JP7647849B2 - Silicon carbide semiconductor epitaxial substrate - Google Patents

Description

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

Traditionally, power semiconductor elements (power devices) using silicon (Si) have been developed for the purpose of controlling high frequencies and high power, and various improvements have been made to significantly improve element characteristics. However, the element 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 element characteristics.

Silicon carbide (SiC) is attracting attention as a semiconductor material for such power semiconductor elements. Silicon carbide has a dielectric breakdown field 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.

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 to fabricate semiconductor devices in this epitaxial layer. This epitaxial layer is grown on the silicon carbide single crystal 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 while the silicon carbide single crystal substrate is heated.

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 an angle of 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 used when forming epitaxial layers on silicon carbide single crystal substrates. The tilt angle of the (0001) crystal plane with respect to the surface is referred to below as the offset angle.

In addition, Patent Document 2 (JP 2005-311348 A) and Non-Patent Document 1 disclose that basal plane dislocations present in silicon carbide single crystal substrates propagate to the epitaxial layer, thereby reducing the reliability of bipolar elements or unipolar elements incorporating bipolar-type parasitic diodes.

Patent Document 3 (JP Patent Publication 2008-4888) discloses that the surface of a silicon carbide single crystal substrate before epitaxial growth is smoothed to a predetermined roughness value or less by hydrogen etching or chemical mechanical polishing, and the flow rate of the source gas is set to satisfy predetermined conditions, thereby reducing basal plane dislocations propagating to the epitaxial layer.

Patent document 4 (JP Patent Publication 9-321323 A) and non-patent document 2 disclose that, in order to improve the stability of the electrical characteristics of silicon carbide semiconductor elements, an epitaxial layer with a high impurity concentration is provided between the epitaxial layer in which the semiconductor elements are fabricated and the silicon carbide single crystal substrate.

U.S. Pat. No. 4,912,064 JP 2005-311348 A JP 2008-4888 A Japanese Patent Application Publication No. 9-321323

Materials Science Forum, 2007, Vol. 600-603, pp. 1127-1130 Epiworld product information, October 2018 edition, [Retrieved March 13, 2020], Internet <URL: http://www.epiworld-cn.com>

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 degree of reduction in basal plane dislocations can be observed. However, in order to increase the yield rate of semiconductor elements and use silicon carbide as a power semiconductor element to replace silicon semiconductor elements, it is necessary to further reduce the basal plane dislocation density.

The object of the present invention is to provide a method for producing a silicon carbide semiconductor epitaxial substrate with fewer basal plane dislocations by solving the above problems.

Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

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

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 sequentially 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 cm ^-3 or more and 2 × ¹⁰ cm ^-3 or less.

According to a representative embodiment, a silicon carbide semiconductor epitaxial substrate is obtained that has a high-quality epitaxial layer with few basal plane dislocations. The reliability and yield rate of semiconductor devices fabricated using this substrate can be improved.

1 is a cross-sectional view showing a silicon carbide semiconductor epitaxial substrate according to an embodiment. FIG. 1 is a cross-sectional view illustrating a state in which basal plane dislocations of 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. 1A to 1C are cross-sectional views of a silicon carbide semiconductor epitaxial substrate according to an embodiment during a manufacturing process. 4 is a cross-sectional view of the silicon carbide semiconductor epitaxial substrate during the manufacturing process following FIG. 3. 5 is a cross-sectional view of the silicon carbide semiconductor epitaxial substrate during the manufacturing process following FIG. 4 . 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. 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. 11 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. FIG. 2 is a cross-sectional view illustrating basal plane dislocations present in a silicon carbide single crystal substrate as a comparative example. 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. FIG. 1 is a cross-sectional view illustrating a state in which basal plane dislocations of a silicon carbide single crystal substrate are transformed 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.

The following describes in detail an embodiment of the present invention with reference to the drawings. In all drawings used to explain the embodiment, the same reference numerals are used for components having the same functions, and repeated explanations will be omitted. In addition, in the embodiment, explanations of the same or similar parts will not be repeated as a general rule, unless particularly necessary.

In addition, here, a silicon carbide single crystal substrate may simply be referred to as a substrate. In addition, here, 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.

