Abstract:
According to one aspect of the invention, there is provided a semiconductor device fabrication method comprising: forming a gate insulating film on a semiconductor substrate; forming a film containing a predetermined semiconductor material and germanium on the gate insulating film; oxidizing the film to form a first film having a germanium concentration higher than that of the film and a film thickness smaller than that of the film on the gate insulating film, and form an oxide film on the first film; removing the oxide film; forming, on the first film, a second film containing the semiconductor material and having a germanium concentration lower than that of the first film; forming a gate electrode by etching the second and first films; and forming a source region and drain region by ion-implanting a predetermined impurity by using the gate electrode as a mask.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims benefit of priority under 35 USC §119 from the Japanese Patent Application No. 2004-325086, filed on Nov. 9, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device and a method of fabricating the same.  
         [0003]     Recently, the use of silicon germanium (SiGe) obtained by adding germanium (Ge) to silicon (Si) as a gate electrode material of a P-channel MOS transistor (to be referred to as a PMOSFET hereinafter) is proposed. This makes it possible to increase the activation ratio (the ratio of activated impurity atoms to all doped impurity atoms) of boron (B) as a P-type dopant, and thereby decrease the thickness of a depletion layer formed near the interface of the gate electrode and suppress depletion of the gate electrode.  
         [0004]     In the surface portion of the gate electrode, a silicide for reducing the parasitic resistance is formed. Since silicon germanium (SiGe) does not well match a silicide made of cobalt (Co) or nickel (Ni), the resistance of the silicide increases. Therefore, a method is proposed by which a silicon (Si) film as a silicide reaction layer to be reacted with a silicide is formed on a silicon germanium (SiGe) film, thereby forming a stacked structure of the silicon germanium (SiGe) film and silicon (Si) film as a gate electrode.  
         [0005]     If, however, predetermined annealing is performed after the silicon germanium (SiGe) film and silicon (Si) film are formed, germanium (Ge) in the silicon germanium (SiGe) film diffuses in the silicon (Si) film as an upper layer. If the germanium (Ge) concentration in the silicon (Si) film as an upper layer exceeds about 5 at %, the resistance of a silicide to be formed later undesirably increases. Note that at % represents an atomic composition ratio.  
         [0006]     To prevent germanium (Ge) in the silicon germanium (SiGe) film from diffusing in the silicon (Si) film as an upper layer, therefore, the germanium (Ge) concentration in the silicon germanium (SiGe) film as a lower layer is desirably as low as possible, and the film thickness is desirably as small as possible.  
         [0007]     Unfortunately, in a PMOSFET, to suppress depletion of the gate electrode by increasing the activation ratio of ion-implanted boron, it is necessary to increase the germanium (Ge) concentration in the silicon germanium (SiGe) film. Accordingly, the formation of a silicon germanium (SiGe) film having a low germanium (Ge) concentration causes depletion of the gate electrode, and makes it difficult to improve the drivability of the transistor.  
         [0008]     If, therefore, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness is formed near the interface with a gate insulating film, it is possible to prevent an increase in resistance of the silicide because the amount of germanium (Ge) which diffuses in the silicon (Si) film as an upper layer reduces. In addition, depletion of the gate electrode can be suppressed since the activation ratio of boron increases.  
         [0009]     Unfortunately, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness readily causes migration because the melting point of germanium (Ge) is as low as about 945° C. As a consequence, a large number of projections and recesses are formed on the film surface, and this worsens the morphology (the surface state).  
         [0010]     By contrast, when silicon germanium (SiGe) is used as the gate electrode material in an N-channel MOS transistor (to be referred to as an NMOSFET hereinafter), the activation ratio of phosphorus (P) or arsenic (As) as an N-type dopant decreases.  
         [0011]     Accordingly, to form a complementary MOS transistor (to be referred to as a CMOSFET hereinafter) made up of a PMOSFET and NMOSFET, it is desirable to form a stacked structure of a silicon germanium (SiGe) film and silicon (Si) film as a gate electrode of the PMOSFET, and form only a silicon (Si) film as a gate electrode of the NMOSFET.  
