Abstract
A SiC semiconductor device manufacturing method includes a step of etching a surface of a SiC substrate 1 with H.sub.2 gas under Si-excess atmosphere within a temperature range of 1000 C. to 1350 C., a step of depositing, by a CVD method, a SiO.sub.2 film 2 on the SiC substrate 1 at such a temperature that the SiC substrate 1 is not oxidized, and a step of thermally treating the SiC substrate 1, on which the SiO.sub.2 film 2 is deposited, in NO gas atmosphere within a temperature range of 1150 C. to 1350 C.
Claims
1. A SiC semiconductor device manufacturing method, comprising: a step (A) of etching a surface of a SiC substrate with H.sub.2 gas under Si-excess atmosphere within a temperature range of 1000 C. to 1350 C.; a step (B) of depositing, by a CVD method, a SiO.sub.2 film on the SiC substrate at such a temperature that the SiC substrate is not oxidized; and a step (C) of thermally treating the SiC substrate, on which the SiO.sub.2 film is deposited, in NO gas atmosphere within a temperature range of 1150 C. to 1350 C.
2. The SiC semiconductor device manufacturing method according to claim 1, wherein in the step (A), a one-to-three monolayer thick Si layer is formed on the surface of the SiC substrate.
3. The SiC semiconductor device manufacturing method according to claim 1, wherein the step (A) is performed under atmosphere where SiH.sub.4 gas or gas containing a Si atom is added to H.sub.2 gas.
4. The SiC semiconductor device manufacturing method according to claim 1, further comprising: before the step (A), a step of etching away an oxide film formed on the surface of the SiC substrate after sacrificial oxidation of the SiC substrate.
5. The SiC semiconductor device manufacturing method according to claim 1, wherein the SiC substrate includes a SiC substrate having a SiC epitaxial layer on a surface.
6. The SiC semiconductor device manufacturing method according to claim 1, wherein an absolute value of an effective fixed charge density at an interface between the SiO.sub.2 film and the SiC substrate is 410.sup.11 cm.sup.2 or less.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIGS. 1A to 1C show views of a SiC semiconductor device manufacturing method in one embodiment of the present invention.
(2) FIG. 2 shows graphs of a defect density at an interface between a SiO.sub.2 film and a SiC substrate.
(3) FIG. 3 shows a cross-sectional view of the structure of an n-channel MOSFET.
(4) FIG. 4 shows graphs of the drain current-gate voltage characteristics of the n-channel MOSFET.
(5) FIG. 5 shows graphs of the channel mobility of the n-channel MOSFET.
(6) FIG. 6 shows graphs of a nitrogen atom density in the SiO.sub.2 film and at the SiO.sub.2 film/SiC substrate interface.
(7) FIG. 7 shows graphs of a correlation between the nitrogen atom density in the SiO.sub.2 film and an effective fixed charge density at the SiC substrate/SiO.sub.2 film interface.
(8) FIG. 8 shows graphs of the dependency of the channel mobility on a NO thermal treatment temperature.
(9) FIG. 9 shows a graph of the dependency of the channel mobility on a hydrogen etching temperature.
(10) FIG. 10 shows graphs of the dependency of the channel mobility on an acceptor density in a p-type epitaxial growth layer.
DESCRIPTION OF EMBODIMENTS
(11) 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.
(12) FIGS. 1A to 1C are views showing a SiC semiconductor device manufacturing method in one embodiment of the present invention.
(13) As shown in FIG. 1A, a surface of a SiC substrate 1 is etched with high-temperature H.sub.2 gas under Si-excess atmosphere. At this point, an extremely-thin (one-to-three monolayer thcikness) Si film is formed on the SiC substrate 1. Etching with high-temperature H.sub.2 gas is performed by addition of a slight amount of SiH.sub.4 gas, and may be performed, for example, under conditions of a H.sub.2 flow rate: 5000 sccm, a SiH.sub.4 flow rate: 0.2 sccm, a temperature: 1300 C., a pressure: 13 kPa, and a time: 15 minutes. Etching with high-temperature H.sub.2 gas is preferably performed within a temperature range of 1000 C. to 1350 C. 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.sub.2Cl.sub.2 or SiH.sub.3Cl, instead of SiH.sub.4, similar advantageous effects are also obtained.
(14) As the SiC substrate 1, one configured such that a SiC epitaxial layer (not shown) is formed on the SiC substrate 1 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.
