Silicon carbide semiconductor epitaxial substrate

Abstract

To provide a silicon carbide semiconductor epitaxial substrate that has less basal surface dislocation.SOLUTION: This invention disclosed a silicon carbide semiconductor epitaxial substrate, where root mean square roughness Rq(nm) of an outermost surface of a plurality of silicon carbide single crystal epitaxial layers satisfies a relation of Rq(nm)<0.326, donor concentration of a first epitaxial layer coming into contact with a silicon carbide single crystal substrate of the plurality of silicon carbide single crystal epitaxial layers is 5×1018 cm-3 or more and 2×1019 cm-3 or less, and a ratio of the silicon carbide single crystal substrate to basal surface dislocation density, of the basal surface dislocation density of a second epitaxial layer formed on the first epitaxial layer of the plurality of silicon carbide single crystal epitaxial layers, is 0.1% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention can be used in a method for producing a silicon carbide semiconductor epitaxial substrate. [Background technology]

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

[0003] Silicon carbide (SiC) has attracted attention as a semiconductor material for such power semiconductor elements. Silicon carbide has a dielectric breakdown field strength that is at least one order of magnitude higher than that of silicon, making it suitable for use in high-voltage devices. It is also expected to have semiconductor properties that are far superior to silicon, such as excellent heat resistance.

[0004] When fabricating power semiconductor devices using silicon carbide, it is possible 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 semiconductor devices in 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 (US Pat. No. 4,912,064) discloses a method of using a substrate in which the (0001) crystal plane of a silicon carbide single crystal is tilted at 3 to 12 degrees with respect to the surface in order to prevent the generation of heterogeneous phases during epitaxial growth. Currently, this method is widely adopted when forming an epitaxial layer on a silicon carbide single crystal substrate. The tilt angle of the (0001) crystal plane with respect to the surface is hereinafter referred to as the offset angle.

[0006] Furthermore, Patent Document 2 (JP 2005-311348 A) and Non-Patent Document 1 disclose that basal plane dislocations present in a silicon carbide single crystal substrate propagate to an epitaxial layer, thereby reducing the reliability of a bipolar element or a unipolar element incorporating a bipolar-type parasitic diode.

[0007] Patent Document 3 (JP 2008-4888 A) discloses that the number of basal plane dislocations propagating to the epitaxial layer can be reduced by smoothing the surface of a silicon carbide single crystal substrate prior to epitaxial growth to a predetermined roughness value or less by hydrogen etching, chemical mechanical polishing, or the like, and further setting the flow rate of the source gas to satisfy predetermined conditions.

[0008] Patent Document 4 (JP Patent Publication No. 9-321323) and Non-Patent Document 2 disclose that in order to increase the stability of the electrical characteristics of a silicon carbide semiconductor element, an epitaxial layer with a high impurity concentration is provided between the epitaxial layer in which the semiconductor element is fabricated and a silicon carbide single crystal substrate. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Pat. No. 4,912,064 [Patent Document 2] JP 2005-311348 A [Patent Document 3] JP 2008-4888 A [Patent Document 4] Japanese Patent Application Publication No. 9-321323 [Non-patent literature]

[0010] [Non-Patent Document 1] Materials Science Forum, 2007, Vol. 600-603, pp. 1127-1130 [Non-Patent Document 2] Epiworld product information, October 2018 edition, [Retrieved March 13, 2020], Internet<URL:http: / / www.epiworld-cn.com> Summary of the Invention [Problem to be solved by the invention]

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

[0012] An object of the present invention is to solve the above problems and to provide a method for producing a silicon carbide semiconductor epitaxial substrate having fewer basal plane dislocations.

