Grinding method

Abstract

The present invention provides a grinding method that can detect the timing at which irregularities have been removed from the edge face of a wafer. [Solution] The grinding method includes a holding step of holding the other end face of the wafer on a chuck table, and a grinding step of grinding one end face of the wafer with a grinding means equipped with a grinding wheel on which grinding wheels are arranged in an annular shape. The grinding step includes a stylus positioning step of positioning the stylus of a contact-type thickness measuring instrument on one end face of the wafer, a first grinding step of rotating the chuck table and the grinding wheel and feeding the grinding means at a first feed rate to grind one end face of the wafer and remove irregularities, and a second grinding step of feeding the grinding means at a second feed rate to grind the one end face of the wafer from which the irregularities have been removed and flatten it. In the first grinding step, when the amplitude of the stylus swing becomes small and reaches a predetermined value, it is assumed that the irregularities have been removed from one end face of the wafer and the process moves to the second grinding step.

Description

Technical Field

[0001] The present invention relates to a grinding method for flattening one end face of a wafer produced from an ingot.

Background Art

[0002] A wafer having a plurality of devices such as ICs and LSIs formed on its surface is thinned by grinding its back surface with a grinding device, and then divided into individual device chips by a dicing device or a laser processing device. The divided device chips are used in electrical devices such as mobile phones and personal computers.

[0003] The grinding device is also used when flattening the end face of a wafer produced by slicing an ingot (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When slicing an ingot to produce a wafer, cutting means or laser beam irradiation means is used. Since the end face (sliced surface) of the wafer sliced from the ingot by the cutting means or the laser beam irradiation means is uneven, the end face of the wafer is ground by grinding means provided with a grinding wheel to remove the unevenness. In the grinding mode for removing the unevenness of the end face of the wafer, the grinding means is fed at a relatively high feed rate. However, if the grinding feed of the grinding means is continued at the relatively high feed rate even after the removal of the unevenness of the wafer is completed, there are problems such as damaging the wafer or causing distortion in the grinding means.

[0006] On the other hand, if the grinding mode for removing irregularities is ended before the wafer's surface irregularities have been completely removed, the wafer thickness measurement in the subsequent planar grinding mode will be inaccurate, leading to deviations from the target thickness and inconsistent quality.

[0007] In particular, with high-hardness SiC ingots, when a delamination layer is formed by irradiating the SiC ingot with a laser beam using the technology proposed by the present applicant (see, for example, Japanese Patent Publication No. 2016-111143), and a SiC wafer is produced by slicing the SiC ingot from the delamination layer, the edge surface of the SiC wafer becomes a sharp, uneven surface like a bed of nails. Therefore, it is difficult to determine the timing when the unevenness has been removed, and the aforementioned problems are likely to occur.

[0008] The object of the present invention is to provide a grinding method that can detect the timing at which irregularities are removed from the edge face of a wafer. [Means for solving the problem]

[0009] According to the present invention, the following grinding method is provided that solves the above problems. That is, A grinding method for flattening one end face of a wafer produced from an ingot, A holding step of holding the other end face of the wafer on a chuck table, The process includes grinding one end face of a wafer held on a chuck table using a grinding means equipped with a grinding wheel in which grinding wheels are arranged in an annular pattern, The grinding process is, A stylus positioning step in which the stylus of a contact-type thickness measuring instrument is positioned on one end face of the wafer, A first grinding step involves rotating the chuck table and the grinding wheel to feed the grinding means at a first feed rate to grind one end face of the wafer and remove irregularities, The system comprises a second grinding step in which the grinding means is fed at a second feed rate to grind and flatten one end face of the wafer from which the irregularities have been removed, A grinding method is provided in which, in the first grinding step, when the amplitude of the stylus decreases to a predetermined value, the method proceeds to the second grinding step, assuming that the irregularities have been removed from one end face of the wafer.