(15) Next, as shown in FIG. 1B, a SiO.sub.2 film 2 is deposited on the SiC substrate 1 by a plasma CVD method. The SiO.sub.2 film 2 may be deposited at such a temperature that the SiC substrate 1 is not oxidized, and for example, may be deposited under conditions of a tetraethoxysilane (TEOS) flow rate: 0.3 seem, an O.sub.2 flow rate: 450 seem, a temperature: 400 C., a pressure: 34 Pa, a radio-frequency power: 100 W, and a time: 30 minutes. The SiO.sub.2 film 2 is preferably deposited within a temperature range of 300 C. to 450 C.
(16) Note that the SiO.sub.2 film 2 may be deposited using a thermal CVD method. In this case, the SiO.sub.2 film 2 may be deposited under conditions of a SiH.sub.4 flow rate: 5 seem, a N.sub.2O flow rate: 300 seem, a N.sub.2 flow rate: 3000 seem, a temperature: 720 C., a pressure: 15 Pa, and a time: 4 minutes.
(17) Next, as shown in FIG. 1C, the SiC substrate 1 on which the SiO.sub.2 film 2 is deposited is thermally treated in NO gas atmosphere. Conditions for the thermal treatment may be, for example, a NO flow rate: 300 seem, a N.sub.2 flow rate: 2700 seem, a temperature: 1250 C., a pressure: 1 atm, and a time: 60 minutes. The thermal treatment in NO gas atmosphere is preferably performed within a temperature range of 1150 C. to 1350 C. Here, in NO gas atmosphere includes atmosphere in which NO gas is diluted with dilution gas such as N.sub.2 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: 10%; N.sub.2 flow rate: 90%) with N.sub.2 gas.
(18) (Analysis of Interface Defect Density)
(19) A MOS capacitor was formed on the SiO.sub.2 film 2 deposited on the SiC substrate 1 by the method shown in FIGS. 1A to 1C, and a defect density (interface state density) at an interface between the SiO.sub.2 film 2 and the SiC substrate 1 was obtained using analysis (high-low method) of CV characteristics. For the SiC substrate 1, an n-type 4HSiC(0001) substrate was used, and a donor concentration in a SiC epitaxial growth layer was 510.sup.15 cm.sup.3. Moreover, the thickness of the SiO.sub.2 film 2 was about 30 nm.
(20) Note that for comparison, a sample obtained in such a manner that by a method disclosed in Non-Patent Document 2, a SiO.sub.2 film 2 is formed on a SiC substrate 1 having a surface subjected to high-temperature H.sub.2 etching and the SiC substrate 1 is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere and a sample obtained in such a manner that a SiO.sub.2 film 2 is formed on a surface of a SiC substrate 1 by thermal oxidation and the SiC substrate 1 is subsequently thermally treated in high-temperature NO gas atmosphere were also formed.
(21) FIG. 2 shows graphs of results, the horizontal axis indicating an energy (ET) from a conduction band edge (E.sub.C) 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.sub.2 film 2 is deposited on the SiC substrate 1 by the method shown in FIGS. 1A to 1C and the SiC substrate 1 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 2, the SiO.sub.2 film is formed on the SiC substrate and the SiC substrate is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere. Further, the graph indicated by C shows the results for the sample obtained in such a manner that the SiO.sub.2 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.
(22) FIG. 2 shows that the samples (graphs A, B) whose SiC substrates were subjected to etching with high-temperature H.sub.2 gas in Si-excess atmosphere as pretreatment before formation of the SiO.sub.2 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.sub.2 gas as pretreatment.
(23) According to these results, many defects remain on the surface of the SiC substrate 1 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 1 is etched with high-temperature H.sub.2 gas in Si-excess atmosphere so that the interface state density can be significantly reduced.
(24) (Characterization of Fabricated SiC MOSFET)
(25) An n-channel MOSFET was fabricated with the SiO.sub.2 film 2 deposited on the SiC substrate 1 by the method shown in FIGS. 1A to 1C, 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 2, the SiO.sub.2 film is formed on the SiC substrate and the SiC substrate is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere was also formed.
