Wafer laser processing method

A wafer laser processing method for forming a groove along streets in a wafer by moving the wafer at a predetermined feed rate while a laser beam whose focal spot is elliptic is applied along the streets formed on the wafer, comprising: a groove forming step for forming a groove along the streets by applying a first laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 30 to 60:1, along the streets formed on the wafer; and a debris removing step for removing debris accumulated in the groove by applying a second laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 1 to 20:1, along the groove formed by the groove forming step; the groove forming step and the debris removing step being repeated alternately.

FIELD OF THE INVENTION

The present invention relates to a wafer laser processing method for forming a groove in a wafer such as an optical device wafer or the like.

DESCRIPTION OF THE PRIOR ART

In the production process of a semiconductor device, a plurality of areas are sectioned by dividing lines called “streets” arranged in a lattice pattern on the front surface of a substantially disk-like silicon substrate, and a device such as IC or LSI is formed in each of the sectioned areas. Individual semiconductor chips are manufactured by cutting this semiconductor wafer along the streets to divide it into the areas having a device formed therein. An optical device wafer having light receiving elements such as photodiodes or light emitting elements such as laser diodes laminated on the front surface of a sapphire or silicon carbide substrate is also cut along streets to be divided into individual optical devices such as photodiodes or laser diodes which are widely used in electric appliances.

Although the wafer can be easily cut from the silicon substrate into individual chips by a cutting machine, it is difficult to cut a wafer comprising a substrate made of a material hard to cut such as sapphire, silicon carbide, gallium arsenide or lithium tantalate by the cutting machine. As a means of dividing a wafer comprising a substrate made of the above materials hard to cut, along streets, JP-A 2000-156358 discloses a method in which a groove is formed by applying a pulse laser beam of a wavelength having absorptively for the wafer along the streets formed on the wafer and the wafer is divided along the grooves.

In the method in which the laser beam is applied along the streets formed on the wafer, however, there arises a problem in that when a groove is formed by applying a laser beam to a wafer having a thickness of 300 μm or more, scattered debris accumulates in the grooves to prevent processing by means of the laser beam. According to experiments conducted by the inventors of the present invention, when a pulse laser beam was applied along the streets of a wafer having a thickness of 350 μm 400 times under the conditions of a wavelength of 355 nm, a repetition frequency of 10 kHm, an average output of 7 W and a long axis (D1) of the focal spot of 100 μm, a short axis (D2) of 10 μm and a processing-feed rate of 50 mm/sec, a groove having a depth required for division could not be formed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wafer laser processing method capable of forming a groove having a depth required for division even when the thickness of the wafer is 300 μm or more.

To attain the above object, according to the present invention, there is provided a wafer laser processing method for forming a groove along streets in a wafer by moving the wafer at a predetermined processing-feed rate while a laser beam whose focal spot is elliptic is applied along the streets formed on the wafer, comprising:

a groove forming step for forming a groove along the streets by applying a first laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 30 to 60:1, along the streets formed on the wafer; and

a debris removing step for removing debris accumulated in the groove by applying a second laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 1 to 20:1, along the groove formed by the groove forming step; the groove forming step and the debris removing step being repeated alternately.

The long axis of the elliptic focal spot of the first laser beam is set to 300 to 600 μm, and the long axis of the elliptic focal spot of the second laser beam is set to 10 to 200 μm.

In the present invention, the groove forming step is carried out with the first laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 30 to 60:1 suitable for the formation of a groove, the debris removing step is carried out with the second laser beam whose elliptic focal spot has a ratio of the long axis to the short axis of 1 to 20:1, and the groove forming step and the debris removing step are repeated alternately. Therefore, even when the wafer is thick, a groove having a required depth can be formed without being influenced by the debris.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in detail hereinunder with reference to the accompanying drawings.

FIG. 1is a perspective view of a laser beam processing machine for carrying out the wafer laser processing method according to the present invention. The laser beam processing machine1shown inFIG. 1comprises a stationary base2, a chuck table mechanism3for holding a workpiece, which is mounted on the stationary base2in such a manner that it can move in a processing-feed direction indicated by an arrow X, a laser beam application unit support mechanism4mounted on the stationary base2in 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 unit5mounted on the laser beam application unit support mechanism4in such a manner that it can move in a direction indicated by an arrow Z.

