Patent Publication Number: US-10332754-B2

Title: Method of manufacturing nitride semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese patent applications No. 2015-192663 filed on Sep. 30, 2015, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     Technical Field 
     The invention relates to a method of manufacturing a nitride semiconductor device. 
     Description of the Related Art 
     A known configuration of a semiconductor device includes a semiconductor layer that is made of a nitride semiconductor, for example, gallium nitride (GaN) (for example, JP 2014-086698A). Ion implantation may be employed to form a P-type region in a specified area of the semiconductor layer. 
     JP 2014-086698A describes a method that forms a silicon dioxide (SiO 2 ) layer of 50 nm in thickness on a gallium nitride layer to serve as a protective layer for protecting the surface of the gallium nitride layer, performs ion implantation and performs heat treatment at temperature of 800° C. to 900° C. JP 2014-041917A describes a method that performs ion implantation into a gallium nitride layer without providing a protective layer, forms an aluminum nitride (AlN) layer to protect the surface of the gallium nitride layer after performing the ion implantation and performs heat treatment. The relevant techniques are described in WO 2015/029578 and JP 2009-126727A. 
     In the technique described in JP 2014-086698A, however, the protective layer is formed rather thin to allow for ion permeation and has insufficient thickness as the protective layer in heat treatment. There is accordingly a likelihood that nitrogen present in the gallium nitride layer is dropped off during heat treatment to roughen the surface of the gallium nitride layer. In the technique described in JP 2014-041917A, on the other hand, the protective layer may be denatured by heat during heat treatment. This may make it difficult to remove the protective layer. There is accordingly a need to expose the gallium nitride layer to rather extreme conditions for removal of the protective layer. This may result in roughening the surface of the gallium nitride layer. 
     There is accordingly a demand for a technique that suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     SUMMARY 
     In order to solve at least part of the problems described above, the invention may be implemented by aspects described below. 
     (1) According to one aspect of the invention, there is provided a method of manufacturing a nitride semiconductor device. The method of manufacturing the nitride semiconductor device comprises: a first film forming process that forms a first film on a nitride semiconductor layer; an ion implantation process that implants a P-type impurity into the nitride semiconductor layer through the first film by ion implantation; a second film forming process that forms a second film on the first film, after the ion implantation process; and a heat treatment process that processes the nitride semiconductor layer by heat treatment after the second film forming process. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (2) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the first film may have thickness of 1 nm to 100 nm. The method of manufacturing the nitride semiconductor device of this aspect allows for efficient ion implantation. 
     (3) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the second film may be formed by metal organic chemical vapor deposition at temperature of not lower than 300° C. and not higher than 800° C. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (4) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the P-type impurity may be either magnesium or beryllium. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (5) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the heat treatment process may be performed at temperature of not lower than 900° C. and not higher than 1600° C. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (6) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the first film may be made of a nitride semiconductor containing at least one of aluminum and indium. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (7) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the second film may be made of a nitride semiconductor. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (8) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the second film may be made of a nitride semiconductor containing at least one of aluminum and indium. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (9) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the second film forming process and the heat treatment process may be performed in an identical apparatus. The method of manufacturing the nitride semiconductor device of this aspect does not need a process of moving the nitride semiconductor layer with the first film and the second film formed thereon into another apparatus. This reduces the total number of manufacturing processes. 
     (10) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the first film may contain substantially no silicon. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (11) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the heat treatment process may be performed at temperature of not lower than 900° C. and not higher than 1200° C. in an ammonia-containing atmosphere. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (12) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the second film may contain silicon. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (13) According to one embodiment, the method of manufacturing the nitride semiconductor device of the above aspect may further comprise a film removal process that removes the first film and the second film, after the heat treatment process. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     (14) According to one embodiment of the method of manufacturing the nitride semiconductor device of the above aspect, the film removal process may include a process of performing wet etching. The method of manufacturing the nitride semiconductor device of this aspect forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
     The invention may be implemented by any of various aspects other than the method of manufacturing the nitride semiconductor device described above, for example, a manufacturing apparatus for manufacturing a nitride semiconductor device by the above manufacturing method. 
