Patent Publication Number: US-8536583-B2

Title: MOSFET and method for manufacturing MOSFET

Description:
TECHNICAL FIELD 
     The present invention relates to a MOSFET and a method for manufacturing the MOSFET. 
     BACKGROUND ART 
     Conventionally, a semiconductor device using a silicon carbide (SiC) has been known (for example, WO01/018872 (hereinafter, referred to as Patent Document 1)). Patent Document 1 describes that a SiC substrate of 4H (Hexagonal) poly type having a plane orientation of almost {03-38} is used to form a Metal-Oxide-Semiconductor Field-effect Transistor (MOSFET). It is also described that in the MOSFET, a gate oxide film is formed by means of dry oxidation (thermal oxidation). Patent Document 1 describes that such a MOSFET achieves large channel mobility (approximately 100 cm 2 /Vs). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: WO 01/018872 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, as a result of analysis and review, the present inventor has found that the channel mobility in the above-described MOSFET occasionally fails to be sufficiently large. When the channel mobility is not large, excellent characteristics of the semiconductor device thus employing SiC cannot be exhibited stably. 
     The present invention is made to solve the foregoing problem, and an object of the present invention is to provide a MOSFET having improved channel mobility and a method for manufacturing such a MOSFET. 
     Means for Solving the Problems 
     The present inventor has diligently diagnosed what renders channel mobility small, in order to achieve large channel mobility in the MOSFET with good reproducibility as described above. As a result, the present invention has been accomplished. Specifically, the present inventor has found that the channel mobility becomes small due to a trap (hereinafter, also referred to as “interface state” or “interface state density”) existing in an interface between the gate oxide film and the SiC semiconductor film positioned below the gate oxide film. This can be also presumed from a fact that the above-described MOSFET has a threshold voltage much higher than its theoretical value. To achieve a MOSFET with reduced influence of such an interface state, the present inventor has diligently studied and accordingly arrived at the present invention. 
     Specifically, a MOSFET of the present invention includes: a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; a semiconductor layer formed on the main surface of the SiC substrate; and an insulating film formed in contact with a surface of the semiconductor layer, the MOSFET having a sub-threshold slope of not more than 0.4 V/Decade. 
     A method of the present invention for manufacturing a MOSFET includes the steps of: preparing a silicon carbide (SiC) substrate having a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane; forming a semiconductor layer on the main surface of the SiC substrate; and forming an insulating film in contact with a surface of the semiconductor layer, the MOSFET having a sub-threshold slope of not more than 0.4 V/Decade. 
     The present inventor has focused attention on the sub-threshold slope, which is associated with the interface state, and diligently studied a range of the sub-threshold slope so as to improve the mobility. As a result, the present inventor has found that by setting the sub-threshold slope at not more than 0.4 V/Decade, the interface state density can be reduced with good reproducibility near the interface between the insulating film and the semiconductor layer. In this way, most of carriers, which are to serve as an inversion channel layer, are prevented from being trapped in the interface state within the semiconductor layer at a region facing the insulating film. Therefore the channel mobility can be improved. 
     The lower limit of the off angle is set at 50° because it was observed that the carrier mobility is significantly increased as the off angle is increased in the course from a (01-14) plane in which the off angle is 43.3° to a (01-13) plane in which the off angle is 51.5° and because there is no natural plane in the range of the off angle between the (01-14) plane and the (01-13) plane. 
     Further, the upper limit of the off angle is 65° because it was observed that the carrier mobility is significantly decreased as the off angle is increased in the course of a (01-12) plane in which the off angle is 62.1° to a (01-10) plane in which the off angle is 90°, and because there is no natural plane in the range of the off angle between the (01-12) plane and the (01-10) plane. 
