Laser processing method for a wafer

Abstract

A processing method for a wafer (W) having a device region (W1) in which a plurality of devices (D) are formed, and a peripheral edge region (W2) surrounding the device region (W1), the plurality of devices (D) being respectively formed at a plurality of regions divided by a plurality of dividing lines (S) formed on a front side of the wafer (W), the processing method comprising: a modified layer forming step in which the wafer (W) is held on a chuck table (2) in the state where a back surface (Wb) of the wafer (W) is exposed, and in which a laser beam having a transmission wavelength is applied to the wafer (W) toward the back surface (Wb) of the wafer (W) along the dividing lines (S) in the state where the focal point of the laser beam is set to the inside of the wafer (W), thereby forming a modified layer as a dividing start point inside the wafer (W) along each dividing line (S); and a transporting step in which the wafer (W) is unloaded from the chuck table (2) and the wafer (W) is transported to a position where a next step to be performed is carried out; wherein the modified layer is not formed in the peripheral edge region (W2) of the wafer (W) in the modified layer forming step, thereby forming a reinforcing portion of the peripheral edge region (W2) which inhibits breakage of the wafer (W) at the modified layer during the transporting step.

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a laser processing method for a wafer, comprising the step of applying a laser beam having a transmission wavelength to the wafer toward the back side of the wafer along division lines in the state where the focal point of the laser beam is set inside the wafer, thereby forming a modified layer in the wafer along each division line, whereby the modified layer is not formed in a peripheral marginal area (PERIPHERAL MARGINAL AREA) in which no devices are formed.Prior ArtThere is a wafer having a plurality of devices formed on the front side to be divided by a plurality of division lines. The wafer is cut along the division lines to obtain the individual devices. For example, an optical device wafer is formed by forming an epitaxial layer of gallium nitride (GaN) or the like on the front surface of a sapphire substrate, a SiC substrate, or the like, the epitaxial layer including a plurality of optical devices such as LEDs divided by a plurality of division lines. The wafer for the optical device is divided into the individual optical devices by applying a laser beam along the division lines. The optical devices thus obtained are used in various devices such as a liquid crystal television (LCD) and a computer.As a method for separating such a wafer, there is a method using a laser beam having an absorption wavelength (for example, 266 nm) on a sapphire substrate to thereby form a separation groove along each division line on the front side of the sapphire substrate by ablation, and then applying an external force to each separation groove to thereby divide the wafer into the individual optical devices (see Japanese Patent Publication JP H10-305 420 A). In this method, however, a material fused due to ablation is disposed on the periphery of each optical device, thereby causing a reduction in the luminous power of each optical device.In order to solve this problem, a method for practical use has been introduced, comprising the steps of applying a laser beam having a transmission wavelength (for example, 1064 nm) to a sapphire substrate along the division lines toward the back side where the epitaxial layer is not formed in the state where the focal point of the laser beam is set to the inside of the wafer, thereby forming a modified layer in the wafer along each division line, and next applying an external force to the wafer, thereby dividing the wafer along each division line. According to this method, the generation of a fused material can be suppressed (see, for example, Japanese Patent JP 3 408 805 B2).DE 10 2010 039 798 A1 discusses a wafer processing method including the steps of forming a modified layer in a wafer along each division line and forming an annular modified layer in the wafer along the boundary between a circular recess and an annular reinforcing portion formed on the back side of the wafer.US 2007 / 0 123 002 A1 discusses grinding the back side of a device region of a workpiece to form a recessed portion and an annular reinforcing part.US 2007 / 0 202 619 A1 discusses a laser processing apparatus which has a laser light source which simultaneously emits laser beams having two wavelengths.SUMMARY OF THE INVENTIONHowever, in the case that the modified layer is formed in the wafer along each division line, there is a possibility that the wafer is broken off from the modified layer in the following transporting step. Further, in the case where a reflection film made of gold, aluminum, etc. is formed on the back surface of a wafer for optical device, the laser beam cannot be applied toward the back surface of the wafer.It is therefore an object of the present invention to provide a laser processing method for a wafer which can eliminate the possibility that the wafer breaks off from the modified layer when transporting the wafer. It is another object of the present invention to provide a laser processing method for a