Selection method of silicon carbide semiconductor epitaxial substrate

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] The present invention can be used in a method for manufacturing a silicon carbide semiconductor epitaxial substrate.

Background Art

[0002] Conventionally, for the purpose of controlling high frequencies and high power, the development of power semiconductor devices (power devices) using silicon (Si) has been promoted, and significant improvements in device characteristics have been achieved through various improvements. However, currently, the device performance of such power semiconductor devices is approaching the theoretical limit values calculated from the physical property values of silicon. For this reason, power semiconductor devices using new semiconductor materials are being studied for the purpose of further improving device characteristics.

[0003] As such a semiconductor material for power semiconductor devices, silicon carbide (SiC) has attracted attention. Since silicon carbide has a breakdown electric field that is more than one order of magnitude higher than that of silicon, it is considered applicable to high-voltage devices, and is expected to have semiconductor characteristics that are far superior to those of silicon, such as excellent heat resistance.

[0004] When manufacturing a power semiconductor device using silicon carbide, it is conceivable to epitaxially grow a silicon carbide single-crystalline thin film on a silicon carbide single-crystalline substrate using a method called chemical vapor deposition, and fabricate a semiconductor device in this epitaxial layer. This epitaxial layer is grown on a silicon carbide single-crystalline substrate, for example, by introducing monosilane (SiH 4 ) gas for supplying silicon (Si) atoms and propane (C 3 H 8 ) gas for supplying carbon (C) atoms in a heated state of the silicon carbide single-crystalline substrate.

[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 occurrence 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] Also, 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 with a high impurity concentration is provided between the epitaxial layer in which the semiconductor device is formed 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

Non-Patent Literature

[0010]

Non-Patent Literature 1

Non-Patent Literature 2

Summary of the Invention

Problems to be Solved by the Invention

[0011] As disclosed in Patent Document 3, if the surface roughness of a silicon carbide single crystal substrate before epitaxial growth is smoothed and the formation conditions of the epitaxial layer are set, a certain decrease in basal plane dislocations can be observed. However, in order to increase the yield rate of semiconductor elements and use silicon carbide as a semiconductor element for power instead of silicon semiconductor elements, 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 provide a method for manufacturing a silicon carbide semiconductor epitaxial substrate with 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 Problems

[0014] Among the embodiments disclosed in the present application, the outline of a representative one will be briefly described as follows. 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 sequentially growing a plurality of epitaxial layers made of silicon carbide on the silicon carbide single crystal substrate by chemical vapor deposition. The root 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. The growth conditions of the plurality of epitaxial layers are set. Among the plurality of epitaxial layers, 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 or more and 2 × 10 19 cm -3 or less.

Advantages of the Invention

[0016] According to a representative embodiment, a silicon carbide semiconductor epitaxial substrate having an epitaxial layer with few basal plane dislocations and good quality can be obtained. The reliability and yield of semiconductor devices fabricated using this substrate can be improved.

Brief Description of the Drawings

[0017]

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Figure 10

Figure 11

Mode 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 for explaining the embodiments, members having the same function are denoted by the same reference numerals, and repeated explanations thereof are omitted. Further, in the embodiments, unless particularly necessary, explanations of the same or similar parts are not repeated in principle.

[0019] Also, here, the silicon carbide single crystal substrate may sometimes be simply referred to as a substrate. Also, here, 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 room for improvement) The details of the room for improvement will be described below.

[0021] The present inventors formed epitaxial layers on a silicon carbide single crystal substrate (silicon carbide semiconductor substrate) under various conditions, and examined in detail 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 a predetermined condition is effective for 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 within 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. 9 to 11.

