JP7476915B2 - Distortion evaluation device for hexagonal compound semiconductor and manufacturing method for silicon carbide epitaxial substrate - Google Patents

Description

This disclosure relates to a method for manufacturing a hexagonal compound semiconductor.

JP 2013-203653 A discloses the use of a photoelastic method to evaluate the residual strain in a compound semiconductor substrate.

JP 2013-203653 A

JP 2013-203653 A discloses Group III nitride semiconductors such as gallium nitride (GaN), but does not particularly consider residual strain after epitaxial growth of a compound semiconductor film on the surface of a substrate made of the hexagonal compound semiconductor. In the above-mentioned hexagonal compound semiconductors such as GaN, silicon carbide (SiC), and aluminum nitride (AlN), problems such as substrate cracking occur due to residual strain during manufacturing processes including the epitaxial growth process of the compound semiconductor film, and compound semiconductors with accurately evaluated strain are required.

The objective of this disclosure is to provide a method for manufacturing a hexagonal compound semiconductor in which distortion is accurately evaluated.

A method for manufacturing a hexagonal compound semiconductor according to one aspect of the present invention includes a step of preparing a substrate, a step of calculating a first phase difference, a step of homoepitaxially growing a compound semiconductor layer, and a step of calculating a second phase difference. In the step of preparing a substrate, a hexagonal compound semiconductor substrate having a hexagonal crystal structure, birefringence, and including a first main surface, a second main surface opposite the first main surface, and a side surface is prepared. In the step of calculating the first phase difference, the first main surface is irradiated with light that transmits through the substrate, the light is transmitted through the substrate along the <0001> direction in the substrate, and the first phase difference caused by the birefringence is calculated. In the step of homoepitaxially growing the compound semiconductor layer, the compound semiconductor layer is homoepitaxially grown on the first main surface based on predetermined growth conditions. In the step of calculating the second phase difference, light is irradiated onto the surface of the compound semiconductor layer, the light is transmitted through the compound semiconductor layer and the substrate along the <0001> direction in the substrate, and the second phase difference caused by the birefringence is calculated.

According to the present disclosure, for a hexagonal compound semiconductor having a hexagonal crystal structure, by transmitting light along the <0001> direction through the hexagonal compound semiconductor, the effect of natural birefringence due to the crystal structure of the hexagonal compound semiconductor can be reduced and the phase difference of the birefringence due to the photoelastic effect caused by strain can be measured. This makes it possible to obtain a hexagonal compound semiconductor in which strain is accurately evaluated.

FIG. 1 is a flow chart showing a method for evaluating strain in a compound semiconductor. FIG. 2 is a schematic diagram for explaining the basic configuration of the evaluation device. FIG. 3 is a schematic diagram for explaining a configuration example of the evaluation device. FIG. 4 is a schematic diagram for explaining the direction of light passing through a compound semiconductor. FIG. 5 is a flowchart showing a method for manufacturing a compound semiconductor.

[Description of the embodiment of the present invention]
First, the embodiments of the present invention will be listed and described.

(1) A method for manufacturing a hexagonal compound semiconductor according to one aspect of the present invention includes the steps of preparing a substrate, calculating a first phase difference, homoepitaxially growing a compound semiconductor layer, and calculating a second phase difference. In the step of preparing a substrate, a hexagonal compound semiconductor substrate having a hexagonal crystal structure, birefringence, and including a first main surface, a second main surface opposite the first main surface, and a side surface is prepared. In the step of calculating the first phase difference, the first main surface is irradiated with light that transmits through the substrate, the light is transmitted through the substrate along the <0001> direction in the substrate, and the first phase difference caused by the birefringence is calculated. In the step of homoepitaxially growing the compound semiconductor layer, the compound semiconductor layer is homoepitaxially grown on the first main surface based on predetermined growth conditions. In the step of calculating the second phase difference, light is irradiated onto the surface of the compound semiconductor layer, and the light is transmitted through the compound semiconductor layer and the substrate along the <0001> direction in the compound semiconductor layer and the substrate, and the second phase difference caused by birefringence is calculated. The compound semiconductor is, for example, silicon carbide (SiC), gallium nitride (GaN), aluminum nitride (AlN), etc.

In this way, the phase difference due to birefringence is calculated when light is transmitted through the hexagonal compound semiconductor along the <0001> direction, so data can be obtained in which the influence of natural birefringence caused by the crystal structure of the hexagonal compound semiconductor is reduced. Therefore, a hexagonal compound semiconductor can be obtained in which the distortion is evaluated based on the phase difference caused by birefringence in a state in which the influence of the natural birefringence is reduced. In addition, since the distortion can be evaluated based on the phase difference caused by birefringence before and after the compound semiconductor layer is formed on the compound semiconductor substrate, the influence on distortion due to the compound semiconductor layer formation process can be evaluated, and the occurrence of distortion in the compound semiconductor substrate (substrate with epitaxial layer) on which the compound semiconductor layer is formed can be confirmed. Then, by comparing the distortion after the compound semiconductor layer is formed with the distortion before the compound semiconductor layer is formed to grasp the amount of distortion generated during epitaxial growth, it becomes possible to determine whether the growth conditions in the epitaxial growth are appropriate.

(2) In the above-mentioned method for manufacturing a hexagonal compound semiconductor, the first main surface of the substrate is inclined with respect to the {0001} plane. In this case, since the substrate is a so-called off-substrate, it is difficult to transmit light along the <0001> direction inside the substrate even if light is simply irradiated from a direction perpendicular to the first main surface. Therefore, by measuring the inclination angle (off angle) and inclination direction (off direction) that the first main surface of the substrate makes with respect to the {0001} plane in advance, the method for manufacturing a hexagonal compound semiconductor according to this embodiment is effective in that it is possible to set the light transmission direction inside the substrate to the <0001> direction, taking into account the refraction of light at the interface between the first main surface and the surrounding air, etc.

(3) In the method for manufacturing a hexagonal compound semiconductor, in the step of calculating the first phase difference, the angle of incidence of the light with respect to the first principal surface is determined based on information on the inclination angle and inclination direction of the first principal surface with respect to the {0001} plane that has been measured in advance. Furthermore, in the step of calculating the second phase difference, the angle of incidence of the light with respect to the surface of the compound semiconductor layer is determined based on the information.

