Wafer dividing method and machining method, and wafer dividing device

Abstract

Provided are a wafer dividing method, wafer machining method, and wafer dividing device that make it possible to increase the semiconductor material utilization efficiency, mainly in power semiconductor usages. Specifically, provided is a wafer dividing method for obtaining thin wafers that includes attaching fixing plates to both sides of a wafer comprising at least a semiconductor crystal, and cutting the wafer near the middle in the thickness direction while holding the fixing plates on both sides, thereby obtaining two thin wafers attached to the fixing plates.

Description

Wafer dividing method, processing method, and wafer dividing device

[0001] The present invention relates to a wafer dividing method, a processing method, and a wafer dividing device for obtaining thin wafers, and more particularly to a wafer dividing method, a processing method, and a wafer dividing device suitable for obtaining thin wafers suitable for manufacturing power semiconductors.

[0002] Various semiconductor devices are formed on a disk-shaped wafer and then diced into individual pieces. Wafers are obtained by slicing a cylindrical or other ingot (made of semiconductor) (see, for example, Patent Document 1).

[0003] This process is shown in Figure 9, where Figure 9(a) shows the shape of a cylindrical ingot 1 obtained by semiconductor crystal growth. Figure 9(b) shows the state at which slicing of the ingot 1 begins, with wires 170 arranged at equal intervals in a direction approximately perpendicular to the longitudinal direction of the ingot. Here, the wires 170 constitute a wire saw, and are made of a piano wire or the like with a diameter of 120 μm to 300 μm to which hard particles (diamond, etc.) with an abrasive size of 10 μm to 30 μm are attached. The wires 170 are run in one direction or both directions to slice the ingot 1. Figure 9(c) shows the state after slicing is completed, and each wafer 10 can be separated as shown in Figure 9(d).

[0004] In the semiconductor crystals forming the wafer 10, there are crystal orientations suitable for subsequent device formation, and in the example of the wafer 10 shown in FIG. 9(d), the wafer surface 10S is the surface suitable for device formation.

[0005] 10 and 11 are diagrams for explaining a semiconductor process using the wafer 10 shown in FIG. 9(d), showing an example of a process for obtaining a power semiconductor device.

[0006] 10(a) shows a state in which a backside fixing plate 2 is placed on the opposite side (backside) of the wafer 10 from the wafer front side 10S. Here, the backside fixing plate 2 is formed by laminating an adhesive film 21 on a substrate 20. As shown in FIG. 10(b), the backside of the wafer 10 is attached to the backside fixing plate 2 by closely adhering the adhesive film 21. Incidentally, the wafer front side 10S is a surface created by slicing using wires 170, but it is not necessarily a flat surface, and therefore cannot be used for device formation in this state. Therefore, as a planarization process, the wafer front side 10S is polished, ground, etc., followed by chemical mechanical polishing (CMP) to obtain a wafer 101 having an atomically flat wafer front side 101S (FIG. 10(c)).

[0007] Thereafter, a device process such as photolithography is performed on the wafer surface 101S to form the surfaces of a large number of devices 6 (FIG. 10(d)). Thereafter, the opposite side of the wafer surface 101S is processed.

[0008] The preparation steps for this are shown in Figures 10(e) to 11(c). Here, the device protection fixing plate 4 is formed by laminating an adhesive film 41 on a substrate 40. The adhesive film 41 is placed opposite the wafer front surface 101S (Figure 10(e)) and then adhered to the wafer front surface 101S (Figure 11(a)). After that, the rear surface fixing plate 2 is peeled off (Figures 11(b) to 11(c)).

[0009] At this stage, the wafer back surface 101B, which is the surface opposite to the wafer front surface 101S, of the wafer 101 is exposed (FIG. 11(d)). The wafer back surface 101B is not flat because it has been sliced ​​by the wires 170. Therefore, a planarization process is performed on the wafer back surface 101B to obtain a wafer 102 having a flat wafer back surface 102B as shown in FIG. 11(e). A metal film is then formed on the wafer back surface 102B to form an electrode surface 102M (FIG. 11(f)). The device protection fixing plate 4 is then peeled off, and a dicing process is performed to separate the devices 6 into individual pieces.

[0010] 12 is a cross-section showing an example of the structure of an individual device 6, in which a p region 60, an n+ region 61, a gate oxide film 62, a gate electrode 63, and a source electrode 64 are formed in the device process leading to Fig. 10(d). Also, the electrode surface 102M on which the metal film is formed in Fig. 11(f) becomes a drain electrode 6102M in each device 6.

