LASER PROCESSING DEVICE

Abstract

Laser processing device (2) comprising: a holding unit (22) configured to hold a plate-shaped workpiece (10); a laser beam application unit (8) configured to apply a laser beam (LB) to the workpiece (10) held by the holding unit (22) in order to process the workpiece (10); and a processing feeder unit configured to bring the holding unit (22) and the laser beam application unit (8) into relative processing feeder alignment, wherein the laser beam application unit (8) comprises a laser oscillator (82) configured to emit a laser beam (LB), a condenser (86) configured to focus the laser beam (LB) emitted by the laser oscillator (82) and thereby apply the laser beam (LB) to the workpiece (10) held by the holding unit (22), and a liquid layer imager (40) arranged and configured at a lower end section of the condenser (86).to form a layer of liquid (W) on a surface of the workpiece (10), the liquid layer images (40), a housing (42) with a bottom wall (422d) that forms a gap (S) between itself and the surface of the workpiece (10), a liquid supply section (43) that is configured to supply the liquid (W) to the housing (42) in order to fill the gap (S) with the liquid (W) through a beam aperture (422e) formed in the bottom wall (422d) in order to form the layer of liquid (W) and to cause the liquid (W) to flow downwards, and having a transparent section (423) formed on the bottom wall (422d) in a direction perpendicular to a processing feed direction adjacent to the beam aperture (422e) and which allows the transmission of the laser beam (LB) through it, and the laser beam (LB) through the transparent section (423) and the layer of liquid (W),which fills the gap (S) and is applied to the workpiece (10).

Description

Technical FieldThe present invention relates to a laser processing apparatus for applying a laser beam to a plate-shaped workpiece to process the workpiece.Description of the Prior ArtA wafer having a plurality of devices such as integrated circuits (IC) and large scale integrations (LSI) formed on a front side divided by division lines (streets) is divided into individual device chips by a laser processing apparatus, and the divided device chips are used for electrical devices such as mobile phones, PCs, and lighting apparatuses.As examples of the laser processing apparatus, there are those of a type in which a laser beam of such a wavelength to be absorbed in a workpiece is applied to the workpiece with a focal point of the laser beam positioned on a front side of the workpiece, grooves serving as starting points of separation are formed by ablation (see, e.g., Japanese Patent Laid-Open No. JP H10-305 420 A discloses a type in which a laser beam of such a wavelength to be transmitted through a workpiece is applied to the workpiece with a focal point of the laser beam positioned in the interior of the workpiece, whereby modified layers serving as starting points of separation are formed in the interior of the workpiece (see, for example, Japanese Patent JP 3 408 805 B2 and a type in which a laser beam of such a wavelength to be transmitted through a workpiece is applied to the workpiece with a focal point of the laser beam positioned at the peripheral part of the workpiece, whereby a plurality of shielding tunnels each having a minute hole extending from a front side to a back side of the workpiece and serving as a starting point of separation, and an amorphous region, which surrounds the minute hole (see, e.g., Japanese Patent Laid-Open No. JP 2014-221 483 A. One of these types of laser processing apparatuses is selected according to the kind of the workpiece, the processing accuracy, and the like.Among the above-mentioned laser processing apparatuses, the ablation type one has a problem in particular in that dirt particles (laser processing chips) generated when the laser beam is applied to the front surface of the wafer may scatter and adhere to the front surfaces of the devices formed on the wafer, thereby degrading the quality of the devices. In order to overcome this problem, a technology has been proposed in which a liquid resin allowing the laser beam used for processing to be transmitted therethrough is applied to the front side of the wafer before laser processing to thereby prevent adhesion of dirt particles, and the liquid resin is removed after laser processing (see, for example, Japanese Patent Laid-Open No. JP 2004-188 475 AUS 2001 / 0 040 152 A1 relates to a method for cutting a semiconductor wafer, wherein a mounting tape is bonded to a back side of the semiconductor wafer, and the mounting tape comprises a resin base and a net-like member made of a material harder than that of the resin base and embedded in the resin base. A liquid jet is ejected toward a front surface of the semiconductor wafer. At the same time, a laser beam is axially transmitted through the liquid jet, so that the semiconductor wafer is cut into individual pieces by the laser beam.JP 2016-36 818 A relates to a laser processing apparatus having a holding means for the object to be processed and a laser beam irradiation means that irradiates the object with a laser beam. The laser beam irradiation means includes a