In this case, even if the substrate is warped or has an uneven shape, by obtaining information such as the substrate's off-angle and off-direction in advance, it is possible to perform an accurate evaluation of the distortion with reduced effects of natural birefringence caused by the crystal structure of the hexagonal compound semiconductor. This makes it possible to accurately determine whether the growth conditions for epitaxial growth are appropriate.

(4) A method for manufacturing a hexagonal compound semiconductor according to one embodiment of the present invention includes the steps of preparing a substrate, calculating a first phase difference, calculating a first maximum value and a first standard deviation, homoepitaxially growing a compound semiconductor layer, calculating a second phase difference, calculating a second maximum value and a second standard deviation, and changing a growth condition. In the step of preparing a substrate, a hexagonal compound semiconductor substrate having a hexagonal crystal structure, having birefringence, and including a first main surface, a second main surface opposite to the first main surface, and a side surface is prepared. In the step of calculating the first phase difference, light that transmits through the substrate is irradiated to a plurality of locations on the first main surface, the light is transmitted through the substrate along the <0001> direction in the substrate, and the first phase difference caused by the birefringence is calculated. In the step of calculating the first maximum value and the first standard deviation, the first maximum value and the first standard deviation are calculated for the first phase difference at the plurality of locations. In the step of homoepitaxially growing the compound semiconductor layer, the compound semiconductor layer is homoepitaxially grown on the first main surface. In the step of calculating the second phase difference, light is irradiated to a plurality of locations on the surface of the compound semiconductor layer, and the light is transmitted through the compound semiconductor layer and the substrate along the <0001> direction in the compound semiconductor layer and the substrate, and a second phase difference caused by birefringence is calculated. In the step of calculating the second maximum value and the second standard deviation, the second maximum value and the second standard deviation are calculated for the second phase differences at the plurality of locations. In the step of changing the growth conditions, the growth conditions are changed when the conditions that the second maximum value is greater than the first maximum value and the second standard deviation is greater than the first standard deviation are met.

In this way, light passes through the hexagonal compound semiconductor along the <0001> direction, so that the natural birefringence caused by the crystal structure of the hexagonal compound semiconductor can be reduced. In addition, by calculating the phase difference at multiple locations on the first main surface of the substrate, the magnitude and variation of the phase difference are correlated with the magnitude and variation of the strain in the substrate, so that the distribution state of strain within the surface of the compound semiconductor substrate can be evaluated. In other words, it is possible to obtain a hexagonal compound semiconductor in which the distribution state of strain within the surface of the substrate can be evaluated based on the phase difference caused by birefringence, with the effects of the natural birefringence reduced.

In addition, because the distortion can be evaluated based on the phase difference caused by birefringence before and after the compound semiconductor layer is formed on the compound semiconductor substrate, it is possible to evaluate the effect of the compound semiconductor layer formation process on distortion and to confirm the occurrence of distortion in the compound semiconductor substrate (substrate with epitaxial layer) on which the compound semiconductor layer is formed.

Furthermore, by comparing the maximum value and standard deviation of the phase difference before and after the compound semiconductor layer is formed, it is possible to evaluate the effect of distortion due to the film formation process of the compound semiconductor layer and adjust the growth conditions in the film formation process so as to suppress the increase in distortion. As a result, it is possible to obtain a substrate with an epitaxial layer in which distortion is suppressed.

(5) In the above-mentioned method for manufacturing a hexagonal compound semiconductor, the first main surface of the substrate is inclined with respect to the {0001} plane. In this case, since the substrate is a so-called off-substrate, it is difficult to transmit light along the <0001> direction inside the substrate even if light is simply irradiated from a direction perpendicular to the first main surface. Therefore, by measuring the inclination angle (off angle) and inclination direction (off direction) that the first main surface of the substrate makes with respect to the {0001} plane in advance, the method for manufacturing a hexagonal compound semiconductor according to this embodiment is effective in that it is possible to set the light transmission direction inside the substrate to the <0001> direction, taking into account the refraction of light at the interface between the first main surface and the surrounding air, etc.

(6) In the method for manufacturing a hexagonal compound semiconductor, in the step of calculating the first phase difference, the angle of incidence of light with respect to the first principal surface is determined for a plurality of points on the first principal surface based on information on the inclination angle and inclination direction of the first principal surface with respect to the {0001} plane that has been previously measured. Furthermore, in the step of calculating the second phase difference, the angle of incidence of light with respect to the surface of the compound semiconductor layer is determined for a plurality of points on the surface of the compound semiconductor layer based on the information.

In this case, even if the substrate is warped or has an uneven shape, by obtaining information such as the substrate's off-angle and off-direction in advance, it is possible to accurately evaluate the distortion with reduced effects of natural birefringence caused by the crystal structure of the hexagonal compound semiconductor. This makes it possible to accurately determine the distribution of distortion within the substrate surface and the suitability of the growth conditions for epitaxial growth.

(7) In the above compound semiconductor manufacturing method, the diameter of the substrate is 100 mm or more. Since there is concern about problems caused by distortion in compound semiconductor substrates with such a relatively large diameter, it is preferable to apply the above compound semiconductor manufacturing method.

(8) In the above-mentioned compound semiconductor manufacturing method, the compound semiconductor is a silicon carbide single crystal. Since problems such as cracking of the substrate due to distortion occur with silicon carbide (SiC), it is preferable to apply the above-mentioned manufacturing method.

[Details of the embodiment of the present invention]
Hereinafter, details of an embodiment of the present disclosure (hereinafter, referred to as the present embodiment) will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and the description thereof will not be repeated.

<Outline of evaluation method>
As shown in FIG. 1, the evaluation method of the silicon carbide substrate according to the present embodiment is an example of a strain evaluation method of a hexagonal compound semiconductor, and includes a preparation step (S10) and an evaluation step (S20). The preparation step (S10) is a step of preparing a substrate of a hexagonal compound semiconductor. In the preparation step (S10), a substrate made of a hexagonal compound semiconductor having a hexagonal crystal structure and birefringence is prepared. The substrate includes a first main surface, a second main surface opposite to the first main surface, and a side surface. As the substrate, for example, a silicon carbide substrate made of silicon carbide (SiC) single crystal is placed on the stage of the measuring device. Note that a GaN substrate or an AlN substrate may be used as the substrate. As the substrate prepared here, an off-substrate whose surface is inclined with respect to the (0001) plane may be used. In addition, the diameter of the substrate may be 100 mm or more, 150 mm or more, or 200 mm or more. The diameter of the substrate may be 400 mm or less, or 300 mm or less. The substrate may be a base substrate and a substrate with an epitaxial layer formed on the surface of the base substrate. Alternatively, the substrate may be a substrate made of a compound semiconductor without an epitaxial layer. In the following, a silicon carbide substrate is used as a representative example.

