Gallium oxide substrate processing method

Abstract

[Problem] To provide a practical method for processing a gallium oxide substrate that allows a gallium oxide substrate to be cut vertically and horizontally in a grid pattern. [Solution] Using a gallium oxide substrate in which the main surface is the (001) plane: mechanical scribing is carried out in which kerfs are engraved using a scribing tool on the main surface along predetermined cutting lines parallel to the X direction, which is the direction parallel to the line of intersection between the main surface and the (100) plane; and laser scribing for converting is carried out by scanning a laser beam along predetermined cutting lines parallel to the Y direction. Cutting is performed so as to cause breaking along the predetermined cutting lines of the X direction and Y direction following mechanical scribing and laser scribing.

Description

[Technical Field]

[0001] The present invention relates to a method for processing a gallium oxide (Ga2O3) substrate, and more particularly to a substrate processing method that enables a gallium oxide substrate to be cut into a lattice pattern. [Background technology]

[0002] Electronic devices, which are created by forming a grid-like pattern of device regions on a semiconductor or glass substrate and then cutting out each individual device region, are used in a wide range of fields as products such as optical devices, display devices, control devices, and power devices. Among these, for power semiconductor devices used for power control, etc., it is desirable to use substrate materials that can reduce power loss and have excellent voltage resistance properties.

[0003] The performance of substrate materials used in power semiconductors is evaluated using the Varigar performance index (εμEc 3 (where ε is the dielectric constant, μ is the electron mobility, and Ec is the dielectric breakdown field strength) is used as one of the performance indices, and materials with high index values ​​are evaluated as excellent materials for use as power semiconductors. Comparing the Varigar figures of merit of major semiconductor materials, where Si is set to 1, SiC has a value of about 340, GaN has a value of about 870, and Ga2O3 has a value of 1500 to 3000, and SiC, GaN, and Ga2O3 are all attracting attention in the market as next-generation power semiconductor materials.

[0004] Of these, gallium oxide (Ga2O3) has an outstanding Varigar figure of merit, and since gallium oxide with a beta gallium structure can be efficiently grown into high-quality bulk single crystals using melt growth methods such as the Edge Defined Film Fed Growth (EFG) method, it is particularly expected to be used as a power semiconductor material.

[0005] This β-gallium oxide has an a-axis lattice constant of 12.2 Å, a b-axis lattice constant of 3.0 Å, a c-axis lattice constant of 5.8 Å, an α-angle (the angle between the b-axis and the c-axis) of 90°, a β-angle (the angle between the c-axis and the a-axis) of 103.8°, and a γ-angle (the angle between the a-axis and the b-axis) of 90°, resulting in a monoclinic crystal structure as shown in Figure 1.

[0006] Furthermore, in gallium oxide single crystals having a β-gallia structure (hereinafter, gallium oxide having a β-gallia structure will be simply referred to as gallium oxide), the (100) plane of the above crystal structure is the first cleavage plane and exhibits particularly strong cleavage properties, followed by the (001) plane, which is the second cleavage plane and exhibits weak cleavage properties.

[0007] In the melt growth method, in which a seed crystal is grown by contacting it with a melt, the cleavage properties of the single crystal vary greatly depending on the crystal surface (growth surface) that comes into contact with the melt. If a single crystal with strong cleavage properties is produced, the processability of the substrate can be significantly impaired. Therefore, as a method for growing gallium oxide crystals that solves the problem of impairing the processability of the substrate, it has been disclosed that if the seed crystal is pulled in the c-axis direction during crystal growth, the crystal growth surface is the (001) plane, and the crystal is grown by the EFG method so that the (100) plane stands vertically to obtain a single crystal, it is possible to weaken the cleavage tendency and improve the processability (see Patent Document 1).