(Embodiment)
<Details of areas for improvement>
The areas for improvement are described in detail below.

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 to 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.

Here, we explain basal plane dislocations in silicon carbide single crystal substrates and epitaxial layers using Figures 9 to 11.

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, the silicon carbide single crystal substrate 1 has a back surface and a main surface (front surface) opposite the back surface. As the silicon carbide single crystal substrate 1, for example, a substrate having an inclination angle (offset angle) θ of about 2 to 8° is used. Basal plane dislocations (BPDs) are linear crystal defects that occur in the silicon carbide single crystal substrate 1 parallel to the (0001) crystal plane. In FIG. 9, the cross section of silicon carbide single crystal substrate 1 is shown from a direction in which the inclined (0001) crystal plane appears as a straight line, so the basal plane dislocations that occur 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. This method of illustrating basal plane dislocations is the same in FIG. 10, FIG. 11, and FIG. 2, which will be used in the following explanation.

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, about 100 to 3000 cm ^-2 . Since the (0001) crystal plane is inclined 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 inclined (0001) crystal plane.

The basal plane dislocations 12 caused by processing occur only near the surface of the silicon carbide single crystal substrate 1. Therefore, the basal plane dislocations 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.

However, it is extremely difficult to physically remove basal plane dislocations 11 that occur deep inside the silicon carbide single crystal substrate 1. When an epitaxial layer is formed on such a silicon carbide single crystal substrate 1 that has basal plane dislocations, the basal plane dislocations 11 exposed on the surface of the silicon carbide single crystal substrate 1 and the disturbance in the atomic arrangement around the basal plane dislocations 11 propagate to the epitaxial layer.

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, the 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, the 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 the interface 4a between the silicon carbide single crystal substrate 1 and the epitaxial layer 4. On the other hand, the 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 extending into the epitaxial layer 4. In this case, even if the basal plane dislocation 11 is transformed into a threading edge dislocation 43 in the epitaxial layer 4, the basal plane dislocation 44 remains in the epitaxial layer 4.

The properties of basal plane dislocations and threading edge dislocations are very different from each other. When observed closely, basal plane dislocation 41 is separated into two partial dislocations spaced apart by a distance of several nanometers. 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 basal plane dislocation 41 by the recombination of electrons and holes in 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 that crosses this planar stacking fault, the electrical resistance is higher than the electrical resistance in a location where no stacking fault exists.

When a bipolar element or a unipolar element incorporating a bipolar parasitic diode is energized, electrons and holes recombine in the epitaxial layer. For this reason, if basal plane dislocations occur in the region that functions as a semiconductor element, as described above, stacking faults expand over time as current is applied, 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 semiconductor elements.

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 is harmless and does not adversely affect the device characteristics or reliability characteristics of semiconductor devices.

The threading edge dislocation 43 is harmless in itself, just like the threading edge dislocation 42, but since it is accompanied by a basal plane dislocation 44 in the epitaxial layer 4, the basal plane dislocation 44 portion affects the reliability of the element as described above.

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 to form a highly reliable semiconductor device.

In light of the above, there is room for improvement in silicon carbide semiconductor epitaxial substrates in terms of increasing the reliability of semiconductor elements.

Therefore, in this embodiment, a technique is implemented to resolve the above-mentioned room for improvement. Below, we will explain the growth method that reduces the density of basal plane dislocations in the epitaxial layer as the technical idea of this embodiment that incorporates this technique.

<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.

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.

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 having 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 x V (μm/h) + 0.074 (1)

The silicon carbide semiconductor epitaxial substrate of the present embodiment used to form a semiconductor element has a structure in which at least two silicon carbide epitaxial layers are laminated 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 the epitaxial substrate, it is considered that the thickness of the first epitaxial layer on the silicon carbide single crystal substrate is often about 0.5 μm, and the donor concentration is often about 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 specifications of the withstand voltage 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 about 1×10 ¹⁶ cm ⁻³ . Since the function of the semiconductor element is 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.

While 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 density of basal plane dislocations in the second epitaxial layer.

