Selection method for silicon carbide semiconductor epitaxial substrates

Abstract

To provide a silicon carbide semiconductor epitaxial substrate of which a basal surface inversion is smaller.SOLUTION: A silicon carbide semiconductor epitaxial substrate includes: a silicon carbide single crystal substrate 1 having a front surface of which an off-set angle is 8° or less, and a mean square roughness is 0.1 nm; and a silicon carbide single-crystal epitaxial layer. A first epitaxial layer 2 contacted to the silicon carbide single crystal substrate is formed under a gas supply condition that C / Si satisfies 1.0 or more and is 1.4 or less, a donor concentration is 5×1018 cm-3 or more, a ratio to a basal surface inversion density of the silicon carbide single crystal substrate to the basal surface inversion density in the second epitaxial layer 3 to be formed onto the first epitaxial layer is 0.1% or less, and a mean square roughness Rq(nm) of the outermost surface of the silicon carbide single-crystal epitaxial layer satisfies a relation of Rq<0.007×V+0.074 against a growing speed V(μm / h).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention can be used in a method for manufacturing silicon carbide semiconductor epitaxial substrates. [Background technology]

[0002] Traditionally, power semiconductor devices (power devices) using silicon (Si) have been developed for the purpose of controlling high frequencies and high power, and various improvements have been made to significantly enhance their characteristics. However, currently, the performance of these power semiconductor devices is approaching the theoretical limit calculated from the physical properties of silicon. Therefore, power semiconductor devices using new semiconductor materials are being considered in order to further improve their characteristics.

[0003] Silicon carbide (SiC) is attracting attention as a semiconductor material for such power semiconductor devices. Because silicon carbide has a dielectric breakdown field that is more than an order of magnitude higher than silicon, it is expected to be applicable to high-voltage devices, and it is also expected to have far superior semiconductor properties compared to silicon, such as excellent heat resistance.

[0004] When fabricating power semiconductor devices using silicon carbide, one possible method is to epitaxially grow a silicon carbide single crystal thin film on a silicon carbide single crystal substrate using a method called chemical vapor deposition, and then fabricate the semiconductor device within this epitaxial layer. This epitaxial layer is grown on the silicon carbide single crystal substrate, for example, by introducing monosilane (SiH4) gas to supply silicon (Si) atoms and propane (C3H8) gas to supply carbon (C) atoms while the silicon carbide single crystal substrate is heated.

[0005] Patent Document 1 (U.S. Patent No. 4,912,064) discloses a method of using a substrate in which the (0001) crystal plane of a silicon carbide single crystal is inclined 3 to 12° with respect to the surface in order to prevent the generation of heterophase during epitaxial growth. Currently, this method is widely adopted when forming an epitaxial layer on a silicon carbide single crystal substrate. The inclination angle of the (0001) crystal plane with respect to the surface is hereinafter referred to as the offset angle.

[0006] In addition, Patent Document 2 (Japanese Patent Application Laid-Open No. 2005-311348) and Non-Patent Document 1 disclose that the basal plane dislocations present in a silicon carbide single crystal substrate propagate to the epitaxial layer, thereby reducing the reliability of a bipolar element or a unipolar element incorporating a bipolar-type parasitic diode.

[0007] Patent Document 3 (Japanese Patent Application Laid-Open No. 2008-4888) discloses that by smoothing the surface of a silicon carbide single crystal substrate before epitaxial growth to a roughness value below a predetermined value by hydrogen etching or chemical mechanical polishing, etc., and further setting the flow rate of the source gas to satisfy predetermined conditions, the basal plane dislocations propagating to the epitaxial layer are reduced.

[0008] Patent Document 4 (Japanese Patent Application Laid-Open No. 9-321323) and Non-Patent Document 2 disclose that in order to enhance the stability of the electrical characteristics of a silicon carbide semiconductor device, an epitaxial layer having a high impurity concentration is provided between the epitaxial layer in which the semiconductor device is fabricated and the silicon carbide single crystal substrate.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0010] [Non-Patent Document 1] Materials Science Forum, 2007, Vol. 600-603, pp. 1127-1130. [Non-Patent Document 2] Product information from Epiworld, October 2018 edition, [Accessed March 13, 2020], Internet<URL:http: / / www.epiworld-cn.com> [Overview of the project] [Problems that the invention aims to solve]

[0011] As disclosed in Patent Document 3, if the surface roughness of the silicon carbide single crystal substrate is smoothed before epitaxial growth and the conditions for epitaxial layer formation are set, a certain reduction in basal plane dislocations can be observed. However, in order to increase the yield rate of semiconductor devices and to use silicon carbide as a power semiconductor device instead of silicon semiconductor devices, it is necessary to further reduce the basal plane dislocation density.

[0012] The object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor epitaxial substrate with fewer basal plane dislocations by solving the above-mentioned problems.