         [0012]     If, however, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness is formed, the morphology worsens, so a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a large film thickness must be formed below the gate electrode of the PMOSFET. In this case, the heights of the gate electrodes of the PMOSFET and NMOSFET are largely different, and this makes the formation of these gate electrodes impossible.  
         [0013]     A reference concerning a MOSFET which uses silicon germanium (SiGe) as the gate electrode material is as follows.  
         [0014]     Reference 1: Japanese Patent Laid-Open No. 2002-343881  
       SUMMARY OF THE INVENTION  
       [0015]     According to one aspect of the present invention, there is provided a semiconductor device fabrication method comprising:  
         [0016]     forming a gate insulating film on a semiconductor substrate;  
         [0017]     forming a film containing a predetermined semiconductor material and germanium on the gate insulating film;  
         [0018]     oxidizing the film to form a first film having a germanium concentration higher than that of the film and a film thickness smaller than that of the film on the gate insulating film, and form an oxide film on the first film;  
         [0019]     removing the oxide film;  
         [0020]     forming, on the first film, a second film containing the semiconductor material and having a germanium concentration lower than that of the first film;  
         [0021]     forming a gate electrode by etching the second and first films; and  
         [0022]     forming a source region and drain region by ion-implanting a predetermined impurity by using the gate electrode as a mask.  
         [0023]     According to one aspect of the present invention, there is provided a semiconductor device comprising:  
         [0024]     a gate insulating film selectively formed on a predetermined region of a semiconductor substrate;  
         [0025]     a gate electrode comprising a first film formed on said gate insulating film and containing a predetermined semiconductor material and germanium, and a second film formed on said first film, containing the semiconductor material, and having a germanium concentration lower than that of said first film; and  
         [0026]     a source region and drain region formed in a surface portion of said semiconductor substrate on two sides of a channel region positioned below said gate electrode.  
         [0027]     According to one aspect of the present invention, there is provided a semiconductor device fabrication method comprising:  
         [0028]     forming a first-conductivity-type semiconductor region and second-conductivity-type semiconductor region in a surface portion of a semiconductor substrate;  
         [0029]     forming a gate insulating film on the semiconductor substrate;  
         [0030]     forming a film containing a predetermined semiconductor material and germanium on the gate insulating film;  
         [0031]     oxidizing the film to form a first film having a germanium concentration higher than that of the film and a film thickness smaller than that of the film on the gate insulating film, and form an oxide film on the first film;  
         [0032]     removing the oxide film;  
         [0033]     forming a mask pattern having a pattern corresponding to the second-conductivity-type semiconductor region, and etching the first film by using the mask pattern, thereby removing the first film formed on the first-conductivity-type semiconductor region via the gate insulating film;  
         [0034]     removing the mask pattern, and forming a second film containing the semiconductor material and having a germanium concentration lower than that of the first film, on the gate insulating film formed on the first-conductivity-type semiconductor region, and on the first film formed on the second-conductivity-type semiconductor region;  
         [0035]     forming a first gate electrode by etching the second and first films formed on the second-conductivity-type semiconductor region, and forming a second gate electrode by etching the second film formed on the first-conductivity-type semiconductor region; and  
         [0036]     forming a first source region and first drain region by ion-implanting a first-conductivity-type impurity in the second-conductivity-type semiconductor region by using the first gate electrode as a mask, and forming a second source region and second drain region by ion-implanting a second-conductivity-type impurity in the first-conductivity-type semiconductor region by using the second gate electrode as a mask. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]      FIG. 1  is a longitudinal sectional view showing the sectional structure of a device in a process of a PMOSFET fabrication method according to the first embodiment of the present invention;  
         [0038]      FIG. 2  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0039]      FIG. 3  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0040]      FIG. 4  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0041]      FIG. 5  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0042]      FIG. 6  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0043]      FIG. 7  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0044]      FIG. 8  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0045]      FIG. 9  is a longitudinal sectional view showing the sectional structure of a device in a process of the PMOSFET fabrication method;  
         [0046]      FIG. 10  is a longitudinal sectional view showing the sectional structure of a device in a process of a CMOSFET fabrication method according to the second embodiment of the present invention;  
         [0047]      FIG. 11  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0048]      FIG. 12  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0049]      FIG. 13  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0050]      FIG. 14  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0051]      FIG. 15  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0052]      FIG. 16  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0053]      FIG. 17  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0054]      FIG. 18  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method;  
         [0055]      FIG. 19  is a longitudinal sectional view showing the sectional structure of a device in a process of the CMOSFET fabrication method; and  
         [0056]      FIG. 20  is a graph showing the relationship between a partial pressure ratio P H2O /P H2  of steam to hydrogen in a gas system containing both steam and hydrogen, and the temperature of the gas system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0057]     Embodiments of the present invention will be described below with reference to the accompanying drawings.  
