Wafer generation method

Abstract

To provide a wafer generation method that can efficiently peel a wafer.SOLUTION: A wafer generation method includes a peeling starting point forming step of positioning the focal point of a laser beam with a wavelength that is transparent to an ingot 2 at a depth corresponding to the thickness of a wafer to be generated from the end face of the ingot 2, and irradiating the ingot 2 with a laser beam to form a peeling starting point 26, and a peeling step of peeling off the wafer to be generated from the ingot 2 from the peeling starting point 26. In the peeling step, degassed water W' is supplied to the end face of the ingot 2 to generate a layer of degassed water W', and ultrasonic waves are applied to destroy the peeling starting point 26.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for producing a wafer by generating a wafer from an ingot.

Background Art

[0002] Devices such as ICs, LSIs, and LEDs are formed by laminating a functional layer on the surface of a wafer made of a material such as Si (silicon) or Al2O3 (sapphire) and partitioning it by a planned division line. Also, power devices, LEDs, etc. are formed by laminating a functional layer on the surface of a wafer made of SiC (silicon carbide) and partitioning it by a planned division line.

[0003] The wafer on which the device is formed is processed along the planned division line by a cutting device or a laser processing device and divided into individual device chips, and each divided device chip is used in an electrical device such as a mobile phone or a personal computer.

[0004] The wafer on which the device is formed is generally produced by thinly cutting a columnar ingot with a wire saw. The front and back surfaces of the produced wafer are polished to a mirror finish (see, for example, Patent Document 1).

[0005] However, when the ingot is cut with a wire saw and the front and back surfaces of the cut wafer are polished, most (70 to 80%) of the ingot is discarded, which is uneconomical. In particular, in the case of a SiC ingot, it has a high hardness and is difficult to cut with a wire saw, requiring a considerable amount of time, resulting in poor productivity. In addition, the unit price of the ingot is high, and there are problems in efficiently producing wafers.

[0006] Therefore, the applicant proposed a technique in which a laser beam with a wavelength that is transparent to SiC is focused inside a SiC ingot, the laser beam is irradiated onto the SiC ingot to form a peeling point on the cutting surface, and the wafer is peeled off from the ingot along the cutting surface where the peeling point has been formed (see, for example, Patent Document 2).

[0007] Furthermore, the applicant has also proposed a technique for applying ultrasound to an ingot via a layer of water to facilitate the separation of wafers to be produced from the ingot (see, for example, Patent Document 3). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-94221 [Patent Document 2] Japanese Patent Publication No. 2016-111143 [Patent Document 3] Japanese Patent Publication No. 2016-146446 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, while applying ultrasound to an ingot with a delamination point improves delamination, it still takes some time for the wafer to delaminate from the ingot. Therefore, there is a need to shorten the time from the start of ultrasound application to the completion of wafer delamination.

[0010] Such problems can also occur when a laser beam with a wavelength that penetrates silicon, sapphire, or other ingots is focused inside the ingot, and the laser beam is irradiated onto the ingot to create a peeling point, thereby peeling off a wafer from the silicon, sapphire, or other ingot.

[0011] The object of the present invention is to provide a method for producing wafers that can be efficiently peeled off. [Means for solving the problem]

[0012] According to the present invention, the following wafer manufacturing method is provided that solves the above problems. That is, A method for generating wafers from ingots, A delamination initiation step is performed by positioning the focal point of a laser beam with a wavelength that is transparent to the ingot at a depth corresponding to the thickness of the wafer to be produced from the edge face of the ingot, and irradiating the ingot with the laser beam to form a modified layer and delamination initiation point, A peeling step in which the wafer to be produced from the ingot is peeled off from the peeling starting point, 、 Includes, In the peeling process, degassed water is supplied to the end face of the ingot to create a layer of degassed water, and ultrasonic waves are applied to destroy the peeling initiation point. In this process, the degassed water is produced by storing water in a separately prepared vacuum tank and reducing the pressure inside the vacuum tank. A method for generating wafers is provided.

[0013] Applicable In the peeling process, it is desirable to generate degassed water with an oxygen content of 2.0 mg / liter or less.