laser beam oscillation means, a condenser including a condenser lens, a cover glass that protects the condenser lens and is disposed at a downstream end part in the laser beam irradiation direction from the condenser, and a water layer formation means that forms a water layer between an upper surface of the object and the cover glass and is disposed at the downstream side in the laser beam irradiation direction of the condenser.JP 2015-85 347 A relates to a laser processing apparatus including a laser beam irradiation means, a processing supply means for moving a chuck table in a processing supply direction, and a placement supply means for moving the chuck table in a placement supply direction orthogonal to the processing supply direction. The laser beam radiating means comprises a laser beam oscillating means, a collector, a polygon mirror means, and an incident position changing means for changing an incident position on the polygon mirror means.SUMMARY OF THE INVENTIONAccording to the method disclosed in Japanese Patent Laid-Open No. JP 2004-188 475 A enables coating with the liquid resin to prevent dirt particles from adhering to the front sides of the structural elements, thereby ensuring the machining quality. However, this technology requires the step of applying the liquid resin and the step of removing the liquid resin after processing, and has a problem in productivity. Further, since the liquid resin cannot be used repeatedly, the technology is uneconomical.A technology has also been proposed in which application of a laser beam is performed with a wafer immersed in water to cause dirt particles to float in the water, thereby preventing adhesion of the dirt particles to the front side of the wafer. However, in the case where the laser beam is applied to the wafer with the wafer immersed in water, minute bubbles are generated from the part of the wafer to which the laser beam is applied, which leads to a problem that the propagation of the laser beam is obstructed by the bubbles and the desired processing cannot be achieved. It is therefore an object of the present invention to provide a laser processing apparatus with which a laser beam can be applied to a plate-shaped workpiece to process the workpiece without hindering application of the laser beam to the workpiece.According to an aspect of the present invention, there is provided a laser processing apparatus including: a holding unit configured to hold a plate-shaped workpiece; a laser beam applying unit configured to apply a laser beam to the workpiece held by the holding unit to process the workpiece; and a processing supplying unit configured to bring the holding unit and the laser beam applying unit into relative processing supply, wherein the laser beam applying unit includes a laser oscillator configured to emit a laser beam, a condenser configured to focus the laser beam emitted from the laser oscillator and thereby apply the laser beam to the workpiece held by the holding unit, and a liquid layer image disposed at a lower end portion of the condenser and configured to form a layer of liquid on a surface of the workpiece, wherein the liquid layer images include a housing having a bottom wall forming a gap therebetween and the surface of the workpiece, a liquid supply portion configured to supply the liquid to the housing to fill the gap with the liquid through a jet hole formed in the bottom wall, thereby forming the layer of the liquid and causing the liquid to flow downward, and a transparent portion formed on the bottom wall in a direction perpendicular to a machining supply direction adjacent to the jet hole and allowing the transmission of the laser beam therethrough, and the laser beam is applied to the workpiece through the transparent portion and the layer of the liquid filling the gap.Preferably, the jet opening has a slit extending in the machining feed direction. Preferably, the laser beam applying unit further includes a scattering means that scatters the laser beam oscillated from the laser oscillator.According to the present invention, the liquid layer image includes: the housing having the bottom wall forming the gap between itself and the surface of the workpiece; the liquid supply portion configured to supply the liquid to the housing to fill the gap with the liquid through the jet hole formed in the bottom wall, thereby forming the layer of the liquid and causing the liquid to flow downward; and the transparent portion formed on the bottom wall adjacent to the jet hole and allowing the transmission of the laser beam therethrough. Further, the laser beam is applied to the workpiece through the transparent portion and the layer of the liquid filling the gap. Accordingly, there is provided a laser processing apparatus in which the application of the laser beam to the workpiece is not inhibited. Further, in the case where the present invention is applied to a laser processing apparatus for performing ablation, adhesion of dirt particles generated at the time of laser processing to devices can be suppressed, and the processing quality of the devices is prevented from decreasing without coating the front surface of the wafer with a liquid resin.The foregoing and other objects, features and advantages of the present invention and the manner in which they are practiced will become more apparent, and the invention itself will be best understood from a study of the following description and the appended claims with reference to the accompanying drawings which show a preferred embodiment of the invention.