JP2022066972A - METHOD FOR EVALUATING SiC SUBSTRATE, METHOD FOR PRODUCING SiC EPITAXIAL WAFER, AND METHOD FOR PRODUCING SiC DEVICE - Google Patents

Abstract

To provide a method for evaluating an SiC substrate that can identify penetration failures at low cost and with efficiency.SOLUTION: A method for evaluating an SiC substrate includes: an image acquisition step for acquiring photoluminescent images of two or more SiC substrates cut from an ingot grown from the same seed crystal; an extraction step for extracting the position of a failure in each of the two or more SiC substrates; and a determination step for determining whether the failure is a penetration failure by comparison of the position of the failure and an S/N ratio. The determination step includes a first determination stage for determining whether the failure of a wafer for comparison is within a predetermined range relative to a failure of a wafer for reference, and a second determination step for determining whether the S/N ratio of the failure of the wafer for comparison, which has been determined to be within the predetermined range in the first determination stage, is lower than or equal to the S/N ratio of the wafer for reference.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for evaluating a SiC substrate, a method for manufacturing a SiC epitaxial wafer, and a method for manufacturing a SiC device.

Silicon carbide (SiC) has an insulation breakdown electric field that is an order of magnitude larger and a bandgap that is three times larger than that of silicon (Si). Further, silicon carbide (SiC) has characteristics such as a thermal conductivity 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 and the like. For this reason, in recent years, SiC epitaxial wafers have come to be used for the above-mentioned semiconductor devices.

The SiC epitaxial wafer is manufactured by growing a SiC epitaxial film, which is an active region of a SiC device, on a SiC substrate by a chemical vapor deposition (CVD) method.

The SiC substrate is manufactured by cutting out a SiC ingot. Defects in the SiC substrate adversely affect the SiC epitaxial wafer and the SiC device. Penetration defects are one of the defects that have a large effect on SiC epitaxial wafers and SiC devices. In addition, a SiC device having a penetration defect may cause a withstand voltage defect.

Patent Document 1 describes a method of identifying a micropipe, which is one of the penetration defects, from the coordinate position of the defect.

Japanese Unexamined Patent Publication No. 2020-31076

However, some penetration defects are small in size and may not be detected by a surface image obtained by an optical microscope. In addition, the coordinate positions may happen to match, and the accuracy of identifying penetration defects is low.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for evaluating a SiC substrate that can efficiently identify through defects at low cost.

The present invention provides the following means for solving the above problems.

(1) The method for evaluating a SiC substrate according to the first aspect is an image acquisition step of acquiring a photoluminescence image of two or more SiC substrates among a plurality of SiC substrates cut out from an ingot grown from the same seed crystal. And, between the extraction step of extracting the position of the defect in each of the two or more SiC substrates and the two or more SiC substrates, the position of the defect and the S / N ratio of the defect in the photoluminescence image are compared. The determination step includes a determination step of determining whether or not the defect is a penetration defect, and the determination step compares the substrate on the seed crystal side of the two or more SiC substrates as a reference wafer. In the first determination step of determining whether the position of the defect on the wafer is within a predetermined range with respect to the position of the defect on the reference wafer, and the first determination step, it is determined that the position is within the predetermined range. It has a second determination step of determining whether the S / N ratio of the defect of the comparison wafer is equal to or less than the S / N ratio of the reference wafer in the defect of the reference wafer and the defect of the comparison wafer.

(2) In the method for evaluating a SiC substrate according to the above aspect, the seed crystal does not have an offset angle, and in the first determination step, the position of the defect of the comparative wafer to be compared is the position of the defect in the reference wafer. It may be determined whether or not they match.

(3) In the image acquisition step of the method for evaluating a SiC substrate according to the above embodiment, each of the two or more SiC substrates is irradiated with excitation light having a wavelength of 200 nm or more and 380 nm or less to acquire the photoluminescence image. May be good.

(4) In the image acquisition step of the method for evaluating a SiC substrate according to the above aspect, the photoluminescence image may be acquired via a long pass filter that allows a wavelength of 600 nm or more to pass through.

(5) The method for evaluating a SiC substrate according to the above embodiment is to evaluate an ingot grown from the same seed crystal based on the position of a defect determined to be a penetration defect in the reference wafer or the comparison wafer in the determination step. Further, an estimation step for estimating the position of the penetration defect in another SiC substrate may be provided.