wafer which can form the modified layer inside the wafer by applying a laser beam even in the case of forming a reflective film on the back side of the wafer.According to an aspect of the present invention, there is provided a processing method for a wafer having a device area in which a plurality of devices are formed and a peripheral edge area surrounding the device area, the plurality of devices being respectively divided at a plurality of areas divided by a plurality of division lines formed on the front side of the wafer, the processing method including a modified layer forming step of holding the wafer on a chuck table in the state where the back side of the wafer is exposed and a step of applying the laser beam having a transmission wavelength to the wafer toward the back side of the wafer along the division lines in the state, in which the focal point of the laser beam is set inside the wafer (inside the wafer), thereby forming a modified layer as a separation start point inside the wafer along each division line; and comprising a transporting step in which the wafer is unloaded from the chuck table and in which the wafer is transported to a position where a next step to be performed is performed; wherein the modified layer is not formed in the peripheral edge portion of the wafer in the modified layer forming step, to thereby form a reinforcing portion in the peripheral edge portion that inhibits breakage of the wafer at the modified layer during the transporting step.In the case where the wafer is an optical device wafer having a sapphire substrate and an epitaxial layer formed on the front surface of the sapphire substrate, the epitaxial layer including a plurality of optical devices each formed at a plurality of regions divided by a plurality of division lines, the wafer is transported by the transporting step to a position for performing a back surface processing step in which a reflective film is formed on the back surface of the wafer.Preferably, the processing method further includes a separation step of applying an external force to the division lines of the wafer to thereby divide the wafer into the individual devices after performing the modified layer forming step.According to the present invention, the reinforcing portion having no modified layers is formed in the peripheral edge portion of the wafer in the modified layer forming step. Accordingly, there is no possibility that the wafer may be broken from the modified layers in terms of a breaking start point in the following transporting step. Further, even in the case where a reflective film is formed on the back surface of the wafer, the reflective film has not yet been formed at the time of performing the modified layer forming step. Accordingly, regardless of whether the wafer is a wafer to be processed to form a reflective film on the back side of the wafer, the modified layer may be formed in the wafer along each division line. Further, the wafer may be transported to the position for performing the backside processing step in the state where the breaking of the wafer from the modified layers is prevented. Accordingly, the reflective film can be formed on the back surface of the wafer in the back surface processing step, thereby facilitating the manufacture of an optical device wafer.The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent by studying the following specification and the appended claims with reference to the accompanying drawings which show some preferred embodiments of the invention.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a perspective view showing a manner of fixing a protection member to the front side of a wafer; FIG. 2 is a perspective view showing a manner of holding the wafer on a chuck table so that the protective member attached to the front side of the wafer comes into contact with the upper surface of the chuck table; FIG. 3 is a perspective view showing a manner of forming a plurality of modified layers in the wafer; FIG. 4 is a sectional view showing a manner of forming a plurality of modified layers in the wafer; FIG. 5 is a perspective view of the wafer in which the modified layers are formed; FIG. 6 is a sectional view showing a circumferential amplifying portion and one of the modified layers formed in the laser shown in FIG. 5 ; FIG. 7 is a side view showing a manner of transporting the wafer held on the chuck table; FIG. 8 is a side view of the wafer in the state where a reflection film is formed on the back side of the wafer; FIG. 9 is a perspective view of the wafer in the state where the reflective sheet formed on the back side of the wafer is attached to a separation board supported on an annular frame and the protection board attached on the front side of the wafer is removed; FIG. 10 is a sectional view showing a separating step; and FIG. 11 is an enlarged plan view showing a modification of the wafer.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSReferring to FIG. 1, the wafer W is an optical device wafer formed by forming a light emitting layer (epitaxial layer) on the front surface of a sapphire substrate. A plurality of intersecting division lines S are formed on a front side Wa of the wafer W to thereby divide a plurality of rectangular regions, and a plurality of optical devices D are respectively formed. The optical devices D (optical devices) are formed in a device area W 1 (device area) and are not formed in a peripheral marginal area W 2 (peripheral marginal area). The device area W 1 is surrounded by the peripheral edge area W 2.