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a perspective view of a laser processing apparatus according to an embodiment of the present invention; FIG. 2 is an exploded perspective view illustrating the laser processing apparatus illustrated in FIG. 1 with a part in a disassembled state; FIG. 3A is a perspective view of a liquid layer image mounted on the laser processing apparatus illustrated in FIG. 1 ; FIG. 3B is an exploded perspective view of the liquid layer image; FIG. 4 is a block diagram for explaining an optical system of a laser beam applying unit mounted on the laser processing apparatus illustrated in FIG. 1 ; and FIG. 5 is a partially enlarged sectional view illustrating an operation state of the liquid layer image at the time of processing mounted on the laser processing apparatus illustrated in FIG. 1.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTA laser processing apparatus according to an embodiment of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 illustrates a perspective view of a laser processing apparatus 2 according to the present embodiment. The laser processing apparatus 2 includes: a holding unit 22 disposed on a base 21 and configured to hold a plate-shaped workpiece (e.g., a wafer 10 made of silicon); a moving mechanism 23 configured to move the holding unit 22; a frame body 26 formed of a vertical wall portion 261 erected on a lateral side of the moving mechanism 23 on the base 21 in a Z direction indicated by the arrow Z and a horizontal wall portion 262 extending from an upper end portion of the vertical wall portion 261 in a horizontal direction; a liquid supply mechanism 4; and a laser beam applying unit 8. Note that the laser processing apparatus 2 as a whole is covered by a housing or the like that is omitted for convenience of explanation, so that dust and the like cannot enter the interior of the laser processing apparatus 2.FIG. 2 is a perspective view of the laser processing apparatus 2 illustrated in FIG. 1, illustrating a state in which a liquid recovery pool 60 constituting a part of the liquid supply mechanism 4 is detached and disassembled from the laser processing apparatus 2.Referring to FIG. 2, the laser processing apparatus 2 according to the present embodiment will be described in detail. Inside the horizontal wall portion 262 of the frame body 26, an optical system constituting the laser beam applying unit 8 configured to apply a laser beam to the wafer 10 held by the holding unit 22 is housed. On the lower surface side of a tip portion of the horizontal wall portion 262, a condenser 86 forming part of the laser beam applying unit 8 is disposed, and an aligning unit 88 is disposed at a position adjacent to the condenser 86 in a direction indicated by the arrow X in the figure. The alignment unit 88 is used for imaging the workpiece held by the holding unit 22 to detect an area to be laser processed, and for performing alignment between the condenser 86 and a processing position of the workpiece.The alignment unit 88 includes a charge-coupled device (CCD)) that uses visible rays to image the front side of the wafer 10. However, depending on the material forming the wafer 10, the alignment unit 88 preferably includes infrared (IR) radiation application means that apply IR beams, an optical system configured to detect the IR beams applied by the IR radiation application means, and an imaging element (IR-CCD) configured to output an electrical signal corresponding to the IR beams detected by the optical system.The holding unit 22 includes: a rectangular X-direction movable plate 30 mounted on the base 21 so as to be movable in the X-direction indicated by arrow X in FIG. 2 ; a rectangular Y-direction movable plate 31 mounted on the X-direction movable plate 30 so as to be movable in a Y-direction indicated by arrow Y in FIG. 2 ; a cylindrical support column 32 fixed to a surface of the Y-direction movable plate 31; and a rectangular cover plate 33 fixed to an upper end of the support column 32. On the cover plate 33, a chuck table 34 is disposed which extends upward through a recess formed above the cover plate 33. The chuck table 34 holds the circular workpiece and is rotatable by a rotational driving means (not shown). On one surface of the chuck table 34, there is disposed a circular suction chuck table 35 formed of a porous material and extending substantially horizontally. The suction chuck table 35 is connected to a suction means (not shown) via a flow passage extending through the support column 32, and four clamps 36 are uniformly arranged in the periphery of the suction chuck table 35. The clamps 36 grip the frame F holding the wafer 10 at the time of mounting the wafer 10 on the chuck table 34. the X direction is the direction indicated by arrow X in FIG. 2, and the Y direction is the direction indicated by arrow Y and is