[0013] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0014] A brief summary of a representative embodiment of the present invention will be described below. It is as follows:

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

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

[0017] [Figure 1] 1 is a cross-sectional view showing a silicon carbide semiconductor epitaxial substrate according to an embodiment. [Diagram 2] FIG. 1 is a cross-sectional view illustrating how basal plane dislocations in a silicon carbide single crystal substrate are transformed into threading edge dislocations in a silicon carbide semiconductor epitaxial substrate having two epitaxial layers on a silicon carbide single crystal substrate. [Diagram 3] 1A to 1C are cross-sectional views of a silicon carbide semiconductor epitaxial substrate according to an embodiment during a manufacturing process. [Figure 4] 4 is a cross-sectional view of the silicon carbide semiconductor epitaxial substrate during the manufacturing process following FIG. 3. [Diagram 5]5 is a cross-sectional view of the silicon carbide semiconductor epitaxial substrate during the manufacturing process following FIG. 4. [Figure 6] 4 is a graph showing the results of Example 1, illustrating the relationship between the amount of nitrogen supplied during epitaxial growth and the donor concentration of the epitaxial layer. [Figure 7] 1 is a graph showing the results of Example 1, illustrating the relationship between the ratio of the nitrogen supply amount to the carbon supply amount during epitaxial growth and the donor concentration of the epitaxial layer. [Figure 8] 13 is a graph showing the results of 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] FIG. 2 is a cross-sectional view illustrating basal plane dislocations present in a silicon carbide single crystal substrate as a comparative example. [Figure 10] FIG. 2 is a cross-sectional view illustrating the propagation of basal plane dislocations of a silicon carbide single crystal substrate in an epitaxial layer formed on the silicon carbide single crystal substrate as a comparative example. [Figure 11] FIG. 1 is a cross-sectional view illustrating a state in which basal plane dislocations of a silicon carbide single crystal substrate are converted into threading edge dislocations at the interface between a silicon carbide single crystal substrate and an epitaxial layer, or in the epitaxial layer, which is a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings for explaining the embodiments, the same reference numerals are used for the members having the same functions, and the repeated explanations are omitted. In addition, in the embodiments, the explanations of the same or similar parts are not repeated as a rule, except when it is particularly necessary.

[0019] Herein, the silicon carbide single crystal substrate may be simply referred to as a substrate. Herein, a substrate having a layered 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 areas for improvement are described in detail below.

[0021] The present inventors formed an epitaxial layer on a silicon carbide single crystal substrate (silicon carbide semiconductor substrate) under various conditions, and conducted detailed studies on the conditions under which the basal plane dislocation density in the epitaxial layer is reduced. 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 a predetermined condition is effective in reducing basal plane dislocations. In addition, the present inventors found that it is important to form at least two epitaxial layers on the silicon carbide single crystal substrate and set the donor concentration of the first epitaxial layer in contact with the silicon carbide single crystal substrate to a predetermined range in order to further reduce basal plane dislocations.

[0022] Here, the basal plane dislocations in the silicon carbide single crystal substrate and the epitaxial layer will be described with reference to FIGS.

[0023] As disclosed in Patent Document 1, when forming an epitaxial layer on a silicon carbide single crystal substrate, it is possible 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 surface of the silicon carbide single crystal substrate. As shown in FIG. 9 as a comparative example, a silicon carbide single crystal substrate 1 has a back surface and a main surface (front surface) opposite to 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. A basal plane dislocation (BPD) is a linear crystal defect that occurs in the silicon carbide single crystal substrate 1 parallel to the (0001) crystal plane. 9 shows a cross section of silicon carbide single crystal substrate 1 from a direction in which the inclined (0001) crystal plane appears as a straight line, and therefore basal plane dislocations occurring in a direction parallel to the (0001) crystal plane are shown as straight lines extending obliquely from the back side to the front side of silicon carbide single crystal substrate 1. The manner in which basal plane dislocations are shown in this manner is the same in FIGS. 10 and 11, and in FIG. 2 which will be used in the following explanation.

[0024] The basal plane dislocations include those that were originally present in the single crystal mass before the silicon carbide single crystal substrate 1 was cut out, as shown by basal plane dislocations 11, and those that were generated when the flat silicon carbide single crystal substrate 1 was cut out from the single crystal mass and processed, as shown by basal plane dislocations 12. The density of basal plane dislocations 11 originally present in silicon carbide single crystal substrate 1 is, for example, 100 to 3000 cm -2 Since the (0001) crystal plane is tilted by an offset angle θ with respect to the surface of silicon carbide single crystal substrate 1, basal plane dislocations 11 penetrate silicon carbide single crystal substrate 1 from the back side to the front side along this tilted (0001) crystal plane.

