Crystal evaluation methods, crystal evaluation equipment, SiC substrates, SiC devices, SiC epitaxial wafers
The EBSD-based crystal evaluation method efficiently detects and classifies multiple crystalline states in SiC substrates, enhancing accuracy and enabling high-quality SiC devices and epitaxial wafers with uniform crystal structures.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- RESONAC CORP
- Filing Date
- 2024-10-29
- Publication Date
- 2026-05-15
AI Technical Summary
Existing methods for evaluating SiC substrates are limited in their ability to efficiently detect multiple types of crystalline states, leading to decreased accuracy when combining evaluation methods, making it difficult to assess the crystal state accurately.
A crystal evaluation method using electron beam backscatter diffraction (EBSD) to acquire and analyze Kikuchi patterns, employing machine learning to identify and classify various crystalline states such as fine heterogeneous polytypes, in-plane crystal orientation misalignment, surface oxidation, processing damage, and fine epitaxial defects, with high spatial resolution and accuracy.
Enables efficient and accurate detection of multiple crystalline states in SiC crystals in a short time, preventing accuracy loss from combined methods, and ensuring high-quality SiC substrates, devices, and epitaxial wafers with uniform crystal structures.
Smart Images
Figure 2026078891000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a crystal evaluation method, a crystal evaluation apparatus, a SiC substrate, a SiC device, and a SiC epitaxial wafer.
Background Art
[0002] Silicon carbide (SiC) has a breakdown electric field one order of magnitude larger and a bandgap three times larger than that of silicon (Si). Also, silicon carbide (SiC) has a thermal conductivity about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be applied to power devices, high-frequency devices, high-temperature operation devices, etc. The SiC substrate applied to such devices is required to be high-quality without damage such as cracks and with few parts in an unintended crystal state.
[0003] Examples of crystal states other than the intended ones that may exist in SiC crystals include fine heterogeneous polytypes, crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, and fine epi defects.
[0004] Conventionally, as a method for detecting the crystal state of such SiC crystals, for example, Patent Document 1 discloses a method for evaluating a wafer in which a gas is supplied to one surface of a SiC wafer and through defects are detected based on the presence or absence of a change in internal pressure.
Prior Art Documents
Patent Documents
[0005]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0006] However, Patent Document 1 could only detect one type of crystalline state present in the SiC substrate and could not detect the multiple types of crystalline states that may exist in the SiC substrate as described above. In order to detect and evaluate these multiple types of crystalline states, it was necessary to combine evaluation methods specialized for detecting each crystalline state. However, when multiple evaluation methods were performed sequentially, the accuracy of the location information of the identified crystalline states decreased, making it difficult to evaluate the crystalline state. For this reason, there was a need for an evaluation method for SiC substrates that could efficiently detect multiple types of crystalline states.
[0007] The present invention has been made in view of the above technical background, and aims to provide a crystal evaluation method, a crystal evaluation apparatus capable of efficiently detecting and evaluating multiple types of crystal states that may exist in a SiC crystal in a short time, and a SiC substrate, a SiC device, and a SiC epitaxial wafer in which the crystal states evaluated by such a crystal evaluation method are below a certain level. [Means for solving the problem]
[0008] To solve the above problems, the crystal evaluation method, crystal evaluation apparatus, SiC substrate, SiC device, and SiC epitaxial wafer of one embodiment of the present invention propose the following means. (1) A crystal evaluation method according to embodiment 1 of the present invention is a crystal evaluation method for evaluating the crystalline state of a SiC crystal, comprising at least a detection step of acquiring Kikuchi patterns at a plurality of measurement points of a SiC crystal by electron beam backscatter diffraction, and an evaluation step of evaluating the crystalline state of the SiC crystal by performing image analysis on the Kikuchi patterns at each measurement point obtained in the detection step.
[0009] (2) Aspect 2 of the present invention is the crystal evaluation method of aspect 1, wherein the evaluation step includes a step of comparing a pre-prepared reference Kikuchi pattern with the Kikuchi pattern of each measurement point and calculating the area ratio of the measurement points that are determined to be identical.
[0010] (3) A third aspect of the present invention is the crystal evaluation method of the first aspect, wherein the evaluation step includes a step of classifying the Kikuchi pattern for at least one crystal state from among fine heterogeneous polytype, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, and fine epitaxial defects using machine learning, and displaying the distribution of the crystal states of the SiC crystal as an image mapping.