[0011] 13 and 14 show how the cross section of the region corresponding to each device 6 changes, with FIGS. 13(a) to 13(e) corresponding to the steps in FIGS. 10(a) to 10(e), and FIGS. 14(a) to 14(f) corresponding to the steps in FIGS. 11(a) to 11(f).

[0012] Patent No. 7100864

[0013] 12 shows the basic structure of a vertical diffusion MOSFET (abbreviated as VD-MOSFET) used as a so-called power semiconductor, in which carrier electrons controlled by a voltage applied to a gate electrode 63 flow from a source electrode 64 to a drain electrode 6102M (current flows from the drain electrode 6102 to the source electrode 64). That is, in the device 6 of FIG. 12, current flows in the thickness direction (d direction).

[0014] However, compared to logic semiconductors, power semiconductors control a larger current when energized and a higher voltage when cutoff, so they are required to have low resistance when energized and high withstand voltage when cutoff.

[0015] 12, it is preferable to reduce the thickness d to reduce the resistance when current is applied, but a minimum thickness d is required to ensure a sufficient withstand voltage. For example, in a device using silicon carbide as a semiconductor, a thickness of about 300 μm was previously required.

[0016] However, recently, it has become possible to obtain high-quality crystals with few defects even in compound semiconductors such as silicon carbide, making it possible to ensure practical voltage resistance even with thicknesses of several tens of micrometers. As a result, the thickness of power semiconductors made of silicon carbide has been reduced from the previous thickness of 300 micrometers or more to 100 micrometers or less.

[0017] On the other hand, if the thickness of the wafer 10 sliced ​​from the ingot 1 as shown in FIG. 9( d ) is reduced, warping and distortion will occur, making it difficult to bond the wafer to the backside fixing plate 2 as shown in FIG. 10( a ) without any gaps, which will cause problems in subsequent processes and affect the yield.

[0018] For this reason, it was necessary to set the thickness of the wafer 10 to 300 μm or more and to thin the thickness by polishing the wafer through more than just a simple planarization process, using at least one of the planarization processes shown in Figures 10(c) and 13(c) and the planarization processes shown in Figures 11(e) and 14(e). As a result, the utilization efficiency of the semiconductor crystal was extremely low.

[0019] The present invention has been made in consideration of the above problems, and provides a wafer dividing method, processing method, and wafer dividing device that can increase the utilization efficiency of semiconductor materials, mainly in power semiconductor applications.

[0020] In order to solve the above problem, the invention described in claim 1 is a method for dividing a wafer to obtain thin wafers, which comprises attaching fixing plates to both sides of a wafer made of at least semiconductor crystal, and cutting the wafer near the middle in the thickness direction while holding the fixing plates on both sides, thereby obtaining two thin wafers attached to the fixing plates.

[0021] The invention described in claim 2 is a wafer processing method in which, of two thin wafers obtained by the wafer dividing method described in claim 1, a thin wafer having an exposed surface on a predetermined crystal orientation side is attached to a fixing plate, and then a flattening process is performed on the surface on the predetermined crystal orientation side, and then a plurality of devices are formed on the flattened surface.

[0022] The invention described in claim 3 is a wafer processing method in which, of two thin wafers obtained by the wafer dividing method described in claim 1, a thin wafer having a surface with a predetermined crystal orientation attached to a fixing plate is attached with its exposed surface to a fixing plate, and then the fixing plate is peeled off from the surface with the predetermined crystal orientation, and a planarization process is performed on the surface with the predetermined crystal orientation, and then a plurality of devices are formed on the surface that has been planarized.

[0023] The invention described in claim 4 is a wafer dividing device that affixes fixing plates to both sides of a wafer and divides the wafer into two thin wafers while holding the fixing plates on both sides, and is equipped with a wafer rotation means that clamps the wafer from both sides via the fixing plates and rotates the wafer in a plane parallel to the wafer surface, and a wire saw with a wire arranged perpendicular to the rotation axis of the wafer rotation means.

[0024] The invention described in claim 5 is the wafer dividing device described in claim 4, further comprising a laser irradiation means for irradiating the wafer with laser light, focusing the laser light on the point where the wire contacts the wafer.

[0025] A sixth aspect of the present invention is the wafer dividing device according to the fifth aspect, wherein the wafer is irradiated with laser light before the wire comes into contact with the wafer.

[0026] The invention described in claim 7 is the wafer dividing device described in claim 6, further comprising a fluid ejection means for ejecting gas or liquid, and ejecting gas or liquid toward the portion of the wafer that has come into contact with the wire saw.

