Patent Description:
In a MOS transistor (SiC MOSFET) using a SiC substrate, in a case where a SiO<NUM> film (gate insulating film) is formed on a surface of the SiC substrate by thermal oxidation, there is a problem that a defect density at an interface between the SiO<NUM> film and the SiC substrate is extremely high. If the interface defect density is high, sufficient performance of the SiC MOSFET, such as a channel mobility, cannot be obtained.

As a method for reducing the interface defect density, Patent Document <NUM> discloses a method in which instead of directly forming a SiO<NUM> film on a surface of a SiC substrate by thermal oxidation, a Si thin film is deposited on a surface of a SiC substrate and is subsequently oxidized and a SiO<NUM> film is formed accordingly.

Patent Document <NUM> discloses a method of fabricating a SiC semiconductor device including the steps of preparing a silicon carbide semiconductor including a first surface having impurities implanted at least partially, forming a second surface by dry etching the first surface of the silicon carbide semiconductor using gas including hydrogen gas, and forming an oxide film constituting the silicon carbide semiconductor device on the second surface.

Patent Document <NUM> discloses an SiC semiconductor element and a manufacturing method for an SiC semiconductor element in which the interface state density of the interface of the insulating film and the SiC is reduced, and channel mobility is improved.

Patent Document <NUM> discloses a method of manufacturing a semiconductor device that includes the treating the surface of a SiC semiconductor substrate prior to forming a gate oxide film on the SiC semiconductor substrate in order to etch the SiC semiconductor substrate by several nm to <NUM> mum with hydrogen in a reaction furnace. The treating is conducted a reduced pressure in the furnace, at a temperature of <NUM>° C.

Non-Patent Document <NUM> discloses a method (interface nitridation) in which thermal treatment is performed in nitrogen monoxide (NO) gas atmosphere after a SiO<NUM> film has been formed on a surface of a SiC substrate by thermal oxidation and an interface between the SiO<NUM> film and the SiC substrate is nitrided accordingly.

In these methods, the defect density at the interface between the SiO<NUM> film and the SiC substrate can be reduced, but the defect density is still high. For this reason, the performance of the SiC MOSFET is greatly limited. Further, in the method in which the interface between the SiO<NUM> film and the SiC substrate is nitrided by the NO thermal treatment, not only interface nitridation but also oxidation proceeds. For this reason, the interface defect density cannot be sufficiently reduced.

As another method for reducing the interface defect density, Non-Patent Document <NUM> discloses a method in which a SiO<NUM> film is formed on a SiC substrate after a surface of the SiC substrate has been etched with high-temperature H<NUM> gas and the SiC substrate formed with the SiO<NUM> film is subsequently thermally treated in high-temperature N<NUM> gas atmosphere. Here, the SiO<NUM> film is formed in such a manner that a Si thin film is deposited on the SiC substrate and is subsequently thermally oxidized at such a temperature that the SiC substrate is not oxidized.

According to the method disclosed in Non-Patent Document <NUM>, the defect density at the interface between the SiO<NUM> film and the SiC substrate can be significantly reduced, but in a case where a gate insulating film of SiO<NUM> is formed on the SiC substrate by this method and a SiC MOSFET is formed accordingly, a high channel mobility is obtained, but normally-on characteristics with a negative threshold voltage are easily brought.

The present invention has been made in view of the above-described points, and a main object thereof is to provide a SiC semiconductor device manufacturing method capable of forming a SiC MOSFET having a high channel mobility and normally-off characteristics.

The SiC semiconductor device manufacturing method according to the present invention includes a step of etching a surface of a SiC substrate with H<NUM> gas under Si-excess atmosphere within a temperature range of <NUM> to <NUM>, a step of depositing, by a CVD method, a SiO<NUM> film on the SiC substrate at such a temperature that the SiC substrate is not oxidized, and a step of thermally treating the SiC substrate, on which the SiO<NUM> film is deposited, in NO gas atmosphere within a temperature range of <NUM> to <NUM>.

According to the present invention, the SiC semiconductor device manufacturing method capable of forming the SiC MOSFET having the high channel mobility and the normally-off characteristics can be provided.