     The MOSFET preferably further includes a region including a nitrogen atom and interposed between the semiconductor layer and the insulating film. 
     In the MOSFET, a maximum value of nitrogen concentration is preferably 1×10 21  cm −3  or greater in the region at a portion distant away by 10 nm or smaller from an interface between the semiconductor layer and the insulating film. 
     In the method for manufacturing the MOSFET, the step of forming the insulating film preferably includes the steps of: forming the insulating film through dry oxidation; and thermally treating the insulating film using gas including a nitrogen atom as atmospheric gas. 
     The present inventor has found that the influence of the interface state is reduced by increasing the concentration of nitrogen atom near the interface between the semiconductor layer and the insulating film. This achieved a MOSFET allowing for further improved channel mobility. 
     In the MOSFET, preferably, the semiconductor layer is formed of SiC. SiC, which has a large band gap, has a maximum dielectric breakdown electric field and a heat conductivity both larger than those of silicon (Si), and allows for carrier mobility as large as that in silicon. Also, in SiC, saturation drift velocity of electrons and withstand voltage are large. Accordingly, a MOSFET can be achieved which allows for high efficiency, high voltage, and large capacitance. 
     In the MOSFET, the main surface of the SiC substrate may have an off orientation falling within a range of ±5° of a &lt;11-20&gt; direction. 
     The &lt;11-20&gt; direction is a representative off orientation in the SiC substrate. Variation of the off orientation, which is caused by variation, etc., in a slicing process in a step of manufacturing the substrate, is set to be ±5°, thereby facilitating formation of an epitaxial layer on the SiC substrate, and the like. In this way, the MOSFET can be manufactured readily. 
     In the MOSFET, the main surface of the SiC substrate may have an off orientation falling within a range of ±5° of a &lt;01-10&gt; direction. 
     As with the &lt;11-20&gt; direction described above, the &lt;01-10&gt; direction is a representative off orientation in the SiC substrate. Variation of the off orientation, which is caused by variation, etc., in a slicing process in a step of manufacturing the substrate, is set to be ±5°, thereby facilitating formation of an epitaxial layer on the SiC substrate, and the like. In this way, the MOSFET can be manufactured readily. 
     In the MOSFET, the main surface of the SiC substrate can have an off angle of not less than −3° and not more than +5° relative to a (0-33-8) plane in the &lt;01-10&gt; direction. 
     In this way, the channel mobility can be further improved. Here, the off angle is thus set at not less than −3° and not more than +5° relative to the plane orientation {03-38} because particularly high channel mobility was obtained in this range as a result of inspecting a relation between the channel mobility and the off angle. 
     Here, the state in which “the off angle is not less than −3° and not more than +5° relative to plane orientation {03-38}” refers to a state in which the orthogonal projection of a normal line of the main surface to a flat plane defined by the &lt;0001&gt; direction and the &lt;01-10&gt; direction serving as a reference for the off orientation forms an angle of not less than −3° and not more than +5° relative to a normal line of the {03-38} plane. The sign of a positive value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;01-10&gt; direction whereas the sign of a negative value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;0001&gt; direction. 
     It should be noted that the plane orientation of the main surface thereof is more preferably substantially {03-38} and the plane orientation of the main surface thereof is further preferably {03-38}. Here, the expression “the plane orientation of the main surface is substantially {03-38}” indicates that the plane orientation of the main surface of the substrate is included in a range of the off the angle in which the plane orientation of the substrate can be regarded as substantially {03-38} in consideration of precision of processing the substrate and the like. The range of the off angle in this case is a range in which the off angle is ±2° relative to {03-38}, for example. In this way, the above-described channel mobility can be improved further. 
     In the MOSFET, the main surface of the substrate has an off angle of not less than −3° and not more than +5° relative to a (0-33-8) plane in the &lt;01-10&gt; direction. 