[0014] The ingot may be a SiC ingot. The SiC ingot has a first face, a second face opposite the first face, a c-axis extending from the first face to the second face, and a c-plane perpendicular to the c-axis, and the c-axis is inclined with respect to the perpendicular to the first face, forming an off-angle between the c-plane and the first face. The peeling initiation step preferably includes a modified layer formation step in which the focal point of the laser beam and the SiC ingot are moved relative to each other in a direction perpendicular to the direction in which the off-angle is formed to form a linear modified layer, and an indexing step in which the focal point of the laser beam and the SiC ingot are moved relative to each other in the direction in which the off-angle is formed to feed a predetermined index amount. [Effects of the Invention]

[0015] The wafer production method of the present invention is The laser beam having a wavelength that is transmissive to the ingot is irradiated onto the ingot with the focus point of the laser beam positioned at a depth corresponding to the thickness of the wafer to be formed from the end face of the ingot, to form a modified layer and form a peeling starting point in a peeling starting point forming step. A peeling step of peeling the wafer to be formed from the ingot from the peeling starting point 、 and in the peeling step, deaerated water is supplied to the end face of the ingot to form a layer of deaerated water and ultrasonic waves are applied to break the peeling starting point. In this process, the degassed water is produced by storing water in a separately prepared vacuum tank and reducing the pressure inside the vacuum tank. Therefore, the wafer can be peeled efficiently.

Brief Description of the Drawings

[0016] [Figure 1] (a) Perspective view of the ingot, (b) Plan view of the ingot shown in (a), (c) Front view of the ingot shown in (a). [Figure 2] (a) Perspective view showing the peeling starting point forming step, (b) Front view showing the peeling starting point forming step, (c) Cross-sectional view of the ingot in which the peeling starting point is formed. [Figure 3] Schematic diagram showing the state of generating deaerated water. [Figure 4] Schematic diagram showing an example of the state of peeling the wafer from the ingot. [Figure 5] Schematic diagram showing another example of the state of peeling the wafer from the ingot.

Mode for Carrying Out the Invention

[0017] Hereinafter, a preferred embodiment of the method for producing a wafer of the present invention will be described with reference to the drawings.

[0018] (Ingot 2) In FIG. 1, a columnar ingot 2 to be processed by the method for producing a wafer of the present invention is shown. The illustrated ingot 2 is formed of single crystal SiC (silicon carbide).

[0019] Ingot 2 has a circular first surface 4, a circular second surface 6 located opposite the first surface 4, a circumferential surface 8 located between the first surface 4 and the second surface 6, a c-axis extending from the first surface 4 to the second surface 6, and a c-plane perpendicular to the c-axis (see Figure 1(c)). At least the first surface 4 is flattened by grinding or polishing to the extent that it does not obstruct the incidence of the laser beam.

[0020] In ingot 2, the c-axis is inclined with respect to the perpendicular 10 of the first face 4, and an off-angle α (for example, α = 1, 3, 6 degrees) is formed between the c-face and the first face 4. The direction in which the off-angle α is formed is indicated by arrow A in Figure 1.

[0021] On the circumferential surface 8 of ingot 2, a rectangular first orientation flat 12 and a second orientation flat 14 are formed, both indicating the crystal orientation. The first orientation flat 12 is parallel to the direction A in which the off-angle α is formed, and the second orientation flat 14 is perpendicular to the direction A in which the off-angle α is formed. As shown in Figure 1(b), when viewed from above, the length L2 of the second orientation flat 14 is shorter than the length L1 of the first orientation flat 12 (L2 <L1)。

[0022] The ingot processed by the wafer manufacturing method of the present invention is not limited to ingot 2 described above, and may be a SiC ingot in which the c-axis is not tilted with respect to the perpendicular to the first surface and the off-angle α between the c-plane and the first surface is 0 degrees (i.e., the perpendicular to the first surface and the c-axis coincide), or it may be an ingot formed from a material other than SiC, such as Si (silicon), Al2O3 (sapphire), or GaN (gallium nitride).

[0023] (Peel-starting point formation process) In the illustrated embodiment, first, a laser beam with a wavelength that is transparent to the ingot 2 is focused at a depth corresponding to the thickness of the wafer to be produced from the end face of the ingot 2, and a delamination initiation step is performed in which the laser beam is irradiated onto the ingot 2 to form a modified layer and delamination initiation point.

