Magnesium oxide substrate manufacturing method

Abstract

To provide a method for manufacturing an MgO substrate, capable of peeling the substrate by a modified layer formed by irradiating the magnesium oxide (MgO) substrate having the main surface of a {111} plane with a laser beam.SOLUTION: A method for manufacturing an MgO substrate comprises the steps of: arranging a laser collection part so as to face the main surface of the MgO substrate having the main surface of a (111) plane; forming a processing mark and a modified layer extending in the laser-beam scanning direction from the processing mark at a predetermined depth from the main surface by relatively scanning the laser collection part and the MgO substrate in a two dimensional shape while irradiating the main surface of the MgO substrate with a laser beam B from the laser collection part to collect the laser beam in the inside of the MgO substrate; and connecting cracks extending along the {100} plane of a MgO single crystal from the modified layer to form a cleavage plane.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a magnesium oxide substrate, and more particularly to a method for manufacturing a magnesium oxide substrate by processing using laser light. [Background technology]

[0002] Magnesium oxide (hereinafter also referred to as MgO) substrates formed from single crystals are used in the fields of semiconductors, displays, energy, etc. MgO substrates can be manufactured by growing the crystal in bulk and cutting it into substrate shapes, or by epitaxially growing it into a thin film (see Patent Document 1).

[0003] Meanwhile, diamond is considered a suitable semiconductor for high-frequency, high-power electronic devices, and its synthesis method, the chemical vapor deposition (CVD) method, uses an MgO substrate as the base substrate (see Patent Document 2). Heteroepicapital growth using this CVD method results in bulk single-crystal diamond grown in the same orientation as the underlying MgO substrate. That is, if the underlying MgO substrate has a crystal orientation of

[0100] , a bulk diamond crystal with a crystal orientation of

[0100] is obtained, and if the underlying MgO substrate has a crystal orientation of

[0111] , a bulk diamond crystal with a crystal orientation of

[0111] is obtained.

[0004] Diamond is also used in magnetic sensors. For magnetic sensors, it is necessary to form a high density of NV centers in single crystal diamond and align the orientation axes of the NV centers. As the technology for orienting high density NV centers in the

[0111] direction using the CVD method has been established, there is an increasing need for (111) bulk crystals made of single crystal diamond with the main surface being the (111) plane.

[0005] In the manufacture of the diamond substrate, the MgO substrate used as the base substrate is expensive, but for example, after vapor-phase synthesis of single-crystal diamond, the MgO substrate can be peeled off and separated while leaving the necessary thickness as the base substrate, so that the MgO substrate can be reused as the base substrate. For this reason, a technology has been proposed in which a laser beam is focused and irradiated to a predetermined depth from the main surface of the MgO substrate, and scanned two-dimensionally to form a modified layer with a modified crystal structure, and the MgO substrate is then peeled off at this modified layer (see Patent Documents 3 to 5).

[0006] This technique utilizes the property that MgO single crystals tend to cleave along the {100} plane, and peels off an MgO substrate whose main surface is the {100} plane along this plane. The cleavage plane of MgO single crystal, which is an NaCl-type ionic crystal, is the {100} plane, and this technique is useful when the peeling surface of the MgO substrate and the cleavage plane coincide with the {100} plane. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-080996 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-59069 [Patent Document 3] Japanese Patent Application Publication No. 2018-183801 [Patent Document 4] Japanese Patent Application Publication No. 2019-69870 [Patent Document 5] Japanese Patent Application Publication No. 2019-160915 Summary of the Invention [Problem to be solved by the invention]

[0008] However, it was difficult to apply this technique to MgO substrates with {111} planes as their main surfaces because cleavage along the {111} plane does not occur. Furthermore, when a focused laser beam was irradiated at a predetermined depth from the main surface of an MgO substrate with {111} planes, processing marks were formed in the {100} plane direction, and cleavage propagated toward the main surface, making it difficult to peel off the {111} plane MgO substrate, i.e., to create a new MgO substrate.