[0025] Basal plane dislocations 12 caused by processing occur only near the surface of silicon carbide single crystal substrate 1. For this reason, basal plane dislocations 12 can be removed together with the surface region of silicon carbide single crystal substrate 1 by hydrogen gas etching or chemical mechanical polishing of the substrate surface before epitaxial growth.

[0026] However, it is extremely difficult to physically remove basal plane dislocations 11 occurring deep inside silicon carbide single crystal substrate 1. When an epitaxial layer is formed on such silicon carbide single crystal substrate 1 having basal plane dislocations, basal plane dislocations 11 exposed on the surface of silicon carbide single crystal substrate 1 and disturbances in the atomic arrangement around basal plane dislocations 11 propagate to the epitaxial layer.

[0027] As shown in FIG. 10 as a comparative example, when an epitaxial layer 4 made of silicon carbide is formed on a silicon carbide single crystal substrate 1, a basal plane dislocation 11 may extend as a basal plane dislocation 41 in the epitaxial layer 4. Also, as shown in FIG. 11 as a comparative example, when an epitaxial layer 4 made of silicon carbide is formed on a silicon carbide single crystal substrate 1, a basal plane dislocation 11 may be converted into a threading edge dislocation (TED) 42 or 43. The threading edge dislocation 42 is a basal plane dislocation 11 that is converted into a threading edge dislocation at an 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 a basal plane dislocation 11 that is converted into a threading edge dislocation 43 in the epitaxial layer 4 after it extends into the epitaxial layer 4. In this case, even if basal plane dislocation 11 is transformed into threading edge dislocation 43 in epitaxial layer 4, basal plane dislocation 44 remains in epitaxial layer 4.

[0028] The properties of a basal plane dislocation and a threading edge dislocation are significantly different from each other. When observed closely, a basal plane dislocation 41 is separated into two partial dislocations at an interval of several nm. The narrow region between the two partial dislocations is a planar crystal defect called a stacking fault. When a certain amount of energy is imparted to the basal plane dislocation 41 by the recombination of electrons and holes in the epitaxial layer 4, the region of this stacking fault gradually spreads along a plane parallel to the (0001) crystal plane. When a current flows in a direction crossing this planar spreading stacking fault, the electrical resistance is higher than the electrical resistance in a portion where no stacking fault exists.

[0029] When a bipolar element or a unipolar element incorporating a bipolar parasitic diode is energized, electrons and holes recombine in the epitaxial layer. Therefore, if basal plane dislocations occur in the region that functions as a semiconductor element, stacking faults expand over time as the current passes, as described above, and the characteristics of the semiconductor element change over time. This means that basal plane dislocations have a significant adverse effect on the reliability characteristics of the semiconductor element.

[0030] On the other hand, threading edge dislocation 42 is a linear defect perpendicular to the (0001) crystal plane. Since it is stable and does not decompose into partial dislocations accompanied by stacking faults, the defect does not expand over time. For this reason, threading edge dislocation 42 does not adversely affect the device characteristics or reliability characteristics of semiconductor devices and is harmless.

[0031] Although threading edge dislocation 43 is harmless in itself, like threading edge dislocation 42, it is accompanied by basal plane dislocation 44 in epitaxial layer 4, and therefore, the portion of basal plane dislocation 44 affects the reliability of the element as described above.

[0032] Therefore, reducing the number of basal plane dislocations 41, 44 in the epitaxial layer 4, in other words, converting as many basal plane dislocations 11 as possible into threading edge dislocations 42 at the interface 4a between the silicon carbide single crystal substrate 1 and the epitaxial layer, is an important requirement for an epitaxial layer for forming a highly reliable semiconductor device.

[0033] In view of the above, there is room for improvement in silicon carbide semiconductor epitaxial substrates from the viewpoint of increasing the reliability of semiconductor elements.

[0034] In the present embodiment, therefore, a technique is implemented to solve the above-mentioned room for improvement. In the following, a growth method for reducing the density of basal plane dislocations in the epitaxial layer will be described as a technical concept of the present embodiment.

[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.