     The method of manufacturing the nitride semiconductor device according to any of the above aspects forms the first film prior to the ion implantation process and forms the second film prior to the heat treatment process. This suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically illustrating the configuration of a semiconductor device according to a first embodiment; 
         FIG. 2  is a process chart showing a method of manufacturing the semiconductor device; 
         FIG. 3  is a diagram schematically illustrating formation of a first film; 
         FIG. 4  is a diagram schematically illustrating ion implantation; 
         FIG. 5  is a diagram schematically illustrating formation of a second film; 
         FIG. 6  is a diagram schematically illustrating formation of a third film; 
         FIGS. 7A to 7C  are diagrams schematically illustrating removal of the first film and the second film in a film removal process; and 
         FIG. 8  is a process chart showing a method of manufacturing a semiconductor device according to a second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A. First Embodiment 
     A-1. Configuration of Semiconductor Device 
       FIG. 1  is a sectional view schematically illustrating the configuration of a semiconductor device  10  according to a first embodiment. XYZ axes orthogonal to one another are illustrated in  FIG. 1 . The semiconductor device  10  is a nitride semiconductor device including a nitride semiconductor layer  120 . 
     Among the XYZ axes of  FIG. 1 , the X axis denotes a left-right axis on the sheet surface of  FIG. 1 . +X-axis direction denotes a rightward direction on the sheet surface, and −X-axis direction denotes a leftward direction on the sheet surface. Among the XYZ axes of  FIG. 1 , the Y axis denotes a front-back axis on the sheet surface of  FIG. 1 . +Y-axis direction denotes a backward direction on the sheet surface, and −Y-axis direction denotes a forward direction on the sheet surface. Among the XYZ axes of  FIG. 1 , the Z axis denotes a top-bottom axis on the sheet surface of  FIG. 1 . +Z-axis direction denotes an upward direction on the sheet surface, and −Z-axis direction denotes a downward direction on the sheet surface. 
     The semiconductor device  10  is a GaN-based semiconductor device formed using gallium nitride (GaN). The semiconductor device  10  includes a substrate  110  and a nitride semiconductor layer  120 . 
     The substrate  110  of the semiconductor device  10  is a semiconductor layer extended along the X axis and the Y axis. According to this embodiment, the substrate  110  is mainly made of gallium nitride (GaN). Another material, for example, sapphire (Al 2 O 3 ), silicon carbide (SiC), silicon (Si) or aluminum gallium nitride (AlGaN) may also be used as the material of the substrate  110 . In the description hereof, the expression of “being mainly made of” denotes “containing 90% or a higher at molar fraction. 
     The nitride semiconductor layer  120  of the semiconductor device  10  is an n-type semiconductor layer made of a nitride semiconductor and extended along the X axis and the Y axis. According to this embodiment, the nitride semiconductor layer  120  is mainly made of gallium nitride (GaN) and contains silicon (Si) as the donor. The nitride semiconductor layer  120  is laid on a +Z-axis direction side surface of the substrate  110 . Non-doped gallium nitride (GaN) or aluminum gallium nitride (AlGaN) may also be used for the nitride semiconductor layer  120 , but gallium nitride (GaN) is used preferably. 
     A P-type semiconductor region  125  of the semiconductor device  10  is part of a +Z-axis direction side area of the nitride semiconductor layer  120  and is formed by ion implantation. The semiconductor in the P-type semiconductor region  125  mainly has P-type characteristics. According to this embodiment, the P-type semiconductor region  125  is mainly made of gallium nitride (GaN), like the nitride semiconductor layer  120 . 
     A-2. Method of Manufacturing Semiconductor Device 
       FIG. 2  is a process chart showing a method of manufacturing the semiconductor device  10 . In manufacture of the semiconductor device  10 , the manufacturer forms a nitride semiconductor layer  120  on a substrate  110  by epitaxial growth at process P 110 . According to this embodiment, the manufacturer forms the nitride semiconductor layer  120  on the substrate  110  by epitaxial growth using an MOCVD apparatus performing metal organic chemical vapor deposition (MOCVD). 
     After forming the nitride semiconductor layer  120  (process P 110 ), the manufacturer forms a first film  130  on (more specifically, on the +Z-axis direction side of) the nitride semiconductor layer  120  at process P 115 , as shown in  FIG. 3 . The process P 115  is also called first film forming process. 