[0010] Preferably, a laser beam of a penetrating wavelength is focused at a predetermined depth from the end face of the SiC ingot, and the SiC ingot is irradiated with the laser beam to form a release layer, after which the wafer is peeled off from the release layer. In this holding step, it is desirable to place a resin that absorbs irregularities on the other end face of the wafer and hold the other end face of the wafer on the chuck table. Alternatively, one end face of the wafer may be swapped with the other end face, and the other end face may be ground to flatten it. [Effects of the Invention]

[0011] The grinding method of the present invention, A grinding method for flattening one end face of a wafer produced from an ingot, A holding step of holding the other end face of the wafer on a chuck table, The process includes grinding one end face of a wafer held on a chuck table using a grinding means equipped with a grinding wheel in which grinding wheels are arranged in an annular pattern, The grinding process is, A stylus positioning step in which the stylus of a contact-type thickness measuring instrument is positioned on one end face of the wafer, A first grinding step involves rotating the chuck table and the grinding wheel to feed the grinding means at a first feed rate to grind one end face of the wafer and remove irregularities, The system comprises a second grinding step in which the grinding means is fed at a second feed rate to grind and flatten one end face of the wafer from which the irregularities have been removed, In the first grinding step, when the swing width of the stylus becomes small and reaches a predetermined value, it is assumed that the unevenness has been removed from one end face of the wafer, and the process proceeds to the second grinding step. Therefore, it is possible to detect the timing at which the unevenness has been removed from the end face of the wafer. Even though the removal of the unevenness of the wafer has been completed, due to continuing the grinding feed of the grinding means at a relatively high feed rate, it is possible to prevent damage to the wafer or distortion of the grinding means. Also, if the first grinding step for removing the unevenness is ended even though the removal of the unevenness of the wafer has not been completed, the thickness measurement of the wafer in the next second grinding step will become inaccurate, resulting in a deviation from the target thickness and unstable quality, and this problem is solved.

Brief Description of the Drawings

[0012] [Figure 1] Perspective view of a wafer with one end face being an uneven surface. [Figure 2] (a) Plan view of an ingot, (b) Front view of the ingot shown in (a), (c) Perspective view of the ingot shown in (a). [Figure 3] (a) Perspective view when forming a release layer on an ingot, (b) Front view when forming a release layer on an ingot, (c) Cross-sectional view of the ingot with the release layer formed. [Figure 4] Schematic diagram when peeling a wafer from a release layer. [Figure 5] Schematic diagram when disposing resin on the other end face of a wafer. [Figure 6] Schematic diagram showing a holding process. [Figure 7] Schematic diagram showing the stylus positioning step of the grinding process. [Figure 8] Schematic diagram showing the first grinding step of the grinding process. [Figure 9] Graph showing an example of the relationship between the thickness of a wafer and the grinding time. [Figure 10] Schematic diagram showing the second grinding step of the grinding process.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, a preferred embodiment of the grinding method according to the present invention will be described with reference to the drawings.

[0014] (Wafer 2) FIG. 1 shows a disk-shaped wafer 2 that can be ground by the grinding method of the present embodiment. The wafer 2 can be formed from an appropriate semiconductor material such as SiC (silicon carbide). The diameter of the wafer 2 is, for example, about 200 mm, and the thickness of the wafer 2 is, for example, about 700 μm to 750 μm. On the outer periphery of the wafer 2, a first orientation flat 4 and a second orientation flat 6 indicating the crystal orientation are formed. Further, at least one end face 2a (the upper face in FIG. 1) of the wafer 2 is an uneven face. On the other hand, the other end face 2b of the wafer 2 may be an uneven face or a flat face.

