Evaluation method for silicon carbide substrates

Abstract

The present invention addresses the problem of providing a novel evaluation method suitable for large-diameter SiC substrates. The present invention is an evaluation method for SiC substrates, the method being characterized by: including an image acquisition step for acquiring an image I by projecting onto a SiC substrate 10 an electron beam PE inclined at an incident angle θ with respect to a normal N that is normal to (0001) plane of the SiC substrate 10; and the incident angle θ being at most 10 degrees.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating a silicon carbide substrate. [Background Art]

[0002] Silicon carbide (SiC) is known as a material that crystallographically exhibits a polytype phenomenon in which various laminated structures are formed along the c-axis with the same composition. More than 200 types of polytypes of this SiC have been confirmed. Among these, 3C-SiC, 4H-SiC, and 6H-SiC are known as polytypes that have a high occurrence probability and are important for applications.

[0003] FIG. 6 is a schematic diagram showing the crystal structure of 4H-SiC. FIG. 6(a) shows the crystal structure of 4H-SiC viewed from the <11-20> direction. FIG. 6(b) shows the crystal structure of 4H-SiC viewed from the

[0001] direction. Note that in the present specification, in the notation of Miller indices, "-" means a bar added to the index immediately following it.

[0004] Furthermore, the notation "A, B, C" in the figure means the occupied positions of three types of atoms (corresponding to Si-C pairs) in a hexagonal close-packed structure. In FIG. 6 showing the crystal structure of 4H-SiC, the stacking of atoms at positions A and C corresponds to a zinc-blende structure, and the stacking of atoms at position B corresponds to a wurtzite structure.

[0005] In 4H-SiC and 6H-SiC, since the zinc-blende structure and the wurtzite structure are periodically stacked, the stacking direction changes at the wurtzite structure. On the other hand, in 3C-SiC, since the zinc-blende structure is continuously stacked, the stacking direction does not change.

[0006] Techniques have been proposed for evaluating the quality of SiC substrates, such as 4H-SiC and 6H-SiC, where the stacking direction changes, by irradiating them with an electron beam. For example, Patent Documents 1 and 2 describe a technique for evaluating the quality of a SiC substrate based on an image obtained by irradiating the surface of the SiC substrate with an incident electron angle tilted with respect to the normal to the {0001} plane of the SiC substrate. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2015-179082 [Patent Document 2] Japanese Patent Publication No. 2019-158888 [Overview of the project] [Problems that the invention aims to solve]

[0008] Figure 7 shows a schematic diagram of the evaluation method for SiC substrates described in Patent Documents 1 and 2. Figure 7(a) shows the SiC substrate 10 to be evaluated placed on the stage 20 of a scanning electron microscope. Figure 7(b) shows the crystal structure of the surface of the SiC substrate 10.

[0009] As shown in Figure 7, in conventional evaluation methods for SiC substrates, an electron beam PE was incident on the SiC substrate 10 at an incident angle θ that was tilted 30° to 40° with respect to the normal N of the {0001} plane of the SiC substrate 10, and an image was acquired.

[0010] Incidentally, one of the most pressing challenges in the widespread adoption of SiC semiconductor devices is cost reduction. Similar to the development of Si semiconductor devices, cost reduction largely depends on increasing the diameter of semiconductor substrates. This is because increasing the substrate (wafer) area allows for a greater number of semiconductor chips to be produced from a single substrate, thereby lowering the manufacturing cost per chip. Therefore, methods for evaluating large-diameter SiC substrates are becoming increasingly important.

[0011] However, the evaluation methods for SiC substrates described in Patent Documents 1 and 2 required tilting the stage 20 by 30° to 40° to acquire image I. This resulted in the problem of the measuring device (particularly the chamber for housing the SiC substrate 10 in the scanning electron microscope) becoming large.

[0012] Furthermore, in the evaluation methods for SiC substrates described in Patent Documents 1 and 2, there was a problem in that the electron beam PE easily lost focus when scanning a large-diameter SiC substrate 10.