The above chuck table mechanism3comprises a pair of guide rails31and31which are mounted on the stationary base2and arranged parallel to each other in the processing-feed direction indicated by the arrow X, a first sliding block32mounted on the guide rails31and31in such a manner that it can move in the processing-feed direction indicated by the arrow X, a second sliding block33mounted on the first sliding block32in such a manner that it can move in the indexing-feed direction indicated by the arrow Y, a cover table35supported on the second sliding block33by a cylindrical member34, and a chuck table36as a workpiece holding means. This chuck table36comprises an adsorption chuck361made of a porous material, and a workpiece, for example, a disk-like semiconductor wafer is held on the adsorption chuck361by a suction means that is not shown. The chuck table36constituted as described above is rotated by a pulse motor (not shown) installed in the cylindrical member34. The chuck table36is provided with clamps362for fixing an annular frame which will be described later.

The above first sliding block32has, on its undersurface, a pair of to-be-guided grooves321and321to be fitted to the above pair of guide rails31and31and, on the top surface, a pair of guide rails322and322formed parallel to each other in the indexing-feed direction indicated by the arrow Y. The first sliding block32constituted as described above can move along the pair of guide rails31and31in the processing-feed direction indicated by the arrow X by fitting the to-be-guided grooves321and321to the pair of guide rails31and31, respectively. The chuck table mechanism3in the illustrated embodiment comprises a processing-feed means37for moving the first sliding block32along the pair of guide rails31and31in the processing-feed direction indicated by the arrow X. The processing-feed means37comprises a male screw rod371that is arranged between the above pair of guide rails31and31parallel thereto, and a drive source such as a pulse motor372for rotary-driving the male screw rod371. The male screw rod371is, at its one end, rotatably supported to a bearing block373fixed on the above stationary base2and is, at the other end, transmission-coupled to the output shaft of the above pulse motor372. The male screw rod371is 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 block32. Therefore, by driving the male screw rod371in a normal direction or reverse direction with the pulse motor372, the first sliding block32is moved along the guide rails31and31in the processing-feed direction indicated by the arrow X.

The above second sliding block33has, on its undersurface, a pair of to-be-guided grooves331and331to be fitted to the pair of guide rails322and322on the top surface of the above first sliding block32and can move in the indexing-feed direction indicated by the arrow Y by fitting the to-be-guided grooves331and331to the pair of guide rails322and322, respectively. The chuck table mechanism3in the illustrated embodiment comprises a first indexing-feed means38for moving the second sliding block33along the pair of guide rails322and322provided on the first sliding block32in the indexing-feed direction indicated by the arrow Y. The first indexing-feed means38has a male screw rod381which is arranged between the above pair of guide rails322and322parallel thereto, and a drive source such as a pulse motor382for rotary-driving the male screw rod381. The male screw rod381is, at its one end, rotatably supported to a bearing block383fixed on the top surface of the above first sliding block32and is, at the other end, transmission-coupled to the output shaft of the above pulse motor382. The male screw rod381is 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 block33. Therefore, by driving the male screw rod381in a normal direction or reverse direction with the pulse motor382, the second sliding block33is moved along the guide rails322and322in the indexing-feed direction indicated by the arrow Y.

The above laser beam application unit support mechanism4comprises a pair of guide rails41and41mounted on the stationary base2and arranged parallel to each other in the indexing-feed direction indicated by the arrow Y and a movable support base42mounted on the guide rails41and41in such a manner that it can move in the direction indicated by the arrow Y. This movable support base42consists of a movable support portion421movably mounted on the guide rails41and41and a mounting portion422mounted on the movable support portion421. The mounting portion422is provided with a pair of guide rails423and423extending parallel to each other in the direction indicated by the arrow Z on one of its flanks. The laser beam application unit support mechanism4in the illustrated embodiment comprises a second indexing-feed means43for moving the movable support base42along the pair of guide rails41and41in the indexing-feed direction indicated by the arrow Y. This second indexing-feed means43has a male screw rod431that is arranged between the above pair of guide rails41and41parallel thereto, and a drive source such as a pulse motor432for rotary-driving the male screw rod431. The male screw rod431is, at its one end, rotatably supported to a bearing block (not shown) fixed on the above stationary base2and is, at the other end, transmission-coupled to the output shaft of the above pulse motor432. The male screw rod431is 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 portion421constituting the movable support base42. Therefore, by driving the male screw rod431in a normal direction or reverse direction with the pulse motor432, the movable support base42is moved along the guide rails41and41in the indexing-feed direction indicated by the arrow Y.