[0011] (4) A crystal evaluation apparatus according to aspect 4 of the present invention is a crystal evaluation apparatus for evaluating the crystalline state of a SiC crystal, comprising a scanning electron microscope equipped with an electron backscatter diffractometer, and an analysis apparatus for performing image analysis of Kikuchi pattern image data output from the electron backscatter diffractometer, wherein the analysis apparatus performs image analysis of the Kikuchi pattern at the measurement points of the SiC crystal, compares it with a pre-stored reference Kikuchi pattern, and evaluates the crystalline state of the SiC substrate.
[0012] (5) The SiC substrate according to embodiment 5 of the present invention has a crystal structure in which, on one main surface of the SiC substrate, more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron backscatter diffraction are identical to each other.
[0013] (6) The SiC device according to embodiment 6 of the present invention includes a SiC substrate and has a crystal structure in which 90% or more of the Kikuchi patterns at multiple measurement points obtained by electron backscatter diffraction are identical to each other on one main surface of the SiC substrate.
[0014] (7) The SiC epitaxial wafer of embodiment 7 of the present invention has a SiC epitaxial layer on one main surface side of the substrate having a crystal structure in which more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron beam backscatter diffraction are identical to each other. [Effects of the Invention]
[0015] According to one embodiment of the present invention, it is possible to provide a crystal evaluation method, a crystal evaluation apparatus capable of efficiently detecting and evaluating multiple types of crystal states that may exist in a SiC crystal in a short time, and a SiC substrate, SiC device, and SiC epitaxial wafer in which the crystal states evaluated by such a crystal evaluation method are below a certain level. [Brief explanation of the drawing]
[0016] [Figure 1] These are photographs showing the defect-free Kikuchi pattern in the examples, as well as examples of Kikuchi patterns for various crystal states. [Figure 2] This is a photograph showing examples of Kikuchi patterns for various crystalline states in the examples. [Figure 3] This is a photograph showing the mapping results of micro-epitaxial defects in the example. [Modes for carrying out the invention]
[0017] Embodiments of the present invention will be described in detail below with reference to the drawings. Note that, for the sake of clarity, the drawings used in the following description may show enlarged versions of key features, and the dimensional ratios of each component may not be the same as those in reality. Furthermore, the materials, dimensions, etc., exemplified in the following description are merely examples, and the present invention is not necessarily limited to them. It can be implemented with appropriate modifications without altering its effects.
[0018] [Crystal evaluation method] A crystal evaluation method relating to one embodiment of the present invention will be described. One embodiment of the present invention provides a crystal evaluation method that detects multiple types of unintended crystal states that may be present in a SiC (silicon carbide) crystal, such as fine heterogeneous polytypes, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, and fine epitaxial defects, and evaluates the crystal state of the SiC crystal.
[0019] The Kikuchi pattern used in the following explanation is obtained through the following procedure. Using an electron backscatter diffraction apparatus (EBSD apparatus) comprising a vacuum chamber (chamber), an electron beam irradiation mechanism, and a screen composed of a CCD sensor, a SiC sample is set in the vacuum chamber. When an electron beam is irradiated onto the surface of this sample, diffracted electrons based on Bragg's law (λ = 2dsinθ) are generated from the sample surface. By receiving these diffracted electrons with a screen composed of a CCD sensor, an image unique to the crystal surface can be obtained. This image is called a Kikuchi pattern.
[0020] For example, crystal states such as fine heterogeneous polytypes and crystal disorder caused by heterogeneous polytypes have conventionally been determined by evaluation methods such as microscopic observation and X-ray topography. However, in the crystal evaluation method of this embodiment, detection and evaluation are performed by the electron backscatter diffraction method (hereinafter referred to as the EBSD method). A unique Kikuchi pattern occurs for fine heterogeneous polytypes, which is different from the defect-free region and the existence region of other crystal states. Therefore, the crystal disorder caused by heterogeneous polytypes can be grasped as a unique disorder in the image of the Kikuchi pattern.
