Method of fabrication of gallium oxidation substrate

Abstract

A practical method for processing a gallium oxide substrate is provided, which can longitudinally and transversely cut a gallium oxide substrate into a grid pattern. Using a gallium oxide substrate with a main surface (001), a mechanical scribing process is performed with a scribing tool to etch grooves on the main surface along a predetermined cutting line parallel to the X direction, taking the direction parallel to the intersection of the main surface and the surface (100) as the X direction. A laser scribing process is also performed, in which a laser beam is scanned along a predetermined cutting line parallel to the Y direction to degrade the substrate. The substrate is then cut in such a way that it is disconnected along the predetermined cutting lines in the X and Y directions after the mechanical scribing and laser scribing processes.

Description

[Technical Field]

[0001] The present invention relates to a method for processing gallium oxide (Ga2O3) substrates, and more specifically, to a method for processing a substrate that can cut a gallium oxide substrate into a grid pattern. [Previous Technology]

[0002] Electronic components, which are made by patterning component areas into a grid on a semiconductor substrate or glass substrate and cutting out individual component areas, are used in a wide range of fields as optical components, display components, control components, power components, etc. Among them, power semiconductor components used in power control and the like are preferably made of substrate materials that can suppress power loss and have excellent voltage resistance.

[0003] The performance evaluation of substrate materials used in power semiconductors uses the Baliga Figure of Merits (εμEc3: where ε is the dielectric constant, μ is the electron mobility, and Ec is the dielectric breakdown electric field strength) as one of the performance indicators. Materials with a high value of this index are considered to be excellent materials for power semiconductors. Comparing the Baliga Figure of Merits of major semiconductor materials, when Si is set to 1, SiC has a value of around 340, GaN around 870, and Ga2O3 between 1500 and 3000. SiC, GaN, and Ga2O3 are all attracting attention in the market as next-generation power semiconductor materials.

[0004] Among them, gallium oxide (Ga2O3) has excellent Balic performance index, and if it is gallium oxide with β Gallia structure, it can be efficiently grown into high-quality bulk single crystal by melt growth method such as EFG (Edge Defined Film Fed Growth), so it is particularly expected to be used as a power semiconductor material.

[0005] This β-Gallia structure of 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 formed by the b-axis and the c-axis) of 90°, a β angle (the angle formed by the c-axis and the a-axis) of 103.8°, and a γ angle (the angle formed by the a-axis and the b-axis) of 90°, and has a monoclinic crystal structure as shown in Figure 1.

[0006] In addition, the gallium oxide single crystal with β-Gallia structure (hereinafter referred to as gallium oxide with β-Gallia structure) has a strong cleavage property as the first cleavage plane on the surface (100) of the crystal structure, and a weaker cleavage property as the second cleavage plane on the surface (001).

[0007] In the melt growth method where a seed crystal is brought into contact with the molten liquid for growth, the cleavage property of a single crystal varies greatly depending on the crystallization surface (growth surface) in contact with the molten liquid. When a single crystal with strong cleavage property is produced, the processability of the substrate is significantly damaged. Therefore, as a method for growing gallium oxide crystals to solve the problem of damaged processability of the substrate, it has been revealed that if the lifting direction of the seed crystal during crystal growth is set to the c-axis direction and the crystal growth surface is set as the surface (001), and the single crystal is obtained by growing it using the EFG method so that the surface (100) is vertically erected, the cleavage property can be reduced and the processability can be improved (see Patent Document 1).

[0008] Although the gallium oxide single crystal growth method described in Patent Document 1 can obtain a single crystal that is grown in the c-axis direction, when performing the circle punching and slicing processes to cut out the circular substrate (wafer) from the single crystal, and the orientation plane process to form the orientation plane that shows the crystal orientation, if the main surface (substrate surface) is cut out to become a surface (100), the surface (100) will still show a strong splitting property as a splitting surface, and defects (cracking, cracking, peeling) will occur.