[0027] An eighth aspect of the present invention is the wafer dividing device according to any one of the fourth to seventh aspects, wherein the wire has a thickness of 30 μm or more and 100 μm or less.

[0028] The invention described in claim 9 is the wafer dividing device described in claim 8, wherein the wire has abrasive grains attached to its surface, and the maximum size of the abrasive grains is 5 μm or more and 10 μm or less.

[0029] A tenth aspect of the present invention is the wafer dividing device according to any one of the fourth to seventh aspects, wherein the wafer is a silicon carbide wafer.

[0030] An eleventh aspect of the present invention is the wafer dividing device according to any one of the fifth to seventh aspects, wherein the wafer is a silicon carbide wafer and the wavelength of the laser light is 430 nm or more and 470 nm or less.

[0031] A twelfth aspect of the present invention is the wafer dividing device according to any one of the fourth to seventh aspects, wherein the thickness of the wafer is 250 μm or more and 800 μm or less.

[0032] The present invention can improve the utilization efficiency of semiconductor crystals in the process of fabricating wafers from semiconductor crystal ingots into devices, and is particularly effective in the manufacture of power semiconductor devices in which current flows in the thickness direction of the device.

[0033] 1A and 1B are diagrams illustrating an embodiment of a wafer dividing method according to the present invention, showing (a) a state in which fixing plates are placed opposite both sides of a wafer sliced ​​from a semiconductor ingot, (b) a state in which fixing plates are attached to both sides of the wafer, (c) a state in which the wafer in the same state is sliced ​​with a wire, and (d) a diagram in which the wafer has been divided into two thin wafers.

[0023] This embodiment of a wafer dividing method according to the present invention illustrates a process of changing a thin wafer divided into two thin wafers with an exposed back surface to a state in which an exposed front surface is formed, showing (a) a state in which a fixing plate is placed opposite the back surface, (b) a state in which a fixing plate is attached to the back surface, (c) a state in which the fixing plate on the front surface side is peeled off, and (d) a diagram in which the front surface side is exposed. This embodiment of a wafer dividing device according to the present invention is illustrated, showing (a) an external view of the components, and (b) an external view showing the wafer being divided. 1A and 1B are cross-sectional views illustrating a process in which an embodiment of the wafer dividing device of the present invention cuts and divides a wafer by grinding, where (a) shows a state before grinding starts, (b) shows a state after grinding has started, and (c) shows a state after grinding has finished and the wafer has been divided.

[0034] FIGS. 1A and 1B show modified examples of the embodiment of the wafer dividing device of the present invention, where (a) Modified Example 1 shows a state before grinding starts, (b) Modified Example 1 shows a state after grinding has finished, (c) Modified Example 2 shows a state before grinding starts, and (d) Modified Example 2 shows a state after grinding has finished.

[0035] FIGS. 1A and 1B are diagrams illustrating modified example 2A of the modified example of the wafer dividing device of the present invention, and modified example 2B of the modified example of the wafer dividing device of the present invention.

[0036] FIGS. 1A and 1B show modified examples of the embodiment of the wafer dividing device of the present invention, where (a) Modified Example 3 shows a state before grinding starts, (b) Modified Example 4 shows a state before grinding starts, (c) Modified Example 5 shows a state before grinding starts, and (d) Modified Example 6 shows a state before grinding starts. 1A and 1B are diagrams illustrating a current method for obtaining wafers from an ingot, in which (a) shows an example of the shape of an ingot made of a semiconductor, (b) shows the state in which the ingot is sliced ​​with wire, (c) shows the state in which the ingot has been sliced, and (d) shows the state in which the ingot has been separated into multiple wafers by slicing.1A and 1B are diagrams illustrating a semiconductor manufacturing process using a wafer sliced ​​from an ingot, the diagram illustrating (a) a state in which a fixing plate is placed opposite the back surface of the wafer, (b) a state in which the back surface is held by the fixing plate, (c) a state in which a planarization process has been performed on the front surface, (d) a state in which a number of semiconductor devices have been formed on the planarized surface, and (e) a state in which a fixing plate is placed opposite the surface on which a number of semiconductor devices have been formed. 1A and 1B are diagrams illustrating a semiconductor manufacturing process using a wafer, the diagram illustrating (a) a state in which a fixing plate is attached to the surface on which a number of semiconductor devices have been formed, (b) a state in which the entire wafer has been inverted from the same state, (c) a state in which the fixing plate is being peeled off from the back surface of the wafer, (d) a state in which the fixing plate has been peeled off from the back surface of the wafer, (e) a state in which the back surface of the wafer has been thinned by grinding and then planarized, and (f) a state in which an electrode film has been formed on the back surface of the planarized wafer. This is a cross-sectional view showing an example of the structure of a power semiconductor device. 1A and 1B are diagrams illustrating a semiconductor manufacturing process using a wafer in a cross section of one device region, in which (a) a fixing plate is placed opposite the back surface of the wafer, (b) a state in which the back surface is held by the fixing plate, (c) a state in which a planarization process has been performed on the front surface, (d) a state in which semiconductor devices have been formed on the planarized surface, and (e) a state in which a fixing plate is placed opposite the surface on which the semiconductor devices have been formed. 1A and 1B are diagrams illustrating a semiconductor manufacturing process using a wafer in a cross section of one device region, in which (a) a fixing plate is attached to the surface on which the semiconductor devices have been formed, (b) a state in which the entire wafer has been inverted from the same state, (c) a state in which the fixing plate is being peeled off from the back surface of the wafer, (d) a state in which the fixing plate has been peeled off from the back surface of the wafer, (e) a state in which the back surface of the wafer has been thinned by grinding and then planarized, and (f) a state in which an electrode film has been formed on the back surface of the planarized wafer.