     In particular, a structure is employed in which a semiconductor layer and an insulating film are formed on a surface close to the (0-33-8) plane, which is a plane close to the C (carbon) plane in the {03-38} plane. In this way, the carrier mobility is improved significantly. 
     Here, in the present application, the (0001) plane of single-crystal silicon carbide of hexagonal crystal is defined as the silicon plane whereas the (000-1) plane is defined as the carbon plane. Meanwhile, the “off angle relative to the (0-33-8) plane in the &lt;01-10&gt; direction” refers to an angle formed by the orthogonal projection of a normal line of the main surface to a flat plane defined by the &lt;000-1&gt; direction and the &lt;01-10&gt; direction serving as a reference for the off orientation, and a normal line of the (0-33-8) plane. The sign of a positive value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;01-10&gt; direction, whereas the sign of a negative value corresponds to a case where the orthogonal projection approaches in parallel with the &lt;000-1&gt; direction. Further, the expression “the main surface having an off angle of not less than −3° and not more than +5° relative to the (0-33-8) plane in the &lt;01-10&gt; direction” indicates that the main surface corresponds to a plane, at the carbon plane side, which satisfies the above-described conditions in the silicon carbide crystal. It should be noted that in the present application, the (0-33-8) plane includes an equivalent plane, at the carbon plane side, which is expressed in a different manner due to determination of an axis for defining a crystal plane, and does not includes a plane at the silicon plane side. 
     Effects of the Invention 
     As described above, the MOSFET and the method for manufacturing the MOSFET according to the present invention allow for improved channel mobility by setting the sub-threshold slope at not more than 0.4 V/Decade. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view schematically showing a MOSFET of an embodiment of the present invention. 
         FIG. 2  illustrates a {03-38} plane in the embodiment of the present invention. 
         FIG. 3  is a flowchart showing a method for manufacturing the MOSFET in the embodiment of the present invention. 
         FIG. 4  is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention. 
         FIG. 5  is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention. 
         FIG. 6  is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention. 
         FIG. 7  is a schematic cross sectional view illustrating a step of the method for manufacturing the MOSFET of the embodiment of the present invention. 
         FIG. 8  is a cross sectional view schematically showing each of MOSFETs of the present invention&#39;s examples 1 and 2. 
         FIG. 9  shows a relation between mobility and sub-threshold slope in a first example. 
         FIG. 10  is a cross sectional view schematically showing a MOS capacitor fabricated in a second example. 
         FIG. 11  shows a relation between energy and interface state density in the second example. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     The following describes an embodiment of the present invention with reference to figures. It should be noted that in the below-mentioned figures, the same or corresponding portions are given the same reference characters and are not described repeatedly. It should be also noted that in the present specification, an individual orientation is represented by [ ], a group orientation is represented by &lt; &gt;, an individual plane is represented ( ), by and a group plane is represented by { }. In addition, crystallographically, a negative index is supposed to be indicated by putting a bar “−” above a numeral, but instead is indicated by putting a negative sign before the numeral in the present specification. 
     Referring to  FIG. 1 , a MOSFET  1  of one embodiment of the present invention will be described. MOSFET  1  of the present embodiment is a MOSFET of vertical type. 
     MOSFET  1  includes a substrate  2 , a semiconductor layer  21 , well regions  23 , source regions  24 , contact regions  25 , an insulating film  26 , a gate electrode  10 , a source electrode  27 , an interlayer insulating film  28 , and a drain electrode  12 . 
     Substrate  2  is, for example, an n +  SiC substrate. Substrate  2  has a main surface having an off angle of not less than 50° and not more than 65° relative to a {0001} plane. Preferably, the main surface thereof is a {03-38} plane. Here, as shown in  FIG. 2 , the {03-38} plane is a plane having a tilt of approximately 55°(54.7°) relative to the {0001 } plane. In other words, the {03-38} plane is a plane having a tilt of approximately 35° (35.3°) relative to the direction of a &lt;0001&gt; axis. 