[0036] According to Patent Document 3, the conversion of basal plane dislocations to threading edge dislocations is closely 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 epitaxial layer during growth, the surface roughness of the epitaxial layer during growth is roughly proportional to the surface roughness of the formed epitaxial layer. Based on this, Patent Document 3 discloses that an epitaxial layer with low basal plane dislocation density can be formed by using a silicon carbide single crystal substrate with a surface root mean square roughness of 0.1 nm or less, and controlling the growth conditions of the epitaxial layer so that the root mean square roughness Rq (nm) of the epitaxial layer surface after growth satisfies the following formula (1), where V (μm / h) is the growth rate of the epitaxial layer. Rq(nm)<0.007×V(μm / h)+0.074 (1)

[0038] The silicon carbide semiconductor epitaxial substrate of the present embodiment used for forming a semiconductor element has a configuration in which at least two silicon carbide epitaxial layers are laminated on a silicon carbide single crystal substrate 1, as shown in Fig. 1. A 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 the epitaxial substrate, the thickness of the first epitaxial layer on the silicon carbide single crystal substrate is set to about 0.5 µm, and the donor concentration is set to 1 × 10 18 cm -3 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 -3Since the functions of a semiconductor element are realized by forming a structure such as a pn junction 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 element.

[0039] In investigating conditions for reducing basal plane dislocations in the second epitaxial layer, the inventors conducted a detailed investigation into 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 of the first epitaxial layer was set to 1×10 18 cm -3 In particular, it was found that increasing the donor concentration of the first epitaxial layer to 5×10 is effective in reducing basal plane dislocations in the second epitaxial layer. 18 cm -3 It was found that when the thickness is equal to or greater than this, the effect of reducing basal plane dislocations in the second epitaxial layer is large.

[0041] Hereinafter, we will explain the conversion of basal plane dislocations in two epitaxial layers on a silicon carbide single crystal substrate.

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

[0043] 2, defects (dislocations) that may occur in an epitaxial substrate have been described using the epitaxial substrate of this embodiment as an example. This embodiment satisfies formula (1) and achieves a reduction in basal plane dislocations 33, 34 in the second epitaxial layer 3 by setting the donor concentration of the first epitaxial layer relatively high as follows. The donor 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 present inventors have found the following through experiments regarding the formation of one epitaxial layer 4 on a silicon carbide single crystal substrate 1 as in the comparative example shown in Fig. 10 or 11. That is, the present inventors have found that the greater the difference in donor concentration between the silicon carbide single crystal substrate 1 and the epitaxial layer 4, the greater the rate (conversion rate) of conversion of basal plane dislocations 11 to threading edge dislocations 42 at the interface 1a between the silicon carbide single crystal substrate 1 and the epitaxial layer, compared to when the difference is small.

[0045] Typically, the donor concentration of the silicon carbide single crystal substrate 1 is 5×10 18 cm -3 In the epitaxial substrate shown in FIG. 2, the donor concentration of the first epitaxial layer 2 is set to about 1×10 18 cm -3 From 2 × 10 19 cm -3 Even if the donor concentration of the first epitaxial layer 2 is changed to about 3×10, the difference in concentration 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 threading 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 usually about 3×10 15 cm -3 From 1×10 16 cm -3 Therefore, the donor concentration of the first epitaxial layer 2 is set to about 1×10 18 cm -3 , 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, it is expected that the conversion rate from basal plane dislocations to threading edge dislocations at the interface 2a will increase. In other words, the threading edge dislocations 31 in FIG. 2 will increase. As a result, there are two cases: basal plane dislocations 33 and basal plane dislocations 34 that transform into threading edge dislocations. As a result, the number of cases where dislocation 32 occurs decreases, and the sum of basal plane dislocations 33 and 34 in second epitaxial layer 3 decreases.