[0034] An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating an embodiment of a wafer dividing method of the present invention. In FIG. 1( a), a wafer 10 is a disk-shaped wafer sliced ​​from an ingot and having a thickness that is free of warping or distortion. While this varies depending on the wafer size, a thickness of less than 350 μm for a 6-inch wafer is prone to warping, and a thickness of less than 250 μm for a 4-inch wafer is prone to warping. Therefore, the thickness of the wafer 10 is preferably 250 μm or greater. On the other hand, if the thickness after division is greater than the thickness that does not cause warping or distortion (alone), the present invention is not worth implementing. Therefore, the thickness of the wafer 10 in FIG. 1( a) is preferably 800 μm or less.

[0035] The wafer 10 is made of semiconductor crystal, and the material is preferably silicon carbide (SiC), which is suitable for power semiconductors, but compound semiconductors such as gallium nitride (GaN) and gallium oxide (Ga2O3) may also be used. Furthermore, the present invention is also effective for silicon (Si). Furthermore, the wafer is not limited to pure semiconductor crystals, and may contain dopants to make it a P-type or N-type semiconductor. In other words, the wafer 10 can be said to be made of at least semiconductor crystals.

[0036] Generally, electron mobility varies depending on the crystal orientation. Therefore, in the device structure shown in FIG. 12, the surface on which the device is formed must be aligned with a predetermined crystal orientation so that electron mobility in the thickness direction (d direction) is high. For this reason, the wafer surface 10S is aligned with the predetermined crystal orientation.

[0037] 1( a), the backside fixing plate 2 is formed by laminating an adhesive film 21 on a substrate 20. As shown in FIG. 1( b), the backside of the wafer 10 (the surface opposite the wafer surface 10S) is brought into close contact with the adhesive film 21, thereby fixing the wafer 10 to the backside fixing plate 2. The frontside fixing plate 3 is formed by laminating an adhesive film 31 on a substrate 30. The wafer 10 is fixed to the frontside fixing plate 3 by bringing the wafer surface 10S into close contact with the adhesive film 31. Here, the surface shapes of the backside fixing plate 2 and the frontside fixing plate 3 are circular and the same size as the wafer 10, but are not limited to this. However, it is desirable for the backside fixing plate 2 to fix the entire backside, and it is desirable for the frontside fixing plate 3 to fix the entire wafer surface 10S. It is also desirable for the adhesive film to have the property of being easily peeled off by applying some kind of treatment; for example, a thermal release sheet such as REVALPHA (registered trademark) is preferably used.

[0038] The substrates 20 and 30 are made of a material and have a thickness that allows them to be flat and rigid. It is also desirable that the adhesive films 21 and 31 are flat. Furthermore, it is desirable that the adhesive film 21 generates as little degassing as possible even under vacuum.