[0013] Other purposes and novel features will become apparent from the description and accompanying drawings herein. [Means for solving the problem]

[0014] A brief overview of some of the representative embodiments disclosed in this application is as follows: It is as follows:

[0015] A typical embodiment of the method for manufacturing a silicon carbide semiconductor epitaxial substrate includes a first step of preparing a silicon carbide single crystal substrate having a surface with an offset angle of 0° to 8° and a mean square roughness of 0.1 nm or less, and a second step of sequentially growing a plurality of epitaxial layers made of silicon carbide on the silicon carbide single crystal substrate by chemical vapor deposition, wherein the growth conditions of the plurality of epitaxial layers are set such that the mean square roughness Rq (nm) of the outermost surface of the plurality of epitaxial layers satisfies the relationship Rq (nm) < 0.007 × V (μm / h) + 0.074, where V (μm / h) is the growth rate of the plurality of epitaxial layers, and the donor concentration of the first epitaxial layer in contact with the silicon carbide single crystal substrate is set to 5 × 10 18 cm -3 The above is 2 x 10 19 cm -3 The settings are as follows: [Effects of the Invention]

[0016] According to a typical embodiment, a silicon carbide semiconductor epitaxial substrate having a high-quality epitaxial layer with few basal plane dislocations can be obtained. The reliability and yield rate of semiconductor devices fabricated using this substrate can be improved. [Brief explanation of the drawing]

[0017] [Figure 1] This is a cross-sectional view showing a silicon carbide semiconductor epitaxial substrate, which is an embodiment of the product. [Figure 2] This is a cross-sectional view illustrating how basal plane dislocations in a silicon carbide single crystal substrate are converted into through-edge dislocations in a silicon carbide semiconductor epitaxial substrate, which comprises two epitaxial layers on a silicon carbide single crystal substrate. [Figure 3] This is a cross-sectional view of the manufacturing process of a silicon carbide semiconductor epitaxial substrate, which is an embodiment of the product. [Figure 4] Figure 3 shows a cross-sectional view of the manufacturing process of a silicon carbide semiconductor epitaxial substrate. [Figure 5]Figure 4 shows a cross-sectional view of the manufacturing process of a silicon carbide semiconductor epitaxial substrate. [Figure 6] This graph shows the results from Example 1, illustrating the relationship between nitrogen supply during epitaxial growth and the donor concentration in the epitaxial layer. [Figure 7] This graph shows the results from Example 1, illustrating the relationship between the ratio of nitrogen supply to carbon supply during epitaxial growth and the donor concentration in the epitaxial layer. [Figure 8] This graph shows the results from Example 2, illustrating the relationship between the donor concentration in the first epitaxial layer and the basal plane dislocation density in the second epitaxial layer. [Figure 9] This is a cross-sectional view illustrating a basal plane dislocation present in a silicon carbide single crystal substrate, which is a comparative example. [Figure 10] This is a cross-sectional view illustrating how basal plane dislocations of a silicon carbide single crystal substrate extend into an epitaxial layer formed on a silicon carbide single crystal substrate, which is a comparative example. [Figure 11] This is a cross-sectional view illustrating the process by which basal plane dislocations of the silicon carbide single crystal substrate are converted into through-edge dislocations at the interface between the silicon carbide single crystal substrate and the epitaxial layer, or within the epitaxial layer, as a comparative example. [Modes for carrying out the invention]

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings used to describe the embodiments, the same reference numerals are used for members having the same function, and repeated descriptions of them will be omitted. In addition, in the embodiments, descriptions of the same or similar parts will not be repeated unless particularly necessary.

[0019] In this context, a silicon carbide single crystal substrate may simply be referred to as a substrate. Furthermore, a substrate having a laminated structure in which an epitaxial layer is formed on a semiconductor substrate is referred to as an epitaxial substrate or a silicon carbide semiconductor epitaxial substrate.

[0020] (Embodiment) <Details of areas for improvement> The following details areas for improvement.

[0021] The inventors of this invention formed an epitaxial layer on a silicon carbide single crystal substrate (silicon carbide semiconductor substrate) under various conditions and conducted a detailed study on the conditions under which the basal plane dislocation density in the epitaxial layer decreases. As a result, as disclosed in Patent Document 3, it was confirmed that smoothing the surface of the silicon carbide single crystal substrate before epitaxial growth and selecting the growth rate of the epitaxial layer so that the surface roughness of the epitaxial layer after growth satisfies predetermined conditions is effective in reducing basal plane dislocations. In addition, the inventors found that forming at least two epitaxial layers on the silicon carbide single crystal substrate and setting the donor concentration of the first epitaxial layer in contact with the silicon carbide single crystal substrate within a predetermined range is important for further reducing basal plane dislocations.

[0022] Here, basal plane dislocations in silicon carbide single crystal substrates and epitaxial layers will be explained using Figures 9 to 11.

[0023] As disclosed in Patent Document 1, when forming an epitaxial layer on a silicon carbide single crystal substrate, it is conceivable to use a silicon carbide single crystal substrate in which the (0001) crystal plane (sometimes called the basal plane) is inclined with respect to the main plane of the silicon carbide single crystal substrate. As shown in Figure 9 as a comparative example, the silicon carbide single crystal substrate 1 has a back surface and a main plane (front surface) on the opposite side of the back surface. As the silicon carbide single crystal substrate 1, for example, a substrate having an inclination angle (offset angle) θ of about 2 to 8° is used. Basal plane dislocations (BPDs) are linear crystal defects that occur in the silicon carbide single crystal substrate 1 parallel to the (0001) crystal plane. In Figure 9, a cross-section of the silicon carbide single crystal substrate 1 is shown from a direction in which the inclined (0001) crystal plane appears as a straight line. Therefore, basal plane dislocations occurring in a direction parallel to the (0001) crystal plane are represented as straight lines extending diagonally from the back side to the front side of the silicon carbide single crystal substrate 1. This method of illustrating basal plane dislocations is the same in Figures 10, 11, and Figure 2, which will be used in a later explanation.

