LED wafer processing method

An LED wafer processing method includes a dividing step of rotatably mounting a first cutting blade having a first width in a first cutting unit, holding an LED wafer on a holding table, and then relatively moving the first cutting unit and the holding table to cut the wafer along each division line formed on the wafer, thereby forming a full-cut groove along each division line to thereby divide the wafer into individual chips. The method further includes rotatably mounting a second cutting blade having a second width larger than the first width in a second cutting unit after performing the dividing step, and then relatively moving the second cutting unit and the holding table to thereby polish the opposed side surfaces of the full-cut groove formed along each division line, whereby a polished groove larger in width than the full-cut groove is formed along each full-cut groove.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a light emitting diode (LED) wafer processing method for dividing an LED wafer into a plurality of individual LED chips, in which the LED wafer is formed from a sapphire substrate, and a plurality of LEDs are formed on the front side of the sapphire substrate so as to be separated from each other by a plurality of division lines.

Description of the Related Art

A plurality of crossing division lines are formed on the front side of a sapphire substrate to thereby define a plurality of separate regions where a plurality of LEDs are respectively formed, thus forming an LED wafer having the plural LEDs on the front side. The LED wafer is divided along the division lines by using a laser processing apparatus to obtain a plurality of individual LED chips respectively including the plural LEDs. The LED chips thus obtained are used in electrical equipment such as mobile phones and illumination equipment.

In general, a sapphire substrate has high hardness and much time is therefore required for cutting of the sapphire substrate by using a cutting blade mounted in a cutting apparatus. Accordingly, it is difficult to efficiently divide the LED wafer into the individual LED chips by using a cutting apparatus. For this reason, a laser processing apparatus is used to divide the LED wafer into the individual LED chips.

It is known that such a laser processing apparatus for realizing the processing of a sapphire substrate may be classified into two types. One of the two types is such that a laser beam having an absorption wavelength to sapphire is applied to the sapphire substrate along each division line, thereby performing ablation along each division line to form a division groove (laser processed groove) along each division line (see JP Hei 10-305420A, for example). The other type is such that a laser beam having a transmission wavelength to sapphire is first applied to the sapphire substrate along each division line, thereby forming a modified layer as a division start point inside the sapphire substrate along each division line, and an external force is next applied to the sapphire substrate to thereby divide the sapphire substrate along each division line (see Japanese Patent No. 3408805, for example). By using any type of laser processing apparatus, the LED wafer can be divided into the individual LED chips.

SUMMARY OF THE INVENTION

According to the above laser processing apparatus, the LED wafer formed from a sapphire substrate can be divided into the individual LED chips more efficiently as compared with the case of using a cutting blade. However, the side surface of each LED chip is modified by the application of a laser beam, causing a reduction in luminance of each LED chip.

It is therefore an object of the present invention to provide an LED wafer processing method which can improve the luminance of each LED chip.

In accordance with an aspect of the present invention, there is provided an LED wafer processing method for dividing an LED wafer along a plurality of crossing division lines to obtain a plurality of individual LED chips, the LED wafer being formed from a sapphire substrate having a front side, the plurality of crossing division lines being formed on the front side of the sapphire substrate to thereby define a plurality of separate regions where a plurality of LEDs are respectively formed, the LED chips respectively including the LEDs, the LED wafer processing method including a cutting blade preparing step of preparing a first cutting blade having an annular first cutting edge having a first width and a second cutting blade having an annular second cutting edge having a second width larger than the first width, the first cutting edge containing diamond abrasive grains having a first grain size, the second cutting edge containing diamond abrasive grains having a second grain size smaller than the first grain size; a dividing step of rotatably mounting the first cutting blade in first cutting means, holding the LED wafer on a holding table in the condition where a front side of the LED wafer is exposed upward, and then relatively moving the first cutting means and the holding table to cut the front side of the LED wafer along each division line, thereby forming a full-cut groove along each division line, so that the full-cut groove has a depth reaching a back side of the LED wafer, whereby the LED wafer is divided into the individual LED chips; and a polishing step of rotatably mounting the second cutting blade in second cutting means after performing the dividing step, and then relatively moving the second cutting means and the holding table holding the LED wafer to thereby polish opposed side surfaces of the full-cut groove formed along each division line, whereby a polished groove larger in width than the full-cut groove is formed along each full-cut groove.