[0009] The present invention has been proposed in view of the above-mentioned circumstances, and aims to provide a method for manufacturing an MgO substrate, in which a laser beam is focused and irradiated to a predetermined depth from the main surface of an MgO substrate having a {111} plane, and the laser beam is scanned in a predetermined two-dimensional direction to form a modified layer with a modified crystal structure, thereby separating the MgO substrate having a {111} plane as its main surface. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the method for manufacturing a magnesium oxide substrate according to this application includes the steps of: arranging a laser focusing unit for focusing laser light so as to face the main surface of a magnesium oxide single crystal substrate, the main surface of which is a (111) plane; irradiating the main surface of the substrate from the laser focusing unit with laser light to focus the laser light inside the substrate while scanning the laser focusing unit and the substrate relatively in a predetermined direction two-dimensionally to form a processed mark at a predetermined depth from the main surface and a modified layer extending from the processed mark along the laser light scanning direction; and forming a cleavage plane by connecting cracks extending from the modified layer along the {100} plane of the magnesium oxide single crystal.

[0011] The step of forming the modified layer includes: <110> The step of forming the modified layer may include scanning the laser beam along at least two directions selected from the above directions. <111> The step of forming the modified layer may include scanning the focused laser beam along at least two directions selected from the direction of the {100} plane, and generating cracks along the {100} plane by slip in the slip direction starting from processing marks formed by scanning the focused laser beam along at least two directions.

[0012] The step of forming the modified layer may include a step of scanning the focused laser light along at least two directions and a step of moving the laser focusing unit and the substrate relatively in the line pitch direction by a predetermined distance. The step may further include a step of peeling off a portion of the substrate from the main surface to a depth reaching the modified layer and a portion deeper than the modified layer.

[0013] The laser light may be a high-intensity laser light, and the pulse width of the laser light may be in the range of several ps to several tens of ns. [Effects of the Invention]

[0014] According to this invention, a laser beam is focused and irradiated to a predetermined depth from the main surface of an MgO substrate whose main surface is a {111} plane, and scanned two-dimensionally to form a modified layer with a modified crystal structure, and the MgO substrate can be peeled off at this modified layer. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a perspective view showing a schematic configuration of a processing device. [Figure 2] FIG. 1 is a diagram showing the crystal structure of an MgO single crystal. [Figure 3] FIG. 1 is a diagram illustrating cleavage that occurs in an MgO single crystal. [Figure 4] FIG. 2 is a top view showing an MgO substrate. [Figure 5] FIG. 1 is a diagram illustrating a (111) plane formed by {100} planes. [Figure 6] FIG. 10 is a top view illustrating scanning of an MgO substrate with laser light. [Figure 7] 1A to 1C are cross-sectional views illustrating a process for forming a modified layer on an MgO substrate. [Figure 8] 10A to 10C are cross-sectional views illustrating another embodiment of processing for forming a modified layer on an MgO substrate. [Figure 9] 10A and 10B are top views illustrating another mode of scanning the MgO substrate with laser light. [Figure 10] 1 is a micrograph of an MgO substrate of Experimental Example 1 on which a modified layer is formed. [Figure 11] 11 is a further enlarged micrograph of the MgO substrate shown in FIG. 10. [Figure 12] 1 is a micrograph of an MgO substrate of Experimental Example 2 on which a modified layer is formed. [Figure 13] 13 is a micrograph further enlarged from the micrograph of the MgO substrate shown in FIG. 12. [Figure 14] 14 is a micrograph further enlarged from the micrograph of the MgO substrate shown in FIG. 13. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of a method for manufacturing an MgO substrate will be described in detail with reference to the drawings. In the drawings, identical or similar parts are designated by identical or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationships between thicknesses and planar dimensions, the thickness ratios of each layer, etc., may differ from the actual ones. Therefore, specific thicknesses and dimensions should be determined with reference to the following explanation. Furthermore, it goes without saying that the drawings may include parts with different dimensional relationships and ratios.