[0021] Also, as crystal states caused by the substrate surface state, surface oxidation and surface processing damage exist. Conventionally, different detection methods for crystal states were required, such as X-ray photoelectron spectroscopy (XPS) for surface oxidation and X-ray diffraction (XRD) for surface processing damage. However, in the crystal evaluation method of this embodiment, detection and evaluation are performed by the EBSD method. Unique Kikuchi patterns occur for surface oxidation and surface processing damage, respectively, which are different from the defect-free region and the existence region of other crystal states. Therefore, the crystal disorder caused by surface oxidation and surface processing damage can be grasped as a unique disorder in the image of the Kikuchi pattern.
[0022] Furthermore, it is known that 3C polytypes exist among the possible crystalline states that can occur during the epitaxial film deposition process. Conventionally, these were distinguished by methods such as fluorescence (PL) mapping, but in the crystal evaluation method of this embodiment, detection and evaluation are performed by the EBSD method. This EBSD method also provides information on the crystal orientation of the SiC substrate. In the conversion from 4H (0001) to 3C (111), there are two types of 3C crystal orientations (orientations reversed by 180° from each other), and conventional PL mapping methods could not distinguish between them, but the EBSD method can distinguish between these two crystal orientations, thereby improving the accuracy of the analysis.
[0023] The procedure for crystal evaluation involves first placing the SiC crystal to be evaluated, such as a SiC substrate or SiC epitaxial wafer (sample), on the sample stage of a scanning electron microscope (SEM) equipped with an electron beam backscatter diffraction (EBSD) device. Then, an electron beam is irradiated onto the sample surface within the SEM. This yields a Kikuchi pattern, which is a picture unique to the crystal surface (detection step). The Kikuchi pattern obtained at this time differs depending on the crystal structure and crystal orientation, so multiple types of Kikuchi patterns corresponding to different crystal states are formed.
[0024] At this time, the SEM's electron beam can be scanned along the sample surface to acquire the Kikuchi pattern from all measurement points in any given measurement area. This allows for the acquisition of a mapping image of the Kikuchi pattern in the measurement area of the sample surface.
[0025] Next, the image data of the Kikuchi pattern at the measurement point output from the EBSD device is input into an analysis device consisting of, for example, an image processing computer, to perform image analysis of the Kikuchi pattern at the measurement point. This process determines whether or not the above-mentioned crystalline states (fine heterogeneous polytype, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, fine epitaxial defects, etc.) exist at the measurement point, and if so, identifies the type of crystalline state, thereby evaluating the crystalline state of the sample (evaluation step).
[0026] Specifically, the analysis device's memory stores a reference Kikuchi pattern for defect-free samples, as well as reference Kikuchi patterns specific to each type of crystal state (e.g., fine heterogeneous polytype, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, fine epitaxial defects, etc.). These reference Kikuchi patterns can be obtained by pre-training a machine learning model to create specific Kikuchi patterns for various conditions, such as fine heterogeneous polytypes, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, and fine epitaxial defects.
[0027] The analysis device then sequentially compares the image of the Kikuchi pattern at the input measurement point with images of several types of reference Kikuchi patterns that have been stored in advance, and detects Kikuchi patterns that are similar to or match each other. This allows the device to determine whether the aforementioned crystalline state exists at the measurement point of the SiC substrate (sample), and, if so, what type of crystalline state it is. The crystal evaluation result at this measurement point is then displayed, for example, on a monitor.
[0028] Furthermore, in this evaluation process, it is also preferable to calculate the area ratio of measurement points where the Kikuchi pattern of the measurement point is determined to be identical to the reference Kikuchi pattern. This makes it possible to obtain data on the area ratios present for each of the above-mentioned types of crystal states on one main surface of the SiC substrate (sample).
[0029] Furthermore, in this evaluation process, it is also preferable to scan the sample surface with the SEM electron beam, acquire Kikuchi patterns from all measurement points in an arbitrary measurement area, compare these Kikuchi patterns with a reference Kikuchi pattern, and display the distribution of each of the aforementioned crystal states on one main surface of the SiC substrate as an image mapping.