(6) The method for manufacturing a SiC epitaxial wafer according to the second aspect includes an evaluation method for a SiC substrate according to the above aspect.

(7) The method for manufacturing a SiC device according to a third aspect includes an evaluation method for a SiC substrate according to the above aspect.

The method for evaluating a SiC substrate, the method for manufacturing a SiC epitaxial wafer, and the method for manufacturing a SiC device according to the above aspects can identify penetration defects at low cost and efficiently.

It is a flow chart of the evaluation method of the SiC substrate which concerns on this embodiment. It is a schematic diagram of an ingot crystal grown from a seed crystal. An example of a coordinate system for extracting the position of a defect from a SiC substrate is shown. It is an enlarged image of defects in the photoluminescence image. It is a photoluminescence image of a defect that is the same penetration defect as the defect of FIG. 4 and is located farther from the seed crystal than the defect of FIG.

Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios of each component may differ from the actual ones. be. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

FIG. 1 is a flow chart of an evaluation method for a SiC substrate according to this embodiment. The method for evaluating a SiC substrate includes, for example, a preparation step S0, an image acquisition step S1, an extraction step S2, and a determination step S3. The evaluation method of the SiC substrate is performed in the order of, for example, the preparation step S0, the image acquisition step S1, the extraction step S2, and the determination step S3. The order of the image acquisition step S1 and the extraction step S2 may be reversed.

In the preparation step S0, the ingot is first prepared. FIG. 2 is a schematic diagram of an ingot Ig crystal grown from a seed crystal. In FIG. 2, the height direction of the ingot Ig is referred to as the z direction, one direction orthogonal to the z direction is referred to as the x direction, and the directions orthogonal to the x direction and the z direction are referred to as the y direction. Ingot Ig is a SiC single crystal crystal grown from the same seed crystal.

The seed crystal may have a growth plane facing the raw material having an offset angle with respect to the crystal plane, or may have no offset angle. When it has an offset angle, the offset angle is, for example, 2 ° or more and 6 ° or less. The ingot grows on the growth plane of the seed crystal, for example, by the sublimation method.

The ingot Ig may have a penetration defect 1 inside. Penetration defect 1 is a defect extending in the crystal growth direction of the ingot Ig. The penetration defect 1 penetrates from the front surface to the back surface of the SiC substrate W after cutting out the SiC substrate W from the ingot Ig. Penetration defects 1 include penetration defects derived from the seed crystal originating from the interface with the seed crystal and penetration defects derived from bulk growth originating from a point inside the SiC ingot (a point other than the interface with the seed crystal). However, the proportion of penetration defects derived from seed crystals is high.

The penetration defect 1 is often formed along the crystal growth direction. For example, when a seed crystal having an offset angle is used, the penetration defect 1 is often inclined with respect to the z direction of the ingot Ig. The inclination angle θ of the ingot Ig of the penetration defect 1 with respect to the z direction coincides with, for example, the offset angle. When the seed crystal does not have an offset angle, the inclination angle θ is, for example, 0 °.

Next, the SiC substrate W is cut out from the ingot Ig. The SiC substrate W may be cut out in parallel with the xy plane or may be cut out at an angle with respect to the xy plane. For example, the SiC substrate W may be cut out so as to be orthogonal to the penetration defect 1. By cutting out the ingot Ig, a plurality of SiC substrates W can be obtained.

Next, the image acquisition step S1 is performed. In the image acquisition step S1, photoluminescence inspection is performed on two or more SiC substrates W among the plurality of cut out SiC substrates. By performing a photoluminescence test, a photoluminescence image can be obtained. The photoluminescence inspection may be performed on all the cut-out SiC substrates, or may be performed on a part of the plurality of cut-out SiC substrates.

The photoluminescence method is a method of irradiating a substance with excitation light and measuring the light emitted when the excited electrons return to the ground state. The first surface of the SiC substrate W is irradiated with excitation light having an energy larger than the band gap of SiC, and the intensity of photoluminescence emitted from the SiC substrate W is measured. By applying the photoluminescence method to the SiC substrate W, defects of the SiC substrate W, aggregation points of impurities, and the like are identified. The photoluminescence test can be performed using, for example, SICA88 manufactured by Lasertec Co., Ltd.

In the photoluminescence inspection, the excitation light irradiating the SiC substrate has, for example, a wavelength of 200 nm or more and 380 nm or less, preferably 313 nm. Further, when obtaining a photoluminescence image, it is preferable to detect the light emitted by the SiC substrate W through a long pass filter that passes a wavelength of 600 nm or more.