(26) FIG. 3 is a cross-sectional view showing the structure of the formed n-channel MOSFET. A p.sup.-type SiC epitaxial growth layer 10A is formed on a p-type 4HSiC(0001) substrate 10, and n.sup.+-type source region 11 and drain region 12 are formed on a surface of the epitaxial growth layer 10A. A gate insulating film 20 formed of a SiO.sub.2 film is formed on the surface of the epitaxial growth layer 10A between the source region 11 and the drain region 12. A source electrode 30, a drain electrode 31, and a gate electrode 32 are each formed on the source region 11, the drain region 12, and the gate insulating film 20.
(27) Note that an acceptor concentration in the p.sup.-type SiC epitaxial growth layer 10A was 110.sup.15 cm.sup.3 and a donor concentration in the source region 11 and the drain region 12 was 810.sup.19 cm.sup.3. Moreover, the thickness of the gate insulating film 20 was 30 nm.
(28) (A) Drain Current-Gate Voltage Characteristics
(29) FIG. 4 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 FIGS. 1A to 1C, the gate insulating film 20 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 2, the gate insulating film 20 is formed and the SiC substrate is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere.
(30) FIG. 4 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.sub.2 gas atmosphere shows normally-on characteristics (negative threshold voltage).
(31) (B) Channel Mobility
(32) FIG. 5 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 FIGS. 1A to 1C, the gate insulating film 20 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 2, the gate insulating film 20 is formed and the SiC substrate is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere.
(33) FIG. 5 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.sub.2 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.
(34) According to these results, the SiC substrate 1 is etched with high-temperature H.sub.2 gas in Si-excess atmosphere before deposition of the SiO.sub.2 film 2 on the SiC substrate 1 and the SiC substrate 1 is thermally treated in high-temperature NO gas atmosphere after deposition of the SiO.sub.2 film 2, so that a MOSFET having a high drain current and a high channel mobility and having normally-off characteristics can be obtained.
(35) (Nitrogen Atom Density in SiO.sub.2 Film and at SiO.sub.2 Film/SiC Interface)
(36) FIG. 6 shows graphs of measurement results of a nitrogen atom density in the SiO.sub.2 film 2 and at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 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.sub.2 film 2 and the SiC substrate 1, a positive side indicates a position in the SiC substrate 1, and a negative side indicates a position in the SiO.sub.2 film 2. Moreover, the vertical axis indicates the nitrogen atom density.
(37) A graph indicated by A shows results for the sample obtained in such a manner that the SiO.sub.2 film 2 is deposited on the SiC substrate 1 by the method shown in FIGS. 1A to 1C and the SiC substrate 1 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 2, the SiO.sub.2 film 2 is formed on the SiC substrate 1 and the SiC substrate 1 is subsequently thermally treated in high-temperature N.sub.2 gas atmosphere.
(38) FIG. 6 shows that for both the samples, a sufficient density of nitrogen atoms is introduced into the interface between the SiO.sub.2 film 2 and the SiC substrate 1. Thus, it is assumed that the defect density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is sufficiently reduced.
(39) On the other hand, it shows that the nitrogen atom density in the SiO.sub.2 film 2 is extremely low in the sample (graph A) whose the SiC substrate 1 was thermally treated in high-temperature NO gas atmosphere while a high density of nitrogen atoms is present in the SiO.sub.2 film 2 in the sample (graph B) whose SiC substrate 1 was thermally treated in high-temperature N.sub.2 gas atmosphere.
(40) (Correlation Between Nitrogen Atom Density in SiO.sub.2 Film and Effective Fixed Charge Density at Interface)
(41) FIG. 7 shows graphs of a correlation between the nitrogen atom density in the SiO.sub.2 film 2 and an effective fixed charged density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1. Here, the nitrogen atom density in the SiO.sub.2 film 2 indicates the average of nitrogen atom densities in the SiO.sub.2 film 2 in a region of 5 to 20 nm 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 1 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 1 was thermally treated in high-temperature N.sub.2 gas atmosphere. A numerical value near each symbol indicates a thermal treatment temperature.
(42) FIG. 7 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.sub.2 film 2 and thermally treated in high-temperature N.sub.2 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.sub.2 film 2 and thermally treated in high-temperature NO gas atmosphere.
(43) According to these results, it is assumed as follows. If the nitrogen atom density in the SiO.sub.2 film 2 is extremely high, the nitrogen atoms and impurity atoms are bound to each other, and a positive fixed charge is generated in the SiO.sub.2 film 2. Accordingly, the MOSFET shows normally-on characteristics. Conversely, if the nitrogen atom density in the SiO.sub.2 film 2 is low, a positive fixed charge is less likely to be generated in the SiO.sub.2 film 2, 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.