[0015] (Ingot 8) FIGS. 2(a) to 2(c) show a columnar ingot 8 for generating the wafer 2. The ingot 8 can be formed from an appropriate semiconductor material such as SiC (silicon carbide). The ingot 8 has a circular first end face 10, a circular second end face 12 located on the opposite side of the first end face 10, a peripheral face 14 located between the first end face 10 and the second end face 12, a c-axis extending from the first end face 10 to the second end face 12, and a c-plane (see FIG. 2(b)) orthogonal to the c-axis. At least the first end face 10 is flattened by grinding or polishing to such an extent that it does not prevent the incidence of laser light. Further, in the ingot 8, the c-axis is inclined with respect to the perpendicular 16 of the first end face 10, and an off-angle α (for example, α = 1, 3, 6 degrees) is formed between the c-plane and the first end face 10. The direction in which the off-angle α is formed is indicated by an arrow A.

[0016] On the circumferential surface 14 of the ingot 8, a first orientation flat 18 and a second orientation flat 20 are formed, both indicating the crystal orientation. The first orientation flat 18 is parallel to the direction A in which the off-angle α is formed. The second orientation flat 20 is perpendicular to the direction A in which the off-angle α is formed. As shown in Figure 2(a), when viewed from above, the length L2 of the second orientation flat 20 is shorter than the length L1 of the first orientation flat 18 (L2 <L1)。

[0017] The ingot used to produce wafer 2 is not limited to the ingot 8 described above. For example, it may be a SiC ingot in which the c-axis is not tilted with respect to the perpendicular to the first end face and the off-angle α between the c-plane and the first end face is 0 degrees (i.e., the perpendicular to the first end face coincides with the c-axis). Alternatively, it may be an ingot formed from a material other than SiC, such as Si (silicon), Al2O3 (sapphire), or GaN (gallium nitride).

[0018] (Forming a delamination layer on ingot 8) When producing wafer 2 from ingot 8, first, a laser beam of a penetrating wavelength is focused at a predetermined depth from the first end face 10 of ingot 8, and the laser beam is irradiated onto ingot 8 to form a delamination layer.

[0019] The peeled layer can be formed, for example, using the laser processing apparatus 22 shown in Figure 3(a). The laser processing apparatus 22 includes a holding table 24 for holding the ingot 8, an oscillator (not shown) that emits a pulsed laser beam LB with a wavelength that is transparent to the ingot 8, and a concentrator 26 that focuses the pulsed laser beam LB emitted by the oscillator and irradiates the ingot 8 held on the holding table 24 with the pulsed laser beam LB.

[0020] When forming the release layer, first, the ingot 8 is held in place by the holding table 24. At this time, with the first end face 10 facing upwards, the ingot 8 is fixed to the upper surface of the holding table 24 using an appropriate adhesive (for example, an epoxy resin adhesive). If multiple suction holes are formed in the upper surface of the holding table 24, a suction force may be generated on the upper surface of the holding table 24 to hold the ingot 8 in place by suction.

[0021] Once the ingot 8 is held in the holding table 24, the orientation of the ingot 8 is adjusted, as is the position of the focusing point of the laser beam LB. At this time, the imaging means (not shown) of the laser processing device 22 is used to image the ingot 8 from above, and the imaged ingot 8 Based on the image, the second orientation flat 20 is aligned in the X-axis direction indicated by arrow X in Figure 3(a). That is, the direction perpendicular to the direction A in which the off-angle α is formed is aligned in the X-axis direction. In addition, the focal point FP of the laser beam LB (see Figure 3(b)) is positioned at a depth corresponding to the thickness of the wafer 2 from the first end face 10 of the ingot 8. Note that the Y-axis direction indicated by arrow Y in Figure 3(a) is perpendicular to the X-axis direction, and the XY plane defined by the X-axis and Y-axis directions is essentially horizontal.

