Patent Publication Number: US-2022219263-A1

Title: Laser processing machine

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a laser processing machine that applies a pulsed laser beam to a workpiece which is held on a chuck table, to process the workpiece. 
     Description of the Related Art 
     A semiconductor wafer with a plurality of devices such as integrated circuits (ICs) or large-scale integration (LSI) circuits formed in a front surface thereof divided by a plurality of intersecting streets is singulated into individual device chips by a laser processing machine or a dicing machine, and the thus-singulated device chips are used in electrical equipment such as mobile phones or personal computers. 
     The devices are formed of a plurality of circuit layers stacked on the front surface of the semiconductor wafer, and an insulating layer of low dielectric constant (low-k film) is also stacked on the streets. When the streets are cut by a cutting blade of the dicing machine, peeling of the insulating layer may occur, and may then spread to the circuit layers, so that the devices may be damaged. Before cutting the semiconductor wafer by the cutting blade, a laser beam is therefore applied along each street to form a plurality of laser-processed grooves at a predetermined interval therebetween in an indexing direction (Y-axis direction). The insulating layer that still remains between the laser-processed grooves is cut by the cutting blade. As the remaining insulating layer has been cut off at both sides, its cutting does not give any effect to the circuit layers (see, for example, JP 2005-064231A). 
     SUMMARY OF THE INVENTION 
     With the technique described in JP 2005-064231A cited above, however, there is a need to perform the application of the laser beam by repeatedly moving the wafer, which is held on a chuck table, and a laser beam application unit relative to each other in an X-axis direction such that the plurality of laser-processed grooves is formed for each single street, thereby leading to a problem of low productivity. 
     The present invention therefore has as an object thereof the provision of a laser processing machine that can effectively remove an insulating layer from each street. 
     In accordance with an aspect of the present invention, there is provided a laser processing machine including a chuck table configured to hold a workpiece, a laser beam application unit configured to apply a pulsed laser beam to the workpiece held on the chuck table, an X-axis direction feed mechanism configured to perform relative processing feed of the chuck table and the laser beam application unit in an X-axis direction, and a Y-axis direction feed mechanism configured to perform relative processing feed of the chuck table and the laser beam application unit in a Y-axis direction that intersects the X-axis direction at right angles. The laser beam application unit includes a laser oscillator that oscillates pulsed laser and emits a pulsed laser beam, a spot shaper configured to shape a spot profile of the pulsed laser beam emitted from the laser oscillator such that the spot profile becomes long in the Y-axis direction and short in the X-axis direction, a polygon mirror that disperses, in the X-axis direction, the spot which has been shaped by the spot shaper, and a condenser that focuses, on the workpiece held on the chuck table, the pulsed laser beam which has been dispersed by the polygon mirror. 
     Preferably, the workpiece may be a semiconductor wafer with a plurality of devices formed in a front surface thereof divided by a plurality of intersecting streets, and the spot profile which has been shaped at the spot shaper may have, in the Y-axis direction, a length corresponding to a width of the streets. Preferably, the laser beam application unit may further include a water film-forming unit configured to form a water film between the condenser and the workpiece held on the chuck table. 
     According to the laser processing machine of the present invention, the length in the Y-axis direction of the spot profile can be set corresponding to the width of the streets by the spot shaper, and the pulsed laser beam can be applied after dispersing it in the X-axis direction by the polygon mirror. The insulating layer can therefore be efficiently removed from each street, so that the productivity is improved. 