[0030] As described above, according to the crystal evaluation method of this embodiment, by comparing the Kikuchi pattern of the SiC crystal (sample) obtained by the EBSD method with the reference Kikuchi patterns pre-stored for each type of crystal state described above, multiple types of crystal states can be identified with a single detection method, making it possible to evaluate SiC crystals with high accuracy and in a short time. For example, since it is not necessary to combine multiple different detection methods for each type of crystal state as in the conventional method, crystal evaluation can be accelerated, and the decrease in detection accuracy of crystal states caused by combining multiple detection methods can be prevented.
[0031] [Crystal evaluation equipment] The crystal evaluation apparatus of this embodiment comprises a scanning electron microscope (SEM) equipped with an electron beam backscatter diffractometer (EBSD) and an analysis apparatus that performs image analysis of the Kikuchi pattern output from the electron beam backscatter diffractometer. The analysis apparatus performs image analysis of the Kikuchi pattern at the measurement points of the SiC crystal and compares it with a pre-stored reference Kikuchi pattern to evaluate the crystal state of the SiC substrate. The specific crystal evaluation method using the analysis apparatus should be carried out in accordance with the evaluation steps of the crystal evaluation method described above.
[0032] In these crystal evaluation devices, the spatial resolution improves when the distance between the objective lens and the sample in the SEM is reduced. In typical SEMs, the distance between the objective lens and the sample is set to a few centimeters. Also, a common setup for EBSD analysis is to tilt the sample at 60° to 70°.
[0033] However, with these standard SEM setups, if a large SiC substrate (sample), such as a 6-inch sample, is tilted at an angle of 60° to 70°, the objective lens interferes with the sample, making it difficult to measure the entire surface of the sample.
[0034] Each crystalline state present in the SiC substrate (e.g., fine heterogeneous polytypes, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, fine epitaxial defects, etc.) is several micrometers or larger in size, so a spatial resolution of several nanometers, as found in typical SEMs, is not necessary. For this reason, the crystal evaluation apparatus of this embodiment is configured to reduce spatial resolution by increasing the distance between the sample and the objective lens so that the sample does not interfere with the objective lens even when a sample of about 6 to 10 inches is tilted at 60 to 70 degrees. Furthermore, by setting the sample's tilt angle to 10° to 45°, the distance between the sample and the objective lens can be shortened.
[0035] [SiC substrate] The SiC substrate of this embodiment has a crystal structure in which, on one main surface of the SiC substrate, more than 90% of the Kikuchi patterns obtained by electron backscatter diffraction at multiple measurement points are identical to each other.
[0036] These measurement points can be set, for example, at least 10, preferably 20, 30, 40, 50, and more preferably 100, locations at equal intervals on one main surface of the SiC substrate.
[0037] Furthermore, the percentage of identical Kikuchi patterns should be at least 90%, preferably 92%, 94%, 96%, 98%, more preferably 99%, or 100%.
[0038] In such a SiC substrate, using the embodiment of the crystal evaluation method described above, more than 100 Kikuchi patterns are acquired at equal intervals extending in the X and Y directions along one main plane of the SiC substrate, and the images of all these Kikuchi patterns are compared with each other. If more than 90% of the more than 100 Kikuchi patterns are identical, for example, if more than 90% of the images of defect-free Kikuchi patterns are found, then the substrate can be used as a high-quality semiconductor substrate.
[0039] [SiC devices] The SiC device of this embodiment includes a SiC substrate, and on one main surface of the SiC substrate, it has a crystal structure in which more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron backscatter diffraction are identical to each other.
[0040] These measurement points can be set, for example, at least 10, preferably 20, 30, 40, 50, and more preferably 100, locations at equal intervals on one main surface of the SiC substrate.
[0041] Furthermore, the percentage of identical Kikuchi patterns should be at least 90%, preferably 92%, 94%, 96%, 98%, more preferably 99%, or 100%.
[0042] Such SiC devices maintain uniformity of the crystal structure even in minute regions, such as the fine SiC chip substrates that make up the SiC device cut out from the SiC substrate by dicing, enabling the realization of SiC devices for power devices with excellent semiconductor properties.
[0043] [SiC epitaxial wafer] The SiC epitaxial wafer of this embodiment has a SiC epitaxial layer on one main surface side of the substrate, having a crystal structure in which more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron beam backscatter diffraction are identical to each other.
[0044] These measurement points can be set, for example, at least 10, preferably 20, 30, 40, 50, and more preferably 100, locations at equal intervals on one main surface of the SiC substrate.