     It should be noted that the main surface of substrate  2  may have an off orientation falling within a range of ±5° of the &lt;11-20&gt; direction or a range of ±5° of the &lt;01-10&gt; direction. Further, the plane orientation of the main surface of substrate  2  may have an off angle of not less than −3° and not more than +5° relative to the plane orientation {03-38}. Furthermore, the main surface of substrate  2  may have an off angle of not less than −3° and not more than +5° relative to the (0-33-8) plane in the &lt;01-10&gt; direction. In these cases, channel mobility can be improved. In particular, by setting the plane orientation of the main surface of substrate  2  to (0-33-8), the channel mobility can be improved more. 
     On the main surface of substrate  2 , a semiconductor layer  21  formed of, for example, an n type SiC is formed. Each of well regions  23  is positioned in a portion of the main surface of semiconductor layer  21  so as to form a pn junction with semiconductor layer  21 . Well region  23  is a p type SiC, for example. Each of source regions  24  is positioned in a portion of the main surface thereof within well region  23  so as to form a pn junction with well region  23 . Source region  24  is a SiC, for example. Each of contact regions  25  is positioned in a portion of the main surface thereof within well region  23  so as to form a pn junction with source region  24 . Contact region  25  is a SiC, for example. 
     Semiconductor layer  21  is of the same conductive type (n) as that of source region  24 , and has an impurity concentration lower than that of source region  24 . Semiconductor layer  21  has a thickness of for example 10 μm. It is not particularly limited as to which one of the impurity concentration of semiconductor layer  21  and the impurity concentration of source region  24  is higher or lower. Source region  24  preferably has an impurity concentration higher than that of semiconductor layer  21 , for example, has an impurity concentration of 1×10 18  cm −3  to 1×10 20  cm −3 . Examples of the n type impurity usable are: nitrogen (N), phosphorus (P), and the like. 
     Further, well region  23  is of a second conductive type (p) different from that of semiconductor layer  21 . Examples of the p type impurity usable are: aluminum (Al), boron (B), and the like. Well region  23  has an impurity concentration of, for example, 5×10 15  cm 3  to 5×10 18  cm 31 3 . 
     A region between source region  24  and semiconductor layer  21  in well region  23  serves as a channel of MOSFET  1 . In the present embodiment, the conductive type thereof is determined to form an n channel, but the first and second conductive types may be determined in a manner opposite to that in the above-described case, so as to form a p channel. 
     Insulating film  26  insulates between semiconductor layer  21  and gate electrode  10 , and is formed on and in contact with well region  23  between source region  24  and semiconductor layer  21 . 
     Gate electrode  10  is formed on insulating film  26  to face at least well region  23  between source region  24  and semiconductor layer  21 . It should be noted that gate electrode  10  may be also formed on another region as long as it is formed above well region  23  so as to face well region  23  between source region  24  and semiconductor layer  21 . 
     On source region  24  and contact region  25 , source electrode  27  is formed and is electrically connected to source region  24  and contact region  25 . Source electrode  27  is electrically insulated from gate electrode  10  by insulating film  26 . Further, drain electrode  12  is formed on an opposite surface of substrate  2  to its surface making contact with semiconductor layer  21 , and is thus electrically connected to substrate  2 . 
     MOSFET  1  has a sub-threshold slope of 0.4 V/Decade or smaller. This leads to reduced interface state density, thereby achieving a large mobility. 
     Now, the sub-threshold slope will be described. The term “sub-threshold slope (also referred to as “sub-threshold swing”, “S value”, or the like)” indicates a gate voltage not more than a threshold voltage and required to increase current flowing between the source and the drain by one digit. The sub-threshold slope is expressed by the following Formula 1 with the gate voltage being represented by V G  and the drain current being represented by I D . 
     