     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 an overall perspective view of a laser processing machine according to an embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of the laser processing machine of  FIG. 1 , with a section of the laser processing machine being depicted in an exploded fashion; 
         FIG. 3A  is a perspective view of a water film-forming unit disposed in the laser processing machine of  FIG. 1 ; 
         FIG. 3B  is an exploded perspective view depicting, in an exploded fashion, the water film-forming unit of  FIG. 3A ; 
         FIG. 4A  is a block diagram illustrating an outline of an optical system of a laser beam application unit disposed in the laser processing machine depicted in  FIG. 1 ; 
         FIG. 4B  is a plan view of a spot shaped by a spot shaper illustrated in  FIG. 4A ; 
         FIG. 5  is a perspective view illustrating how laser processing is applied to a wafer by the laser processing machine depicted in  FIG. 1 ; 
         FIG. 6A  is a cross-sectional view of the water film-forming unit and the wafer during the laser processing illustrated in  FIG. 5 ; and 
         FIG. 6B  is a plan view illustrating a manner in which the spot is dispersed in an X-axis direction during the laser processing illustrated in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the attached drawings, a description will hereinafter be made in detail about a laser processing machine according to an embodiment of the present invention. 
       FIG. 1  depicts an overall perspective view depicting a laser processing machine  2  of the present embodiment. The laser processing machine  2  includes a holding unit  22  disposed on a bed  21  and configured to hold a slice-shaped workpiece (for example, a silicon-made wafer  10 ), a moving mechanism  23  configured to move the holding unit  22 , a frame body  26  formed from a vertical wall section  261 , which is disposed upright in a Z-axis direction indicated by an arrow Z beside the moving mechanism  23  on the bed  21 , and a horizontal wall section  262 , which extends in a horizontal direction from an upper end portion of the vertical wall section  261 , and a laser beam application unit  8 . As depicted in the figure, the wafer  10  is supported, for example, on an annular frame F via an adhesive type T, and is held on the holding unit  22 . Actually, the above-described laser processing machine  2  is covered in its entirety by a housing or the like a depiction of which is omitted in the figure for the convenience of description, and is configured to prevent penetration of fine powder, dust, and the like thereinto. 
     In the laser processing machine  2  of the present embodiment, a water film-forming unit  4  may be disposed as needed in addition to the above-described configuration. The water film-forming unit  4  is configured to form a water film between a condenser  86  disposed in the laser beam application unit  8  and the wafer  10  held on the holding unit  22 .  FIG. 2  is a perspective view depicting the laser processing machine  2  of  FIG. 1 , in which a water recovery pool  60  that configures a section of the water film-forming unit  4  has been detached from the laser processing machine  2  and a part of the detached section is depicted in an exploded fashion. 
     With reference to  FIG. 2 , the laser processing machine  2  according to the present embodiment will be described further. Inside the horizontal wall section  262  of the frame body  26 , an optical system which will be described in detail subsequently herein is accommodated. The optical system configures the laser beam application unit  8  that applies a pulsed laser beam to the wafer  10  held on the holding unit  22 . On the side of a lower surface of a distal end portion of the horizontal wall section  262 , the condenser  86  that configures a portion of the laser beam application unit  8  is disposed, and an alignment unit  88  is also disposed at a location adjacent to the condenser  86  in an X-axis direction indicated by an arrow X in the figure. The alignment unit  88  is used to perform an alignment between the condenser  86  and a processing position on the wafer  10  by imaging the wafer  10  held on the holding unit  22  and detecting a region to which laser processing is to be applied. 
     The alignment unit  88  includes an imaging device (charge coupled device (CCD)) that uses a visible beam to image a front surface  10   a  (see  FIG. 6A ) of the wafer  10 . Depending on the material that forms the wafer  10 , the alignment unit  88  may preferably include infrared ray application means for applying an infrared ray, an optical system that captures the infrared ray applied by the infrared ray application means and reflected on the front surface  10   a  of the wafer  10 , and an imaging device (infrared CCD) that outputs electrical signals corresponding to the infrared ray captured by the optical system. 