[0009] Therefore, in order to obtain a gallium oxide substrate that does not produce defects, a gallium oxide substrate and its manufacturing method have been disclosed (see Patent Document 2). The substrate has a main surface other than (100) as the main surface of the circular gallium oxide substrate, and the main surface and an orientation plane orthogonal to the surface (100) are arranged in a point-symmetric manner with the center point of the main surface as the symmetry point, so that the first and second orientation planes are formed on the periphery of the main surface.

[0010] In the invention patent document 2, the gallium oxide substrate and its manufacturing method are described. Although the main surface can be any surface other than surface (100), it is preferred to use surface (101), surface (110), and surface (111) as the main surface.

[0011] In addition, similar to Patent Document 2, as a method for suppressing the generation of defects and manufacturing a disc-shaped gallium oxide substrate from a gallium oxide single crystal ingot, a method has been disclosed for manufacturing a gallium oxide substrate by cutting out a cylindrical block by wire discharge processing, including orientation planar processing, and slicing the cylindrical block (see Patent Document 3).

[0012] In the gallium oxide substrate and its manufacturing method described in Patent Document 3, the main surface is described as a surface different from the surface (100), for example, surface (-201), surface (101), or surface (001). Furthermore, when the main surface is surface (001), homo epitaxial growth of a high-quality Ga2O3 semiconductor layer can be performed, making it suitable as a substrate for excellent electronic components. Furthermore, it is disclosed that the orientation plane is preferably perpendicular to the main surface, arranged along a direction parallel to the intersection of the main surface and surface (100). The number of orientation planes formed when cutting out a cylindrical block by wire discharge machining, including orientation plane machining, can be one (or two). [Prior Art Documents] [Patent Documents]

[0013] Invention Patent Document 1: Japanese Patent Application Publication No. 2006-312571; Invention Patent Document 2: Japanese Patent Application Publication No. 2013-67524; Invention Patent Document 3: Japanese Patent No. 5816343 [Summary of the Invention]

[0014] [The problem the invention aims to solve]

[0015] As also described in the above-mentioned Patent Document 3, among substrates with a different orientation than the surface (100) as the main surface (substrate surface), gallium oxide substrates with the main surface as the surface (001) are more superior as substrates for electronic components. If used as substrates for power semiconductor components, excellent power components can be manufactured.

[0016] However, when a power semiconductor device is patterned on a gallium oxide substrate, similar to the conventional device processing formed on a Si substrate or a glass substrate, since each device area must be cut without causing adverse effects on the device area (patterning area), a cutting step is required to be performed without causing adverse effects on the device area by mechanical scribing using a cutter wheel or laser scribing using laser irradiation.

[0017] In the cutting process of the circular Si substrate, conventionally, it is cut into a grid pattern in both directions based on the orientation plane. Specifically, it is set that the direction parallel to the intersection of the main surface and the orientation plane is taken as the X direction, and the direction orthogonal to the X direction on the main surface is taken as the Y direction, and it is cut into a grid pattern along the X and Y directions.

[0018] However, since the gallium oxide substrate has a first cleaving surface (100) exhibiting strong cleaving properties, it is known that if the same scribe pattern established on Si substrates or glass substrates is applied directly to cut into a lattice shape, the strong cleaving surface will affect the cutting process, making it difficult to achieve the desired lattice shape. Specifically, when mechanically scribes to cut in the X and Y directions, even if grooves are formed on the main surface, if the grooves are in a direction different from the cleaving surface, cracks will not penetrate into the thickness direction, making it impossible to cut. Furthermore, when laser scribes to cut in the X and Y directions, even if laser is irradiated along the predetermined cutting line, the position of the cutting surface on the main surface often becomes unstable, making it impossible to cut in accordance with the predetermined cutting line, resulting in a problem where high-precision cutting cannot be achieved.