orthogonal to the X direction. A plane defined by the X direction and the Y direction is substantially horizontal.The moving mechanism 23 includes an X-direction moving unit 50 and a Y-direction moving unit 52. the X-direction moving unit 50 converts a rotational movement of a motor 50 ainto a rectilinear movement, and transmits the rectilinear movement to the X-direction movable plate 30 through a ball screw 50 b, thereby causing the X-direction movable plate 30 to advance and retract in the X-direction along guide rails 27 on the base 21. The Y-direction moving unit 52 converts a rotational motion of a motor 50 ainto a rectilinear motion and transmits the rectilinear motion to the Y-direction movable plate 31 through a ball screw 52 b, thereby causing the Y-direction movable plate 31 to advance and retract in the Y-direction along guide rails 27 on the X-direction movable plate. Note that, although illustration is omitted, the X-direction moving unit 50 and the Y-direction moving unit 52 are each provided with position detecting means, whereby it is possible to accurately detect the X-direction position, the Y-direction position, and the circumferential rotation position of the chuck table 34, and to accurately position the chuck table 34 at an arbitrary position and an arbitrary angle by driving the X-direction moving unit 50, the Y-direction moving unit 52, and the rotation driving means (not illustrated). The X-direction moving unit 50 is a machining feed unit configured to move the holding unit 22 in a machining feed direction, and the Y-direction moving unit 52 is an indexing feed unit configured to move the holding unit 22 in an indexing feed direction.Referring to Figs. 1 to 3B, the liquid supply mechanism 4 will be described. As illustrated in FIG. 1, the liquid supply mechanism 4 includes a liquid layer former 40, a liquid supply pump 44, a filter 45, the liquid recovery basin 60, a pipe 46 athat connects the liquid layer former 40 and the liquid supply pump 44, and a pipe 46 bthat connects the liquid recovery basin 60 and the filter 45. It should be noted that each of the conduits 46a and 46b is preferably partially or completely comprised of a flexible tube.As illustrated in FIG. 3A, the liquid layer image 40 is disposed at a lower end portion of the condenser 86. An exploded view of the liquid layer image 40 is shown in FIG. 3B. As shown in FIG. 3B, the liquid film forming apparatus 40 includes a housing 42 and a liquid supply portion 43. the housing 42 is substantially rectangular in plan view and is composed of an upper housing member 421 and a lower housing member 422.The upper case 421 divided into two regions 421 aand 421 bin a Y direction indicated by the arrow Y in the figure is formed with a circular opening 421 cto fit the condenser 86 in the depth side region 421 ain the figure, and is formed with a plate-shaped cross section 421 din the viewer side region 421 b. The lower case 422 is formed, in a region facing the opening 421 cof the upper case 421, with a cylindrical opening 422 awhich has the same shape as the opening 421 cand is disposed at a position in alignment with the position of the opening 421 cin plan view. At a lower portion of the opening 422a, a circular plate-shaped transparent portion 423 closing the lower portion of the opening 422a is provided. The transparent portion 423 has such a property as to allow transmission of a laser beam LB described later therethrough, and is made of, for example, a glass plate. The housing lower 422 is formed, in a region facing the plate-shaped portion 421 dof the upper housing member 421 d, with a liquid passage portion 422 bfor ejecting a liquid from a bottom wall 422 dof the housing 42. The liquid passage portion 422 bis a space defined by the plate-shaped portion 421 dof the upper case 421, the side walls 422 c, and the bottom wall 422 d. The bottom wall 422 dof the liquid passage portion 422 bis formed with a slit-shaped jet hole 422 eextending in a machining feed direction indicated by arrow X, and a side surface on the side for connection to the liquid supply portion 43 is formed with a liquid supply hole 422 ffor supplying a liquid to the liquid passage portion 422 b. A bottom surface of the transparent portion 423 is formed to be flush with the slit-shaped jet hole 422 eextending in the machining feed direction, and the transparent portion 423 forms a part of the bottom wall 422 dof the lower case member 422.The liquid supply portion 43 includes a supply port 43 athat is supplied with a liquid W, an outlet port (omitted in the figure) formed at such a position as to face the liquid supply port 422 fformed in the housing 42, and a communication channel (omitted in the figure) for providing communication between the supply port 43 aand the outlet port. The liquid supply portion 43 is mounted on the housing 42 in the Y direction, thereby forming the liquid layer image 40.The liquid layer image 40 is configured as described above. The liquid W discharged from the liquid supply pump 44 is supplied to the liquid supply port 422 fof the housing 42 through the supply port 43 aof the liquid supply portion 43, the communication channel in the inside of the liquid supply