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

Below, we explain the transformation of basal plane dislocations in two epitaxial layers on a silicon carbide single crystal substrate.

Figure 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 are transformed 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 progress 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 progress 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, the 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.

In FIG. 2, defects (dislocations) that may occur in an epitaxial substrate are 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 shown below. 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.

The inventors have found through experiments that, as in the comparative example shown in FIG. 10 or FIG. 11, when one epitaxial layer 4 is formed on a silicon carbide single crystal substrate 1, the following is true. That is, the 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.

Usually, the donor concentration of the silicon carbide single crystal substrate 1 is considered to be set to about 5×10 ¹⁸ cm ⁻³ . In the epitaxial substrate shown in FIG. 2, even if the donor concentration of the first epitaxial layer 2 is changed from 1×10 ¹⁸ cm ⁻³ to about 2×10 ¹⁹ cm ⁻³ , the concentration difference between the donor concentration of the first epitaxial layer 2 and the donor concentration of the silicon carbide single crystal substrate 1 does not change much. Therefore, it is considered that the conversion rate from the basal plane dislocation 11 to the threading edge dislocation 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, it is considered that the donor concentration of the second epitaxial layer 3 is usually set to about 3×10 ¹⁵ cm ⁻³ to 1×10 ¹⁶ cm ⁻³ . For this reason, when the donor concentration of the first epitaxial layer 2 increases from 1×10 ¹⁸ cm ⁻³ , the donor concentration difference (the donor concentration difference between the first epitaxial layer 2 and the second epitaxial layer 3) at the interface 2a 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 increases. That is, the threading edge dislocations 31 in FIG. 2 increase. As a result, the number of cases in which basal plane dislocations 33 and basal plane dislocations 34 become threading edge dislocations 32 decreases, and the sum of basal plane dislocations 33 and 34 in the second epitaxial layer 3 decreases.

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 the donor concentration of the first epitaxial layer 2. However, if the donor concentration of the first epitaxial layer 2 exceeds 2×10 ¹⁹ cm ^-3 , stacking faults tend to occur in the first epitaxial layer 2, which is not preferable. Therefore, the donor concentration of the first epitaxial layer 2 is desirably 5×10 ¹⁸ cm ^-3 or more and 2×10 ¹⁹ cm ^-3 or less.

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, it is more desirable to set the donor concentration of the first epitaxial layer 2 to 8×10 ¹⁸ cm ^-3 or more and 2×10 ¹⁹ cm ^-3 or less. From the same viewpoint, it is even more desirable to set the donor concentration of the first epitaxial layer 2 to 1×10 ¹⁹ cm ^-3 or more and 2×10 ¹⁹ cm ^-3 or less.

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, which is also effective in reducing the basal plane dislocation density in the second epitaxial layer 3. However, on the other hand, increasing the thickness of the first epitaxial layer 2 increases the time required for the growth of the first epitaxial layer 2, and increases the risk of defects occurring due to flaking 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 the 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 making the thickness of the first epitaxial layer 2 10 μm or less, it is possible to reduce the possibility of defects occurring on the surface of the first epitaxial layer 2 due to flaking material falling from the inner wall of the growth furnace, and to reduce manufacturing costs. It is preferable that the thickness of the first epitaxial layer 2 be 3 μm or more.

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

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°.

Here, if the offset angle is less than 2°, the terrace portion becomes too wide, and silicon carbide (3C-SiC) with a structure different from that of the epitaxial layer grows on the terrace portion, and if this is mixed into the epitaxial layer, it may become a crystal defect. Also, 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 for the offset angle of the silicon carbide single crystal substrate 1 to be in the range of 2° to 5°.

The method for manufacturing a silicon carbide semiconductor epitaxial substrate according to this embodiment is described in detail below.

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 the (0001) crystal plane, one surface is the (0001) Si plane with silicon atoms exposed on the outermost surface. The other surface parallel to this is the (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. Below, a case where the (0001) Si plane is used as the growth surface is 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 for this offset angle is 2° to 5°.

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 with a diameter of 6 inches and a thickness of about 350 μm to 400 μm.