     The holding unit  22  includes a rectangular X-axis direction moving plate  30  mounted on the bed  21  movably in the X-axis direction, a rectangular Y-axis direction moving plate  31  mounted on the X-axis direction moving plate  30  movably in a Y-axis direction that is indicated by an arrow Y and that intersects the X-axis direction at right angles in  FIG. 2 , a cylindrical post  32  fixed on an upper surface of the Y-axis direction moving plate  31 , and a rectangular cover plate  33  fixed on an upper end of the post  32 . On the cover plate  33 , a chuck table  34  is disposed extending upward through an elongated hole formed in the cover plate  33 . The chuck table  34  is configured to hold the wafer  10  and to be rotatable by undepicted rotary drive means. On an upper surface of the chuck table  34 , a suction chuck  35  is disposed. The suction chuck  35  has a circular shape, is formed from a porous material, and extends substantially horizontally. The suction chuck  35  is connected to an undepicted suction source via a flow passage that extends through the post  32 , and four clamps  36  are arranged at equal intervals around the suction chuck  35 . The clamps  36  grip the frame F with the wafer  10  held thereon when fixing the wafer  10  on the chuck table  34 . A plane defined by the X-axis direction and the Y-axis direction forms a substantially horizontal plane. 
     The moving mechanism  23  includes at least an X-axis direction feed mechanism  50  and a Y-axis direction feed mechanism  52 . The X-axis direction feed mechanism  50  is configured to perform relative processing feed of the holding unit  22  and the laser beam application unit  8  in the X-axis direction, and the Y-axis direction feed mechanism  52  is configured to perform relative processing feed of the holding unit  22  and the laser beam application unit  8  in the Y-axis direction. The X-axis direction feed mechanism  50  converts a rotary motion of a motor  50   a  to a linear motion via a ball screw  50   b , and transmits the linear motion to the X-axis direction moving plate  30 , whereby the X-axis direction moving plate  30  is advanced or retracted in the X-axis direction along guide rails  27  on the bed  21 . The Y-axis direction feed mechanism  52  converts a rotary motion of a motor  52   a  to a linear motion via a ball screw  52   b , and transmits the linear motion to the Y-axis direction moving plate  31 , whereby the Y-axis direction moving plate  31  is advanced or retracted in the Y-axis direction along guide rails  37  on the X-axis direction moving plate  30 . Although not depicted in the figures, the chuck table  34 , the X-axis direction feed mechanism  50 , and the Y-axis direction feed mechanism  52  each include position detecting means, and therefore positions of the chuck table  34  in the X-axis and Y-axis directions and an angular position of the chuck table  34  in a peripheral direction (rotation direction) are detected accurately. The X-axis direction feed mechanism  50 , the Y-axis direction feed mechanism  52 , and the undepicted rotary drive means for the chuck table  34  are then driven by a control unit a depiction of which is omitted in the figures, thereby enabling to accurately position the chuck table  34  at desired positions and angle. 
     With reference to  FIGS. 1 to 3B , a description will be made about the water film-forming unit  4 . As depicted in  FIG. 1 , the water film-forming unit  4  includes a water film former  40 , a pump  44 , a filter  45 , the water recovery pool  60 , a pipe  46   a  connecting the water film former  40  and the pump  44  to each other, and a pipe  46   b  connecting the water recovery pool  60  and the filter  45  to each other. Preferably, the pipe  46   a  and the pipe  46   b  may each be formed by a flexible hose in its part or entirety. 
     As depicted in  FIG. 3A , the water film former  40  is disposed on a lower end portion of the condenser  86 . An exploded view of the water film former  40  is presented in  FIG. 3B . As appreciated from  FIG. 3B , the water film former  40  includes a casing  42  and a water supply portion  43 . The casing  42  has a substantially rectangular shape as seen in plan, and is configured of an upper casing member  421  and a lower casing member  422 . 
     The upper casing member  421  is divided into two portions  421   a  and  421   b  in the Y-axis direction indicated by the arrow Y in the figure, the portion  421   a  on the farther side in the figure defines a circular opening  421   c  for insertion of the condenser  86 , and the portion  421   b  on the nearer side in the figure is formed as a plate-shaped portion  421   d . In the lower casing member  422 , a cylindrical opening  422   a  is formed in a region opposite the opening  421   c  of the upper casing member  421 . The cylindrical opening  422   a  has the same shape as that of the opening  421   c , and is coincident in disposed position with the opening  421   c  as seen in plan. The opening  422   a  includes a disc-shaped transparent portion  423  in a bottom portion thereof, and is closed by the transparent portion  423 . The transparent portion  423  has characteristics that allow a pulsed laser beam LB, which will be described subsequently herein, to pass therethrough, and is formed, for example, from a glass plate. 
