounting portion  422  is provided with a pair of guide rails  423  and  423  extending parallel to each other in the direction indicated by the arrow Z on one of its flanks. The laser beam application unit support mechanism  4  in the illustrated embodiment comprises a second indexing-feed means  43  for moving the movable support base  42  along the pair of guide rails  41  and  41  in the direction indicated by the arrow Y. This second indexing-feed means  43  comprises a male screw rod  431  arranged between the above pair of guide rails  41  and  41  parallel thereto, and a drive source such as a pulse motor  432  for rotary-driving the male screw rod  431 . The male screw rod  431  is, at its one end, rotatably supported to a bearing block (not shown) fixed on the above stationary base  2  and is, at the other end, transmission-coupled to the output shaft of the above pulse motor  432  via a speed reducer that is not shown. The male screw rod  431  is screwed into a threaded through-hole formed in a female screw block (not shown) projecting from the undersurface of the center portion of the movable support portion  421  constituting the movable support base  42 . Therefore, by driving the male screw rod  431  in a normal direction or reverse direction with the pulse motor  432 , the movable support base  42  is moved along the guide rails  41  and  41  in the indexing-feed direction indicated by the arrow Y. 
   The laser beam application unit  5  in the illustrated embodiment comprises a unit holder  51  and a laser beam application means  52  secured to the unit holder  51 . The unit holder  51  has a pair of to-be-guided grooves  511  and  511  to be slidably fitted to the pair of guide rails  423  and  423  on the above mounting portion  422  and is supported in such a manner that it can move in the direction indicated by the arrow Z by fitting the to-be-guided grooves  511  and  511  to the above guide rails  423  and  423 , respectively. 
   The illustrated laser beam application means  52  comprises a cylindrical casing  521  that is secured to the above unit holder  51  and extends substantially horizontally. In the casing  521 , there are installed a pulse laser beam oscillation means  522  and a transmission optical system  523 , as shown in  FIG. 2 . The pulse laser beam oscillation means  522  comprises a pulse laser beam oscillator  522   a  composed of a YAG laser oscillator or YVO4 laser oscillator and a repetition frequency setting means  522   b  connected to the pulse laser beam oscillator  522   a . The transmission optical system  523  comprises suitable optical elements such as a beam splitter, etc. A condenser  53  for converging a laser beam oscillated from the above laser beam oscillation means  522  and transmitted via the optical transmission system  523 , is attached to the end of the above casing  521 . 
   A first embodiment of the condenser  53  will be described with reference to  FIG. 3 . 
   The condenser  53  shown in  FIG. 3  comprises a direction changing mirror  532 , a first prism  533 , a second prism  534  and an image forming lens  535  all of which are installed in a case  531  (see  FIG. 2 ). The direction changing mirror  532  changes the direction of a laser beam L that is oscillated from the above pulse laser beam oscillation means  522  (see  FIG. 2 ) and irradiated via the transmission optical system  523 , to a downward direction, that is, toward the first prism  533 , as shown in  FIG. 3 . The first prism  533  splits the laser beam L oscillated from the pulse laser beam oscillation means  522  into a first laser beam L 1  and a second laser beam L 2  both having a semicircular section and interchanges the first laser beam L 1  with the second laser beam L 2 . The second prism  534  corrects the optical paths of the first laser beam L 1  and the second laser beam L 2  split by the first prism  533  to become parallel to each other. The image forming lens  535  focuses the first laser beam L 1  and the second laser beam L 2  whose optical paths have been corrected by the second prism  534  into spots. 