Next, the evaluation step (S20) is performed. In the evaluation step (S20), which corresponds to the step of transmitting light through the substrate and the step of calculating the phase difference, the distortion of the substrate is evaluated using the photoelastic effect. Specifically, in the evaluation step (S20), the surface of the substrate is irradiated with light, and the light is transmitted through the substrate along the <0001> direction, thereby evaluating the distortion using photoelasticity in a state in which the effect of natural birefringence is reduced. From a different perspective, in the step of transmitting light through the substrate included in the evaluation step (S20), the first main surface is irradiated with light that transmits through the substrate, and the light is transmitted through the substrate along the <0001> direction in the substrate. In the step of calculating the phase difference, the phase difference caused by birefringence is calculated. The phase difference is used to evaluate the distortion.

In addition, in the evaluation step (S20), the surface of the silicon carbide substrate is irradiated with light at multiple locations to evaluate the distortion at multiple locations of the silicon carbide substrate. Any method can be used as a specific method for evaluating the distortion using photoelasticity, and an example of a specific evaluation method will be described later.

<Configuration of Evaluation Device>
The basic configuration of a measuring device for carrying out the above-mentioned evaluation method is shown in Fig. 2. As shown in Fig. 2, the measuring device 100 includes a controller 10, an incident side optical system 41, a light receiving side optical system 42, a stage 24 on which a silicon carbide substrate 1 serving as a sample is mounted, and optical axis adjustment units 43 to 45. The incident side optical system 41 includes a light source 21, a lens 22, and a polarizer 23. The light receiving side optical system 42 includes an analyzer 25, a lens 26, and a light receiver 27. Between the light source 21 and the light receiver 27, the lens 22, the polarizer 23, the stage 24, the analyzer 25, and the lens 26 are arranged in this order from the light source 21 side.

The optical axis adjustment unit 43 changes the angle (incident angle) that the incident light from the incident side optical system 41 to the silicon carbide substrate 1 makes with respect to the surface of the silicon carbide substrate 1 in order to adjust the optical axis when the light incident from the incident side optical system 41 to the silicon carbide substrate 1 passes through the silicon carbide substrate 1. For example, the optical axis adjustment unit 43 may be configured to move the incident side optical system 41 relative to the stage 24. The optical axis adjustment unit 44 moves the light receiving side optical system 42 relative to the stage 24 so that the optical axis of the light transmitted through the silicon carbide substrate 1 held on the stage 24 overlaps with the optical axis set to be received by the light receiving side optical system 42. The optical axis adjustment unit 45 moves the stage 24 to adjust the optical axis when passing through the silicon carbide substrate 1. For example, the optical axis adjustment unit 45 may move the stage 24 relative to the incident side optical system 41 in order to change the above-mentioned incident angle. The optical axis adjustment units 43 to 45 are connected to the control controller 10 and are controlled by control signals from the control controller 10.

In addition, in the measuring device 100, only either the set of optical axis adjustment units 43, 44 or the optical axis adjustment unit 45 may be installed. By using these optical axis adjustment units 43 to 45 to change the angle of incidence of light on the silicon carbide substrate 1 mounted on the stage 24, the direction of the optical axis of the light passing through the silicon carbide substrate 1 can be changed. As a result, it becomes possible to make the traveling direction of the light passing through the silicon carbide substrate 1 be along the <0001> direction.

The optical axis adjustment units 43 to 45 may have any configuration. For example, a cylinder or an electric motor driven by fluid pressure may be used as the optical axis adjustment units 43 to 45. For example, a hydraulic cylinder connected to a stand on which the incident optical system 41 is mounted may be used as the optical axis adjustment unit 43 so as to change the inclination angle of the stand on which the incident optical system 41 is mounted with respect to the stage 24. Similarly, a hydraulic cylinder connected to a stand on which the light receiving optical system 42 is mounted may be used as the optical axis adjustment unit 44 so as to change the inclination angle of the stand on which the light receiving optical system 42 is mounted with respect to the stage 24. The optical axis adjustment unit 43 may be an optical module including a mirror arranged between the polarizer 23 and the stage 24 to change the traveling direction of light, and a driving unit such as a motor that adjusts the position of the mirror to change the traveling direction of light. The optical axis adjustment unit 44 may be an optical module including a mirror and a driving unit arranged between the stage 24 and the analyzer 25, similar to the optical axis adjustment unit 43.

Also, as the optical axis adjustment unit 45, for example, a motor that changes the tilt angle of the stage 24 with respect to the incident light from the incident side optical system 41 may be used. In this case, two motors may be arranged so that the tilt angle of the stage 24 with respect to the incident light can be changed in two intersecting directions. Specifically, a configuration may be adopted in which a first motor that rotates the stage 24 around a first axis and the stage 24 connected to the rotation axis of the first motor are mounted on a first base, and a second motor that rotates the first base around a second axis extending in a direction different from the first axis is connected to the first base.

The light source 21, polarizer 23, and analyzer 25 are configured to be rotatable in a direction intersecting (preferably perpendicular to) the optical axis of the evaluation light that passes from the light source 21 through the silicon carbide substrate on the stage 24 and reaches the light receiver 27. The rotation angle of the polarizer 23 and analyzer 25 can be controlled by the control controller 10. For example, a driving device such as a motor that rotates the polarizer 23 and analyzer 25 is controlled by the control controller 10.

The light source 21 emits light for evaluation in response to a control signal from the controller 10. Any light source can be used as the light source 21, and for example, a light source such as a laser diode can be used. The stage 24 is configured to be movable in a direction intersecting the optical axis of the light. The movement of the stage 24 is controlled by the controller 10. For example, a conventionally known driving device such as a linear guide can be used to move the stage 24, and the driving device is controlled by the controller 10. For example, the driving device may be configured to be capable of moving both the optical axis adjustment unit 45 and the stage 24 described above in a direction intersecting the optical axis of the light.