[0045] Furthermore, the percentage of identical Kikuchi patterns should be at least 90%, preferably 92%, 94%, 96%, 98%, more preferably 99%, or 100%.
[0046] By forming a SiC epitaxial layer on one main surface of a SiC substrate, the surface crystal structure becomes denser, and the number of the aforementioned crystal states (e.g., fine heterogeneous polytypes, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, fine epitaxial defects, etc.) can be reduced.
[0047] Although one embodiment of the present invention has been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Examples]
[0048] The Kikuchi patterns for each crystal state of a SiC substrate were actually obtained using the crystal evaluation method of this embodiment. Figure 1 shows the Kikuchi patterns for 4H defect-free, surface processing damage, and fine epitaxial defects, and Figure 2 shows the Kikuchi patterns for fine heterogeneous polytypes, each as photographs.
[0049] Figures 1 and 2 show that the Kikuchi pattern is disordered for each crystal state compared to the Kikuchi pattern of defect-free areas. It was confirmed that by comparing these Kikuchi patterns, which are unique to each crystal state, with the Kikuchi patterns obtained at measurement points using image processing, it is possible to identify the type of crystal state present in the SiC substrate.
[0050] Next, the SEM electron beam was scanned along the sample surface, and fine epitaxial defects were mapped based on the Kikuchi pattern from all measurement points in an arbitrary measurement area. The results are shown photographically in Figure 3. As shown in Figure 3, the regions where micro-epitaxial defects exist are mapped with a different color from the defect-free regions, confirming that the spatial distribution of micro-epitaxial defects can be easily detected.
[0051] Next, the crystal structure of a SiC device was actually evaluated using the crystal evaluation method of this embodiment. EBSD measurements were performed at 100 locations per area corresponding to a 5mm square chip on one main surface of an 8-inch SiC substrate.
[0052] As a result, we identified a region where the proportion of 4H-SiC Kikuchi patterns exceeded 99%. We fabricated a SiC device using this identified region, which corresponds to a single 5mm square chip. The fabricated SiC device exhibits excellent uniformity of the crystal structure at a minute level that had not been considered in conventional SiC devices, enabling the realization of a SiC device with superior long-term reliability.
Claims
1. A crystal evaluation method for evaluating the crystalline state of SiC crystals, A detection step in which the Kikuchi pattern is obtained at multiple measurement points of a SiC crystal by electron beam backscatter diffraction, An evaluation step is performed to evaluate the crystalline state of the SiC crystal by performing image analysis on the Kikuchi patterns of each measurement point obtained in the detection step, A crystal evaluation method having at least the following.
2. The crystal evaluation method according to claim 1, wherein the evaluation step includes a step of comparing a pre-prepared reference Kikuchi pattern with the Kikuchi patterns of each measurement point and calculating the area ratio of measurement points that are determined to be identical.
3. The crystal evaluation method according to claim 1, wherein the evaluation step includes a process of classifying the Kikuchi pattern for at least one crystal state from among fine heterogeneous polytype, in-plane crystal orientation misalignment, surface oxidation, processing damage, crystal disorder, and fine epitaxial defects using machine learning, and displaying the distribution of the crystal states of the SiC crystal as an image mapping.
4. A crystal evaluation apparatus for evaluating the crystalline state of SiC crystals, The system comprises a scanning electron microscope equipped with an electron backscatter diffractometer, and an analysis device that performs image analysis of the Kikuchi pattern output from the electron backscatter diffractometer. The aforementioned analysis device is a crystal evaluation device that performs image analysis on the Kikuchi pattern of measurement points of a SiC crystal, compares it with a pre-stored reference Kikuchi pattern, and evaluates the crystalline state of the SiC crystal.
5. A SiC substrate having a crystal structure in which more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron backscatter diffraction on one main surface of the SiC substrate are identical to each other.
6. A SiC device comprising a SiC substrate, wherein, on one main surface of the SiC substrate, more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron beam backscatter diffraction are identical to each other in its crystal structure.
7. A SiC epitaxial wafer having a SiC epitaxial layer on one main surface side of the substrate, having a crystal structure in which more than 90% of the Kikuchi patterns at multiple measurement points obtained by electron beam backscatter diffraction are identical to each other.