Preferably, a depth of cut by the first cutting blade in the dividing step is stepwise increased to cut the front side of the LED wafer along each division line in plural stages. Preferably, the average grain size of the diamond abrasive grains contained in the first cutting edge is set in a range of #300 to #500 as the first grain size, and the average grain size of the diamond abrasive grains contained in the second cutting edge is set in a range of #1800 to #2200 as the second grain size. Preferably, the width of the first cutting edge is set in a range of 0.15 to 0.24 mm as the first width, and the width of the second cutting edge is set in a range of 0.25 to 0.34 mm as the second width.

Preferably, the LED wafer processing method further includes a V-blade preparing step of preparing a V-blade having an annular cutting edge whose outer circumferential portion has a V-shaped cross section; and a chamfering step of rotatably mounting the V-blade in cutting means, holding the LED wafer on the holding table in the condition where the back side of the LED wafer is exposed upward, and then relatively moving the cutting means and the holding table to form a chamfered portion on the back side of the LED wafer in an area corresponding to each division line formed on the front side of the LED wafer. Preferably, the V-blade includes a plurality of V-blades having a plurality of annular cutting edges whose outer circumferential portions have different V-shaped cross sections such that the plurality of annular cutting edges of the plurality of V-blades have different tip angles, and the chamfering step is performed by stepwise mounting the plurality of V-blades in the cutting means. More preferably, the plurality of V-blades include three kinds of V-blades having three kinds of cutting edges, and the different tip angles of the three kinds of cutting edges include a first tip angle of 110 to 130 degrees, a second tip angle of 80 to 100 degrees, and a third tip angle of 50 to 70 degrees.

Preferably, the cutting edge of the V-blade contains diamond abrasive grains having an average grain size of #1800 to #2200. Preferably, a depth of cut by the plurality of V-blades is stepwise increased in the chamfering step every time the V-blades are stepwise mounted in the cutting means, thereby cutting the back side of the LED wafer along each division line in plural stages. More preferably, a depth of cut to be stepwise increased in the chamfering step is set in a range of 0.04 to 0.06 mm per stage.

According to the present invention, an LED wafer conventionally difficult to cut by using a cutting blade can be divided into individual LED chips. Further, no modified layer is formed on the side surface of each LED chip. Accordingly, as compared with a processing method for dividing an LED wafer by using a laser beam, the luminance of each LED chip can be improved. Further, by performing a chamfering step using a V-blade having an annular cutting edge whose outer circumferential portion has a V-shaped cross section to cut the back side of the LED wafer in an area corresponding to each division line, the luminance can be further improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will now be described an LED wafer processing method according to a first preferred embodiment of the present invention with reference toFIGS. 1 to 6B. In performing the LED wafer processing method according to the first preferred embodiment, an LED wafer10as a workpiece is prepared as depicted inFIG. 1. The LED wafer10is composed of a substantially circular sapphire substrate and a GaN layer formed on the front side (upper surface) of the sapphire substrate. The sapphire substrate has a thickness of 0.45 mm. The GaN layer is partitioned by a plurality of crossing division lines12to thereby define a plurality of separate regions where a plurality of LEDs14are respectively formed. The plural crossing division lines12are composed of a plurality of parallel division lines12extending in a first direction and a plurality of parallel division lines12extending in a second direction perpendicular to the first direction. The outer circumference of the LED wafer10is formed with a straight portion, i.e., so-called orientation flat16indicating crystal orientation. The LED wafer10has a front side10aand a back side10bopposite to the front side10a, in which the LEDs14are formed on the front side10aof the LED wafer10.

After preparing the LED wafer10, a disk-shaped support substrate20slightly larger in diameter than the LED wafer10is prepared as depicted inFIG. 1. The support substrate20is a substrate having a predetermined rigidity, and it is formed of polyethylene terephthalate (PET), for example. As depicted inFIG. 1, the support substrate20has a front side (upper surface)20aand a back side (lower surface)20bopposite to the front side20a. The back side10bof the LED wafer10is attached to the front side20aof the support substrate20in the condition where the center of the LED wafer10coincides with the center of the support substrate20, thereby forming a wafer unit30composed of the LED wafer10and the support substrate20. In attaching the LED wafer10to the support substrate20, a wax or the like is applied to the back side10bof the LED wafer10, so as to improve an adhesion strength.

