Abstract:
A manufacturing method for a CMOS semiconductor device in which gate electrodes are adjusted to have different work function values comprises forming an device region of a first and second conductivity type for forming first and second MOS semiconductor element devices, respectively, in a semiconductor substrate, forming a gate insulator, forming a laminated film comprising a molybdenum film and nitrogen containing film for doping nitrogen into molybdenum, doping nitrogen from the nitrogen containing film into molybdenum, processing the laminated film into gate electrodes of the first and second MOS semiconductor element devices, removing the nitrogen containing film from the gate electrodes of the second MOS semiconductor element device and covering the gate electrode of the first MOS semiconductor element devices with a nitrogen diffusion preventing film, and reducing the nitrogen concentration in molybdenum of the gate electrodes of the second MOS semiconductor element device. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/926,711, filed Aug. 25, 2004, which is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-311387, filed Sep. 3, 2003, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a MOS semiconductor device and a manufacturing method thereof, more particularly, to a MOS semiconductor device in which work functions of gate electrodes are adjusted to different values for MOS transistors having different conductivity types in a dual-gate structure using a metal as a gate electrode and a manufacturing method thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     In a miniaturized CMOS semiconductor device, it is general to use a so-called dual-gate structure. In the dual-gate structure, a polysilicon highly doped with an n-type impurity (dopant) (hereinafter referred to as n +  polysilicon), for example, phosphorus, is used to a gate electrode in an n-type MOS semiconductor device. Further, a polysilicon highly doped with a p-type dopant (hereinafter referred to as p +  polysilicon), for example, boron, is used to a gate electrode in a p-type MOS semiconductor device. This is based on characteristics that the work function of polysilicon can be controlled by type and amount of dopant. In a designing of a surface-channel-type MOS semiconductor device, the dual-gate structure using polysilicon is widely used to attain an acceptable trade-off relationship with respect to major design factors such as the thickness of gate insulators and the substrate doping concentration.  
         [0006]     However, the influence of the depletion layer formed in the polysilicon gate electrode has caused a more serious problem as devices have become further miniaturized in recent years. As described above, since the polysilicon electrode is doped with dopant with a high impurity concentration, the depletion layer in the polysilicon is extremely thin. However, in a next-generation device, in which the gate insulator thickness is as thin as 2 nm or less in equivalent thickness of a silicon oxide film (SiO 2 ), the depletion layer causes an increase in the capacitance corresponding to several 10% or more of the effective gate insulator thickness. Further, enhancement of the performance by a reduction in the gate insulator thickness may be hindered.  
         [0007]     In order to avoid the above problems, studies have been conducted to use metals as the gate electrodes.  
         [0008]     The metal used to a gate electrode is required to match the MOS semiconductor device manufacturing process. For example, it is required the following characteristics to be attained: a metal film can be easily deposited; an etching process can be easily performed to form gate electrodes; an etching selectivity to the gate insulator is high; and the gate electrode is thermally stable against the gate insulator in heat treatments after forming the gate electrode. Since the work functions of the metals are different depending on respective materials and have distributed widely, it is possible to form gate electrodes having work functions equivalent to those of n +  and p +  polysilicon by selecting two kinds of adequate metals. For example, the use of titanium (Ti) to a gate electrode of an n-type MOS semiconductor device and of molybdenum (Mo) to a gate electrode of a p-type MOS semiconductor device has been reported by Q. Lu et al. in “2000 Symposium on VLSI Technology Digest of Technical Papers,” pp. 72-73. However, when this technique is applied to a CMOS semiconductor device production in practice, the manufacturing process becomes too complicated and is not suitable for mass production.  
         [0009]     It has been observed that Mo is thermally stable in the heat treatment usually employed in the latest manufacturing process of a MOS semiconductor device even when Mo is formed on an insulator, such as SiO 2  film, silicon nitride film (SiN) and hafnium oxide film (HfO 2 ), which is used or may be used as a gate insulator. Therefore, Mo is expected to be used as a gate electrode material of future CMOS semiconductor devices. Further, a technique for implanting nitrogen ions into Mo to adjust the work function is reported by P. Ranade et al. in  Mat. Res. Soc. Symp.,  Vol. 611, 2000, pp. C3.2.1-C3.2.6. However, it has been pointed out problems by the inventors and others based on their studies. That is, this method is not practical because of problems that the gate insulator and an interface between the gate electrode and the gate insulator are damaged in the implantation.  