The light receiver 27 receives the light that has passed through the lens 22, the polarizer 23, the silicon carbide substrate 1, the analyzer 25, and the lens 26. The light detection data from the light receiver 27 is transmitted to the control controller 10. The control controller 10 evaluates the distortion at the position on the silicon carbide substrate 1 where the light is irradiated, based on data such as the emission intensity of the light from the light source 21, the light detection data from the light receiver 27 (data on the received light intensity), and the rotation angles of the polarizer 23 and the analyzer 25.

An example of the configuration of the measuring device shown in FIG. 2 is shown in FIG. 3. As shown in FIG. 3, in the example of the configuration of the measuring device, the control controller 10 is connected to the light source 21, the polarizer 23, the stage 24, and the analyzer 25 via the control IO bus 11 and the drive circuits 31, 33, 34, and 35. The control controller 10 is also connected to the light receiver 27 via the control IO bus 11, the AD converter 38, the variable gain amplifier 37, and the IV (current-voltage) converter 36. The control controller 10 is also connected to the optical axis adjustment units 43 to 45 via the control IO bus 11 and the drive circuit 39. The incident side optical system 41 is mounted on a stand. An optical axis adjustment unit 43 including a hydraulic cylinder is connected to the stand so as to change the tilt angle of the stand relative to the stage 24. The light receiving side optical system 42 is also mounted on another stand. An optical axis adjustment unit 44 including a hydraulic cylinder is connected to the stand so as to change the tilt angle of the stand relative to the stage. As described above, the optical axis adjustment unit 45 including two motors is connected to the stage 24. The light source 21 can be any light source, for example a laser diode. The light receiver 27 can be any configuration, for example a photodiode.

The stage 24 is movable in the X-axis direction perpendicular to the plane of the paper in FIG. 3 and in the Y-axis direction along the vertical direction of the paper. As described above, the stage 24 is configured to be tiltable relative to the optical axis of the light by the optical axis adjustment units 43 to 45. The polarizer 23 and the analyzer 25 can be rotated, for example, by controlling a pulse motor (not shown) by the drive circuits 33 and 35. The controller 10 may be connected to a display for displaying data, a storage device for recording measurement data, and the like.

<Details of evaluation method>
An example of a method for evaluating a silicon carbide substrate using the measuring device shown in FIG. 3 will now be described in detail.

Preparation step (S10):
First, silicon carbide substrate 1 to be evaluated is placed on stage 24 of the measurement device shown in Fig. 3. As silicon carbide substrate 1, an off-substrate is used in which surface 4 is inclined by an off angle with respect to the (0001) plane indicated by dotted line 2 as shown in Fig. 4. Note that as silicon carbide substrate 1, a substrate in which surface 4 is not inclined with respect to the (0001) plane may also be used.

Step (S30) of adjusting the incident direction of light with respect to the silicon carbide substrate:
Next, the relative positions of the incident side optical system 41, stage 24, and light receiving side optical system 42 are determined so as to satisfy predetermined conditions in order to obtain measurement data in a state in which the direction in which evaluation light is incident on silicon carbide substrate 1 is the <0001> direction perpendicular to the (0001) plane, as shown by arrow 3 in Fig. 4. Note that in this step, the stage 24 may be fixed and the incident side optical system 41 may be moved, or the incident side optical system 41 may be fixed and the stage 24 may be moved.

Measurement step (S21):
Light is irradiated from the light source 21 to the silicon carbide substrate 1 set on the stage 24. For example, infrared light can be used as the irradiated light. At this time, the polarizer 23 and the analyzer 25 are synchronously rotated while their orientations are parallel, and the transmitted light intensity at the silicon carbide substrate 1 is measured as a function of the polarization angle Φ. Furthermore, the polarizer 23 and the analyzer 25 are synchronously rotated while their orientations are orthogonal to each other, and the transmitted light intensity at the silicon carbide substrate 1 is measured as a function of the polarization angle Φ.

Evaluating birefringence phase difference and principal vibration azimuth angle (S22):
The phase difference δ (unit: radian) of birefringence and the principal vibration azimuth angle Ψ are calculated from the data at the above-mentioned measurement points. Any calculation method can be used, but for example, the following method is used.

The transmitted light intensity of the sample when the orientations of the polarizer 23 and the analyzer 25 are orthogonal and parallel are defined as I _⊥ (Φ) and _I∥ (Φ), respectively. Furthermore, the polarization angle when the polarizer 23 and the analyzer 25 are synchronously rotated as described above is defined as Φ. In this case, the relationship shown in the following formula (1) holds between the phase difference δ of the birefringence of the silicon carbide substrate 1 as the sample and the principal vibration azimuth angle Ψ.

_Ir (Φ)= _I⊥ (Φ)/{ _I⊥ (Φ)+ _I∥ (Φ)}= ^sin2 {2(Φ-Ψ)}{ ^sin2 (δ/2)} Formula (1)
The birefringence phase difference δ and the principal vibration azimuth angle Ψ are obtained by sine and cosine transforming I _r (Φ) shown in the above formula. For example, the phase difference δ and the principal vibration azimuth angle Ψ at the measurement point can be obtained based on the following formulas (2) to (5).

δ = 2 sin ^-1 {16 (I _sin ² + I _cos ² )} ^1/4 Formula (2)
Ψ=(1/4)tan ⁻¹ (I _sin /I _cos ) Equation (3)
I _sin = (-1/4) sin4Ψ { ^sin2 (δ/2)} Equation (4)
I _cos = (-1/4) cos 4 Ψ {sin ² (δ/2)} Equation (5)
In a state where the traveling direction of light in the silicon carbide substrate 1 is set to the direction along the <0001> direction of the silicon carbide substrate 1 as shown in Fig. 4, the natural birefringence caused by the crystal structure of silicon carbide can be reduced, and the above-mentioned phase difference value becomes relatively small. Since the measurement is performed under conditions in which the natural birefringence is suppressed as described above, the strain can be evaluated with high accuracy by the phase difference caused by the birefringence using photoelasticity. On the other hand, when the traveling direction of light in the silicon carbide substrate 1 is not along the <0001> direction, the influence of the natural birefringence becomes relatively large, and the above-mentioned phase difference value becomes relatively large, making it difficult to accurately measure the strain.