Next, the extraction step S2 is performed. In the extraction step S2, the positions of defects in each of the two or more SiC substrates W are extracted. The location of the defect is measured, for example, by a confocal microscope. In the extraction step S2, for example, the position of the defect may be extracted from the photoluminescence image detected in the image acquisition step S1, or the defect may be extracted by defect measurement (for example, surface inspection using a microscope) performed separately from photoluminescence. The position may be extracted. Further, the position of the defect may be extracted by combining the photoluminescence image and the result of the defect inspection different from the photoluminescence. When the photoluminescence image is not used in the extraction step S2, the order of the image acquisition step S1 and the extraction step S2 can be reversed.

FIG. 3 shows an example of a coordinate system for extracting the position of a defect from the SiC substrate W. The SiC substrate W shown in FIG. 3 has an orientation flat OF. For example, the position of the defect D is extracted with the direction parallel to the orientation flat OF as the x direction and the direction orthogonal to the x direction as the y direction. The x-direction is, for example, [11-20], and the y-direction is, for example, [1-100]. When [11-20] and [1-100] are different from the main surface direction of the SiC substrate W, the x component is the orthographic projection direction component of [11-20] and the y component is the orthographic projection direction of [1-100]. It may be used as an ingredient.

Next, the determination step S3 is performed. In the determination step S3, the positions of the defects of the different SiC substrates W and the S / N ratios of the defects in the photoluminescence image are compared. The determination step includes, for example, a first determination step S3a and a second determination step S3b.

The first determination step S3a compares the positional relationships of defects of different SiC substrates W. First, the SiC substrate on the seed crystal side of the plurality of SiC substrates W whose defect positions are compared is designated as the reference wafer W1 (see FIG. 2). As described above, the penetration defect has a high proportion of the penetration defect derived from the seed crystal generated from the interface with the seed crystal, and may be blocked during the growth. By using the SiC substrate W on the side closer to the seed crystal as the reference wafer W1, the probability of overlooking the penetration defect existing in the ingot Ig can be reduced. Further, since the penetration defect derived from the seed crystal is continuous over the seed crystal and the ingot Ig, it is easy to predict the position where the penetration defect is likely to exist on the SiC substrate W on the side close to the seed crystal.

Then, the SiC substrate W cut out from a position away from the seed crystal from the reference wafer W1 is referred to as a comparison wafer W2. In the first determination step S3a, the positions of defects of the reference wafer W1 and the comparison wafer W2 are compared, and it is determined whether or not the defects are within a predetermined range.

As shown in FIG. 2, the penetration defects 1 are continuous in the crystal growth direction. Therefore, when the defect whose position is specified in the extraction step S2 is the penetration defect 1, the defect position on the reference wafer W1 and the defect position on the comparison wafer W2 substantially coincide with each other.

When the seed crystal has an offset angle and the penetration defect 1 has an inclination angle θ with respect to the z direction, the position of the penetration defect on the reference wafer W1 and the position of the penetration defect on the comparison wafer W2 are slightly deviated from each other. .. When the seed crystal does not have an offset angle and the inclination angle θ of the ingot Ig of the penetration defect 1 with respect to the z direction is 0 °, the defect position on the reference wafer W1 and the defect position on the comparison wafer W2 coincide with each other.

The amount of deviation between the two defects is a value obtained by multiplying the distance h in the z direction between the reference wafer W1 and the comparison wafer W2 in the ingot Ig by tan θ. The direction in which the defect position shifts is, for example, the [11-20] direction.

When the amount of deviation between the defect of the reference wafer W1 and the defect of the comparison wafer W2 is htan θ or more, it is highly possible that these defects are not the penetrating defects 1 that communicate with each other. Regardless of the positional relationship between the reference wafer W1 and the comparison wafer W2 in the z direction, if the amount of displacement between the positions of the two defects is 0.6 mm or more, it is possible that these defects are not the penetrating defects 1 that communicate statistically. Is high. In this case, the "predetermined range" is, for example, htan θ or 0.6 mm.