         [0010]     As another method for adjusting the work function of the Mo gate electrode, a method for thermally diffusing nitrogen from a titanium nitride (TiN) film deposited on Mo to underlying Mo has been reported by R. J. P. Lander et al. in  Mat. Res. Soc. Symp. Proc . Vol. 716, 2002, pp. B5.11.1-B5.11.6. However, no study of a method for applying the technique to a CMOS semiconductor device with gate electrodes adjusting to have different work function values on the same semiconductor substrate has been reported.  
         [0011]     Therefore, in order to realize a miniaturized CMOS semiconductor device, such as a device having a feature size of the 65 nm or less and having gate electrodes whose work functions are adjusted to different respective values, the following criteria must be met: (a) the gate electrodes of MOS semiconductor devices of different electrical conductivity types must be adjusted to have different work function values, respectively, (b) no depletion layer can be formed in the gate electrode in the operation of the MOS semiconductor device, (c) the manufacturing process must be simple and practicable, and (d) side effects must not occur in a MOS semiconductor device and/or a semiconductor manufacturing process, for example, damage to the gate insulator must be negligible.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     The above criteria are satisfactorily met in the semiconductor device and manufacturing method thereof disclosed herein.  
         [0013]     According to an aspect of the invention, a MOS semiconductor device manufacturing method comprises forming an device region of a first conductivity type for forming first MOS semiconductor element devices and a device region of a second conductivity type for forming second MOS semiconductor element devices in a semiconductor substrate, forming a gate insulator, forming a laminated film comprising a molybdenum film and a nitrogen containing film which is to be used to dope nitrogen into the moly00bdenum film, doping nitrogen from the nitrogen containing film into the molybdenum film, processing the laminated film into gate electrodes of the first and second MOS semiconductor element devices, removing the nitrogen containing film from the gate electrodes of the second MOS semiconductor element device and covering the gate electrode of the first MOS semiconductor element devices with a nitrogen diffusion preventing film, and reducing the nitrogen concentration in the molybdenum film of the gate electrodes of the second MOS semiconductor element device.  
         [0014]     In another aspect of the invention, a MOS semiconductor device manufacturing method comprises forming an device region of a first conductivity type for forming first MOS semiconductor element devices and an device region of a second conductivity type for forming second MOS semiconductor element devices in a semiconductor substrate, sequentially forming a gate insulator and a molybdenum film, processing the molybdenum film into gate electrodes of the first and second MOS semiconductor element devices, forming a nitrogen containing film which is to be used to dope nitrogen into the molybdenum film of the gate electrodes, doping nitrogen into the molybdenum film from the nitrogen containing film, removing the nitrogen containing film from the gate electrodes of the second MOS semiconductor element devices and covering the gate electrodes of the first MOS semiconductor element devices with a nitrogen diffusion preventing film, and reducing the nitrogen concentration in the molybdenum film of the gate electrodes of the second MOS semiconductor element devices.  
         [0015]     In still another aspect of the invention, a MOS semiconductor device manufacturing method comprises forming an device region of a first conductivity type for forming first MOS semiconductor element devices and a device region of a second conductivity type for forming second MOS semiconductor element devices on a semiconductor substrate, sequentially forming a gate insulator and a molybdenum film, processing the molybdenum film into gate electrodes of the first and second MOS semiconductor element devices, forming a nitrogen containing film which is to be used to dope nitrogen into the molybdenum film of the gate electrodes, removing the nitrogen containing film on the gate electrodes of the second MOS semiconductor element devices, and doping nitrogen from the nitrogen containing film into the molybdenum film of the gate electrodes of the first MOS semiconductor element devices.  
         [0016]     In a further another aspect of the invention, a MOS semiconductor device comprises first and second MOS semiconductor element devices, wherein each of the first and second MOS semiconductor element devices having a gate electrode which include a molybdenum film, the molybdenum films in the gate electrode of each of the first and second MOS semiconductor element devices contains different nitrogen concentrations, and work function of the gate electrode of each of the first and second MOS semiconductor element devices is adjusted to different values.  