   The condenser  53  shown in  FIG. 3  is constituted as described above, and its function will be described hereinunder. The direction of the laser beam L having a circular section, which has been oscillated from the above pulse laser beam oscillation means  522  and is illuminated through the transmission optical system  523  is changed to a direction toward the first prism  533  by the direction changing mirror  532 . The laser beam L having a circular section, which has reached the first prism  533 , is split into the first laser beam Ll and the second laser beam L 2  both having a semicircular section by the first prism  533 , and the first laser beam L 1  and the second laser beam L 2  are interchanged by the first prism  533 . As a result, arcuate portions of the first laser beam L 1  and the second laser beam L 2  are located on the inner side and linear portions are located on the outer side, respectively. The optical paths of the first laser beam L 1  and the second laser beam L 2  thus split are then corrected to become parallel to each other after they pass through the second prism  534 . The first laser beam L 1  and the second laser beam L 2  whose optical paths have been corrected to become parallel to each other by the second prism  534  are focused at a predetermined image forming position “P” after they pass through the image forming lens  535 . Since the image forming position “P” is located on the downstream side of the focal point “f” of the image forming lens  535  at this point, the first laser beam L 1  and the second laser beam L 2  are inverted and focused into spots having linear portions on the outer side and arcuate portions on the inner side. It is important that the arcuate portions of the spots of the first laser beam L 1  and the second laser beam L 2  focused by this image forming lens  535  should be partially overlapped with each other to form a rectangular spot. 
   In the above embodiment, the image forming spots at the image forming position “P” on the downstream side of the focal point “f” of the image forming lens  535  are used. Spots at a distance of ±100 μm from the focal point “f” can be used for laser processing. 
   When the distance between the apex of the first prism  533  and the image forming lens  535  is represented by “a”, the distance between the image forming lens  535  and the image forming position “P” is represented by “b” and the focal distance of the image forming lens  535  is represented by “f” in  FIG. 3 , the equation (1/a+1/b=1/f) is established. As shown in  FIG. 4 , the diameter “d” (length) of the spots of the first laser beam L 1  and the second laser beam L 2  both having semicircular shape is determined by magnification (m=b/a). The width “E” made by the spots of the first laser beam L 1  and the second laser beam L 2  is changed by varying the interval between the first prism  533  and the second prism  534 . That is, as shown in  FIG. 5 , when the interval between the first prism  533  and the second prism  534  is “D 1 ”, the width (E) is “E 1 ” between the first laser beam L 1  and the second laser beam L 2  passing through the second prism  534 . When the interval between the first prism  533  and the second prism  534  expands to “D 2 ”, the width (E) is “E 2 ” between the first laser beam L 1  and the second laser beam L 2  passing through the second prism  534 . Therefore, the width (E) of the spots of the first laser beam L 1  and the second laser beam L 2  focused at the predetermined image forming position “P” (see  FIG. 3 ) after they pass through the image forming lens  535  also changes. For instance, when the diameter of the laser beam L entered to the first prism  533  is 1 mm, the magnification “m” is 1/50, and the interval “D” between the first prism  533  and the second prism  534  is changed from 1 mm to 5 mm, the width (E) of the spots of the first laser beam L 1  and the second laser beam L 2  changes in a range from 5 to 20 μm. 
   A description will be subsequently given of a second embodiment of the condenser  53  with reference to  FIG. 6 . 
   In the condenser  53  shown in  FIG. 6 , a relay lens  536  is interposed between the second prism  534  and the image forming lens  535  in the first embodiment shown in  FIG. 3 . Since other constituent members of the condenser  53  shown in  FIG. 6  are the same as the constituent members of the first embodiment shown in  FIG. 3 , the same members are given the same reference symbols and their descriptions are omitted. 
   In the condenser  53  shown in  FIG. 6 , the back-focus position “f 1 ” of the relay lens  536  is aligned with the apex position of the first prism  533  to form an infinite correction optical system. Therefore, as the interval “c” between the relay lens  536  and the image forming lens  535  can be freely changed, the degree of design freedom is high. In the condenser  53  shown in  FIG. 6 , the magnification can be freely changed by a combination of the relay lens  536  and the image forming lens  535 . In the condenser  53  shown in  FIG. 6 , the image forming position “P” of the first laser beam L 1  and the second laser beam L 2  becomes the focal point “f” of the image forming lens  535 . “f 2 ” in  FIG. 6  is the focal point obtained by a combination of the relay lens  536  and the image forming lens  535 . It is also important in this embodiment that the linear portions of the spots of the first laser beam L 1  and the second laser beam L 2  having a semicircular section focused by the image forming lens  535  should be located on the outer side and the arcuate portions should be located on the inner side and partially overlapped with each other to form a rectangular spot. 