The laser beam application unit5in the illustrated embodiment comprises a unit holder51and a laser beam application means52secured to the unit holder51. The unit holder51has a pair of to-be-guided grooves511and511to be slidably fitted to the pair of guide rails423and423on the above mounting portion422and is supported in such a manner that it can move in the direction indicated by the arrow Z by fitting the guide grooves511and511to the above guide rails423and423, respectively.

The laser beam application unit5in the illustrated embodiment comprises a moving means53for moving the unit holder51along the pair of guide rails423and423in the direction indicated by the arrow Z. The moving means53comprises a male screw rod (not shown) arranged between the above pair of guide rails423and423and a drive source such as a pulse motor532for rotary driving the male screw rod. By driving the male screw rod (not shown) in a normal direction or reverse direction with the pulse motor532, the unit holder51and the laser beam application means52are moved along the guide rails423and423in the direction indicated by the arrow Z. In the illustrated embodiment, the laser beam application means52is designed to be moved up by driving the pulse motor532in a normal direction and to be moved down by driving the pulse motor532in the reverse direction.

The illustrated laser beam application means52has a cylindrical casing521which is secured to the above unit holder51and extends substantially horizontally. The laser beam application means52comprises a pulse laser beam oscillation means522and a transmission optical system523installed in the casing521as shown inFIG. 2and a processing head6for applying a pulse laser beam oscillated by the pulse laser beam oscillation means522to the workpiece held on the above chuck table36, which is attached to the end of the casing521. The above pulse laser beam oscillation means522comprises a pulse laser beam oscillator522acomposed of a YAG laser oscillator or YVO4 laser oscillator and a repetition frequency setting means522bconnected to the pulse laser beam oscillator522a. The transmission optical system523comprises suitable optical elements such as a beam splitter, etc.

The above processing head6comprises a direction changing mirror61and a condenser7as shown inFIG. 3. The direction changing mirror61changes the direction of the pulse laser beam that is oscillated from the above pulse laser beam oscillation means522and irradiated through the transmission optical system523, toward the condenser7. The condenser7in the illustrated embodiment comprises a condenser lens8opposed to the workpiece held on the above chuck table36, a cylindrical lens unit9arranged on the upstream side in the laser beam application direction of the condenser lens8, that is, between the condenser lens8and the direction changing mirror61, and an interval adjusting mechanism for adjusting the interval between the condenser lens8and the cylindrical lens unit9, which will be described later. The above direction changing mirror61, the cylindrical lens unit9and the interval adjusting mechanism later described are installed in a processing head housing60mounted onto the end of the above casing521as shown inFIG. 4. The above condenser lens8is installed in a lens housing80attached to the bottom of the processing head housing60. The focal distance of the condenser lens8is set to 40 mm in the illustrated embodiment.

A description will be subsequently given of the above cylindrical lens unit9with reference toFIGS. 5 to 7.FIG. 5is a perspective view of the cylindrical lens unit9andFIG. 6is an exploded perspective view of the cylindrical lens unit9shown inFIG. 5.

The cylindrical lens unit9shown inFIG. 5andFIG. 6comprises a cylindrical lens91, a lens holding member92for holding the cylindrical lens91, a first frame93for holding the lens holding member92and a second frame94for holding the first frame93.

The cylindrical lens91is a convex lens having a semicircular section as shown inFIG. 7. The focal distance of this cylindrical lens91is set to 40 mm in the illustrated embodiment. The lens holding member92for holding the cylindrical lens91is circular and made of a synthetic resin in the illustrated embodiment. This cylindrical lens91is embedded in the lens holding member92made of a synthetic resin in such a manner that its top surface and bottom surface are exposed. A projecting piece921is formed projectingly from one position of the side wall of the lens holding member92as shown inFIG. 6.

