Abstract:
A semiconductor device includes: a semiconductor layer formed over a substrate; an insulating film formed over the semiconductor layer; and an electrode formed over the insulating film, wherein the insulating film includes an amorphous film including carbon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority from Japanese Patent Application No. 2011-31108 filed on Feb. 16, 2011, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    Examples relate to a semiconductor device, a power supply device, an amplifier, and a method for manufacturing a semiconductor device. 
       BACKGROUND 
       [0003]    GaN, AlN, and InN included in nitride semiconductors or materials including mixed crystals of those and the like have a wide band gap and are used for high-output electronic devices or short wavelength light emitting devices. Field effect transistors (FET), for example, a High Electron Mobility Transistor (HEMT), are used for the high-output electronic devices. An HEMT including a nitride semiconductor is used for high-output high-efficiency amplifiers, or high-power switching devices, and the like. 
         [0004]    The HEMT having a high drain breakdown voltage and a gate breakdown voltage may include a Metal Insulator Semiconductor (MIS) structure including an insulating film to be used as a gate insulating film. Due to the MIS structure, a semiconductor device suitable for electric power application is generated. 
         [0005]    Related art is disclosed in Japanese Laid-Open Patent Publication No. 2002-359256, Japanese Laid-Open Patent Publication No. 2008-218479, or the like. 
       SUMMARY 
       [0006]    According to one aspect of the embodiments, a semiconductor device includes: a semiconductor layer formed over a substrate; an insulating film formed over the semiconductor layer; and an electrode formed over the insulating film, wherein the insulating film includes an amorphous film including carbon. 
         [0007]    Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an exemplary semiconductor element; 
           [0009]      FIG. 2A  and  FIG. 2B  illustrate an exemplary measurement of a semiconductor element; 
           [0010]      FIG. 3  illustrates an exemplary relationship between an applied voltage and a capacity; 
           [0011]      FIG. 4A  to  FIG. 4C  illustrate an exemplary insulating film; 
           [0012]      FIG. 5  illustrates an exemplary relationship between a thickness of an amorphous carbon film and a threshold voltage variation range; 
           [0013]      FIG. 6  illustrates an exemplary semiconductor device; 
           [0014]      FIG. 7  illustrates an exemplary relationship between a carbon-carbon bond ratio and a film density or a plasmon peak; 
           [0015]      FIG. 8  illustrates an exemplary plasmon peak; 
           [0016]      FIG. 9A  to  FIG. 9E  illustrate an exemplary method for manufacturing a semiconductor device; 
           [0017]      FIG. 10  illustrates an exemplary film forming apparatus; 
           [0018]      FIG. 11  illustrates an exemplary semiconductor device; 
           [0019]      FIG. 12A  and  FIG. 12B  illustrate an exemplary insulating film; 
           [0020]      FIG. 13  illustrates an exemplary semiconductor device; 
           [0021]      FIG. 14A  to  FIG. 14F  illustrate an exemplary method for manufacturing a semiconductor device; 
           [0022]      FIG. 15  illustrates an exemplary discrete-packaged semiconductor device; 
           [0023]      FIG. 16  illustrates an exemplary power supply device; and 
           [0024]      FIG. 17  illustrates an exemplary high frequency amplifier. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    Due to a reduction in on-resistance, a normally-off operation, or an increase in breakdown voltage of switching elements, a high efficiency switching element for power application using a transistor may be provided. The switching element like that may stably perform a switching operation. 
         [0026]      FIG. 1  illustrates an exemplary a semiconductor device. The semiconductor device illustrated in  FIG. 1  may not stably perform a switching operation. The semiconductor device illustrated in  FIG. 1  includes a buffer layer  2  laminated on a substrate  1  including silicon, an electron transit layer  3 , a spacer layer  4 , an electron supply layer  5 , a cap layer  6 , and an insulating film  7  formed on the cap layer  6 . The electron transit layer  3 , the spacer layer  4 , the electron supply layer  5 , and the cap layer  6  may be formed by a Metal-Organic Vapor Phase Epitaxy (MOVPE) method. In order to epitaxially grow the electron transit layer  3  and the like, the buffer layer  2  may be formed on the substrate  1 . By the buffer layer  2  on the substrate  1 , the electron transit layer  3  or the like is epitaxially grown on the buffer layer  2 . 
         [0027]    The electron transit layer  3  may be i-GaN having a thickness of about 3 μm. The spacer layer  4  may be i-AlGaN having a thickness of about 5 nm. The electron supply layer  5  may be n-AlGaN having a thickness of about 5 nm. The n-AlGaN may be doped with silicon (Si) as an impurity element with a concentration of 5×10 18  cm −3 . The cap layer  6  may be n-GaN having a thickness of 10 nm. The n-GaN may be doped with silicon (Si) as an impurity element with a concentration of 5×10 18  cm −3 . At the side near the electron supply layer  5  of the electron transit layer  3 , a 2 dimensional electron gas (2DEG)  3   a  is generated. The insulating film  7  may correspond to a gate insulating film. The insulating film  7  includes a film having an aluminum oxide whose thickness is about 20 nm formed by an Atomic Layer Deposition (ALD) method. 