[0024] These basal plane dislocations include those that were originally present in the single crystal mass before the silicon carbide single crystal substrate 1 was cut, as illustrated by basal plane dislocation 11, and those that were generated when the plate-shaped silicon carbide single crystal substrate 1 was cut from the single crystal mass and processed, as illustrated by basal plane dislocation 12. The density of basal plane dislocations 11 originally present in the silicon carbide single crystal substrate 1 is, for example, 100 to 3000 cm³. -2 To that extent, the (0001) crystal plane is tilted by an offset angle θ with respect to the surface of the silicon carbide single crystal substrate 1, so the basal plane dislocation 11 penetrates the silicon carbide single crystal substrate 1 from the back side to the front side along this tilted (0001) crystal plane.

[0025] The basal plane dislocations 12 generated by the processing occur only near the surface of the silicon carbide single crystal substrate 1. Therefore, the basal plane dislocations 12 can be removed along with the surface region of the silicon carbide single crystal substrate 1 by hydrogen gas etching or chemical mechanical polishing of the substrate surface before epitaxial growth.

[0026] However, basal plane dislocations 11 that occur deep within the silicon carbide single crystal substrate 1 are extremely difficult to remove physically. When forming an epitaxial layer on such a silicon carbide single crystal substrate 1 having basal plane dislocations, the basal plane dislocations 11 exposed on the surface of the silicon carbide single crystal substrate 1 and the disorder of the atomic arrangement around the basal plane dislocations 11 propagate to the epitaxial layer.

[0027] As shown in Figure 10 as a comparative example, when an epitaxial layer 4 made of silicon carbide is formed on a silicon carbide single crystal substrate 1, the basal plane dislocation 11 may extend directly into the epitaxial layer 4 as a basal plane dislocation 41. Also, as shown in Figure 11 as a comparative example, when an epitaxial layer 4 made of silicon carbide is formed on a silicon carbide single crystal substrate 1, the basal plane dislocation 11 may be converted into a threading edge dislocation (TED) 42 or 43. A threading edge dislocation 42 is formed when a basal plane dislocation 11 is converted into a threading edge dislocation at the interface 4a between the silicon carbide single crystal substrate 1 and the epitaxial layer 4. On the other hand, a threading edge dislocation 43 is formed when a basal plane dislocation 11 extends into the epitaxial layer 4 and is then converted into a threading edge dislocation 43 within the epitaxial layer 4. In this case, even if the basal dislocation 11 is converted into a through-edge dislocation 43 in the epitaxial layer 4, the basal dislocation 44 remains in the epitaxial layer 4.

[0028] Basis plane dislocations and through-edge dislocations have significantly different properties. When observed closely, basis plane dislocation 41 is separated into two partial dislocations separated by a few nanometers. The narrow region between these two partial dislocations is a planar crystal defect called a stacking fault. When a predetermined energy is imparted to basis plane dislocation 41 through recombination of electrons and holes in the epitaxial layer 4, this stacking fault region gradually expands along a plane parallel to the (0001) crystal plane. The electrical resistance when current flows across this planarly expanded stacking fault is greater than the electrical resistance in areas where there is no stacking fault.

[0029] When a bipolar element, or a unipolar element containing a bipolar parasitic diode, is energized, recombination of electrons and holes occurs in the epitaxial layer. Therefore, if a basal plane dislocation occurs in the region functioning as a semiconductor element, the stacking fault expands over time, as described above, and the characteristics of the semiconductor element change over time. This means that basal plane dislocations have a significant negative impact on the reliability characteristics of the semiconductor element.

[0030] On the other hand, through-edge dislocations 42 are linear defects perpendicular to the (0001) crystal plane. They are stable and do not decompose into partial dislocations accompanied by stacking faults, so the defects do not expand over time. For this reason, through-edge dislocations 42 do not adversely affect the device characteristics or reliability characteristics of semiconductor devices and are harmless.

[0031] Although the through-edge dislocation 43 is harmless in itself, similar to the through-edge dislocation 42, it is accompanied by a basal plane dislocation 44 in the epitaxial layer 4, and the portion of the basal plane dislocation 44 affects the reliability of the device as described above.

[0032] Therefore, reducing the basal plane dislocations 41 and 44 in the epitaxial layer 4, that is, converting as many basal plane dislocations 11 as possible into through-edge dislocations 42 at the interface 4a between the silicon carbide single crystal substrate 1 and the epitaxial layer, is a crucial requirement for the epitaxial layer in order to form a highly reliable semiconductor device.

[0033] Based on the above, there is room for improvement in silicon carbide semiconductor epitaxial substrates from the perspective of enhancing the reliability of semiconductor devices.

[0034] Therefore, this embodiment incorporates measures to address the aforementioned areas for improvement. Below, we will describe a growth method that reduces the density of basal plane dislocations in the epitaxial layer, as a technical concept in this embodiment that incorporates these measures.

[0035] <Structure and manufacturing method of silicon carbide semiconductor epitaxial substrate> Hereinafter, the structure and manufacturing method of the silicon carbide semiconductor epitaxial substrate of the present embodiment will be described with reference to FIGS. 1 to 5.

[0036] According to Patent Document 3, the conversion of basal plane dislocations into screw dislocations is deeply related to the smoothness of the growing epitaxial layer and the growth rate of the epitaxial layer.