The silicon carbide single crystal substrate 1 is mechanically polished by known procedures to remove any process-induced alteration layers on the surface, and the surface roughness of the front and back surfaces of the substrate is reduced to a specified 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 root-mean-square roughness measured over a 10 μm area of the sample using an atomic force microscope (AFM). If commercially available wafers are used, it may be possible to omit the steps up to this point.

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.

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

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

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

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 hydrogen gas is preferably about 100 slm to 170 slm.

After reaching a predetermined temperature, the silicon carbide single crystal substrate 1 is maintained at that temperature, and a raw material gas is supplied. The growth of the first epitaxial layer 2 is started with the start of the supply of the raw material gas, but the surface 5 (see FIG. 3) of the silicon carbide single crystal substrate 1 may be hydrogen-etched before the growth of the first epitaxial layer 2. 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 (C ₃ H ₈ ) 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 the basal plane dislocations introduced into the substrate surface by processing, and reduce the 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 processing 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 may be about 1 minute or more and 15 minutes or less.

The source gas may include, for example, monosilane (SiH ₄ ) gas as a silicon atom source and propane (C ₃ H ₈ ) gas as a carbon atom source. Nitrogen (N ₂ ) 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).

The epitaxial layer is grown so as to satisfy formula (1) by controlling the supply ratio of the raw material gas (the ratio of carbon atoms to silicon atoms in the raw material gas, expressed as the C/Si ratio) and the supply amount of the raw material 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 conducted in advance to grow an epitaxial layer using the C/Si ratio and the supply amount of the raw material gas as parameters, and the root mean square roughness Rq and growth rate of the surface of the obtained epitaxial layer are measured, and then the C/Si ratio and the supply amount of the raw material gas that satisfy formula (1) are obtained. Then, epitaxial growth is performed using the obtained C/Si ratio and the supply amount of the raw material 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, it is preferable to set the C/Si ratio to 1.0 or more and 1.4 or less.

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

According to the results of experiments conducted by the present inventors, when the donor concentration of the first epitaxial layer 2 is set to 5× ¹⁰ cm ⁻³ or more and 2× ¹⁰ cm ⁻³ or less, and C/Si is set to 1.0 or more and 1.4 or less, the ratio N/C of the flow rate of nitrogen supplied (nitrogen supply amount) to the carbon atoms in the raw material gas (carbon supply amount) is 4.0 or more and 110 or less.

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 equipment. Therefore, the preferred range of the source gas supply rate depends on the chemical vapor deposition equipment. 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.

As shown in FIG. 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, and the growth of the second epitaxial layer 3 is started. When growing the second epitaxial layer 3, the flow rate of the raw material 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 is 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 (top surface) of the epitaxial layer is the root mean square roughness of the upper surface of the second epitaxial layer.

The conditions for growing the second epitaxial layer 3 can be changed as appropriate within the range of the conditions described above as the conditions for growing the first epitaxial layer 2. That is, a gas containing carbon atoms and a gas containing silicon atoms are supplied as raw material gases, and a mixed gas of nitrogen gas and hydrogen gas for controlling the donor concentration of the epitaxial layer is supplied, 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 raw material gas is set to 1.0 or more and 1.4 or less.

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. In this manner, the silicon carbide semiconductor epitaxial substrate of this embodiment, which includes the silicon carbide single crystal substrate 1, the first epitaxial layer 2, and the second epitaxial layer 3, is completed.

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

As such silicon carbide semiconductor epitaxial substrates are repeatedly produced, a coating of silicon carbide or the like accumulates on the inner walls 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 and the inner walls cleaned as appropriate. The optimal growth conditions for the epitaxial layer may change before and after cleaning, but according to the inventors' experiments, the variation was not very large. By conducting an experiment to confirm the donor concentration after cleaning and slightly adjusting the amount of nitrogen supplied, it is possible to form an epitaxial layer similar to that before cleaning.

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).

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 each of the four substrates. The growth conditions were hydrogen supply rate of 120 slm, growth pressure of 20 kPa, growth temperature of 1600° C., SiH ₄ supply rate of 270 sccm, and 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.

A mercury probe was placed in contact with the surface of the epitaxial layer after growth, and a voltage of up to about 1 V 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.