Hereinafter, an embodiment of the present invention will be described in detail based on the drawings. Note that the present invention is not limited to the following embodiment. Moreover, changes can be made as necessary without departing from a scope in which advantageous effects of the present invention are obtained.

<FIG> are views showing a SiC semiconductor device manufacturing method in one embodiment of the present invention.

As shown in <FIG>, a surface of a SiC substrate <NUM> is etched with high-temperature H<NUM> gas under Si-excess atmosphere. At this point, an extremely-thin (one-to-three monolayer thcikness) Si film is formed on the SiC substrate <NUM>. Etching with high-temperature H<NUM> gas is performed by addition of a slight amount of SiH<NUM> gas, and may be performed, for example, under conditions of a H<NUM> flow rate: <NUM> sccm, a SiH<NUM> flow rate: <NUM> sccm, a temperature: <NUM>, a pressure: <NUM> kPa, and a time: <NUM> minutes. Etching with high-temperature H<NUM> gas is preferably performed within a temperature range of <NUM> to <NUM>. Note that optimal gas flow rate, pressure, and time depend on an apparatus that performs this processing. If an extremely-thin Si film is formed on the SiC substrate with gas containing Si, such as SiH<NUM>Cl<NUM> or SiH<NUM>Cl, instead of SiH<NUM>, similar advantageous effects are also obtained.

As the SiC substrate <NUM>, one configured such that a SiC epitaxial layer (not shown) is formed on the SiC substrate <NUM> may be used. Note that preferably when a MOSFET is formed on the SiC epitaxial layer, a surface of the SiC epitaxial layer is oxidized, and thereafter, the oxide film is removed.

Next, as shown in <FIG>, a SiO<NUM> film <NUM> is deposited on the SiC substrate <NUM> by a plasma CVD method. The SiO<NUM> film <NUM> may be deposited at such a temperature that the SiC substrate <NUM> is not oxidized, and for example, may be deposited under conditions of a tetraethoxysilane (TEOS) flow rate: <NUM> sccm, an O<NUM> flow rate: <NUM> sccm, a temperature: <NUM>, a pressure: <NUM> Pa, a radio-frequency power: <NUM> W, and a time: <NUM> minutes. The SiO<NUM> film <NUM> is preferably deposited within a temperature range of <NUM> to <NUM>.

Note that the SiO<NUM> film <NUM> may be deposited using a thermal CVD method. In this case, the SiO<NUM> film <NUM> may be deposited under conditions of a SiH<NUM> flow rate: <NUM> sccm, a N<NUM>O flow rate: <NUM> sccm, a N<NUM> flow rate: <NUM> sccm, a temperature: <NUM>, a pressure: <NUM> Pa, and a time: <NUM> minutes.

Next, as shown in <FIG>, the SiC substrate <NUM> on which the SiO<NUM> film <NUM> is deposited is thermally treated in NO gas atmosphere. Conditions for the thermal treatment may be, for example, a NO flow rate: <NUM> sccm, a N<NUM> flow rate: <NUM> sccm, a temperature: <NUM>, a pressure: <NUM> atm, and a time: <NUM> minutes. The thermal treatment in NO gas atmosphere is preferably performed within a temperature range of <NUM> to <NUM>. Here, "in NO gas atmosphere" includes atmosphere in which NO gas is diluted with dilution gas such as N<NUM> gas. For example, in the present embodiment, in order to reduce the amount of use of NO gas with toxicity, the thermal treatment is performed in atmosphere in which NO gas is diluted (NO flow rate: <NUM>%; N<NUM> flow rate: <NUM>%) with N<NUM> gas.

A MOS capacitor was formed on the SiO<NUM> film <NUM> deposited on the SiC substrate <NUM> by the method shown in <FIG>, and a defect density (interface state density) at an interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> was obtained using analysis (high-low method) of C-V characteristics. For the SiC substrate <NUM>, an n-type <NUM>-SiC(<NUM>) substrate was used, and a donor concentration in a SiC epitaxial growth layer was <NUM> × <NUM><NUM> cm-<NUM>. Moreover, the thickness of the SiO<NUM> film <NUM> was about <NUM>.

