Abstract:
A method for fabricating a field effect transistor includes: forming an insulating film provided on a semiconductor layer, the insulating film having an opening via which a surface of the semiconductor layer is exposed and including silicon oxide; forming a Schottky electrode on the insulating film and in the opening, the Schottky electrode having an overhang portion and having a first contact layer that is provided in a region contacting the insulating film and contains oxygen, and a second contact layer that is provided on the first contact layer and contains a smaller content of oxygen than that of the first contact layer; and removing the insulating film by a solution including hydrofluoric acid

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. Ser. No. 12/027,425, filed Feb. 7, 2008, which is based upon and claims priority of Japanese Patent Application No. 2007-028603 filed Feb. 7, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a field effect transistor and a method for fabricating the same, and more particularly, to a field effect transistor having tungsten silicide as a connecting layer connected to a semiconductor layer and a method for fabricating the same. 2. Description of the Related Art 
         [0004]    There is an FET (Field Effect Transistor) having a Schottky electrode that contacts a semiconductor substrate. An example of this type is MESFET (Metal Semiconductor Field Effect Transistor).  FIGS. 1A and 1B  are respectively cross-sectional views of a conventional FET having a Schottky electrode that contacts a semiconductor substrate  10  containing GaAs (gallium arsenide). This type of FET is described in, for example, Japanese Patent Application Publication No. 6-163605. Referring to  FIG. 1A , a recess  12  is formed on the semiconductor substrate  10 . A silicon oxide (SiO 2 ) film  14  having an opening is formed on the semiconductor substrate  10 . A Schottky electrode  30  is formed in the opening. The Schottky electrode  30  is composed of a contact layer  20  made of tungsten silicide (WSi), and a metal layer  28  made of gold (Au). The Schottky electrode  30  contacts the semiconductor substrate  10  in the recess  12 , and functions as a gate electrode of the FET. There is a parasitic capacitance Cf resulting from a dielectric film, which is the silicon oxide film  14  between the semiconductor substrate  10  and the Schottky electrode  30 . Thus, as shown in  FIG. 1B , the silicon oxide film  14  is removed to reduce the parasitic capacitance Cf between the semiconductor substrate  10  and the Schottky electrode  30 . 
         [0005]    However, there is a problem that contact layer  20  of tungsten silicide is chipped when the silicon oxide film  14  is removed by a solution containing hydrofluoric acid. The contact layer  20  thus chipped degrades the reliability of the Schottky electrode and deteriorates the performance of FET. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention has been made in view of the above circumstances and provides a semiconductor device capable of restraining a contact layer from being chipped and a method for fabricating the same. 
         [0007]    According to an aspect of the present invention, there is provided a method for fabricating a semiconductor device including: forming an insulating film provided on a semiconductor layer, the insulating film having an opening via which a surface of the semiconductor layer is exposed and including silicon oxide; forming a Schottky electrode on the insulating film and in the opening, the Schottky electrode having an overhang portion and having a first contact layer that is provided in a region contacting the insulating film and contains oxygen, and a second contact layer that is provided on the first contact layer and contains a smaller content of oxygen than that of the first contact layer; and removing the insulating film by a solution including hydrofluoric acid. 
         [0008]    According to another aspect of the present invention, there is provided a semiconductor device including: a semiconductor layer; and a Schottky electrode provided on the semiconductor layer and composed of an overhang portion, the Schottky electrode having a contact layer that is provided in a region that contacts a surface of the semiconductor layer and is located below the overhang portion, the contact layer being made of tungsten silicide having an oxygen content of 2.6% or more. 
         [0009]    According to yet another aspect of the present invention, there is provided a semiconductor device including: a semiconductor layer; and a Schottky electrode provided on the semiconductor layer and composed of an overhang portion, the Schottky electrode having a first contact layer that is located below the overhang portion and is made of tungsten silicide containing oxygen, and a second contact layer that is provided on the first contact layer and has a smaller oxygen content than that of the first contact layer. 