[0023] When forming an epitaxial layer on a silicon carbide single crystal substrate, as disclosed in Patent Document 1, it is conceivable to use a silicon carbide single crystal substrate in which the (0001) crystal plane (sometimes also referred to as the basal plane) is inclined with respect to the main surface of the silicon carbide single crystal substrate. As shown as a comparative example in FIG. 9, the silicon carbide single crystal substrate 1 has a back surface and a main surface (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 dislocation (BPD) is a linear crystal defect and occurs parallel to the (0001) crystal plane in the silicon carbide single crystal substrate 1. In FIG. 9, since a cross-section of the silicon carbide single crystal substrate 1 is shown from the direction in which the inclined (0001) crystal plane appears as a straight line, the basal plane dislocation occurring in the direction parallel to the (0001) crystal plane is represented as a straight line extending in an oblique direction from the back side to the front side of the silicon carbide single crystal substrate 1. Such a way of illustrating the basal plane dislocation is the same in FIGS. 10 and 11 and FIG. 2 used in the later description.

[0024] This basal plane dislocation includes those originally present in the single crystal ingot before cutting out the silicon carbide single crystal substrate 1, as illustrated by basal plane dislocation 11, and those generated when cutting out and processing the flat silicon carbide single crystal substrate 1 from the single crystal ingot, as illustrated by basal plane dislocation 12. The density of the basal plane dislocation 11 originally present in the silicon carbide single crystal substrate 1 is, for example, from 100 to 3000 cm -2 or so. Since the (0001) crystal plane is inclined by the offset angle θ with respect to the surface of the silicon carbide single crystal substrate 1, the basal plane dislocation 11 penetrates along this inclined (0001) crystal plane from the back side to the front side of the silicon carbide single crystal substrate 1.

[0025] The basal plane dislocation 12 generated by processing occurs only in the vicinity of the surface of the silicon carbide single crystal substrate 1. Therefore, the basal plane dislocation 12 can be removed together 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, it is very difficult to physically remove the basal plane dislocations 11 that occur deep inside the single-crystalline silicon carbide substrate 1. When an epitaxial layer is formed on the single-crystalline silicon carbide substrate 1 having such basal plane dislocations, the basal plane dislocations 11 exposed on the surface of the single-crystalline silicon carbide substrate 1 and the disorder of the atomic arrangement around the basal plane dislocations 11 are propagated to the epitaxial layer.

[0027] As shown as a comparative example in FIG. 10, when an epitaxial layer 4 made of silicon carbide is formed on the single-crystalline silicon carbide substrate 1, the basal plane dislocations 11 may extend as basal plane dislocations 41 in the epitaxial layer 4 as they are. Further, as shown as a comparative example in FIG. 11, when an epitaxial layer 4 made of silicon carbide is formed on the single-crystalline silicon carbide substrate 1, the basal plane dislocations 11 may be converted into threading edge dislocations (TED) 42 or 43. The threading edge dislocation 42 is formed by the basal plane dislocation 11 being converted into a threading edge dislocation at the interface 4a between the single-crystalline silicon carbide substrate 1 and the epitaxial layer 4. On the other hand, the threading edge dislocation 43 is formed by the basal plane dislocation 11 extending in the epitaxial layer 4 and then being converted into a threading edge dislocation 43 in the epitaxial layer 4. In this case, even if the basal plane dislocation 11 is converted into a threading edge dislocation 43 in the epitaxial layer 4, basal plane dislocations 44 remain in the epitaxial layer 4.

[0028] The basal plane dislocations and the threading edge dislocations are greatly different in nature from each other. When the basal plane dislocation 41 is observed in detail, it is separated into two partial dislocations at intervals of several nm. The narrow region between the two partial dislocations is a planar crystal defect called a stacking defect. When a predetermined 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 defect gradually spreads along a plane parallel to the (0001) crystal plane. The electrical resistance when a current flows in a direction crossing this planar stacking defect is larger than the electrical resistance at a location where no stacking defect exists.

[0029] When a bipolar element or a unipolar element incorporating a bipolar parasitic diode is in an energized state, recombination of electrons and holes occurs in the epitaxial layer. Therefore, if basal plane dislocations are present in the region functioning as a semiconductor element, as described above, stacking defects expand with the passage of energization time, 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, the through-thickness edge dislocation 42 is a linear defect in a direction perpendicular to the (0001) crystal plane. Since it is stable without decomposing into partial dislocations with stacking defects, the defect does not expand over time. Therefore, the through-thickness edge dislocation 42 is harmless and does not affect the element characteristics and reliability characteristics of the semiconductor element.