[0024] The peeling point formation process can be carried out, for example, using the laser processing apparatus 16 shown in Figure 2. The laser processing apparatus 16 comprises a chuck table 18 for holding the ingot 2 by suction, an oscillator (not shown) that emits a pulsed laser beam LB with a wavelength that is transparent to the ingot 2, and a concentrator 20 that focuses the pulsed laser beam LB emitted by the oscillator and irradiates the ingot 2, which is held by suction on the chuck table 18, with the pulsed laser beam LB.

[0025] The chuck table 18 is configured to rotate freely around an axis extending in the vertical direction, and is also configured to move freely in the X-axis direction, indicated by arrow X in Figure 2(a), and in the Y-axis direction, which is perpendicular to the X-axis direction (indicated by arrow Y in Figure 2(a)). The XY plane defined by the X-axis and Y-axis directions is essentially horizontal.

[0026] Continuing the explanation with reference to Figure 2, in the peeling point formation process, first, the ingot 2 is held by suction on the upper surface of the chuck table 18 with the first surface 4 facing upwards. Next, the ingot 2 is imaged from above by the imaging means (not shown) of the laser processing apparatus 16, and based on the image of the ingot 2 captured by the imaging means, the orientation of the ingot 2 is adjusted to a predetermined orientation, and the positional relationship between the ingot 2 and the light concentrator 20 is adjusted.

[0027] When adjusting the orientation of ingot 2 to a predetermined orientation, the second orientation flat 14 is aligned in the X-axis direction, as shown in Figure 2(a). This aligns the direction perpendicular to the direction A in which the off-angle α is formed with the X-axis direction, and also aligns the direction A in which the off-angle α is formed with the Y-axis direction.

[0028] Next, the focal point FP (see Figure 2(b)) of the laser beam LB is positioned on the first surface 4 of the ingot 2 at a depth corresponding to the thickness of the wafer to be produced. Then, while moving the focal point FP and the ingot 2 relatively in the X-axis direction (the direction perpendicular to the direction A in which the off-angle α is formed), the laser beam LB with a wavelength that is transparent to the ingot 2 is irradiated onto the ingot 2 from the focuser 20. As a result, as shown in Figure 2(c), a linear modified layer 22 in which SiC is separated into Si (silicon) and C (carbon) can be formed along the X-axis direction. In addition, cracks 24 extending along the c-plane are also formed from the modified layer 22 (modified layer formation step).

[0029] Next, the focusing point FP and the ingot 2 are indexed relative to each other in the Y-axis direction (direction A where the off-angle is formed) (indexing step). The indexing amount Li is set to a length that does not exceed the width of the crack 24, so that adjacent cracks 24 in the Y-axis direction overlap when viewed vertically. Then, by alternately repeating the modified layer formation step and the indexing step, a delamination starting point 26 having multiple modified layers 22 and cracks 24 is formed at a depth (planned cutting surface) corresponding to the thickness of the wafer to be produced.

[0030] Such a peeling point formation process can be carried out, for example, under the following processing conditions. Wavelength of pulsed laser beam: 1064nm Repeat frequency: 80kHz Average output: 3.2W Pulse width: 4ns Diameter of the focal point: 10 μm Numerical Aperture (NA): 0.45 Index amount: 400 μm Wafer thickness to be produced: 700 μm

[0031] (Peeling process) After performing the delamination point formation process, a delamination process is carried out to delaminate the wafer to be produced from ingot 2 from the delamination point 26.

[0032] In the peeling process, first, ultrasonic waves are applied to the water stored in the vacuum tank to reduce the pressure inside the tank and generate degassed water. When generating degassed water, for example, the vacuum tank 28 shown in Figure 3 can be used.

[0033] The vacuum chamber 28 comprises a bottom plate 30, side walls 32 extending upward from the periphery of the bottom plate 30, and a top plate 34 provided at the upper end of the side walls 32. The side walls 32 have a supply port 36 for supplying water W before degassing to the vacuum chamber 28, and an outlet port 38 for discharging the degassed water from the vacuum chamber 28. The top plate 34 is provided with a suction hole 40, which is connected to a vacuum pump (not shown). An ultrasonic oscillator 42 is also installed inside the vacuum chamber 28.