In order to make the direction of light travel in the silicon carbide substrate 1 follow the <0001> direction, it is necessary to take into account the difference in refractive index between the atmosphere surrounding the silicon carbide substrate 1 and the silicon carbide substrate 1. That is, light is refracted when it enters the silicon carbide substrate 1 from the atmosphere side. The refraction of light at the interface between the atmosphere and the silicon carbide substrate 1 can be considered, for example, based on Snell's law.

Here, in a compound semiconductor substrate such as silicon carbide substrate 1, the substrate may have a shape defect such as warping. In such a case, the inclination of the main surface of silicon carbide substrate 1 mounted on stage 24 with respect to the reference plane of stage 24 varies depending on the position within the main surface. Therefore, even if the incident direction of light from incident side optical system 41 to silicon carbide substrate 1 is determined in advance using Snell's law so that the traveling direction of light in silicon carbide substrate 1 is the <0001> direction, the actual traveling direction of light in silicon carbide substrate 1 may deviate from the <0001> direction.

Therefore, in this embodiment, the off-direction and off-angle from the (0001) plane at each position on the main surface of the silicon carbide substrate 1 are previously examined for one measurement point on the main surface of the silicon carbide substrate 1. The incident angle of the incident light is determined so that the traveling direction of the light in the silicon carbide substrate 1 is the <0001> direction, and the above steps (S30), (S21), and (S22) are performed. The off-direction and off-angle from the (0001) plane at each position on the main surface of the silicon carbide substrate 1 can be measured, for example, by X-ray diffraction. In this way, even if the shape of the silicon carbide substrate 1 is not flat, data on the phase difference δ and the principal vibration azimuth angle Ψ with the influence of natural birefringence reduced can be obtained at the measurement point on the surface of the silicon carbide substrate 1.

For example, consider the case where light with a wavelength of 950 nm is irradiated onto a silicon carbide substrate with an off-angle of 4°. In this case, for example, the refractive index n1 of air on the incident side is set to 1, and the refractive index n2 of silicon carbide on the exit side is set to 2.2. Applying these conditions to Snell's law, it is considered that when light is irradiated onto the main surface of the silicon carbide substrate 1 from a direction inclined at 9° to the normal to the main surface of the silicon carbide substrate 1, the traveling direction of the light in the silicon carbide is approximately parallel to the <0001> direction. Note that for each of the multiple measurement points on the main surface of the silicon carbide substrate 1, the light irradiation direction is determined in advance taking into account the shape, such as warpage, of the silicon carbide substrate 1 mounted on the stage 24.

The above-mentioned two types of transmitted light intensity data are measured at multiple measurement points on the surface of the silicon carbide substrate 1. The measurement points may be arranged, for example, in a grid pattern on the surface of the silicon carbide substrate 1. The distance between the measurement points may be, for example, 0.1 mm or more and 50 mm or less, 0.3 mm or more and 2 mm or less, or 0.4 mm or more and 0.6 mm or less.

By determining the birefringence phase difference δ for each measurement point in the manner described above, the maximum value and standard deviation of the phase difference δ can be determined for multiple measurement points across the entire substrate.

Residual strain evaluation step (S23):
Next, the residual strain of the silicon carbide substrate 1 is evaluated. Any method can be adopted as the evaluation method, but for example, the method disclosed in Japanese Patent No. 3156382 (JP Patent Publication No. 5-339100) can be used. Specifically, the residual strain of the silicon carbide substrate can be calculated from the absolute value |Sr-St| of the difference between the radial strain Sr and the tangential strain St. In this calculation, |Sr-St| can be calculated as shown in the following formulas (6) and (7).

|Sr-St|=kδ[{cos2Ψ/( _P11 - _P12 )} ² +{(sin2Ψ)/ _P44 } ^1/ 2Equation (6)
However, in the above formula (6),
k = λ / (πdn ₀ ³ ) Equation (7)
Here, the wavelength of light used in the measurement is λ, the thickness of the silicon carbide substrate 1 used in the measurement is d, the refractive index of the silicon carbide substrate 1 is _n0 , and the photoelastic constants of the silicon carbide substrate 1 are _P11 , _P12 , and _P44 . Here, the refractive index _n0 is the refractive index when light is transmitted along the <0001> direction.

In this way, the phase difference δ and the principal vibration azimuth angle Ψ caused by the birefringence of the silicon carbide substrate 1 can be obtained using the photoelastic method, thereby obtaining the distortion of the silicon carbide substrate 1. Note that the above-mentioned measurement process (S21) to the residual distortion evaluation process (S23) correspond to the evaluation process (S20) shown in FIG. 1.

<Effects of the evaluation method>
According to the evaluation method as described above, data can be obtained as measurement results when evaluation light is transmitted through silicon carbide substrate 1 along the <0001> direction, so that distortion (residual distortion) can be evaluated while reducing the influence of natural birefringence caused by the crystal structure of silicon carbide substrate 1.

Here, cubic compound semiconductor crystals are optically isotropic. Therefore, there is no anisotropy in the refractive index. Therefore, no natural birefringence occurs regardless of the direction from which light is transmitted, making it easy to accurately measure the distortion of the crystal.

On the other hand, in a hexagonal compound semiconductor, like silicon carbide described above, the refractive index differs between when light is transmitted parallel to the <0001> direction and when it is transmitted along a direction intersecting the <0001> direction. As a result, when the optical axis of light transmitted through a hexagonal compound semiconductor deviates from the <0001> direction by a certain angle or more, the effect of natural birefringence cannot be ignored, and it is not easy to accurately evaluate the distortion of the crystal. The direction in which light is transmitted through a hexagonal compound semiconductor is preferably within ±3° from the <0001> direction, more preferably within ±2°, and even more preferably within ±1°.

In the evaluation method, in the evaluation step (S20), the strain at multiple locations on the silicon carbide substrate 1 is evaluated by irradiating light to multiple measurement points on the surface of the silicon carbide substrate 1. Specifically, the evaluation step (S20) includes a step of determining the phase difference δ caused by birefringence at the multiple locations, and a step of calculating the maximum value and standard deviation for the phase difference δ at the multiple locations. From a different perspective, in the step (S21) of transmitting light through the substrate, light is irradiated to multiple locations on the first main surface. In the step (S22) of calculating the phase difference caused by birefringence, the phase difference at multiple locations is calculated. Note that the evaluation step (S20) may include a step of determining the principal vibration azimuth angle Ψ at the multiple locations. In this case, the distribution state of strain on the surface of the silicon carbide substrate 1 can be evaluated.