[0037] Although it is difficult to measure the surface roughness of the growing epitaxial layer surface, the surface roughness of the growing epitaxial layer surface is generally proportional to the surface roughness of the formed epitaxial layer surface. Therefore, in Patent Document 3, a silicon carbide single crystal substrate with a root mean square roughness of 0.1 nm or less on the surface is used, and the root mean square roughness Rq (nm) of the epitaxial layer surface after growth is expressed as a function of the growth rate V (μm / h) of the epitaxial layer, and it is disclosed that an epitaxial layer with a low basal plane dislocation density can be formed by controlling the growth conditions of the epitaxial layer so as to satisfy the following formula (1). Rq (nm) < 0.007 × V (μm / h) + 0.074 ····(1)

[0038] As shown in FIG. 1, the silicon carbide semiconductor epitaxial substrate of the present embodiment used for forming a semiconductor element has a structure in which at least two layers of silicon carbide epitaxial layers are stacked on a silicon carbide single crystal substrate 1. The first epitaxial layer (first silicon carbide epitaxial layer) 2 in contact with the silicon carbide single crystal substrate 1 has a relatively high donor concentration (impurity concentration). In an epitaxial substrate, the thickness of the first epitaxial layer on the silicon carbide single crystal substrate is about 0.5 μm, and the donor concentration is 1×10 18 cm -3 It is considered that this is often the case. On the other hand, the film thickness and donor concentration of the second epitaxial layer (second silicon carbide epitaxial layer) 3 are appropriately designed according to the withstand voltage specifications of the semiconductor element formed on the epitaxial substrate. For example, in the case of an element with a withstand voltage of 1 kV, the film thickness of the second epitaxial layer 3 is 10 μm, and the donor concentration is 1×10 16 cm -3To that extent, since the function of the semiconductor device is realized by creating structures such as pn junctions in the second epitaxial layer 3, it is important to reduce basal plane dislocations in the second epitaxial layer 3 from the viewpoint of improving the reliability of the semiconductor device.

[0039] In their investigation of conditions for reducing basal plane dislocations in the second epitaxial layer, the inventors particularly investigated the effect of the donor concentration in the first epitaxial layer on the basal plane dislocation density in the second epitaxial layer.

[0040] As a result, the donor concentration in the first epitaxial layer was 1 × 10⁻⁶ 18 cm -3 We found that increasing the concentration of donor in the first epitaxial layer was effective in reducing basal plane dislocations in the second epitaxial layer. In particular, we found that increasing the donor concentration in the first epitaxial layer by 5 × 10⁻⁶ 18 cm -3 In the above cases, it was found that the effect of reducing basal dislocations in the second epitaxial layer was significant.

[0041] The following describes the transformation of basal plane dislocations in two epitaxy layers on a silicon carbide single crystal substrate.

[0042] Figure 2 illustrates the transformation of basal plane dislocations in stacked epitaxial layers. Most of the basal plane dislocations 11 in the silicon carbide single crystal substrate 1 are transformed into through-edge dislocations 21 at the interface 1a between the silicon carbide single crystal substrate 1 and the first epitaxial layer 2. The remaining basal plane dislocations that were not transformed propagate through the first epitaxial layer 2, and some of them are transformed into through-edge dislocations 22 in the first epitaxial layer 2. The remaining basal plane dislocations reach the interface 2a between the first epitaxial layer 2 and the second epitaxial layer 3, and some of these basal plane dislocations are then transformed into through-edge dislocations 31 near the interface 2a. The remaining basal plane dislocations then propagate through the second epitaxial layer, and some of these basal plane dislocations 34 are transformed into through-edge dislocations 32 in the second epitaxial layer 3. However, the basal dislocations 34 that propagated through the second epitaxial layer 3 before being converted into through-edge dislocations 32 remain as they are. The remaining basal dislocations 33 that were not converted into through-edge dislocations reach the surface 3a of the second epitaxial layer.

[0043] Figure 2 illustrates the defects (dislocations) that may occur in an epitaxial substrate, using the epitaxial substrate of this embodiment as an example. This embodiment satisfies equation (1) and reduces basal plane dislocations 33 and 34 in the second epitaxial layer 3 by setting the donor concentration of the first epitaxial layer relatively high as described below. The donor referred to here is, for example, N (nitrogen), and the silicon carbide single crystal substrate 1, the first epitaxial layer 2, and the second epitaxial layer 3 all have n-type conductivity.

[0044] The inventors have experimentally discovered the following regarding the case where a single epitaxial layer 4 is formed on a silicon carbide single crystal substrate 1, as shown in the comparative example in Figure 10 or Figure 11. Specifically, the inventors have found that the larger the difference between the donor concentrations of the silicon carbide single crystal substrate 1 and the epitaxial layer 4, the greater the rate at which basal plane dislocations 11 are converted to through-edge dislocations 42 at the interface 1a between the silicon carbide single crystal substrate 1 and the epitaxial layer (conversion rate), compared to the case where the difference is small.

[0045] Typically, the donor concentration of silicon carbide single crystal substrate 1 is 5 × 10⁻⁶. 18 cm -3 It is thought that this is often set to a certain degree. In the epitaxial substrate shown in Figure 2, the donor concentration of the first epitaxial layer 2 is set to 1 × 10⁻¹⁶. 18 cm -3 From 2 x 10 19 cm -3 Even if the degree of variation is changed, the concentration difference between the donor concentration of the first epitaxial layer 2 and the donor concentration of the silicon carbide single crystal substrate 1 does not change significantly. Therefore, it is considered that the conversion rate from basal plane dislocations 11 to through-edge dislocations 21 at the interface 1a between the silicon carbide single crystal substrate 1 and the epitaxial layer does not change significantly depending on the donor concentration of the first epitaxial layer 2. On the other hand, the donor concentration of the second epitaxial layer 3 is typically 3 × 10⁻⁶. 15 cm -3 From 1 x 10 16 cm -3 It is thought that this is often set to a certain degree. For this reason, the donor concentration in the first epitaxial layer 2 is 1 × 10⁻⁶ 18 cm -3 As the concentration increases, the donor concentration difference at the interface 2a between the first epitaxial layer 2 and the second epitaxial layer 3 (the donor concentration difference between the first epitaxial layer 2 and the second epitaxial layer 3) increases significantly. As a result, the conversion rate from basal plane dislocations to through-edge dislocations at interface 2a is expected to increase. That is, the number of through-edge dislocations 31 in Figure 2 increases. As a result, there are cases where basal plane dislocations 33 are formed, and cases where basal plane dislocations 34 are converted to through-edge dislocations. The number of cases where dislocation is at position 32 decreases, and the sum of basal dislocations 33 and basal dislocations 34 in the second epitaxial layer 3 decreases.