A similar correlation was also investigated for the cases of SiH ₄ supply amount of 270 sccm, C ₃ H ₈ supply amount of 90 sccm (C/Si=1.0), and C ₃ H ₈ supply amount of 126 sccm (C/Si=1.4). The results are plotted against the ratio of the nitrogen supply amount to the carbon supply amount (N/C) in FIG. 7. The horizontal axis of FIG. 7 is the ratio of the nitrogen supply amount to the carbon supply amount (N/C), and the vertical axis of FIG. 7 is the donor concentration of the first epitaxial layer. From FIG. 7, it was found that the nitrogen supply amount required to set the donor concentration in the range of 5×10 ¹⁸ cm ⁻³ to 2×10 ¹⁹ cm ⁻³ in the range of C/Si=1.0 to 1.4 is in the range of N/C=4 to 110 (the range of the thick arrow shown in FIG. 7).

Example 2
Three 6-inch diameter 4H-SiC single crystal substrates, substrates A, B, and C, were prepared. All of them 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 were considered to be at the same level.

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 of them.

Two epitaxial layers were formed on each substrate. The thickness of the first epitaxial layer was 3 μm. The donor concentrations of the first epitaxial layers were 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 layers had a thickness of 30 μm for all substrates A, B, and C, and a donor concentration of 3×10 ¹⁵ cm ^-3 .

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, and are as follows: the growth conditions are hydrogen supply amount 120 slm, growth pressure 20 kPa, growth temperature 1600° C., SiH ₄ supply amount 270 sccm, and C ₃ H ₈ supply amount 108 sccm (C/Si=1.2).

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

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

After forming the second epitaxial layer, the surface roughness RMS of the second epitaxial layer was evaluated using an atomic force microscope. Under the conditions of substrate A, it was 0.288 nm, under the conditions of substrate B, it was 0.300 nm, and under the conditions of substrate C, it was 0.295 nm, and there was no significant difference between the substrates.

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 substrates A, B, and C.

The basal plane dislocation density was determined by photographing the images of basal plane dislocations in the epitaxial layer and counting the number of dislocations by a method called photoluminescence imaging. The images of basal plane dislocations obtained by this method are limited to those in the epitaxial layer with a low donor concentration. Therefore, basal plane dislocations in the substrate and the first epitaxial layer with a high donor concentration are not observed, and only basal plane dislocations in the second epitaxial layer with a low donor concentration, that is, those corresponding to basal plane dislocations 33 and basal plane dislocations 34 in FIG. 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 FIG. 8. The horizontal axis of the graph in FIG. 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.

8, as the donor concentration of the first epitaxial layer increases, the basal plane dislocation density in the second epitaxial layer decreases. By satisfying formula (1) and setting the donor concentration of the first epitaxial layer to 5×10 ¹⁸ cm ^-3 or more, the basal plane dislocation density of the second epitaxial layer can be set to 0.1% or less of the basal plane dislocation density of the substrate. This can improve the stability of the semiconductor element formed on the second epitaxial layer 3.

The invention made by the inventors has been specifically described above based on the embodiments thereof, but the invention is not limited to the above-mentioned embodiments and can be modified in various ways without departing from the gist of the invention.

Reference Signs List 1 Silicon carbide single crystal substrate 2 First epitaxial layer 3 Second epitaxial layer 4 Epitaxial layer

Claims (4)

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, a first epitaxial layer in contact with the silicon carbide single crystal substrate has a donor concentration of 5×10 ¹⁸ cm ⁻³ or more and 2×10 ¹⁹ cm ⁻³ or less;
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. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein the Rq (nm) is 0.288 to 0.326. 2. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein the first epitaxial layer has a donor concentration of not less than 8×10 ¹⁸ cm ⁻³ and not more than 2×10 ¹⁹ cm ⁻³ . 2. The silicon carbide semiconductor epitaxial substrate according to claim 1, wherein a donor concentration of the first epitaxial layer in contact with the silicon carbide single crystal substrate is not less than 1×10 ¹⁹ cm ⁻³ and not more than 2×10 ¹⁹ cm ⁻³ .

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