[0046] Thus, it is expected that the conversion rate of basal plane dislocations to threading edge dislocations at the interface 2a between the first epitaxial layer 2 and the second epitaxial layer 3 increases with increasing donor concentration of the first epitaxial layer 2. However, when the donor concentration of the first epitaxial layer 2 is 2×10 19 cm -3If the donor concentration exceeds 5×10, stacking faults are likely to occur in the first epitaxial layer 2, which is undesirable. 18 cm -3 That's it, 2 x 10 19 cm -3 It is desirable to do the following:

[0047] From the viewpoint of increasing the conversion rate of basal plane dislocations to threading 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 18 cm -3 That's it, 2 x 10 19 cm -3 From the same viewpoint, it is more preferable that the donor concentration of the first epitaxial layer 2 is 1×10 19 cm -3 That's it, 2 x 10 19 cm -3 It is even more preferable that:

[0048] In addition, increasing the thickness of the first epitaxial layer 2 increases the chance of transformation into threading edge dislocations 22 (see FIG. 2) in the first epitaxial layer 2, and this 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 the growth of the first epitaxial layer 2, and increases the risk of defects occurring due to flaking material falling from the inner wall of the growth furnace onto the surface of the first epitaxial layer 2 during growth. In addition, there is also a problem of increased manufacturing costs. From this perspective, 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 for the first epitaxial layer 2 is sufficient to obtain the effect of dislocation transformation. Therefore, by setting the thickness of the first epitaxial layer 2 to 10 μm or less, it is possible to suppress the possibility of defects occurring on the surface of the first epitaxial layer 2 due to peeling material falling from the inner wall of the growth furnace, and to reduce the manufacturing cost. Note that the thickness of the first epitaxial layer 2 is preferably 3 μm or more.

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

[0050] In order to increase the conversion rate from basal plane dislocations 11 to threading 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 a silicon carbide single crystal substrate with an inclined (0001) crystal plane has a step structure consisting of terraces made of the (0001) crystal plane and their edges. The offset angle is, for example, in the range of 0° to 8°.

[0051] Here, if the offset angle is less than 2°, the terrace portion becomes too wide, and silicon carbide (3C-SiC) having a structure different from that of the epitaxial layer grows on the terrace portion, and if this grows into the epitaxial layer, it may cause crystal defects. If the offset angle is more than 5°, the conversion rate at the interface 1a between the silicon carbide single crystal substrate 1 and the first epitaxial layer 2 may decrease, and the basal plane dislocation density in the first epitaxial layer 2 may increase. Therefore, it is particularly desirable that the offset angle of the silicon carbide single crystal substrate 1 be in the range of 2° to 5°.

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

[0053] First, a silicon carbide single crystal substrate 1 is prepared as shown in FIG. 3. The silicon carbide single crystal constituting the silicon carbide single crystal substrate 1 is preferably 4H-SiC. If the surface of the silicon carbide single crystal substrate 1 is a (0001) crystal plane, one surface is a (0001) Si plane with silicon atoms exposed on the outermost surface. The other surface parallel to this is a (000-1) C plane with carbon atoms exposed. Regardless of which surface is used as the main surface, it is possible to form a silicon carbide epitaxial layer thereon, but the optimal formation conditions may differ. The following describes 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 of 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 disk-shaped silicon carbide single crystal substrate having a diameter of 6 inches and a thickness of about 350 μm to 400 μm.

[0055] The silicon carbide single crystal substrate 1 is mechanically polished by a known procedure to remove any process-affected layer formed on the surface, and the surface roughness of the front and back surfaces of the substrate is reduced to 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, the surface roughness RMS refers to the value obtained by measuring the root-mean-square roughness of a 10 μm area of ​​the sample using an atomic force microscope (AFM). When using a commercially available wafer, it may be possible to omit the steps up to this point.

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

[0057] If the surface roughness RMS of surface 5 exceeds 0.1 nm, there is a possibility that basal plane dislocations that exist only in the vicinity of surface 5 of silicon carbide single crystal substrate 1 cannot be completely removed and remain. Furthermore, no matter how the growth conditions of the epitaxial layer are controlled, the surface roughness of the second epitaxial layer cannot be sufficiently small, and the condition of formula (1) cannot be satisfied.