The above first frame93is square with a side length E, and a circular hollow931for accepting the above lens holding member92and a working chamber932for accepting the projecting piece921formed on the lens holding member92are formed in the top surface of the first frame93, as shown inFIG. 6. A hole931bis formed in the center portion of the bottom wall931aof the circular hollow931. A recess932bwhich is a spring seat is formed in a wall932aforming the working chamber932. A screw hole932cis formed on the axis line of the recess932bin the first frame93. The lens holding member92is fitted in the circular hollow931of the first frame93constituted as described above, as shown inFIG. 5, and the projecting piece921is housed in the working chamber932. Therefore, the lens holding member92fitted in the circular hollow931of the first frame93can turn along the side wall (inner peripheral face) of the circular hollow931in the range where the projecting piece921can move within the working chamber932. A helical compression spring95is interposed between the above recess932band the projecting piece921. A first adjustment screw96is screwed into the above screw hole932c, and the end of the first adjustment screw96is designed to be brought into contact with the projecting piece921. Therefore, when the first adjustment screw96is moved forward by turning in one direction, the lens holding member92is turned in one direction against the spring force of the helical compression spring95, and when the first adjustment screw96is moved backward by turning in the other direction, the lens holding member92is turned in the other direction by the spring force of the helical compression spring95. Thus, the projecting piece921formed on the lens holding member92, the first adjustment screw96and the helical compression spring95function as a turning adjustment means for turning the lens holding member92along the inner peripheral face of the circular hollow931.

The above second frame94is of a rectangular shape, and a rectangular hollow941for accepting the first frame93is formed in the top surface of the second frame94, as shown inFIG. 6. This rectangular hollow941has a width A corresponding to the side length E of the above square first frame93and a length B larger than the side length E of the first frame93. The rectangular hollow941is sectioned by a bottom wall942aand side walls942b,942c,942dand942e. A hole942fis formed in the center portion of the bottom wall942a. A recess942gwhich is a spring seat is formed in the inner surface of the side wall942dsectioning the rectangular hollow941. A screw hole942his formed in the side wall942eopposite to the side wall942dhaving the recess942g. A prolonged hole942jfor accepting the above first adjustment screw96is formed in the side wall942bof the second frame94. The above first frame93is fitted in the rectangular hollow941of the second frame94constituted as described above, as shown inFIG. 5. A helical compression spring97is interposed between the recess942gformed in the inner surface of the above side wall942dand the side wall of the first frame93. A second adjustment screw98is screwed into the screw hole942hformed in the side wall942e, and the end of the second adjustment screw98is brought into contact with the side wall of the first frame93. Therefore, when the second adjustment screw98is moved forward by turning in one direction, the first frame93is moved in one direction against the spring force of the helical compression spring97and when the second adjustment screw98is moved backward by turning in the other direction, the first frame93is moved in the other direction by the spring force of the helical compression spring97. Thus, the second adjustment screw98and the helical compression spring97function as a moving adjustment means for moving the first frame93relative to the second frame94in a direction perpendicular to the converging direction of the cylindrical lens91.

The cylindrical lens unit9constituted as described above is set in the interval adjustment mechanism10shown inFIG. 8. The interval adjustment mechanism10will be described hereinbelow.

The interval adjustment mechanism10shown inFIG. 8comprises a support board11, a condenser lens support plate12installed at the lower end of the support board11, and a support table13arranged such that it can move in the vertical direction along the front surface of the support board11.

A guide groove111is formed in the center portion of the front surface of the support board11in the vertical direction. The condenser lens support plate12projects from the front surface of the support board11at a right angle. A hole121is formed in the center portion of this condenser lens support plate12. The lens housing80in which the condenser lens8is installed is situated at a position corresponding to the hole121of the undersurface of the condenser lens support plate12constituted as described above.

The above support table13is composed of a support portion14and a table portion15installed at the lower end of the support portion14. The support portion14has, on the back, a to-be-guided rail141that is fitted to the guide groove111formed in the above support board11. By fitting this to-be-guided rail141to the guide groove111, the support table13is supported to the support board11in such a manner that it can move along the guide groove111in the vertical direction. The above table portion15projects from the front surface of the support portion14at a right angle. A hole151is formed in the center portion of the table portion15. Positioning rails152and153extending at a right angle from the front surface of the support board11are formed at both side ends of the table portion15. The interval between the positioning rails152and153is set to a size corresponding to the width direction of the second frame94constituting the above cylindrical lens unit9.