         [0010]    According to a further aspect of the present invention, there is provided a method for fabricating a semiconductor device including: forming an insulating film provided on a semiconductor layer, the insulating film having an opening via which a surface of the semiconductor layer is exposed and including silicon oxide; forming a Schottky electrode on the insulating film and in the opening, the Schottky electrode having an overhang portion and having a contact layer that is provided in a region contacting the insulating film and is composed of a portion made of tungsten silicide having an oxygen content of 2.6% or more; and removing the insulating film by a solution including hydrofluoric acid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A and 1B  are cross-sectional views of a conventional FET; 
           [0012]      FIGS. 2A through 2C  are cross-sectional views of steps of a process for fabricating an FET in accordance with a first embodiment; 
           [0013]      FIGS. 3A and 3B  are cross-sectional views of steps of the process following the steps shown in  FIGS. 2A through 2C ; 
           [0014]      FIG. 4  shows an oxygen content and an argon content as a function of sputtering pressure in tungsten silicide; 
           [0015]      FIG. 5  is a schematic cross-sectional view of sample A; 
           [0016]      FIG. 6  is a schematic cross-sectional view of sample B; 
           [0017]      FIG. 7  is a schematic cross-sectional view of sample C; 
           [0018]      FIG. 8A  is a cross-sectional view of a step of a process for fabricating an FET in accordance with a second embodiment, and  FIG. 8B  is a graph of an oxygen content with respect to the height of a contact layer of the FET; 
           [0019]      FIG. 9A  is a cross-sectional view of a step of a process for fabricating an FET in accordance with the second embodiment, and  FIG. 9B  is a graph of an oxygen content with respect to the height of a contact layer of the FET; 
           [0020]      FIG. 10  schematically shows sputtering of tungsten silicide in a third embodiment; and 
           [0021]      FIG. 11  is a cross-sectional view of a step of a process for fabricating the FET in accordance with the third embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    A description will now be given of embodiments of the present invention with reference to the accompanying drawings. 
       First Embodiment 
       [0023]    A description will now be given, with reference to  FIGS. 2A through 3B , of a method for fabricating an FET in accordance with a first embodiment of the present invention. Referring to  FIG. 2A , the semiconductor substrate  10  (semiconductor layer) is formed by epitaxially growing a channel layer of InGaAs (indium gallium arsenide) and an electron supply layer of AlGaAs (aluminum gallium arsenide) on a GaAs substrate. The recess  12  is formed on the semiconductor substrate  10 . The silicon oxide film  14  (insulating film) is formed on the semiconductor substrate  10  by CVD (Chemical Vapor Deposition). The opening  16  is formed in a recess portion of the silicon oxide film  14 . This process defines the silicon oxide film  14  having the opening  16  through which the surface of the semiconductor substrate  10  is exposed. The silicon oxide film  14  has a thickness of approximately 300 nm, and the length of the opening  16  on the bottom of the silicon oxide film  14  that contacts the semiconductor substrate  10  is approximately 0.2 μm. The length of the opening  16  on the bottom of the silicon oxide film  14  is less than that of the opening on the top thereof. This is because the opening  16  is formed by a sidewall method. 
         [0024]    Referring to  FIG. 2B , tungsten silicide is grown to a thickness of approximately 0.15 μm on the silicon oxide film  14  and in the opening  16  by DC sputtering, so that a contact layer  20  of tungsten silicide can be formed. The inventors investigated the quality of tungsten silicide grown as the contact layer  20  shown in  FIG. 2B . The investigation used a sputtering power of 2 kW and sputtering gas of argon (Ar). The content of oxygen (mole %) and the content of argon were evaluated while the sputtering pressure that is the pressure of the sputtering gas in the sputtering apparatus was changed. Then, the semiconductor substrate  10  is annealed at 500° C. for 30 minutes. 
         [0025]    Referring to  FIG. 2C , the metal layer  28  of Au is formed on the contact layer  20  by plating. The contact layer  20  is removed with the metal layer  28  being as a mask, so that the Schottky electrode  30  composed of the contact layer  20  and the metal layer  28  can be completed. 
         [0026]    Referring to  FIG. 3A , the silicon oxide film  14  is removed from predetermined regions, and a source electrode  32  and a drain electrode  34 , which are ohmic contacts, are formed by evaporation. 
         [0027]    Referring to  FIG. 3B , the silicon oxide film  14  is removed by an HF solution, which may be buffered hydrofluoric acid composed of hydrofluoric acid and ammonium fluoride. In this step, the silicon oxide film  14  located below an overhang portion  31  of the Schottky electrode  30  is completely removed.  FIG. 3B  shows the completed FET. 