(44) As shown in FIG. 7, when the nitrogen atom density in the SiO.sub.2 film 2 is low, there are cases where the effective fixed charge density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is negative and positive. This is because of the following reasons.
(45) The effective fixed charge density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is represented as the sum of a positive charge due to an impurity or a defect in the SiO.sub.2 film 2 (location close to the interface with the SiC substrate 1) 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.sub.2 film 2, 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.
(46) 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.sub.2 film 2, but the negative charge is relatively low because of a low interface state density. As a result, the effective fixed charge density is positive.
(47) 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.sub.2 film 2 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.
(48) As shown in FIG. 7, the absolute value of the effective fixed charge density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is preferably 410.sup.1 cm.sup.2 or less in order for the MOSFET to show normally-off characteristics.
(49) (Dependency of Channel Mobility on NO Thermal Treatment Temperature)
(50) FIG. 8 shows a graph (graph indicated by A) of measurement results of the channel mobility when a temperature of thermally treating the SiC substrate 1 in high-temperature NO gas atmosphere is changed within a range of 1100 C. to 1350 C. in a case where the n-channel MOSFET having, as the gate insulating film 20, the SiO.sub.2 film 2 deposited on the SiC substrate 1 is formed by the method shown in FIGS. 1A to 1C. Moreover, a graph indicated by B shows results in a case where the SiC substrate 1 is etched with high-temperature H.sub.2 gas without addition of a slight amount of SiH.sub.4 gas and such etching is not performed under Si-excess atmosphere.
(51) FIG. 8 shows that when the SiC substrate 1 is etched with high-temperature H.sub.2 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.
(52) That is, an effect of reducing the interface defect density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 can be expected in such a manner that the SiC substrate 1 is etched with high-temperature H.sub.2 gas before formation of the SiO.sub.2 film 2 on the SiC substrate 1. However, in a case where the SiO.sub.2 film 2 is deposited on the SiC substrate 1 by the CVD method, reaction gas contains 02 gas or N.sub.2O gas, and for this reason, the surface of the SiC substrate 1 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 1 in such a manner that etching with high-temperature H.sub.2 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 1 can be prevented. Thus, the interface defect density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is significantly reduced, and a high channel mobility is obtained.
(53) On the other hand, in a case where etching with high-temperature H.sub.2 gas is not performed under Si-excess atmosphere, no extremely-thin Si films are formed on the surface of the SiC substrate 1, and for this reason, even if the SiO.sub.2 film is deposited under optimal conditions and the high-temperature NO thermal treatment is performed, the surface of the SiC substrate 1 is oxidized at an initial stage of depositing the SiO.sub.2 film. As a result, the interface defect density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 is not sufficiently reduced, and a low channel mobility is obtained.
(54) FIG. 8 shows that a high channel mobility is obtained in such a manner that the NO thermal treatment for the SiC substrate 1 is performed within a temperature range of 1150 C. to 1350 C. If the NO thermal treatment temperature is lower than 1150 C., this is not preferable because a sufficient density of nitrogen atoms is not introduced into the interface between the SiO.sub.2 film 2 and the SiC substrate 1, 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 1350 C., this is not preferable because oxidation of the SiC substrate due to NO gas progresses and new interface defects are generated.
(55) (Dependency of Channel Mobility on Hydrogen Etching Temperature)
(56) FIG. 9 shows a graph of measurement results of the channel mobility when a temperature of hydrogen-etching the SiC substrate 1 in Si-excess atmosphere is changed within a range of 900 C. to 1400 C. before formation of the SiO.sub.2 film 2 in a case where the n-channel MOSFET having, as the gate insulating film 20, the SiO.sub.2 film 2 deposited on the SiC substrate 1 is formed by the method shown in FIGS. 1A to 1C.
(57) FIG. 9 shows that a high channel mobility is obtained in such a manner that hydrogen etching for the SiC substrate 1 is performed within a temperature range of 1000 C. to 1350 C. If the hydrogen etching temperature is lower than 1000 C., this is not preferable because the surface of the SiC substrate 1 cannot be sufficiently cleaned and an effect of reducing the interface defect density cannot be obtained. If the hydrogen etching temperature exceeds 1400 C. close to a Si melting point (1420 C.), this is not preferable because it is difficult to form the extremely-thin Si film on the surface of the SiC substrate 1 and an effect of reducing the interface defect density cannot be obtained.