[0008] The method for growing gallium oxide single crystals described in Patent Document 1 produces single crystals grown in the c-axis direction. However, when performing circular punching and slicing to cut this single crystal into disk-shaped substrates (wafers) and orientation flat processing to form orientation flats that indicate the crystal orientation, if the main surface (substrate surface) is cut to be the (100) plane, the crystal will still exhibit strong cleavage with the (100) plane as the cleavage plane, resulting in defects (chipping, cracks, peeling).

[0009] In order to obtain a gallium oxide substrate that is free of defects, a gallium oxide substrate and a manufacturing method thereof have been disclosed in which a surface other than the (100) plane is used as the main surface of a circular gallium oxide substrate, and orientation flats perpendicular to this main surface and the (100) plane are formed around the periphery of the main surface such that first and second orientation flats are arranged in point symmetry with respect to the center point of the main surface (see Patent Document 2).

[0010] In the gallium oxide substrate and its manufacturing method described in Patent Document 2, the primary surface may be any surface except the (100) plane, but it is specifically described that the primary surface is preferably the (101), (110), or (111) plane.

[0011] Similarly to Patent Document 2, a method for producing a disk-shaped gallium oxide substrate from a gallium oxide single crystal ingot while suppressing the occurrence of defects has been disclosed in which a cylindrical block is cut out by wire electric discharge machining, including orientation flat machining, and the cylindrical block is sliced ​​to produce a gallium oxide substrate (see Patent Document 3).

[0012] In the gallium oxide substrate and its manufacturing method described in Patent Document 3, the primary surface is a plane other than the (100) plane, such as the (-201) plane, the (101) plane, or the (001) plane. It is also described that when the primary surface is the (001) plane, homoepitaxial growth of a high-quality Ga2O3-based semiconductor layer is possible, and the substrate can be used as an excellent substrate for electronic devices. It also states that the orientation flat should be arranged in a direction parallel to the intersection of the main surface and the (100) plane, and is preferably perpendicular to the main surface, and that the number of orientation flats formed when cutting out a cylindrical block, including orientation flat machining by wire electric discharge machining, may be one (although two may be formed). [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-312571 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-67524 [Patent Document 3] Patent No. 5816343 Summary of the Invention [Problem to be solved by the invention]

[0014] As described in Patent Document 3, among substrates having a principal surface (substrate surface) with a plane orientation other than the (100) plane, a gallium oxide substrate having a principal surface in the (001) plane is excellent as a substrate for electronic devices, and when used as a substrate for power semiconductor devices, excellent power devices can be manufactured.

[0015] Incidentally, when a power semiconductor device is patterned on a gallium oxide substrate, it is necessary to cut each device area without adversely affecting the device area (pattern formation area), just as in conventional device processing on Si substrates or glass substrates. Therefore, a cutting process is required to cut the device area without adversely affecting the device area (pattern formation area) by mechanical scribing using a cutting edge such as a cutter wheel or laser scribing using laser irradiation.

[0016] In the conventional cutting process for a disk-shaped Si substrate, the substrate is cut lengthwise and widthwise in a grid pattern based on the orientation flat. Specifically, the direction parallel to the intersection line between the main surface and the orientation flat is defined as the X direction, and the direction perpendicular to the X direction on the main surface is defined as the Y direction, and the substrate is cut in a grid pattern along the X and Y directions.

[0017] However, because gallium oxide substrates have a first cleavage plane (100) that exhibits strong cleavage properties, it was found that if an attempt was made to cut the substrate into a lattice pattern using a scribe similar to the cutting method previously established for Si substrates and glass substrates, the strong cleavage plane would have an effect, making it difficult to achieve the expected lattice-pattern cutting process. Specifically, when attempting to cut in the X and Y directions with a mechanical scribe, even if a kerf is formed on the main surface, if the kerf is in a direction different from the direction of the cleavage plane, the crack does not penetrate in the thickness direction, making it impossible to cut. Also, when attempting to cut in the X and Y directions with a laser scribe, even though the laser is irradiated along the planned cutting line, the position of the cutting surface on the main surface is not fixed, and there are many cases where cutting cannot be performed in line with the planned cutting line, resulting in the problem of being unable to achieve high-precision cutting.