(1) Modified Layer Forming StepAs shown in FIG. 1, a protection member such as a protection tape is attached to the front side Wa of the wafer W. As shown in FIG. 2, the wafer W in which the protective member 1 is fixed to the front side Wa is inverted and placed on a chuck table 2 of a laser processing apparatus in the state where the protective member 1 fixed to the front side Wa comes into contact with the upper surface of the chuck table 2. Subsequently, the wafer W is held by applying negative pressure to the upper surface of the chuck table 2 at its suction portion 20 in the state where the back surface Wb of the wafer is directed upward or exposed. At this time, a reflective film has not yet been formed on the back surface Wb.In this step, the division lines S to which a laser beam is to be applied are detected. This determination can be made by providing a conventional camera on the chuck table 2 side opposite to the front side Wa of the wafer W and by exposing the front side Wa. Alternatively, this detection may be performed by providing an infrared camera over the back surface Wb of the wafer W and exposing the front surface Wa from the back surface Wb through the wafer W by using infrared light.As shown in FIG. 3, a laser beam 30 is applied from a laser beam head 3 toward the back side Wb of the wafer W held on the chuck table 2 when the chuck table 2 is moved in the X direction. The laser beam 30 has a transmission wavelength toward the wafer W. As shown in FIG. 4, the laser beam 30 is applied toward the back side Wb of the wafer W along the division lines S formed on the front side Wa in the state where the focal point 31 of the laser beam 30 is set inside the wafer W. For this reason, the laser processing is performed in the following states.Light source: YAG pulsed laserWavelength: 1,064 nmAverage power 0.1 to 0.4 WRepetition frequency: 100 kHzFeed speed: 300 to 800 mm / SThickness of sapphire substrate: 120 μmPosition of the focal point: 60 μm from the back sideWidth of the modified layer: 30 μmThis laser processing is performed along a predetermined one of the division lines S extending in a first direction, thereby forming a modified layer 32. Then, the laser head 3 is indexed in the Y direction by the pitch of the division lines to similarly perform the laser processing along the remaining division lines S extending in the first direction. Subsequently, the chuck table 2 is rotated by 90° to similarly perform the laser processing along all the division lines S extending in a second direction perpendicular to the first direction, thereby forming a plurality of modified layers 32 in the wafer W along all the division lines S extending in the second direction. As a result, the intersecting modified layers 32 are formed in the wafer W along all the intersecting division lines S extending in the first and second directions as shown in FIG. 5. Each modified layer 32 acts as a separation start point in separating the wafer W.In forming the modified layers 32, the laser beam 30 is applied only to the device region W 1 of the wafer W, and the laser beam 30 is not applied to the peripheral edge region W 2 of the wafer W. Accordingly, as shown in FIGS. 5 and 6, an annular reinforcing portion W 3 having no modified layers is formed in the peripheral edge region W 2. That is, if the modified layers 32 are also formed in the peripheral edge portion W 2, the wafer W easily breaks. However, such easy breakage of the wafer W can be prevented by the reinforcing portion W 3. This modified layer forming step is performed before forming a metal foil (reflective foil) on the back surface Wb of the wafer W. Accordingly, there is no obstacle to the wafer W when applying the laser beam to the wafer W toward the back side Wb, so that the laser beam can be reliably focused inside the wafer W.(2) Transporting StepAs shown in FIG. 7, the wafer W having the modified layers 32 formed in the facility area W 1 is unloaded from the chuck table 2 and transported to a position where the next step is to be performed. For example, this transporting step is performed by using a transporting device 4 having an arm portion 40 and a suction portion 41 provided at the lower end of the arm portion 40. That is, the back surface Wb of the wafer W is held by negative pressure of the suction portion 41 of the transfer device 40, and the arm portion 40 is moved to thereby transfer the wafer W held by the suction portion 41 to a next step performing apparatus. In the wafer W to be transferred by the transferring step, the modified layers 32 are formed only in the device area W 1, and the reinforcing portion W 3 having no modified layers is formed around the device area W 1. Accordingly, even if any external force is applied to the wafer W in the transporting step, there is no possibility that the wafer W can be broken along the modified layers 32.