       FIG. 3  is a diagram illustrating formation of the first film  130 . The first film  130  serves to suppress a +Z-axis direction side surface of the nitride semiconductor layer  120  from being contaminated in a subsequent ion implantation process and suppress recoil of ions implanted by ion implantation. The first film  130  also provides a high dose impurity of the ion in the vicinity of the +Z-axis direction side surface of the nitride semiconductor layer  120  in the ion implantation process. 
     The film thickness of the first film  130  is preferably not less than 1 nm and is more preferably not less than 2 nm, in order to sufficiently cover the +Z-axis direction side surface of the nitride semiconductor layer  120 . In order to allow for sufficient permeation of ions implanted by ion implantation, on the other hand, the film thickness of the first film  130  is preferably not greater than 100 nm and is more preferably not greater than 50 nm. The first film  130  having the film thickness in the above preferable range ensures more efficient ion implantation. According to this embodiment, the first film  130  has the film thickness of 30 nm. 
     In the case of ion implantation of a P-type impurity such as magnesium (Mg), it is preferable that the first film  130  contains no N-type impurity. For example, it is preferable that the first film  130  contains substantially no silicon (Si). In the case where the first film  130  contains silicon (Si) as an N-type impurity in the form of, for example, silicon dioxide (SiO 2 ) or silicon nitride (SiN), ions that are implanted by ion implantation and permeate through the first film  130  are likely to collide with the N-type impurity present in the first film  130 , so that the collided N-type impurity is likely to be unintentionally implanted into the nitride semiconductor layer  120 . This may result in decreasing a P-type carrier or producing an N-type carrier in the nitride semiconductor layer  120 . The expression of “containing substantially no” denotes “containing less than 1% at molar fraction”. 
     The first film  130  may be made of a nitride semiconductor containing at least one of aluminum (Al) and indium (In). According to this embodiment, aluminum nitride (AlN) is used as the material of the first film  130 . Other available materials for the first film  130  include, for example, indium nitride (In), aluminum oxide (Al 2 O 3 ), hafnium oxide (HMO, zirconium oxide (ZrO 2 ), magnesium oxide (MgO) and gallium oxide (Ga 2 O 3 ). According to this embodiment, the first film  130  is formed by sputtering. According to another embodiment, the first film  130  may be formed by metal organic chemical vapor deposition (MOCVD) or chemical vapor deposition (CVD). 
     After the first film forming process (process P 115 ), the manufacturer implant ions of P-type impurity into the nitride semiconductor layer  120  through the first film  130  at process P 120 . The process P 120  is also called ion implantation process. 
       FIG. 4  is a diagram illustrating ion implantation. According to this embodiment, magnesium (Mg) is used as the P-type impurity for ion implantation. The P-type impurity for ion implantation may be another element, for example, beryllium (Be). The temperature of ion implantation is not specifically limited but may be room temperature. The temperature of ion implantation is preferably not lower than 500° C., in order to reduce possible crystal defects during ion implantation and facilitate the P-type impurity implanted by ion implantation to serve as the acceptor. The temperature of ion implantation is also preferably not higher than 600° C., in order to suppress nitrogen (N) from being dropped off from the surface of the nitride semiconductor layer  120 . 
     After the ion implantation process (process P 120 ), the manufacturer introduces the substrate  110  with the nitride semiconductor layer  120  and the first film  130  formed thereon into the MOCVD apparatus at process P 125 . 
     At subsequent process P 130 , the manufacturer forms a second film  140  on the first film  130 . The process P 130  is also called second film forming process. 
       FIG. 5  is a diagram illustrating formation of the second film  140 . The second film  140  serves to suppress the +Z-axis direction side surface of the nitride semiconductor layer  120  from being roughened by subsequent heat treatment. For example, in the case where the first film  130  fails to cover part of the +Z-axis direction side surface of the nitride semiconductor layer  120 , the second film  140  covers the part of the +Z-axis direction side surface of the nitride semiconductor layer  120  that is not covered by the first film  130 . As a result, the second film  140  serves to suppress exposure of the +Z-axis direction side surface of the nitride semiconductor layer  120 . 