         [0028]      FIG. 4  is a graph of the content of oxygen and the content of argon as a function of sputtering pressure. The contact layer  20  contains oxygen (O 2 ) along with argon (Ar) that is sputtering gas. This is because residual oxygen in a chamber of the sputtering apparatus is captured by tungsten silicide. As the sputtering pressure is decreased, the content of argon is increased and the content of oxygen is also increased. 
         [0029]      FIGS. 5 and 6  are respectively schematic cross-sectional views of gate electrodes and their vicinities of samples A and B of FETs respectively having the contact layers  20  grown at sputtering pressures of 2.0 Pa and 1.0 Pa and respectively having oxygen contents of 2.0% and 2.6% (see  FIG. 4 ). Referring to  FIG. 5 , chips  50  are observed in the contact layer  20  of tungsten silicide of sample A. In contrast, no chips are observed in the contact layer  20  of tungsten silicide of sample B. The chips  50  formed in the contact layer  20  of sample A degrade the reliability of the Schottky electrode  30  and deteriorate electrical performance. In contrast, sample B is highly reliable and is electrically stable in the absence of chips in the tungsten silicide contact layer  20 . 
         [0030]    As described above, an increased content of oxygen in tungsten silicide restrains chipping of tungsten silicide. This improvement may be explained as follows. 
         [0031]    Etching of SiO 2  by HF is expressed by reaction formulas (1) and (2): 
         [0000]      2HF+H 2 O→H 3 O + +HF 2   −   (1)
 
         [0000]      SiO 2 +2H 3 O + +2HF 2   − →SiF 4 +4H 2 O  (2)
 
         [0032]    Formula (1) shows that the reaction does not progress in the absence of H 2 O, and formula (2) shows that H 2 O is generated in the progress of the reaction. 
         [0033]    If electrons e −  are supplied to H 2 O in liquid phase due to a certain factor, OH −  is generated by a reaction expressed by formula (3) by taking oxygen in the air: 
         [0000]      O 2 +2H 2 O+4e − →4OH −   (3)
 
         [0034]    It may be considered that a reaction described by formula (4) etches tungsten silicide from OH −  generated by formula (3) and HF 2   −  generated by formula (1): 
         [0000]      WSi+4HF 2   − +4OH − →WF 4 +SiF 4 +4H 2 O  (4)
 
         [0035]    In a case where the surface of the semiconductor substrate  10  is exposed to hydrofluoric acid, electrons in the conduction band are effused from the surface of the semiconductor substrate  10  into an aqueous solution, and the reaction described in formula (3) takes places. Thus, the reactions of formulas (2) and (4) may be caused. In order to investigate which one of the reactions of formulas (2) and (4) is faster than the other, sample C was prepared. Sample C was produced by growing the contact layer  20  under the same condition as that for sample A, and the silicon oxide film  14  was etched so as not to fully expose the overhang portion  31  of the Schottky electrode  30  in  FIG. 3B . 
         [0036]      FIG. 7  is a cross-sectional view of sample C. The silicon oxide film  14  remains below the overhang portion  31  of the contact layer  20  (indicated by reference numerals  52  in  FIG. 7 ). In  FIG. 7 , chips in the tungsten silicide layer as shown in  FIG. 5  are not observed. It may be considered from the above experimental results that 4HF 2   −  generated in formula (1) is spent in the reaction of formula (2) during the time when SiO 2  remains, and the reaction of formula (4) does not take place so that tungsten silicide is not etched because the reaction of formula (2) is faster than that of formula (4). 
         [0037]    Oxygen that is mixed with tungsten silicide when the contact layer  20  is formed shown in  FIG. 2B , reacts with silicon of tungsten silicide by annealing and results in silicon oxide. Thus, the contact layer  20  of sample B contains silicon oxide in addition to tungsten silicide. It may be considered that, when there is a large amount of silicon oxide (that is, a large oxygen content), 4HF 2   −  is used in the reaction with silicon oxide and tungsten silicide is not etched, as has been considered previously. It is however guessed that sample A does not have a sufficient amount of silicon oxide in tungsten silicide of the contact layer  20 , which may cause the chips  50  in the contact layer  20  of tungsten silicide. 