[0039] After fixing both sides of the wafer 10 with the backside fixing plate 2 and the frontside fixing plate 3 as shown in FIG. 1( b), a wire 70 as shown in FIG. 1( c) is placed near the middle of the wafer 10 in the thickness direction, and the wafer 10 is then ground and cut into two pieces. FIG. 1( d) shows the state after division, in which the wafer 10 is divided into two pieces, the thin wafer 11 and the thin wafer 12. Here, the thicknesses of the thin wafers 11 and 12 are less than half the thickness of the wafer 10 (because at least a thickness corresponding to the thickness of the wire 70 is removed during grinding). Therefore, while both the thin wafer 11 and the thin wafer 12 would warp or distort when used alone, as shown in FIG. 1( d), the thin wafer 11 is fixed to the flat backside fixing plate 2 and the thin wafer 12 is fixed to the flat frontside fixing plate 3, so warping or distortion does not occur.

[0040] In this embodiment, the wire 70 constitutes the wire saw 7 (not shown), and is a high-strength steel wire, such as piano wire, with abrasive grains attached to it. Diamond, which has excellent hardness, is preferably used as the material for the hard particles used as abrasive grains. However, when the wafer 11 is modified using a laser, as described below, zirconia (ZrO2), silicon carbide, etc. can also be used. The maximum size of the abrasive grains is the abrasive grain size, and the maximum width of the abrasive grains attached to the base wire is the wire thickness. To maintain wire strength while grinding thin wafers without damaging them, the abrasive grain size is preferably 5 μm to 10 μm, and the wire thickness is preferably 30 μm to 100 μm. That is, the wire 70 is thinner and has a smaller abrasive grain size than the wire 170 shown in FIG. 9(b). This reduces the amount of material removed during cutting and minimizes unevenness on the cut surface after division.

[0041] After dividing as shown in Fig. 1(d), the thin wafer surface 11S of the thin wafer 11 is a surface suitable for device formation, and since it is fixed to the backside fixing plate 2 shown in Fig. 10(b), the same processes as those shown in Fig. 10(c) to Fig. 11(f) can be carried out. However, since the thin wafer 11 is thinner than the wafer 10, the thickness to be polished can be reduced in Fig. 11(d) to Fig. 11(e), and loss of semiconductor material can be reduced.

[0042] On the other hand, in the thin wafer 12 shown in Fig. 1(d), the wafer surface 10S of the wafer 10 is covered by the surface fixing plate 3, and the exposed back surface is not suitable for device formation. For this reason, the thin wafer 12 also needs to be in the state shown in Fig. 10(b), that is, in a state in which the back surface is fixed while the thin wafer surface 12S (= wafer surface 10S) is exposed. Fig. 2 shows the process of fixing the thin wafer 12 fixed to the surface fixing plate 3 to the back surface fixing plate 5 to expose the thin wafer surface 12S.

[0043] 2(a) shows a state in which a back fixing plate 5 is placed near the back surface of a thin wafer 12 fixed to a front fixing plate 3. The back fixing plate 5 is formed by laminating an adhesive film 51 on a substrate 50, and as shown in FIG. 2(b), the back surface of the thin wafer 12 is adhered to the adhesive film 51. Here, when forming devices on the thin wafer 12, the back fixing plate 5 is required to have the same function as the back fixing plate 2. In other words, the characteristics required for the substrate 50 and adhesive film 51 of the back fixing plate 5 are the same as the characteristics required for the substrate 20 and adhesive film 21 of the back fixing plate 2.

[0044] As shown in Fig. 2(b), an adhesive film 51 is adhered to the back surface of the thin wafer 12 and fixed with a back surface fixing plate 5, and then the front surface fixing plate 3 is peeled off as shown in Fig. 2(c). As a result, the thin wafer 12 fixed to the back surface fixing plate 5 has the thin wafer front surface 12S (= wafer front surface 10S) exposed as shown in Fig. 2(d). Because the back surface fixing plate 5 has the same function as the back surface fixing plate 2, the thin wafer 12 can be processed in the same manner as the thin wafer 11, as shown in Fig. 10(c) to Fig. 11(f).

[0045] The thin wafers 11 and 12 obtained by dividing the wafer 10 are free from warping or distortion because one of their surfaces is fixed by a flat fixing plate. Therefore, the thin wafers 11 and 12 can be further divided to obtain ultrathin wafers. For example, the thin wafer 12 shown in FIG. 2(b) may be cut by grinding with a wire 70 from the same state as the wafer 10 shown in FIG. 1(c) to obtain two ultrathin wafers. The thin wafer 11 can also be cut by attaching a fixing plate to the thin wafer surface 11S side and then cutting it to obtain two ultrathin wafers. Here, if the thickness of the thin wafer 11 or ultrathin wafer 12, or even the thickness of the ultrathin wafer, is 100 μm or more, dividing it into two is not difficult from the perspective of the grinding margin of the wire 70 and positional accuracy. Furthermore, since the thickness of the wafer 10 is 800 μm or less, the upper limit of the thickness of a thin wafer with one surface fixed to a fixing plate is 400 μm. Therefore, when the present invention is applied to a thin wafer (or an ultra-thin wafer) bonded to a flat fixing plate, it can be preferably used if the thickness is 100 μm or more and 400 μm or less.