[0019] While other cutting techniques (such as wire discharge machining) as described in Patent Document 3 can be considered for longitudinal and transverse cutting, there is a risk of adverse effects on the component areas after the component areas have been patterned on the main surface of the substrate. Furthermore, in order to mass-produce the components, the processing time or processing cost required for cutting must also be considered. In addition, it is more ideal to use a highly reliable processing technique that has been used in the cutting of Si substrates or glass substrates for cutting.

[0020] Therefore, the object of the present invention is to provide a practical method for processing a gallium oxide substrate that can longitudinally and transversely cut a disc-shaped gallium oxide substrate into a grid pattern. Furthermore, the object of the present invention is to provide a processing method for cutting each element region on the gallium oxide substrate in a grid pattern without affecting the element regions, when element regions are patterned into a grid pattern on a disc-shaped gallium oxide substrate. [Means for solving the problem]

[0021] The gallium oxide substrate processing method of the present invention, which is carried out to solve the above-mentioned problems, is a gallium oxide substrate processing method in which a gallium oxide substrate with a β Gallia structure is cut along the X and Y directions orthogonal to each other on the main surface. The method is configured to use the aforementioned main surface as surface (001) of the gallium oxide substrate, and take the direction parallel to the intersection line of the aforementioned main surface and surface (100) as the X direction, perform mechanical scribing processing to etch grooves on the aforementioned main surface by means of the tip of a scribing tool along a predetermined cutting line parallel to the X direction, and perform laser scribing processing to degrade the substrate by means of a scanning laser beam along a predetermined cutting line parallel to the Y direction, so as to break the substrate along the predetermined cutting lines in the X and Y directions after the aforementioned mechanical scribing processing and the aforementioned laser scribing processing.

[0022] Here, the term "grinding tool" refers to a tool with a sharp tip formed of a material with a hardness higher than that of the substrate material, such as diamond or superhard alloy. The tool is used to scribble grooves on the substrate by simultaneously pressing the tip against it and scanning the substrate. The tip can be a rotating tool or a stationary tool.

[0023] In the "laser marking process", although the laser light source used is a laser light source with a wavelength that can be absorbed by the gallium oxide substrate, it is better to select a laser wavelength or lens optical system that can be absorbed not only by the substrate surface but also by the substrate interior as much as possible, so as to perform laser irradiation to modify the material to a deeper position in the thickness direction. Specifically, an infrared laser can be used as the laser light source.

[0024] In the "breaking" step, stress is applied mechanically or by heating along the etched cutting line. While there are no particular limitations on the method of stress application, breaking devices conventionally used for breaking Si substrates or glass substrates can be used. Specifically, a mechanical breaking device can be used, in which a break bar is placed against the back of the cutting line to flex the substrate for breaking; or a breaking device can be used, in which hot or cold air is sprayed to heat the substrate for breaking. Alternatively, a simple breaking method, such as bending the substrate manually or using a simple jig, can also be used.

[0025] According to the present invention, when a groove is etched on the main surface along a predetermined cutting line parallel to the X direction using the tip of a scriber, since the X direction is the intersection of the main surface and the surface (100), the surface (100) containing the groove will split, and it can be cut along that surface (100). In contrast, there is no surface orientation showing splitting in the Y direction, which is orthogonal to the X direction. Therefore, even if a groove is etched on the main surface by scriber in the Y direction, cracking will not penetrate. If a deep groove is etched along the Y direction and cut with all force, the surface layer will peel off or the substrate will be damaged. Therefore, regarding the Y direction, laser scriber processing is performed by heating and deteriorating the material by scanning a laser beam along the predetermined cutting line. This allows the substrate to be altered from its surface to its interior, weakening the bonds between non-cleaving lattices. A laser beam directly below the predetermined cutting line can then be used to cut along the locally altered vertical plane. However, when a laser beam is irradiated along the predetermined cutting line in the X direction, although cleaving occurs along the cleaving surface of the surface (100), the starting point of the cleavage is not limited to the predetermined cutting line on the main surface. Cleavage can occur near the laser irradiation position, deviating from the predetermined cutting line. Therefore, cutting cannot be performed correctly along the predetermined cutting line, significantly reducing processing accuracy. [Effects of the Invention]