         [0028]      FIG. 2A  and  FIG. 2B  illustrate an exemplary measurement of a semiconductor element. In  FIG. 2 , an anode electrode  8  and a cathode electrode  9  including mercury are provided on the insulating film  7 . The capacity between the anode electrode  8  and the cathode electrode  9  may be measured. The anode electrode  8  may have a disk shape having a diameter of about 500 μm. The cathode electrode  9  may have a doughnut shape having an inner diameter of about 1500 μm and an outer diameter of about 2500 μm. The cathode electrode  7  may be disposed so that the center of the cathode electrode  9  matches the center of the anode electrode  8 . The cathode electrode  9  may be grounded, so that a ground potential may be achieved.  FIG. 2A  illustrates a top view of a semiconductor element.  FIG. 2B  illustrates a cross sectional view taken along the dashed line IIB-IIB of  FIG. 2A . 
         [0029]      FIG. 3  illustrates an exemplary relationship between an applied voltage and a capacity. For example,  FIG. 3  may illustrate the capacity detected between the anode electrode  8  and the cathode electrode  9  when the voltage applied to the anode electrode  8  varies. For example, the cathode electrode  9  is grounded, and an AC component being 100 kHz and 25 mV and superimposed on a given voltage is applied to the anode electrode  8 , and the capacity is measured. 
         [0030]    As illustrated in  FIG. 3 , when the voltage applied to the anode electrode  8  gradually increases from −30 V to 10V, the capacity sharply increases from 0 at around −7 V. When the voltage further increases, the capacity is substantially constant without largely varying. However, when the voltage reaches about 0 V, the capacity increases. After this, the capacity increases with an increase in the applied voltage, and gradually approaches a certain value. When the voltage applied to the anode electrode  8  gradually decreases from 10 V to −30 V, the capacity decreases with a reduction in the applied voltage. However, when the voltage reaches near 7 V, the capacity becomes substantially constant. When the voltage decreases to 0 V, the capacity may not vary. After this, with a further reduction in the applied voltage, the capacity decreases when the voltage have passed 0 V, and when the voltage reaches near −1.5 V, the capacity becomes 0, after that, when the voltage decreases, the capacity is still 0 and does not vary. For example, when the insulating film  7  of the semiconductor element illustrated in  FIG. 1  includes aluminum oxide, the relationship between the applied voltage and the capacity during an increase in the voltage and the relationship between the applied voltage and the capacity during a reduction in the voltage are different from each other. 
         [0031]    When the applied voltage increases from a low voltage, the depletion layer thickness decreases, and a capacity is produced at a time when 2DEG  3   a  is formed in the electron transit layer  3 , and then the capacity sharply increases. When the applied voltage decreases from a high voltage, the depletion layer thickness increases, and the capacity decreases with a reduction in the 2DEG  3   a . Since electrons and the like trapped at a trap level formed in the insulating film  7  affect the distribution of the 2DEG  3   a , the curve during the increase in the applied voltage may be almost overlapped by a shift with the curve during the reduction in the applied voltage. For example, when a trap level is formed in the insulating film  7  so that electrons and the like are trapped, the capacity to be detected varies. The capacity detected during the increase in the voltage and the capacity detected during the reduction in the voltage may be different when substantially the same voltage is applied. 
         [0032]    When the relationship between the voltage and the capacity varies depending on the history of the previously applied voltages, a stable switching operation may not be achieved, so that the reliability of the semiconductor device may decrease. The shift amount between a capacity curve during the increase in the voltage and a capacity curve during the reduction in the voltage may be referred to as a threshold voltage variation range. For example, when the insulating film  7  of the semiconductor device illustrated in  FIG. 1  includes an aluminum oxide, the threshold voltage variation range may be about 5.4 V. 
         [0033]    For example, when the aluminum oxide film of the insulating film  7  is an amorphous film of a compound, a trap level may be formed. When the insulating film  7  includes an amorphous film or the like which is a compound, such as an oxide or a nitride, a trap level may be formed. 