After forming the wafer unit30by attaching the LED wafer10to the support substrate20, the wafer unit30is transferred to a cutting apparatus40depicted inFIG. 2(a part of the cutting apparatus40being depicted). The cutting apparatus40includes a circular holding table42having a circular vacuum chuck42afor holding the wafer unit30under suction. The wafer unit30transferred to the cutting apparatus40is first placed on the vacuum chuck42aof the holding table42in the condition where the support substrate20is oriented downward, that is, the back side20bof the support substrate20is in contact with the upper surface of the vacuum chuck42a. The vacuum chuck42ais slightly smaller in diameter than the support substrate20. The vacuum chuck42ais connected to suction means (not depicted) producing a vacuum. In the above condition where the wafer unit30is placed on the vacuum chuck42a, the suction means is operated to hold the wafer unit30on the vacuum chuck42aunder suction.

After holding the wafer unit30on the holding table42of the cutting apparatus40under suction as mentioned above, a cutting blade preparing step is performed as a preliminary step for a dividing step and a polishing step to be hereinafter described. The cutting blade preparing step will now be described with reference toFIG. 3.

The cutting apparatus40further includes a spindle unit50as cutting means. As depicted inFIG. 3, the spindle unit50includes a spindle housing52and a spindle54rotatably supported to the spindle housing52. The spindle housing52is mounted on a movable base (not depicted) movable both in an indexing direction and a cutter feeding direction (vertical direction), in which the indexing direction is a direction perpendicular to a work feeding direction in a horizontal plane. Accordingly, the spindle housing52is movable both in the indexing direction and in the cutter feeding direction. The spindle54is adapted to be rotated about its horizontal axis extending in the indexing direction by a rotational drive mechanism (not depicted). In this cutting blade preparing step, a first cutting blade61and a second cutting blade62are adapted to be selectively mounted on the spindle54at its front end portion. The first cutting blade61or the second cutting blade62mounted on the spindle54is fixed by threadedly engaging a fastening nut56to the front end portion of the spindle54. The first cutting blade61has an annular first cutting edge61aformed along the entire outer circumference, and the second cutting blade62also has an annular second cutting edge62aformed along the entire outer circumference. Both the first cutting edge61aand the second cutting edge62aare formed by fixing diamond abrasive grains with a metal bond or the like. The first cutting edge61ahas a width (thickness) smaller than that of the second cutting edge62a. In other words, the second cutting edge62ahas a width (thickness) larger than that of the first cutting edge61a.

In this preferred embodiment, the width of the first cutting edge61ais set to 0.2 mm, and the outer diameter of the first cutting edge61ais set to 50 mm. Further, the width of the second cutting edge62ais set to 0.3 mm, and the outer diameter of the second cutting edge62ais set to 50 mm. The diamond abrasive grains constituting the first cutting edge61ahas an average grain size of #400 (30 μm in diameter), and the diamond abrasive grains constituting the second cutting edge62ahas an average grain size of #2000 (4 μm in diameter). The width of the first cutting edge61ais preferably selected in the range of 0.14 to 0.24 mm suitable for efficient division of the sapphire substrate, and the average grain size of the diamond abrasive grains of the first cutting edge61ais preferably selected in the range of #300 to #500 which is a relatively large size. In contrast, the width of the second cutting edge62ais preferably selected in the range of 0.25 to 0.34 mm slightly larger than the width of the first cutting edge61a, and the average grain size of the diamond abrasive grains of the second cutting edge62ais preferably selected in the range of #1800 to #2200 which is a relatively small size and suitable for polishing of the side surfaces of each LED chip14′ (seeFIG. 5A) obtained by dividing the LED wafer10, thereby improving the luminance of each LED chip14′. In this manner, the cutting blade preparing step is completed.