       
         
           
             
               
                 
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     Further, a region including nitrogen atoms is preferably formed at an interface between semiconductor layer  21  and insulating film  26 . For example, a region distant away from the interface between semiconductor layer  21  and insulating film  26  by 10 nm or smaller preferably has a nitrogen concentration of 1×10 21  cm −3  at maximum. In this case, mobility (channel mobility) can be at a sufficiently large value in a channel region having a channel length (region between well regions  23  in semiconductor layer  21 ). 
     This is considered to be attained due to the following reasons. That is, when insulating film  26  is formed through thermal oxidation or the like, a multiplicity of interface states are formed in the interface between insulating film  26  and semiconductor layer  21 . This will result in extremely small channel mobility in the channel region, if nothing is done therefor. To counteract this problem, the nitrogen atoms are introduced into the region at the interface between insulating film  26  and semiconductor layer  21  as described above. In this way, the channel mobility can be improved while reducing the influences of the interface states. 
     The following describes a method for manufacturing MOSFET  1  in the present embodiment. 
     First, as shown in  FIG. 3 , a substrate preparing step (S 10 ) is performed. In this step, a SiC substrate whose conductive type is n type is prepared as substrate  2 . The SiC substrate thus prepared has a main surface having an off angle of not less than 50° and not more than 65° relative to a plane orientation {0001}, for example, having a plane orientation of (03-38) or (0-33-8). Such a substrate can be obtained by slicing an ingot having the (0001) plane as its main surface into substrate  2  so that the (03-38) plane or (0-33-8) plane thereof is exposed as the main surface, for example. In this step, for further improvement of the channel mobility in MOSFET  1  to be manufactured, it is particularly preferable to prepare substrate  2  having the (0-33-8) plane as its main surface. Further, as substrate  2 , a substrate having a specific resistance of 0.02 Ωcm may be used, for example. 
     Then, a semiconductor layer forming step (S 20 ) is performed. Specifically, as shown in  FIG. 4 , semiconductor layer  21  is formed on the main surface of substrate  2 . Semiconductor layer  21  is formed of SiC whose conductive type is n type, and has a thickness of 10 μm, for example. Further, n type impurity in semiconductor layer  21  can have a concentration of 1×10 16  cm −3 . 
     Then, an injecting step (S 30 ) is performed. Specifically, impurity (for example, Al) whose conductive type is p type is injected into semiconductor layer  21 , using as a mask an oxide film formed by means of photolithography and etching. In this way, well regions  23  are formed as shown in  FIG. 5 . Thereafter, the oxide film thus used is removed and an oxide film having a new pattern is formed using photolithography and etching. Using this oxide film as a mask, n type conductive impurity (for example, P) is injected into predetermined regions to form source regions  24 . In a similar way, conductive impurity whose conductive type is p type is injected to form contact regions  25 . As a result, a structure shown in  FIG. 5  is obtained. 
     After such an injecting step (S 30 ), activation annealing treatment is performed. This activation annealing treatment can be performed under conditions that, for example, argon (Ar) gas is employed as atmospheric gas, heating temperature is in a range of 1700-1800° C., and heating time is 30 minutes. This activation annealing activates the impurity in the ion-injected region, and restores crystallinity. 
     Next, a gate insulating film forming step (S 40 ) is performed. Specifically, as shown in  FIG. 6 , insulating film  26  is formed to cover semiconductor layer  21 , well regions  23 , source regions  24 , and contact regions  25 . The formation of insulating film  26  may be performed through dry oxidation (thermal oxidation), for example. This dry oxidation can be performed under conditions that, for example, heating temperature is 1200° C., heating time is 30 minutes, and the like. The insulating film can be formed to have a thickness of, for example, 40 nm. 
     Then, a nitrogen annealing step (S 50 ) is performed. Specifically, gas including nitrogen (N) atoms, such as nitrogen monoxide (NO) gas or dinitrogen oxide (N 2 O) gas, is used as atmospheric gas for heat treatment. The atmospheric gas is preferably nitrogen oxide. 
     The heat treatment can be performed under conditions that, for example, heating temperature is not less than 1100° C. and not more than 1300° C. and heating time is not less than 30 minutes and not more than 120 minutes. As a result, the nitrogen atoms can be introduced into the vicinity of the interface between insulating film  26  and each of semiconductor layer  21 , well regions  23 , source regions  24 , and contact regions  25 , each of which is positioned below insulating film  26 . 
     After this nitrogen annealing step, additional annealing may be performed using Ar gas, which is inert gas. Specifically, the annealing may be performed using the Ar gas as the atmospheric gas, under conditions that heating temperature is 1100° C. and heating time is 60 minutes. 
     In addition, after the nitrogen annealing step, surface cleaning may be performed, such as organic cleaning, acid cleaning, or RCA cleaning. 