     In the lower casing member  422 , a water flow channel portion  422   b  is formed in a region opposite the plate-shaped portion  421   d  of the upper casing member  421  to eject liquid (water W in the present embodiment) from a bottom wall  422   d  of the casing  42 . The water flow channel portion  422   b  is a space formed by the plate-shaped portion  421   d  of the upper casing member  421 , side walls  422   c , and the bottom wall  422   d . In the bottom wall  422   d  of the water flow channel portion  422   b , a slit-shaped ejection port  422   e  is formed extending in the X-axis direction, and in a side wall on a side to which the water supply portion  43  is connected, a water supply port  422   f  is formed to supply the water W to the water flow channel portion  422   b . The above-described transparent portion  423  has a lower surface formed in flush with the slit-shaped ejection port  422   e  which extends in a processing feed direction, and the transparent portion  423  forms a portion of the bottom wall  422   d  of the lower casing member  422 . 
     The water supply portion  43  includes a supply port  43   a  through which the water W is supplied, a discharge port (a depiction of which is omitted in the figure) formed at a position opposite the water supply port  422   f  formed in the casing  42 , and a communication channel (a depiction of which is omitted in the figure) which communicates the supply port  43   a  and the discharge port with each other. By assembling the water supply portion  43  to the casing  42  in the Y-axis direction, the water film former  40  is formed. 
     The water film former  40  has such a configuration as described above, and the water W delivered from the pump  44  is supplied to the casing  42  via the water supply portion  43 , and is ejected from the ejection port  422   e  formed in the bottom wall  422   d  of the casing  42 . As depicted in  FIG. 1 , the water film former  40  is attached to the lower end portion of the condenser  86  such that the water supply portion  43  and the casing  42  extend along the Y-axis direction. As a consequence, the ejection port  422   e  formed in the bottom wall  422   d  of the casing  42  is positioned to extend along the X-axis direction. 
     Referring back to  FIG. 2 , a description will be made about the water recovery pool  60 . As depicted in  FIG. 2 , the water recovery pool  60  includes an outer frame member  61  and two water covers  66 . 
     The outer frame member  61  includes a pair of outer side walls  62   a  extending in the X-axis direction, a pair of outer side walls  62   b  extending in the Y-axis direction, pairs of inner side walls  63   a  and  63   b  disposed on inner sides of and in parallel with the outer side walls  62   a  and  62   b  with a predetermined interval from the outer side walls  62   a  and  62   b , and a bottom wall  64  connecting the outer side walls  62   a  and  62   b  and the inner side walls  63   a  and  63   b  together at lower ends thereof. The outer side walls  62   a  and  62   b , the inner side walls  63   a  and  63   b , and the bottom wall  64  form a rectangular water recovery channel  70  which has long sides extending along the X-axis direction and short sides extending along the Y-axis direction. On inner sides of the inner side walls  63   a  and  63   b  that form the water recovery channel  70 , openings are formed extending vertically. The bottom wall  64  which forms the water recovery channel  70  has a slight inclination. At a corner portion located at a lowest position of the water recovery channel  70  (a left corner portion in the figure), a water drain hole  65  is disposed. The pipe  46   b  is connected to the water drain hole  65 , so that the water drain hole  65  is connected to the filter  45  via the pipe  46   b.    