[0022] After adjusting the orientation of the ingot 8 and the position of the focal point FP of the laser beam LB, the laser beam LB is irradiated onto the ingot 8 to form a delamination layer. During this process, the focal point FP and the ingot 8 are moved relative to each other in the X-axis direction (the direction perpendicular to the direction A in which the off-angle α is formed), while the laser beam LB with a wavelength that is transparent to the ingot 8 is irradiated onto the ingot 8 from the concentrator 26. This allows for the formation of a linear modified layer 28 along the X-axis direction in which SiC is separated into Si (silicon) and C (carbon), as shown in Figure 3(c). Cracks 30 extending along the c-plane are also formed from the modified layer 28. Next, the focal point FP and the ingot 8 are indexed relative to each other in the Y-axis direction (the direction A in which the off-angle α is formed). The index amount Li is set to a length that does not exceed the width of the crack 30, so that adjacent cracks 30 overlap when viewed vertically in the Y-axis direction. Then, by repeatedly alternating between forming the modified layer 28 and index feeding, a delamination layer 32 having multiple modified layers 28 and cracks 30 is formed to a depth corresponding to the thickness of the wafer 2 to be produced.

[0023] (The wafer 2 is generated by peeling it off from the delamination layer 32.) After forming a delamination layer 32 on the ingot 8, the wafer 2 is produced by delaminating it from the delamination layer 32. Delamination of the wafer 2 from the delamination layer 32 can be performed, for example, using a delamination apparatus 34 shown in Figure 4. The delamination apparatus 34 comprises an arm 36 that extends substantially horizontally, a motor 38 attached to the tip of the arm 36, and a disc-shaped suction piece 40 attached to the lower surface of the motor 38. The suction piece 40 is designed to adsorb the upper surface of the ingot 8 with its lower surface. The suction piece 40 also incorporates an ultrasonic vibration applying means (not shown) that applies ultrasonic vibration to the lower surface of the suction piece 40.

[0024] When peeling the wafer 2 from the delamination layer 32, first, the ingot 8 held by the holding table 24 is positioned below the suction piece 40 of the delamination device 34. Next, the arm 36 is lowered by the lifting mechanism (not shown) of the delamination device 34, bringing the lower surface of the suction piece 40 into contact with the first end face 10 of the ingot 8. Then, an suction force is generated on the lower surface of the suction piece 40, causing the first end face 10 of the ingot 8 to adhere to the suction piece 40. Next, the ultrasonic vibration applying mechanism is activated to apply ultrasonic vibration to the lower surface of the suction piece 40. The motor 38 is also activated to rotate the suction piece 40. This allows the wafer 2 to be peeled from the delamination layer 32 of the ingot 8. One end face 2a of the wafer 2 (the lower surface in Figure 4), which is the delamination surface, is an uneven surface.

[0025] As described above, when a laser beam LB is irradiated onto an ingot 8 to form a delamination layer 32, and a wafer 2 is produced by delaminating from the delamination layer 32, the first end face 10 of the ingot 8 is a flat surface that does not obstruct the incidence of the laser beam LB, so the other end face 2b (top surface in Figure 4) of the wafer 2 delaminated from the ingot 8 is a flat surface. However, when a cutting means such as a wire saw or an inner saw is used to produce the wafer 2 from the ingot 8, both one end face 2a and the other end face 2b of the wafer 2 may be uneven surfaces.

[0026] (holding process) After peeling the wafer 2 from the release layer 32, a holding step is performed to hold the other end face 2b of the wafer 2 (the end face opposite the release surface) on a chuck table. In the holding step, it is preferable to first apply a resin that absorbs irregularities (for example, epoxy resin) to the other end face 2b of the wafer 2. This allows the resin to absorb the irregularities of the other end face 2b if it is an uneven surface, thereby allowing the wafer 2 to be properly held on the chuck table. When applying a resin that absorbs irregularities to the other end face 2b of the wafer 2, for example, a liquid resin 44 can be placed between the disc-shaped substrate 42 shown in Figure 5 and the other end face 2b of the wafer 2, then the wafer 2 is pressed against the substrate 42 to spread the resin 44 evenly between the substrate 42 and the wafer 2, and then the resin 44 is cured. Furthermore, the substrate 42 is made of glass, for example, and its diameter should be slightly larger than the diameter of wafer 2.