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

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

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

[0061] After the temperature reaches a predetermined value, the silicon carbide single crystal substrate 1 is maintained at that temperature and a source gas is supplied. The growth of the first epitaxial layer 2 starts with the start of the supply of the source gas, but the surface 5 (see FIG. 3) of the silicon carbide single crystal substrate 1 may be hydrogen-etched before the first epitaxial layer 2 is grown. The hydrogen etching may be performed, for example, by maintaining the silicon carbide single crystal substrate 1 at a constant temperature in the above-mentioned hydrogen atmosphere in an epitaxial growth furnace. The hydrogen gas may contain a hydrocarbon such as propane (C3H8) or a hydrogen halide gas such as hydrogen chloride (HCl). This makes it possible to remove the process-affected layer on the substrate surface, remove basal plane dislocations introduced into the substrate surface by processing, and reduce basal plane dislocations propagating to the epitaxial layer. The hydrogen etching is preferably performed at a temperature of 1300°C or higher and 1700°C or lower. If the hydrogen etching temperature is less than 1300°C, the process-affected layer cannot be completely removed, and basal plane dislocations introduced by the process may remain on the substrate surface. A temperature of 1700°C is sufficient to remove the process-affected layer, and temperatures higher than this may actually impair the flatness of the substrate surface. The time required for hydrogen etching is approximately 1 to 15 minutes.

[0062] The source gas may include, for example, monosilane (SiH4) gas as a source of silicon atoms and propane (C3H8) gas as a source of carbon atoms. Nitrogen (N2) gas is supplied simultaneously with the source gas to control the donor concentration of the epitaxial layer. The source gas may include hydrogen halide gas such as hydrogen chloride (HCl).

[0063] An epitaxial layer is grown so as to satisfy formula (1) by controlling the supply ratio of the source gas (represented by the ratio of carbon atoms to silicon atoms in the source gas, C / Si ratio) and the supply amount of the source gas. Both the root mean square roughness Rq of the surface of the epitaxial layer and the growth rate V of the epitaxial layer in formula (1) are characteristics that can be measured after the epitaxial layer is formed. For this reason, an experiment is first conducted in which an epitaxial layer is grown using the C / Si ratio and the supply amount of the source gas as parameters, and the root mean square roughness Rq and growth rate of the obtained epitaxial layer are measured, and then the C / Si ratio and the supply amount of the source gas that satisfy formula (1) are obtained. Then, epitaxial growth is performed using the obtained C / Si ratio and the supply amount of the source gas as the conditions for epitaxial growth. Taking into consideration the uniformity of the epitaxial layer thickness or donor concentration, or the suppression of morphological defects on the epitaxial layer surface, the C / Si ratio is preferably set to 1.0 or more and 1.4 or less.

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

[0065] According to the results of the experiments by the present inventors, the donor concentration of the first epitaxial layer 2 is set to 5×10 18 cm -3 That's it, 2 x 10 19 cm -3 When the ratio of C / Si was set to 1.0 or more and 1.4 or less, the ratio N / C of the nitrogen flow rate (nitrogen supply amount) to the carbon atoms in the raw material gas (carbon supply amount) was 4.0 or more and 110 or less.

[0066] The growth rate of the epitaxial layer is proportional to the flow rate of the source gas, but the proportionality constant differs depending on the structure of the chemical vapor deposition apparatus. Therefore, the preferred range of the source gas supply rate depends on the chemical vapor deposition apparatus. The relationship between the source gas supply rate and the growth rate of the epitaxial layer is understood through preliminary experiments, and the growth rate to be adopted is set. The growth time required to deposit the required film thickness is calculated from the set value of the growth rate.

[0067] 5, after a predetermined time for forming the first epitaxial layer 2 has elapsed, the nitrogen flow rate is changed to a flow rate required for the second epitaxial layer 3, thereby starting the growth of the second epitaxial layer 3. When growing the second epitaxial layer 3, the flow rate of the source gas and the C / Si ratio may be changed from the conditions for forming the first epitaxial layer 2 within a range that satisfies formula (1). With respect to formula (1), the growth rate V here refers to the growth rate of the stacked epitaxial layers including the first epitaxial layer 2 and the second epitaxial layer 3, and the root mean square roughness Rq of the surface (outermost surface) of the epitaxial layer refers to the root mean square roughness of the upper surface of the second epitaxial layer.