[0018] It is possible to cut the substrate lengthwise and crosswise using other cutting techniques (e.g., wire electric discharge machining) as described in Patent Document 3, but this may have a negative effect on the device region after the device region has been patterned on the main surface of the substrate. Furthermore, for mass production of devices, the processing time and cost required for cutting must also be taken into consideration. It is also desirable to be able to cut the substrate using highly reliable processing techniques that have been used in cutting silicon substrates and glass substrates.

[0019] Therefore, an object of the present invention is to provide a practical method for processing a gallium oxide substrate that enables a disk-shaped gallium oxide substrate to be cut lengthwise and crosswise into a lattice pattern. Another object of the present invention is to provide a processing method for cutting out each device region on a gallium oxide substrate into a lattice shape without affecting the device region, for example, when the device region is patterned in a lattice shape on a disk-shaped gallium oxide substrate. [Means for solving the problem]

[0020] The method for processing a gallium oxide substrate of the present invention, which has been made to solve the above problems, is a method for processing a gallium oxide substrate, which cuts a gallium oxide substrate having a β-gallium structure along an X direction and a Y direction which are orthogonal to each other on a main surface, A gallium oxide substrate whose main surface is a (001) plane is used, and the direction parallel to the intersection line between the main surface and the (100) plane is defined as the X direction. Mechanical scribing is performed to carve kerfs on the main surface with the cutting edge of a scribing tool along a planned cutting line parallel to the X direction, and laser scribing is performed to modify the substrate by scanning a laser beam along the planned cutting line parallel to the Y direction, so that the substrate breaks along the planned cutting lines in the X and Y directions after the mechanical scribing and laser scribing processes.

[0021] Here, the term "scribing tool" refers to a tool with a sharp cutting edge made of a material, such as diamond or cemented carbide, that is harder than the substrate material, and that scribes grooves by scanning the substrate while pressing the cutting edge against it. The cutting edge may be a rotary blade or a fixed blade.

[0022] The laser light source used in "laser scribing" is a laser light source with a wavelength that can be absorbed by the gallium oxide substrate, but it is more preferable to select a laser wavelength and lens optical system that is absorbed not only on the surface of the substrate but also as much as possible inside the substrate, and irradiate the laser so that it is altered deep into the thickness direction. Specifically, an infrared laser can be used as the laser light source.

[0023] In the "breaking" process, mechanical or thermal stress is applied along the planned cutting line after scribing. There are no particular limitations on the method of applying stress, but a breaking device that has traditionally been used to break Si substrates or glass substrates can be used. Specifically, a mechanical breaking device can be used that applies a breaking bar to the back side of the planned cutting line to bend the substrate and cut it, or a breaking device that thermally breaks the substrate by blowing cold or hot heat onto it. Simple breaking, such as bending the substrate manually or using a simple jig, is also acceptable.

[0024] According to the present invention, when a kerf is formed by scribing the main surface with the cutting edge of a scribing tool along a planned cutting line parallel to the X direction, the X direction is the direction of the intersection between the main surface and the (100) plane, and therefore the (100) plane containing the kerf is cleaved, and the wafer can be cut out along this (100) plane. In contrast, there is no plane orientation that exhibits cleavage in the Y direction, which is perpendicular to the X direction. Therefore, even if a scribing tool is used to scribe grooves in the Y direction on the main surface, cracks will not penetrate. If a deep groove is carved along the Y direction and an attempt is made to cut it out by force, the surface layer will peel off and the substrate will be destroyed. Therefore, in the Y direction, laser scribing is performed by scanning a laser beam along the intended cutting line to thermally modify the material. This makes it possible to modify the material from the surface to the interior of the substrate, weakening the bonds between the crystal lattices on non-cleavable surfaces, and the laser beam is irradiated directly below the intended cutting line, allowing cutting along the locally modified vertical surface. When the laser beam is irradiated along the line to cut in the X direction, cleavage occurs along the cleavage plane of the (100) plane, but the starting point of the cleavage is not limited to the line to cut on the main surface, and cleavage may occur at a position near the laser irradiation position that is off the line to cut. As a result, it is not possible to cut accurately along the line to cut, and the processing accuracy decreases significantly. [Effects of the Invention]