[0027] In the holding process, after a resin 44 that absorbs irregularities is placed on the other end face 2b of the wafer 2, the substrate 42 side is held by the chuck table. In the holding process, for example, the chuck table 46 shown in Figure 6 can be used. A disc-shaped suction chuck 48 is positioned at the upper end of the chuck table 46. The suction chuck 48 is made of a porous material such as porous ceramics and is connected to a suction means (not shown). The chuck table 46 is also configured to rotate freely with its axis in the vertical direction. Then, with the substrate 42 facing downwards, the wafer 2 is placed on the chuck table 46, and a suction force is generated on the upper surface of the suction chuck 48 by the suction means, causing the other end face 2b of the wafer 2 to be held by the chuck table 46 through the substrate 42.

[0028] (Grinding process) After the holding process is performed, a grinding process is carried out in which one end face 2a of the wafer 2 held on the chuck table 46 is ground using a grinding means equipped with a grinding wheel in which grinding wheels are arranged in an annular shape.

[0029] (Stylus positioning step in the grinding process) In the grinding process, first, as shown in Figure 7, a stylus positioning step is performed in which the stylus 52 of the contact-type thickness measuring instrument 50 is positioned on one end face 2a of the wafer 2. The contact-type thickness measuring instrument 50 can be any commonly used thickness measuring instrument, and by contacting the measuring stylus 52 with one end face 2a of the wafer 2 without rotating the chuck table 46 and determining the height of the end face 2a, the approximate starting position for feeding the grinding means can be determined.

[0030] (The first grinding step in the grinding process) In the grinding process, after performing the stylus positioning step, the chuck table 46 is rotated and the grinding wheel is rotated to feed the grinding means at a first feed rate to grind one end face 2a of the wafer 2 and perform the first grinding step to remove irregularities.

[0031] The first grinding step of the grinding process can be carried out using, for example, the grinding means 54 shown in Figure 8. The grinding means 54 comprises a spindle 56 extending in the vertical direction, a spindle motor (not shown) for rotating the spindle 56, and a disc-shaped wheel mount 58 fixed to the lower end of the spindle 56. An annular grinding wheel 62 is fastened to the lower surface of the wheel mount 58 by bolts 60. Multiple grinding wheels 64 are fixed to the outer circumference of the lower surface of the grinding wheel 62, arranged in an annular pattern at intervals in the circumferential direction.

[0032] In the first grinding step of the grinding process, the spindle 56 is first rotated at a predetermined rotational speed (for example, 4500 rpm) in the direction indicated by arrow R1. The chuck table 46 is also rotated at a predetermined rotational speed (for example, 200 rpm) in the direction indicated by arrow R2. Next, the spindle 56 is lowered, bringing the grinding wheel 64 into contact with one end face 2a of the wafer 2, and grinding fluid is supplied to the portion where the grinding wheel 64 is in contact with the end face 2a. Then, the spindle 56 is lowered at a first feed rate (for example, 5.0 μm / s). This grinds one end face 2a of the wafer 2 and removes irregularities.

[0033] In the first grinding step, when the amplitude of the stylus 52 is initially large and gradually decreases to a predetermined value, it is assumed that the irregularities have been removed from one end face 2a of the wafer 2, and the process moves to the second grinding step. As shown in Figure 9, the first grinding step is performed from the start time t0 of the first grinding step until the time t1 when the amplitude of the stylus 52 decreases to a predetermined value (for example, 50 seconds to 150 seconds). Then, at the time t1 when the amplitude of the stylus 52 decreases to a predetermined value in the first grinding step, it is assumed that the irregularities have been removed from one end face 2a of the wafer 2, and the process moves to the second grinding step.

[0034] (The second grinding step in the grinding process) In the grinding process, after performing the first grinding step, the grinding means 54 is fed at a second feed rate to perform a second grinding step in which one end face 2a of the wafer 2 from which the irregularities have been removed is ground to flatten it (see Figures 9 and 10).