       (1) FIRST EMBODIMENT  
       [0058]     FIGS.  1  to  9  show a PMOSFET fabrication method according to the first embodiment of the present invention. First, as shown in  FIG. 1 , a desired resist pattern is formed on a p-type semiconductor substrate  10  by photolithography, and used as a mask to ion-implant phosphorus (P), arsenic (As), or antimony (Sb). After that, annealing is performed to form an N-type semiconductor region  110  about 1 μm deep.  
         [0059]     As shown in  FIG. 2 , element isolation oxide films  120 A and  120 B about 400 nm thick are formed in desired regions on the semiconductor substrate  100 . As shown in  FIG. 3 , a protective oxide film  130  about 8 nm thick is formed, and ion implantation for adjusting the gate threshold voltage of a PMOSFET is performed. After that, the protective oxide film  130  is removed.  
         [0060]     As shown in  FIG. 4 , a gate insulating film  140  made of, e.g., a silicon oxide (SiO 2 ) film about a few nm thick is formed on the surface of the semiconductor substrate  100 . Note that the gate insulating film  140  need not be a silicon oxide (SiO 2 ) film, and may also be, e.g., an oxynitride film containing 0 to about a few % of nitrogen, a high-dielectric material such as a tantalum oxide (TaO 2 ) film, a zirconium oxide (ZrO x ) film, or hafnium oxide (HfO x ) film (where x is a positive integer), or a silicate film of any of these materials.  
         [0061]     As shown in  FIG. 5 , polysilicon germanium (SiGe) about 60 nm thick containing about 10 at % of germanium (Ge) is deposited on the gate insulating film  140  by using CVD (Chemical Vapor Deposition) or the like, thereby forming a polysilicon germanium (SiGe) film  150 .  
         [0062]     The morphology can be improved by thus forming the polysilicon germanium (SiGe) film  150  having a low germanium (Ge) concentration of about 10 at % and a large film thickness of about 60 nm.  
         [0063]     As shown in  FIG. 6 , selective oxidation which produces silicon oxide (SiO 2 ) and produces no germanium oxide (GeOs) is performed by preferentially oxidizing silicon (Si) in the polysilicon germanium (SiGe) film  150 .  
         [0064]     When this silicon oxide (SiO 2 ) is produced, germanium (Ge) is pushed down and deposited on the gate insulating film by a so-called plowing effect. Consequently, a polysilicon germanium (SiGe) film  160  having a high germanium (Ge) concentration of 30 at % and a small film thickness of about 10 nm and a silicon oxide (SiO 2 ) film  170  about 80 nm thick are formed without deteriorating the morphology.  
         [0065]     Note that to preferentially oxidize silicon (Si) in the polysilicon germanium (SiGe) film  150 , selective oxidation is preferably performed by using a gas system containing both steam (H 2 O) as an oxidizer and hydrogen (H 2 ) as a reducer, and setting a partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) and the temperature of the gas system within a desired range.  
         [0066]      FIG. 20  shows the relationship between the partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) in the gas system containing both steam (H 2 O) and hydrogen (H 2 ), and the temperature of the gas system. Referring to  FIG. 20 , a range R 10  positioned below a curve L 10  indicates a range over which neither silicon (Si) nor germanium (Ge) oxidizes, a range R 20  positioned below a curve L 20  and above the curve L 10  indicates a range over which silicon (Si) oxidizes and germanium (Ge) does not oxidize, and a range R 30  positioned above the curve L 20  indicates a range over which both silicon (Si) and germanium (Ge) oxidize.  