[0025] According to the present invention, a substrate with a specified orientation of the main surface and cutting direction is used, and mechanical scribing is performed with a scribing tool in the X direction, while laser scribing is performed in the Y direction, which is orthogonal to the X direction. This allows cutting in the X direction by utilizing cleavage (in a direction almost perpendicular to the main surface at an inclination of about 14° with respect to the XY plane (main surface)), and cutting in the Y direction by weakening the bonds between crystal lattices in a direction in which there is no cleavage plane. This makes it possible to precisely cut the main surface of a gallium oxide substrate in the X and Y directions, which are orthogonal to each other.

[0026] In the above invention, the laser scribing process on the line to cut parallel to the Y direction may be performed by repeating the scanning of the laser beam a plurality of times on one line to cut. By repeating the laser beam scan multiple times, the bonds between the crystal lattices at the positions inside the substrate where the laser beam passed can be further weakened as the number of scans increases, making it easier to perform the subsequent breaking process in the Y direction.When the thickness of the substrate is increased, the ease of cutting in the Y direction can be adjusted by increasing the number of laser beam scans depending on the thickness of the substrate.

[0027] Furthermore, using the above-described method for processing a gallium oxide substrate, the main surface may be processed into a parallelepiped shape in which the main surface is a (001) plane, the side surface in the X direction is a (100) plane that intersects obliquely with the main surface (at a theoretical angle of 103.8°), the side surface in the Y direction is a plane orthogonal to the main surface and the side surface in the X direction, and the main surface is rectangular. According to the present invention, when cutting is performed using cleavage in the X direction, it is possible to cut out a parallelepiped shape with the main surface and back surface being rectangular (including square). In conventional manufacturing processes for devices using Si substrates, the main surface of the substrate is scribed in the X and Y directions to cut out a rectangular parallelepiped shape. However, the gallium oxide substrate of the present invention can also be cut out into a parallelepiped shape that is close to a rectangular parallelepiped by scribing the main surface in the X and Y directions. Therefore, device processing techniques that have traditionally been used for Si substrates can be applied almost directly to device processing on gallium oxide substrates. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 2 is a diagram illustrating the plane orientation and crystal axes of a gallium oxide crystal having a monoclinic β-gallium structure. [Figure 2] 1A to 1C are diagrams ((a) plan view, (b) front view, (c) right side view) illustrating the plane orientation index and orientation flat crystal orientation of a gallium oxide substrate used in the present invention. [Figure 3] 3 is a diagram showing cutting lines in the X and Y directions along which a gallium oxide substrate is cut according to the present invention. FIG. [Figure 4] These are enlarged views showing the cross section cut by mechanical scribing in the X direction, where (a) is a cross-sectional photograph seen from the front, and (b) is a cross-sectional photograph seen from the side. [Figure 5] 10A and 10B are enlarged views showing the cross section cut by laser scribing in the X direction, where (a) is a cross-sectional photograph seen from the front and (b) is a cross-sectional photograph seen from the side. [Figure 6] This is an enlarged photograph of the main surface after laser scribing in the X direction, taken from above. [Figure 7] These are enlarged views showing the cut surface by laser scribing in the Y direction, where (a) is a cross-sectional photograph seen from the front, and (b) is a cross-sectional photograph seen from the side. DETAILED DESCRIPTION OF THE INVENTION

[0029] (Gallium oxide substrate surface orientation index and orientation flat orientation index) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figure 2 is a diagram illustrating the plane orientation index and orientation flat crystal orientation of a gallium oxide substrate used in the substrate processing method of the present invention. The disk-shaped gallium oxide substrate 1 has a (001) plane orientation on both the main surface S, which is the substrate front surface, and the back surface of the substrate. The orientation flat is a plane parallel to the intersection L between the (001) plane, which is the main surface S, and the (100) plane, and is formed as an orientation flat OF, which is a plane perpendicular to the main surface S. Therefore, the (100) plane is inclined at a theoretical angle of 13.8° from the orientation flat OF. The method for manufacturing a substrate having such a defined orientation of the main surface S and orientation flat OF is not particularly limited, but it can be manufactured by the manufacturing method described in Patent Document 3, for example.