[0031] The through-thickness edge dislocation 43 is harmless in itself, similar to the through-thickness edge dislocation 42, but since it is accompanied by basal plane dislocations 44 in the epitaxial layer 4, the portion of the basal plane dislocations 44 affects the reliability of the element as described above.

[0032] Therefore, reducing the basal plane dislocations 41 and 44 in the epitaxial layer 4, that is, how to convert as many basal plane dislocations 11 into through-thickness edge dislocations 42 at the interface 4a between the silicon carbide single crystal substrate 1 and the epitaxial layer, becomes an important requirement for the epitaxial layer for forming a highly reliable semiconductor element.

[0033] From the above, there is room for improvement in the silicon carbide semiconductor epitaxial substrate from the viewpoint of enhancing the reliability of semiconductor elements.

[0034] Therefore, in the present embodiment, a device is provided to solve the above-mentioned room for improvement. Hereinafter, as a technical idea in the present embodiment provided with this device, a growth method for reducing the density of basal plane dislocations in the epitaxial layer will be described.

[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, the surface roughness of the growing epitaxial layer is generally proportional to the surface roughness of the formed epitaxial layer. 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 surface of the epitaxial layer after growth is expressed as a function of the growth rate V (μm / h) of the epitaxial layer, and the growth conditions of the epitaxial layer are controlled so as to satisfy the following formula (1), thereby forming an epitaxial layer with a low basal plane dislocation density. 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 structure in which at least two layers of silicon carbide epitaxial layers are stacked on a silicon carbide single crystal substrate 1 as shown in FIG. 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 -3is the degree. Since the function of the semiconductor device is manifested by creating a structure such as a pn junction in the second epitaxial layer 3, it is important to reduce the basal plane dislocations in the second epitaxial layer 3 from the viewpoint of enhancing the reliability of the semiconductor device.

[0039] In examining the conditions for reducing the basal plane dislocations in the second epitaxial layer, the present inventors specifically investigated in detail the influence of the donor concentration of the first epitaxial layer on the basal plane dislocation density in the second epitaxial layer.

[0040] As a result, it was found that increasing the donor concentration of the first epitaxial layer to higher than 1×10 18 cm -3 is effective in reducing the basal plane dislocations in the second epitaxial layer. In particular, when the donor concentration of the first epitaxial layer is 5×10 18 cm -3 or higher, it was found that the effect of reducing the basal plane dislocations in the second epitaxial layer is large.

[0041] Hereinafter, the conversion of the basal plane dislocations in the two-layer epitaxial layer on the silicon carbide single crystal substrate will be described.

[0042] FIG. 2 illustrates the conversion of basal plane dislocations in the stacked epitaxial layers. Most of the basal plane dislocations 11 in the silicon carbide single crystal substrate 1 are converted 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 are not converted progress through the first epitaxial layer 2, and a part of them is converted 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, a part of those basal plane dislocations is converted into threading edge dislocations 31 near the interface 2a. Then, the remaining basal plane dislocations progress through the second epitaxial layer, and a part of the basal plane dislocations 34 is converted into threading edge dislocations 32 in the second epitaxial layer 3. However, the basal plane dislocations 34 that have progressed through the second epitaxial layer 3 before being converted into the threading edge dislocations 32 remain as they are. The remaining basal plane dislocations 33 that are not converted into threading edge dislocations reach the surface 3a of the second epitaxial layer.

[0043] In FIG. 2, taking the epitaxial substrate of the present embodiment as an example, defects (dislocations) that can occur in the epitaxial substrate are described. The present embodiment satisfies the formula (1) and realizes the reduction of the 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 follows. The donor mentioned 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 an n-type conductivity type.