(58) As described above, the SiC semiconductor device manufacturing method in the present embodiment includes a step of etching the surface of the SiC substrate 1 with H.sub.2 gas under Si-excess atmosphere within a temperature range of 1000 C. to 1350 C., a step of depositing, by the CVD method, the SiO.sub.2 film 2 at such a temperature that the SiC substrate 1 is not oxidized, and a step of thermally treating the SiC substrate 1 formed with the SiO.sub.2 film 2 in NO gas atmosphere within a temperature range of 1150 C. to 1350 C. With this configuration, the defect density at the interface between the SiO.sub.2 film 2 and the SiC substrate 1 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.sub.2 film as the gate insulating film 20 is formed.
(59) In the above-described embodiment, the example where the MOSFET is formed on the 4HSiC(0001) 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 (11-20) plane or a (1-100) plane, characteristics better than those in the case of a (0001) plane are obtained.
(60) Actually, in a case where the MOSFET is formed in such a manner that the gate insulating film 20 is formed by the method shown in FIGS. 1A to 1C and the SiC substrate is subsequently thermally treated in NO gas atmosphere at 1250 C., the MOSFET formed on the (11-20) plane showed excellent characteristics of a channel mobility of 164 cm.sup.2/Vs and a threshold voltage of 1.21 V and the MOSFET formed on the (1-100) plane showed excellent characteristics of a channel mobility of 158 cm.sup.2/Vs and a threshold voltage of 1.28 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.sup.-type SiC epitaxial growth layer 10A in the MOSFET formed herein was 110.sup.16 cm.sup.3.
(61) 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 (0001) plane, and therefore, the MOS channel needs to be formed on a (11-20) plane (A-plane) or a (1-100) 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 10.sup.7 to 108 cm.sup.3.
(62) 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. 3 were formed using SiC substrates having a (11-20) plane and a (1-100) plane as surfaces by the method shown in FIGS. 1A to 1C while an acceptor density in a p-type epitaxial growth layer is changed within a range of 10.sup.17 to 10.sup.18 cm.sup.3, and a channel mobility was measured. Moreover, as comparative examples, MOSFETs were formed, using SiC substrates having a (11-20) plane and a (1-100) plane as surfaces, in such a manner that a gate insulating film (SiO.sub.2 film) 20 is formed on a SiC substrate 1 by thermal oxidation and the SiC substrate 1 is subsequently thermally treated in high-temperature NO gas atmosphere. Here, the thickness of the gate insulating film was 30 nm.
(63) FIG. 10 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).
(64) As shown in a graph A1, the MOSFET formed on the (11-20) plane exhibited a high channel mobility of about 130 cm.sup.2/Vs within an acceptor density of 10.sup.17 to 10.sup.18 cm.sup.3. As shown in a graph A2, the MOSFET formed on the (1-100) plane also exhibited a high channel mobility of 80 to 110 cm.sup.2/Vs within an acceptor density of 10.sup.17 to 10.sup.18 cm.sup.3. 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 110.sup.18 cm.sup.3, an extremely-high channel mobility 6 to 80 times as high as that in the typical method was obtained.
(65) 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).
(66) 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.sub.2 film is formed on the SiC epitaxial layer. However, the SiO.sub.2 film may be directly formed on the SiC substrate.
(67) 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.
(68) In the above-described embodiment, the SiO.sub.2 film 2 is deposited on the SiC substrate 1 by the CVD method. However, the SiO.sub.2 film may be formed in such a manner that a Si thin film is deposited by the CVD method and is subsequently thermally oxidized at such a temperature that the SiC substrate 1 is not oxidized. In this case, the Si thin film is formed on the surface of the SiC substrate 1 before formation of the SiO.sub.2 film 2, and therefore, etching of the SiC substrate 1 with high-temperature H.sub.2 as pretreatment is not necessarily performed under Si-excess atmosphere. Etching of the SiC substrate 1 with high-temperature H.sub.2 is preferably performed within a temperature range of 1200 C. to 1350 C.
DESCRIPTION OF REFERENCE CHARACTERS
(69) 1 SiC Substrate 2 SiO.sub.2 Film 10 p-type SiC Substrate 10A p.sup.-type SiC Epitaxial Growth Layer 11 Source Region 12 Drain Region 20 Gate Insulating Film 30 Source Electrode 31 Drain Electrode 32 Gate Electrode