[0026] According to the present invention, a substrate with a specific orientation having a main surface and a cutting direction is provided. Mechanical cutting is performed in the X direction using a cutting tool, and laser cutting is performed in the Y direction, which is orthogonal to the X direction. Therefore, the X direction can be cut by cleaving (inclined at about 14° relative to the XY plane (main surface) and approximately perpendicular to the main surface), and the Y direction can be cut in a way that weakens the bonding between lattices in the direction without cleaving. The process can be performed to cut the main surface of the gallium oxide substrate in the X and Y directions, which are orthogonal to each other, with good accuracy.

[0027] In the above invention, the laser marking process for a predetermined cutting line parallel to the aforementioned Y direction can be configured to repeatedly scan the aforementioned laser beam several times for a single predetermined cutting line. By repeatedly scanning the laser beam several times, the bonding between the lattice layers at the location through which the laser beam passes within the substrate is further weakened with the increase in the number of scans, making it easier to perform the subsequent Y-direction disconnection process. When the thickness of the substrate needs to be increased, the number of laser beam scans can be increased according to the substrate thickness, thereby adjusting the ease of cutting in the Y direction.

[0028] Alternatively, the gallium oxide substrate processing method described above can be used, wherein the aforementioned main surface is surface (001), the side surface in the X direction is surface (100) that is inclined and intersecting the aforementioned main surface (theoretically at an angle of 103.8°), and the side surface in the Y direction is surface that is orthogonal to the aforementioned main surface and the aforementioned side surface in the X direction, and is processed into a parallelepiped shape that makes the aforementioned main surface rectangular. According to the present invention, when cutting by splitting in the X direction, a parallelepiped shape that makes the main surface and the back surface rectangular (including square) can be cut out. In the conventional manufacturing process of devices using Si substrates, although the main surface of the substrate is etched in the XY direction and cut into a cuboid shape, the gallium oxide substrate of the present invention also etches the main surface in the XY direction, thereby cutting out a parallelepiped shape that is close to a cuboid shape. Therefore, the device processing technology that has been used for Si substrates can be applied almost directly to the device processing of gallium oxide substrates.

Implementation Method

[0030] (Orientation index of the surface of the gallium oxide substrate and orientation index of the orientation plane) The embodiments of the present invention will be described below with reference to the figures. Figure 2 is a diagram illustrating the orientation index of the surface of the gallium oxide substrate and the crystal orientation of the orientation plane used in the substrate processing method of the present invention. The circular gallium oxide substrate 1 is configured with the orientation of the main surface S of the substrate surface and the orientation of the back surface of the substrate as (001). Furthermore, the orientation plane is formed along the direction parallel to the intersection line L of the surface (001) and the surface (100) which is the main surface S, and is perpendicular to the main surface S as the orientation plane OF. Therefore, the surface (100) is tilted from the orientation plane OF by a theoretical angle of 13.8°. In addition, there is no particular limitation on the method of manufacturing a substrate whose orientation of the main surface S and the orientation plane OF is defined in this way, but it can be manufactured by, for example, the manufacturing method described in Patent Document 3.

[0031] (Verification Experiment of Cutting Process) The verification experiment of the present invention will be described below. Taking the direction parallel to the intersection line L of the main surface S and the surface (100), that is, the direction parallel to the intersection line of the main surface S and the orientation plane OF (orientation index <010>) as the reference and setting it as the X direction, and setting the direction on the main surface S orthogonal to the X direction as the Y direction, as shown in FIG3, the process of cutting into a grid shape along the predetermined cutting lines set on the main surface S in the X and Y directions is performed. The thickness of the substrate is 0.7 mm.