[0030] (Cutting process verification experiment) The verification experiment of the present invention will be described below. The direction parallel to the intersection line L between the main surface S and the (100) plane, that is, the direction parallel to the intersection line between the main surface S and the orientation flat OF (orientation index <010> ) was used as the reference, which was defined as the X direction, and the direction perpendicular to this on the main surface S was defined as the Y direction, and the substrate was cut into a grid pattern along the planned cutting lines in the X and Y directions set on the main surface S as shown in Figure 3. The thickness of the substrate used was 0.7 mm.

[0031] (Mechanical scribing in the X direction) The X-direction processing was performed as follows: in the first step, a scribing tool was used to perform mechanical scribing to form a kerf on the main surface S, and then in the second step, a breaking process was performed to mechanically apply stress along the intended cutting line. The scribing conditions were as follows: Scribing tool: Cutter wheel (rotary blade) Scribe load: 0.1MPa Scribe speed: 100mm / s

[0032] As a result, cutting was possible starting from the planned cutting line on the main surface S. Figure 4(a) is a cross-sectional photograph of the cut surface seen from the front, and Figure 4(b) is a cross-sectional photograph seen from the side. The cut surface was inclined at an angle of approximately 104° to the main surface S (approximately 14° to the orientation flat surface) (chipping occurred), and cutting was possible along the cleavage plane of the (100) plane. Furthermore, similar cutting was possible when the scribe load was set to 0.05 MPa to 0.2 MPa.

[0033] (Mechanical scribing in the Y direction) As with the X direction, the Y direction was processed by mechanical scribing using a scribing tool in the first step, followed by breaking in the second step to mechanically apply stress along the intended cutting line. The scribing conditions were as follows: Scribing tool: Cutter wheel (rotary blade) Scribe load: 0.1MPa Scribe speed: 100mm / s

[0034] As a result, it was not possible to cut at all. If an attempt was made to deepen the kerf by increasing the scribe load, peeling of the main surface S (surface layer) would occur. Since there was no cleavage plane in the Y direction, the crack could not penetrate in the thickness direction, and cutting along the planned cutting line was not possible.

[0035] (Laser scribing in the X direction) The X-direction processing was performed by laser scribing using an infrared laser in the first step, and then by breaking along the intended cutting line in the second step. The scribing conditions were as follows: Laser light source: Infrared laser Oscillation wavelength: 1064nm Lens optics: Combination lens (multifocal lens)

[0036] As a result, cutting was achieved with a flat cut surface. Figure 5(a) is a cross-sectional photograph of the cut surface seen from the front, and Figure 5(b) is a cross-sectional photograph seen from the side. The cut surface was inclined at an angle of approximately 103.9° to the main surface S (chipping occurred), and cutting was achieved along the cleavage plane of the (100) plane. However, there were many cases where the position of the cutting plane on the main surface S was not parallel to the planned cutting line, making it difficult to determine the position of the cutting plane. Figure 6 is a plan view of the main surface S taken from above. There is a misalignment between the scribe line L (the same line as the planned cutting line), which is the trace left by the laser irradiation on the main surface S, and the cutting line C that was actually cut on the main surface S.

[0037] This phenomenon is thought to be caused by the fact that the position of the starting crack, which determines the cutting surface, changes stochastically depending on the positional relationship between the weak lattice bonds that exist irregularly inside the substrate and the position where the laser beam passes through the substrate, making it difficult to determine the position of the cutting surface.As a result, when scribing in the X direction using laser irradiation, it was not possible to cut accurately along the intended cutting line.