[0013] This measurement method evaluates the surface of a SiC substrate at a depth of 0.5 to several nm. However, in conventional methods, image I is acquired by tilting stage 20 by 30° to 40°. Therefore, it is necessary to precisely control stage 20 so that the surface of the SiC substrate 10 does not deviate from the focal point of the electron beam PE.

[0014] However, when scanning a wide area with the electron beam PE while tilted at 30° to 40°, it is difficult to maintain focus on the surface position of the SiC substrate 10. In other words, if the stage 20 is moved while maintaining a large tilt angle of 30° to 40°, movement along the direction of irradiation of the electron beam PE (movement in the height direction of the stage 20) occurs, causing the focus to easily be lost.

[0015] Figure 8 shows a schematic diagram illustrating the amount of movement when scanning the SiC substrate 10. Figure 8(a) shows the evaluation of a small-area SiC substrate 10. Figure 8(b) shows the evaluation of a large-area SiC substrate 10. Thus, as the area of ​​the SiC substrate 10 to be evaluated increases, a larger chamber is required, and there is a problem that the focus is more likely to deviate.

[0016] In view of the above-mentioned problems, the problem that the present invention aims to solve is to provide a novel evaluation method suitable for evaluating large-diameter SiC substrates. [Means for solving the problem]

[0017] The present invention, which solves the above-mentioned problems, includes an image acquisition step in which an image is acquired by incidenting an electron beam at an incident angle inclined with respect to the normal to the {0001} plane of a SiC substrate. The aforementioned incident angle is 10° or less, and this is a method for evaluating a SiC substrate.

[0018] In this way, by setting the incidence angle to 10° or less, the chamber housing the SiC substrate can be miniaturized. Furthermore, when scanning a large-diameter SiC substrate, it is possible to suppress the deviation of the SiC substrate surface from the electron beam's focal point.

[0019] In a preferred embodiment of the present invention, the process includes an oxide film removal step to remove the oxide film from the SiC substrate, and the image acquisition step is performed after the oxide film removal step. In this way, by removing the oxide film, information about the surface of the SiC substrate can be obtained with high accuracy.

[0020] In a preferred embodiment of the present invention, the image acquisition step is a step of acquiring the image within 60 minutes of the time the SiC substrate is exposed to the atmosphere after the oxide film removal step. Thus, by limiting the exposure time of the SiC substrate to air to 60 minutes or less, information about the surface of the SiC substrate can be obtained with high accuracy.

[0021] In a preferred embodiment of the present invention, the image has a plurality of brightness information reflecting the stacking direction of atoms, and the invention includes a brightness comparison step of comparing the brightness information.

[0022] In a preferred embodiment of the present invention, the image has a first brightness information reflecting a first stacking direction and a second brightness information reflecting a second stacking direction, and the process includes a brightness comparison step of comparing the first brightness information and the second brightness information.

[0023] In a preferred embodiment of the present invention, the incident angle is inclined in the <1-100> direction with respect to the normal.

[0024] In a preferred embodiment of the present invention, the acceleration voltage of the electron beam is 1.0 kV or less.

[0025] In a preferred embodiment of the present invention, the diameter of the silicon carbide substrate is 4 inches or more. Effects of the Invention

[0026] According to the disclosed technique, a novel evaluation method suitable for evaluating large-diameter SiC substrates can be provided.