         [0034]      FIG. 4A  to  FIG. 4C  illustrate an exemplary insulating film.  FIG. 4A  illustrates the insulating film  7  having a thickness of about 20 nm and including an aluminum oxide.  FIG. 4B  and  FIG. 4C  illustrate an insulating film including materials other than oxides and nitrides. For example, an insulating film  7   a  illustrated in  FIG. 4B  may include an amorphous carbon film. An insulating film  7   b  illustrated in  FIG. 4C  illustrates an insulating film  7   b  in which an amorphous carbon film and an aluminum oxide film are laminated. In the insulating film  7   a  illustrated in  FIG. 4B , an amorphous carbon film of about 20 nm is formed. In the insulating film  7   b  illustrated in  FIG. 4C , an aluminum oxide film of about 10 nm is formed by an ALD method, and then an amorphous carbon film of about 10 nm is formed. The amorphous carbon film may be an amorphous film including carbon as the main ingredient, and the amorphous carbon film may be formed by Filtered Cathodic Arc (FCA) which is an arc vapor deposition method. 
         [0035]      FIG. 5  illustrates an exemplary relationship between a thickness of an amorphous carbon film and a threshold voltage variation range. When the thickness of the amorphous carbon film becomes large, the threshold voltage variation range becomes small. When the insulating film is the amorphous carbon film, the threshold voltage variation range may become substantially 0. 
         [0036]    For example, when the insulating film includes the amorphous carbon film, the threshold voltage variation range becomes small. When the entire insulating film is the amorphous carbon film, the threshold voltage variation range becomes substantially 0. When the insulating film corresponding to the gate insulating film includes the amorphous carbon film, a switching operation may be stably performed. For example, when the insulating film corresponding to the gate insulating film is partially or entirely the amorphous carbon film, the switching operation may be stably performed, so that a highly reliable semiconductor device may be provided. 
         [0037]      FIG. 6  illustrates an exemplary semiconductor device. The semiconductor device illustrated in  FIG. 6  may be an HEMT. The semiconductor device includes a buffer layer  20  formed on the substrate  10  including a semiconductor or the like and an electron transit layer  21 , an electron supply layer  22 , and a cap layer  23  including a semiconductor layer laminated on the buffer layer  20  by epitaxial growth. On the cap layer  23 , an insulating film  30  is formed. On the insulating film  30 , a gate electrode  41  is formed. A source electrode  42  and a drain electrode  43  are coupled to the electron transit layer  21 . On the exposed insulating film  30 , a protective film  50  including an insulator is formed. 
         [0038]    The substrate  10  may include a Si substrate, a SiC substrate, a sapphire (Al 2 O 3 ) substrate, or the like. For example, the substrate  10  is a Si substrate and the buffer layer  20  is formed on the substrate. When the substrate  10  includes other materials, the buffer layer  20  may not be formed. The electron transit layer  21  may be i-GaN, the electron supply layer  22  may be n-AlGaN, and the cap layer  23  may be n-GaN. A two-dimensional electron gas (2DEG)  21   a  is formed at the side near the electron supply layer  22  of the electron transit layer  21 . A non-illustrated spacer layer may be formed between the electron transit layer  21  and the electron supply layer  22 . 
         [0039]    The gate electrode  41 , the source electrode  42 , and the drain electrode  43  may include metal materials. The insulating film  30  to be a gate insulating film may include an amorphous carbon film having a thickness of 20 nm. The protective film  50  may include an aluminum oxide (Al 2 O 3 ) film formed by a plasma ALD. 
         [0040]    The amorphous carbon film as the insulating film  30  may be an amorphous film including carbon as the main ingredient, and may be referred to as Diamond Like Carbon (DLC). The amorphous carbon film has high density, high insulation properties, and high surface smoothness. The hydrogen content in the amorphous carbon film having high insulating properties, high density, or the like may be reduced, and the amorphous carbon film may be a diamond like film. For example, the film density of the amorphous carbon film may be high and the sp3 content may be higher than the sp2 content in the carbon-carbon bond of the amorphous carbon film. The amorphous carbon film in a state where the sp3 content is higher than the sp2 content in the carbon-carbon bond becomes a film in a state close to a high-density diamond. Therefore, the formation of a trap level or the like may be reduced and the threshold voltage variation range may become small. For example, the insulating film  30  including the amorphous carbon film may stably perform a switching operation. 
         [0041]    The carbon-carbon bond includes bonding manners sp2 and sp3. Graphite is formed with the carbon-carbon bond sp2, and a diamond is formed with the carbon-carbon bond sp3. When the amorphous carbon film is further a diamond-like film, the sp3 bond content is higher than the sp2 bond content. For example, the carbon-carbon bond content may be sp2≦sp3. 