     The two water covers  66  each include a resin-made bellows member  66   b  and fixtures  66   a  that have a flattened square U-shape and that are fixedly secured on opposite ends of the corresponding bellows member  66   b . The fixtures  66   a  are formed with dimensions sufficient to straddle the two inner side walls  63   a  of the outer frame member  61 , the two inner side walls  63   a  being disposed opposing each other in the Y-axis direction. One of the fixtures  66   a  of each bellows member  66   b , specifically, the outer fixture  66   a  as viewed in the X-axis direction is fixed on the inner side wall  63   b  of the outer frame member  61 , the inner side wall  63   b  being disposed opposing the outer fixture  66   a  in the X-axis direction. The water recovery pool  60  configured as described above is fixed on the bed  21  of the laser processing machine  2  by undepicted fixtures. The cover plate  33  of the holding unit  22  is attached such that the cover plate  33  is held between the inner fixtures  66   a  of the two bellows members  66   b . Owing to the above-described configuration, the cover plate  33  moves along the inner side walls  63   a  of the water recovery pool  60  when the cover plate  33  is moved in the X-axis direction by the X-axis direction feed mechanism  50 . 
       FIG. 4A  depicts a block diagram illustrating an outline of the optical system of the laser beam application unit  8 . As illustrated in  FIG. 4A , the laser beam application unit  8  includes a laser oscillator  81 , an attenuator  82 , a spot shaper  83 , a polygon mirror  91 , and the condenser  86 . The laser oscillator  81  emits the pulsed laser beam LB. The attenuator  82  adjusts as needed the output of the pulsed laser beam LB emitted from the laser oscillator  81 . The spot shaper  83  is configured to shape the profile of a spot S of the pulsed laser beam LB emitted from the laser oscillator  81  such that, as illustrated in  FIG. 4B , the spot profile S becomes long in the Y-axis direction and short in the X-axis direction on the holding unit  22 . The polygon mirror  91  functions to disperse the spot S, which has been shaped by the spot shaper  83 , in the X-axis direction on the holding unit  22 . The condenser  86  focuses the pulsed laser beam LB, which has been dispersed by the polygon mirror  91  in the X-axis direction, on the wafer  10  held on the holding unit  22 . 
     The polygon mirror  91  disposed in an upper part of the condenser  86  includes an unillustrated motor that rotates the polygon mirror  91  at a high speed (for example, 10,000 rpm) in a direction indicated by an arrow R 1 . Inside the condenser  86 , a condenser lens (fθ lens)  86   a  is disposed to condense and apply the pulsed laser beam LB to the wafer  10 . As illustrated in the figure, the polygon mirror  91  includes a plurality of mirrors M (18 mirrored facets in the present embodiment) on a side wall surface thereof, and has a polygonal shape as seen in side view. The condenser lens  86   a  is located below the above-described polygon mirror  91 , condenses the pulsed laser beam LB reflected by the mirrors M of the polygon mirror  91  that is rotating in the direction indicated by the arrow R 1 , and applies the condensed pulsed laser beam LB to the wafer  10  on the chuck table  34 . Owing to the rotation of the polygon mirror  91 , the application angle of the pulsed laser beam LB reflected by the mirrors M continuously changes in a predetermined range, so that spot S formed by the pulsed laser beam LB is dispersed in a predetermined range in the X-axis direction indicated by an arrow R 2 . 
     As the spot shaper  83 , a diffractive optical element (DOE) is adopted, for example. By the adoption of the DOE, a diffraction of the pulsed laser beam LB guided from the attenuator  82  is controlled such that, as illustrated in  FIG. 4B , the profile of the spot S formed on the chuck table  34  of the holding unit  22  becomes long in the Y-axis direction and short in the X-axis direction. As the profile of the spot S, the dimensions in the X-axis direction and the Y-axis direction are set, for example, at 10 μm and 50 μm, respectively. It is to be noted that, corresponding to a width dimension (approximately 55 μm) of streets  14  dividing the front surface  10   a  (see  FIG. 5 ) of the wafer  10 , the streets  14  being to be described subsequently herein, this dimension of the length in the Y-axis direction is set as a dimension slightly smaller than the width dimension. In the embodiment described above, the DOE is adopted as the spot shaper  83 . Without being limited to the DOE, however, the present invention can adopt another known technique that can shape the spot profile of the pulsed laser beam LB. Using, for example, a digital micromirror device (DMD), a spatial light modulator (SLM), a cylindrical lens, a mask, a phase plate, or the like as the known technique, the profile of the spot S of the pulsed laser beam LB can be changed to a desired profile. 