[0068] The conditions for growing the second epitaxial layer 3 can be appropriately changed within the range of the conditions for growing the first epitaxial layer 2 described above. That is, a gas containing carbon atoms and a gas containing silicon atoms, and a mixed gas of nitrogen gas and hydrogen gas for controlling the donor concentration of the epitaxial layer are supplied as source gases, and the pressure is maintained at 10 kPa or more and 30 kPa or less. Under these conditions, the second epitaxial layer is grown on a silicon carbide single crystal substrate maintained at a temperature of 1500°C or more and 1700°C or less. The ratio C / Si of the amount of carbon atoms supplied to the amount of silicon atoms supplied in the source gas is set to 1.0 or more and 1.4 or less.

[0069] After a predetermined time for forming the second epitaxial layer 3 has elapsed, the supply of the source gas and nitrogen gas is stopped to stop the growth. While maintaining a predetermined pressure in the hydrogen gas flow, the 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 are deposited is cooled. Through the above steps, the silicon carbide semiconductor epitaxial substrate of the present embodiment including the silicon carbide single crystal substrate 1, the first epitaxial layer 2 and the second epitaxial layer 3 is completed.

[0070] The number of basal plane dislocations in second epitaxial layer 3 of the silicon carbide semiconductor epitaxial substrate thus fabricated is 0.1% or less of the number of basal plane dislocations present in silicon carbide single crystal substrate 1. That is, the silicon carbide semiconductor epitaxial substrate of the present embodiment has a second epitaxial layer with excellent crystal quality. Therefore, a semiconductor device fabricated using the silicon carbide semiconductor epitaxial substrate of the present embodiment has excellent reliability.

[0071] In addition, as the production of such silicon carbide semiconductor epitaxial substrates is repeated, a coating such as silicon carbide is deposited on the inner wall of the growth furnace of the chemical vapor deposition apparatus. This coating eventually peels off and falls onto the surface of the epitaxial layer being grown, causing defects in the epitaxial layer, so the growth furnace must be opened to the atmosphere as appropriate to clean the inner wall. The optimal growth conditions for the epitaxial layer may change before and after cleaning, but according to the experiments of the present inventors, the fluctuations were not so large. By conducting an experiment to confirm the donor concentration after cleaning and finely adjusting the amount of nitrogen supply, it is possible to form an epitaxial layer similar to that before cleaning.

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

[0073] First, five 6-inch diameter 4H-SiC single crystal substrates were prepared. The offset angle for each substrate was 4°. A silicon carbide epitaxial layer with a thickness of 10 μm was grown on the (0001) Si surface of these four substrates. The growth conditions were hydrogen supply rate of 120 slm, growth pressure of 20 kPa, growth temperature of 1600°C, SiH4 supply rate of 270 sccm, and C3H8 supply rate of 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 placed in contact with the surface of the epitaxial layer after growth, and a voltage of up to about 1V was applied between the front (main) and back surfaces 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 amount of nitrogen supplied during epitaxial layer growth and the donor concentration of the epitaxial layer. The horizontal axis of Figure 6 shows the amount of nitrogen supplied when growing the first epitaxial layer, and the vertical axis of Figure 6 shows the donor concentration of the grown first epitaxial layer. In Figure 6, both the vertical and horizontal axes are plotted logarithmically, and the correlation between the amount of nitrogen supplied and the donor concentration is a straight line with a slope of approximately 1. This shows that the amount of nitrogen supplied and the donor concentration are approximately proportional to each other.

[0075] A similar correlation was also investigated for the cases where the SiH4 supply rate was 270 sccm, the C3H8 supply rate was 90 sccm (C / Si=1.0), and the C3H8 supply rate was 126 sccm (C / Si=1.4). The results are plotted against the ratio of the nitrogen supply rate to the carbon supply rate (N / C) in Figure 7. The horizontal axis of Figure 7 is the ratio of the nitrogen supply rate to the carbon supply rate (N / C), and the vertical axis of Figure 7 is the donor concentration in the first epitaxial layer. From Figure 7, it can be seen that when the donor concentration is increased by 5×10 18 cm -3 From 2 × 10 19 cm -3 The amount of nitrogen required to keep the range of N / C from 4 to 110 (thick arrow in Fig. 7) It was found that the range was

[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 cm -2 Since they were cut from the same block, other crystal qualities are likely to be at the same level.