(3) Backside Processing StepThe wafer W is transported in the transporting step toward an apparatus for performing a back side processing step in which a reflective film is formed on the back side Wb of the wafer W. As shown in FIG. 8, a reflective film 5 made of gold, aluminum, etc. is formed on the back surface Wb of the wafer W in the back surface processing step. This reflecting sheet 5 is formed to improve the luminance of each optical device. For example, the reflective film 5 is formed by evaporation, sputtering, CVD, etc. Since the wafer W is not broken in the transporting step as described above, the reflective sheet 5 can be reliably formed on the back surface Wb of the wafer W.(4) Separating StepAfter performing the backside processing step, the wafer is attached to a dicing tape T (Dicing tape) in the state where the reflective sheet 5 formed on the backside Wb comes into contact with the dicing tape T. An annular frame F is preliminarily fixed to the peripheral portion of the separation chamber T so that the wafer is supported by the separation chamber T on the annular frame F. Further, the protection member 1 is removed from the front side Wa of the wafer W at this time.In the state where the wafer W is supported by the separation gap T on the annular frame F as described above, the reflective sheet 5 formed on the back surface Wb of the wafer W is held under vacuum at each adjacent optical device D 1 and D 2 by means of a pair of vacuum portions 60 and 61 through the separation gap T as shown in FIG. 10. In this suction holding state, the vacuum portions 60 and 61 are moved in opposite horizontal directions B and C as shown in FIG. 10, to thereby apply a horizontal external force to the parting line S between these adjacent optical devices D 1 and D 2. As a result, the modified layer 32 extending along the parting line S between the adjacent optical devices D1 and D2 starts to break, thereby separating the optical devices D1 and D2 from each other. Similarly, all the other optical devices are separated along the division lines S, thereby dividing the wafer W into the individual optical devices S. The reflecting sheet 5 remains on the back side of each optical device D.In the case of a wafer Wx shown in FIG. 11, a part of the optical devices D is formed in the vicinity of the outer periphery Wc of the wafer Wx, and the width of the peripheral edge portion W 2 is thus very small. In this case, the modified layers 32 are formed in the modified layer forming step so that the opposite ends of each modified layer 32 do not reach the outer periphery of the device region W 1. That is, the modified layers 32 are formed so that the optical devices D are not completely separated along the modified layers 32, whereby the width of the reinforcing portion W 3 becomes larger than the width of the peripheral edge region W 2. In the next separating step, the pulling force for separating the adjacent optical devices from each other along each modified layer 32 is used as a breakage starting point to cut the reinforcing portion W 3 along the extension lines 32 aof the modified layers 32 as shown in FIG. 11. In such a case that a part of the optical devices D is formed in the vicinity of the outer periphery Wc near the wafer Wx, the reinforcing portion W 3 is formed to extend inward from the peripheral edge region W 2 into the device region W 1.

Claims

A processing method for a wafer (W) having a device area (W1) in which a plurality of devices (D) are formed, and a peripheral edge area (W2) surrounding the device area (W1), wherein the plurality of devices (D) are respectively formed at a plurality of areas divided by a plurality of division lines (S) formed on a front side of the wafer (W), the processing method comprising: a modified layer forming step in which the wafer (W) is held on a chuck table (2) in the state in which a back side (Wb) of the wafer (W) is exposed, A laser beam having a transmission wavelength is applied to the wafer (W) toward the back side (Wb) of the wafer (W) along the division lines (S) in the state where the focal point of the laser beam is set to the inside of the wafer (W), thereby forming a modified layer as a division start point inside the wafer (W) along each division line (S); and a transporting step in which the wafer (W) is unloaded from the chuck table (2) and the wafer (W) is transported toward a position where a next step to be performed is performed; wherein the modified layer is not formed in the peripheral edge portion (W 2) of the wafer (W) in the modified layer forming step, to thereby form a reinforcing portion of the peripheral edge portion (W 2) that inhibits breakage of the wafer (W) on the modified layer during the transporting step.The processing method for a wafer according to claim 1, wherein: the wafer (W) comprises an optical device wafer (D) formed of a sapphire substrate and a plurality of optical devices (D) formed on a front surface of the sapphire substrate at a plurality of regions divided by a plurality of division lines (S); and wherein the step to be performed next after performing the transporting step comprises a back surface processing step in which a reflection film (5) is formed on a back surface (Wb) of the optical device wafer (W).The processing method for a wafer according to claim 1, further comprising a separating step of applying an external force to the division lines (S) of the wafer (W) to thereby divide the wafer (W) into the individual devices (D) after performing the modified layer forming step.

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