In the above evaluation method, the silicon carbide substrate 1 may be an off-substrate in which the surface 4 as the first main surface is inclined with respect to the {0001} plane. In this case, in the evaluation step (S20), light passes through the silicon carbide substrate 1 along the <0001> direction, which corresponds to the off angle that is the angle at which the surface 4 is inclined with respect to the {0001} plane as shown in FIG. 4 (the direction indicated by the arrow 3) inclined with respect to the surface 4. The above evaluation method may include a step (S30) of setting the light irradiation direction to a tilted direction with respect to the surface 4 of the silicon carbide substrate 1 so that the light passes through the silicon carbide substrate 1 along the <0001> direction as shown by the arrow 3 in FIG. In this case, the strain of the silicon carbide substrate 1, which is an off-substrate, can be evaluated under conditions that reduce the effect of natural birefringence.

From a different perspective, in the above-mentioned distortion evaluation method, in the evaluation step (S20), the angle of incidence of light with respect to the principal surface is determined for each measurement point on the principal surface based on information on the inclination angle (off angle) and inclination direction (off direction) of the principal surface with respect to the {0001} plane that have been previously measured. In this case, even when the shape of the silicon carbide substrate 1 is not flat, for example because the silicon carbide substrate 1 is warped, the off angle and off direction (apparent off angle and off direction) that take the shape into consideration are previously obtained for each measurement point, and as a result, an accurate distortion evaluation can be performed with reduced effects of natural birefringence due to the crystal structure of silicon carbide.

In the above evaluation method, the substrate to be evaluated may include a silicon carbide substrate 1 as a compound semiconductor substrate, and a silicon carbide epitaxial layer as a compound semiconductor layer homoepitaxially grown on the silicon carbide substrate. In this case, it is possible to confirm the occurrence of distortion in the silicon carbide substrate on which the epitaxial layer is formed (silicon carbide substrate with epitaxial layer).

<Method of Manufacturing Compound Semiconductor>
Hereinafter, a method for manufacturing a hexagonal compound semiconductor to which the above-mentioned evaluation method is applied will be described. As shown in FIG. 5, in the method for manufacturing a hexagonal compound semiconductor, a substrate preparation step (S100) is first performed. In this step (S100), a compound semiconductor substrate used for manufacturing a compound semiconductor device is prepared. Hereinafter, silicon carbide will be described as an example of a hexagonal compound semiconductor. In the step (S100), a hexagonal compound semiconductor substrate having a hexagonal crystal structure, birefringence, and including a surface 4 as a first main surface, a second main surface opposite to the first main surface, and a side surface is prepared. Hereinafter, a silicon carbide substrate is prepared as an example of a compound semiconductor substrate. As the silicon carbide substrate, an off substrate having an off angle of the surface with respect to the (0001) plane (C plane) of more than 0° and not more than 8° is used. The diameter of the silicon carbide substrate is, for example, 100 mm or more. The diameter of the silicon carbide substrate may be 150 mm or more, or may be 200 mm or more. The thickness of the silicon carbide substrate may be, for example, 200 μm or more and 1 mm or less. The silicon carbide substrate may have a diameter of 300 mm or less.

Next, an inspection step (S200) is performed as a step of calculating the first phase difference. In this step (S200), the distortion of the silicon carbide substrate is evaluated using the evaluation method shown in Figures 1 to 4. Specifically, the off-direction and off-angle from the (0001) plane at each position on the main surface of the silicon carbide substrate are previously examined for multiple locations on the surface of the silicon carbide substrate. Based on the off-angle and off-direction, the incident angle of the incident light is determined in advance so that the traveling direction of the light in the silicon carbide substrate is the <0001> direction, and the above steps (S30), (S21), and (S22) are performed. As a result, a phase difference value in which natural birefringence at the measurement point is suppressed is obtained. In addition, the principal vibration azimuth angle Ψ may be obtained at the same time. Thereafter, the distortion is obtained as described in the above-mentioned residual distortion evaluation step (S23).

From a different perspective, in this step (S200), the surface 4 as the first main surface is irradiated with light that passes through the substrate, and the light is transmitted through the substrate along the <0001> direction in the substrate as shown in FIG. 4, and the first phase difference δ caused by birefringence is calculated. The above step (S200) may be performed at a plurality of measurement points as a plurality of locations on the surface 4 as the first main surface. Note that the maximum value and standard deviation of the birefringence phase difference δ may be obtained for each of the plurality of measurement points, and the variation in the distribution of strain and the magnitude of strain may be evaluated. In this way, the step (S210) of obtaining the above maximum value and standard deviation corresponds to the step of calculating the first maximum value and first standard deviation.

Next, a film formation step (S300) is carried out as a step for homoepitaxially growing a compound semiconductor layer. In this step (S300), a silicon carbide epitaxial layer is formed as a compound semiconductor layer on the surface 4 of the silicon carbide substrate based on predetermined growth conditions. Any conventionally known conditions can be used as the film formation conditions for the silicon carbide epitaxial layer.

Next, an inspection step (S400) after the film formation is performed. In the step (S400) for calculating the second phase difference, the same evaluation method as in the inspection step (S200) is used to evaluate the distortion of the silicon carbide epitaxial substrate in which a silicon carbide epitaxial layer is formed on the surface of the silicon carbide substrate as the base substrate prepared in the substrate preparation step. From a different perspective, in this step (S400), light is irradiated onto the surface of the silicon carbide epitaxial layer as the compound semiconductor layer, and the light is transmitted through the silicon carbide epitaxial layer and the substrate along the <0001> direction in the silicon carbide epitaxial layer and the substrate to calculate the second phase difference δ caused by birefringence. The above step (S400) may be performed at multiple measurement points, which are multiple locations on the surface of the silicon carbide epitaxial layer.

In the evaluation of the distortion in this inspection step (S400), the birefringence phase difference may be evaluated as in the above inspection step (S200), or the distortion value may be evaluated directly. Alternatively, the maximum value and standard deviation of the birefringence phase difference δ may be evaluated for multiple measurement points. The step (S410) of determining the maximum value and standard deviation in this manner corresponds to the step of calculating the second maximum value and second standard deviation.