         [0038]    In terms of the above considerations, the contact layer  20  made of tungsten silicide having an oxygen content of 2.6% is formed in a region (a lower portion of the overhang portion  31 ) in which the contact layer  20  may contact the silicon oxide film  14 . With this structure, the contact layer  20  of tungsten silicide is not chipped as shown in  FIG. 6  even by removing the silicon oxide film  14  formed along the side surface of the opening  16  by a solution containing hydrofluoric acid. It is thus possible to improve the reliability of the Schottky electrode  30  and stabilize the electric performance. The contact layer  20  of tungsten silicide can be restrained from being chipped for an oxygen content of 2.6% or more. Only a part of a contact region over which the contact layer  20  of tungsten silicide contacts the silicon oxide film  14  may satisfy an oxygen content of 2.6% or more. Preferably, the entire contact region over which the contact layer  20  contacts the silicon oxide film  14  satisfies an oxygen content of 2.6% or more in order to more effectively restrain chipping of tungsten silicide. A portion of the contact layer  20  that contacts the semiconductor substrate  10  may have an oxygen content of 2.6% or more. 
         [0039]    The first embodiment has the Schottky electrode  30  (contact layer  20 ) having the overhang portion  31 , and the silicon oxide film  14  below the overhang portion  31  of the contact layer  20  is removed in  FIG. 3B . The Schottky electrode  30  may have a rectangular shape. The Schottky electrode  30  having the overhang portion  31  has a large parasitic capacitance Cf between the Schottky electrode  30  and the semiconductor substrate  10  shown in  FIG. 1A , as compared to the Schottky electrode formed into the rectangular shape. Thus, there is a strong demand to remove the silicon oxide film  14  that contacts the Schottky electrode  30 . Thus, the contact layer  20  of tungsten silidie is liable to be chipped, and the structure of containing oxygen in tungsten silicide is particularly effective. 
         [0040]    The metal layer  28  is formed on the contact layer  20 , as shown in  FIG. 2C . The metal layer  28  is provided to reduce the resistance of the Schottky electrode  30 . When the contact layer  20  and the metal layer  28  are made of different materials, electromotive force is generated at an interface between the contact layer  20  and the metal layer  28  due to a difference in electron affinity. This difference may facilitate the reaction of formula (3) and chipping of the tungsten silicide layer. Thus, it is particularly effective to contain oxygen in tungsten silicide. Particularly, the metal layer  28  made of gold may help to generate an electromotive force at the interface between the contact layer  20  and the metal layer  28 . Thus, the structure of containing oxygen in tungsten silicide is particularly effective. 
         [0041]    As shown in  FIG. 3B , the source electrode  32  and the drain electrode  34 , which are ohmic electrode, are in contact with the semiconductor substrate  10  when the silicon oxide film  14  is removed. In this case, electrons may be supplied from the ohmic electrodes and may help the reaction of formula (3). Thus, the tungsten silicide layer may be more liable to be chipped. Thus, it is more effective to contain oxygen in tungsten silicide. 
       Second Embodiment  
       [0042]    A second embodiment changes the content of oxygen in the contact layer.  FIG. 8A  corresponds to the production step shown in  FIG. 2B  and shows a step of forming the contact layer  20  of tungsten silicide. A first contact layer  22  having a thickness t 1  of 0.03 μm is grown on the silicon oxide film  14  and in the opening  16 . A second contact layer  24  having a thickness of 0.12 μm is grown on the first contact layer  22 . The conditions for growth of the first and second contact layers  22  and  24  are the same as those for samples B and A described with reference to  FIG. 4 . The first and second contact layers  22  and  24  form a contact layer  20   a.  As shown in  FIG. 8B , the oxygen content in the first contact layer  22  is comparatively high, and that in the second contact layer  24  is comparatively low. 
         [0043]    In the second embodiment, the first contact layer  22  made of tungsten silicide containing oxygen is provided in a region that the silicon oxide film  14  contacts (this region contacts an upper surface S 1  of the silicon oxide film  14  and a side surface S 2  of the opening  16 ), and the second contact layer  24  made of tungsten silicide containing oxygen having a lower content than that of the first contact layer  22  is provided on the first contact layer  22 . The tungsten silicide layer grown under the condition for sample B has a comparatively large oxygen content and has great effects of restraining chipping of the tungsten silicide layer. However, the above tungsten silicide layer may contain a large amount of an impurity such as argon, and has a high resistivity. With the above in mind, in  FIG. 3B , the first contact layer  22  having a comparative large content of oxygen is arranged in the region that contacts the silicon oxide film  14  and the semiconductor substrate exposed to an aqueous solution that primarily contains hydrofluoric acid, and the second contact layer  24  having a comparatively small contact of oxygen is arranged in a region further than the above region from the semiconductor substrate  10  and the silicon oxide film  14 . With the above structure, it is possible to restrain increase in the resistance of the contact layer  20   a  and restrain chipping of the tungsten silicide layer. 