On the other hand, when the amount of deviation between the defect of the reference wafer W1 and the defect of the comparison wafer W2 is htan θ or less, these defects may be a communication defect 1. If the amount of displacement between the positions of the two defects is 0.2 mm or less regardless of the positional relationship between the reference wafer W1 and the comparison wafer W2 in the z direction, these defects may be statistically communicating penetration defects 1. Is high. In this case, the "predetermined range" is, for example, htan θ or 0.2 mm.

The first determination step S3a determines that, for example, when the positions of the two defects are out of the predetermined range, these defects are not associated with the same penetration defect 1. The first determination step S3a roughly screens a defect associated with the same penetration defect 1 and a defect that is not a defect associated with the same penetration defect 1.

Next, the second determination step S3b is performed for the defect whose positional relationship is determined to be within a predetermined range in the first determination step S3a. The second determination step S3b compares the S / N ratio of the defect of the reference wafer W1 with the S / N ratio of the defect of the comparison wafer W2 in the photoluminescence image. The defect for which the S / N ratio is compared is a defect whose positional relationship is determined to be within a predetermined range in the first determination step S3a.

FIG. 4 is an enlarged image of defects in the photoluminescence image. FIG. 5 is a photoluminescence image of a defect that is the same penetration defect as the defect of FIG. 4 and is located farther from the seed crystal than the defect of FIG. Penetration defects appear in the photoluminescence image as points of different luminance than their surroundings. The brightness of the defect may be higher than the brightness of the surroundings, or the brightness of the defect may be lower than the brightness of the surroundings. The ratio (S / N ratio) to the brightness around the defective bright spot is, for example, 1.5 or more or 0.75 or less. In the extraction step S2, the S / N ratio of the defect extracted from the position of the defect is measured. The signal-to-noise ratio is the ratio of the brightness between the center of the defect and the periphery of the defect.

The S / N ratio of the defect caused by the penetration defect 1 tends to be larger in the early stage of crystal growth (closer to the seed crystal) and smaller in the later stage of crystal growth (farther from the seed crystal). The defect shown in FIG. 5 is caused by the same penetration defect as the defect shown in FIG. 4, but the S / N ratio of the defect is different because the cutting position of the SiC substrate W is different. When the defect is the penetration defect 1, the S / N ratio of the defect of the comparison wafer W2 is equal to or less than the S / N ratio of the defect of the reference wafer W1.

In the second determination step S3b, for example, when the S / N ratio of the defect of the comparison wafer W2 is equal to or less than the S / N ratio of the defect of the reference wafer W1, it is determined that the defect is a defect associated with the same penetration defect 1. do. On the contrary, in the second determination step S3b, for example, when the S / N ratio of the defect of the comparison wafer W2 is larger than the S / N ratio of the defect of the reference wafer W1, the defect is associated with the same penetration defect 1. Judge that it is not a defect.

As described above, according to the SiC substrate evaluation method according to the present embodiment, the penetration defect 1 which is one of the killer defects can be easily estimated without destroying the SiC substrate W.

Further, the method for evaluating a SiC substrate according to this embodiment can be incorporated as one step of a method for manufacturing a SiC epitaxial wafer and a method for manufacturing a SiC device. Specifically, after cutting out the SiC substrate W from the ingot Ig and before forming the epitaxial film, an evaluation step using the evaluation method is performed. By performing the evaluation process, the SiC substrate W, which may be a defective product, can be evaluated at an early stage, and the yield of the product can be increased.

Further, from the results obtained from the comparison between the reference wafer W1 and the comparison wafer W2, the positions of penetration defects in other SiC substrates of the ingot grown from the same seed crystal can be estimated. Hereinafter, the process is referred to as an estimation process.

In the estimation process, the position of the penetration defect in the other SiC substrate of the ingot grown from the same seed crystal is estimated based on the position of the defect determined to be the penetration defect in the determination step on the reference wafer W1 or the comparison wafer W2. do.

In the estimation step, the position of the penetration defect in the other SiC substrate is estimated based on the position of the penetration defect in the reference wafer W1 or the comparison wafer W2, the step flow direction, and the offset angle of the SiC substrate. The position of the penetration defect in the other SiC substrate is, for example, the penetration in the reference wafer W1 or the comparison wafer W2 from the distance H in the thickness direction between the other SiC substrate and the reference wafer W1 or the comparison wafer W2 and the offset angle θ of the seed crystal. It can be estimated that the position is deviated by Htan θ from the position of the defect.