[0044] The inventors have experimentally found the following about the case of forming a single epitaxial layer 4 on the silicon carbide single crystal substrate 1 as in the comparative example shown in FIG. 10 or FIG. 11. That is, the inventors have found that the larger the difference between the donor concentration of the silicon carbide single crystal substrate 1 and the epitaxial layer 4, the larger the ratio (conversion rate) at which the basal plane dislocations 11 are converted into threading edge dislocations 42 at the interface 1a between the silicon carbide single crystal substrate 1 and the epitaxial layer compared to the case where 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 -3When it exceeds this value, stacking defects are likely to occur in the first epitaxial layer 2, which is not preferable. Therefore, the donor concentration of the first epitaxial layer 2 is 5×10 18 cm -3 or more and 2×10 19 cm -3 or less, which is desirable.

[0047] From the viewpoint of increasing the conversion rate from 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 8×10 18 cm -3 or more and 2×10 19 cm -3 or less, which is more desirable. Also, from the same viewpoint, the donor concentration of the first epitaxial layer 2 being 1×10 19 cm -3 or more and 2×10 19 cm -3 or less is even more desirable.

[0048] In addition, increasing the thickness of the first epitaxial layer 2 increases the chance of conversion to threading edge dislocations 22 (see FIG. 2) in the first epitaxial layer 2, so 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 the risk of debris falling from the inner wall of the growth furnace onto the surface of the growing first epitaxial layer 2 and causing defects increases. Also, there is a problem of increasing the manufacturing cost. From such a viewpoint, an appropriate upper limit of the thickness can be considered for the first epitaxial layer 2. From the experiments of the inventors, a thickness of 10 μm for the first epitaxial layer 2 is sufficient to obtain a sufficient effect of dislocation conversion. Therefore, by setting the thickness of the first epitaxial layer 2 to 10 μm or less, the possibility of debris falling from the inner wall of the growth furnace onto the surface of the first epitaxial layer 2 and causing defects can be suppressed, and the manufacturing cost can be reduced. Note that the thickness of the first epitaxial layer 2 is preferably 3 μm or more.

[0049] Furthermore, selecting a single-crystalline silicon carbide substrate 1 with fewer basal plane dislocations 11 contained therein is also effective in reducing the basal plane dislocation density in the 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 single-crystalline silicon carbide substrate 1 and the first epitaxial layer 2, it is desirable to reduce the offset angle of the single-crystalline silicon carbide substrate 1. The surface of a single-crystalline silicon carbide substrate with a tilted (0001) crystal plane has a terrace structure composed of terraces formed by the (0001) crystal plane and steps formed by their edges. The offset angle is, for example, in the range of 0° or more and 8° or less.

[0051] Here, when 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 in the terrace portion, and if this is mixed into the epitaxial layer, it may become a crystal defect. Further, when the offset angle is greater than 5°, the conversion rate at the interface 1a between the single-crystalline silicon carbide substrate 1 and the first epitaxial layer 2 decreases, and there is a risk that the basal plane dislocation density in the first epitaxial layer 2 increases. Therefore, the offset angle of the single-crystalline silicon carbide substrate 1 is particularly preferably in the range of 2° or more and 5° or less.

[0052] Hereinafter, the method for manufacturing a silicon carbide semiconductor epitaxial substrate according to the present embodiment will be described in detail.

[0053] First, as shown in FIG. 3, a single-crystalline silicon carbide substrate 1 is prepared. The single-crystalline silicon carbide constituting the single-crystalline silicon carbide substrate 1 is preferably 4H-SiC. When the surface of the single-crystalline silicon carbide substrate 1 is the (0001) crystal plane, one surface becomes the (0001)Si plane where silicon atoms are exposed on the outermost surface. The other surface parallel to this becomes the (000-1)C plane where carbon atoms are exposed. It is possible to form a silicon carbide epitaxial layer on either surface used as the main surface, but the optimal formation conditions may be different. Hereinafter, the case where the (0001)Si plane is the growth surface will be described. 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° or more and 5° or less.

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

[0055] The single-crystalline silicon carbide substrate 1 is mechanically polished by a known procedure until the processed damaged layer formed on the surface is removed and the surface roughness of the front and back surfaces of the substrate reaches a predetermined value. Further, the surface 5 of the single-crystalline silicon carbide substrate 1 on which epitaxial growth is to be performed is mirror-polished with abrasive grains such as diamond until the surface roughness RMS becomes 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 with an atomic force microscope (AFM: Atomic Force Microscope). When using a commercially available wafer, the above steps may be omitted.