After completing the cutting blade preparing step, a dividing step is performed to divide the LED wafer10. The dividing step will now be described with reference toFIGS. 4, 5A, and 5B. In performing the dividing step, the first cutting blade61is first mounted rotatably on the front end portion of the spindle54of the cutting apparatus40, thereby configuring first cutting means60A depicted inFIG. 4. After configuring the first cutting means60A, alignment means including an imaging camera (not depicted) or the like in the cutting apparatus40is used to perform alignment between the first cutting blade61and the division lines12of the LED wafer10held on the holding table42under suction.

After performing the alignment, the first cutting blade61is positioned above one end of a predetermined one of the division lines12extending in the first direction as a cutting start position. In this condition, the rotational drive mechanism for rotating the spindle54is operated to rotate the spindle54and accordingly rotate the first cutting blade61fixedly mounted on the spindle54. The first cutting blade61is rotated at a speed of 15,000 rpm, for example, by the rotational drive mechanism. Thereafter, the first cutting blade61is lowered to cut the front side10aof the LED wafer10until a predetermined depth from the front side10a, e.g., a depth of 0.15 mm. At the same time, the holding table42is moved in the feeding direction depicted by an arrow X inFIG. 4at a feed speed of 2 mm/s, for example. Accordingly, the front side10aof the LED wafer10is cut along the predetermined division line12by the first cutting blade61to form a half-cut groove99having a depth of 0.15 mm. Thereafter, the first cutting blade61is moved in the indexing direction depicted by an arrow Y inFIG. 4to the position above the next division line12adjacent to the above predetermined division line12where the half-cut groove99has been formed. Thereafter, the cutting operation by the first cutting blade61is similarly performed along the next division line12to thereby form a similar half-cut groove99. In this manner, the cutting operation is similarly performed along all of the other division lines12extending in the first direction to thereby form a plurality of similar half-cut grooves99. Thereafter, the holding table42is rotated 90 degrees to similarly perform the cutting operation along all of the other division lines12extending in the second direction perpendicular to the first direction, thereby forming a plurality of similar half-cut grooves99. Thus, the plural half-cut grooves99are respectively formed on the front side10aof the LED wafer10along all of the plural crossing division lines12, in which each half-cut groove99has a depth of 0.15 mm and a width of 0.2 mm.

As described above, the LED wafer10has a thickness of 0.45 mm, and each half-cut groove99formed in the dividing step has a depth of 0.15 mm. Accordingly, the LED wafer10is cut by the depth equal to ⅓ of the thickness of the LED wafer10in the above first stage of the dividing step. Thereafter, the spindle housing52is further lowered by 0.15 mm to thereby increase the depth of cut by 0.15 mm. In this condition, the first cutting blade61is similarly operated to further cut the front side10aof the LED wafer10along all of the crossing division lines12, that is, all of the half-cut grooves99previously formed. Accordingly, the depth of each half-cut groove99is increased to 0.3 mm by this second stage of the dividing step. That is, the LED wafer10is further cut by the depth equal to ⅓ of the thickness of the LED wafer10to increase the depth of each half-cut groove99to an amount equal to ⅓ of the thickness of the LED wafer10in the above second stage of the dividing step. Thereafter, the spindle housing52is further lowered by 0.15 mm to thereby increase the depth of cut by 0.15 mm. In this condition, the first cutting blade61is similarly operated to further cut the front side10aof the LED wafer10along all of the crossing division lines12, that is, along all of the half-cut grooves99previously increased in depth.

As a result, as depicted inFIG. 5B, which is an enlarged side view of an essential part of the wafer unit30depicted inFIG. 5A, a full-cut groove100having a depth of 0.45 mm and a width of 0.2 mm is formed along each division line12, in which this depth is a depth extending from the front side10aof the LED wafer10to the back side10bthereof, that is, reaching the front side20aof the support substrate20. Accordingly, the LED wafer10is fully cut by the plural full-cut grooves100formed along all of the crossing division lines12, thereby obtaining a plurality of individual LED chips14′ as depicted inFIG. 5A. In summary, the dividing step according to this preferred embodiment is composed of three stages to be sequentially performed in such a manner that the depth of cut by the first cutting blade61is stepwise increased by 0.15 mm per stage to cut the LED wafer10along each division line12, so that the full-cut groove100having a depth of 0.45 mm reaching the support substrate20is formed along each division line12to thereby divide the LED wafer10into the individual LED chips14′ respectively including the LEDs14. The depth of cut to be stepwise increased is not limited to 0.15 mm per stage, but it may be suitably adjusted. For example, the depth of cut to be stepwise increased may be set to 0.225 mm per stage. In this case, the LED wafer10having a thickness of 0.45 mm is cut stepwise in two stages to obtain the individual LED chips14′. Further, the depth of cut to be stepwise increased may be suitably adjusted according to the thickness of the LED wafer10. In this manner, the dividing step is completed.