     Next, an electrode forming step (S 60 ) is performed. Specifically, on insulating film  26 , a layer of high-concentration n type poly Si or the like, which is to be gate electrode  10 , is formed using a CVD (chemical vacuum deposition) method. On this layer, a resist film having a pattern provided with an opening at a region other than the region to be gate electrode  10  is formed using the photolithography method. Using this resist film as a mask, the layer&#39;s portion exposed from the pattern is removed by means of RIE (Reactive Ion Etching) or the like. In this way, gate electrode  10  can be formed as shown in  FIG. 7 . 
     Then, an insulating film formed of SiO 2  or the like, which is to be interlayer insulating film  28 , is formed using the CVD method so as to cover gate electrode  10 . For example, silicon oxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) may be deposited using the CVD method or a plasma CVD method. For example, SiO 2  may be deposited using the plasma CVD method thereon by for example 1 μm, under conditions that a raw material gas of tetraethoxysilane (TEOS) and oxygen (O 2 ) is used and heating temperature is 350° C. On the insulating film, a resist film having a pattern provided with an opening at a region other than the region to be interlayer insulating film  28  is formed using the photolithography method. Using the resist film as a mask, the insulating film&#39;s portion exposed from the pattern is removed using the RIE. In this way, interlayer insulating film  28  having openings can be formed as shown in  FIG. 7 . 
     Next, on interlayer insulating film  28 , a resist film is formed using the photolithography method. The resist film has a pattern for exposing a portion of each source region  24  and each contact region  25 . On the pattern and the resist, a conductive film of Ni or the like is formed. Thereafter, by removing (lifting off) the resist, a portion of each source electrode  27  can be formed in contact with source region  24  and contact region  25  each of which is exposed from insulating film  26  and interlayer insulating film  28 . Further, drain electrode  12  is formed on the back-side surface of substrate  2 . For drain electrode  12 , nickel (Ni) can be used, for example. After forming source electrode  27  and drain electrode  12 , heat treatment for alloying is performed, for example. In this way, a portion of source electrode  27  and drain electrode  12  can be formed as shown in  FIG. 7 . 
     Then, on the formed portion of source electrode  27 , upper source electrode  27  is formed. Upper source electrode  27  can be formed by means of, for example, lift-off, etching, or the like. 
     As described above, MOSFET  1  of the present embodiment includes: SiC substrate  2  having a main surface having an off angle of not less than 50° and not more than 65° relative to the {0001} plane, preferably, having the {03-38} plane; semiconductor layer  21  formed on SiC substrate  2 ; and insulating film  26  formed in contact with the surface of semiconductor layer  21 , and MOSFET  1  has a sub-threshold slope of not more than 0.4 V/Decade. 
     Meanwhile, the method for manufacturing MOSFET  1  in the present embodiment includes: the substrate preparing step (S 10 ) of preparing SiC substrate  2  having a main surface having an off angle of not less than 50° and not more than 65° relative to the {0001} plane, preferably, having a {03-38} plane as the main surface; the semiconductor layer forming step (S 20 ) of forming semiconductor layer  21  on SiC substrate  2 ; and the gate insulating film forming step (S 40 ) of forming insulating film  26  in contact with the surface of semiconductor layer  21 , and MOSFET  1  has a sub-threshold slope of not more than 0.4 V/Decade. 
     The present inventor has found that by setting the sub-threshold slope at not more than 0.4 V/Decade, interface state density in the vicinity of the interface between insulating film  26  and semiconductor layer  21  can be reduced effectively. This restrains most of carriers, which are to serve as an inversion channel layer, from being trapped in the interface states at the region facing insulating film  26  in semiconductor layer  21 . This further prevents trapped carriers from behaving as fixed charges. Accordingly, applied voltage (threshold voltage) to the gate electrode can be maintained to be small, whereby most of the carriers contribute to a current between the source and the drain. Thus, MOSFET  1  allowing for improved channel mobility is obtained. As such, in the present embodiment, a large channel mobility can be realized with good reproducibility, thus allowing the excellent characteristics of MOSFET  1  to be exhibited stably. 
     First Example 
     In the present example, the effect of improving mobility in a MOSFET by setting the sub-threshold slope thereof at not more than 0.4 V/Decade was examined. 
     The Present Invention&#39;s Examples 1 and 2 
     As each of the MOSFETs of the present invention&#39;s examples 1 and 2, a MOSFET  3  of lateral type basically as shown in  FIG. 8  was manufactured. 
     Specifically, first, in the substrate preparing step (S 10 ), as substrate  2 , a 4H—SiC substrate was prepared which had the (03-38) plane as the main surface thereof. 
     Next, in the semiconductor layer forming step (S 20 ), as semiconductor layer  31 , a p type SiC layer was formed which had a thickness of approximately 0.8 μm and had an impurity concentration of 1×10 16  cm −3 . The main surface of the p type SiC layer corresponded to the (03-38) plane. 