     In addition, the laser beam application unit  8  includes unillustrated focal point position adjusting means. Although an illustration of a specific configuration of the focal point position adjusting means in the figure is omitted, the focal point position adjusting means may be configured having, for example, a ball screw which includes nut portions fixed on the condenser  86  and extends in the Z-axis direction indicated by the arrow Z and a motor connected to one end portion of the ball screw. Owing to such a configuration, a rotary motion of the motor is converted to a linear motion, and the condenser  86  is moved along guide rails (an illustration of which is omitted in the figure) disposed in the Z-axis direction, whereby the position in the Z-axis direction of the focal point of the pulsed laser beam LB condensed by the condenser  86  is adjusted. 
     The laser processing machine  2  of the present embodiment has the configuration as generally described above, and its functions and operations will be described hereinafter. 
     When laser processing by the laser processing machine  2  of the present embodiment is performed, the wafer  10  supported on the annular frame F via the adhesive tape T as depicted in  FIG. 5  is provided. The wafer  10  is made of a silicon substrates, and carries a plurality of devices  12  formed in the front surface  10   a  divided by the streets  14 . Over the streets  14  on the front surface  10   a  of the wafer  10 , an insulating layer of low dielectric constant (low-k film) is stacked. After the wafer  10  has been provided, the wafer  10  is placed, with the front surface  10   a  facing upward, on the suction chuck  35  of the above-described chuck table  34 . The wafer  10  is then fixed by the clamps  36 , and at the same time the unillustrated suction source is operated to produce a suction force on the suction chuck  35 , so that the wafer  10  is held under suction. It is to be noted that the chuck table  34 , the suction chuck  35 , and the clamps  36  are omitted in  FIG. 5 . 
     After the wafer  10  has been held on the chuck table  34 , the chuck table  34  is moved by the above-described moving mechanism  23  as needed to position the wafer  10  right below the alignment unit  88 . After the wafer  10  has been positioned right below the alignment unit  88 , the wafer  10  is imaged from above by the alignment unit  88 . Based on an image of the wafer  10  as captured by the alignment unit  88 , an alignment between a processing position (predetermined one of the streets  14 ) on the wafer  10  and the condenser  86  is next performed by a method such as pattern matching. By moving the chuck table  34  on the basis of position information acquired by this alignment, the condenser  86  is positioned together with the water film former  40  above the predetermined street  14  of the wafer  10  as illustrated in  FIG. 5 . Next, the condenser  86  is moved in the Z-axis direction by the unillustrated focal point position adjusting means, whereby the spot S is formed at a surface height of one end portion of the predetermined street  14  which is an application staring point by the pulsed laser beam LB on the wafer  10 .  FIG. 6A  depicts a schematic cross-sectional view taken through the water film former  40  together with the wafer  10  in the Y-axis direction. As appreciated from  FIG. 6A , the water film former  40  of the water film-forming unit  4  is disposed on the lower end portion of the condenser  86 , and a clearance P of, for example, approximately 0.5 to 2.0 mm is formed between the bottom wall  422   d  of the casing  42  which makes up the water film former  40  and the front surface  10   a  of the wafer  10 . 
     After the alignment between the condenser  86  and the wafer  10  has been performed, the water W is replenished to the water film-forming unit  4  as needed and sufficiently via the water recovery channel  70  of the water recovery pool  60 , and the pump  44  is operated. The water W which circulates in the water film-forming unit  4  is, for example, pure water. 