         [0044]    In order to restrain the tungsten silicide layer from being chipped, it is preferable that the oxygen content of the first contact layer  22  is equal to or greater than 2.6%. The first contact layer  22  is arranged in the region that the silicon oxide film  14  contacts. In order to more effectively restrain the tungsten silicide layer from being chipped, the first contact layer  22  is arranged in the entire region that contacts the silicon oxide film  14 . The first contact layer  22  may be arranged in a region that contacts the semiconductor substrate  10 . In order to restrain the tungsten silicide layer from being chipped, the first contact layer  22  is preferably 0.03 μm thick or more. 
         [0045]    Preferably, the second contact layer  24  is formed by sputtering with a higher sputtering pressure than that used for forming the first contact layer  22 . Thus, the oxygen content of the first contact layer  22  can be made greater than that of the second contact layer  24 , as shown in  FIG. 4 . 
         [0046]      FIG. 9A  is a cross-sectional view of the semiconductor substrate  10  observed in a process for fabricating an FET in accordance with a variation of the second embodiment. The sputtering pressure is gradually increased in the process of forming the contact layer  20   b.  The oxygen content gradually decreases as the film growing process goes on, as shown in  FIG. 4 . Thus, as shown in  FIG. 9B , the oxygen content as a function of the height of the contact layer  20   b  decreases evenly. 
         [0047]    As in the case of the variation of the second embodiment, the content of oxygen in the contact layer  20   b  is set so as to gradually decrease as the position of interest is further away from the region that contacts the silicon oxide film  14  and the semiconductor substrate  10 . That is, the oxygen content that decreases continuously may be realized by the process for forming a first contact layer  22   b  and a second contact layer  24   b.  The oxygen content of a contact layer composed of the first and second contact layers  22   b  and  24   b  continuously decreases from the lower side to the upper side of the overhang portion  31 . Thus, like the second embodiment, the present variation is capable of restraining the resistance of the contact layer  20   b  from increasing and restraining the tungsten silicide layer from being chipped. In  FIG. 9A , the contact layer  20   b  is illustrated so as to be composed of the two separate contact layers  22   b  and  24   b  for the sake of simplicity. It is to be noted that, in actually, as shown in  FIG. 9B , the oxygen content changes continuously. However, the present variation allows a discontinuity in the oxygen content at the interface between the first contact layer  22   b  and the second contact layer  24   b.    
         [0048]    In the second embodiment and its variation, when the contact layer  20   a  or  20   b  is formed, the sputtering pressure used to form the region that contacts the silicon oxide film  14  and the semiconductor substrate  10  (the above region has a height of zero or close to zero in  FIGS. 8B and 9B ) is made lower than the sputtering pressure used to form the region away from the region that contacts the silicon oxide film  14  and the semiconductor substrate  10  (the above region has a height t 2  in  FIGS. 8B and 9B ). It is thus possible to obtain a larger oxygen content in the region that contacts the silicon oxide film  14  and the semiconductor substrate  10  than that in the region away from the silicon oxide film  14  and the semiconductor substrate  10 . The sputtering power may be raised instead of lowering the sputtering pressure. Oxygen gas may be added to the sputtering gas instead of lowering the sputtering pressure and the concentration of oxygen gas may be increased. 
       Third Embodiment  
       [0049]    A third embodiment has an arrangement in which silicon oxide in the silicon oxide film  14  is taken in the contact layer  20 .  FIG. 10  schematically illustrates a step in which the contact layer of tungsten silicide is being grown. When molecules  54  of WSi hit the silicon oxide film  14 , if the WSi molecules  54  have a large kinetic energy, SiO 2  molecules  56  are sputtered. 