[0046] Thus, the conversion rate from basal plane dislocations to through-edge dislocations at the interface 2a between the first epitaxial layer 2 and the second epitaxial layer 3 is expected to increase as the donor concentration of the first epitaxial layer 2 increases. However, if the donor concentration of the first epitaxial layer 2 is 2 × 10⁻¹⁰ 19 cm -3If it exceeds this value, stacking faults are more likely to occur in the first epitaxial layer 2, which is undesirable. Therefore, the donor concentration of the first epitaxial layer 2 should be 5 × 10⁻¹⁰. 18 cm -3 The above is 2 x 10 19 cm -3 The following is preferable.

[0047] From the viewpoint of increasing the conversion rate from basal plane dislocations to through-edge dislocations at the interface 2a between the first epitaxial layer 2 and the second epitaxial layer 3, the donor concentration of the first epitaxial layer 2 is set to 8 × 10⁻¹⁰. 18 cm -3 The above is 2 x 10 19 cm -3 It is more desirable to do the following. Also, from the same perspective, the donor concentration in the first epitaxial layer 2 should be 1 × 10⁻⁶ 19 cm -3 The above is 2 x 10 19 cm -3 The following is even more desirable:

[0048] Furthermore, increasing the thickness of the first epitaxial layer 2 increases the opportunities for dislocation conversion to through-edge dislocations 22 (see Figure 2) within the first epitaxial layer 2, which is also effective in reducing the basal plane dislocation density in the second epitaxial layer 3. However, on the other hand, increasing the thickness of the first epitaxial layer 2 increases the time required for its growth, and increases the risk of defects occurring due to detached material falling from the inner wall of the growth furnace onto the surface of the first epitaxial layer 2 during growth. This also leads to increased manufacturing costs. From these perspectives, an appropriate upper limit for the thickness of the first epitaxial layer 2 can be considered. From the inventors' experiments, a thickness of 10 μm in the first epitaxial layer 2 is sufficient to obtain the dislocation conversion effect. Therefore, by setting the thickness of the first epitaxial layer 2 to 10 μm or less, the possibility of defects occurring due to detached material falling from the inner wall of the growth furnace onto the surface of the first epitaxial layer 2 can be suppressed, and manufacturing costs can be reduced. However, it is preferable that the thickness of the first epitaxial layer 2 be 3 μm or more.

[0049] Furthermore, selecting a silicon carbide single crystal substrate 1 that contains fewer basal plane dislocations 11 is also effective in reducing the basal plane dislocation density in the second epitaxial layer 3.

[0050] To increase the conversion rate from basal plane dislocations 11 to through-edge dislocations 21 at the interface 1a between the silicon carbide single crystal substrate 1 and the first epitaxial layer 2, it is desirable to reduce the offset angle of the silicon carbide single crystal substrate 1. The surface of the silicon carbide single crystal substrate with inclined (0001) crystal planes has a step structure consisting of terraces made of (0001) crystal planes and their edges. The offset angle is, for example, in the range of 0° to 8°.

[0051] If the offset angle is less than 2°, the terrace becomes too wide, and silicon carbide (3C-SiC) with a different structure from the epitaxial layer may grow in the terrace. If this is mixed into the epitaxial layer, it may result in crystal defects. Also, if the offset angle is greater than 5°, the conversion rate at the interface 1a between the silicon carbide single crystal substrate 1 and the first epitaxial layer 2 becomes low, which may increase the basal plane dislocation density in the first epitaxial layer 2. Therefore, an offset angle of 2° to 5° is particularly desirable for the silicon carbide single crystal substrate 1.

[0052] The method for manufacturing a silicon carbide semiconductor epitaxial substrate according to this embodiment will be described in detail below.

[0053] First, a silicon carbide single crystal substrate 1 is prepared as shown in Figure 3. Preferably, the silicon carbide single crystal constituting the silicon carbide single crystal substrate 1 is 4H-SiC. If the surface of the silicon carbide single crystal substrate 1 is considered a (0001) crystal plane, one surface becomes a (0001)Si plane where silicon atoms are exposed at the outermost surface. The other surface, parallel to this, becomes a (000-1)C plane where carbon atoms are exposed. While it is possible to form a silicon carbide epitaxial layer on either surface, the optimal formation conditions may differ. Below, we will describe the case where the (0001)Si plane is used as the growth surface. The offset angle of the (0001)Si plane with respect to the substrate surface is in the range of 0° to 8°. A more preferable range for this offset angle is 2° to 5°.

[0054] The silicon carbide single crystal substrate 1 may be cut from a block of single-crystal silicon carbide using a known method, or a commercially available wafer may be purchased. The wafer prepared here is, for example, a disc-shaped silicon carbide single crystal substrate with a diameter of 6 inches and a thickness of about 350 μm to 400 μm.