[0027] Other problems, features and advantages will become apparent upon reading the following description of modes for carrying out the invention when considered in conjunction with the drawings and the claims. Brief Description of the Drawings

[0028] [Figure 1] It is a schematic diagram of the evaluation method for a SiC substrate according to an embodiment. [Figure 2] It is an example of an image I (SEM image) obtained by the evaluation method for a SiC substrate according to an embodiment. [Figure 3] It is a schematic diagram comparing the evaluation method for a SiC substrate according to an embodiment and a conventional evaluation method for a SiC substrate. [Figure 4] These are images I of Example 1 and Example 2. [Figure 5] It is a graph showing the relationship between the incident angle θ of an electron beam PE and brightness information. [Figure 6] It is a schematic diagram of the crystal structure of 4H-SiC. [Figure 7] It is a schematic diagram of a conventional evaluation method for a SiC substrate. [Figure 8] It is a schematic diagram of a conventional evaluation method for a SiC substrate. Modes for Carrying Out the Invention

[0029] A preferred embodiment of the SiC substrate evaluation method according to the present invention will be described in detail below with reference to the attached drawings. The technical scope of the present invention is not limited to the embodiments shown in the attached drawings, and modifications can be made as appropriate within the scope described in the claims. In the following description of embodiments and in the attached drawings, the same reference numerals are used for similar components, and redundant explanations are omitted.

[0030] Figure 1 is a schematic diagram of the evaluation method for a SiC substrate 10 according to the present invention. Figure 1(a) shows the SiC substrate 10 to be evaluated placed on a stage 20, and an electron beam PE incident on the SiC substrate 10 at an incident angle θ of 8°. Figure 1(b) shows the crystal structure of the surface of the SiC substrate 10 in Figure 1(a).

[0031] The present invention provides a method for evaluating a SiC substrate, characterized by acquiring an image I by irradiating the SiC substrate 10 with an electron beam PE at an incidence angle θ of 10° or less with respect to the normal N of the {0001} plane.

[0032] The SiC substrate evaluation method according to this embodiment includes an oxide film removal step of removing the oxide film on the surface of the SiC substrate 10, an image acquisition step of acquiring an image I by injecting an electron beam PE at an incident angle θ tilted with respect to the normal N of the {0001} plane of the SiC substrate 10, and a brightness comparison step of comparing the brightness information L contained in the image I.

[0033] The diameter of the SiC substrate 10 is preferably 4 inches or more, more preferably 6 inches or more, and even more preferably 8 inches or more. The following describes each step in detail according to embodiments of the present invention.

[0034] <Oxide film removal process> The oxide film removal process involves removing the oxide film formed on the surface of the SiC substrate 10, thereby exposing the Si and C crystal structure on the surface of the SiC substrate 10. Methods for removing this oxide film include using solutions such as hydrogen fluoride (HF) or ammonium fluoride (NH4F), or using gas-phase methods such as dry etching or plasma.

[0035] Furthermore, the oxide film removal process includes etching and crystal growth methods for the SiC substrate 10. That is, no oxide film is formed on the surface of the SiC substrate 10 after etching or growth. Therefore, etching and crystal growth methods for the SiC substrate 10 can naturally be used as oxide film removal processes.

[0036] Examples of etching methods include hydrogen etching, which uses hydrogen gas as the etching gas, and Si vapor pressure etching (SiVE), which involves heating under a Si atmosphere.

[0037] Examples of crystal growth methods include chemical vapor deposition (CVD), physical vapor transport (PVT), and metastable solvent epitaxy (MSE).

[0038] <Image acquisition process> The image acquisition process, as shown in Figure 1, involves injecting an electron beam PE into the SiC substrate 10 at an incident angle θ tilted with respect to the normal N of the {0001} plane of the SiC substrate 10, and acquiring an image I. This incident angle θ is preferably 10° or less, more preferably 9° or less, and even more preferably 8° or less. Furthermore, this incident angle θ is preferably 5° or more, more preferably 6° or more, even more preferably 7° or more, and even more preferably 8° or more.

[0039] The image acquisition process includes a setup step of placing the SiC substrate 10 to be evaluated on the stage 20 of the scanning electron microscope, a tilting step of tilting the stage 20 to a tilt angle φ such that the electron beam PE is incident on the SiC substrate 10 at an incident angle θ of 10° or less with respect to the normal N of the {0001} plane of the SiC substrate 10, and an electron beam irradiation step of irradiating the SiC substrate 10 with the electron beam PE to obtain an image I.