         [0050]      FIG. 11  is a cross-sectional view of the semiconductor substrate  10  observed in a process for fabricating an FET in accordance with the third embodiment. The SiO 2  molecules  56  are sputtered from the silicon oxide film  14 . Thus, a contact layer  20   c  close to the interface with the silicon oxide film  14  includes a first contact layer  22   c  that is a region containing a large mount of oxygen. That is, the surface of the silicon oxide film  14  is sputtered and the resultant oxygen (for example, molecules of silicon oxide) included in the silicon oxide film  14  are taken in the region that contacts the silicon oxide film  14  (this region contacts the upper surface S 1  of the silicon oxide film  14  and the side surface S 2  of the opening  16 ), so that the first contact layer  22   c  containing oxygen can be formed in the above region. In  FIG. 3C , the region of the contact layer  20  exposed to the aqueous solution containing hydrofluoric acid used for removing the silicon oxide film  14  contacts the silicon oxide film  14 . Thus, the third embodiment is capable of restraining the tungsten silicide layer from being chipped. Further, silicon oxide of the silicon oxide film  14  is taken in the contact layer  20 . It is thus possible to stably take oxygen in the contact layer  20 , as compared to the first embodiment in which residual oxygen in the chamber of the sputtering apparatus is used. Furthermore, the present embodiment does not need new sputtering gas such as oxygen gas, and reduces the production cost. 
         [0051]    In  FIG. 10 , the tungsten silicide molecules  54  may have an increased kinetic energy when sputtered from the target with higher energy. However, referring to  FIG. 11 , after the first contact layer  22   c  in which the silicon oxide molecules  56  are taken is formed, the silicon oxide film  14  is already covered with the tungsten silicide film, and a second contact layer  24   c  formed on the first contact layer  22   c  does not have an increased oxygen content even when the sputtering power is raised. There is a further problem. When the sputtering power is high, argon of the sputtering gas is taken in the contact layer  20   c  and increases the resistance of the contact layer  20   c.  Thus, when the contact layer  20   c  is formed, the sputtering power used to form the region that contacts the silicon oxide film  14  is preferably made greater than the sputtering power used to form the region away from the silicon oxide film  14 . The sputtering pressure may be lowered instead of raising the sputtering power. 
         [0052]    When the contact layer  20   c  is formed with a high sputtering power, the semiconductor substrate  10  may be damaged. Thus, it is preferable that annealing is performed after the first contact layer  22   c  is formed in order to restore the damaged surface of the semiconductor substrate  10  in the bottom of the opening  16 . The annealing may be performed at 500° C. for 30 minutes. The semiconductor substrate  10  may be annealed before or after forming the second contact layer  24   c.    
         [0053]    The FETs of the first and second embodiments have oxygen that has reacted with silicon in the side and bottom surfaces of the contact layer  20  (which contact the silicon oxide film  14  and the semiconductor substrate  10 , respectively). In contrast, the FET of the third embodiment has a large amount of oxygen that has reacted with silicon in the side surface of the contact layer  20  (which contacts the silicon oxide film  14 ), but does not have a large amount of oxygen that has reacted with silicon in the bottom surface. The region of the contact layer  20  exposed to the solution containing hydrofluoric acid for removing the silicon oxide film  14  is the side surface of the contact layer  20 . Thus, at least the side surface of the contact layer  20  is required to contain oxygen that has reacted with silicon. 
         [0054]    As described above, in  FIG. 2B , the tungsten silicide layer can be restrained from being chipped by forming the contact layer  20  so that a region that contacts the silicon oxide film  14  and the semiconductor substrate  10  contains oxygen. 
         [0055]    The first through third embodiments are the exemplary FETs using the GaAs substrate. The present invention includes semiconductor devices having Schottky electrodes other than the FETs. The first through third embodiments use the semiconductor layer that has the layer epitaxially grown on the GaAs substrate. The present invention is not limited to the above semiconductor layer but may be a GaAs substrate, an AlGa substrate or an Si substrate. That is, the surface of the semiconductor layer is the surfaces of these substrates. The solution that contains hydrofluoric acid is not limited to the buffered hydrofluoric acid that contains hydrofluoric acid and ammonium fluoride, but may be an aqueous solution of hydrofluoric acid. The insulating film is not limited to the silicon oxide film  14  but may be an insulating film that contains silicon oxide removable by a solution containing hydrofluoric acid. For example, the insulating film may be silicon oxy-nitride (SiON). 
         [0056]    The present invention is not limited to the specifically disclosed embodiments, but may include other embodiments and variations without departing from the scope of the present invention. 
         [0057]    The present application is based on Japanese Patent Application No. 2007-028603 filed on Feb. 7, 2007, the entire disclosure of which is hereby incorporated by reference.