[0032] (Mechanical scribing in the X direction) The first step of the X-direction machining is to perform mechanical scribing using a scribing tool to form a groove on the main surface S. The second step involves mechanically applying stress along a predetermined cutting line to break the surface. The scribing conditions are as follows: Scribing tool: Reamer wheel (rotary cutter); Scribing load: 0.1 MPa; Scribing speed: 100 mm / s

[0033] As a result, cutting can be performed starting from the predetermined cutting line of the main surface S. Figure 4(a) is a cross-sectional photograph of the cut surface viewed from the front, and Figure 4(b) is a cross-sectional photograph viewed from the side. The cutting surface is inclined at an angle of about 104° relative to the main surface S (about 14° relative to the orientation plane) (fracture occurs), and cutting can be performed along the splitting surface of the surface (100). In addition, cutting can also be performed when the scribing load is set to 0.05MPa to 0.2MPa.

[0034] (Mechanical scribing in the Y direction) The Y-direction machining system is similar to that in the X direction. In the first step, mechanical scribing is performed using a scribing tool. Then, as the second step, mechanical stress-applied breaking is performed along a predetermined cutting line. The scribing conditions are as follows: Scribing tool: Reamer wheel (rotary cutter) Scribing load: 0.1MPa Scribing speed: 100mm / s

[0035] As a result, complete cutting was not achieved. If the scribing load is increased to deepen the groove, peeling of the main surface S (surface layer) will occur. Since there is no cleaving surface in the Y direction, the crack cannot penetrate in the thickness direction, and cutting along the predetermined cutting line cannot be performed.

[0036] (Laser marking in the X direction) The X-direction marking is performed in the first step by laser marking with an infrared laser, followed by a mechanical stress-applied breaking process along a predetermined cutting line in the second step. The marking conditions are as follows: Laser source: Infrared laser oscillation wavelength: 1064nm Lens optical system: Combined lens (multifocal lens)

[0037] As a result, it is possible to cut on a flat cutting surface. Figure 5(a) is a cross-sectional photograph of the cutting surface viewed from the front, and Figure 5(b) is a cross-sectional photograph viewed from the side. The cutting surface is inclined at an angle of about 103.9° relative to the main surface S (fracture occurs), and it is possible to cut along the split surface of the surface (100). However, it often happens that the position of the cutting surface on the main surface S is offset from the predetermined cutting line, and the position of the cutting surface is not fixed. Figure 6 is a top view photograph of the main surface S taken from above. The scribed line L (the same line as the predetermined cutting line) which is the mark left on the main surface S after laser irradiation, and the cutting line C which is actually cut on the main surface S, are offset in position parallel to each other.

[0038] This phenomenon can be attributed to the fact that the location of the crack at the starting point of the cut surface changes probabilistically due to the positional relationship between the weakly bonded lattice regions irregularly present within the substrate and the location of the laser beam passing through the substrate, thus making the position of the cut surface unpredictable. Therefore, in X-axis scribing processes using laser irradiation, it is impossible to perform cuts along the predetermined cutting line with good accuracy.

[0039] (Laser marking in the Y direction) The Y-direction marking is performed by infrared laser in the first step, followed by a mechanical stress-applied breaking process along a predetermined cutting line in the second step. The marking conditions are as follows: Laser source: Infrared laser oscillation wavelength: 1064nm Lens optical system: Combined lens (multifocal lens)

[0040] As a result, a cut can be made on a flat cutting surface. Figure 7(a) is a cross-sectional photograph of the cut surface viewed from the front, and Figure 7(b) is a cross-sectional photograph viewed from the side. The cutting surface can be cut perpendicular to the main surface S, and can be cut in a direction without splitting. The position of the cutting surface can also be accurately cut along the predetermined cutting line.