The interval adjustment mechanism10in the illustrated embodiment has a moving means16for moving the support table13downward along the guide groove111of the support board11. The moving means16comprises a male screw rod161arranged in the vertical direction on one side of the support portion14of the support table13and a pulse motor162for rotary-driving the male screw rod161. The male screw rod161is screwed into a threaded screw hole163aformed in a movable plate163fixed to the upper end of the support portion14, and the lower end of the male screw rod161is rotatably supported to a bearing164fixed to the side surface of the support board11. The pulse motor162is attached to the support board11, and its drive shaft162ais connected to the upper end of the male screw rod161. Therefore, the support table13is moved down by driving the male screw rod161in the normal direction with the pulse motor162and moved up by driving the male screw rod161in the reverse direction. The moving means16can suitably adjust the interval between the table portion15of the support table13and the condenser lens support plate12by driving the pulse motor162in the normal direction or reverse direction.

The above cylindrical lens unit9is set on the table portion15of the support table13of the interval adjustment mechanism10constituted as described above, as shown inFIG. 9. That is, the second frame94of the cylindrical lens unit9is placed between the positioning rails152and153of the table portion15constituting the support table13. The cylindrical lens unit9placed at a predetermined position on the table portion15of the support table13is fixed on the table portion15of the support table13by a suitable fixing means that is not shown. The converging direction of the cylindrical lens91of the cylindrical lens unit9arranged on the table portion15of the support table13as described above is set to the processing-feed direction indicated by the arrow X inFIG. 1andFIG. 9.

Returning toFIG. 1, an image pick-up means17for detecting the area to be processed by the above laser beam application means52is mounted on the front end portion of the casing521constituting the above laser beam application means52. This image pick-up means17is constituted by an image pick-up device (CCD), etc. and supplies an image signal to a control means100. This control means100is composed of a computer, receives an image signal from the above image pick-up means17and supplies control signals to the above pulse motor372, pulse motor382, pulse motor432, pulse motor532, pulse motor162, and the like.

The laser beam processing machine in the illustrated embodiment is constituted as described above, and its function will be described hereinunder.

The shape of the focal spot of a laser beam applied by the above-described laser beam application means52will be described with reference toFIGS. 10(a) to10(c) andFIGS. 11(a) to11(c).

A description is first given of a case where the interval (d1) between the cylindrical lens91and the condenser lens8is set to 40 mm which is the same as the focal distance (f2) of the cylindrical lens91, as shown inFIGS. 10(a) and10(b). In this case, the laser beam L is converged in the Y direction not by the cylindrical lens91but only by the condenser lens8. That is, as shown inFIG. 10(a), the laser beam L passing through the cylindrical lens91is focused at a focal point P140 mm below the condenser lens8, which is the focal distance (f1) of the condenser lens8.

Meanwhile, the laser beam L is converged in the X direction by the cylindrical lens91. That is, since the focal distance (f2) of the cylindrical lens91is set to 40 mm, the focal point P2of the laser beam L focused in the X direction by the cylindrical lens91is at the center position of the condenser lens8, as shown inFIG. 10(b). The laser beam L focused at the center position of the condenser lens8expands toward the undersurface of the condenser lens8and is focused again at the above focal point P1from the undersurface of the condenser lens8. When the interval (d1) between the cylindrical lens91and the condenser lens8is made the same as the focal distance (f2) of the cylindrical lens91, the laser beam L with a circular section entering the cylindrical lens91is converged by the cylindrical lens91in the X direction and converged by the condenser lens8in the Y direction, whereby a focal spot S1having a circular section shown in the enlarged view ofFIG. 10(c) is formed at the focal point P1. Therefore, when the workpiece is placed at the position of the focal point P1, it can be processed by means of the focal spot S1having a circular section.

A description is next given of a case where the interval (d1) between the cylindrical lens91and the condenser lens8is set to 20 mm which is half of the focal distance (f2) of the cylindrical lens91, as shown inFIGS. 11(a) and11(b). Also in this case, the laser beam L is converged in the Y direction not by the cylindrical lens91but only by the condenser lens8. That is, as shown inFIG. 11(a), the laser beam L passing through the cylindrical lens91is focused at the focal point P140 mm below the condenser lens8, which is the focal distance (f1) of the condenser lens8.