Note that for comparison, a sample obtained in such a manner that by a method disclosed in Non-Patent Document <NUM>, a SiO<NUM> film <NUM> is formed on a SiC substrate <NUM> having a surface subjected to high-temperature H<NUM> etching and the SiC substrate <NUM> is subsequently thermally treated in high-temperature N<NUM> gas atmosphere and a sample obtained in such a manner that a SiO<NUM> film <NUM> is formed on a surface of a SiC substrate <NUM> by thermal oxidation and the SiC substrate <NUM> is subsequently thermally treated in high-temperature NO gas atmosphere were also formed.

<FIG> shows graphs of results, the horizontal axis indicating an energy (ET) from a conduction band edge (EC) and the vertical axis indicating the interface defect density. The graph indicated by A shows the results for the sample obtained in such a manner that the SiO<NUM> film <NUM> is deposited on the SiC substrate <NUM> by the method shown in <FIG> and the SiC substrate <NUM> is subsequently thermally treated in high-temperature NO gas atmosphere. Moreover, the graph indicated by B shows the results for the sample obtained in such a manner that by the method disclosed in Non-Patent Document <NUM>, the SiO<NUM> film is formed on the SiC substrate and the SiC substrate is subsequently thermally treated in high-temperature N<NUM> gas atmosphere. Further, the graph indicated by C shows the results for the sample obtained in such a manner that the SiO<NUM> film is formed on the surface of the SiC substrate by thermal oxidation and the SiC substrate is subsequently thermally treated in high-temperature NO gas atmosphere.

<FIG> shows that the samples (graphs A, B) whose SiC substrates were subjected to etching with high-temperature H<NUM> gas in Si-excess atmosphere as pretreatment before formation of the SiO<NUM> film on the SiC substrate have interface state densities significantly lower than that of the sample (graph C) whose SiC substrate was not subjected to etching with high-temperature H<NUM> gas as pretreatment.

According to these results, many defects remain on the surface of the SiC substrate <NUM> from which the oxide film has been removed after sacrificial oxidation of the surface, and in order to efficiently eliminate these defects, the surface of the SiC substrate <NUM> is etched with high-temperature H<NUM> gas in Si-excess atmosphere so that the interface state density can be significantly reduced.

An n-channel MOSFET was fabricated with the SiO<NUM> film <NUM> deposited on the SiC substrate <NUM> by the method shown in <FIG>, and transistor characteristics were evaluated. Note that for comparison, the sample obtained in such a manner that by the method disclosed in Non-Patent Document <NUM>, the SiO<NUM> film is formed on the SiC substrate and the SiC substrate is subsequently thermally treated in high-temperature N<NUM> gas atmosphere was also formed.

<FIG> is a cross-sectional view showing the structure of the formed n-channel MOSFET. A p--type SiC epitaxial growth layer 10A is formed on a p-type <NUM>-SiC(<NUM>) substrate <NUM>, and n+-type source region <NUM> and drain region <NUM> are formed on a surface of the epitaxial growth layer 10A. A gate insulating film <NUM> formed of a SiO<NUM> film is formed on the surface of the epitaxial growth layer 10A between the source region <NUM> and the drain region <NUM>. A source electrode <NUM>, a drain electrode <NUM>, and a gate electrode <NUM> are each formed on the source region <NUM>, the drain region <NUM>, and the gate insulating film <NUM>.

Note that an acceptor concentration in the p--type SiC epitaxial growth layer 10A was <NUM> × <NUM><NUM> cm-<NUM> and a donor concentration in the source region <NUM> and the drain region <NUM> was <NUM> × <NUM><NUM> cm-<NUM>. Moreover, the thickness of the gate insulating film <NUM> was <NUM>.

<FIG> shows graphs of the drain current-gate voltage characteristics of the fabricated n-channel MOSFET. A graph indicated by A shows results in a case where by the method shown in <FIG>, the gate insulating film <NUM> is formed and the SiC substrate is subsequently thermally treated in high-temperature NO gas atmosphere. Moreover, a graph indicated by B shows results in a case where by the method disclosed in Non-Patent Document <NUM>, the gate insulating film <NUM> is formed and the SiC substrate is subsequently thermally treated in high-temperature N<NUM> gas atmosphere.

