Wafer processing processes

Abstract

Method for processing a wafer formed by cutting a single crystal ingot (2), comprising: a crystal stress removal step for removing crystal stresses remaining in a circumferential section (24) of the wafer (2), In the crystal stress removal step, a laser beam (LB) having a wavelength such that it is transmitted through the wafer (2) is applied from one side (2a) of the wafer (2) to positions arranged along an edge of the wafer (2) and spaced inwards from the edge by a predetermined distance, so that fine openings (221) and amorphous regions shielding the fine openings (221) are formed over a region from one side (2a) to the other side (2b) of the wafer (2), thereby forming shielding tunnels (22) in an annular pattern, and an external force is exerted on the wafer (2) along the shielding tunnels (22) to fracture the wafer (2) in a region of the shielding tunnels (22), thereby removing the circumferential section (24) in which the crystal stresses remain.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a method for processing a wafer made of silicon carbide (SiC), gallium nitride (GaN) or the like to become a substrate on which a light-emitting layer for forming optical devices such as LEDs is laminated. Description of the state of the art

[0002] A wafer of silicon carbide (SiC), gallium nitride (GaN), or the like, on which a light-emitting layer for forming optical devices such as LEDs is to be laminated, is formed by cutting a single-crystal ingot of silicon carbide (SiC), gallium nitride (GaN), or the like into a predetermined thickness by a wire saw or the like (see, for example, Japanese Patent Application Laid-Open No. 2010-207988 A). The front surface and / or the back surface of the wafer formed by cutting the single-crystal ingot is ground and finished to a mirror surface.

[0003] Further prior art helpful for understanding the present invention can be found in the following documents: JP 2006- 339 431 A relates to a method for processing a substrate produced by a hybrid vapor phase epitaxy process. JP 2004- 39 808 A relates to a semiconductor substrate manufacturing method and an apparatus therefor. SUMMARY OF THE INVENTION

[0004] However, when grinding the front or back of a wafer, warpage and eventual breakage of the wafer may occur, resulting in low productivity. Furthermore, when a wafer warpage occurs, it is impossible to uniformly deposit a light-emitting layer on the wafer, resulting in variations in the quality of the final products, namely optical devices such as LEDs. Warpage is believed to occur during grinding the front or back of a wafer because crystal stress remains in a peripheral portion of the wafer during ingot production.

[0005] Accordingly, it is an object of the present invention to provide a wafer processing method capable of processing a wafer formed by slicing a single crystal ingot while preventing warpage of the wafer.

[0006] According to a first aspect of the present invention, there is provided a method for processing a wafer formed by slicing a single crystal ingot, which includes a crystal strain removing step for removing crystal strains remaining in a peripheral portion of the wafer.

[0007] According to the first aspect, in the crystal stress removing step, a laser beam having a wavelength such that it is transmitted through the wafer is applied to the wafer from one side of the wafer at positions arranged along an edge of the wafer and spaced inward from the edge by a predetermined distance, so that fine openings and amorphous regions shielding the fine openings are caused to form over a range from one side to the other side of the wafer, thereby forming shielding tunnels in an annular pattern, and preferably, an external force is applied to the wafer along the shielding tunnels to break the wafer in a range of the shielding tunnels, thereby removing the peripheral portion where the crystal stress remains.

[0008] Preferably, a mark forming step for forming a mark indicating a crystal orientation is performed at a position on an inner side of a region where a laser beam is applied on the one side of the wafer.

[0009] In the method for processing a wafer formed by slicing a single-crystal ingot according to the present invention, the crystal strain removal step is performed to remove the crystal strains remaining in the peripheral portion of the wafer. Therefore, even when one side or the other side of the wafer is ground, warpage of the wafer due to the existence of the residual crystal strains is prevented. Accordingly, when a light-emitting layer is laminated on the ground side of the wafer in a subsequent step, a light-emitting layer with a uniform thickness can be formed due to the absence of warpage.