[0017] Furthermore, the embodiments shown below are merely examples of devices and methods for embodying the technical ideas of this invention, and the materials, shapes, structures, arrangements, etc. of the components of the embodiments of this invention are not limited to those described below. Various modifications can be made to the embodiments of this invention within the scope of the claims.

[0018] In this embodiment, the MgO substrate is made of MgO single crystal, and unless otherwise specified, it is assumed that the upper surface is the (111) plane of the main surface. This main surface is not limited to the (111) plane, and may be any other plane as long as it is a {111} plane. Furthermore, since there are restrictions on the characters that can be used in this specification, for convenience, the overscores attached to the numbers in the Miller indices will be replaced with a minus sign "-" before the numbers.

[0019] 1 is a perspective view showing a schematic configuration of a processing apparatus 100. The processing apparatus 100 has a stage 110 on which an MgO substrate 10 is placed, a stage support 120 that supports the stage 110 so that it can move in the X and Y directions in a horizontal plane, and a fixture 130 that fixes the MgO substrate 10. The fixture 130 can be an adhesive layer, a mechanical chuck, an electrostatic chuck, a vacuum chuck, or the like.

[0020] An MgO substrate 10 as an object to be processed is fixed on the stage 110 with the (111) plane of the main surface 10a facing up. The shape of the object to be processed is not limited to this, and it may be, for example, an MgO single crystal ingot or a block cut to a certain length from an ingot, as long as the main surface 10a is a (111) plane.

[0021] The processing apparatus 100 also has a laser light source 160 that generates pulsed laser light, and a laser focusing unit 190 that includes an objective lens 170 and an aberration adjustment unit 180, so that the laser light B emitted from the laser light source 160 is irradiated via the laser focusing unit 190 toward the (111) plane of the main surface 10a of the MgO substrate 10.

[0022] The laser light source 160 generates a high-intensity laser beam. The high-intensity laser beam is specified by its peak power and power density. Here, the peak power is the value obtained by dividing the pulse energy by the pulse width, and the power density is the value of the energy per unit time per unit area. The high-intensity laser beam has a peak power of 10 kW or more, and a power density calculated from the peak power of 1000 W / cm. 2 Furthermore, a shorter pulse width is more effective for increasing the power density, and a pulse width of 10 ns or less is preferred, and a pulse width of 100 ps or less is even more preferred. A pulse width of 15 ps or less is even more preferred.

[0023] The high-intensity laser beam is preferably a laser beam with a short pulse width that irradiates the substrate with low laser output and does not damage the substrate. Furthermore, to increase the power density, a laser beam with a short pulse width (e.g., a laser beam with a pulse width of 10 ns or less) is preferred. By irradiating a laser beam with a short pulse width, it is easy to significantly increase the power density of the high-intensity laser beam.

[0024] FIG. 2 is a diagram showing the crystal structure of an MgO single crystal. As shown in FIG. 2(a), the unit cell of an MgO single crystal is composed of magnesium cations (Mg 2+ ) and the oxygen anion (O 2- ) form a face-centered cubic lattice structure and are also called NaCl-type ionic crystals.

[0025] Figure 2(b) shows the (111) and (11-1) planes in the unit cell of a MgO single crystal. 2+ Or O 2- The ions are aligned along the {111} plane. Therefore, cleavage along the {111} plane is difficult for reasons explained later. The [1-10] direction is also shown in the figure. The [1-10] direction is the direction in which one side of the triangle formed by the outline of the (111) plane extends in the unit cell.

[0026] Figure 3 is a diagram illustrating the cleavage that occurs in an MgO single crystal. As shown in Figure 3(a), magnesium cations, indicated by "+", and oxygen anions, indicated by "-", are arranged to form an ionic crystal. Figure 3(a) shows the arrangement of magnesium ions and oxygen ions on one of the {100} planes of the face-centered cubic lattice of MgO shown in Figure 2(a). Within this plane, the magnesium ions and oxygen ions are arranged alternately.