[0041] In laser marking in the Y direction, when the number of laser marking scans is increased to multiple times, as the number of scans increases, cutting can be performed during the break-off process in the second step even with reduced stress along the predetermined cutting line. Compared to laser marking with only one scan, a tendency to easily perform the break-off process can be observed. By reducing the stress applied during the break-off process, the risk of induced splitting of the main surface (001) can be suppressed during break-off.

[0042] (Summary of Verification Results) The verification experimental results above are summarized as follows: (a) A gallium oxide substrate with an orientation plane formed on a surface (preferably perpendicular to the main surface S) having a main surface (001) as the surface and along a direction (<010> direction) containing the intersection line parallel to the main surface and surface (100). (b) Mechanical scribing is performed on the main surface S by using a scribing tool to etch grooves along a predetermined cutting line parallel to the X direction, with the intersection line direction (<010> direction) as the X direction. (c) Laser scribing is performed by scanning a laser beam along a predetermined cutting line orthogonal to the X direction, thereby altering the laser beam. By satisfying the above conditions, the main surface S can be cut into a grid shape along mutually orthogonal X and Y directions.

[0043] By using the above processing method to cut a grid-like substrate from the main surface S, since a parallelepiped can be cut with the main surface S (substrate surface) and the back surface of the substrate as surfaces (001), a pair of side surfaces perpendicular to the main surface S (010), and another pair of side surfaces inclined at a theoretical angle of 103.8° relative to the main surface S, a cuboid shape can be almost cut out, and the device processing technology in the conventional Si substrate can be applied (continuing). [Industrial Applicability]

[0044] The present invention can be used as a cutting process for gallium oxide substrates suitable for power devices. [Simplified Explanation of the Diagram]

[0029] Figure 1 is a diagram illustrating the planar orientation and crystal axis of gallium oxide crystals belonging to the β-Gallia structure of the monoclinic crystal system. Figure 2 is a diagram illustrating the planar orientation index and crystal orientation of the orientation plane of the gallium oxide substrate used in this invention ((a) top view, (b) front view, (c) right side view). Figure 3 shows a diagram of the predetermined cutting lines in the X and Y directions of the gallium oxide substrate of this invention. Figure 4 is an enlarged view of the cut surface processed by mechanical scribing in the X direction, (a) is a cross-sectional photograph viewed from the front, and (b) is a cross-sectional photograph viewed from the side. Figure 5 is an enlarged view of the cut surface processed by laser scribing in the X direction, (a) is a cross-sectional photograph viewed from the front, and (b) is a cross-sectional photograph viewed from the side. Figure 6 is an enlarged photograph taken from above of the main surface processed by laser scribing in the X direction. Figure 7 shows an enlarged view of the cross-section formed by laser engraving in the Y direction. (a) is a cross-sectional photograph viewed from the front, and (b) is a cross-sectional photograph viewed from the side.

Claims

1. A method for processing a gallium oxide substrate, wherein the β-Gallia structure gallium oxide substrate is cut along X and Y directions orthogonal to each other on the main surface, the method is to use the gallium oxide substrate with the main surface as surface (001), and to perform mechanical scribing by using the tip of a scribing tool to etch grooves on the main surface along a predetermined cutting line parallel to the X direction as the X direction, and to perform laser scribing by scanning a laser beam along the predetermined cutting line parallel to the Y direction to degrade it, and to cut along the predetermined cutting lines in the X and Y directions after the mechanical scribing and the laser scribing.

2. The method for processing a gallium oxide substrate as described in claim 1, wherein, For laser marking of a predetermined cutting line parallel to the aforementioned Y direction, the laser beam is scanned repeatedly along the predetermined cutting line.

3. A method for processing a gallium oxide substrate, wherein the gallium oxide substrate is processed using the method described in claim 1 or 2, wherein the main surface is a surface (001), the side surface in the X direction is a surface (100) that is inclined and intersecting the main surface, and the side surface in the Y direction is a surface that is orthogonal to the main surface and the side surface in the X direction, and the main surface is processed into a rectangular parallelepiped shape.

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