[0034] When generating degassed water, first, water W before degassing is supplied into the vacuum tank 28 from the supply port 36. Next, the ultrasonic oscillator 42 is activated to apply ultrasound (for example, around 0.1 MHz to 1.0 MHz) to the water W. At the same time, the vacuum pump is activated to reduce the pressure inside the vacuum tank 28. As a result, as shown in Figure 3, the gas dissolved in the water W appears as bubbles, and the gas can be removed from the water W. In this way, degassed water can be generated by applying ultrasound to the water W stored in the vacuum tank 28 and reducing the pressure inside the vacuum tank 28.

[0035] When generating degassed water, it is preferable to have a low air pressure in the depressurization tank 28. This is because degassing is promoted as the pressure is reduced. The relationship between the air pressure in the depressurization tank 28 and the lower limit of the oxygen content of the degassed water is shown below.

[0036] Air pressure in the depressurized tank (atm) Lower limit of oxygen content in degassed water (mg / liter) 1.0 8.1 0.7 6.55 0.65 5.8 0.6 5.48 0.5 4.97 0.4 4.08 0.3 3.1 0.2 1.96 0.1 1.14 0.03 0.36

[0037] Once degassed water is generated, the degassed water is supplied to the end face of the ingot 2 to create a layer of degassed water, and ultrasonic waves are applied to destroy the delamination initiation point 26. The destruction of the delamination initiation point 26 can be carried out, for example, using the delamination device 44 shown in Figure 4.

[0038] The peeling device 44 comprises a water tank 46, a rod 48 positioned to move up and down above the water tank 46, and an ultrasonic emitting member 50 attached to the lower end of the rod 48. A holding table 52 for holding the ingot 2 is provided inside the water tank 46. An outlet 54 is formed at the lower end of the water tank 46 for discharging degassed water after the wafer has been peeled from the ingot 2.

[0039] When destroying the delamination point 26, first, the wafer to be generated is positioned upwards (i.e., with the first surface 4, which is the end face closest to the delamination point 26, facing upwards), and the ingot 2 is held by the holding table 52. In this case, an adhesive (for example, an epoxy resin adhesive) may be interposed between the second surface 6 of the ingot 2 and the upper surface of the holding table 52 to fix the ingot 2 to the holding table 52, or an attractive force may be generated on the upper surface of the holding table 52 to hold the ingot 2 by suction.

[0040] Next, degassed water W' is supplied into the water tank 46 until the water level is higher than the top surface of the ingot 2. Then, the rod 48 is lowered to position the ultrasonic emitting member 50 slightly above the first surface 4 of the ingot 2. The distance between the first surface 4 and the ultrasonic emitting member 50 can be about 2 to 3 mm. Then, ultrasonic waves are emitted from the ultrasonic emitting member 50, and the delamination point 26 is destroyed by the ultrasonic waves through the layer of degassed water W' between the first surface 4 and the ultrasonic emitting member 50. This makes it possible to separate the wafer to be produced from the ingot 2 from the delamination point 26.

[0041] In the example described above, a method was explained in which degassed water W' is stored in a water tank 46. However, as shown in Figure 5, a layer of degassed water W' may also be created by supplying degassed water W' from a supply nozzle 56 between the first surface 4 of the ingot 2 and the ultrasonic oscillating member 50.

[0042] In this process, the wafer to be produced is positioned upwards, and the ingot 2 is held by the holding table 52. Then, the ultrasonic oscillating member 50 is positioned slightly above the first surface 4. Degassed water W' is then supplied from the supply nozzle 56 between the first surface 4 and the ultrasonic oscillating member 50 to create a layer of degassed water W'. Ultrasound is then emitted from the ultrasonic oscillating member 50, and the delamination point 26 is destroyed by the ultrasound through the layer of degassed water W' between the first surface 4 and the ultrasonic oscillating member 50. This allows the wafer to be produced from the ingot 2 to be separated from the delamination point 26.