     Next, in the injecting step (S 30 ), SiO 2  was used as the mask material. Using P as n type impurity, source regions  24  and drain regions  29  were formed to have an impurity concentration of 1×10 19  cm −3 . Meanwhile, using Al as p type impurity, contact regions  25  were formed to have an impurity concentration of 1×10 19  cm −3 . 
     After the injecting step (S 30 ), activation annealing treatment was performed. The conditions of the activation annealing treatment were: Ar gas was used as an atmospheric gas, heating temperature was 1700-1800° C., and heating time was 30 minutes. 
     Then, in the gate insulating film forming step (S 40 ), a gate oxide film was formed as insulating film  26  by means of dry oxidation, under conditions that heating temperature was 1200° C., and heating time was 30 minutes in the present invention&#39;s example 1 and 45 minutes in the present invention&#39;s example 2. In addition, surface cleaning was performed. 
     Then, in the nitrogen annealing step (S 50 ), in an atmosphere including NO, heat treatment was performed under conditions that heating temperature was 1100° C. in the present invention&#39;s example 1 and 1200° C. in the present invention&#39;s example 2 and heating time was 120 minutes. 
     Next, in the electrode forming step (S 60 ), gate electrode  10  formed of poly Si, source electrode  27  formed of Ni, and drain electrode  12  formed of Ni were formed. 
     By performing the steps (S 10 -S 60 ), MOSFETs  3  of the present invention&#39;s examples 1 and 2 were manufactured. 
     Comparative Example 1 
     A MOSFET of a comparative example 1 was manufactured in basically the same way as the MOSFET of the present invention&#39;s example 1, but was different therefrom in conditions in that the main surface of the substrate was the (0001) plane, the nitrogen annealing step (S 50 ) was not performed, and heating temperature was 1300° C. and heating time was 20 minutes in the gate insulating film forming step (S 40 ). 
     Comparative Example 2 
     A MOSFET of a comparative example 2 was manufactured in basically the same way as the MOSFET of the present invention&#39;s example 1, but was different in that the main surface of the substrate was the (0001) plane, heating temperature was 1300° C. and heating time was 30 minutes in the gate insulating film forming step (S 40 ), and heating temperature was 1300° C. and heating time was 60 minutes in the nitrogen annealing step (S 50 ). 
     Comparative Example 3 
     A MOSFET of a comparative example 3 was manufactured in basically the same way as the MOSFET of the present invention&#39;s example 1, but was different therefrom in that the main surface of the substrate was the (0001) plane, heating temperature was 1300° C. and heating time was 30 minutes in the gate insulating film forming step (S 40 ), and heating temperature was 1200° C. and heating time was 60 minutes in the nitrogen annealing step (S 50 ). 
     Measuring Method 
     The mobility and sub-threshold slope of each of the MOSFETs of the present invention&#39;s examples 1 and 2 and comparative examples 1-3 were measured. 
     Specifically, for the mobility thereof a source-drain current I DS  was measured while applying a gate voltage V G  with a source-drain voltage V DS =0.1 V (gate voltage dependency was measured). Then, the maximum value of the mobility relative to the gate voltage was determined as follows: 
     Channel mobility μ=g m ×(L×d)/(W×∈×V DS ), where g m =(δI Ds )/(δV G ), L indicates a gate length, d indicates the thickness of the oxide film, W indicates a gate width, and ε indicates the permittivity of the oxide film. 
     The sub-threshold slope was measured as follows. While applying gate voltage V G  with source-drain voltage V DS =0.1V, the sub-threshold slope was measured using Formula 1 in a range in which gate voltage V G  was equal to or smaller than the threshold voltage at a straight line region in a semilogarithmic plot of source-drain current I DS  relative to the gate voltage. The result is shown in  FIG. 9 . 
     As shown in  FIG. 9 , the MOSFETs of the present invention&#39;s examples 1 and 2, each of which has a sub-threshold slope of 0.4, achieved a high mobility of not less than 74 (cm 2 /Vs) and not more than 92 (cm 2 /Vs). On the other hand, the MOSFETs of comparative examples 1-3, each of which has a sub-threshold slope of 0.9-1.0, had a low mobility of not less than 2.5 (cm 2 Ns) and not more than 20 (cm 2 /Vs). 
     As such, according to the present example, it has been found that the mobility can be improved by setting the sub-threshold slope at not more than 0.4 V/Decade. 
     In the present example, the SiC substrate having the (03-38) plane as its main surface was employed as substrate  2 , but the present inventor has found that the mobility can be further improved when using a SiC substrate having the (0-33-8) plane as its main surface. 
     Second Example 
     In the present example, the effect of achieving reduced interface state density by setting the sub-threshold slope at not more than 0.4 V/Decade was examined. 
     Generally, the interface state density is determined from the sub-threshold slope (S value) as follows. In Formula 2 described below, a theoretical S value obtained upon interface state density D it =0 can be determined using a known insulating film capacity C ox , and a depletion layer capacity C d  upon strong inversion (calculated from the maximum width of the depletion layer upon strong inversion, for example). 
     