         [0042]      FIG. 7  illustrates an exemplary relationship between a carbon-carbon bond ratio and a film density or a plasmon peak. As illustrated in  FIG. 7 , the carbon-carbon bond sp3 ratio and the film density of the amorphous carbon film have a correlation, and when the carbon-carbon bond sp3 ratio becomes high, the film density also becomes high. The carbon-carbon bond sp3 ratio and the plasmon peak of the amorphous carbon film have a correlation, and when the carbon-carbon bond sp3 ratio becomes high, the plasmon peak also becomes high. The amorphous carbon film where carbon-carbon bond sp3 ratio is 50% or more, for example, The amorphous carbon film where the sp3 bond ratio is higher than the sp2 bond ratio and hydrogen is hardly included, may have a film density of 2.6 g/cm 3  or more and a plasmon peak may be 28 eV or more. The film density may be calculated based on the results obtained by Rutherford backscattering spectroscopy in the amorphous carbon film formed on the silicon substrate and the film thickness obtained by cross section length measuring by Transmission Electron Microscope (TEM). 
         [0043]    When the film density of the amorphous carbon film is 2.6 g/cm 3  or more and the plasmon peak of the film is 28 eV or more, the amorphous carbon film may be formed by an FCA method of an arc vapor deposition method. For example, the film density of the amorphous carbon film formed by the FCA method may be 3.2 g/cm 3 . The density of diamond may be 3.56 g/cm 3 . The film densities of the amorphous carbon film may be 2.6 g/cm 3  or more and 3.56 g/cm 3  or lower. The hydrogen content of the amorphous carbon film formed by the FCA method is lower than the hydrogen content of the amorphous carbon film formed by CVD. The hydrogen content in the amorphous carbon film formed by the FCA method may be 1 atm % or lower. The highest film density of the amorphous carbon film including hydrogen formed by Chemical Vapor Deposition (CVD) may be lower than about 2.6 g/cm 3 . 
         [0044]      FIG. 8  illustrates an exemplary plasmon peak.  FIG. 8  illustrates the plasmon peaks of amorphous carbon films formed by the FCA method or the CVD method. The plasmon peak  8 A of the amorphous carbon film formed by the FCA method may be about 30 eV and is 28 eV or more. The plasmon peak  8 B of the film formed by the CVD method may be about 23 eV and may be lower than 28 eV. The plasmon peak of the film formed by the FCA method may be 28 eV or more. The insulating film  30  may be an amorphous carbon film formed by the FCA method. The carbon-carbon bond of the amorphous carbon film may be sp2≦sp3, the density may be 2.6 g/cm 3  or more and 3.56 g/cm 3  or lower, and the plasmon peak may be 28 eV or more. 
         [0045]    The thickness of the amorphous carbon film formed as the insulating film  30  may be 2 nm or more and 200 nm or lower, for example, 10 nm or more and 30 nm or lower. When the amorphous carbon film covers the entire surface, the thickness of the amorphous carbon film may be at least several atom layers or more. An amorphous carbon film whose thickness is lower than 2 nm may not be able to cover the entire surface. For example, the thickness of the amorphous carbon film may be 10 nm or more as illustrated in  FIG. 5 . Since the stress of the amorphous carbon film is high, film separation may occur due to the stress in an amorphous carbon film whose thickness exceeds 30 nm. The thickness of the amorphous carbon film may be 30 nm or lower. 
         [0046]      FIG. 9A  to  FIG. 9E  illustrate an exemplary method for manufacturing a semiconductor device. 
         [0047]    As illustrated in  FIG. 9A , the buffer layer  20  is formed on the substrate  10  and a semiconductor layer including the electron transit layer  21 , the electron supply layer  22 , the cap layer  23 , and the like is formed on the buffer layer  20  by epitaxial growth by Metal-Organic Vapor Phase Epitaxy (MOVPE) or the like. The substrate  10  may be a substrate including Si, SiC, sapphire (Al 2 O 3 ), or the like. On the substrate  10 , the buffer layer  20  may be formed in order to epitaxially grow the electron transit layer  21  and the like. The buffer layer  20  may be a non-doped i-AlN layer having a thickness of about 0.1 μm, for example. The electron transit layer  21  may be a non-doped i-GaN layer having a thickness of about 3 μm. The electron supply layer  22  may be an n-Al 0.25 Ga 0.75 N layer having a thickness of about 30 nm, which may be doped with Si as an impurity element with a concentration of 5×10 18  cm −3 . The cap layer  23  may be an n-GaN layer having a thickness of about 10 nm, which may be doped with Si as an impurity element with a concentration of 5×10 18  cm −3 . The semiconductor layer may be formed by crystal growth, such as Molecular Beam Epitaxy (MBE). 