[0038] (Laser scribing in the Y direction) The Y-direction processing was performed by laser scribing using an infrared laser in the first step, and then by breaking along the intended cutting line in the second step. The scribing conditions were as follows: Laser light source: Infrared laser Oscillation wavelength: 1064nm Lens optics: Combination lens (multifocal lens)

[0039] As a result, it was possible to cut with a flat cut surface. Figure 7(a) is a cross-sectional photograph of the cut surface seen from the front, and Figure 7(b) is a cross-sectional photograph seen from the side. The cut surface was perpendicular to the main surface S, and it was possible to cut in a direction without cleavage planes. The cut surface was also precisely positioned along the planned cutting line.

[0040] In laser scribing in the Y direction, increasing the number of laser scribe scans to multiple times enabled cutting with reduced stress applied along the intended cutting line during the breaking process in the second step, and the breaking process tended to be easier than when laser scribing was performed with only one scan. By reducing the stress applied during the breaking process, it was possible to reduce the risk of inducing cleavage of the (001) plane, the main surface, during breaking.

[0041] (Summary of verification results) The results of the above verification experiments can be summarized as follows: (a) The main surface is a (001) plane, and the direction parallel to the intersection line between the main surface and the (100) plane ( <010> a gallium oxide substrate having an orientation flat formed along a plane (preferably a plane perpendicular to the main surface S) including the orientation flat (direction); (b) The direction of the above intersection line ( <010> direction) is set as the X direction, and a mechanical scribing process is performed in which a kerf is carved on the main surface S using a scribing tool along a planned cutting line parallel to the X direction. (c) performing laser scribing by scanning a laser beam along a line to be cut that is parallel to the Y direction perpendicular to the X direction; By satisfying all of the above, the main surface S could be cut into a grid pattern along the X and Y directions, which are orthogonal to each other.

[0042] The substrate cut into a grid pattern from the main surface S by the above processing method can be cut out as a parallelepiped in which the main surface S (substrate front surface) and the back surface of the substrate are (001) planes, one pair of side surfaces are (010) planes perpendicular to the main surface S, and the other pair of side surfaces are inclined at a theoretical angle of 103.8° with respect to the main surface S. Therefore, it can be cut out into an approximately rectangular parallelepiped shape, making it possible to apply (repurpose) conventional device processing techniques for Si substrates. [Industrial Applicability]

[0043] The present invention can be used as a cutting method for a gallium oxide substrate suitable for use in a power device. [Explanation of symbols]

[0044] 1. Gallium oxide substrate S main surface OF Orientation Flat C Cut surface L Laser scribe irradiation marks (planned cutting line)

Claims

1. A method for processing a gallium oxide substrate, comprising cutting a gallium oxide substrate having a β-gallium structure along an X direction and a Y direction orthogonal to each other on a main surface, Using a gallium oxide substrate whose main surface is a (001) plane, a direction parallel to an intersection line between the main surface and a (100) plane is defined as an X direction, and a mechanical scribing process is performed in which a cutting groove is carved on the main surface with a cutting edge of a scribing tool along a planned cutting line parallel to the X direction; A laser scribing process is performed by scanning a laser beam along a cutting line parallel to the Y direction, thereby modifying the material. A processing method for a gallium oxide substrate, which breaks the substrate along planned cutting lines in the X and Y directions after the mechanical scribing process and the laser scribing process.

2. The method for processing a gallium oxide substrate according to claim 1, wherein the laser scribing process on the line to be cut parallel to the Y direction comprises repeating scanning of the laser beam multiple times on one line to be cut.

3. Using the method for processing a gallium oxide substrate according to claim 1 or 2, A method for processing a gallium oxide substrate, wherein the main surface is a (001) plane, the side surface in the X direction is a (100) plane that intersects obliquely with the main surface, the side surface in the Y direction is a plane that is perpendicular to the main surface and the side surface in the X direction, and the main surface is processed into a parallelepiped shape that is rectangular.

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