[0010] The above and other objects, features and advantages of the present invention and the mode for carrying them out will become more apparent and the invention itself will be best understood by studying the following description and the appended claims with reference to the accompanying drawings which show some preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a wafer formed by cutting a single crystal ingot; Fig. 2 is a perspective view of a main part of a laser processing apparatus for performing a first embodiment of a crystal stress removing step; Fig. 3A to 3C are views for explaining the first embodiment of the crystal strain removing step; Fig.4 is a view for explaining a mark forming step for forming a mark indicating a crystal orientation of the wafer subjected to the first embodiment of the crystal stress removing step; Fig. 5 is a view for explaining a peripheral portion removing step in the crystal strain removing step; Fig. 6 is a perspective view of a main part of a laser processing apparatus for performing a second embodiment of the crystal stress removing step; Fig. 7A and Fig. 7B are views for explaining the second embodiment of the crystal strain removing step; and Fig. 8A and Fig. 8B are views for explaining a grinding step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The wafer processing method according to the present invention will be described in more detail below with reference to the drawings. Fig. 1 shows a perspective view of a wafer to be processed by the wafer processing method of the present invention. Fig. The wafer 2 shown in FIG. 1 is formed by cutting a single-crystal ingot of silicon carbide (SiC), gallium nitride (GaN), or the like into a predetermined thickness (for example, 700 to 800 µm) with a wire saw or the like, and is provided with a notch 21 on its periphery indicating the crystal orientation. The wafer 2 thus formed by cutting a single-crystal ingot has crystal stresses remaining in a peripheral portion thereof.

[0012] In the wafer processing method of the present invention, a crystal stress removal step is performed to remove the crystal stresses remaining in the peripheral portion of the wafer 2. A first embodiment of the crystal stress removal step is carried out using a Fig. 2 shown laser processing device 3. The Fig. The laser processing device 3 shown in Figure 2 includes a clamping table 31 for holding a workpiece thereon and a laser beam application means 32 for applying a laser beam to the workpiece held on the clamping table 31. The clamping table 31 is configured to hold the workpiece by suction and in the Fig. 2 to be rotated in the direction indicated by the arrow 31a by a rotating mechanism (not shown).

[0013] The laser beam application means 32 includes a substantially horizontally arranged cylindrical housing 321. A pulse laser beam oscillation means (not shown) including a pulse laser beam oscillator and a repetition frequency adjusting means is arranged within the housing 321. A focusing device 322 having a focusing lens 322a (see Fig.3) for focusing the pulsed laser beam oscillated by the pulse laser beam oscillation means is attached to a tip portion of the casing 321. The focusing lens 322a of the focusing device 322 has a numerical aperture (NA) set as follows. The numerical aperture (NA) of the focusing lens 322a is set so that a value obtained by dividing the numerical aperture (NA) by the refractive index (N) of the single-crystal substrate is within the range of 0.05 to 0.2 (numerical aperture setting step). Note that the laser beam application means 32 includes focus position adjusting means (not shown) for adjusting the focus position of the pulsed laser beam focused by the focusing lens 322a of the focusing device 322.

[0014] The first embodiment of the crystal stress removing step performed by using the laser processing apparatus 3 described above will be described with reference to Fig. 3A to 3C. In the first embodiment of the crystal strain removal step, first, as shown in Fig.3A, the wafer 2 is placed on the chuck table 31 of the laser processing apparatus 3 with one side 2a of the chuck table 31 facing upward and the other side 2b of the chuck table 31 facing downward, and is held by suction on the chuck table 31. The chuck table 31 with the wafer 2 held thereon by suction is moved by a moving mechanism (not shown) into a processing area in which the focusing device 322 is arranged, and the chuck table 31 is arranged so that a position spaced inward from the periphery (edge) of the wafer 12 by a predetermined distance is located immediately below the focusing device 322.Subsequently, the focus position adjusting means (not shown) is operated to move the focusing device 322 in an optical axis direction so that the focus P of a pulsed laser beam LB focused by the focusing lens 322a of the focusing device 322 is arranged at a desired position in the thickness direction of the wafer 2 (arranging step). Note that in this embodiment, the focus P of the pulsed laser beam is arranged at a desired position within the wafer 2 with respect to the upper surface (one side 2a) to which the pulsed laser beam is to be applied (the desired position is, for example, a position spaced from one side 2a by 5 to 10 μm toward the other side 2b of the wafer 2).

[0015] Subsequently, while the pulsed laser beam LB having a wavelength such that it is transmitted through the wafer 2 is applied by the focusing device 322, the chuck table 31 is rotated in the direction of the arrow 31a in Fig. 3A (shielding tunnel formation step). After the chuck table 31 has been rotated one revolution, the application of the pulsed laser beam is stopped and the rotation of the chuck table 31 is stopped, as shown in Fig. 3B is shown.