         [0067]     Accordingly, by setting the partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) and the temperature of the gas system within the range R 20 , silicon (Si) oxidizes, and germanium (Ge) does not oxidize, or, even if germanium (Ge) oxidizes, the oxide reduces and returns to germanium (Ge).  
         [0068]     Note that the gas system containing both an oxidizer and reducer need only be a system containing at least one of steam (H 2 O), carbon dioxide (CO 2 ), and oxygen (O 2 ) as an oxidizer, and at least one of hydrogen (H 2 ) and carbon monoxide (CO) as a reducer.  
         [0069]     Note also that normal thermal oxidation may also be performed by setting the partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) and the temperature of the gas system within the range R 30 . In this case, the production amount of germanium oxide (GeO 2 ) slightly increases. Since, however, the oxidation rate of silicon (Si) is higher than that of germanium (Ge), it is possible to form, on the gate insulating film, a polysilicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness, as in the case of selective oxidation.  
         [0070]     Referring back to  FIG. 6 , the silicon oxide (SiO 2 ) film  170  is removed by, e.g., a dilute hydrofluoric acid solution. After that, as shown in  FIG. 7 , a polysilicon (Si) film  180  about 80 nm thick is formed by CVD or the like, thereby forming a stacked structure of the polysilicon germanium (SiGe) film  160  and polysilicon (Si) film  180 .  
         [0071]     Note that even when germanium (Ge) in the polysilicon germanium (SiGe) film  160  diffuses in its overlying film by annealing, if the germanium (Ge) concentration in this overlying film does not exceed about 5 at %, a polysilicon germanium (SiGe) film having a low germanium (Ge) concentration may also be formed on the polysilicon germanium (SiGe) film  160 .  
         [0072]     As shown in  FIG. 8 , a photoresist step, RIE (Reactive Ion Etching) step, and the like are executed to form a gate electrode  190  made up of a polysilicon germanium (SiGe) film  190 A and polysilicon (Si) film  190 B and a gate insulating film  195 .  
         [0073]     As shown in  FIG. 9 , a P-type dopant such as boron (B), boron fluoride (BF 2 ), or indium (In) is ion-implanted, and the ion-implanted boron (B) is activated by predetermined annealing, thereby forming a lightly doped source extension region  198 A and drain extension region  198 B having shallow junctions.  
         [0074]     After gate electrode side walls  200 A and  200 B are formed on the side surfaces of the gate electrode  190 , a P-type dopant such as boron (B), boron fluoride (BF 2 ), or indium (In) is ion-implanted again. Subsequently, annealing which diffuses boron (B) is performed to activate the boron (B) which is ion-implanted into the gate electrode  190 , and form a source region  210 A and drain region  210 B.  
         [0075]     Then, a metal film made of, e.g., nickel (Ni) or platinum (pt) is formed by sputtering. After that, annealing is performed to form silicides  220 A to  220 C for reducing the parasitic resistance on the surface of the gate electrode  190  and in the surface portions of the source region  210 A and drain region  210 B.  
         [0076]     Subsequently, an interlayer dielectric film (not shown) is formed, and an interconnection step is performed by forming a contact plug (not shown) in this interlayer dielectric film, thereby forming a PMOSFET  300 .  
         [0077]     As shown in  FIG. 9 , in the PMOSFET  300  fabricated by the above method, the element isolation oxide films  120 A and  120 B for element isolation are formed in the surface portion of the semiconductor substrate  100 . Near the central portion of the element region isolated by the element isolation oxide films  120 A and  120 B, the gate electrode  190  made up of the polysilicon germanium (SiGe) film  190 A and polysilicon (Si) film  190 B is formed via the gate insulating film  195  formed on the surface of the semiconductor substrate  100 .  
         [0078]     On the side surfaces of the gate electrode  190 , the gate electrode side walls  200 A and  200 B as insulating films are formed. Also, a channel region  230  in which an electric current flows is formed near the surface of the semiconductor substrate  100  below the gate electrode  190 .  