<FIG> shows that both the samples have high drain currents, but the sample (graph A) whose SiC substrate was thermally treated in high-temperature NO gas atmosphere shows normally-off characteristics (positive threshold voltage) while the sample (graph B) whose SiC substrate was thermally treated in high-temperature N<NUM> gas atmosphere shows normally-on characteristics (negative threshold voltage).

<FIG> shows graphs of the channel mobility of the formed n-channel MOSFET. A graph indicated by A shows results in a case where by the method shown in <FIG>, the gate insulating film <NUM> is formed and the SiC substrate is subsequently thermally treated in high-temperature NO gas atmosphere. Moreover, a graph indicated by B shows results in a case where by the method disclosed in Non-Patent Document <NUM>, the gate insulating film <NUM> is formed and the SiC substrate is subsequently thermally treated in high-temperature N<NUM> gas atmosphere.

<FIG> shows that both the samples have high channel mobilities, but the sample (graph A) whose SiC substrate was thermally treated in high-temperature NO gas atmosphere shows normally-off characteristics (positive threshold voltage) while the sample (graph B) whose SiC substrate was thermally treated in high-temperature N<NUM> gas atmosphere shows normally-on characteristics (negative threshold voltage). Note that the graph B shows a decrease in the channel mobility at a high gate voltage.

According to these results, the SiC substrate <NUM> is etched with high-temperature H<NUM> gas in Si-excess atmosphere before deposition of the SiO<NUM> film <NUM> on the SiC substrate <NUM> and the SiC substrate <NUM> is thermally treated in high-temperature NO gas atmosphere after deposition of the SiO<NUM> film <NUM>, so that a MOSFET having a high drain current and a high channel mobility and having normally-off characteristics can be obtained.

<FIG> shows graphs of measurement results of a nitrogen atom density in the SiO<NUM> film <NUM> and at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> by a secondary ion mass spectrometry (SIMS) method. The horizontal axis indicates a position in a film thickness direction, zero indicates the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM>, a positive side indicates a position in the SiC substrate <NUM>, and a negative side indicates a position in the SiO<NUM> film <NUM>. Moreover, the vertical axis indicates the nitrogen atom density.

A graph indicated by A shows results for the sample obtained in such a manner that the SiO<NUM> film <NUM> is deposited on the SiC substrate <NUM> by the method shown in <FIG> and the SiC substrate <NUM> is subsequently thermally treated in high-temperature NO gas atmosphere. Moreover, a graph indicated by B shows results for the sample obtained in such a manner that by the method disclosed in Non-Patent Document <NUM>, the SiO<NUM> film <NUM> is formed on the SiC substrate <NUM> and the SiC substrate <NUM> is subsequently thermally treated in high-temperature N<NUM> gas atmosphere.

<FIG> shows that for both the samples, a sufficient density of nitrogen atoms is introduced into the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM>. Thus, it is assumed that the defect density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is sufficiently reduced.

On the other hand, it shows that the nitrogen atom density in the SiO<NUM> film <NUM> is extremely low in the sample (graph A) whose the SiC substrate <NUM> was thermally treated in high-temperature NO gas atmosphere while a high density of nitrogen atoms is present in the SiO<NUM> film <NUM> in the sample (graph B) whose SiC substrate <NUM> was thermally treated in high-temperature N<NUM> gas atmosphere.

<FIG> shows graphs of a correlation between the nitrogen atom density in the SiO<NUM> film <NUM> and an effective fixed charged density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM>. Here, the nitrogen atom density in the SiO<NUM> film <NUM> indicates the average of nitrogen atom densities in the SiO<NUM> film <NUM> in a region of <NUM> to <NUM> from the interface. Moreover, the effective fixed charge density at the interface was obtained from a voltage shift of an actual measurement value of the capacitance-voltage characteristics of the MOS capacitor from theoretical characteristics. A rectangular mark in the figure indicates a result for the sample whose SiC substrate <NUM> was thermally treated in high-temperature NO gas atmosphere. Moreover, a circular mark in the figure indicates a result for the sample whose SiC substrate <NUM> was thermally treated in high-temperature N<NUM> gas atmosphere. A numerical value near each symbol indicates a thermal treatment temperature.