portion 43, and the discharge port, then flows through the liquid passage portion 422 bof the housing 42, and is discharged from the jet port 422 eformed in the bottom wall 422 d. As illustrated in FIG. 1, the liquid layer image 40 includes the liquid supply portion 43 and the housing 42 fixed to a lower end portion of the condenser 86 so as to be aligned in the Y direction. Accordingly, the jet hole 422 eformed in the bottom wall 422 dof the housing 42 is positioned to extend along an X direction, which is the machining feed direction.Referring again to Figs. 1 and 2, the liquid recovery basin 60 will be described. As shown in FIG. 2, the liquid recovery basin 60 includes an outer frame body 61 and two waterproof covers 66.The outer frame body 61 includes: outer walls 62 aextending in the X direction indicated by arrow X in the figure; outer walls 62 bextending in the Y direction indicated by arrow Y in the figure; inner walls 63 aand 63 bdisposed parallel to the outer walls 62 aand 62 bon the inner sides of the outer walls 62 aand 62 bwith a predetermined interval therebetween; and a bottom wall 64 connecting lower ends of the outer walls 62 aand 62 band the inner walls 63 aand 63 b. The outer walls 62a and 62b, the inner walls 63a and 63b, and the bottom wall 64 define a rectangular liquid recovery channel 70 whose longitudinal direction is in the X direction and whose transverse direction is in the Y direction. On the inside of the inner walls 63a and 63b constituting the liquid recovering passage 70, an opening is formed penetrating in the vertical direction. The bottom wall 64 constituting the liquid recovery passage 70 is provided with slight slopes in the X and Y directions, and a liquid discharge port 65 is disposed at a corner portion (a corner portion on the left side in the figure) corresponding to a lowermost position of the liquid recovery passage 70. The conduit 46b is connected to the liquid discharge port 65, and the liquid discharge port 65 is connected to the filter 45 through the conduit 46b. Note that the outer frame body 61 is preferably formed of a stainless steel sheet material that is highly corrosion- and rust-resistant.Each of the two waterproof covers 66 includes gate-shaped metallic devices 66 aand a cover member 66 bmade of resin, which has a bellows-like shape and is fixed to the metallic devices 66 aat both ends. The metallic devices 66 aare formed in such a size that they can bridge the two inner walls 63 adisposed facing each other in the Y direction of the outer frame body 61. One of the metallic devices 66 aof each of the two waterproof covers 66 is fixed to each of the inner walls 63 adisposed facing each other in the X direction of the outer frame body 61. The liquid recovery pool 60 thus configured is fixed to the base 21 of the laser processing apparatus 2 by devices (not illustrated). The cover plate 33 of the holding unit 22 is mounted so as to be clamped between the metal devices 66 aof the two waterproof covers 66. Note that the end surfaces with respect to the X direction of the cover member 33 have the same gate shape as the metallic devices 66 aand are sized to straddle the inner walls 63 aof the outer frame body 61 in the Y direction as the metallic devices 66 a; therefore, the cover member 33 is fixed to the waterproof covers 66 after the outer frame body 61 of the liquid recovery basin 60 is disposed on the base 21. According to the above configuration, when the cover plate 33 is moved in the X direction by the X-direction moving mechanism 50, the cover plate 33 is moved along the inner walls 63 aof the liquid recovery basin 60. Note that the method of fixing the waterproof covers 66 and the cover member 33 is not limited to the above-mentioned method; for example, a method may be adopted in which the cover member 33 is previously fixed to the inner walls 63 bof the outer frame body 61 before fixing the two waterproof covers 66, and the waterproof covers 66 are fixed to the outer frame body 61 previously mounted to the base 21 together with the cover member 33.Referring back to FIG. 1 for the continuation of the description, when the liquid supply mechanism 4 is configured as described above, the liquid W discharged from the discharge port 44 aof the liquid supply pump 44 is supplied to the liquid layer image 40 via the conduit 46 a. The liquid W supplied to the liquid layer image 40 is ejected downward through the jet opening 422 eformed in the bottom wall 422 dof the housing 42 of the liquid layer image 40. The liquid W ejected from the liquid layer image 40 flows on the cover plate 33 or the waterproof covers 66 and flows into the liquid recovery basin 60, and the liquid W that has flowed down into the liquid recovery basin 60 flows through the liquid recovery channel 70 to be collected into the liquid discharge port 65 provided at the lowermost position of the liquid recovery channel 70. The liquid W collected in the liquid discharge port 65 is guided through the pipe 46 bto the filter 45 where laser processing waste (dirt particles) and dust and the like are removed, and the liquid W is returned to the liquid supply pump 44. In this way, the liquid W discharged from the liquid supply pump 44 