[0055] The silicon carbide single crystal substrate 1 is subjected to a known procedure to remove the processed altered layer on its surface and to mechanical polishing until the surface roughness of both the front and back surfaces reaches a predetermined value. Furthermore, the surface 5 of the silicon carbide single crystal substrate 1 on which epitaxial growth is performed is mirror-polished with abrasive grains such as diamond until the surface roughness RMS is 0.2 to 2 nm. Here, surface roughness RMS refers to the value measured by atomic force microscope (AFM) for the mean square roughness of a 10 μm area of ​​the sample. If commercially available wafers are used, it may be possible to omit these steps.

[0056] After mechanical polishing, the surface 5 is further smoothed by chemical mechanical polishing or reactive ion etching, etc., to achieve a surface roughness RMS of 0.1 nm or less.

[0057] If the surface roughness RMS of surface 5 exceeds 0.1 nm, basal plane dislocations present only near surface 5 of the silicon carbide single crystal substrate 1 may not be completely removed and may remain. Furthermore, no matter how the growth conditions of the epitaxial layer are controlled, the surface roughness of the second epitaxial layer may not become sufficiently small, making it impossible to satisfy the conditions of equation (1).

[0058] The surface roughness RMS of surface 5 is more preferably 0.05 nm or less. The smaller the surface roughness RMS of surface 5, the wider the range of growth conditions for the epitaxial layer satisfying equation (1), and the larger the process margin. This makes it possible to manufacture high-quality silicon carbide semiconductor epitaxial substrates more stably.

[0059] Next, as shown in Figure 4, a first epitaxial layer 2 is formed on the silicon carbide single crystal substrate 1 by epitaxial growth.

[0060] Epitaxial growth is carried out by chemical vapor deposition. Specifically, a silicon carbide single crystal substrate 1 is placed in a growth furnace for epitaxial growth, and while supplying hydrogen gas to maintain the furnace pressure at 10 kPa to 30 kPa, the silicon carbide single crystal substrate 1 is heated to 1500°C to 1700°C. The total flow rate of hydrogen gas is preferably 100 slm to 170 slm.

[0061] After reaching a predetermined temperature, the silicon carbide single crystal substrate 1 is held at that temperature and a raw material gas is supplied. The growth of the first epitaxial layer 2 begins with the start of the raw material gas supply, but the surface 5 (see Figure 3) of the silicon carbide single crystal substrate 1 may be hydrogen-etched before the growth of the first epitaxial layer 2. Hydrogen etching can be performed, for example, in an epitaxial growth furnace by holding the silicon carbide single crystal substrate 1 at a constant temperature under the hydrogen atmosphere. The hydrogen gas may contain hydrocarbons such as propane (C3H8) or hydrogen halides such as hydrogen chloride (HCl). This removes the processed altered layer on the substrate surface, removes basal plane dislocations introduced to the substrate surface by processing, and reduces basal plane dislocations propagating to the epitaxial layer. Hydrogen etching is preferably performed at a temperature of 1300°C to 1700°C. If the hydrogen etching temperature is below 1300°C, the processed altered layer may not be completely removed, and basal plane dislocations introduced by the process may remain on the substrate surface. To remove the processed altered layer, a temperature of 1700°C is sufficient; temperatures exceeding this may actually impair the flatness of the substrate surface. The time required for hydrogen etching should be between 1 and 15 minutes.

[0062] For example, monosilane (SiH4) gas is used as the source of silicon atoms, and propane (C3H8) gas is used as the source of carbon atoms. In addition, nitrogen (N2) gas is supplied simultaneously with the source gas to control the donor concentration in the epitaxial layer. The source gas may also contain hydrogen halides such as hydrogen chloride (HCl).

[0063] An epitaxial layer is grown to satisfy equation (1) by controlling the supply ratio of the raw material gas (the ratio of carbon atoms to silicon atoms in the raw material gas, expressed as the C / Si ratio) and the amount of raw material gas supplied. Both the mean square roughness Rq of the surface of the epitaxial layer in equation (1) and the growth rate V of the epitaxial layer are properties that can be measured after the epitaxial layer has been formed. Therefore, an experiment is conducted in advance to grow an epitaxial layer using the C / Si ratio and the amount of raw material gas supplied as parameters. After measuring the mean square roughness Rq and growth rate of the obtained epitaxial layer, the C / Si ratio and the amount of raw material gas supplied that satisfy equation (1) are determined. Then, the obtained C / Si ratio and amount of raw material gas supplied are used as the conditions for epitaxial growth, and epitaxial growth is performed. Taking into consideration factors such as the uniformity of the epitaxial layer thickness or donor concentration, or the suppression of morphological defects on the epitaxial layer surface, it is preferable to set the C / Si ratio to 1.0 or higher and 1.4 or lower.

[0064] The donor concentration in the epitaxial layer is proportional to the nitrogen flow rate supplied and is also correlated with the C / Si ratio mentioned above. The relationship between nitrogen flow rate and donor concentration at a given C / Si ratio should be determined through preliminary experiments, and the nitrogen supply amount should then be set accordingly.

[0065] According to the results of our experiments, the donor concentration in the first epitaxial layer 2 was 5 × 10⁻¹⁰ 18 cm -3 The above is 2 x 10 19 cm -3 When the following conditions were met, the ratio N / C (the ratio of nitrogen flow rate to carbon atoms in the raw gas) was between 4.0 and 110, provided that the C / Si ratio was between 1.0 and 1.4.