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

[0057] When the RMS surface roughness of the surface 5 exceeds 0.1 nm, the basal plane dislocations existing only in the vicinity of the surface 5 of the single-crystalline silicon carbide substrate 1 may not be completely removed and may remain. Also, no matter how the growth conditions of the epitaxial layer are controlled, the surface roughness of the surface of the second epitaxial layer cannot be made sufficiently small, and the conditions of formula (1) cannot be satisfied.

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

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

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

[0061] After reaching a predetermined temperature, the single-crystalline silicon carbide substrate 1 is held at that temperature, and a source gas is supplied. Growth of the first epitaxial layer 2 starts with the supply of the source gas, but the surface 5 (see FIG. 3) of the single-crystalline silicon carbide substrate 1 may be hydrogen-etched before growing the first epitaxial layer 2. The hydrogen etching can be performed, for example, by holding the single-crystalline silicon carbide substrate 1 at a constant temperature in the above hydrogen atmosphere in an epitaxial growth furnace. Propane (C 3 H 8)Hydrocarbons such as or hydrogen halide gases such as hydrogen chloride (HCl) may be included. Thereby, the processed and modified layer on the substrate surface can be removed, the basal plane dislocations introduced on the substrate surface by processing can be removed, and the basal plane dislocations propagating to the epitaxial layer can be reduced. Hydrogen etching is preferably performed at a temperature of 1300 °C or higher and 1700 °C or lower. When the temperature of hydrogen etching is less than 1300 °C, the processed and modified layer may not be completely removed, and there is a risk that the basal plane dislocations introduced by processing remain on the substrate surface. To remove the processed and modified layer, a temperature of 1700 °C is sufficient, and a temperature exceeding this may rather impair the flatness of the substrate surface. The time required for hydrogen etching may be about 1 minute or more and 15 minutes or less.

[0062] For the source gas, for example, monosilane (SiH 4 ) gas is used as a source of silicon atoms, and propane (C 3 H 8 ) gas is used as a source of carbon atoms. Also, nitrogen (N 2 ) gas is supplied simultaneously with the source gas for controlling the donor concentration of the epitaxial layer. The source gas may contain hydrogen halide gases such as hydrogen chloride (HCl).

[0063] By controlling the supply ratio of the source gas (represented by the ratio of carbon atoms to silicon atoms in the source gas, the C / Si ratio) and the supply amount of the source gas, an epitaxial layer is grown so as to satisfy formula (1). Both the root mean square roughness Rq of the surface of the epitaxial layer of formula (1) and the growth rate V of the epitaxial layer are characteristics that can be measured after forming the epitaxial layer. For this reason, an experiment of growing an epitaxial layer using the C / Si ratio and the supply amount of the source gas as parameters is performed in advance. After measuring the root mean square roughness Rq and the growth rate of the surface of the obtained epitaxial layer, the C / Si ratio and the supply amount of the source gas when formula (1) is satisfied are obtained. Then, epitaxial growth is performed using the obtained C / Si ratio and source gas supply amount as the conditions for epitaxial growth. Considering the uniformity of the film thickness or donor concentration of the epitaxial layer, or the suppression of the occurrence of morphological defects on the surface of the epitaxial layer, etc., the C / Si ratio is preferably set to 1.0 or more and 1.4 or less.

[0064] The donor concentration of the epitaxial layer is proportional to the supplied nitrogen flow rate and also has a correlation with the above C / Si ratio. The relationship between the nitrogen flow rate and the donor concentration at a predetermined C / Si ratio is grasped by preliminary experiments, and the supply amount of nitrogen is set.