For example, when the seed crystal does not have an offset angle and the inclination angle θ of the ingot Ig of the penetration defect 1 with respect to the z direction is 0 °, the position substantially coincides with the position of the penetration defect in the reference wafer W1 and the comparison wafer W2. , It can be estimated that there is a penetration defect in other SiC substrates. S / N ratio of penetration defects The S / N ratio tends to decrease toward the growth direction of the crystal, but when the S / N ratio becomes close to 1, it becomes difficult to detect in the image acquisition step S1. However, the position of the penetration defect can be estimated by the estimation process.

By performing the estimation step, it is possible to identify the position where the device killer defect of the SiC substrate obtained from the same SiC ingot can exist without checking each SiC substrate, SiC epitaxial wafer, SiC chip, etc. one by one. Screening in the estimation process improves the throughput for making SiC epitaxial wafers. The method for evaluating a SiC substrate according to the present embodiment is preferable as the number of SiC substrates used is larger from the viewpoint of performing the estimation process with high accuracy. On the other hand, considering the time and effort for observing, it is preferable that the number of SiC substrates used is close to two.

Although an example of the present embodiment has been illustrated above, the present invention is not limited to these embodiments. For example, a combination of characteristic configurations of each embodiment, addition of other configurations, and the like may be performed.

(Example 1)
Ten SiC ingots with different seed crystals were prepared, and the above-mentioned determination method was applied to each of the ingots.
There were 63 defects whose defect positions were within a predetermined range and the S / N ratio of the defects of the comparison wafer was equal to or less than the S / N ratio of the reference wafer. When it was examined by X-ray topography whether or not each of the extracted defects was a penetration defect, 60 penetration defects were confirmed, and the ratio of penetration defects was 95%.

(Comparative Example 1)
In Comparative Example 1, the ingot was judged without screening based on the S / N ratio.
There were 75 defects whose defect positions were within the predetermined range. That is, the ratio of penetration defects was 80%.

As described above, Example 1 was able to extract penetration defects with a higher probability than Comparative Example 1.

S0 ... Preparation process, S1 ... Image acquisition process, S2 ... Extraction process, S3 ... Judgment process, S3a ... First determination process, S3b ... Second determination process, Ig ... Ingot, 1 ... Penetration defect, W ... SiC substrate, W1 ... reference wafer, W2 ... comparative wafer, D ... defect, OF ... orientation flat

Claims (7)

An image acquisition process for acquiring a photoluminescence image of two or more SiC substrates among a plurality of SiC substrates cut out from an ingot grown from the same seed crystal,
An extraction step for extracting the position of a defect in each of the two or more SiC substrates, and an extraction step.
It has a determination step of comparing the position of a defect and the S / N ratio of the defect in the photoluminescence image between the two or more SiC substrates and determining whether or not the defect is a penetration defect. ,
The determination step is
The substrate on the seed crystal side of the two or more SiC substrates is used as the reference wafer, and it is determined whether the position of the defect of the comparison wafer to be compared is within a predetermined range with respect to the position of the defect in the reference wafer. 1 Judgment process and
In the defect of the reference wafer and the defect of the comparison wafer whose position is determined to be within the predetermined range in the first determination step, the S / N ratio of the defect of the comparison wafer is the S / N ratio of the reference wafer. A method for evaluating a SiC substrate, comprising a second determination step of determining whether or not the ratio is N or less. The seed crystal does not have an offset angle
The method for evaluating a SiC substrate according to claim 1, wherein the first determination step determines whether or not the defects of the comparative wafers to be compared match the defects in the reference wafer. The method for evaluating a SiC substrate according to claim 1 or 2, wherein in the image acquisition step, each of the two or more SiC substrates is irradiated with excitation light having a wavelength of 200 nm or more and 380 nm or less to acquire the photoluminescence image. .. The method for evaluating a SiC substrate according to any one of claims 1 to 3, wherein in the image acquisition step, the photoluminescence image is acquired through a long pass filter that passes a wavelength of 600 nm or more. An estimation step of estimating the position of a penetration defect in another SiC substrate of an ingot grown from the same seed crystal based on the position of the defect determined to be a penetration defect in the determination step on the reference wafer or the comparison wafer. The method for evaluating a SiC substrate according to any one of claims 1 to 4, further comprising. A method for manufacturing a SiC epitaxial wafer using the method for evaluating a SiC substrate according to any one of claims 1 to 5. A method for manufacturing a SiC device using the method for evaluating a SiC substrate according to any one of claims 1 to 5.

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