         [0079]     The source region  210 A is formed between the channel region  230  and element isolation oxide film  120 A, and the drain region  210 B is formed between the channel region  230  and element isolation oxide film  120 B.  
         [0080]     Furthermore, the suicides  220 A to  220 C for reducing the parasitic resistance are formed on the surface of the gate electrode  190  and on the surfaces of the source region  210 A and drain region  210 B.  
         [0081]     In this embodiment as described above, the polysilicon germanium (SiGe) film  150  having a low germanium (Ge) concentration and a large film thickness is formed such that the morphology is uniform, and then silicon (Si) in the polysilicon germanium (SiGe) film  150  is preferentially oxidized. Consequently, the polysilicon germanium (SiGe) film  160  having a high germanium (Ge) concentration and a small film thickness can be formed with a good morphology on the gate insulating film  140 .  
         [0082]     This makes it possible to reduce the amount of germanium (Ge) in the polysilicon germanium (SiGe) film  190 A, which diffuses in the polysilicon (Si) film  190 B, thereby preventing an increase in resistance of the silicide  220 A. In addition, depletion of the gate electrode  190  can be suppressed by increasing the activation ratio of boron (B).  
       (2) SECOND EMBODIMENT  
       [0083]     FIGS.  10  to  19  show a CMOSFET fabrication method according to the second embodiment of the present invention. First, a desired resist pattern is formed on a semiconductor substrate  400  by photolithography, and used as a mask to ion-implant boron (B), gallium (Ga), or indium (In).  
         [0084]     Similarly, a desired resist pattern is formed on the semiconductor substrate  400  by photolithography, and used as a mask to ion-implant phosphorus (P), arsenic (As), or antimony (Sb). After that, as shown in  FIG. 10 , annealing is performed to form a P-type semiconductor region  410  and N-type semiconductor region  420  about 1 μm deep.  
         [0085]     As shown in  FIG. 11 , an element isolation oxide film  430  about 400 nm thick are formed in a desired region on the semiconductor substrate  400 . As shown in  FIG. 12 , a protective oxide film  440  about 10 nm thick is formed, and ion implantation for adjusting the gate threshold voltage is performed. After that, the protective oxide film  440  is removed.  
         [0086]     As shown in  FIG. 13 , a gate insulating film  450  made of, e.g., a silicon oxide (SiO 2 ) film about a few nm thick is formed on the surface of the semiconductor substrate  400 . Note that the gate insulating film  450  need not be a silicon oxide (SiO 2 ) film, and may also be, e.g., an oxynitride film containing 0 to about a few % of nitrogen, a high-k material such as a tantalum oxide (TaO 2 ) film, zirconium oxide (ZrO x ) film, or hafnium oxide (HfO x ) film (where x is a positive integer), or a silicate film of any of these materials.  
         [0087]     As shown in  FIG. 14 , CVD (Chemical Vapor Deposition) or the like is used to deposit, on the gate insulating film  450 , seed silicon (Si) (not shown) about a few nm thick and polysilicon germanium (SiGe) about 30 nm thick containing about 10 at % of germanium (Ge), thereby forming a polysilicon germanium (SiGe) film  460 .  
         [0088]     The morphology can be improved by thus forming the polysilicon germanium (SiGe) film  460  having a low germanium (Ge) concentration of about 10 at % and a large film thickness of about 30 nm.  
         [0089]     As shown in  FIG. 15 , as in the first embodiment, selective oxidation which produces silicon oxide (SiO 2 ) and produces no germanium oxide (GeOs) is performed by preferentially oxidizing silicon (Si) in the polysilicon germanium (SiGe) film  460 .  
         [0090]     When this silicon oxide (SiO 2 ) is produced, germanium (Ge) is pushed down and deposited on the gate insulating film  450  by a so-called plowing effect. Consequently, a polysilicon germanium (SiGe) film  470  having a high germanium (Ge) concentration of 30 at % and a small film thickness of about 10 nm and a silicon oxide (SiO 2 ) film  480  about 40 nm thick are formed without deteriorating the morphology.  