[0040] In the tilting process, it is preferable to tilt the stage so that the electron beam PE is tilted in the <1-100> direction with respect to the normal N of the SiC substrate 10.

[0041] Furthermore, the inclination process is preferably carried out so that the incident angle θ of the electron beam PE is in the range of 5° to 10°, more preferably in the range of 7° to 9°, and even more preferably 8°.

[0042] In the electron beam irradiation process, it is preferable to irradiate the PE with an electron beam at an acceleration voltage of 1.0 kV or less.

[0043] Image I is created based on electrons reflected from the surface of the SiC substrate 10 by the electron emission unit of a scanning electron microscope (primary electrons), electrons emitted after interacting with the SiC substrate 10 (backscattered electrons), and electrons generated during the interaction process (secondary electrons).

[0044] In other words, a detector placed inside the scanning electron microscope detects backscattered electrons and / or secondary electrons, and an image I is created based on the positional information of the SiC substrate 10 and the detection results of each electron.

[0045] When acquiring image I in the image acquisition process, the time during which the SiC substrate 10 is exposed to the atmosphere after the oxide film removal process is preferably 60 minutes or less, more preferably 50 minutes or less, even more preferably 40 minutes or less, and even more preferably 30 minutes or less.

[0046] In other words, when the incident angle θ is 10° or less, it is more susceptible to the effects of the oxide film compared to the conventional case where the angle is 30° to 40°. Therefore, by measuring the SiC substrate 10 while it is exposed to air for 60 minutes or less, the brightness information L in image I becomes clearer.

[0047] <Brightness comparison process> The brightness comparison step is a step of comparing multiple brightness information L reflected in image I. Brightness information L is information about contrast that reflects the stacking direction of atoms. By comparing these multiple brightness information L, surface information of the SiC substrate 10 can be obtained.

[0048] For example, the brightness comparison process is a process of comparing a first brightness information L1 that reflects the stacking direction of atoms with a second brightness information L2 that can be compared with this first brightness information L1. Specifically, it is a process of obtaining information about the contrast, such as whether the first brightness information L1 is white or black, by comparing the contrast between the first brightness information L1 and the second brightness information L2.

[0049] Figure 2 shows an example of image I obtained in the image acquisition process. Image I has multiple brightness information L that reflect the number of SiC unit layers and / or the stacking direction of the SiC unit layers. There are multiple variations of brightness information L, which are combinations of the number of SiC unit layers and the first stacking direction D1 (the direction in which Si atoms are stacked A→B→C) and the second stacking direction D2 (the direction in which Si atoms are stacked C→B→A).

[0050] Figure 2 shows a first brightness information L1 that reflects a crystal structure in which two SiC unit layers are stacked in a first stacking direction D1 (a crystal structure in which Si atoms are stacked in the order A→B→C), and a second brightness information L2 that reflects a crystal structure in which two SiC unit layers are stacked in a second stacking direction D2 (a crystal structure in which Si atoms are stacked in the order C→B→A).

[0051] Although Figures 1 and 2 show the case where 4H-SiC has two types of brightness information L, it may also have a third type of brightness information L3 that reflects the crystal structure formed by stacking a single SiC unit layer.

[0052] Furthermore, the evaluation method for SiC substrates according to the present invention can be used to evaluate any polytype substrate in which the stacking direction changes. For example, in 6H-SiC, the variation in brightness information L is determined by the combination of three SiC unit layers and the first stacking direction (the direction in which Si atoms are stacked in the order A→B→C→A) and the second stacking direction (the direction in which Si atoms are stacked in the order A→C→B→A).

[0053] According to the present invention, the electron beam PE is incident at an incident angle θ of 10° or less with respect to the normal N of the {0001} plane of the SiC substrate 10. This makes it possible to miniaturize the measuring device.