[0043] In the example shown in Figure 4, there is time required to store the degassed water W' in the water tank 46 and time required to discharge the used degassed water W' from the water tank 46 after wafer peeling. In contrast, in the example shown in Figure 5, a layer of degassed water W' can be immediately generated by supplying the degassed water W' from the supply nozzle 56 between the first surface 4 and the ultrasonic oscillating member 50, and the used degassed water W' can be discharged simultaneously with the application of ultrasound to the ingot 2. Therefore, the peeling process time can be shortened in the example in Figure 5 compared to the example in Figure 4.

[0044] As described above, in the illustrated embodiment, degassed water W' is supplied to the end face of the ingot 2 to create a layer of degassed water W', and ultrasound is applied to the ingot 2 through the layer of degassed water W' to destroy the peeling initiation point 26. Therefore, the energy of the ultrasound is not converted into cavitation, and the energy of the ultrasound can be effectively applied to the ingot 2. Thus, wafers can be efficiently peeled from the ingot 2.

[0045] <Experiment> The inventors generated multiple layers of degassed water by changing the air pressure in a vacuum chamber, applied ultrasound to an ingot through the generated layers of degassed water, and measured the time it took for the delamination point to be destroyed and the wafer to delaminate from the ingot. They also measured the sound pressure (amplitude) of the ingot when ultrasound was applied to it. The frequency of the ultrasound used to generate the degassed water was 0.1 MHz, and the frequency of the ultrasound applied to the ingot to destroy the delamination point was 25 kHz. The temperature of the degassed water was 20°C.

[0046] <Experimental Results> Oxygen content of degassed water (mg / liter), peeling time (seconds), sound pressure (V) 8.1 1352 1.54 6.55 1223 1.56 5.8 1123 1.66 5.48 1082 1.68 4.97 1002 1.88 4.08 815 1.88 3.1 753 1.88 1.96 356 2.20 1.14 243 2.32 0.36 236 3.12

[0047] As can be understood by referring to the experimental results above, the lower the oxygen content of the degassed water, the shorter the time it takes for the wafer to detach from the ingot, and the higher the sound pressure of the ingot. Furthermore, the detachment time was 753 seconds when the oxygen content of the degassed water was 3.1 mg / liter, and 356 seconds when the oxygen content of the degassed water was 1.96 mg / liter. When the oxygen content of the degassed water changed from 3.1 mg / liter to 1.96 mg / liter, the detachment time was reduced to less than half. Therefore, from the viewpoint of efficiently producing wafers from ingots, it is preferable to produce degassed water with an oxygen content of 2.0 mg / liter or less. [Explanation of Symbols]

[0048] 2: Ingot 4: First side 6: The second side 10: Perpendicular line 22: Modified layer 26: Starting point of delamination 28: Pressure Reducing Tank LB: Laser beam FP: Focus point W': Degassed water

Claims

1. A method for generating wafers from ingots, A delamination initiation step involves positioning the focal point of a laser beam with a wavelength that is transparent to the ingot at a depth corresponding to the thickness of the wafer to be produced from the edge face of the ingot, and irradiating the ingot with the laser beam to form a modified layer and delamination initiation point, The process includes a peeling step of peeling off the wafer to be produced from the ingot from the peeling starting point, A wafer manufacturing method in which, in the peeling process, degassed water is supplied to the end face of the ingot to create a layer of degassed water, and ultrasonic waves are applied to destroy the peeling starting point, wherein the degassed water is generated by storing water in a separately prepared vacuum tank and reducing the pressure inside the vacuum tank.

2. The wafer production method according to claim 1, wherein degassed water with an oxygen content of 2.0 mg / liter or less is produced in the peeling step.

3. The method for producing a wafer according to claim 1, wherein the ingot is a SiC ingot.

4. The SiC ingot has a first face, a second face opposite the first face, a c-axis extending from the first face to the second face, and a c-face perpendicular to the c-axis, wherein the c-axis is inclined with respect to the perpendicular to the first face and an off-angle is formed between the c-face and the first face. The delamination starting point formation step includes a modified layer formation step in which the focusing point of the laser beam and the SiC ingot are moved relative to each other in a direction perpendicular to the direction in which the off-angle is formed to form a linear modified layer, A method for producing a wafer according to claim 3, comprising an index step of moving the focal point of the laser beam and the SiC ingot relative to each other in the direction in which the off-angle is formed, thereby feeding the index by a predetermined amount.

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