       
         
           
             
               
                 
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                       ( 
                       
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                         + 
                         
                           
                             
                               C 
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                             + 
                             
                               C 
                               it 
                             
                           
                           
                             C 
                             OX 
                           
                         
                       
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                   [ 
                   
                     Formula 
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     In Formula 2, k indicates a Boltzmann&#39;s constant, T indicates an absolute temperature, C d  indicates the depletion layer capacity upon strong inversion, C it  indicates the interface state capacity (C it =qD it ), and C ox  indicates an oxide film capacity. 
     By comparing the S values in Formula 2 and the first example, interface state capacity C it  can be calculated to derive interface state density D it . However, it was found that D it  determined in accordance with Formula 2 is not so precise. In view of this, in the present example, a MOS capacitor  30  shown in  FIG. 10  was fabricated as described below. In accordance with the capacitance/voltage characteristics thereof, interface state density D it  was determined with improved precision and was examined. 
     The Present Invention&#39;s Example 3 
     Specifically, first, as substrate  2 , a substrate similar to that in the substrate preparing step (S 10 ) of the present invention&#39;s example 1 was used. 
     Next, a semiconductor layer  21  similar to that in the semiconductor layer forming step (S 20 ) of the present invention&#39;s example 1 was formed on substrate  2 . 
     Then, an insulating film  26  similar to that in the gate insulating film forming step (S 40 ) of the present invention&#39;s example 1 was formed on semiconductor layer  21 . 
     Then, the nitrogen annealing step (S 50 ) was performed in a manner similar to that in the present invention&#39;s example 1, except that heating temperature was 1100° C. and heating time was 60 minutes. 
     Then, on insulating film  26 , a gate electrode  10  similar to that in the electrode forming step (S 60 ) of the present invention&#39;s example 1 was formed. Further, as back-side contact electrode  18 , Ni was formed. 
     In this way, the MOS capacitor of the present invention&#39;s example 3 was manufactured. 
     Comparative Example 4 
     A MOS capacitor of a comparative example 4 was manufactured in basically the same way as the MOS capacitor of the present invention&#39;s example 3, but was different therefrom in that heating temperature was 1200° C. and heating time was 30 minutes in the gate insulating film forming step (S 40 ) and the nitrogen annealing step (S 50 ) was not performed. 
     Comparative Example 5 
     A MOS capacitor of a comparative example 5 was manufactured in basically the same way as the MOS capacitor of the present invention&#39;s example 3, but was different therefrom in that the main surface of the substrate was the (0001) plane and heating temperature was 1300° C. and heating time was 60 minutes in the nitrogen annealing step (S 50 ). 
     Measuring Method 
     Energy and interface state density in each of the MOS capacitors of the present invention&#39;s example 3 and comparative examples 4 and 5 were measured. It should be noted that the energy herein refers to energy in a band gap with respect to the bottom of the conduction band at the semiconductor layer side of the MOS interface (interface between semiconductor layer  21  and insulating film  26 ). 
     The interface state density was measured in accordance with capacitance C/voltage V characteristics by means of a High-Low method. The result thereof is shown in  FIG. 11 . 
     As shown in  FIG. 11 , in the MOS capacitor of the present invention&#39;s example 3, the interface states at the MOS interface were low. From this fact, it was found that insulating film  26  was thermally treated using gas including nitrogen atoms as the atmospheric gas, thereby achieving reduced interface state density. 
     Further, when manufacturing a MOSFET under the conditions for the MOS capacitor of the present invention&#39;s example 3, the MOSFET had a sub-threshold slope of not more than 0.4. As such, it was found that the interface state density can be reduced by setting the sub-threshold slope at not more than 0.4. 
     It is considered that such a reduced interface state density provides the following effect. That is, inversion electrons can be reduced which does not contribute to the current between the source and the drain and are trapped in the interface states. This can reduce an applied gate voltage, i.e., threshold voltage, required to form inversion channel electrons necessary to let a current flow sufficiently between the source and the drain. Because the interface state density can be reduced by setting the sub-threshold slope at not more than 0.4 as such, it is considered that the mobility therein can be improved. 
     On the other hand, in the MOS capacitors of comparative examples 4 and 5, the interface states at the MOS interface were high. When manufacturing a MOSFET under the conditions for the MOS capacitor of comparative examples 4 and 5, the MOSFET had a sub-threshold slope of more than 0.4. Accordingly, it is considered that the threshold voltage is large because when inversion channel electrons are trapped in the MOS interface, the mobility in the MOSFET is decreased and they accordingly behave as negative fixed charges. In Patent Document 1, the nitrogen annealing step (S 50 ) is not performed as in comparative example 4. Hence, it is considered that in Patent Document 1, the interface state density is as large as that in comparative example 4. Accordingly, it can be said that high mobility is hardly achieved with good reproducibility in the MOSFET of Patent Document 1. 
     As described above, according to the present example, it has been found that by setting the sub-threshold slope at not more than 0.4 V/Decade, the interface state density can be reduced to improve the mobility. In addition, it is considered that such a large channel mobility can be realized with good reproducibility. 
     Although the embodiments and examples of the present invention have been described thus far, it is initially expected to appropriately combine features of the embodiments and the examples. In addition, the embodiments and examples disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     Industrial Applicability 
     The present invention is advantageously applied to a MOSFET in which a semiconductor layer made of SiC is formed in contact with an insulating film. 
     DESCRIPTION OF THE REFERENCE SIGNS 
       1 ,  3 : MOSFET;  2 : substrate;  10 : gate electrode;  12 : drain electrode;  18 : back-side contact electrode;  21 ,  31 : semiconductor layer;  23 : well region;  24 : source region;  25 : contact region;  26 : insulating film;  27 : source electrode;  28 : interlayer insulating film;  29 : drain region;  30 : MOS capacitor.