[0065] According to the results of the experiments by the present inventors, when the donor concentration of the first epitaxial layer 2 is 5×10 18 cm -3 or more and 2×10 19 cm -3 or less, the ratio N / C of the supplied nitrogen flow rate (nitrogen supply amount) to the carbon atoms (carbon supply amount) in the source gas when C / Si is 1.0 or more and 1.4 or less becomes 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 varies depending on the structure of the chemical vapor deposition apparatus. Therefore, the preferred range of the source gas supply amount depends on the chemical vapor deposition apparatus. From preliminary experiments, grasp the relationship between the source gas supply amount and the growth rate of the epitaxial layer, and set the growth rate to be adopted. Calculate the growth time required to deposit the required film thickness from the set value of the growth rate.

[0067] As shown in FIG. 5, after a predetermined time for forming the first epitaxial layer 2 has elapsed, by changing the nitrogen flow rate to the flow rate required for the second epitaxial layer 3, the growth of the second epitaxial layer 3 is started. 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 when forming the first epitaxial layer 2 within the range satisfying formula (1). Regarding formula (1), the growth rate V referred to here is the growth rate of the stacked epitaxial layer including the first epitaxial layer 2 and the second epitaxial layer 3, and the root mean square roughness Rq of the surface (the outermost surface) of the epitaxial layer is the root mean square roughness of the upper surface of the second epitaxial layer.

[0068] Also, the conditions for growing the second epitaxial layer 3 can be appropriately changed within the range of the conditions described above as the conditions for growing the first epitaxial layer 2. That is, supply a mixed gas of a gas containing carbon atoms and a gas containing silicon atoms as the source gas, and nitrogen gas and hydrogen gas for controlling the donor concentration of the epitaxial layer, and maintain the atmospheric pressure at 10 kPa or more and 30 kPa or less. Under such conditions, grow the second epitaxial layer on a silicon carbide single crystal substrate maintained at a temperature of 1500 °C or more and 1700 °C or less. Also, set the ratio C / Si of the carbon atom supply amount in the source gas to the silicon atom supply amount 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 the nitrogen gas is stopped to halt the growth. While maintaining a predetermined pressure in a hydrogen gas stream, 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. Thus, 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 basal plane dislocations in the second epitaxial layer 3 of the silicon carbide semiconductor epitaxial substrate fabricated in this manner are 0.1% or less of the basal plane dislocations present in the silicon carbide single crystal substrate 1. That is, the silicon carbide semiconductor epitaxial substrate of the present embodiment is provided with a second epitaxial layer having excellent crystal quality. Therefore, a semiconductor device fabricated using the silicon carbide semiconductor epitaxial substrate of the present embodiment has excellent reliability.

[0071] Note that as the fabrication of such a silicon carbide semiconductor epitaxial substrate is repeated, a film such as silicon carbide is deposited 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, generating defects in the epitaxial layer. Therefore, it is necessary to appropriately open the growth furnace to the atmosphere and clean the inner wall. Before and after the cleaning, the optimal growth conditions of the epitaxial layer may change, but according to the experiments of the present inventors, the variation was not so large. By conducting an experiment to confirm the donor concentration after cleaning and finely adjusting the supply amount of nitrogen, it is possible to form an epitaxial layer similar to that before cleaning.

[0072] <Example 1> The present inventors 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 of each substrate was 4°. A silicon carbide epitaxial layer with a thickness of 10 μm was grown on the (0001)Si surface of four of these substrates. The growth conditions were a hydrogen supply rate of 120 slm, a growth pressure of 20 kPa, a growth temperature of 1600 °C, a SiH 4 supply rate of 270 sccm, and a C 3 H 8 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 brought into contact with the surface of the grown epitaxial layer, and a voltage of up to about 1 V was applied between the surface (main surface) and the back surface to measure the correlation between capacitance and voltage. The results were analyzed to calculate the donor concentration. The relationship between the nitrogen supply rate during epitaxial layer growth and the donor concentration of the epitaxial layer is shown in Fig. 6. The horizontal axis in Fig. 6 indicates the nitrogen supply rate when growing the first epitaxial layer, and the vertical axis indicates the donor concentration of the grown first epitaxial layer. In Fig. 6, both the vertical axis and the horizontal axis are plotted logarithmically, and the correlation between the nitrogen supply rate and the donor concentration is a straight line with a slope of approximately 1. From this, it was found that the nitrogen supply rate and the donor concentration are approximately proportional.