     As the water film-forming unit  4  has the above-described configuration, the water W delivered from a delivery port  44   a  of the pump  44  is supplied to the water film former  40  via the pipe  46   a . The water W supplied to the water film former  40  is ejected downward from the ejection port  422   e  formed in the bottom wall  422   d  of the casing  42  of the water film former  40 . As illustrated in  FIG. 6A , the water W ejected from the ejection port  422   e  forms a layer of the water W while filling up the clearance P formed between the bottom wall  422   d  of the casing  42  and the wafer  10 , specifically, between the transparent portion  423  and the wafer  10 . Thereafter, the water W flows downward, and is recovered in the water recovery pool  60 . The water W recovered in the water recovery pool  60  is guided to the filter  45  by way of the above-described pipe  46   b , is cleaned at the filter  45 , and is returned to the pump  44 . In this manner, the water W delivered by the pump  44  is allowed to circulate in the water film-forming unit  4 . 
     Upon lapse of a predetermined time (several minutes, approximately) after initiation of operation of the water film-forming unit  4 , the clearance P between the bottom wall  422   d  of the casing  42 , specifically, the transparent portion  423  and the wafer  10  is filled up with the water W to form the layer of the water W, thereby creating a state in which the water W stably circulates in the water film-forming unit  4 . 
     With the water W stably circulating in the water film-forming unit  4 , the X-axis direction feed mechanism  50  is operated while operating the laser beam application unit  8 , whereby the chuck table  34  is moved at a predetermined moving speed in the X-axis direction (in a direction indicated by an arrow Xl in  FIG. 5 ) that is the processing feed direction. The pulsed laser beam LB emitted from the condenser  86  passes through the transparent portion  423  and the layer of the water W, and is applied to the predetermined street  14  which is the processing position of the wafer  10 . When the pulsed laser beam LB is applied to the wafer  10  as described above, the pulsed laser beam LB is dispersed in the X-axis direction owing to the above-described rotation of the polygon mirror  91  as illustrated in  FIG. 6B . As a result, in a state in which the spot S is dispersed and the pulsed laser beam LB is applied to the spot S as indicated by an arrow R 3  on the predetermined street  14  along the X-axis direction on the wafer  10 , the wafer  10  is moved in the direction indicated by the arrow Xl. 
     The above-described laser processing conditions for the laser processing machine  2  can be realized, for example, under the following specific processing conditions. 
     Wavelength of pulse laser beam: 355 nm 
     Average output: 11 W 
     Repetition frequency: 2.7 MHz 
     Processing feed rate: 100 mm/s 
     Subsequent to the application of the pulsed laser beam LB to a predetermined one of the mirrors M, the pulsed laser beam LB is applied to a next one of the mirrors M, the next mirror M being located on a downstream side with respect to a rotation direction of the polygon mirror  91  as indicated by the arrow R 1 , so that the pulsed laser beam LB is continuously dispersed and applied to the wafer  10 . While the pulsed laser beam LB is emitted from the laser oscillator  81  and the polygon mirror  91  is rotated as described above, laser processing is performed along the streets  14 . Here, it is to be noted that, in the present embodiment, the profile of the spot S of the pulsed laser beam LB is shaped to be long in the Y-axis direction and short in the X-axis direction on each street  14  of the wafer  10  as described based on  FIGS. 4A and 4B . Especially, in the present embodiment, the spot S is shaped in such a manner as to, corresponding to the width (55 μm) of the streets  14 , have a length of 50 μm in the Y-axis direction. In other words, without needing to apply the pulsed laser beam LB to each single street  14  while relatively and repeatedly moving the wafer  10  held on the holding unit  22  and the laser beam application unit  8  in the X-axis direction, an insulating layer  16  on the street  14  can be efficiently removed in a wide range to form a processed groove  100  by a single stroke of laser processing. 