[0027] Suppose an external force is applied as shown in Figure 3(a), and an adjacent layer is displaced along another {100} plane perpendicular to one of the {100} planes by a distance equivalent to one ion, as shown in Figure 3(b). In this case, ions of the same polarity face each other and exert a repulsive force, resulting in cleavage along this plane. In contrast, as shown in Figure 2(b), ions of the same polarity are arranged in the {111} plane. Therefore, even if an adjacent layer is displaced along another {111} plane perpendicular to one of the {111} planes by a distance equivalent to one ion, the ion arrangement remains unchanged, and no cleavage occurs. For this reason, in MgO single crystals, the {100} plane is the cleavage plane, and cleavage does not occur along the {111} plane.

[0028] Figure 4 is a top view of MgO 10. The MgO substrate 10 processed by the processing apparatus of Figure 1 has a roughly rectangular plate-like shape with a predetermined thickness. The top surface of the MgO substrate 10 is the (111) plane of the main surface 10a, and notches are formed at both ends of one side of the periphery of the roughly rectangular shape, indicating that the direction toward this side is the [1-10] direction.

[0029] The figure also shows the orientations of the (100), (001), and (001) planes that make up the {001} plane of the MgO substrate 10. The (100), (001), and (001) planes form a regular triangular pyramid with the (111) plane as the base. The [1-10] direction is the direction in which the sides formed by the (001) and (111) planes extend. Note that the orientations of the (100), (001), and (001) planes will be indicated appropriately in the following drawings for reference.

[0030] Figure 5 is a diagram illustrating the (111) plane composed of {100} planes. Figure 5(a) shows the orientation of the (100), (010), and (001) planes that make up the {001} plane when viewed from above the (111) plane. The (100), (010), and (001) planes are tilted at approximately 55 degrees from the (111) plane on the surface, pointing in three directions and forming the slopes of a regular triangular pyramid with the (111) plane as the base.

[0031] Each slope of this regular triangular pyramid is extended beyond the base, so that each slope forms a rhombus. The contour of this figure, when orthogonally projected onto the (111) plane, forms a regular hexagon, and it intersects with the (111) plane at an equilateral triangle that corresponds to the base of the regular triangular pyramid. Therefore, this figure can be said to extend along the (111) plane. Hereinafter, for convenience, a figure formed by three slopes, each of which has a rhombus shape, will be referred to as a unit figure.

[0032] As shown in Figure 5(b), using one unit figure as a base, unit figures can be connected around this unit figure along the (111) plane. Strictly speaking, this plane is composed of the inclined surfaces of the (100), (010), and (001) planes that make up the {100} plane. However, because each unit figure contains a (111) plane, the surface formed by tiling the unit figures can be said to form a (111) plane. Here, the (100), (010), and (001) planes of the unit figures constitute the {100} plane that cleaves in MgO single crystals. Therefore, by appropriately connecting the cleavage planes created by cracks along the {100} plane to connect the unit figures, as shown in Figure 5(c), an MgO substrate can be formed with a {100} cleavage plane present on its surface along the (111) plane.

[0033] 6 is a top view illustrating laser beam scanning on an MgO substrate. As shown in the figure, the entire (111) plane of main surface 10a is scanned with a predetermined offset in a first scanning direction along the [-110] or [1-10] direction where the (001) plane and the (111) plane intersect, a second scanning direction along the [0-11] or [01-1] direction where the (100) plane and the (111) plane intersect, and a third scanning direction along the [10-1] or [-101] direction where the (010) plane and the (111) plane intersect.

[0034] MgO single crystals have a face-centered cubic lattice structure, and the {110} plane <110> It has a slip system in the direction. <110> When a processed mark is formed by irradiating a laser beam along the [-110] or [1-10] direction, [-1-11] or [11-1] and [1-1-1] or [-111] direction, which are the first to third scanning directions included in the direction, defects and dislocations are generated from the processed mark, and the {110} plane and <110> Cracks are formed on the (100), (010), and (001) planes of the unit figure starting from slip in the direction. As shown in Figure 3, in MgO single crystal, cleavage occurs along the {100} planes, which include the (100), (010), and (001) planes.