     The film thickness of the second film  140  is preferably not less than 10 nm and not greater than 100 nm. The second film  140  having the film thickness of not less than 10 nm can sufficiently cover the +Z-axis direction side surface of the nitride semiconductor layer  120 . The second film  140  having the film thickness of not greater than 100 nm, on the other hand, can be readily removed in a subsequent film removal process. According to this embodiment, the second film  140  has the film thickness of 70 nm. 
     The second film  140  may be made of a nitride semiconductor, for example, a nitride semiconductor containing at least one of aluminum (Al) and indium (In). According to this embodiment, aluminum nitride (AlN) is used as the material of the second film  140 . The materials of the first film  130  and the second film  140  are not specifically limited. For example, in the case where indium nitride (InN) is used as the material of the first film  130 , gallium nitride (GaN) may be used as the material of the second film  140 . In the case where indium nitride (InN) is used as the material of the second film  140 , indium nitride (InN) is likely to be sublimated and removed by subsequent heat treatment. It is accordingly preferable to add a nitride semiconductor containing aluminum (Al) and gallium (Ga) to the material of the second film  140 . The material of the second film  140  preferably includes, for example, indium aluminum nitride (In x Al 1-x N (0&lt;x&lt;1), indium gallium nitride (In x Ga 1-x N (0&lt;x&lt;1), aluminum nitride (AlN) or gallium nitride (GaN). 
     According to this embodiment, the second film  140  may be formed by metal organic chemical vapor deposition (MOCVD) at temperature of not lower than 300° C. and not higher than 800° C. In general, in terms of forming a dense film, it is preferable to form a film by epitaxial growth at temperature of not lower than 1100° C. According to this embodiment, however, ion implantation is likely to roughen a +Z-axis direction side surface of the first film  130 , this makes it difficult to form a dense film by epitaxial growth. In terms of forming a dense film, it is thus preferable to form the second film  140  by metal organic chemical vapor deposition at temperature of not lower than 300° C. and not higher than 800° C. 
     After the second film forming process (process P 130 ), the manufacturer processes the nitride semiconductor layer  120  by heat treatment at process P 135 . The process P 135  is also called heat treatment process. According to this embodiment, the heat treatment process (process P 135 ) is performed subsequent to the second film forming process (process P 130 ) in the same MOCVD apparatus. Accordingly the substrate  110  is not taken out of the MOCVD apparatus from the second film forming process (process P 130 ) to the end of the heat treatment process (process P 135 ). This heat treatment process (process P 135 ) activates the P-type impurity implanted into the nitride semiconductor layer  120  by ion implantation. More specifically, the heat treatment process moves the implanted P-type impurity to adequate lattice positions in the nitride semiconductor layer  120  and recovers the nitride semiconductor layer  120  from damage by ion implantation, thus producing a P-type carrier. 
     The heat treatment temperature is preferably not lower than 900° C., in terms of more effectively activating the P-type impurity. In terms of suppressing nitrogen (N) from being dropped off from the nitride semiconductor layer  120  or the second film  140 , the heat treatment temperature is not higher than 1400° C., and the atmosphere of heat treatment is a nitrogen (N)-containing atmosphere, and is preferably an ammonia (NH 3 )-containing atmosphere. Preferably, the heat temperature is not higher than 1200° C. When heat treatment is performed in the ammonia (NH 3 )-containing atmosphere, (i) it is preferable that the flow rate of ammonia gas is preferably not lower than 10 slm and not higher than 50 slm; and (ii) the internal pressure of the heat treatment space is preferably not lower than 10 Torr and not higher than 760 Torr and is more preferably not lower than 200 Torr and not higher than 400 Torr. The heat treatment time is preferably not shorter than 1 minute and not longer than 60 minutes. For example, at the heat treatment temperature of 1050° C., the heat treatment time of not shorter than 5 minutes is preferable to more effectively activate the P-type impurity in the nitride semiconductor layer  120 . 