     After the above-described laser processing has been performed along the predetermined single street, the above-described moving mechanism  23  is operated to position the condenser  86  above one end portion of the next street  14  which is adjacent in the Y-axis direction to the predetermined street  14  that has been already subjected to the laser processing and that extends in a first direction, the next street  14  having not been processed, and laser processing similar to the above-described laser processing is performed, whereby the insulating layer  16  on the next street  14  that extends in the first direction is removed to form another processed groove  100 . After such laser processing has been performed along all the streets  14  extending in the same direction as the streets  14  subjected to the laser processing, the chuck table  34  is rotated by 90 degrees, and similar laser processing is also performed along the unprocessed streets  14  extending in a second direction that intersects at right angles the above-processed streets  14  in the first direction. In the manner as described above, the processed grooves  100  can be formed, with the insulating layer  16  removed, along all the streets  14  on the wafer  10 . 
     After the processed grooves  100  have been formed along all the streets  14  on the wafer  10  as descried above in the present embodiment, the wafer  10  is transferred to a dicing machine a depiction of which is omitted in the figures, where the wafer  10  is cut along the streets  14  by a cutting blade disposed in the dicing machine to singulate the wafer  10  into individual device chips. In the present embodiment, the processed grooves  100  are formed, with the insulating layer  16  on the streets  14  having been efficiently removed, in the wide range (50 μm) corresponding to the width dimension (55 μm) of the streets  14  as described above. Formation of device chips through cutting of the streets  14  of the wafer  10  by a cutting blade of a thickness (for example, 30 μm) smaller than the width dimension of the streets  14  therefore eliminates the problem that peeling of the insulating layer may occur, may spread to the circuit layers of the devices  12 , and may damage the devices  12 . 
     When the above-described laser processing is performed, bubbles occur in the water W that is present at a position of the wafer  10  where the pulsed laser beam LB is applied. However, in the present embodiment, the water W is allowed to always flow at a predetermined flow rate through the clearance P formed over the wafer  10 , as described based on  FIG. 6A . As a consequence, bubbles occurred in a vicinity of an application position of the pulsed laser beam LB are allowed to promptly flow downward together with the water W out of the clearance P formed over the wafer  10 , so that the bubbles are expelled. In particular, according to the present embodiment, the ejection port  422   e  formed in the bottom wall  422   d  of the casing  42  is formed at a position adjacent in the Y-axis direction to the transparent portion  423  also disposed in the bottom wall  422   d , and in a slit shape extending in the processing feed direction. Owing to the configuration described above, the water W is supplied from the Y-axis direction that intersects at right angles the X-axis direction in which the pulse laser beam LB is dispersed, and removes the bubbles occurred at the position where the pulsed laser beam LB is applied. As a consequence, the pulsed laser beam LB can be applied to the wafer  10  without interference by bubbles occurred through laser processing, so that good ablation processing can be continuously performed. 
     In addition, owing to the continuous flow of the water W through the clearance P over the wafer  10 , debris particles occurred and released into the water W through ablation processing are promptly expelled together with bubbles from the front surface  10   a  of the wafer  10 . The water W with the above-described bubbles and debris particles contained therein is guided to the filter  45  via the pipe  46   b , and the filtered water W is again supplied to the pump  44 . Since the water W circulates in the water film-forming unit  4  as described above, debris particles, fine powder, dust, and the like are appropriately captured by the filter  45 , and hence the water W is maintained in a clean state. The laser processing machine  2  of the present embodiment is provided with the water film-forming unit  4  as described above, and hence there is no need to apply a protective tape or a protective film of a water-soluble resin or the like to the front surface  10   a  of the wafer  10 , leading to a further improvement in productivity. 
     In the embodiment described above, with respect to the profile of the spot S formed by the laser beam application unit  8 , the length in the Y-axis direction is set at 50 μm corresponding to the width (55 μm) of the streets  14 . The present invention is however not limited to such a length. When shaping the profile of the spot S by the spot shaper  83  disposed in the laser beam application unit  8 , it is important to shape the spot S such that the spot S is dimensioned to be smaller than the width dimension of the streets  14  and to be greater than the thickness of a cutting blade to be used when the wafer  10  is singulated along the streets  14 . 
     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.