[0035] FIG. 7 is a cross-sectional view illustrating a process for forming a modified layer on an MgO substrate 10. As shown in FIG. 7(a), pulsed laser light B is scanned across the MgO substrate 10 in a predetermined direction, leaving processed marks by the laser light B at predetermined intervals. The laser light B is focused and irradiated onto the interior of the MgO substrate 10 at a predetermined depth from the main surface 10a via a laser focusing unit 190 facing the main surface 10a of the MgO substrate 10. As described above, slip from the processed mark can be used as a starting point to generate cracks along at least one of the (100), (010), and (001) planes of the unit figure. Experiments have confirmed that these cracks propagate along the {100} plane in the scanning direction of the laser light B.

[0036] Next, as shown in Figure 7(b), the MgO substrate 10 is scanned with laser light B in the direction opposite to the predetermined direction, forming processing marks by laser light B at predetermined intervals. The positions of these processing marks may be the same as the positions of the processing marks formed in the first scan in Figure 7(a), or may be other positions, such as the center of adjacent processing marks formed in the first scan. As described above, cracks can be generated starting from slippage from the processing marks along any of the (100), (010), and (001) faces of the unit figure where no cracks have occurred.

[0037] In this way, by scanning the laser beam B in two stages, cracks can be generated along the (100), (010), and (001) planes of the unit figure. The cracks generated from the processing marks connect with each other and spread along the (111) plane, and a modified layer containing cracks and with changed crystalline properties is formed along the (111) plane. The MgO substrate 10 can then be peeled off along this modified layer.

[0038] In the second scan shown in Fig. 7(b), the scanning is performed in the opposite direction to the first scan shown in Fig. 7(a), but this is not limiting. The second scan may also be performed in the same direction as the first scan.

[0039] 8A and 8B are cross-sectional views illustrating another embodiment of processing for forming a modified layer on an MgO substrate 10. As shown in FIG. 8A, pulsed laser light B is scanned across the MgO substrate 10 in a predetermined direction, leaving processed marks by the laser light B at predetermined intervals. The laser light B is focused and irradiated onto the interior of the MgO substrate 10 at a predetermined depth from the main surface 10a via a laser focusing unit 190 facing the main surface 10a of the MgO substrate 10. As described above, slippage of the {100} plane from the processed mark can be used as an initiation point to generate cracks along at least one of the (100), (010), and (001) planes of the unit figure.

[0040] Next, as shown in Figure 8(b), the MgO substrate 10 is scanned in the predetermined direction with laser light B irradiated from the back surface of the MgO substrate 10 opposite the main surface 10a, leaving processing marks by the laser light B at predetermined intervals. The positions of these processing marks may be the same as those of the processing marks left by the first scan in Figure 7(a), or may be at other positions, such as the center of adjacent processing marks left by the first scan. As described above, cracks can be generated starting from slippage from the processing marks along any of the (100), (010), and (001) planes of the unit figure where no cracks have occurred.

[0041] 1, the MgO substrate 10 is fixed on a stage 110 with the (111) plane of the main surface 10a as the upper surface. In order to irradiate the back surface of the MgO substrate 10 opposite the main surface 10a with the laser light B, it is necessary to temporarily remove the MgO substrate 10 attached to the stage 110, turn the MgO substrate 10 upside down so that the back surface faces upward, and then attach it again to the stage 110.

[0042] 9 is a top view illustrating another mode of scanning the MgO substrate with laser light. As shown in the figure, the entire (111) plane of main surface 10a is scanned at a predetermined line pitch in three directions: a first scanning direction in the [11-2] or [-1-12] direction perpendicular to the [-110] or [1-10] direction where the (001) plane and the (111) plane intersect; a second scanning direction in the [2-1-1] or [-211] direction perpendicular to the [0-11] or [01-1] direction where the (100) plane and the (111) plane intersect; and a third scanning direction in the [-12-1] or [1-21] direction perpendicular to the [10-1] or [-101] direction where the (010) plane and the (111) plane intersect.