As described above, the first cutting blade61used in performing the dividing step has the annular first cutting edge61a, which contains diamond abrasive grains having a relatively large grain size and has a relatively small width. Accordingly, the time required for cutting of the LED wafer10can be reduced, so that the LED wafer10can be divided efficiently. However, the LED wafer10is cut along each division line12by using the first cutting edge61acontaining diamond abrasive grains having a relatively large grain size, that is, containing coarse abrasive grains, so that each full-cut groove100has a pair of opposed side surfaces100aeach having a large surface roughness, causing a reduction in luminance of each LED chip14′. To cope with this problem, a polishing step is performed to polish each side surface100aafter performing the dividing step. This polishing step will now be described with reference toFIGS. 6A and 6B.

After performing the dividing step by using the first cutting blade61set in the cutting apparatus40, the first cutting blade61is removed from the front end portion of the spindle54constituting the first cutting means60A, and the second cutting blade62previously prepared in the cutting blade preparing step is next mounted on the front end portion of the spindle54, thereby configuring second cutting means60B depicted inFIG. 6A.

After configuring the second cutting means60B, the depth of cut by the second cutting blade62is set to 0.45 mm reaching the bottom of each full-cut groove100. In this condition, the second cutting blade62is rotated at a speed of 15,000 rpm as in the dividing step, and the holding table42holding the wafer unit30is moved in the feeding direction depicted by an arrow X inFIG. 6Ato thereby make the second cutting blade62cut the LED wafer10along each division line12, that is, along each full-cut groove100having a depth of 0.45 mm. Accordingly, the opposed side surfaces100aof each full-cut groove100are polished by the second cutting blade62to thereby form a polished groove102depicted inFIG. 6B. As described above, the second cutting edge62aof the second cutting blade62has a width of 0.3 mm larger than the width of the first cutting edge61a. Further, the average grain size of the diamond abrasive grains constituting the second cutting edge62ais set to #2000 (4 μm in diameter) smaller than that of the diamond abrasive grains constituting the first cutting edge61a. Accordingly, as depicted inFIG. 6Bwhich is an enlarged side view of an essential part of the LED wafer10depicted inFIG. 6A, the opposed side surfaces100aof each full-cut groove100having a width of 0.2 mm depicted inFIG. 5Bare polished by the second cutting edge62ato form each polished groove102having a width of 0.3 mm, in which each polished groove102has a pair of opposed side surfaces102aeach having a relatively small surface roughness. That is, each side surface100aof each full-cut groove100is polished by an amount of 0.05 mm. Each side surface102acan provide a smooth side surface of each LED chip14′, thereby allowing a good luminance. In this manner, the polishing step is completed.

By performing the cutting blade preparing step, the dividing step, and the polishing step as mentioned above, the LED wafer10conventionally difficult to cut by using a cutting blade can be efficiently divided into the individual LED chips14′ by using the first cutting blade61and the second cutting blade62. Further, no modified layer is formed on the side surface of each LED chip14′. Accordingly, as compared with a processing method for dividing an LED wafer by using a laser beam, the luminance of each LED chip14′ can be improved.

A second preferred embodiment of the LED wafer processing method according to the present invention will now be described. The second preferred embodiment further includes a V-blade preparing step of preparing a V-blade having an annular cutting edge whose outer circumferential portion has a V-shaped cross section and a chamfering step of cutting the back side10bof the LED wafer10in an area corresponding to each division line12by using the V-blade prepared above to thereby form a chamfered portion, in which the V-blade preparing step and the chamfering step are performed after performing the polishing step mentioned above. The V-blade preparing step and the chamfering step will now be described with reference toFIGS. 7 to 11C.