[0054] Figure 3 is an explanatory diagram showing the movement amounts of the SiC substrate 10 and the stage 20. Figure 3(a) shows scanning of the SiC substrate 10 at an incident angle θ of 30°, which corresponds to a conventional evaluation method for SiC substrates. Figure 3(b) shows scanning of the SiC substrate 10 at an incident angle θ of 8°, which corresponds to the evaluation method for SiC substrates according to the present invention.

[0055] Although the SiC substrate 10 and stage 20 in Figure 3 are the same size, the range over which the SiC substrate 10 moves (the area indicated by the dashed line) is smaller in the SiC substrate evaluation method according to the present invention. In other words, by setting the incident angle θ to 10° or less, the size of the chamber housing the SiC substrate 10 can be reduced, thereby lowering the equipment cost.

[0056] Furthermore, according to the present invention, the electron beam PE is incident on the SiC substrate 10 at an incident angle θ of 10° or less with respect to the normal N of the {0001} plane of the SiC substrate 10. This makes it possible to suppress the focal point from deviating from the surface of the SiC substrate 10 as the stage 20 moves.

[0057] For example, when scanning a 6-inch wafer (15.24 cm in diameter) from edge to edge while tilted at 30°, the stage 20 will move 7.62 cm in the height direction. In contrast, when scanning while tilted at 8°, the stage 20 will move 2.12 cm in the height direction.

[0058] Furthermore, when scanning an 8-inch wafer (20.32 cm in diameter) from edge to edge while tilted at 30°, the stage 20 moves 10.16 cm in the height direction. In contrast, when scanning while tilted at 8°, the stage 20 moves 2.82 cm in the height direction.

[0059] In this way, the amount of movement of the stage 20 in the height direction can be reduced, thereby suppressing deviation of the focal position. [Examples]

[0060] The present invention will be described in more detail below with reference to Examples 1 and 2. However, the present invention is not limited to these examples.

[0061] Example 1 The SiC substrate 10 was evaluated under the following conditions.

[0062] <SiC substrate 10> Semiconductor material: 4H-SiC Off-angle: 4° Off direction: [11-20]

[0063] <Oxide film removal process> Oxide film removal method: Si vapor pressure etching Heating temperature: 1800℃ Heating time: 60 minutes

[0064] <Image acquisition process> Scanning electron microscope: Zeiss Merline Acceleration voltage: 1kV Slope direction: Parallel to <1-100> Incident angle θ:8° Air exposure time: 60 minutes

[0065] <Brightness comparison process> Figure 4(a) shows Image I, obtained during the image acquisition process according to Example 1. When a SiC substrate 10 exposed to air for 60 minutes is measured at an incident angle θ of 8°, the first brightness information L1 and the second brightness information L2 can be obtained. That is, from Image I obtained in Example 1, information on the crystal structure of the outermost surface of the SiC substrate, step shape information, step height information, distance information at any given location, etc., can be obtained.

[0066] Example 2 The SiC substrate 10 was evaluated under the following conditions.

[0067] <SiC substrate 10> A SiC substrate 10 was used under the same conditions as in Example 1.

[0068] <Oxide film removal process> The oxide film was removed under the same conditions as in Example 1.

[0069] <Image acquisition process> Scanning electron microscope: Zeiss Merline Acceleration voltage: 1kV Slope direction: Parallel to <1-100> Incident angle θ:8° Exposure time to the atmosphere: 5 minutes

[0070] <Brightness comparison process> Figure 4(b) shows image I obtained during the image acquisition process according to Example 2. When the SiC substrate 10, which has been exposed to air for 5 minutes, is measured at an incident angle θ of 8°, the first brightness information L1 and the second brightness information L2 can be obtained more clearly than in Example 1. In other words, a comparison between Example 1 and Example 2 shows that brightness information L can be accurately obtained by shortening the exposure time to air.