   A description will be subsequently given of a third embodiment of the condenser  53  with reference to  FIG. 7 . 
   In the condenser  53  shown in  FIG. 7 , a cylindrical lens  537  is arranged on the upstream side in the laser beam application direction of the first prism  533  in the first embodiment shown in  FIG. 3 , that is, between the direction changing mirror  532  and the first prism  533 . Since other constituent members of the condenser  53  shown in  FIG. 7  are the same as the constituent members of the first embodiment shown in  FIG. 3 , the same members are given the same reference symbols and their descriptions are omitted. 
   In the condenser  53  shown in  FIG. 7 , after the laser beam L having a circular section, which has been oscillated from the above pulse laser beam oscillation means  522  and is illuminated through the transmission optical system  523  passes through the cylindrical lens  537 , it reaches the first prism  533  as a laser beam having an elliptic section. As a result, a first laser beam L 1  and a second laser beam L 2  split by the first prism  533  have a semi-elliptic section, and the image forming spots of the first laser beam L 1  and the second laser beam L 2  passing through the second prism  534  and focused by the image forming lens  535  become semi-elliptic as well. Also in this embodiment, the arcuate portions of the spots of the first laser beam L 1  and the second laser beam L 2  focused by the image forming lens  535  are partially overlapped with each other to form a rectangular spot. Thus, the rectangular spot is formed by making the sections of the image forming spots of the first laser beam L 1  and the second laser beam L 2  semi-elliptic and partially overlapping the arcuate portions of the spots with each other, thereby making it possible to increase the ratio of the length “d” to the width “E”. Accordingly, since pulse laser beams are applied sequentially in the direction of the length of the image forming spots, the ratio of the overlapped portions of the image forming spots (pulses) can be increased and hence, a continuous groove  15  can be formed without fail. 
   A description will be subsequently given of a fourth embodiment of the condenser  53  with reference to  FIG. 8 . 
   In the condenser  53  shown in  FIG. 8 , the cylindrical lens  537  in the third embodiment shown in  FIG. 7  is arranged on the downstream side in the laser beam application direction of the second prism  534 , that is, between the second prism  534  and the image forming lens  535 . Since other constituent members of the condenser  53  shown in  FIG. 8  are the same as the constituent members of the third embodiment shown in  FIG. 7 , the same members are given the same reference symbols and their descriptions are omitted. 
   In the condenser  53  shown in  FIG. 8 , the laser beam L having a circular section, which has been oscillated from the above pulse laser beam oscillation means  522  and is illuminated through the transmission optical system  523  is split into the first laser beam L 1  and the second laser beam L 2  both having a semicircular section by the first prism  533 , and the first laser beam L 1  and the second laser beam L 2  are interchanged by the first prism  533 . The optical paths of the first laser beam L 1  and the second laser beam L 2  are then corrected to become parallel to each other after they pass through the second prism  534 . The first laser beam L 1  and the second laser beam L 2  whose optical paths have been corrected to become parallel to each other by the second prism  534  pass through the cylindrical lens  537 , whereby their respective cross sections become semi-elliptic sections, and reach the image forming lens  535 . After the first laser beam L 1  and the second laser beam L 2  pass through the image forming lens  535 , the linear portions of the spots of the first laser beam L 1  and the second laser beam L 2  having a semi-elliptic section are located on the outer side and the arcuate portions of the spots are located on the inner side and partially overlapped with each other to form a rectangular spot. 