         [0048]    As illustrated in  FIG. 9B , the source electrode  42  and the drain electrode  43  are formed. For example, a photoresist is applied onto the cap layer  23 , followed by exposure by an exposing apparatus and development, thereby forming a non-illustrated resist pattern having an opening portion in a region where the source electrode  42  and the drain electrode  43  are to be formed. Dry etching, such as Reactive Ion Etching (RIE) using chlorine gas, is performed to remove the cap layer  23  and the electron supply layer  22  in a region where the resist pattern is not formed, so that the surface of the electron transit layer  21  is exposed. In the dry etching, chlorine gas of about 30 sccm is introduced as an etching gas into a chamber, the pressure in the chamber is set to about 2 Pa, and RF power of 20 W is applied. A metal film including a Ta/Al laminated film or the like is formed by vacuum deposition or the like, and the metal film formed on the resist pattern is lifted-off with the resist pattern and removed by immersion in an organic solvent or the like. The source electrode  42  and the drain electrode  43  are formed in a region where the resist pattern is not formed. After lifting-off, heat treatment with a temperature of 550° C. is performed, for example, and ohmic contact is achieved. The resist pattern in the dry etching may be used for the resist pattern in the lifting-off, however, the dry etching and the lifting-off may be separately carried out. 
         [0049]    As illustrated in  FIG. 9C , the insulating film  30  corresponding to a gate insulating film is formed on the cap layer  23 . The insulating film  30  may be formed by growing an amorphous carbon film by the FCA method. For example, by using a graphite target as a raw material, setting an arc current to 70 A and setting an arc voltage to 26 V, an amorphous carbon film having a thickness of about 20 nm may be formed by the FCA method. 
         [0050]    As illustrated in  FIG. 9D , the gate electrode  41  is formed. For example, a photoresist is applied onto the insulating layer  30 , followed by exposure by an exposing apparatus and development, thereby forming a non-illustrated resist pattern having an opening portion in a region where the gate electrode  41  is to be formed. A metal film, for example, Ni film having a thickness of about 10 nm or Au film having a thickness of about 300 nm, is formed on the entire surface by vacuum deposition, and the metal film formed on the resist pattern is lifted-off with the resist pattern and removed by immersion in an organic solvent or the like. The gate electrode  41  including Ni/Au is formed in a given region on the insulating film  30 . 
         [0051]    As illustrated in  FIG. 9E , the protective film  50  is formed on the insulating film  30 . The protective film  50  may include an aluminum oxide film formed by the ALD method, an amorphous carbon film formed by the FCA method, a silicon nitride film formed by the plasma CVD method, or a laminate of these films, for example. 
         [0052]    Through the above-described processes, a transistor may be formed. The semiconductor layer may include GaN and AlGaN and may also include a nitride semiconductor, such as InAlN or InGaAlN. 
         [0053]      FIG. 10  illustrates an exemplary film forming apparatus. The film forming apparatus illustrated in  FIG. 10  may be an FCA film forming apparatus for use in the FCA method. The FCA film forming apparatus includes a plasma generating portion  110 , a plasma separating portion  120 , a particle trapping portion  130 , a plasma carrying portion  140 , and a film forming chamber  150 . The plasma generating portion  110 , the plasma separating portion  120 , and the particle trap portion  130  may have a cylindrical shape and may be associated in this order. The plasma carrying portion  140  also has a cylindrical shape. One end of the plasma carrying portion  140  is almost perpendicularly coupled to the plasma separating portion  120  and the other end of the plasma carrying portion is coupled to the film forming chamber  150 . Inside the film forming chamber  150 , a stage  152  for disposing a substrate  151  or the like serving as a film forming target is provided. 
         [0054]    An insulating plate  111  is provided on the lower end portion of the case of the plasma generating portion  110 , and a graphite serving as a target (cathode)  112  is provided on the insulating plate  111 . On the periphery of the lower end portion of a case of the plasma generating portion  110 , a cathode coil  114  is provided. On the inner wall surface of the case, an anode  113  is provided. When the amorphous carbon film is formed, arc discharge occurs by the application of a given voltage between the target  112  and the anode  113  from a non-illustrated power source, so that plasma is formed above the target  112 . In the cathode coil  114 , a magnetic field for stabilizing the arc discharge is produced by supplying a given current from another non-illustrated power supply. Due to the arc discharge, carbon, which is the target  112  of graphite, evaporates and is supplied into the plasma as ions of film forming materials. 
         [0055]    An insulating ring  121  is provided on the boundary portion between the plasma generating portion  110  and the plasma separating portion  120 . The case of the plasma generating portion  110  and the case of the plasma separating portion  120  are electrically separated by the insulating ring  121 . On the periphery of the plasma separating portion  120 , guide coils  122   a  and  122   b  are provided which generate a magnetic field for moving the plasma formed in the plasma generating portion  110  in a given direction while converging the plasma to the central portion of the case. In the vicinity of the associating portion of the plasma separating portion  120  and the plasma carrying portion  140 , an oblique magnetic field generating coil  123  is provided which generates a magnetic field which substantially perpendicularly deflects the plasma moving direction. 