[0043] MgO single crystal has {110} plane <110> It has a slip system in the direction. <110> When a processed mark is formed by irradiating a focused laser beam along the first to third scanning directions, which are perpendicular to the [-110] or [1-10] direction, the [0-11] or [01-1] direction, and the [10-1] or [-101] direction, respectively, slippage of the {110} plane from the processed mark can initiate cleavage cracks on the (100), (010), and (001) planes of the unit figure. As shown in Figure 3, in MgO single crystal, cleavage occurs along the {100} plane, which includes the (100), (010), and (001) planes.

[0044] In this way, by scanning the laser beam B in the above crystal direction, cracks can be generated along the (100), (010), and (001) planes that make up the {100} plane of the unit figure. The cracks generated from the processing marks connect to each other as cleavage planes of the {100} planes and spread along the (111) plane, making it possible to peel off the MgO substrate 10 along these connected cleavage planes of the {100} planes.

[0045] As described above, according to this embodiment, even for an MgO substrate 10 whose main surface is the (111) plane, by arranging and connecting unit figures formed of {100} planes such as the (100), (010), and (001) planes that constitute the {100} planes that easily cleave in MgO single crystals along the (111) plane, it is possible to form a modified layer in which cracks extend along the (111) plane. Therefore, the MgO substrate 10 can be peeled off from such a modified layer.

[0046] For example, in the case of MgO substrates used as base substrates for diamond single crystals, if a 180 μm thick MgO substrate can be obtained from a 200 μm thick MgO base substrate and reused, a significant cost reduction can be achieved in the diamond substrate manufacturing process, which is expected to make a significant contribution to reducing the cost of diamond substrates.

[0047] (Experimental Example 1) In Experimental Example 1, the processing apparatus 100 was used under the conditions shown in Table 1. The objective lens correction collar corresponds to the aberration adjustment unit 180 in the processing apparatus 100 of FIG.

[0048] [Table 1]

[0049] The MgO substrate 10 to be processed was as shown in Table 2.

[0050] [Table 2]

[0051] FIG. 10 is a micrograph of the MgO substrate 10 of Experimental Example 1 on which a modified layer was formed. In Experimental Example 1, the MgO substrate 10 was scanned along the [-110] or [1-10] direction, the [0-11] or [01-1] direction, and the [10-1] or [-101] direction as shown in FIG. 6. As shown in FIG. 7, the MgO substrate 10 was scanned with laser light B in two stages. The scanning with laser light B was not performed over the entire main surface 10a of the MgO substrate 10, but only within a predetermined range in the line pitch direction of each scanning direction. In FIG. 10, modified layers formed by scanning with laser light B in the above directions are observed.

[0052] Figure 11 is a further enlarged micrograph of the MgO substrate 10 shown in Figure 10. Each micrograph is an enlarged view of a portion of the modified layer extending along the scanning direction of the laser light B in the [-110] or [1-10] direction, the [0-11] or [01-1] direction, and the [10-1] or [-101] direction. It can be seen that the modified layer extending along the scanning direction is formed with a constant width in the line pitch direction.

[0053] Specifically, cleavage occurs in the (010) plane in Figure 11(a), the (100) plane in Figure 11(b), and the (001) plane in Figure 11(c). In this way, three {100} planes are cleaved in each scanning direction of the laser beam B. That is, according to Experimental Example 1, cleavage occurs in the (001), (010), and (100) planes by irradiating the laser beam B in three directions excluding the six directions or the reverse directions of the six directions under the experimental conditions in Table 1.