As depicted inFIG. 7, the V-blade preparing step is a step of preparing a V-blade having an annular cutting edge whose outer circumferential portion has a V-shaped cross section, in which the cutting edge is formed by fixing diamond abrasive grains with a metal bond or the like. More specifically, the V-blade is adapted to be mounted on the front end portion of the spindle54of the spindle unit50of the cutting apparatus40. The annular cutting edge of this V-blade has an outer diameter of 60 mm and a width (thickness) of 3 mm, for example, larger than the width of each polished groove102. Further, the diamond abrasive grains contained in the cutting edge of this V-blade preferably has an average grain size of #1800 to #2200, more preferably, #2000. In this preferred embodiment, this V-blade is composed of a first V-blade63having a first cutting edge63a, a second V-blade64having a second cutting edge64a, and a third V-blade65having a third cutting edge65a. The outer circumferential portion of the first cutting edge63ahas a tip angle of 120 degrees as viewed in cross section. The outer circumferential portion of the second cutting edge64ahas a tip angle of 90 degrees as viewed in cross section. The outer circumferential portion of the third cutting edge65ahas a tip angle of 60 degrees as viewed in cross section. The first V-blade63, the second V-blade64, and the third V-blade65are adapted to be selectively mounted on the front end portion of the spindle54. As depicted inFIG. 7, the first, second, or third V-blade63,64, or65mounted on the spindle54is fixed by threadedly engaging the fastening nut56with the front end portion of the spindle54.

After performing this V-blade preparing step, the chamfering step is performed to form a chamfered portion on the back side10bof the LED wafer10in an area corresponding to each division line12. Prior to performing the chamfering step, a support substrate changing step is performed in the following manner.

As depicted inFIG. 8, the LED wafer10processed by the polishing step remains attached to the support substrate20in the condition where the front side10aof the LED wafer10is oriented upward, that is, exposed, thus forming the wafer unit30. The chamfering step is performed to the back side10bof the LED wafer10as mentioned above. Therefore, the wafer unit30is once removed from the holding table42of the cutting apparatus40after performing the polishing step. Thereafter, as depicted inFIG. 8, the wafer unit30is inverted in such a manner that the front side10aof the LED wafer10is oriented downward, that is, the back side20bof the support substrate20is oriented upward. In this condition, the LED wafer10of the wafer unit30is attached through any adhesive or the like to another support substrate22having a front side (upper surface)22aand a back side (lower surface)22b. The adhesive is previously applied to the front side10aof the LED wafer10. More specifically, the front side10aof the LED wafer10of the wafer unit30is attached to the front side22aof the support substrate22. The support substrate22has the same shape and size as those of the support substrate20, and it is formed of the same material as that of the support substrate20. After attaching the LED wafer10of the wafer unit30to the support substrate22, the support substrate20is separated from the LED wafer10as depicted inFIG. 9, thereby forming a new wafer unit32composed of the LED wafer10and the support substrate22. As apparent fromFIG. 9, in the condition where the support substrate20has been separated from the LED wafer10, the back side10bof the LED wafer10of the wafer unit32is exposed upward, so that all the polished grooves102formed so as to respectively correspond to all the crossing division lines12are exposed upward. In this manner, the support substrate changing step is completed.