[0075] The same correlation was also investigated for the cases of a SiH 4 supply rate of 270 sccm, a C 3 H 8 supply rate of 90 sccm (C / Si = 1.0) and a C 3 H 8 supply rate of 126 sccm (C / Si = 1.4). The results plotted against the ratio of the nitrogen supply rate to the carbon supply rate (N / C) are shown in Fig. 7. The horizontal axis in Fig. 7 is the ratio of the nitrogen supply rate to the carbon supply rate (N / C), and the vertical axis is the donor concentration of the first epitaxial layer. From Fig. 7, in the range of C / Si = 1.0 to 1.4, the donor concentration ranges from 5×10 18 cm -3 to 2×10 19 cm -3The required nitrogen supply amount to be in the range is found to be in the range of N / C = 4 to 110 (the range of the thick arrow shown in Fig. 7).

[0076] <Example 2> Three substrates A, B, and C, which are 6-inch diameter 4H-SiC single crystal substrates, were prepared. In each case, the offset angle is 4°, and the basal plane dislocation density is about 400 cm -2 . Since they are cut from the same bulk, it is considered that the other crystal qualities are also at the same level.

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

[0078] An epitaxial layer consisting of two layers was formed on each substrate. The thickness of the first epitaxial layer was 3 μm for all. Also, the donor concentration of the first epitaxial layer was 1×10 18 cm -3 for substrate A, 5×10 18 cm -3 for substrate B, and 1×10 19 cm -3 for substrate C. The second epitaxial layer had a film thickness of 30 μm for all of substrates A, B, and C, and the donor concentration was 3×10 15 cm -3 .

[0079] The only difference among 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 and are as follows. That is, the growth conditions are a hydrogen supply amount of 120 slm, a growth pressure of 20 kPa, a growth temperature of 1600 °C, a SiH 4 supply amount of 270 sccm, and a C 3 H 8 supply amount of 108 sccm (C / Si = 1.2).

[0080] Based on the results of Example 1, the nitrogen supply amount during the 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 of substrates A, B, and C.

[0082] After forming the second epitaxial layer, the surface roughness RMS of the second epitaxial layer surface 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 among the substrates.

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

[0084] By means of a method called photoluminescence imaging, the basal plane dislocation images in the epitaxial layer were photographed, and the basal plane dislocation density was obtained by measuring the number thereof. The images of the basal plane dislocations obtained by this method are limited to those in the epitaxial layer with a low donor concentration. Therefore, the basal plane dislocations in the substrate and the first epitaxial layer with a high donor concentration are not observed, and only those corresponding to the basal plane dislocations 33 and 34 in the second epitaxial layer with a low donor concentration, that is, in FIG. 2, are observed. The relationship between the basal plane dislocation density in the second epitaxial layer obtained by such a method and the donor concentration of the first epitaxial layer is shown in FIG. 8. The horizontal axis of the graph in FIG. 8 indicates the donor concentration of the first epitaxial layer, and the vertical axis indicates 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. Satisfying Equation (1) and setting the donor concentration of the first epitaxial layer to 5×10 18cm -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, the stability of the semiconductor element formed on the second epitaxial layer 3 can be improved.

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

Explanation of Reference Numerals

[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; a first epitaxial layer in contact with a (0001) Si face of the silicon carbide single crystal substrate; a second epitaxial layer in contact with the first 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; 1. A method for selecting a silicon carbide semiconductor epitaxial substrate, comprising: (a) the donor concentration of the first epitaxial layer is 5×10 １８ cm －３ The above, and (b) the root mean square roughness Rq (nm) of the outermost surface of the second epitaxial layer, where V is the growth rate of the second epitaxial layer (μm / h), is Rq<0.007×V+0.074; The silicon carbide semiconductor epitaxial substrate is selected on the condition that it satisfies the following: How to select a silicon carbide semiconductor epitaxial substrate.

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