         [0056]    Particles generated in the plasma generating portion  110  enter and go straight into the particle trapping portion  130  with being hardly affected by the magnetic field in the plasma separating portion  120 . On the upper end of the particle trapping portion  130 , a reflector  131  which reflects the particles in a transverse direction and a particle capturing portion  132  which capture the particles reflected by the reflector  131  are provided. In the particle capturing portion  132 , plural fins  133  are disposed obliquely to the inside of the case. The particles entering the particle capturing portion  132  are repeatedly reflected by the fins  133  to lose the kinetic energy. Eventually, the particles adhere to the fins  133  or the wall surface of the case of the particle capturing portion  132  and are captured. 
         [0057]    The plasma separated from the particles in the plasma separating portion  120  enters the plasma carrying portion  140 . The plasma carrying portion  140  is divided into a negative voltage applying portion  142  and an associating portion  146 . An insulating ring  141  is provided between the negative voltage applying portion  142  and the plasma separating portion  120  and between the negative voltage applying portion  142  and the associating portion  146 . The plasma separating portion  120  and the negative voltage applying portion  142  are electrically separated and the associating portion  146  and the negative voltage applying portion  142  are electrically separated. 
         [0058]    The negative voltage applying portion  142  is divided into an inlet portion  143  at the side of the plasma separating portion  120 , an outlet portion  145  at the side of the associating portion  146 , and an intermediate portion  144  between the inlet portion  143  and the outlet portion  145 . On the periphery of the inlet portion  143 ,  143   a  is provided which generates a magnetic field for moving the plasma to the side of the film forming chamber  150  while converging the plasma. Inside the inlet portion  143 , a plurality of fins  143   b  which capture the particles entering the inlet portion  143  are provided obliquely to the case inner surface. 
         [0059]    Apertures  144   a  and  144   b  each having an opening portion for regulating the flow path of the plasma are provided at the inlet portion  143  side and the outlet portion  145  side, respectively, of the intermediate portion  144 . On the periphery of the intermediate portion  144 , a guide coil  144   c  is provided which generates a magnetic field for deflecting the plasma moving direction. 
         [0060]    The diameter of the associating portion  146  may become gradually larger from the negative voltage applying portion  142  side to the film forming chamber  150 . Also inside the associating portion  146 , a plurality of fins  146   a  are provided. On the periphery of the boundary portion of the associating portion  146  and the film forming chamber  150 , a guide coil  146   b  is provided which moves the plasma to the film forming chamber  150  side while converging the plasma. 
         [0061]    In the FCA film forming apparatus, the plasma generating portion  110  discharges arc and generates plasma including carbon ions. The plasma reaches the substrate  151  and the like while the oblique magnetic field generating coil  123  and the like are removing components which become particles. Therefore, an amorphous carbon film is formed on the substrate  151  and the like. 
         [0062]      FIG. 11  illustrates an exemplary semiconductor device. The semiconductor device illustrated in  FIG. 11  includes an insulating film  230 , which includes a laminated structure of an aluminum oxide film  231  and an amorphous carbon film  232 , for example, the insulating film  230  in which the amorphous carbon film  232  is formed on the aluminum oxide film  231 , in place of the insulating film  30  including the amorphous carbon film of the semiconductor device illustrated in  FIG. 6 . An aluminum oxide film  231  having a thickness of about 10 nm may be formed by the ALD method by using trimethyl aluminum (TMA) and pure water (H 2 O) and setting the substrate temperature to 300° C. By the above-described FCA method, an amorphous carbon film  232  having a thickness of about 10 nm may be formed.  FIG. 12A  and  FIG. 12B  each illustrate an exemplary insulating film.  FIG. 12A  may be an enlarged view of the insulating film  230  illustrated in  FIG. 11 . 
         [0063]    In the gate insulating film corresponding to the insulating film  230 , two different material films are laminated. Therefore, the gate leakage current may be reduced. For example, the gate leakage current may decrease by the lamination of an aluminum oxide film whose insulation properties are higher than those of the amorphous carbon film, as compared with the insulating film including only the amorphous carbon film. 
         [0064]    In order to reduce the threshold voltage variation range, the thickness of the amorphous carbon film  232  may be equal to or larger than that of the aluminum oxide film  231  as illustrated in  FIG. 5 . The thickness of the amorphous carbon film  232  may be 10 nm or more. 
         [0065]    In place of the aluminum oxide film  231 , a hafnium oxide film or a silicon nitride film may be formed. As illustrated in  FIG. 12B , by forming the amorphous carbon film  232  and forming the aluminum oxide film  231  on the amorphous carbon film  232 , the insulating film  233  or the insulating film  230  may be formed. Other elements may be substantially the same as or similar to the elements illustrated in  FIG. 9 . 