[0054] (Experimental Example 2) Experimental Example 2 used the same MgO substrate 10 as the workpiece shown in Table 2 of Experimental Example 1 under the conditions shown in Table 1 of Experimental Example 1. Figure 12 is a micrograph of the MgO substrate 10 of Experimental Example 2 on which a modified layer was formed. In Experimental Example 2, the MgO substrate 10 was scanned along the [11-2] or [-1-12] direction, the [2-1-1] or [-211] direction, and the [-12-1] or [1-21] direction as shown in Figure 9. As shown in Figure 7, the MgO substrate 10 was scanned with laser light B in two stages. The scanning with laser light B was not performed over the entire main surface 10a of the MgO substrate 10, but only within a predetermined range in the line pitch direction of each scanning direction. Figure 12 shows modified layers formed by scanning the MgO substrate 10 with laser light B in the above directions.

[0055] FIG. 13 is a micrograph of the MgO substrate 10 shown in FIG. 12 at a further enlargement. FIG. 14 is a micrograph of the MgO substrate 10 shown in FIG. 13 at a further enlargement. FIG. 13(a) is a magnified view of a portion of the modified layer extending along the [1-21] or [-12-1] direction, and FIG. 14(a) is a further enlargement of FIG. 13(a). In FIG. 14(a), the solid arrow indicates the scanning direction of the laser beam B, and the dashed arrow indicates the direction in which the cleavage progresses. This also applies to the following FIGS. 14(b) and 14(c). FIG. 13(b) is a magnified view of a portion of the modified layer extending along the [2-1-1] or [-211] direction, and FIG. 14(b) is a further enlargement of FIG. 13(b). FIG. 13(c) is an enlarged view of a part of the modified layer extending along the [2-1-1] or [-211] direction, and FIG. 14(c) is a further enlarged view of FIG. 13(c).

[0056] In the scanning direction of each laser beam B, cracks were generated at an angle of approximately 60° to the scanning direction. <110> This is thought to be due to slippage in the direction of the scanning direction. Cleavage of the {100} plane is observed to occur starting from this crack. Since the cleavage of the {100} plane for each of the above scanning directions occurs in the (100), (001), and (010) planes, respectively, it is thought that in order to connect the cleavage of the (100), (010), and (001) planes of the unit figure, it is sufficient to operate laser light B in at least two of the above scanning directions. [Explanation of symbols]

[0057] 10 MgO substrates 10a Main surface 100 Processing equipment 160 Laser Light Source 190 Laser focusing unit

Claims

1. a step of placing a laser focusing unit for focusing laser light so as to face a main surface of a magnesium oxide single crystal substrate whose main surface is a (111) plane; a step of irradiating a main surface of the substrate with laser light from the laser focusing unit and focusing the laser light inside the substrate while two-dimensionally scanning the laser focusing unit and the substrate relative to each other in a predetermined direction to form a processed mark at a predetermined depth from the main surface and a modified layer from the processed mark along the laser light scanning direction; and a step of joining cracks extending from the modified layer along the {100} plane of the magnesium oxide single crystal to form a cleavage plane. A method for producing a magnesium oxide substrate, comprising:

2. 2. The method according to claim 1, wherein the step of forming the modified layer comprises scanning the focused laser beam along at least two directions selected from the <110> direction.

3. 3. The method according to claim 2, wherein the step of forming the modified layer includes generating cracks along the {100} plane by slips occurring in a slip direction starting from processing marks formed by scanning the focused laser beam along at least two directions.

4. The method according to claim 2 or 3, wherein the step of forming the modified layer includes a step of scanning focused laser light along the at least two directions, and a step of moving the laser focusing unit and the substrate relatively in the line pitch direction over a predetermined distance.

5. The method according to claim 1 , further comprising the step of peeling off a portion of the substrate from the main surface to a depth reaching the modified layer and a portion deeper than the modified layer.

6. The method according to any one of claims 1 to 5, wherein the laser light is a high-intensity laser light.

7. The method according to any one of claims 1 to 6, wherein the laser light has a pulse width in the range of several ps to several tens of ns.

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