     Samples were heated at different heat treatment temperatures of 1050° C., 1100° C. and 1150° C., and the surface of the nitride semiconductor layer  120  of each sample was evaluated by photoluminescence method. According to the results of evaluation, the sample heated at the heat treatment temperature of 1150° C. had weak emission intensity at a band end and strong emission intensity of yellow luminescence. This result indicates that deep levels due to crystal defects are provided on the surface layer of the nitride semiconductor layer  120  in the sample heated at the heat treatment temperature of 1150° C. and suggests an increase of crystal defects. By taking into account these results, the heat treatment temperature is preferably lower than 1150° C. In terms of accelerating activation of the P-type impurity in the nitride semiconductor layer  120 , the heat treatment temperature is preferably not lower than 900° C. 
     After the heat treatment process (process P 135 ), the manufacturer takes out of the substrate  110  with the nitride semiconductor layer  120 , the first film  130  and the second film  140  formed thereon from the MOCVD apparatus. 
     At subsequent process P 145 , the manufacturer forms a third film  150  on a −Z-axis direction side rear face of the substrate  110 . The third film  150  serves to suppress the −Z-axis direction side rear face of the substrate  110  from being roughened by subsequent etching. 
       FIG. 6  is a diagram illustrating formation of the third film  150 . According to this embodiment, the third film  150  is made of silicon dioxide (SiO 2 ). According to this embodiment, the third film  150  is formed by chemical vapor deposition. According to another embodiment, the third film  150  may be formed by sputtering. 
     At subsequent process P 150 , the manufacturer removes the first film  130 , the second film  140  and the third film  150 . The process P 150  is also called film removal process. According to this embodiment, wet etching is employed for the film removal process. 
     The manufacturer first removes the first film  130  and the second film  140  using a TMAH (tetramethylammonium hydroxide) aqueous solution that is an alkali aqueous solution. According to this embodiment, after the TMAH aqueous solution is heated to 60° C. or higher (more specifically, 85° C. in this embodiment), the substrate  110  is soaked in the TMAH aqueous solution for 15 minutes or longer. This process removes the first film  130  and the second film  140 . Potassium hydroxide (KOH) may be used for the alkali aqueous solution. 
     The manufacturer subsequently removes the third film  150  and the residuals of the first film  130  and the second film  140  using a BHF (buffered hydrogen fluoride) aqueous solution or an HF (hydrofluoric acid) aqueous solution. According to this embodiment, the substrate  110  is soaked in the BHF aqueous solution or the HF aqueous solution for 5 minutes to 15 minutes and is then washed with ultrapure water. 
     The semiconductor device  10  is completed by this series of processes. 
     The method of manufacturing the semiconductor device  10  according to this embodiment forms the first film (process P 115 ) prior to the ion implantation process (process P 120 ). This suppresses the surface of the nitride semiconductor layer  120  from being roughened in the ion implantation process (process P 120 ). The manufacturing method forms the second film (process P 130 ) prior to the heat treatment process (process P 135 ). This suppresses the surface of the nitride semiconductor layer  120  from being roughened in the heat treatment process (process P 135 ). 
     In the case of a semiconductor layer using silicon (Si) as the semiconductor, silicon (Si) is the stable element and is unlikely to be dropped off from the semiconductor layer in the heat treatment process. It is thus unlikely that the surface of the semiconductor layer is roughened by heat treatment. In the case of the nitrogen (N)-containing nitride semiconductor layer  120 , on the other hand, nitrogen (N) is likely to be dropped off from the nitride semiconductor layer  120  in the heat treatment process. Accordingly, forming the second film  140  on the first film  130  at the process P 130  prior to the heat treatment process (process P 135 ) suppresses the +Z-axis direction side surface of the nitride semiconductor layer  120  from being roughened. Accordingly, the problem that the surface of the semiconductor layer is roughened by heat treatment without forming the second film  140  is not found in the silicon (Si) substrate but is characteristic of the semiconductor device including the nitride semiconductor layer  120 . 
     According to this embodiment, the first film  130  and the second film  140  can be readily removed in the film removal process (process P 150 ). The following describes the mechanism of removal. The first film  130  is damaged in the ion implantation process (process P 120 ) and has a higher degree of crystal defects after the ion implantation process (process P 120 ). The first film  130  is thus in the state to be readily removable in the film removal process (process P 150 ). Even when a dense film is formed as the second film  140 , the first film  130  can be readily removed from the nitride semiconductor layer  120  in the film removal process (process P 150 ). 