[0046] Thus far, the embodiment of the wafer dividing method has been described, in which wafer 10 is divided near the middle in the thickness direction to obtain two thin wafers, thin wafer 11 and thin wafer 12. However, the wafer dividing method is not limited to this. For example, as shown in FIG. 1C , wafer 10 may be divided into three or more thin or ultra-thin wafers by repeatedly cutting wafer 10 with wire 70 at a position near backside fixing plate 2 near the middle in the thickness direction. Specifically, as shown in FIG. 1C , wafer 10 is cut with wire 70 at a position near backside fixing plate 2 near the middle in the thickness direction to obtain thin wafers. After that, a backside fixing plate other than backside fixing plate 2 is re-fixed to the cut surface of the wafer remaining on the frontside fixing plate 3 side, and the same process as described above is repeated to cut the wafer near the backside fixing plate in the thickness direction with wire 70, thereby obtaining three or more thin wafers. This modification makes it possible to obtain three or more thin wafers more efficiently than the method of dividing wafer 10 near the middle in the thickness direction. That is, when four thin wafers are obtained by the method described above of dividing wafer 10 near the middle in the thickness direction, the peeling operation of surface fixing plate 3 shown in FIG. 2 must be performed twice. However, with this modified example, the exposed surfaces of the three thin wafers become surfaces suitable for device formation, so the process shown in FIG. 2 is not necessary, and it is sufficient to perform the process once only on the thin wafers that ultimately remain on the surface fixing plate 3 side, making it possible to obtain thin wafers efficiently.

[0047] 1(c) and 1(d) show that the wire 70 cuts and divides the wafer 10 by grinding, and FIG. 3 shows an embodiment of a wafer dividing device that performs the cutting.

[0048] In Fig. 3(a), wafer rotation jig 8S consisting of rotation shaft 80S and presser plate 81S, and wafer rotation jig 8B consisting of rotation shaft 80B and presser plate 81B are arranged to rotate about the same linear rotation axis, and constitute wafer rotation means 8. In Fig. 3, presser plate 81S and presser plate 81B have a square shape, but this is not limited to this and may be a circle or a polygon (other than a square).

[0049] The pressing plate 81S has a function of holding the substrate 30 on the front fixing plate 3. That is, the surface of the pressing plate 81S facing the front fixing plate 3 has a function of holding the substrate 30 by suction or the like. Similarly, the pressing plate 81B has a function of holding the substrate 20 on the back fixing plate 2 by suction or the like.

[0050] The pressing plate 81S holds the substrate 30 on the front surface fixing plate 3, and the pressing plate 81B holds the substrate 20 on the back surface fixing plate 2, so that the rotating jig 8 sandwiches the wafer 10 on both sides via the front surface fixing plate 3 and the back surface fixing plate 2, as shown in Figure 3 (b).

[0051] At least one of the rotation shafts 80S and 80B is connected to a rotational power source (not shown) to rotate the wafer 10. When the rotation jig 8 clamps the wafer 10 from both sides via the front surface fixing plate 3 and the back surface fixing plate 2, it is desirable to align the central axes of the rotation shafts 80S and 80B with the center of the wafer 10 as much as possible.

[0052] 3(b), when the wire 70 of the wire saw 7 is brought into close contact with the outer periphery of the wafer 10 while the wafer 10 is being rotated, the outer periphery of the wafer 10 is ground by abrasive grains adhering to the surface of the wire 70. In this process, abrasive grains may come off the wire 70 or the wire 70 may be damaged during the grinding of the wafer 10. Therefore, the wire 70 is run in sequence, and the portion used to grind the wafer 70 is collected.

[0053] 3(b), if the positions and distance DX between the presser plates 81S and 81B are not stable, it is undesirable because tilting and unevenness will occur on the cut surface. For this reason, while grinding is being performed by the wire 70, control is performed to suppress fluctuations in the positions of the presser plates 81S and 81B (in the X direction).