   In the above third embodiment and the fourth embodiment, the cylindrical lens  537  is provided to increase the ratio of the length “d” to the width “E” of the image forming spots of the first laser beam L 1  and the second laser beam L 2 . The cylindrical lens  537  maybe dislocated at 90°. By dislocating the cylindrical lens  537  as described above, the ratio of the overlapped portions of the image forming spots of the first laser beam L 1  and the second laser beam L 2  can be increased. 
   Returning to  FIG. 1 , an image pick-up means  6  for detecting the area to be processed by the above laser beam application means  52  is mounted on the front end of the casing  521  constituting the above laser beam application means  52 . This image pick-up means  6  comprises an infrared illuminating means for applying infrared radiation to the workpiece, an optical system for capturing the infrared radiation applied by the infrared illuminating means, and an image pick-up device (infrared CCD) for outputting an electric signal corresponding to the infrared radiation captured by the optical system, in addition to an ordinary image pick-up device (CCD) for picking up an image with visible radiation in the illustrated embodiment. An image signal is supplied to a control means that is not shown. 
   The laser beam application unit  5  in the illustrated embodiment comprises a moving means  54  for moving the unit holder  51  along the pair of guide rails  423  and  423  in the direction indicated by the arrow Z. The moving means  54  comprises a male screw rod (not shown) arranged between the pair of guide rails  423  and  423  and a drive source such as a pulse motor  542  for rotary-driving the male screw rod. By driving the male screw rod (not shown) in a normal direction or reverse direction with the pulse motor  542 , the unit holder  51  and the laser beam application means  52  are moved along the guide rails  423  and  423  in the direction indicated by the arrow Z. In the illustrated embodiment, the laser beam application means  52  is moved up in a normal direction by driving the pulse motor  542  and moved down in a reverse direction by driving the pulse motor  542 . 
   The laser beam processing machine in the illustrated embodiment is constituted as described above, and its function will be described hereinbelow. 
   An optical device wafer as a workpiece to be processed by the above laser beam processing machine will be described with reference to  FIG. 9  and  FIG. 10 .  FIG. 9  is a perspective view of the optical device wafer and  FIG. 10  is an enlarged sectional view of the principal portion of the optical device wafer shown in  FIG. 9 . 
   In the optical device wafer  10  shown in  FIG. 9  and  FIG. 10 , a plurality of devices  13  composed of a device layer  12 , in which layers formed from gallium nitride (GaN) or aluminum nitride gallium (AlGaN) or the like are laminated, are formed in a matrix on the front surface of a sapphire substrate  11 . The devices  13  are sectioned by dividing lines  14  formed in a lattice pattern. 
   For the laser processing of the back surface  10   b  of the optical device wafer  10  constituted as described above, a protective tape  20  is affixed to the front surface  10   a  of the optical device wafer  10 , as shown in  FIG. 11  (protective tape affixing step). 
   The above protective tape affixing step is followed by a laser beam application step for forming a groove along the dividing lines  14  on the back surface  10   b  of the optical device wafer  10 . In this laser beam application step, the protective tape  20  side of the optical device wafer  10  is first placed on the chuck table  36  of the laser beam processing machine shown in  FIG. 1  and suction held on the chuck table  36 . Therefore, the optical device wafer  10  is held in such a manner that the back surface  10   b  faces up. 
   The chuck table  36  suction holding the optical device wafer  10  as described above is brought to a position right below the image pick-up means  6  by the processing-feed means  37 . After the chuck table  36  is positioned right below the image pick-up means  6 , alignment work for detecting the area to be processed of the optical device wafer  10  is carried out by the image pick-up means  6  and the unshown control means. That is, the image pick-up means  6  and the unshown control means carry out image processing such as pattern matching, etc. to align a dividing line  14  formed in a predetermined direction of the optical device wafer  10  with the condenser  53  of the laser beam application means  52  for applying a laser beam along the dividing line  14 , thereby performing the alignment to a laser beam application position. The alignment of the laser beam application position is also carried out on dividing lines  14  formed on the optical device wafer  10  in a direction perpendicular to the above predetermined direction. Although the front surface  10   a  having the dividing lines  14  formed thereon of the optical device wafer  10  faces down at this point, as the image pick-up means  6  comprises an infrared illuminating means, an optical system for capturing the infrared radiation and an image pick-up device (infrared CCD) for outputting an electric signal corresponding to the infrared radiation as described above, images of the dividing lines  14  can be picked up through the back surface  10   b.    