After performing the V-blade preparing step and the support substrate changing step, the wafer unit32is transferred to the cutting apparatus40for performing the chamfering step, and the support substrate22of the wafer unit32is held on the holding table42under suction as depicted inFIG. 10. The first V-blade63having the first cutting edge63ahaving a tip angle of 120 degrees is first mounted on the front end portion of the spindle54of the cutting apparatus40, thereby configuring cutting means chamfering the edge portions of each polished groove102. After holding the wafer unit32on the holding table42in the condition where the back side10bof the LED wafer10is oriented upward, the alignment means including an imaging camera (not depicted) in the cutting apparatus40is operated to perform alignment between the first V-blade63and the polished grooves102exposed to the back side10bof the LED wafer10held on the holding table42. After performing this alignment, the rotational drive mechanism for rotating the spindle54is operated to rotate the spindle54at a speed of 30,000 rpm, for example. Further, the first V-blade63is positioned above one end of a predetermined one of the polished grooves102respectively corresponding to the division lines12extending in the first direction as a chamfering start position, in which the polished grooves102are exposed to the back side10bof the LED wafer10. Thereafter, the first V-blade63is lowered to cut the back side10bof the LED wafer10until a predetermined depth from the back side10b, e.g., a depth of 0.05 mm. At the same time, the holding table42is moved in the feeding direction depicted by an arrow X inFIG. 10at a feed speed of 5 mm/s, for example. Accordingly, the back side10bof the LED wafer10is cut along the predetermined polished groove102by the first V-blade63to form a chamfered portion103. More specifically, as depicted inFIG. 11A, a first chamfered portion103aconstituting the chamfered portion103is formed at the edge portions of the predetermined polished groove102exposed to the back side10bof the LED wafer10so as to form a tapering angle of 120 degrees. Thereafter, as similarly to the dividing step and the polishing step mentioned above, a plurality of first chamfered portions103aare formed by the first V-blade63along all of the other polished grooves102exposed to the back side10bof the LED wafer10.

After forming the first chamfered portions103aalong all of the polished grooves102exposed to the back side10bof the LED wafer10as depicted inFIG. 11A, the rotation of the spindle54is once stopped and the spindle unit50is next raised. Thereafter, the first V-blade63is removed from the spindle54, and the second V-blade64having the second cutting edge64ahaving a tip angle of 90 degrees is next mounted on the spindle54. After mounting the second V-blade64on the spindle54, the depth of cut by the second V-blade64is increased by 0.05 mm from the depth of cut (=0.05 mm) by the first V-blade63, that is, the depth of cut by the second V-blade64is set to 0.1 mm from the back side10bof the LED wafer10. In this condition, the back side10bof the LED wafer10is similarly cut along each polished groove102by the second V-blade64. That is, the upper end of each polished groove102previously chamfered by the first V-blade63is further chamfered by the second V-blade64. As a result, a second chamfered portion103bis formed along each polished groove102having the first chamfered portion103aas depicted inFIG. 11B. As apparent fromFIG. 11B, the second chamfered portion103bis formed below the first chamfered portion103a(i.e., toward the front side10aof the LED wafer10) along each polished groove102because the second cutting edge64ahaving a tip angle of 90 degrees is used and the depth of cut by the second V-blade64is increased by 0.05 mm as mentioned above.

After forming the second chamfered portion103balong each polished groove102as mentioned above, the spindle54is stopped again and the spindle unit50is next raised. In this condition, the second V-blade64is removed from the spindle54, and the third V-blade65having the third cutting edge65ahaving a tip angle of 60 degrees is next mounted on the spindle54. After mounting the third V-blade65on the spindle54, the depth of cut by the third V-blade65is further increased by 0.05 mm from the depth of cut (=0.1 mm) by the second V-blade64, that is, the depth of cut by the third V-blade65is set to 0.15 mm from the back side10bof the LED wafer10. In this condition, the back side10bof the LED wafer10is similarly cut along each polished groove102by the third V-blade65. That is, the upper end of each polished groove102previously chamfered by the second V-blade64is further chamfered by the third V-blade65. As a result, a third chamfered portion103cis formed along each polished groove102having the first chamfered portion103aand the second chamfered portion103bas depicted inFIG. 11C. As apparent fromFIG. 11C, the third chamfered portion103cis formed below the second chamfered portion103b(i.e., toward the front side10aof the LED wafer10) along each polished groove102because the third cutting edge65ahaving a tip angle of 60 degrees is used and the depth of cut by the third V-blade65is further increased by 0.05 mm as mentioned above.