         [0091]     Note that to preferentially oxidize silicon (Si) in the polysilicon germanium (SiGe) film  460 , as in the first embodiment, selective oxidation is preferably performed by using a gas system containing both steam (H 2 O) as an oxidizer and hydrogen (H 2 ) as a reducer, and setting a partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) and the temperature of the gas system within the range R 20  shown in  FIG. 20 . Note that normal thermal oxidation may also be performed by setting the partial pressure ratio P H2O /P H2  of steam (H 2 O) to hydrogen (H 2 ) and the temperature of the gas system within the range R 30 .  
         [0092]     Referring back to  FIG. 15 , the silicon oxide (SiO 2 ) film  480  is removed by, e.g., a dilute hydrofluoric acid solution. After that, as shown in  FIG. 16 , a photoresist  490  having a pattern corresponding to the N-type semiconductor region  420  is formed and used as a mask to etch the polysilicon germanium (SiGe) film  470  by using a predetermined alkali-based solution, thereby removing the polysilicon germanium (SiGe) film  470  positioned on the P-type semiconductor region  410 .  
         [0093]     As shown in  FIG. 17 , the photoresist  490  is removed, and a polysilicon (Si) film  500  about 100 nm thick is formed by CVD or the like. In this manner, on the N-type semiconductor region  420  where a PMOSFET is to be formed, a stacked structure of the polysilicon germanium (SiGe) film  470  having a germanium (Ge) concentration of 30 at % and a film thickness of about 10 nm and the polysilicon (Si) film  500  about 100 nm thick is formed. On the other hand, only the polysilicon (Si) film  500  about 100 nm thick is formed on the P-type semiconductor region  410  where an NMOSFET is to be formed.  
         [0094]     Note that even when germanium (Ge) in the polysilicon germanium (SiGe) film  470  diffuses in its overlying film by annealing, if the germanium (Ge) concentration in this overlying film does not exceed about 5 at %, a polysilicon germanium (SiGe) film having a low germanium (Ge) concentration may also be formed instead of the polysilicon (Si) film  500 .  
         [0095]     As shown in  FIG. 18 , a photoresist step, RIE (Reactive Ion Etching) step, and the like are executed to form a gate electrode  510  made up of a polysilicon germanium (SiGe) film  510 A and polysilicon (Si) film  510 B and a gate insulating film  515  on the N-type semiconductor region  420 , and a gate electrode  520  made of a polysilicon germanium (SiGe) film and a gate insulating film  525  on the P-type semiconductor region  410 .  
         [0096]     As shown in  FIG. 19 , on the N-type semiconductor region  420 , a P-type dopant such as boron (B) is ion-implanted, and annealing is performed to diffuse this boron (B), thereby forming a lightly doped source extension region  527 A and drain extension region  527 B having shallow junctions.  
         [0097]     In addition, on the P-type semiconductor region  410 , an N-type dopant such as phosphorus (P) is ion-implanted, and annealing is performed to diffuse this phosphorus (P), thereby forming a lightly doped source extension region  528 A and drain extension region  528 B having shallow junctions.  
         [0098]     Gate electrode side walls  530 A and  530 B are formed on the side surfaces of the gate electrode  510 , and gate electrode side walls  540 A and  540 B are formed on the side surfaces of the gate electrode  520 .  
         [0099]     Subsequently, on the N-type semiconductor region  420 , a P-type dopant such as boron (B) is ion-implanted, and annealing is performed to diffuse this boron (B), thereby activating the boron (B) ion-implanted into the gate electrode  510 , and forming a source region  550 A and drain region  550 B.  
         [0100]     In addition, on the P-type semiconductor region  410 , an N-type dopant such as phosphorus (P) is ion-implanted, and annealing is performed to diffuse this phosphorus (P), thereby activating the phosphorus (P) ion-implanted into the gate electrode  520 , and forming a source region  560 A and drain region  560 B.  