Next, a step (S500) is carried out to determine whether the condition for changing the film formation conditions is satisfied. In this step (S500), the data obtained in the steps (S200) and (S400) are compared, and if the distortion increases due to the film formation in the film formation step (S300), it is determined that the condition for changing the film formation conditions is satisfied. In this step (S500), for example, it may be determined whether the condition that the second maximum value, which is the maximum value of the phase difference δ of the birefringence obtained in the step (S410), is greater than the first maximum value, which is the maximum value of the phase difference δ of the birefringence obtained in the step (S210), is satisfied. In addition, as a determination condition in the step (S500), a condition that the second standard deviation obtained in the step (S410) is greater than the first standard deviation obtained in the step (S210) may be adopted in addition to the condition regarding the maximum value. If the answer is YES in this step (S500), the process proceeds to the step (S600) of changing the film formation conditions, and the film formation conditions in the next step (S300) are changed. After the step (S600) of changing the growth conditions, the process proceeds to the post-processing step (S700) described below. Also, if the answer is NO in step (S500), the process proceeds directly to the post-processing step (S700).

Next, a post-processing step (S700) is performed. In this post-processing step (S700), ion implantation into the formed silicon carbide epitaxial layer and the formation of conductive layers such as electrodes and wiring are performed. A plurality of silicon carbide semiconductor elements are simultaneously formed on the surface of the silicon carbide substrate. The silicon carbide substrate is then divided into individual elements to obtain silicon carbide semiconductor devices as an example of compound semiconductors. The divided elements (silicon carbide semiconductor devices) may be incorporated into a module for achieving a specified function (for example, a module for power control) to form a semiconductor module.

<Effects of compound semiconductor manufacturing method>
According to the above-mentioned method for manufacturing a hexagonal compound semiconductor, the phase difference δ is calculated in a state where light is transmitted through the compound semiconductor along the <0001> direction, so that the phase difference δ in which the influence of natural birefringence caused by the crystal structure of the compound semiconductor is reduced can be obtained. Therefore, a silicon carbide substrate as an example of a compound semiconductor in which the distortion is evaluated by the phase difference δ caused by birefringence can be obtained in a state where the influence of the natural birefringence is reduced. In addition, since the distortion can be evaluated by the phase difference δ caused by birefringence before and after the silicon carbide epitaxial layer as the compound semiconductor layer is formed on the compound semiconductor substrate, the influence on the distortion by the film formation step (S300) of the silicon carbide epitaxial layer can be evaluated. Furthermore, the occurrence state of distortion in the silicon carbide substrate with the epitaxial layer on which the silicon carbide epitaxial layer is formed can be confirmed. Then, by comparing the distortion after the silicon carbide epitaxial layer is formed with the distortion before the silicon carbide epitaxial layer is formed, it becomes possible to determine the suitability of the growth conditions in the epitaxial growth by grasping the amount of distortion generated during the epitaxial growth.

In addition, since the phase difference is calculated for multiple locations on the surface 4 of the substrate, and the magnitude and variation of the phase difference δ are correlated with the magnitude and variation of the distortion in the substrate, the distribution state of the distortion within the surface of the substrate can be evaluated. In other words, it is possible to obtain a compound semiconductor in which the distribution state of the distortion within the surface of the substrate can be evaluated using the phase difference δ caused by birefringence while reducing the influence of the natural birefringence.

In the above-mentioned method for manufacturing a hexagonal compound semiconductor, the substrate has a first main surface, surface 4, that is inclined with respect to the {0001} plane. In step (S200), the angle of incidence of light with respect to surface 4 is determined for each of a plurality of points on surface 4 based on information on the inclination angle (off angle) and inclination direction (off direction) of surface 4 with respect to the {0001} plane that have been previously measured. In step (S400), the angle of incidence of light with respect to the surface of the epitaxial layer, which is a compound semiconductor layer, is determined for each of a plurality of points on the surface of the epitaxial layer based on the above information.

In this case, even if the substrate is warped or otherwise not flat, by obtaining information such as the substrate's off-angle and off-direction in advance, it is possible to accurately evaluate the strain by utilizing the photoelasticity when light passes through the compound semiconductor along the <0001> direction. In other words, it is possible to perform an accurate evaluation of the strain with reduced effects of natural birefringence resulting from the crystal structure of the hexagonal compound semiconductor. This makes it possible to accurately determine the distribution of strain within the substrate surface and the suitability of the growth conditions for epitaxial growth.

The above-mentioned method for manufacturing a hexagonal compound semiconductor includes steps (S500) and (S600), and therefore by comparing the maximum value and standard deviation of the phase difference δ before and after the silicon carbide epitaxial layer is formed, it is possible to evaluate the effect of distortion due to the film formation process of the silicon carbide epitaxial layer and adjust the film formation conditions in step (S300) so as to suppress an increase in distortion. Therefore, it is possible to obtain a substrate with an epitaxial layer in which distortion is suppressed.

In the above-mentioned method for manufacturing a hexagonal compound semiconductor, the above-mentioned steps (S200) and (S400) are performed at multiple measurement points on the substrate surface or on the surface of the silicon carbide epitaxial layer, so that accurate evaluation of strain can be performed with reduced effects of natural birefringence due to the crystal structure of the compound semiconductor within the substrate surface.

In the above-mentioned manufacturing method for hexagonal compound semiconductors, the diameter of the substrate is 100 mm or more. Since there is concern about problems caused by distortion in compound semiconductor substrates with such a relatively large diameter, it is preferable to apply the above-mentioned manufacturing method for compound semiconductors.

In the above-mentioned method for manufacturing a hexagonal compound semiconductor, the hexagonal compound semiconductor is a silicon carbide single crystal. Since cracking of the substrate due to distortion is particularly problematic for silicon carbide, it is preferable to apply the above-mentioned manufacturing method.

(Example)
In order to verify the effectiveness of the above-mentioned evaluation method, the following experiment was carried out.

<Silicon carbide substrate>
As an evaluation target, a silicon carbide substrate of polytype 4H having a diameter of 150 mm and a surface off angle of 4° with respect to the (0001) plane was prepared. The silicon carbide substrate had a thickness of 350 μm.

In addition, after carrying out a distortion evaluation process as described below, a silicon carbide epitaxial layer was formed on the silicon carbide substrate under the following conditions.