[0035] The rotation direction and rotation speed of the spindle 56 and chuck table 46 in the second grinding step of the grinding process may be the same as those in the first grinding step. Therefore, it is possible to transition from the first grinding step to the second grinding step without stopping the rotation of the spindle 56 and chuck table 46. However, the second feed rate in the second grinding step (e.g., 0.5 μm / s) should be smaller than the first feed rate in the first grinding step (e.g., 5.0 μm / s).

[0036] In the second grinding step, it is preferable to detect the load current value of the spindle motor of the grinding means 54 and adjust the feed rate so that the load current value falls within a certain range. This reduces damage to the wafer 2 and distortion to the grinding means 54. For example, based on the load current value of the spindle motor, the second feed rate in the second grinding step may be controlled as follows: 0.5 μm / s from time t1 to time t2 (e.g., 100 seconds), 0.4 μm / s from time t2 to time t3 (e.g., 35 seconds), 0.3 μm / s from time t3 to time t4 (e.g., 40 seconds), 0.27 μm / s from time t4 to time t5 (e.g., 30 seconds), and 0.23 μm / s from time t5 to time t6 (e.g., 45 seconds). Then, when the thickness of wafer 2 reaches a predetermined thickness (for example, 600 μm), the second grinding step is terminated.

[0037] Furthermore, after performing the second step, finish grinding may be performed on one end face 2a of the wafer 2 using a grinding wheel having abrasive grains smaller than those of the grinding wheel 64 used in the first and second grinding steps. Alternatively, after grinding one end face 2a of the wafer 2, the one end face 2a and the other end face 2b may be swapped and the other end face 2b may be ground to flatten it.

[0038] As described above, in this embodiment, the timing at which irregularities are removed from the end face 2a of the wafer 2 can be detected. Therefore, it is possible to prevent damage to the wafer 2 or distortion of the grinding means 54 caused by continuing the grinding feed of the grinding means 54 at a relatively fast feed rate even though the removal of irregularities from the end face 2a of the wafer 2 has been completed. Furthermore, if the first grinding step to remove irregularities is completed even though the removal of irregularities from the wafer 2 is not yet complete, the thickness measurement of the wafer 2 in the next second grinding step will be inaccurate, and the problem of inconsistent quality due to deviations from the target thickness is resolved. [Explanation of symbols]

[0039] 2: Wafer 2a: One end face of the wafer 2b: The other end face of the wafer 8: Ingot 32: Exfoliation layer 44: Resin 46: Chuck Table 50: Thickness measuring instrument 52: Stylus 54: Grinding methods 62: Grinding Wheel 64: Grinding Wheel LB: Laser beam FP: Focus point

Claims

1. A grinding method for flattening one end face of a wafer produced from an ingot, A holding step of holding the other end face of the wafer on a chuck table, The process includes grinding one end face of a wafer held on a chuck table using a grinding means equipped with a grinding wheel in which grinding wheels are arranged in an annular pattern, The grinding process is, A stylus positioning step in which the stylus of a contact-type thickness measuring instrument is positioned on one end face of the wafer, A first grinding step involves rotating the chuck table and the grinding wheel to feed the grinding means at a first feed rate to grind one end face of the wafer and remove irregularities, The system comprises a second grinding step in which the grinding means is fed at a second feed rate to grind and flatten one end face of the wafer from which the irregularities have been removed, A grinding method in which, in the first grinding step, when the amplitude of the stylus decreases to a predetermined value, the irregularities are considered to have been removed from one end face of the wafer, and the process proceeds to the second grinding step.

2. The grinding method according to claim 1, wherein the focal point of a laser beam with a penetrating wavelength is positioned at a predetermined depth from the end face of the SiC ingot, the laser beam is irradiated onto the SiC ingot to form a delamination layer, and then the wafer is produced by peeling it off from the delamination layer.

3. The grinding method according to claim 1, wherein in the holding step, a resin that absorbs irregularities is provided on the other end face of the wafer, and the other end face of the wafer is held on the chuck table.

4. The grinding method according to claim 1, wherein one end face of a wafer is swapped with the other end face, and the other end face is ground to flatten it.

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