[0054] 4A and 4B are cross-sectional views showing the process of cutting wafer 10 by wire 70 through grinding. In Fig. 4A, wire 70 is separated from wafer 10 and grinding has not yet begun, but wafer 10 is being rotated while being sandwiched between presser plates 8 (8S and 8B). Fig. 4B shows the state in which grinding by wire 70 is progressing from the surface of wafer 10 to the interior, and Fig. 4C shows the state in which grinding has reached the center, and wafer 10 has been divided into thin wafers 11 and 12.

[0055] Incidentally, the wafer dividing device shown in Figure 3 can divide one wafer into two thin wafers, improving the efficiency of material utilization, but the time required from the start to the end of grinding is the same as in the conventional device. Therefore, several modifications that were considered as a method of increasing the grinding speed for the embodiment shown in Figures 3 and 4 are shown below.

[0056] FIG. 5( a) shows a first modification of the embodiment, showing the state before grinding begins. In the first modification, a laser irradiation means 71 is provided. Here, by irradiating the wafer with laser light L using the laser irradiation means 71, the material constituting the wafer 10 is altered, allowing the wire 70 to efficiently grind the wafer 10. When irradiating the wafer 10 with the laser light L, the irradiation spot diameter is preferably 80% or more and 120% or less of the width (diameter) of the wire 70, in terms of grinding speed, accuracy, and grinding width. The optimum wavelength of the laser light L varies depending on the material of the wafer 10. In the case of silicon carbide, an absorption wavelength of approximately 450 nm is preferred, and a wavelength of 430 nm or more and 470 nm or less is desirable.

[0057] In the first modification, the laser light emitted by the laser irradiation means 71 is focused on the point (depth) where the wire 70 makes contact. That is, in the process of the wafer 10 being ground by the wire 70, it is preferable to focus on the point of the wafer 10 just before the wire 70 makes contact. Figure 5(b) shows the state where grinding is completed in the first modification of the embodiment.

[0058] As mentioned above, it is preferable to focus the laser light emitted by the laser irradiation means 71 on a point (depth) just before contact with the wire 70, and such a point is the bottom of the groove formed by cutting with the wire 70. In such a groove, shavings may remain inside the groove, and the laser light may not reach the bottom of the groove.

[0059] To address this issue, a second modification of the embodiment shown in FIGS. 5( c) and 5(d) is provided. This second modification includes a fluid ejection means 71 that uses a fluid to remove shavings remaining inside the grooves. The fluid ejection means 71 ejects gas or liquid toward the grooves in the wafer 10 (created by the wire 70 grinding) to remove the shavings remaining in the grooves. The fluid ejection means 71 is positioned along the rotation direction of the wafer 10 between the point where the wire 70 contacts the groove and the point where the laser irradiation means 71 applies the laser light. This allows the shavings created by the wire 70 to be removed from the groove before the laser light is applied to the groove, thereby enabling the laser light to be applied to the bottom of the groove. This allows the wafer 10 to be efficiently ground and cut, and the time from the start of grinding shown in FIG. 5( c) to the completion of grinding shown in FIG. 5(d) can be shortened compared to the embodiment shown in FIG. 4. It is also possible to irradiate the laser light L and proceed with grinding with the wire 70 partway, remove the shavings, and then repeat the irradiation of the laser light L and grinding with the wire 70 again.

[0060] In the figures described so far, an embodiment has been shown in which the wire 70 moves down to grind the wafer 10, but the wire 70 may move up instead of down. However, in an apparatus configuration including a laser irradiation means 71 and a fluid jetting means 72, it is desirable that the location where the laser light is irradiated and the direction in which the fluid (gas or liquid) is jetted onto the wafer 10 remain constant. Therefore, it is preferable to use a configuration in which the laser irradiation means 71 and the fluid jetting means 72 are not changed relative to the wafer 10, but are instead raised as a whole, as in Modification 2A of the embodiment shown in Figure 6. That is, it is preferable to use a configuration in which the wafer rotation means 8 that rotates the wafer 10, the laser irradiation means 71, and the fluid jetting means 72 are moved together, or the wire saw 7 is moved.

[0061] As a further modification of the second modification of the embodiment, a modification 2B is shown in Fig. 7. In the modification 2B, the inclination of the wire 70 is changed in the configuration of the second modification shown in Fig. 5(c) to slightly change the location where the wafer 10 is cut. By changing the inclination in this way, it is possible to prevent shavings from clinging to the wire 70, and a decrease in cutting efficiency is prevented.