is circulated in the liquid supply mechanism 4.FIG. 4 is a block diagram generally illustrating the optical system of the laser beam applying unit 8. As illustrated in FIG. 4, the laser beam applying unit 8 includes: a laser oscillator 82 configured to emit a pulsed laser beam LB; an attenuator (omitted in the figure) configured to control the output of the laser beam LB emitted from the laser oscillator 82; a reflecting mirror (omitted in the figure) configured to appropriately change the optical path of the laser beam LB emitted from the laser oscillator 82; a polygonal mirror 91 as a scattering means that scatters the application direction of the laser beam LB; and the condenser 86. the laser oscillator 82, for example, oscillates a laser beam having such a wavelength as to be absorbed in the workpiece.The polygon mirror 91 disposed at an upper portion of the condenser 86 is provided with a motor (not shown) for rotating the polygon mirror 91 at high speed in a direction indicated by the arrow R. Inside the condenser 86, a focusing lens (fθ lens) 86 ais disposed, which is configured to focus the laser beam LB and thereby apply the laser beam LB to the workpiece. As shown in the figure, the polygon mirror 91 has a plurality of mirrors M concentrically arranged with respect to a rotation axis of the polygon mirror 91. The fθ lens 86a of is positioned below the polygon mirror 91 and serves to focus the laser beam LB reflected from the polygon mirror 91 and apply the laser beam LB to the wafer 10 on the chuck table 34. With the polygonal mirror 91 rotated, the angle of the laser beam LB reflected by the mirrors M is varied within a predetermined range, and the laser beam LB is applied while being dispersed within a predetermined range in the machining feed direction (X direction) on the wafer 10.Further, the laser beam applying unit 8 includes a focus position control means (not shown). While the illustration of the specific configuration of the focal point position control means is omitted, for example, a configuration may be adopted that includes a ball screw extending in the Z direction indicated by arrow Z and having a nut portion fixed to the condenser 86, and a motor connected to an end portion of the ball screw. According to such a configuration, a rotational motion of the motor is converted into a rectilinear motion and the condenser 86 is moved along guide rails (omitted from the figure) arranged in the Z direction, thereby controlling the position of the focal point of the laser beam LB focused by the condenser 86 in the Z direction indicated by the arrow Z.The laser processing apparatus 2 of the present invention is configured generally as described above, and an operation thereof will be described below. In performing laser processing by the laser processing apparatus 2 in the present embodiment, the plate-shaped workpiece, for example, the wafer 10 formed of silicon (Si) and formed with devices on a front side thereof, supported on the annular frame F by the adhesive tape T is prepared. After the wafer 10 is prepared, the wafer 10 is placed on the suction chuck table 35 of the chuck table 34 illustrated in FIG. 1 in a state where the front side thereof formed with the devices is on the upper side, and the wafer 10 is fixed by the clamps 36 or the like. After the wafer 10 is mounted on the suction chuck table 35, a suction source (not shown) is operated to generate a suction force on the suction chuck table 35, with the wafer 10 being held by suction.After the wafer 10 is held by the suction chuck table 35, the chuck table 34 is moved appropriately in the X direction and the Y direction by the moving mechanism 23, and the wafer 10 is thereby positioned on the chuck table 34 at a position below the alignment unit 88. After the wafer 10 is positioned below the alignment unit 88, a surface of the wafer 10 is imaged by the alignment unit 88. Subsequently, based on the image of the wafer 10 captured by the alignment unit 88, alignment between the wafer 10 and the condenser 86 is performed by a technique such as pattern matching. Based on the position information obtained by the alignment, the chuck table 34 is moved, whereby the condenser 86 is positioned at a position on the upper side of a machining start position on the wafer 10. Subsequently, the condenser 86 is moved in the Z direction by the focal point position control means (not illustrated), whereby the focal point is positioned at an area level of an end portion of the dicing line, which is a start position for applying the laser beam LB to the wafer 10. FIG. 5 illustrates a schematic sectional view of the liquid layer image 40 cut in the Y direction. As illustrated in FIG. 5, the liquid film 40 of the liquid supply mechanism 4 is disposed at the lower end portion of the condenser 86, and setting is made such that, when the focal point is positioned at the height of the front side of the wafer 10, for example, a gap S of about 0.5 to 2.0 mm is formed between the bottom wall 422 dof the case 42 constituting the liquid film 40 and the front side of the wafer 10.After the alignment is performed between the condenser 86 and the wafer 10, a sufficient amount of the liquid W is