         [0101]     Then, a metal film made of, e.g., nickel (Ni) or platinum (pt) is formed by sputtering. After that, annealing is performed to form silicides  570 A to  570 C for reducing the parasitic resistance on the surface of the gate electrode  510  and in the surface portions of the source region  550 A and drain region  550 B, and form silicides  580 A to  580 C on the surface of the gate electrode  520  and in the surface portions of the source region  560 A and drain region  560 B.  
         [0102]     Subsequently, an interlayer dielectric film (not shown) is formed, and an interconnection step is performed by forming a contact plug (not shown) in this interlayer dielectric film, thereby forming a CMOSFET  700  including a PMOSFET  700 A and NMOSFET  700 B.  
         [0103]     As shown in  FIG. 19 , in the CMOSFET  700  fabricated by the above method, the element isolation oxide film  430  for element isolation is formed in the surface portion of the semiconductor substrate  400 . Near the central portion of the N-type semiconductor region  420  isolated by the element isolation oxide film  430 , the gate electrode  510  made up of the polysilicon germanium (SiGe) film  510 A and polysilicon (Si) film  510 B is formed via the gate insulating film  515  formed on the surface of the semiconductor substrate  400 .  
         [0104]     On the side surfaces of the gate electrode  510 , the gate electrode side walls  530 A and  530 B as insulating films are formed. Also, a channel region  590  in which an electric current flows is formed near the surface of the semiconductor substrate  400  below the gate electrode  510 .  
         [0105]     The source region  550 A is formed between the channel region  590  and element isolation oxide film  430 , and the drain region  550 B is formed between the channel region  590  and an element isolation oxide film (not shown).  
         [0106]     In addition, the suicides  570 A to  570 C for reducing the parasitic resistance are formed on the surface of the gate electrode  510  and on the surfaces of the source region  550 A and drain region  550 B.  
         [0107]     On the other hand, near the central portion of the P-type semiconductor region  410 , the gate electrode  520  made of the polysilicon (Si) film is formed via the gate insulating film  525  formed on the surface of the semiconductor substrate  400 .  
         [0108]     On the side surfaces of the gate electrode  520 , the gate electrode side walls  540 A and  540 B as insulating films are formed. Also, a channel region  600  in which an electric current flows is formed near the surface of the semiconductor substrate  400  below the gate electrode  520 .  
         [0109]     The source region  560 A is formed between the channel region  600  and an element isolation oxide film (not shown), and the drain region  560 B is formed between the channel region  600  and the element isolation oxide film  430 . In addition, the silicides  580 A to  580 C are formed on the surface of the gate electrode  520  and on the surfaces of the source region  560 A and drain region  560 B.  
         [0110]     In this embodiment as described above, a silicon germanium (SiGe) film having a high germanium (Ge) concentration and a small film thickness can be formed with a good morphology on the gate insulating film  450 .  
         [0111]     When the CMOSFET  700  is formed, therefore, it is possible to form the gate electrode  510  made up of the polysilicon germanium (SiGe) film  510 A and polysilicon (Si) film  510 B in the PMOSFET  700 A, and the gate electrode  520  made of the polysilicon (Si) film in the NMOSFET  700 B, without increasing the difference between the heights of the gate electrode  510  in the PMOSFET  700 A and the gate electrode  520  in the NMOSFET  700 B. As a consequence, the inability to fabricate these gate electrodes can be avoided.  
       (3) OTHER EMBODIMENTS  
       [0112]     Note that the above embodiments are merely examples and do not limit the present invention. For example, silicon carbon (Si:C) as a solid solution may also be used instead of silicon (Si) used in the polysilicon germanium (SiGe) films  150  and  460  and polysilicon (Si) films  180  and  500 . In this case, it is possible to first form a polycrystalline silicon germanium carbon (SiGe:C) film having a low germanium (Ge) concentration and a large film thickness, and then form a polycrystalline silicon germanium carbon (SiGe:C) film having a high germanium (Ge) concentration and a small film thickness on a gate insulating film by preferentially oxidizing silicon carbon (Si:C) of the former polycrystalline carbon germanium (SiGe:C) film.  
         [0113]     The above embodiments can suppress depletion of the gate electrode, and prevent an increase in resistance of a silicide.