       FIGS. 7A to 7C  are diagrams illustrating removal of the first film  130  and the second film  140  in the film removal process (process P 150 ). The third film  150  is omitted from the illustration of  FIGS. 7A to 7C , for the purpose of easy explanation. 
     The film removal process (process P 150 ) first removes part of the second film  140  to make part of the first film  130  exposed as shown in  FIG. 7A . The first film  130  is then preferentially removed as shown in  FIG. 7B . This is attributed to the higher degree of crystal defects in the first film  130  by the damage in the ion implantation process (process P 120 ). Preferential removal of the first film  130  causes the second film  140  from being lifted off from the P-type semiconductor region  125 . As a result, the first film  130  and the second film  140  are removed from the nitride semiconductor layer  120  as shown in  FIG. 7C . 
     According to this embodiment, the first film  130  and the second film  140  are readily removed in the film removal process (process P 150 ). This suppresses the nitride semiconductor layer  120  from being roughened in the film removal process (process P 150 ). 
     According to this embodiment, the heat treatment process (process P 135 ) is performed subsequent to the second film forming process (process P 130 ) in the same MOCVD apparatus. This does not need a process of moving the substrate  110  to another apparatus and thereby reduces the total number of manufacturing processes. 
     B. Second Embodiment 
       FIG. 8  is a process chart showing a method of manufacturing a semiconductor device  10 A according to a second embodiment. The method of manufacturing the semiconductor device  10 A according to the second embodiment differs from the method of manufacturing the semiconductor device  10  according to the first embodiment (shown in  FIG. 2 ) by that a heat treatment process (process P 135 A) is performed in a different apparatus from an apparatus used for the second film forming process (process P 130 ) but is otherwise similar to the method of manufacturing the semiconductor device  10  according to the first embodiment. In other words, the method of manufacturing the semiconductor device  10 A according to the second embodiment differs from the method of manufacturing the semiconductor device  10  according to the first embodiment (shown in  FIG. 2 ) by a series of processes between the ion implantation process (process P 120 ) and the third film forming process (process P 145 ) but is otherwise similar to the method of manufacturing the semiconductor device  10  according to the first embodiment. 
     According to the second embodiment, after the ion implantation process (process P 120 ), the manufacturer introduces the substrate  110  with the nitride semiconductor layer  120  and the first film  130  formed thereon into a film forming apparatus at process P 125 A. The film forming apparatus may be, for example, a sputtering apparatus or an MOCVD apparatus. According to this embodiment, a sputtering apparatus is employed as the film forming apparatus. 
     At subsequent process P 130 A, the manufacturer forms a second film  140 A on the first film  130 . The second film  140 A may be, for example, an insulating film made of aluminum oxide (Al 2 O 3 ), silicon dioxide (SiO 2 ), silicon nitride (SiN), zirconium oxide (ZrO 2 ), magnesium oxide (MgO) or gallium oxide (Ga 2 O 3 ) or a carbon film made of hydrocarbon (C 2 H 2 ). According to this embodiment, the second film  140 A is made of the silicon (Si)-containing material, i.e., silicon dioxide (SiO 2 ). 
     After the second film forming process (process P 130 A), the manufacturer introduces the substrate  110  with the nitride semiconductor layer  120 , the first film  130  and the second film  140 A formed thereon into an RTA (rapid thermal annealing) apparatus. The substrate  110  may be introduced into an MOCVD apparatus, in place of the RTA apparatus. 
     At subsequent process P 135 A, the manufacturer processes the nitride semiconductor layer  120  by heat treatment. According to this embodiment, the second film  140 A is made of the insulating material, i.e., silicon dioxide (SiO 2 ) and does not contain nitrogen (N), so that nitrogen (N) is not dropped off from the second film  140 A itself. There is accordingly no need to perform heat treatment in the ammonia (NH 3 )-containing atmosphere. By taking into account the likelihood that nitrogen (N) included in, for example, the nitride semiconductor layer  120  is dropped off across the second film  140 A, it is preferable to perform heat treatment in a nitrogen (N)-containing atmosphere. The heat treatment temperature is preferably not lower than 900° C., in terms of more effectively activating the P-type impurity. In terms of suppressing the surface roughness of the nitride semiconductor layer  120  caused by drop-off of nitrogen (N) from the nitride semiconductor layer  120 , on the other hand, the heat treatment temperature is preferably not higher than 1600° C. and is more preferably not higher than 1500° C. 