[0066] The growth rate of the epitaxial layer is proportional to the flow rate of the raw material gas, but the proportionality constant differs depending on the structure of the chemical vapor deposition (CVA) system. Therefore, the preferred range of raw material gas supply depends on the CVA system. The relationship between the raw material gas supply and the epitaxial layer growth rate should be determined through preliminary experiments, and the growth rate to be adopted should be set accordingly. From the set growth rate, the growth time required to deposit the desired film thickness should be calculated.

[0067] As shown in Figure 5, after a predetermined time has elapsed for the formation of the first epitaxial layer 2, the nitrogen flow rate is changed to the flow rate required for the second epitaxial layer 3, thereby initiating the growth of the second epitaxial layer 3. During the growth of the second epitaxial layer 3, the flow rate of the raw material gas and the C / Si ratio may be changed from the conditions used for forming the first epitaxial layer 2, within the range that satisfies equation (1). In relation to equation (1), the growth rate V here is the growth rate of the stacked epitaxial layer including the first epitaxial layer 2 and the second epitaxial layer 3, and the mean square roughness Rq of the surface (outermost surface) of the epitaxial layer is the mean square roughness of the upper surface of the second epitaxial layer.

[0068] Furthermore, the conditions for growing the second epitaxial layer 3 can be appropriately changed within the range of the conditions described above for growing the first epitaxial layer 2. Specifically, a gas containing carbon atoms and a gas containing silicon atoms, along with a mixed gas of nitrogen and hydrogen to control the donor concentration of the epitaxial layer, are supplied as raw material gases, and the atmospheric pressure is maintained between 10 kPa and 30 kPa. Under these conditions, the second epitaxial layer is grown on a silicon carbide single crystal substrate maintained at a temperature between 1500°C and 1700°C. In addition, the ratio of carbon atoms supplied in the raw material gas to silicon atoms supplied (C / Si) is set to between 1.0 and 1.4.

[0069] After a predetermined time has elapsed for the formation of the second epitaxial layer 3, the supply of raw material gas and nitrogen gas is stopped to halt growth. While maintaining a predetermined pressure in a hydrogen gas stream, substrate heating is stopped and the silicon carbide single crystal substrate 1, on which the first epitaxial layer 2 and the second epitaxial layer 3 have been deposited, is cooled. As a result, the silicon carbide semiconductor epitaxial substrate of this embodiment, including the silicon carbide single crystal substrate 1, the first epitaxial layer 2, and the second epitaxial layer 3, is completed.

[0070] The basal plane dislocations in the second epitaxial layer 3 of the silicon carbide semiconductor epitaxial substrate fabricated in this manner are less than 0.1% of the basal plane dislocations present in the silicon carbide single crystal substrate 1. In other words, the silicon carbide semiconductor epitaxial substrate of this embodiment has a second epitaxial layer with excellent crystal quality. For this reason, semiconductor devices fabricated using the silicon carbide semiconductor epitaxial substrate of this embodiment have excellent reliability.

[0071] Furthermore, as silicon carbide semiconductor epitaxial substrates are repeatedly fabricated, a film of silicon carbide or other materials accumulates on the inner wall of the growth furnace of the chemical vapor deposition apparatus. This film eventually peels off and falls onto the surface of the growing epitaxial layer, causing defects in the epitaxial layer. Therefore, the growth furnace needs to be opened to the atmosphere periodically to clean the inner wall. The optimal growth conditions for the epitaxial layer may change before and after cleaning, but according to the inventors' experiments, the fluctuation was not very large. After cleaning, an experiment is conducted to check the donor concentration, and by making minor adjustments to the nitrogen supply, it is possible to form an epitaxial layer similar to that before cleaning.

[0072] <Example 1> The inventors investigated the relationship between nitrogen supply amount and donor concentration in order to determine the amount of nitrogen supply when forming the first epitaxial layer 2 (see Figure 4).

[0073] First, five 6-inch diameter 4H-SiC single crystal substrates were prepared. Each substrate had an offset angle of 4°. A 10 μm thick silicon carbide epitaxial layer was grown on the (0001) Si plane of four of these substrates. The growth conditions were: hydrogen supply rate 120 slm, growth pressure 20 kPa, growth temperature 1600°C, SiH4 supply rate 270 sccm, and C3H8 supply rate 108 sccm (C / Si = 1.2). The nitrogen supply rates for the five substrates were 0.4 sccm, 10 sccm, 100 sccm, 400 sccm, and 800 sccm, respectively.

[0074] A mercury probe was brought into contact with the surface of the grown epitaxial layer, and a voltage of approximately 1V was applied between the front (main surface) and back surface to measure the correlation between capacitance and voltage. The results were then analyzed to calculate the donor concentration. Figure 6 shows the relationship between the nitrogen supply amount during epitaxial layer growth and the donor concentration of the epitaxial layer. The horizontal axis of Figure 6 shows the nitrogen supply amount when growing the first epitaxial layer, and the vertical axis shows the donor concentration of the grown first epitaxial layer. In Figure 6, both the vertical and horizontal axes are plotted on a logarithmic scale, and the correlation between nitrogen supply amount and donor concentration is a straight line with a slope of approximately 1. From this, it was found that nitrogen supply amount and donor concentration are approximately proportional.