Meanwhile, since the focal distance (f2) of the cylindrical lens91is set to 40 mm, the laser beam L which is converged in the X direction by the cylindrical lens91as shown inFIG. 11(b) enters the condenser lens8before it is focused, is further converged by the condenser lens8to be focused at a focal point P3and then, expanded in the X direction until it reaches the workpiece. As a result, at the position of the focal point P1, a focal spot S2having an elliptic section is formed as shown in the enlarged view ofFIG. 11(c). The long axis D1of the focal spot S2having an elliptic section is formed in the direction indicated by the arrow X. The ratio (aspect ratio) of the long axis D1to the short axis D2of the elliptic focal spot S2can be adjusted by changing the interval (d1) between the condenser lens8and the cylindrical lens91. That is, as the interval (d1) between the condenser lens8and the cylindrical lens91becomes smaller than the focal distance (f2) of the cylindrical lens91, the ratio (aspect ratio) of the long axis (D1) to the short axis (D2) of the elliptic focal spot S2becomes larger. Therefore, when the workpiece is placed at the position of the focal point P1, it can be processed by means of the focal spot S2having an elliptic section.

A description is subsequently given of a processing method for forming a groove in the workpiece by means of the focal spot S2having an elliptic section shown inFIG. 11(c).

An optical device wafer as the workpiece to be processed by the above laser beam processing machine1will be described with reference toFIG. 12. The optical device wafer20shown inFIG. 12is a silicon carbide (SiC) wafer, a plurality of areas are sectioned by a plurality of streets201formed in a lattice pattern on the front surface20a, and an optical device202such as a photodiode or laser diode is formed in each of the sectioned areas. The rear side of this optical device wafer20is put on a protective tape22which is a synthetic resin sheet made of polyolefin or the like and is mounted on an annular frame21in such a manner that the front surface20afaces up.

To form a groove along the streets201of the above-described optical device wafer20by using the laser beam processing machine1shown inFIG. 1, the optical device wafer20is first placed on the chuck table36of the laser beam processing machine1in such a manner that the front surface20afaces up. The optical device wafer20is suction-held on the chuck table36through the protective tape22by activating a suction means that is not shown. The annular frame21, on which the protective tape22is mounted, is fixed by the clamps362provided on the chuck table36. The chuck table36suction-holding the optical device wafer20is brought to a position right below the image pick-up means17by the processing-feed means37. After the chuck table36is positioned right below the image pick-up means17, alignment work for detecting the area to be processed of the optical device wafer20is carried out by the image pick-up means17and the control means100. That is, the image pick-up means17and the control means100carry out image processing such as pattern matching, etc. to align a street201formed in a predetermined direction of the optical device wafer20with the condenser7of the laser beam application means52for applying a laser beam along the street201, thereby performing the alignment of a laser beam application position. The alignment of the laser beam application position is also carried out on streets201formed on the optical device wafer20in a direction perpendicular to the above predetermined direction.

After the alignment of the laser beam application position is carried out by detecting the street201formed on the optical device wafer20held on the chuck table36as described above, as shown inFIG. 13(a), the chuck table36is moved to a laser beam application area where the condenser7of the laser beam application means52is located so as to bring one end (left end inFIG. 13(a)) of the predetermined street201to a position right below the condenser7. The long axis D1shown inFIG. 11(c) of the focal spot S2having an elliptic section of the laser beam applied from the condenser7is positioned along the street201.