   After the alignment of the laser beam application position is carried out by detecting the dividing line  14  formed on the optical device wafer  10  held on the chuck table  36  as described above, the chuck table  36  is moved to a laser beam application area where the condenser  53  of the laser beam application means  52  is located so as to bring the predetermined dividing line  14  to a position right below the condenser  53  as shown in  FIG. 12(   a ). At this point, as shown in  FIG. 12(   a ), the optical device wafer  10  is positioned such that one end (left end in  FIG. 12(   a )) of the dividing line  14  is located right below the condenser  53 . The moving means  54  is activated to adjust the height position of the laser beam application means  52  so that the image forming position “P” of the first laser beam L 1  and the second laser beam L 2  applied from the condenser  53  is located at the front surface of the dividing line  14 . 
   The chuck table  36 , that is, the optical device wafer  10  is then moved in the direction indicated by the arrow X 1  in  FIG. 12(   a ) at a predetermined feed rate while a laser beam is applied from the condenser  53 . When the other end (right end in  FIG. 12(   b )) of the dividing line  14  reaches a position right below the condenser  53  as shown in  FIG. 12(   b ), the application of the pulse laser beam is suspended, and the movement of the chuck table  36 , that is, the optical device wafer  10  is stopped. As a result, a groove  15  having a width (E) is formed along the predetermined dividing line  14  on the back surface  10   b  of the optical device wafer  10  as shown in  FIG. 13 . Thus, since the linear portions of the image forming spots of the first laser beam L 1  and the second laser beam L 2  for forming the groove  15  are located on the outer side and the arcuate portions of the image forming spots are located on the inner side and partially overlapped with each other to form a rectangular spot, a portion having relatively low energy on the outer sides of the Gaussian distribution of the laser beam is located on the inner side and a portion having high energy on the center portion of the Gaussian distribution of the laser beam is located on the outer side. Accordingly, the groove  15  can be formed with the outermost sides of the image forming spots of the first laser beam L 1  and the second laser beam L 2 , whereby the laser beam does not exceed the width (E) of the groove  15 . Consequently, the device layer  12  formed on the front surface of the substrate  11  is not damaged by the energy of the laser beam. 
   The processing conditions in the above laser beam application step are set as follows, for example.
         Light source of laser beam: YV04 laser or YAG laser   Wavelength: 355 nm   Output: 3.0 W   Cyclic frequency: 100 kHz   Pulse width: 10 ns   Feed rate: 30 mm/sec   Width of dividing line: 50 μm   Width (E) of image forming spot: 10 μm       

   After the above laser beam application step is carried out along all the dividing lines  14  formed in the predetermined direction of the optical device wafer  10 , the chuck table  36 , therefore, the optical device wafer  10  is turned at 90°. The above laser beam application step is carried out along all dividing lines  14  formed on the optical device wafer  10  in a direction perpendicular to the above predetermined direction. 
   After the above laser beam application step is carried out along all the dividing lines  14  formed on the optical device wafer  10  as described above, the optical device wafer  10  is carried to the subsequent dividing step. In the dividing step, as the grooves  15  formed along the dividing lines  14  of the optical device wafer  10  are so deep that the optical device wafer  10  can be easily divided, the optical device wafer  10  can be easily divided by mechanical breaking. 
   While an example in which the present invention is applied to an optical device wafer has been described above, the same effect and function are obtained even when the present invention is applied to laser processing along the dividing lines of a semiconductor wafer having a plurality of circuits on the front surface of a substrate.