         [0066]      FIG. 13  illustrates an exemplary semiconductor device. The semiconductor device illustrated in  FIG. 13  may be an HEMT. The semiconductor device includes a buffer layer  320  formed on a substrate  310  including a semiconductor and the like and an electron transit layer  321 , an electron supply layer  322 , and a cap layer  323  laminated on the buffer layer  320  by epitaxial growth. A source electrode  342  and a drain electrode  343  are coupled to the electron transit layer  321 . A gate electrode  341  is formed through an insulating film  330  in an opening portion formed by partially removing the cap layer  323  and the electron supply layer  322 . The insulating film  330  is also formed on the cap layer  323 , and a protective film  350  including an insulator is formed on the insulating film  330 . 
         [0067]    The substrate  310  may include a Si substrate, a SiC substrate, a sapphire (Al 2 O 3 ) substrate, or the like. When the substrate  310  is a Si substrate, the buffer layer  320  is formed. When the substrate  310  includes other materials, the buffer layer  320  may not be formed. The electron transit layer  321  may be i-GaN, the electron supply layer  322  may be n-AlGaN, and the cap layer  323  may be n-GaN. A two-dimensional electron gas (2DEG)  321   a  is formed at the side near the electron supply layer  322  of the electron transit layer  321 . Between the electron transit layer  321  and the electron supply layer  322 , a non-illustrated spacer layer may be formed. 
         [0068]    The gate electrode  341 , the source electrode  342 , and the drain electrode  343  include metal materials. The insulating film  330  corresponding to the gate insulating film includes an amorphous carbon film having a thickness of 20 nm, for example. The protective film  350  is formed by forming an aluminum oxide (Al 2 O 3 ) film by the plasma ALD. 
         [0069]      FIG. 14A  to  FIG. 14F  illustrate an exemplary method for manufacturing a semiconductor device. 
         [0070]    As illustrated in  FIG. 14A , the buffer layer  320  is formed on the substrate  310 . On the buffer layer  320 , semiconductor layers of the electron transit layer  321 , the electron supply layer  322 , and the cap layer  323  are formed by epitaxial growth, such as MOVPE. The substrate  310  may include Si, SiC, sapphire (Al 2 O 3 ), or the like. In order to form the electron transit layer  321  and the like by epitaxial growth, the buffer layer  320  is formed on the substrate  310 . The buffer layer  320  may be a non-doped i-AlN layer having a thickness of about 0.1 μm, for example. The electron transit layer  321  may be a non-doped i-GaN layer having a thickness of about 3 μm. The electron supply layer  322  may be an n-Al 0.25 Ga 0.75 N layer having a thickness of about 30 nm where Si is doped as an impurity element with a concentration of 5×10 18  cm −3 . The cap layer  323  may be an n-GaN layer having a thickness of about 10 nm where Si is doped as an impurity element with a concentration of 5×10 18  cm −3 . 
         [0071]    As illustrated in  FIG. 14B , the source electrode  342  and the drain electrode  343  are formed. For example, a photoresist is applied onto the cap layer  323 , followed by exposure by an exposing apparatus and development, thereby forming a non-illustrated resist pattern having opening portions in regions where the source electrode  342  and the drain electrode  343  are to be formed. Dry etching, such as RIE using chlorine gas, is performed to remove the cap layer  23  and the electron supply layer  22  in a region where the resist pattern is not formed, so that the surface of the electron transit layer  21  is exposed. A metal film including a Ta/Al laminated film or the like is formed by vacuum deposition or the like, and the metal film formed on the resist pattern is lifted-off with the resist pattern by immersion in an organic solvent or the like. The source electrode  342  and the drain electrode  343  are formed in a region where the resist pattern is not formed. After lifting-off, ohmic contact is achieved by heat treatment at a temperature of 550° C. 
         [0072]    As illustrated in  FIG. 14C , an opening portion  361  is formed. For example, a photoresist is applied onto the cap layer  323 , followed by exposure by an exposing apparatus and development, thereby forming a non-illustrated resist pattern having an opening portion in a region where the opening portion  361  is to be formed. Gas including chlorine is introduced using the resist pattern as a mask, and dry etching by RIE or the like is performed. The cap layer  323  and the electron supply layer  322  in the region where the resist pattern is not formed are partially removed, and the opening portion  361  is formed. The resist pattern is removed. 
         [0073]    As illustrated in  FIG. 14D , the insulating film  330  is formed on the cap layer  323  on the inner surface of the opening portion  361 . The insulating film  330  is formed by the formation of the amorphous carbon film by the FCA method. For example, by using a graphite target as a raw material, setting an arc current to 70 A, and setting an arc voltage to 26 V, an amorphous carbon film having a thickness of about 20 nm is formed by the FCA method, and the insulating film  330  is formed. 