<FIG> shows that the effective fixed charged density at the interface is high for the sample (circular marks) having a high nitrogen atom density in the SiO<NUM> film <NUM> and thermally treated in high-temperature N<NUM> gas atmosphere while the effective fixed charged density at the interface is low for the sample (rectangular marks) having a low nitrogen atom density in the SiO<NUM> film <NUM> and thermally treated in high-temperature NO gas atmosphere.

According to these results, it is assumed as follows. If the nitrogen atom density in the SiO<NUM> film <NUM> is extremely high, the nitrogen atoms and impurity atoms are bound to each other, and a positive fixed charge is generated in the SiO<NUM> film <NUM>. Accordingly, the MOSFET shows normally-on characteristics. Conversely, if the nitrogen atom density in the SiO<NUM> film <NUM> is low, a positive fixed charge is less likely to be generated in the SiO<NUM> film <NUM>, and accordingly, the MOSFET shows normally-off characteristics. Note that the impurity atoms to be bound to the nitrogen atoms are assumed to be, e.g., hydrogen introduced in a thermal treatment step (hydrogen sintering step) performed in atmosphere containing hydrogen at a final stage of fabricating the MOSFET.

As shown in <FIG>, when the nitrogen atom density in the SiO<NUM> film <NUM> is low, there are cases where the effective fixed charge density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is negative and positive. This is because of the following reasons.

The effective fixed charge density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is represented as the sum of a positive charge due to an impurity or a defect in the SiO<NUM> film <NUM> (location close to the interface with the SiC substrate <NUM>) and a negative charge due to electrons trapped at interface states. In a case where the NO treatment temperature is low, the positive charge is low because of a low nitrogen atom density in the SiO<NUM> film <NUM>, but the negative charge is relatively high because of a relatively-high interface state density. As a result, the effective fixed charge density represented by a difference therebetween is negative.

On the other hand, in a case where the NO treatment temperature is high, the positive charge is high because of a high nitrogen atom density in the SiO<NUM> film <NUM>, but the negative charge is relatively low because of a low interface state density. As a result, the effective fixed charge density is positive.

The effective fixed charge density is a great negative value when the interface state density is extremely high, and this is not preferable because the drain current of a SiC MOSFET is lowered. On the other hand, the effective fixed charge density is a great positive value when the nitrogen density in the SiO<NUM> film <NUM> is extremely high, and this is not preferable because normally-on (negative threshold voltage) characteristics are easily brought due to influence of this high positive charge density.

As shown in <FIG>, the absolute value of the effective fixed charge density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is preferably <NUM> × <NUM><NUM> cm-<NUM> or less in order for the MOSFET to show normally-off characteristics.

<FIG> shows a graph (graph indicated by A) of measurement results of the channel mobility when a temperature of thermally treating the SiC substrate <NUM> in high-temperature NO gas atmosphere is changed within a range of <NUM> to <NUM> in a case where the n-channel MOSFET having, as the gate insulating film <NUM>, the SiO<NUM> film <NUM> deposited on the SiC substrate <NUM> is formed by the method shown in <FIG>. Moreover, a graph indicated by B shows results in a case where the SiC substrate <NUM> is etched with high-temperature H<NUM> gas without addition of a slight amount of SiH<NUM> gas and such etching is not performed under Si-excess atmosphere.

<FIG> shows that when the SiC substrate <NUM> is etched with high-temperature H<NUM> gas, the channel mobility is high in a case of performing etching under Si-excess atmosphere (graph A) while the channel mobility is low in a case of not performing etching under Si-excess atmosphere (graph B). This is because of the following reasons.

That is, an effect of reducing the interface defect density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> can be expected in such a manner that the SiC substrate <NUM> is etched with high-temperature H<NUM> gas before formation of the SiO<NUM> film <NUM> on the SiC substrate <NUM>. However, in a case where the SiO<NUM> film <NUM> is deposited on the SiC substrate <NUM> by the CVD method, reaction gas contains O<NUM> gas or N<NUM>O gas, and for this reason, the surface of the SiC substrate <NUM> might be slightly oxidized initially during deposition. However, about one-to-three monolayer thick extremely-thin Si layer is formed on the surface of the SiC substrate <NUM> in such a manner that etching with high-temperature H<NUM> gas is performed under Si-excess atmosphere, and therefore, even in this case, only these extremely-thin Si layers are oxidized and oxidation of the surface of the SiC substrate <NUM> can be prevented. Thus, the interface defect density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is significantly reduced, and a high channel mobility is obtained.