[0016] When the shielding tunnel formation step described above is performed, fine openings 221 and amorphous regions 222 surrounding the fine openings 221, both extending from one side 2a (upper surface) of the wafer 2 toward the other side 2b (lower surface) of the wafer 2, are formed in the vicinity of each focus P of the pulsed laser beam LB, as shown in Fig. 3B and Fig.3C. As a result, amorphous shielding tunnels 22 are formed in a ring-shaped pattern along the periphery or edge of the wafer 2. The shielding tunnels 22 are formed such that the adjacent amorphous regions 222 are connected to each other. Note that the amorphous shielding tunnels 22 formed in the shielding tunnel forming step described above can be formed over the range from one side 2a as the upper surface of the wafer 2 to the other side 2b as the lower surface of the wafer 2. Therefore, even when the wafer 2 is thick, it is sufficient to perform the application of the pulsed laser beam only once. Therefore, the productivity of this step is very high.

[0017] It should be noted that the processing conditions in the shielding tunnel formation step are set as follows: Wavelength of the laser beam: 1030 nm Repetition frequency: 50 kHz Pulse width: 10 hp Average output power: 6 W Focused spot diameter: 3.3 to 13 µm (The focused spot diameter can be obtained by the formula d = (2λ) / (π ·NA). When calculated assuming that the wavelength λ is 1030 nm, d is 3.3 µm when NA is 0.2, and d is approximately 13 µm when NA is 0.05.) Peripheral speed: 200 mm / second.

[0018] It should be noted that it is preferable that a mark forming step for forming a mark 23 indicating the crystal orientation is applied to the upper surface (one side 2a) of the wafer 2 that has been subjected to the above-described shield tunnel forming step by disposing the focusing device 322 at a position immediately above the position corresponding to the notch 21 on the inner side of the laser beam application area, and applying a pulsed laser beam to the wafer 2 as shown in Fig.4. The mark formation step may be performed before the shield tunnel formation step. Note that the means to be used in the mark formation step is not limited to the laser beam, and a mark 23 indicating the crystal orientation may be formed by other means, such as a printing means.

[0019] The shielding tunnels 22 formed in an annular pattern in the peripheral portion of the wafer 2 by performing the above-described shielding tunnel formation step have reduced strength. Therefore, when an external force is applied to the wafer 2 along the shielding tunnels 22 formed in the annular pattern, the wafer 2 is fractured in the region of the shielding tunnels 22, and the peripheral portion 24 in which the crystal stresses remain is removed, as shown in Fig.5 (peripheral portion removal step). Note that, although the crystal orientation indicating notch 21 formed on the periphery of the wafer 2 is removed together with the peripheral portion 24 by the peripheral portion removal step, the crystal orientation indicating mark 23 is formed by the mark forming step at the position (on the inner side of the shielding tunnels 22) corresponding to the notch 21, so that the crystal orientation can be confirmed even after the peripheral portion 24 is removed.

[0020] A second embodiment of the crystal stress removing step for removing crystal stress remaining in a peripheral portion of the wafer 2 will be described below with reference to Fig. 6, Fig. 7A and Fig.7B. It should be noted that a laser processing apparatus 30 for carrying out the second embodiment has substantially the same structure as that shown in Fig. 2, and therefore the same elements as those shown in Fig. 2 are designated by the same reference numerals as in Fig. 2 are used, and their description is omitted here.

[0021] In the second embodiment, a laser beam having a wavelength such that it is absorbed in the wafer 2 is applied to the wafer 2 from one side of the wafer 2 along a line extending along the periphery or edge of the wafer 2 and spaced inward from the edge by a predetermined distance to cause ablation of the wafer 2 and form a laser-machined groove along the edge, thereby removing the peripheral wafer portion in which the crystal stresses remain. Specifically, as shown in Fig. 6 and Fig.7A, a position spaced inward by a predetermined distance from the edge of the wafer 2 held by the chuck table 31 is arranged at a position immediately below the focusing device 322. Subsequently, while a pulsed laser beam having a wavelength such that it is absorbed in the wafer 2 is applied to the wafer 2 by the focusing device 322, the chuck table 31 is rotated by one revolution in the direction of the arrow 31a in Fig. 7A. In this case, the focus P of the pulsed laser beam applied by the focusing device 322 is located in the vicinity of a side 2a as the upper surface of the semiconductor wafer 2. As a result, as shown in Fig.7B, an annular laser-machined groove 25 extending from one side 2a (the upper surface) of the wafer 2 to the other side 2b (the lower surface) of the wafer 2 is formed along a line spaced inward from the edge of the wafer 2 by a predetermined distance, thereby removing the peripheral portion 24 located on the outer side of the annular laser-machined groove 25 and in which the crystal stresses remain (laser-machined groove forming step).

[0022] For example, the machining conditions in the previously described step of forming a laser-machined groove are set as follows: Wavelength of the laser beam: 355 nm Repetition frequency: 10 kHz Pulse width: 50 ns Average output power: 7 W Focused spot diameter: 5 to 15 µm Peripheral speed: 500 mm / second.