         [0074]    As illustrated in  FIG. 14E , the gate electrode  341  is formed. For example, a non-illustrated lower resist, for example, PMGI (trade name) manufactured by U.S. MicroChem. Corp. and a non-illustrated upper resist, for example, PFI32-A8 (trade name) manufactured by Sumitomo Chemical Co., Ltd., are individually formed on the insulating film  330  by the application by a spin coat method or the like. Exposure by an exposing apparatus and development are performed, whereby an opening having a diameter of about 0.8 μm is formed in the upper resist in a region including a portion where the opening portion  361  is formed. The lower resist is wet etched by an alkaline developer using the upper resist as a mask. Metal films, for example, Ni film having a thickness of about 10 nm and Au film having a thickness of about 300 nm, are formed by vacuum deposition on the entire surface, and lifting-off is performed using a warmed organic solvent, whereby the metal film formed on the upper resist is removed with the lower resist and the upper resist. The gate electrode  341  including Ni/Au is formed in the opening portion  361  through the insulating film  330 . 
         [0075]    As illustrated in  FIG. 14F , the protective film  350  is formed on the insulating film  330 . The protective film  350  may include an aluminum oxide film formed by the ALD method, an amorphous carbon film formed by the FCA method, a silicon nitride film formed by the plasma CVD method, or a laminated structure of these films, for example. 
         [0076]      FIG. 15  illustrates an exemplary discrete-packaged semiconductor device. The semiconductor device may be a semiconductor device illustrated in  FIG. 6 ,  FIG. 11 , or  FIG. 13 .  FIG. 15  schematically illustrates the inside of the discrete-packaged semiconductor device, in which the arrangement of electrodes and the like may be different from the electrode arrangement of  FIG. 6 ,  FIG. 11 , or  FIG. 13 . 
         [0077]    For example, the semiconductor device illustrated in  FIG. 6  or  FIG. 11  or  FIG. 13  is cut by dicing or the like, whereby an HEMT semiconductor chip  410  including a GaN-based semiconductor material is formed. The semiconductor chip  410  is fixed by a die attach adhesive  430 , such as a solder, on a lead frame  420 . 
         [0078]    A gate electrode  441  is coupled to a gate lead  421  through a bonding wire  431 . A source electrode  442  is coupled to a source lead  422  through a bonding wire  432 . A drain electrode  443  is coupled to a drain lead  423  through a bonding wire  433 . The bonding wires  431 ,  432 , and  433  may include metal materials, such as Al. The gate electrode  441  may be a gate electrode pad, and is coupled to the gate electrode  41  or  341  of  FIG. 6 ,  FIG. 11 , or  FIG. 13 . The source electrode  442  may be a source electrode pad, and is coupled to the source electrode  42  or  342 . The drain electrode  443  may be a drain electrode pad, and is coupled to the drain electrode  43  or  343 . 
         [0079]    Resin sealing is performed using a mold resin  440  by a transfer mold method. A semiconductor device in which an HEMT including a GaN-based semiconductor material is discrete-packaged is formed. 
         [0080]      FIG. 16  illustrates an exemplary power supply device. A power supply device  460  includes a high-pressure primary circuit  461 , a low-pressure secondary circuit  462 , and a transformer  463  disposed between the primary circuit  461  and the secondary circuit  462 . The primary circuit  461  includes an AC power supply  464 , a bridge rectifier circuit  465 , a plurality of switching elements  466 , for example, four switching elements, and one switching element  467 . The secondary circuit  462  includes a plurality of switching elements  468 , for example, three switching elements. The semiconductor device of  FIG. 6 ,  FIG. 11 , or  FIG. 13  may be used as the switching elements  466  and  467  of the primary circuit  461 . The switching elements  466  and  467  of the primary circuit  461  may be normally-off semiconductor devices. The switching elements  468  at the secondary circuit  462  side may include a metal insulator semiconductor field effect transistor (MISFET) including silicon. 
         [0081]      FIG. 17  illustrates an exemplary high frequency amplifier. A high frequency amplifier  470  may be applied to a power amplifier for base stations of cellular phones, for example. The high frequency amplifier  470  includes a digital predistortion circuit  471 , a mixer  472 , a power amplifier  473 , and a directional coupler  474 . The digital predistortion circuit  471  compensates the nonlinear distortion of an input signal. The mixer  472  mixes the input signal in which the nonlinear distortion has been compensated and an alternating current signal. The power amplifier  473  amplifies the input signal mixed with the alternating current signal. The power amplifier  473  may include the semiconductor device of  FIG. 6 ,  FIG. 11 , or  FIG. 13 . The directional coupler  474  monitors input signals or output signals. For example, the mixer  472  may mix output signals with alternating current signals, and send the mixed signals to the digital predistortion circuit  471  based on switching of the switch. 
         [0082]    Example embodiments of the present invention have now been described in accordance with the above advantages. It will be appreciated that these examples are merely illustrative of the invention. Many variations and modifications will be apparent to those skilled in the art.