introduced into the liquid supply mechanism 4 through the liquid recovery passage 70 of the liquid recovery basin 60, and the liquid supply pump 44 is operated. As the liquid W circulating inside the liquid supply mechanism 4, pure water is used, for example.When the liquid supply mechanism 4 is configured as described above, the liquid W discharged from the discharge port 44 aof the liquid supply pump 44 is supplied to the liquid layer image 40 through the pipe 46 a. The liquid W supplied to the liquid layer image 40 is ejected downward through the jet opening 422 eformed in the bottom wall 422 dof the housing 42 of the liquid layer image 40. The liquid W ejected from the jet opening 422 eform a layer of the liquid W, and fills the gap S formed between the bottom wall 422 dof the case 42 and the wafer 10, specifically, between the transparent portion 423 and the wafer 10, then flows downward, and is recovered by the liquid recovery basin 60. The liquid W that has flowed downward in the liquid recovery basin 60 flows through the liquid recovery channel 70 to be collected into the liquid discharge port 65 provided at the lowermost position of the liquid recovery channel 70. The liquid W collected in the liquid discharge port 65 is guided through the conduit 46b to the filter 45 where the liquid W is clarified and returned to the liquid supply pump 44. In this way, the liquid W discharged from the liquid supply pump 44 is circulated in the liquid supply mechanism 4.With the lapse of a predetermined time (about several minutes) after the start of an operation of the liquid supply mechanism 4, there is a state in which the gap S between the bottom wall 422 dof the case 42, specifically, the transparent portion 423 and the wafer 10 is filled with the liquid W, a layer of the liquid W is formed, and the liquid W is stably circulated in the liquid supply mechanism 4.In a state where the liquid W is stably circulated in the liquid supply mechanism 4, the X-direction moving mechanism 50 is operated to move the chuck table 34 at a predetermined moving speed in the machining feed direction (X direction) while the laser beam applying unit 8 is operated. The laser beam LB applied from the condenser 86 is transmitted through the transparent portion 423 of the liquid layer image 40 and the layer of the liquid W to be applied to the processed position (division line) of the wafer 10. When applying the laser beam LB to the wafer 10, the laser beam LB is applied to the wafer 10 in a diffused manner in association with the rotation of the polygon mirror 91 as described based on FIG. 4. After the laser beam LB is applied to a predetermined mirror M, the laser beam LB is applied to the next mirror M on the downstream side with respect to the rotational direction R of the polygon mirror 91, and the laser beam LB is continuously and dispersedly applied to the wafer 10. When the laser beam LB is oscillated from the laser oscillator 82 and the polygon mirror 91 is rotated, this laser processing is repeated. Note that the number of plates of the mirrors M constituting the polygon mirror 91, the rotational speed of the polygon mirror 91, and are appropriately determined according to the workpiece.Note that the laser processing by the above-mentioned laser processing apparatus 2 can be performed under the following processing conditions, for example.Wavelength of the laser beam: 226 nm, 355 nm, 532 nm, 1064 nmAverage power: 10 to 100 WRepetition frequency: 0 to 300 MHzPulse width: 50 fs to 1 nsMachining feed speed: 10 to 1,000 mm / sWhen ablation is performed in the above state, air bubbles are generated in the liquid W at the location on the wafer 10 where the laser beam LB is applied. In this connection, in the present embodiment, as described based on FIG. 5, the liquid W is caused to constantly flow through the gap formed on the wafer 10 at a predetermined flow speed. As a result, the air bubbles generated in the vicinity of the application position of the laser beam LB are rapidly discharged to the outside through the gap formed on the wafer 10. Specifically, according to the present embodiment, the jet hole 422 eformed in the bottom wall 422 dof the housing 42 is disposed at a position adjacent to the transparent portion 423 formed similarly on the bottom wall in the Y direction, and is formed in a slit shape extending in the machining feed direction. This configuration ensures that the liquid W is supplied from a direction orthogonal to the X direction or the direction in which the laser beam LB is diffused, and the air bubbles generated in the liquid W are removed. As a result, the laser beam LB can be applied to the wafer 10 while avoiding the air bubbles generated by ablation, and favorable ablation can be performed continuously.Further, since the liquid W continuously flows through the gap on the wafer 10, the dirt particles released into the liquid W are quickly removed from the area on the wafer 10 together with the air bubbles. As shown in FIG. 1, the liquid W including the air bubbles and the dirt particles flows on the cover plate 33 and the waterproof covers 66 to be guided into the liquid recovery channel 70 of the liquid recovery basin 60. The liquid W