In this manner, the depth of cut in forming the chamfered portion103is stepwise increased to perform a cutting operation in plural stages, i.e., in three stages in this preferred embodiment. Accordingly, the chamfered portion103is composed of the first chamfered portion103a, the second chamfered portion103b, and the third chamfered portion103c, which are continuously connected in this order to form a pseudo curved surface. Further, the chamfered portion103is formed by using a cutting blade containing diamond abrasive grains having a small grain size similar to that in the polishing step. Accordingly, as similar to the polished surfaces102aof each polished groove102, the chamfered portion103of each polished groove102can contribute to an improvement in luminance of each LED chip14′. The depth of cut to be stepwise increased in the chamfering step is not limited to 0.05 mm per stage, but it may be suitably adjusted. However, the depth of cut to be stepwise increased in the chamfering step is preferably selected in the range of 0.04 to 0.06 mm per stage. Further, the sizes and angles depicted in the drawings are suitably modified for convenience of illustration and they are not actual sizes and angles. Further, the tip angle of the first cutting edge63aof the first V-blade63is preferably selected in the range of 110 to 130 angles. The tip angle of the second cutting edge64aof the second V-blade64is preferably selected in the range of 80 to 100 degrees. The tip angle of the third cutting edge65aof the third V-blade65is preferably selected in the range of 50 to 70 degrees.

The present invention is not limited to the above preferred embodiments, but various modifications may be made. For example, in the above preferred embodiments, the dividing step is performed by using the first cutting blade61having the first cutting edge61ahaving a width of 0.2 mm, and the polishing step is performed by using the second cutting blade62having the second cutting edge62ahaving a width of 0.3 mm. However, the width of the first cutting edge61aand the width of the second cutting edge62amay be suitably adjusted according to the width of each division line12. However, if the difference in width between the first cutting edge61aand the second cutting edge62ais too large, the time required for performing the polishing step becomes long. Therefore, it is desirable to set the difference in width between the first cutting edge61aand the second cutting edge62aas small as possible. Conversely, if the difference in width between the first cutting edge61aand the second cutting edge62ais too small, there is a possibility that polishing of the opposed side surfaces100aof each full-cut groove100may become insufficient. Therefore, the difference in width between the first cutting edge61aand the second cutting edge62ais preferably set in the range of 0.05 to 0.15 mm, for example.

In the above preferred embodiments, the dividing step, the polishing step, and the chamfering step are performed by using the same cutting apparatus40and changing a cutting blade, that is, using different cutting blades. As a modification, a plurality of cutting apparatuses dedicated to the dividing step, the polishing step, and the chamfering step may be prepared. In this case, after finishing the dividing step, the LED wafer10is transferred from the first cutting apparatus dedicated to the dividing step to the second cutting apparatus dedicated to the polishing step. Further, after finishing the polishing step, the LED wafer10is transferred from the second cutting apparatus dedicated to the polishing step to the third cutting apparatus dedicated to the chamfering step.

Further, in the second preferred embodiment, the chamfering step is performed to the polished grooves102formed along the division lines12after performing the dividing step and the polishing step. As a modification, the chamfering step may be performed before performing the dividing step and the polishing step. In this case, the chamfered portion103is formed on the back side10bof the LED wafer10in an area corresponding to each division line12formed on the front side10aof the LED wafer10, before performing the dividing step and the polishing step. After forming the chamfered portion103on the back side10bof the LED wafer10, the dividing step and the polishing step are performed to the front side10aof the LED wafer10to thereby form the polished groove102along each division line12so that the polished groove102reaches the chamfered portion103.

In the second preferred embodiment, the chamfering step is performed by using the three kinds of V-blades, that is, the first V-blade63having a tip angle of 120 degrees, the second V-blade64having a tip angle of 90 degrees, and the third V-blade65having a tip angle of 60 degrees. However, this configuration is merely illustrative, and the chamfering step may be performed by using one kind of V-blade having a suitable tip angle. Further, the chamfering step may be performed by using two kinds of V-blades having different tip angles or by using four or more kinds of V-blades having different tip angles. In this manner, in the case of performing the chamfering step by using a plurality of kinds of V-blades having different tip angles, the tip angles may be suitably adjusted according to the number of the plural V-blades. Owing to the use of the plural V-blades having different tip angles in performing the chamfering step, the chamfered portion103can be made into a pseudo curved surface, thereby contributing to an improvement in luminance of each LED chip14′.

The above-mentioned processing conditions including the rotational speed of the spindle54, the depth of cut to be stepwise increased, and the feed speed in the dividing step, the polishing step, and the chamfering step are merely illustrative and it is needless to say that they may be suitably adjusted.