[0023] It should be noted that it is preferable that, after or before the step of forming the laser-machined groove as described above, a mark forming step of forming a mark 23 (see Fig.6) indicating the crystal orientation is performed on the upper surface (one side 2a) of the wafer 2 by disposing the focusing device 322 at a position immediately above the position corresponding to the notch 21 on the inner side of the laser beam application area, and applying a pulsed laser beam to the upper surface (one side 2a).

[0024] After the crystal stress removal step, which includes the previously described shield tunnel formation step or the previously described laser-machined groove formation step, is performed, a grinding step is performed in which one side of the wafer, whose peripheral portion has been removed, is ground to bring the wafer to a predetermined final thickness. The grinding step is carried out using a Fig. 8A shown grinding device 4. The grinding process in Fig.The grinding device 4 shown in Fig. 8A includes a chuck table 41 for holding a workpiece thereon and a grinding means 42 for grinding the workpiece held on the chuck table 41. The chuck table 41 is adapted to hold the workpiece on its upper surface, which forms a holding surface, under suction and is rotated by a rotary drive mechanism (not shown) in the direction of arrow 41a in Fig.8A. The abrasive 42 includes a spindle housing 421, a rotating spindle 422 rotatably supported by the spindle housing 421 and rotated by the rotation drive mechanism (not shown), an attachment 423 attached to a lower end of the rotating spindle 422, and a grinding wheel 424 attached to a lower surface of the attachment 423. The grinding wheel 424 includes an annular base 425 and grinding stones 426 attached to a lower surface of the base 425 in an annular pattern. The base 425 is attached to the lower surface of the attachment 423 by fastening bolts 427.

[0025] When performing the grinding step by using the grinding device 4 described above, first, the wafer 2 is placed on the chuck table 41 with the other side 2b of the wafer 2 in contact with the upper surface (holding surface) of the chuck table 41, as shown in Fig.8A. Subsequently, a suction means (not shown) is actuated to hold the wafer 2 by suction on the chuck table 41 (wafer holding step). After the wafer 2 is thus held by suction on the chuck table 41, the chuck table 41 is rotated, for example, at 300 rpm in the direction of arrow 41a in Fig. 8A. While the clamping table 41 is rotated in this way, the grinding wheel 424 of the abrasive 42 is rotated at, for example, 6000 rpm in the direction of the arrow 424a in Fig. 8A. While the chuck table 41 and the grinding wheel 424 are rotated in this manner, the grinding stones 426 constituting the grinding wheel 424 are brought into contact with one side 2a as the upper surface of the wafer 2, i.e., the working surface, as shown in Fig.8B, and the grinding wheel 424 is brought in the downward direction (in the direction perpendicular to the holding surface of the chuck table 41) at a grinding feed speed of, for example, 1 µm / second in a predetermined amount of grinding feed as shown by the arrow 424b in Fig. 8A and Fig.8B. As a result, one side 2a as the upper surface of the wafer 2 is ground to form the wafer 2 with a predetermined thickness (for example, 300 μm). Even when the wafer 2 is thus ground to the predetermined thickness, no warpage of the wafer 2 occurs because the peripheral wafer portion where the crystal stress remains has been removed from the wafer 2. Accordingly, when a light-emitting layer is laminated on the ground side of the wafer 2 in a subsequent step, a light-emitting layer with a uniform thickness can be formed due to the absence of warpage.

Claims

[1] A method for processing a wafer (2) formed by cutting a single-crystal ingot, comprising: a crystal stress removal step for removing crystal stresses remaining in a peripheral portion (24) of the wafer (2), in which, in the crystal stress removal step, a laser beam (LB) having a wavelength such that it is transmitted through the wafer (2) is applied to the wafer (2) from one side (2a) of the wafer (2) at positions arranged along an edge of the wafer (2) and spaced inward from the edge by a predetermined distance, so as to cause fine openings (221) and amorphous regions shielding the fine openings (221) to form over a range from one side (2a) to the other side (2b) of the wafer (2), thereby forming shielding tunnels (22) in an annular pattern, and an external force is applied to the wafer (2) along the shielding tunnels (22) to break the wafer (2) in a range of the shielding tunnels (22), thereby removing the peripheral portion (24) in which the crystal stresses remain. [2] A method of processing a wafer according to claim 1, further comprising a mark forming step of forming a mark (23) indicating a crystal orientation at a position on an inner side of a region where a laser beam (LB) is applied on said one side of said wafer (2).

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