[0077] Chemical mechanical polishing was performed on these three substrates (silicon carbide single crystal substrates) A, B, and C. The processing conditions were the same, and the surface roughness RMS after processing was 0.03 nm for all of them.

[0078] Two epitaxial layers were formed on each substrate. The thickness of the first epitaxial layer was 3 μm. The donor concentration of the first epitaxial layer was 1×10 18 cm -3 , and board B is 5×10 18 cm -3 , and substrate C is 1×10 19 cm -3 The second epitaxial layer had a thickness of 30 μm for all substrates A, B, and C, and a donor concentration of 3×10 15 cm -3 It was 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 shown below. That is, the growth conditions are hydrogen supply amount 120 slm, growth pressure 20 kPa, growth temperature 1600°C, SiH4 supply amount 270 sccm, and C3H8 supply amount 108 sccm (C / Si=1.2).

[0080] Based on the results of Example 1, the nitrogen supply amount during formation of the first epitaxial layer was 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] The growth rate under the above conditions 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. It was 0.288 nm under the conditions of substrate A, 0.300 nm under the conditions of substrate B, and 0.295 nm under the conditions of substrate C, and there was no significant difference between the substrates.

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

[0084] The basal plane dislocation density was calculated by taking images of basal plane dislocations in the epitaxial layers and counting their number using a method called photoluminescence imaging. The images of basal plane dislocations obtained by this method are limited to those in the epitaxial layers with low donor concentration. Therefore, basal plane dislocations in the substrate and the first epitaxial layer with high donor concentration are not observed, and only basal plane dislocations in the second epitaxial layer with low donor concentration, that is, those corresponding to basal plane dislocations 33 and 34 in Figure 2, are observed. The relationship between the basal plane dislocation density in the second epitaxial layer obtained by this method and the donor concentration in the first epitaxial layer is shown in Figure 8. The horizontal axis of the graph in Figure 8 shows the donor concentration in the first epitaxial layer, and the vertical axis shows the basal plane dislocation density in the second epitaxial layer.

[0085] As shown in FIG. 8, as the donor concentration of the first epitaxial layer increases, the basal plane dislocation density in the second epitaxial layer decreases. 18 cm -3 By doing so, the basal plane dislocation density of the second epitaxial layer can be made 0.1% or less of the basal plane dislocation density of the substrate, thereby improving the stability of the semiconductor element formed on the second epitaxial layer 3.

[0086] The invention made by the present inventors has been specifically described above based on the embodiments thereof. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the invention. [Explanation of symbols]

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

Claims

1. a silicon carbide single crystal substrate having a surface with an offset angle of 0° or more and 8° or less and a root-mean-square roughness of 0.1 nm or less; a plurality of silicon carbide single crystal epitaxial layers sequentially grown on the silicon carbide single crystal substrate; the root mean square roughness Rq (nm) of the outermost surfaces of the plurality of silicon carbide single crystal epitaxial layers satisfies the relationship Rq (nm) < 0.326; Among the plurality of silicon carbide single crystal epitaxial layers, the donor concentration of a first epitaxial layer in contact with the silicon carbide single crystal substrate is 5×10 １８ cm －３ That's it, 2 x 10 １９ cm －３ is as follows: The thickness of the first epitaxial layer in contact with the silicon carbide single crystal substrate is 3 μm or more and 10 μm or less; a ratio of a basal plane dislocation density in a second epitaxial layer formed on the first epitaxial layer among the plurality of silicon carbide single crystal epitaxial layers to a basal plane dislocation density of the silicon carbide single crystal substrate is 0.1% or less.

2. 2. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein the Rq (nm) is 0.288 to 0.

326.

3. 2. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein the donor concentration of the first epitaxial layer is 8×10 １８ cm －３ That's it, 2 x 10 １９ cm －３ 1. A silicon carbide semiconductor epitaxial substrate comprising:

4. 2. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein the donor concentration of the first epitaxial layer in contact with the silicon carbide single crystal substrate is 1×10 １９ cm －３ That's it, 2 x 10 １９ cm －３ 1. A silicon carbide semiconductor epitaxial substrate comprising:

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