     The method of manufacturing the semiconductor device  10 A according to this embodiment forms the first film (process P 115 ) prior to the ion implantation process (process P 120 ). This suppresses the surface of the nitride semiconductor layer  120  from being roughened in the ion implantation process (process P 120 ). The manufacturing method forms the second film (process P 130 A) prior to the heat treatment process (process P 135 A). This suppresses the surface of the nitride semiconductor layer  120  from being roughened in the heat treatment process (process P 135 A). 
     C. Other Embodiments 
     The invention is not limited to any of the embodiments, the examples and the modifications described above but may be implemented by a diversity of other configurations without departing from the scope of the invention. For example, the technical features of any of the embodiments, the examples and modifications corresponding to the technical features of each of the aspects described in Summary may be replaced or combined appropriately, in order to solve part or all of the problems described above or in order to achieve part or all of the advantageous effects described above. Any of the technical features may be omitted appropriately unless the technical feature is described as essential herein. 
     The embodiment describes above uses the P-type impurity as the ion implantation specifies. The advantageous effects of the invention can be achieved by using an N-type impurity (for example, silicon (Si)) or by using an impurity for element isolation (for example, boron (B), oxygen (O), iron (Fe) or carbon (C)). 
     In the embodiment described above, the film removal process (P 150 ) performs wet etching using the BHF aqueous solution or the HF aqueous solution after wet etching using the TMAH aqueous solution. The invention is, however, not limited to this procedure. For example, any of the following procedures may be employed in the film removal process (P 150 ). 
     According to another embodiment, the film removal process (P 150 ) may perform only the wet etching using the TMAH aqueous solution. The third film  150  serves to suppress formation of irregularities on the −Z-axis direction side rear face of the substrate  110  in the film removal process. In some cases, for example, in the case of forming electrodes on the −Z-axis direction side rear face of the substrate  110 , however, a concavo-convex surface may be allowed for the −Z-axis direction side rear face of the substrate  110 . In such cases, the film removal process (P 150 ) may perform only the wet etching using the TMAH aqueous solution. Only the wet etching using the TMAH aqueous solution, however, provides the possibility that the residuals of the first film  130  and the second film  140  are left on the +Z-axis direction side surface of the nitride semiconductor layer  120 . Accordingly the film removal process (P 150 ) preferably performs the wet etching using the BHF aqueous solution or the HF aqueous solution after the wet etching using the TMAH aqueous solution. 
     According to another embodiment, the film removal process (P 150 ) may sequentially perform (i) wet etching using the TMAH aqueous solution, (ii) wet etching using the BHF aqueous solution or the HF aqueous solution, and (iii) wet etching using hydrochloric acid (HCl). This procedure more effectively suppresses the residuals of the first film  130  and the second film  140  from being left on the +Z-axis direction side surface of the nitride semiconductor layer  120 . 
     According to another embodiment, the film removal process (P 150 ) may perform dry etching. For example, the film removal process (P 150 ) may sequentially perform (i) wet etching using the TMAH aqueous solution, (ii) wet etching using the BHF aqueous solution or the HF aqueous solution, and (iii) dry etching using a chlorine (Cl)-containing etching gas. In the case where aluminum nitride (AlN) is used as the material of the first film  130 , part of aluminum nitride (AlN) of the first film  130  may react with gallium nitride (GaN) of the nitride semiconductor layer  120  to produce aluminum gallium nitride (AlGaN) in the heat treatment process (process P 135 ). This may make it difficult to remove the first film  130 . Aluminum gallium nitride (AlGaN) is not readily removed by using the TMAH aqueous solution but is removable by dry etching using the chlorine (Cl)-containing etching gas. This procedure more effectively suppresses the residuals of the first film  130  and the second film  140  from being left on the +Z-axis direction side surface of the nitride semiconductor layer  120 . 
     The method of manufacturing the nitride semiconductor device described in any of the above embodiments may be employed as part of a method of manufacturing another semiconductor device, for example, MESFET (metal-semiconductor field effect transistor) or HFET (hetero-FET).