Next comes the step of forming a groove along the streets201by applying a first laser beam whose elliptic focal spot has a ratio (aspect ratio) of the long axis to the short axis of 30 to 60:1, along the streets formed on the wafer. It is important that the ratio (aspect ratio) of the long axis to the short axis of the elliptic focal spot of the first laser beam should be set to 30 to 60:1 suitable for the formation of a groove in this groove forming step. As for the adjustment of the aspect ratio of the elliptic focal spot, the pulse motor162of the above interval adjustment mechanism10is controlled to achieve the above aspect ratio set by the control means100. The focal point P1of the pulse laser beam applied from the condenser7is set to a position near the front surface20a(top surface) of the optical device wafer20. The moving means53for moving the laser beam application means52along the guide rails423and423in the direction indicated by the arrow Z is used to set the focal point P1to the position near the front surface20a(top surface) of the optical device wafer20. The chuck table36is then moved in the direction indicated by the arrow X1inFIG. 13(a) at a predetermined processing-feed rate while a pulse laser beam of a wavelength (200 to 600 nm) having absorptivity for the optical device wafer20is applied from the condenser7of the laser beam application means52. When the other end (right end inFIG. 13(a)) of the street201reaches a position right below the condenser7, the application of the pulse laser beam is suspended, and the movement of the chuck table36is stopped. Thereafter, the chuck table36is moved in the direction indicated by the arrow X2inFIG. 13(b) at a predetermined processing-feed rate while a pulse laser beam is applied from the condenser7of the laser beam application means52as shown inFIG. 13(b). When the other end (left end inFIG. 13(b)) of the street201reaches a position right below the condenser7, the application of the pulse laser beam is suspended, and the movement of the chuck table36is stopped. A groove210is formed along the street201in the optical device wafer20as shown inFIG. 13(c) by carrying out this groove forming step a plurality of times (for example, 5 round-trips). Debris220formed by the application of the pulse laser beam accumulates in the groove210.

Since in the above groove forming step, the long axis (D1) of the elliptic focal spot of the first laser beam is set to 500 μm and the aspect ratio is set to 50:1, the overlapping rate of the focal spot becomes large, thereby enhancing the groove processing effect. By carrying out the groove forming step a plurality of times as described above, the groove210is formed along the street201in the optical device wafer20and the debris220accumulates in the groove210at the same time. It has been found that when the debris220accumulates in the groove210, the laser beam is blocked by the debris220to make it impossible to deepen the groove, even though the above groove forming step is repeatedly carried out.

In the present invention, the above groove forming step is followed by the step of removing the debris220accumulated in the groove210. In this debris removing step, it is important that a second laser beam whose elliptic focal spot has an aspect ratio of 1 to 20:1 which is suitable for removing debris should be used. As for the adjustment of the aspect ratio of the elliptic focal spot, the pulse motor162of the above interval adjustment mechanism10is so controlled as to achieve the above aspect ratio set by the control means100. Then, one end (left end inFIG. 14(a)) of the above groove210(street201) is brought to a position right above the condenser7as shown inFIG. 14(a). The long axis of the focal spot having an elliptic section of the laser beam applied from the condenser7is positioned along the groove210(street201). The focal point P1of the pulse laser beam applied from the condenser7is then set to a position near the bottom of the groove210. The moving means53for moving the laser beam application means52along the guide rails423and423in the direction indicated by the arrow Z is used to set the focal point P1to the position near the bottom of the groove210.

The chuck table36is then moved in the direction indicated by the arrow X1inFIG. 14(a) at a predetermined processing-feed rate while the pulse laser beam is applied from the condenser7of the laser beam application means52. When the other end (right end inFIG. 14(a)) of the groove210(street201) reaches a position right below the condenser7, the application of the pulse laser beam is suspended, and the movement of the chuck table36is stopped. Thereafter, the chuck table36is moved in the direction indicated by the arrow X2inFIG. 14(b) at a predetermined processing-feed rate while a pulse laser beam is applied from the condenser7of the laser beam application means52, as shown in FIG.14(b). When the other end (left end inFIG. 14(b)) of the groove210(street201) reaches a position right below the condenser7, the application of the pulse laser beam is suspended, and the movement of the chuck table36is stopped. By carrying out this debris removing step a plurality of times (for example, 5 round-trips), the debris220accumulated in the groove210in the above groove forming step is burned and scattered to be removed and consequentry, the groove210is formed deep in the optical device wafer20, as shown inFIG. 14(c).

In the above debris removing step, as the aspect ratio of the long axis (D1) to the short axis (D2) of the elliptic focal spot of the second laser beam is set to 10:1 which is smaller than the aspect ratio of the long axis (D1) to the short axis (D2) of the focal spot in the above groove forming step and the area of the focal spot becomes small, the power density of the irradiated laser beam becomes high, whereby the debris220accumulated in the groove210in the above groove forming step is burned and scattered to be removed. By repeating the above groove forming step and the above debris removing step, the optical device wafer20can be cut along the streets201.

According to experiments conducted by the inventors of the present invention, when the above groove forming step and the debris removing step were carried out 3 times on a silicon carbide (SiC) wafer having a thickness of 350 μm, the silicon carbide (SiC) wafer could be cut along the predetermined streets.