[0071] Figure 5 is a graph showing the relationship between the incident angle θ and contrast using the SiC substrate 10 of this Example 2. The horizontal axis of Figure 5 is the incident angle θ of the electron beam PE with respect to the normal N of the {0001} plane of the SiC substrate 10. The vertical axis of Figure 5 is a numerical value of the contrast obtained by subtracting the second brightness information L2 (dark areas) from the first brightness information L1 (bright areas) of image I.

[0072] As shown in Figure 5, the contrast value is maximized when the incident angle θ is between 30° and 40°. On the other hand, when measuring at an incident angle θ between 30° and 40°, the measuring device becomes large and the problem of not being able to focus arises when measuring a large-diameter SiC substrate 10. Therefore, the inventors have found that brightness information L can be obtained even at an incident angle θ of 10° or less.

[0073] According to the present invention, the process includes an image acquisition step in which an electron beam PE is incident at an incident angle θ tilted with respect to the normal N of the {0001} plane of the SiC substrate 10 to acquire an image I, wherein the incident angle θ is 10° or less. This makes it possible to miniaturize the measurement chamber of the scanning electron microscope compared to the conventional case where the incident angle θ is set to 30° to 40°.

[0074] Furthermore, according to the present invention, it is possible to suppress focal shifts when measuring large-diameter substrates. By injecting the electron beam PE at a smaller incidence angle θ compared to conventional methods, the travel distance of the stage 20 in the irradiation direction of the electron beam PE can be reduced when scanning a large-diameter SiC substrate 10. This makes it possible to suppress the deviation of the surface of the SiC substrate 10 from the focal position of the electron beam PE when scanning a large-diameter SiC substrate 10.

[0075] Furthermore, the present invention includes an oxide film removal step to remove the oxide film from the SiC substrate 10. By removing the oxide film formed on the SiC substrate 10 in this way, the brightness information L of the image I can be obtained more clearly.

[0076] Furthermore, according to the present invention, the image acquisition step is a step of acquiring image I within 60 minutes of the time the SiC substrate 10 is exposed to the atmosphere after the oxide film removal step. Thus, by limiting the exposure time of the SiC substrate 10 to 60 minutes or less, the brightness information L of image I can be obtained more clearly. [Explanation of Symbols]

[0077] 10 SiC substrates 20 stages 21 Board fixing means D1 First stacking direction D2 Second stacking direction PE electron beam Image I L Brightness information L1 First brightness information L2 Second brightness information N normal θ incident angle φ Tilt angle

Claims

1. An image acquisition step of acquiring an image by incidenting an electron beam at an incident angle inclined with respect to the normal of the {0001} plane of a silicon carbide substrate having a polytype in which the stacking direction changes, and a brightness comparison step of comparing a plurality of brightness information reflecting the stacking direction of atoms in the image, A method for evaluating a silicon carbide substrate, wherein the incident angle of the electron beam in the image acquisition step is 7° to 9°.

2. The process includes an oxide film removal step to remove the oxide film from the silicon carbide substrate, The method for evaluating a silicon carbide substrate according to claim 1, wherein the image acquisition step is performed after the oxide film removal step.

3. The method for evaluating a silicon carbide substrate according to claim 2, wherein the image acquisition step is a step of acquiring the image within 60 minutes after the oxide film removal step during which the silicon carbide substrate is exposed to the atmosphere.

4. The aforementioned image includes a first brightness information reflecting a first stacking direction and a second brightness information reflecting a second stacking direction. The method for evaluating a silicon carbide substrate according to any one of claims 1 to 3, wherein the brightness comparison step involves comparing the first brightness information with the second brightness information.

5. The method for evaluating a silicon carbide substrate according to any one of claims 1 to 4, wherein the image acquisition step is a step of incidenting the electron beam tilted in the <1-100> direction.

6. The method for evaluating a silicon carbide substrate according to any one of claims 1 to 5, wherein the image acquisition step is wherein the acceleration voltage of the electron beam is 1.0 kV or less.

7. The silicon carbide substrate evaluation method according to any one of claims 1 to 6, wherein the diameter of the silicon carbide substrate is 4 inches or more.

Images