introduced into the liquid recovery channel 70 flows through the liquid recovery channel 70 while releasing the air bubbles generated by ablation to the outside, and is discharged through the liquid discharge port 65 formed at the lowermost position of the liquid recovery channel 70. The liquid W discharged through the liquid discharge port 65 is guided to the filter 45 through the pipe 46 band is supplied again to the liquid supply pump 44. The liquid W is thus circulated in the liquid supply mechanism 4, dirt particles, dust and the like are captured by the filter 45 accordingly, and the liquid W is maintained in a clean state.After the ablation is performed for the predetermined division line extending in a first direction, the moving mechanism 23 is operated to position the condenser 86 at an end portion of an unprocessed division line adjacent to the just-previously laser-processed division line in the Y direction, and the same laser processing as the above ablation is performed. After the ablation is performed for all the division lines extending in the first direction, the chuck table 34 is rotated 90 degrees, and the similar ablation is performed for the unprocessed division lines orthogonal to the previously processed division lines extending in the first direction. In this way, ablation can be performed for all division lines on the wafer 10.According to the present embodiment, the laser beam LB is applied to the wafer 10 through the transparent portion 423 disposed in the liquid layer image 40 and the layer of the liquid W to perform the laser processing, and the air bubbles generated from the front side of the wafer 10 and the dirt particles and the like generated by the laser processing are quickly removed together with the liquid W. Accordingly, the air bubbles generated from the front surface of the wafer 10 are prevented from hindering laser processing, adhesion of dirt particles to the devices after processing and the like are prevented, and processing quality is prevented from being deteriorated.Although the transparent portion 423 is formed of a glass plate in the above embodiment, this is not restrictive. The transparent portion 423 only needs to be a transparent plate through which the laser beam LB is transmitted, and may be, for example, a plate made of resin such as an acrylic resin plate.While the laser beam LB applied from the laser oscillator 82 has been guided to the focusing lens 86 while being diffused by the polygon mirror 91 in the above embodiment, this configuration is not restrictive. One or more mirrors fixed in a reflection direction may be used instead of the polygon mirror 91. Further, while an example in which the laser processing applied to the wafer 10 is ablation has been illustrated in the above embodiment, the laser processing may include processing for forming modified layers inside a workpiece (e.g., the laser processing described in Japanese Patent JP 3 408 805 B2) or processing for forming so-called shield tunnels (e.g., the laser processing described in Japanese Patent Laid-Open No. JP 2014-221 483 A).

Claims

A laser processing apparatus (2) comprising: a holding unit (22) configured to hold a plate-shaped workpiece (10); a laser beam applying unit (8) configured to apply a laser beam (LB) to the workpiece (10) held by the holding unit (22) to process the workpiece (10); and a processing supply unit configured to bring the holding unit (22) and the laser beam applying unit (8) into relative processing supply, wherein the laser beam applying unit (8) includes a laser oscillator (82) configured to emit a laser beam (LB), a condenser (86) configured to focus the laser beam (LB) emitted by the laser oscillator (82) and thereby apply the laser beam (LB) to the workpiece (10) held by the holding unit (22), and a liquid layer image (40), which is disposed at a lower end portion of the condenser (86) and is configured to form a layer of a liquid (W) on a surface of the workpiece (10), the liquid layer images (40) include a housing (42) having a bottom wall (422d) forming a gap (S) between itself and the surface of the workpiece (10), a liquid supply portion (43) configured to supply the liquid (W) to the housing (42) to fill the gap (S) with the liquid (W) through a jet opening (422e) formed in the bottom wall (422d), thereby forming the layer of the liquid (W) and causing the liquid (W) to flow downward, and a transparent portion (423), the laser beam (LB) is formed on the bottom wall (422d) in a direction perpendicular to a machining feed direction adjacent to the beam opening (422e) and enabling the transmission of the laser beam (LB) therethrough, and the laser beam (LB) is applied to the workpiece (10) through the transparent portion (423) and the layer of the liquid (W) filling the gap (S).The laser processing apparatus (2) according to claim 1, wherein the jet opening (422e) has a slit extending in the processing feeding direction.The laser processing apparatus (2) according to claim 1 or 2, wherein the laser beam applying unit (8) further includes a scattering means (91) that scatters the laser beam (LB) in the processing feeding direction.

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