On the other hand, in a case where etching with high-temperature H<NUM> gas is not performed under Si-excess atmosphere, no extremely-thin Si films are formed on the surface of the SiC substrate <NUM>, and for this reason, even if the SiO<NUM> film is deposited under optimal conditions and the high-temperature NO thermal treatment is performed, the surface of the SiC substrate <NUM> is oxidized at an initial stage of depositing the SiO<NUM> film. As a result, the interface defect density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> is not sufficiently reduced, and a low channel mobility is obtained.

<FIG> shows that a high channel mobility is obtained in such a manner that the NO thermal treatment for the SiC substrate <NUM> is performed within a temperature range of <NUM> to <NUM>. If the NO thermal treatment temperature is lower than <NUM>, this is not preferable because a sufficient density of nitrogen atoms is not introduced into the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM>, and for this reason, interface nitridation is not sufficiently performed and an effect of reducing the interface defect density cannot be obtained. If the NO thermal treatment temperature exceeds <NUM>, this is not preferable because oxidation of the SiC substrate due to NO gas progresses and new interface defects are generated.

<FIG> shows a graph of measurement results of the channel mobility when a temperature of hydrogen-etching the SiC substrate <NUM> in Si-excess atmosphere is changed within a range of <NUM> to <NUM> before formation of the SiO<NUM> film <NUM> in a case where the n-channel MOSFET having, as the gate insulating film <NUM>, the SiO<NUM> film <NUM> deposited on the SiC substrate <NUM> is formed by the method shown in <FIG>.

<FIG> shows that a high channel mobility is obtained in such a manner that hydrogen etching for the SiC substrate <NUM> is performed within a temperature range of <NUM> to <NUM>. If the hydrogen etching temperature is lower than <NUM>, this is not preferable because the surface of the SiC substrate <NUM> cannot be sufficiently cleaned and an effect of reducing the interface defect density cannot be obtained. If the hydrogen etching temperature exceeds <NUM> close to a Si melting point (<NUM>), this is not preferable because it is difficult to form the extremely-thin Si film on the surface of the SiC substrate <NUM> and an effect of reducing the interface defect density cannot be obtained.

As described above, the SiC semiconductor device manufacturing method in the present embodiment includes a step of etching the surface of the SiC substrate <NUM> with H<NUM> gas under Si-excess atmosphere within a temperature range of <NUM> to <NUM>, a step of depositing, by the CVD method, the SiO<NUM> film <NUM> at such a temperature that the SiC substrate <NUM> is not oxidized, and a step of thermally treating the SiC substrate <NUM> formed with the SiO<NUM> film <NUM> in NO gas atmosphere within a temperature range of <NUM> to <NUM>. With this configuration, the defect density at the interface between the SiO<NUM> film <NUM> and the SiC substrate <NUM> can be significantly reduced, and a SiC MOSFET having a high channel mobility and normally-off characteristics can be achieved in a case where the SiC MOSFET having the SiO<NUM> film as the gate insulating film <NUM> is formed.

In the above-described embodiment, the example where the MOSFET is formed on the <NUM>-SiC(<NUM>) plane has been described. Generally, it has been known that in a case where a SiC MOSFET is formed on a non-basal plane such as a (<NUM>-<NUM>) plane or a (<NUM>-<NUM>) plane, characteristics better than those in the case of a (<NUM>) plane are obtained.

Actually, in a case where the MOSFET is formed in such a manner that the gate insulating film <NUM> is formed by the method shown in <FIG> and the SiC substrate is subsequently thermally treated in NO gas atmosphere at <NUM>, the MOSFET formed on the (<NUM>-<NUM>) plane showed excellent characteristics of a channel mobility of <NUM><NUM>/Vs and a threshold voltage of <NUM> V and the MOSFET formed on the (<NUM>-<NUM>) plane showed excellent characteristics of a channel mobility of <NUM><NUM>/Vs and a threshold voltage of <NUM> V. As described above, the present invention is useful for many crystal planes of SiC substrates for actual use. Note that the acceptor density in the p--type SiC epitaxial growth layer 10A in the MOSFET formed herein was <NUM> × <NUM><NUM> cm-<NUM>.