[0062] Furthermore, although the wire 70 is moved in the vertical direction in the embodiments shown in Figures 4 to 6, this is not limiting. For example, it is possible to grind and cut the wafer 10 by moving the wire 70 in the -Y direction or the -Z direction with the wire 70 oriented as shown in Figures 8(a) to 8(c). Note that Figure 8(a) shows a third variation of the embodiment having the same components as Figure 4(a), Figure 8(b) shows a fourth variation of the embodiment having the same components as Figure 5(a), and Figure 8(c) shows a fifth variation of the embodiment having the same components as Figure 5(c).

[0063] Furthermore, the present invention is also effective in a configuration in which the wire 70 is attached in the vertical direction (Z direction), as in the sixth modified embodiment shown in FIG. 8(d).

[0064] As described above, the semiconductor wafer dividing method of the present invention makes it possible to increase the number of wafers that can be obtained from a semiconductor crystal ingot compared to conventional methods, and the semiconductor wafer processing method of the present invention makes it possible to obtain many devices. Furthermore, the wafer separating apparatus of the present invention makes it possible to increase the grinding speed compared to conventional wire grinding, making it possible to divide wafers at high speed.

[0065] 1 Ingot 2 Back surface fixing plate 3 Front surface fixing plate 4 Device protection fixing plate 5 Back surface fixing plate 6 Semiconductor device 7 Wire saw 8 Wafer rotation means 8S, 8B Wafer rotation jig 10 Wafer (original thickness) 10S Wafer surface 11, 12 Thin wafer 11S, 12S Thin wafer surface 20, 30, 40, 50 Substrate 21, 31, 41, 51 Adhesive film 60 p region 61 n+ region 62 Gate oxide film 63 Gate electrode 64 Source electrode 70, 170 Wire 71 Laser irradiation means 72 Fluid ejection means 80, 80S, 80B Rotation shaft 81, 81S, 81B Pressing plate 6102M Drain electrode F Fluid (liquid flow or air flow) L Laser light RC Rotation shaft

Claims

1. A method for dividing a wafer to obtain thin wafers, comprising the steps of: attaching fixing plates to both sides of a wafer made of at least a semiconductor crystal; and, while holding the fixing plates on both sides, cutting the wafer near the middle in the thickness direction to obtain two thin wafers attached to the fixing plates.

2. A wafer processing method comprising the steps of: attaching one of two thin wafers obtained by the wafer dividing method described in claim 1, which has an exposed surface with a specified crystal orientation, to a fixed plate and then flattening the surface with the specified crystal orientation, followed by forming a number of devices on the flattened surface.

3. A wafer processing method comprising the steps of: attaching the exposed surface of one of two thin wafers obtained by the wafer dividing method described in claim 1, the surface of which facing a specific crystal orientation is attached to a fixing plate; peeling off the fixing plate from the surface facing the specific crystal orientation; flattening the surface facing the specific crystal orientation; and forming a number of devices on the flattened surface.

4. A wafer dividing device which affixes fixing plates to both sides of a wafer and divides the wafer into two thin wafers while holding both sides of the fixing plates, the device comprising: a wafer rotation means which clamps the wafer from both sides via the fixing plates and rotates the wafer in a plane parallel to the wafer surface; and a wire saw having a wire arranged perpendicular to the rotation axis of the wafer rotation means.

5. A wafer dividing apparatus according to claim 4, further comprising a laser irradiation means for irradiating the wafer with a laser beam focused on the point where the wire contacts the wafer.

6. A wafer dividing apparatus according to claim 5, wherein the wafer is irradiated with laser light before the wire comes into contact with the wafer.

7. A wafer dividing device according to claim 6, further comprising a fluid ejection means for ejecting a gas or liquid, the fluid ejection means ejecting the gas or liquid toward the portion of the wafer that has come into contact with the wire saw.

8. A wafer dividing device according to any one of claims 4 to 7, wherein the wire has a thickness of 30 μm or more and 100 μm or less.

9. A wafer dividing device according to claim 8, wherein the wire has abrasive grains attached to its surface, and the maximum size of the abrasive grains is 5 μm or more and 10 μm or less.

10. A wafer dividing apparatus according to any one of claims 4 to 7, wherein the wafer is a silicon carbide wafer.

11. A wafer dividing device according to any one of claims 5 to 7, wherein the wafer is a silicon carbide wafer, and the wavelength of the laser light is 430 nm or more and 470 nm or less.

12. A wafer dividing device according to any one of claims 4 to 7, wherein the thickness of the wafer is 250 μm or more and 800 μm or less.

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