The epitaxial film _{is grown by CVD using silane (SiH4) and propane (C3H8) as source gases, nitrogen (N2 ) or ammonia} ( _NH3 ) as dopant gas, and hydrogen ( _H2 ) as carrier gas. As described above, the main surface is off-axis with respect to the (0001) plane, so the epitaxial film is formed by step-flow growth. Therefore, the epitaxial film is made of 4H-type silicon carbide like the silicon carbide substrate, and the mixture of different polytypes is suppressed. The thickness of the epitaxial film is, for example, about 10 μm to 50 μm.

In the epitaxial film growth process described above, a source gas having a C/Si ratio of less than 1 is used to epitaxially grow the silicon carbide substrate on the main surface. First, the inside of a channel in which the silicon carbide substrate is disposed in a film forming apparatus is replaced with gas. Then, the inside of the channel is adjusted to a predetermined pressure, for example, 60 mbar to 100 mbar (6 kPa to 10 kPa), while flowing a carrier gas. The carrier gas may be, for example, hydrogen (H ₂ ) gas, argon (Ar) gas, helium (He) gas, or the like. The carrier gas flow rate may be, for example, about 50 slm to 200 slm. Here, the unit of flow rate "slm (Standard Liter per Minute)" indicates "L/min" in standard conditions (0° C., 101.3 kPa).

Next, a predetermined alternating current is supplied to an induction heating coil arranged to surround the channel, thereby inductively heating the heating element 6 arranged around the channel. A susceptor is arranged inside the channel. A silicon carbide substrate is placed on the susceptor. This heats the channel and the susceptor on which the silicon carbide substrate is placed to a predetermined reaction temperature. At this time, the susceptor is heated to, for example, about 1500°C to 1750°C.

<Evaluation method>
The silicon carbide substrate was subjected to evaluation of distortion using the evaluation method according to the present embodiment at each timing before and after forming the silicon carbide epitaxial layer as described above. In the evaluation method, a measurement device having a configuration shown in FIG. 3 was used to irradiate the silicon carbide substrate with light having a wavelength of 950 nm to perform measurement. At this time, the off-angle and off-direction were measured in advance for each measurement point using an X-ray diffraction method. As an example, the measurement was performed on a plurality of measurement points arranged in a lattice pattern with 10 mm intervals on the surface of a silicon carbide substrate having a diameter of 150 mm. The number of measurement points was about 170. Then, the maximum value, average value, and standard deviation of the phase difference δ were calculated.

<Evaluation Results>
Before epitaxial growth:
The birefringence phase difference δ (unit: radian) had a maximum value of 0.425, an average value of 0.149, and a standard deviation of 0.065.

After epitaxial growth:
The birefringence phase difference δ had a maximum value of 0.395, an average value of 0.140, and a standard deviation of 0.060.

From the data before and after the epitaxial growth, the change in the maximum value of the phase difference δ due to the epitaxial growth was 0.030, the change in the average value was 0.009, and the change in the standard deviation was 0.005. It can be seen from the data above that the change in distortion before and after the epitaxial growth is sufficiently small. In this way, the evaluation method according to this embodiment was used to evaluate the state of distortion before and after epitaxial growth for a silicon carbide substrate.

The embodiments disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is indicated by the claims rather than the embodiments described above, and is intended to include all modifications within the scope of the claims and meanings equivalent thereto.

1 Silicon carbide substrate 2 Dotted line 3 Arrow 4 Surface 10 Control controller 11 Control IO bus 21 Light source 22, 26 Lens 23 Polarizer 24 Stage 25 Analyzer 27 Photoreceivers 31 to 35, 39 Drive circuit 36 IV converter 37 Variable gain amplifier 38 AD converter 41 Incident side optical system 42 Light receiving side optical system 43 to 45 Optical axis adjustment unit 100 Measurement device

Claims (6)

A strain evaluation device capable of evaluating strain in a hexagonal compound semiconductor substrate having a hexagonal crystal structure, having birefringence, and including a first main surface, a second main surface opposite to the first main surface, and a side surface,
the first main surface of the hexagonal compound semiconductor substrate is inclined with respect to a {0001} plane,
a stage on which the hexagonal compound semiconductor substrate is mounted;
an incident side optical system that irradiates light onto the first principal surface;
a light receiving optical system that receives light transmitted through the hexagonal compound semiconductor substrate;
and an optical axis adjustment unit that tilts an incident angle of the light with respect to the first principal surface so that the light is transmitted through the hexagonal compound semiconductor substrate along a <0001> direction in the hexagonal compound semiconductor substrate . The hexagonal compound semiconductor distortion evaluation device according to claim 1, wherein the optical axis adjustment unit tilts the incident optical system or the stage. 3. The apparatus for evaluating distortion of a hexagonal compound semiconductor according to claim 1, wherein the incident optical system irradiates the light onto a plurality of points on the first main surface. 4. The device for evaluating distortion of a hexagonal compound semiconductor according to claim 1 , wherein the hexagonal compound semiconductor substrate is a substrate with an epitaxial layer, the substrate comprising a base substrate and an epitaxial layer formed on a surface of the base substrate. 5. The apparatus for evaluating strain in a hexagonal compound semiconductor according to claim 1, wherein the hexagonal compound semiconductor substrate is a silicon carbide substrate. A method for manufacturing a silicon carbide epitaxial substrate, comprising the steps of: forming a silicon carbide epitaxial layer on a silicon carbide substrate which is a hexagonal compound semiconductor substrate, the silicon carbide substrate having a diameter of 150 mm, an off angle of 4° with respect to a (0001) plane, and a polytype of 4H;
The silicon carbide epitaxial layer is formed when the maximum value of the phase difference of birefringence at a plurality of points on the surface of the silicon carbide substrate is determined to be 0.425 or less when the thickness of the silicon carbide substrate is 350 μm;
The phase difference of the birefringence at the plurality of points is
an incident side optical system having a light source and a polarizer through which light from the light source passes;
a stage on which the hexagonal compound semiconductor substrate is mounted and which receives the incident light from the polarizer;
a light-receiving optical system including an analyzer on which transmitted light that has been transmitted through the hexagonal compound semiconductor substrate reaches and a light receiver that receives the light that has been transmitted through the analyzer;
and an optical axis adjusting unit for adjusting an optical axis of the light incident on the hexagonal compound semiconductor substrate from the incident side optical system when the light passes through the substrate,
a light axis adjusting unit adjusting the incident light to a direction along a <0001> direction of the silicon carbide substrate, and a wavelength of the light from the light source being measured at 950 nm;

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