It has been known that among SiC power MOSFETs, a trench MOSFET having a MOS channel formed on a trench side wall is advantages in extremely reducing on-resistance. In this case, a SiC substrate surface is a (<NUM>) plane, and therefore, the MOS channel needs to be formed on a (<NUM>-<NUM>) plane (A-plane) or a (<NUM>-<NUM>) plane (M-plane) which is a side wall surface. In an actual SiC power MOSFET, an acceptor density in a p-type epitaxial growth layer is a relatively-high value of about <NUM><NUM> to <NUM><NUM> cm-<NUM>.

Thus, in order to verify whether or not the present invention is also effective for the trench SiC power MOSFET, MOSFETs having the structure shown in <FIG> were formed using SiC substrates having a (<NUM>-<NUM>) plane and a (<NUM>-<NUM>) plane as surfaces by the method shown in <FIG> while an acceptor density in a p-type epitaxial growth layer is changed within a range of <NUM><NUM> to <NUM><NUM> cm-<NUM>, and a channel mobility was measured. Moreover, as comparative examples, MOSFETs were formed, using SiC substrates having a (<NUM>-<NUM>) plane and a (<NUM>-<NUM>) plane as surfaces, in such a manner that a gate insulating film (SiO<NUM> film) <NUM> is formed on a SiC substrate <NUM> by thermal oxidation and the SiC substrate <NUM> is subsequently thermally treated in high-temperature NO gas atmosphere. Here, the thickness of the gate insulating film was <NUM>.

<FIG> shows graphs of results, the vertical axis indicating the channel mobility and the horizontal axis indicating the acceptor density in the p-type epitaxial growth layer (p-type region).

As shown in a graph A1, the MOSFET formed on the (<NUM>-<NUM>) plane exhibited a high channel mobility of about <NUM><NUM>/Vs within an acceptor density of <NUM><NUM> to <NUM><NUM> cm-<NUM>. As shown in a graph A2, the MOSFET formed on the (<NUM>-<NUM>) plane also exhibited a high channel mobility of <NUM> to <NUM><NUM>/Vs within an acceptor density of <NUM><NUM> to <NUM><NUM> cm-<NUM>. In any of these MOSFETs, a channel mobility drop is rather small when the acceptor density in the p-type epitaxial growth layer increases, as compared to the MOSFETs indicated by B1 and B2 and formed by the typical method. At an acceptor density of <NUM> × <NUM><NUM> cm-<NUM>, an extremely-high channel mobility <NUM> to <NUM> times as high as that in the typical method was obtained.

According to the present invention, excellent MOS interface characteristics are obtained, and therefore, the present invention is also effective for formation of other SiC devices using MOS interfaces, such as an insulated-gate bipolar transistor (IGBT).

The present invention has been described above with reference to the preferable embodiment, but such description is not a limited matter and various modifications can be made, needless to say. For example, in the above-described embodiment, the SiC epitaxial layer is formed on the surface of the SiC substrate, and the SiO<NUM> film is formed on the SiC epitaxial layer. However, the SiO<NUM> film may be directly formed on the SiC substrate.

In the above-described embodiment, the SiC substrate from which the oxide film is removed after sacrificial oxidation of the surface is used. However, the manufacturing method of the present invention is also applicable to a SiC substrate not subjected to sacrificial oxidation.

Claim 1:
A SiC semiconductor device manufacturing method, comprising:
a step (A) of etching a surface of a SiC substrate (<NUM>) with H<NUM> gas under Si-excess atmosphere within a temperature range of <NUM> to <NUM>;
a step (B) of depositing, by a CVD method, a SiO<NUM> film (<NUM>) on the SiC substrate (<NUM>) at such a temperature that the SiC substrate (<NUM>) is not oxidized; and
a step (C) of thermally treating the SiC substrate (<NUM>), on which the SiO<NUM> film (<NUM>) is deposited, in NO gas atmosphere within a temperature range of <NUM> to <NUM>.