[0075] Similar correlations were investigated for SiH4 supply of 270 sccm, C3H8 supply of 90 sccm (C / Si=1.0), and C3H8 supply of 126 sccm (C / Si=1.4). Figure 7 plots the results against the ratio of nitrogen supply to carbon supply (N / C). The horizontal axis of Figure 7 represents the ratio of nitrogen supply to carbon supply (N / C), and the vertical axis represents the donor concentration in the first epitaxial layer. From Figure 7, it can be seen that in the range of C / Si=1.0 to 1.4, the donor concentration was 5 × 10⁻⁶. 18 cm -3 From 2 x 10 19 cm -3 It was found that the amount of nitrogen supply required to achieve this range is in the range of N / C = 4 to 110 (the range indicated by the thick arrow in Figure 7).

[0076] <Example 2> Three 6-inch diameter 4H-SiC single crystal substrates, A, B, and C, were prepared. All had an offset angle of 4° and a basal plane dislocation density of approximately 400 cm². -2 Therefore, since they were cut from the same mass, it can be assumed that the quality of the other crystals is at a similar level.

[0077] These three substrates (silicon carbide single crystal substrates) A, B, and C were subjected to chemical mechanical polishing. The processing conditions were identical, and the surface roughness RMS after processing was 0.03 nm for all of them.

[0078] A two-layer epitaxial layer was formed on each substrate. The thickness of the first epitaxial layer was 3 μm in both cases. The donor concentration of the first epitaxial layer was 1 × 10⁻¹⁶ for substrate A. 18 cm -3 , board B is 5×10 18 cm -3 , substrate C is 1 × 10 19 cm -3 This was done. The second epitaxial layer was made with a film thickness of 30 μm on substrates A, B, and C, and the donor concentration was set to 3 × 10⁻⁶. 15 cm -3 That's what I decided.

[0079] The only difference between substrates A, B, and C is the donor concentration of the first epitaxial layer. The growth conditions other than the nitrogen supply amount when forming the first epitaxial layer are the same for all substrates, as follows: The growth conditions are a hydrogen supply of 120 slm, a growth pressure of 20 kPa, a growth temperature of 1600°C, a SiH4 supply of 270 sccm, and a C3H8 supply of 108 sccm (C / Si = 1.2).

[0080] Based on the results of Example 1, the nitrogen supply during the formation of the first epitaxial layer was set to 100 sccm (N / C=0.93) for substrate A, 500 sccm (N / C=4.63) for substrate B, and 1000 sccm (N / C=9.26) for substrate C.

[0081] Under the above conditions, the growth rate was 36 μm / h for all substrates A, B, and C.

[0082] After forming the second epitaxial layer, the surface roughness RMS of the second epitaxial layer was evaluated using an atomic force microscope. The results were 0.288 nm under the conditions for substrate A, 0.300 nm under the conditions for substrate B, and 0.295 nm under the conditions for substrate C, showing no significant differences between the substrates.

[0083] The growth rate is 36 μm / h, so the size is 0.007 × 36 (μm / h) + 0.074 = 0.326 (nm). The outermost surface roughness of the second epitaxial layer satisfies equation (1) in all cases of substrates A, B, and C.

[0084] The basal dislocation density was determined by photographing basal dislocations in the epitaxial layer using a method called photoluminescence imaging and measuring their number. The basal dislocation images obtained by this method are limited to those in the epitaxial layer with low donor concentrations. Therefore, basal dislocations in the substrate and the first epitaxial layer with high donor concentrations are not observed, and only basal dislocations in the second epitaxial layer with low donor concentrations, i.e., those corresponding to basal dislocations 33 and 34 in Figure 2, are observed. Figure 8 shows the relationship between the basal dislocation density in the second epitaxial layer and the donor concentration in the first epitaxial layer obtained by this method. In the graph in Figure 8, the horizontal axis shows the donor concentration in the first epitaxial layer, and the vertical axis shows the basal dislocation density in the second epitaxial layer.

[0085] As shown in Figure 8, as the donor concentration in the first epitaxial layer increases, the basal dislocation density in the second epitaxial layer decreases. Equation (1) is satisfied, and the donor concentration in the first epitaxial layer is 5 × 10⁻⁶. 18 cm -3 By doing so, the basal plane dislocation density of the second epitaxial layer can be reduced to 0.1% or less of the basal plane dislocation density of the substrate. This improves the stability of the semiconductor device formed on the second epitaxial layer 3.

[0086] Although the present inventors have described the invention in detail based on its embodiments, the present invention is not limited to the above embodiments and can be modified in various ways without departing from its essence. [Explanation of symbols]

[0087] 1. Silicon carbide single crystal substrate 2. First Epitaxial Layer 3. Second Epitaxial Layer 4. Epitaxial layer

Claims

[Claim 1] A silicon carbide single crystal substrate and A first epitaxial layer in contact with the (0001)Si surface of the silicon carbide single crystal substrate, The first epitaxial layer is in contact with a second epitaxial layer, A silicon carbide semiconductor epitaxial substrate is selected in which the ratio of the basal plane dislocation density of the second epitaxial layer to the basal plane dislocation density of the silicon carbide single crystal substrate is 0.1% or less. A method for selecting a silicon carbide semiconductor epitaxial substrate, (a) The donor concentration of the first epitaxial layer is 5 × 10 １８ cm －３ The above is 2 x 10¹⁹ cm - 3 or less, and, (b) The mean square roughness Rq (nm) of the outermost surface of the second epitaxial layer is Rq < 0.007 × V + 0.074 with respect to the growth rate V (μm / h) of the second epitaxial layer. The silicon carbide semiconductor epitaxial substrate is selected on the condition that it satisfies the following conditions: A method for selecting silicon carbide semiconductor epitaxial substrates.

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