         [0017]     Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0018]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0019]      FIG. 1  is a cross sectional view for explaining an example of a MOS semiconductor device manufacturing process according to a first embodiment of the present invention;  
         [0020]      FIG. 2  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the first embodiment following  FIG. 1 ;  
         [0021]      FIG. 3  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the first embodiment following  FIG. 2 ;  
         [0022]      FIG. 4  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the first embodiment following  FIG. 3 ;  
         [0023]      FIG. 5  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the first embodiment following  FIG. 4 ;  
         [0024]      FIG. 6  is a cross sectional view for explaining an example of a MOS semiconductor device manufacturing process according to a second embodiment of the present invention;  
         [0025]      FIG. 7  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the second embodiment following  FIG. 6 ;  
         [0026]      FIG. 8  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the second embodiment following  FIG. 7 ;  
         [0027]      FIG. 9  is a cross sectional view for explaining an example of a MOS semiconductor device manufacturing process according to a third embodiment of the present invention;  
         [0028]      FIG. 10  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the third embodiment following  FIG. 9 ;  
         [0029]      FIG. 11  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the third embodiment following  FIG. 10 ;  
         [0030]      FIG. 12  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the third embodiment following  FIG. 11 ;  
         [0031]      FIG. 13  is a cross sectional view for explaining an example of a MOS semiconductor device manufacturing process according to a fourth embodiment of the present invention;  
         [0032]      FIG. 14  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the fourth embodiment following  FIG. 13 ;  
         [0033]      FIG. 15  is a cross sectional view for explaining an example of the MOS semiconductor device manufacturing process according to the fourth embodiment following  FIG. 14 ; and  
         [0034]      FIG. 16  shows a capacitance-voltage (C-V) characteristic showing the effect of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     In order to realize a miniaturized CMOS semiconductor device, such as a device having a feature size of the 65 nm or less and having gate electrodes whose work functions are adjusted to different respective values, the following criteria must be met:  
         [0036]     (a) the gate electrodes of MOS semiconductor devices of different electrical conductivity types must be adjusted to have different work function values, respectively, (b) no depletion layer may be formed in the gate electrode in the operation of the MOS semiconductor device, (c) the manufacturing process must be simple and practicable, and (d) side effects must not occur in a MOS semiconductor device and/or a semiconductor manufacturing process, for example, damage to the gate insulator must be negligible.  
         [0037]     The invention provides a MOS semiconductor device comprising gate electrodes which are suitable for a miniaturized MOS semiconductor device, such as a MOS semiconductor device having a feature size of the 65 nm or less and a manufacturing method thereof. The features of the gate electrodes are that the gate electrodes of the MOS semiconductor devices with electrically different conductivity types have work functions of different values, respectively, no depletion layer may be formed within the gate electrode in the operation of the MOS semiconductor device, the manufacturing process is simple and practicable, and no side effect occur in a MOS semiconductor device and a semiconductor manufacturing process, for example, damage to a gate insulator is negligible.  
         [0038]     In the invention, the work functions of the gate electrodes are adjusted by controlling nitrogen concentrations in the molybdenum gate electrodes by use of a magnificent method. First of all, the feature of the manufacturing method of the MOS semiconductor device according to the invention is outlined with referring to FIGS.  2  to  5 .  
         [0039]     An isolation  13 , for example, a shallow trench isolation (STI), is formed in a silicon substrate  10 . Then, a first semiconductor device region  11  (for example, p-type well) for forming a first semiconductor device  11   n  (for example, n-type MOS semiconductor device) and a second semiconductor device region  12  (for example, n-type well) for forming a second semiconductor devices  12   p  (for example, p-type MOS semiconductor devices) are formed. Next, a gate insulator  101  is formed. Then a gate electrode material, for example, a molybdenum (Mo) film  102 , and a titanium nitride (TiN) film  105  to be used to diffuse nitrogen into the Mo film  103 , for example, are sequentially deposited on the gate insulator  101 . After that, an annealing is performed to diffuse nitrogen through solid phase diffusion into the entire Mo film  103 .  
         [0040]     Then, gate electrodes  110 ,  120  are patterned, and the TiN film  105  formed on one gate electrode  120  (corresponding to the second semiconductor device  12   p  (e.g., p-type MOS semiconductor device)) is removed, while on side surfaces of another gate electrode  110  (corresponding to the first semiconductor device  11   n  (e.g., n-type MOS semiconductor devices)), a film  118  preventing out-diffusion of nitrogen is formed. Next, ion implantations and an annealing are performed to form sources/drains  117 ,  127 .  
         [0041]     In the annealing, in one Mo gate electrode  120  (corresponding to the second semiconductor device, for example) from which the TiN film  105  on the surface is removed, nitrogen is out-diffused from the upper and side surfaces of the gate electrode  120  and the nitrogen concentration in the Mo film  124  is reduced. In the other gate electrode  110  (corresponding to the first semiconductor device  11   n , for example), the nitrogen concentration in the Mo film  113  is remained high since the upper surface of the Mo gate electrode  110  is covered with the TiN film  115  and the side surfaces are covered with out-diffusion preventing films  118 . After that, processes required for forming the MOS semiconductor device, for example, forming a multilevel wiring, is performed to complete the MOS semiconductor device.  
         [0042]     Thus, the nitrogen concentrations in the Mo gate electrodes  113 ,  124  in the n-type and p-type MOS semiconductor devices  11   n ,  12   p  can be adjusted to predetermined values according to the method of the invention. That is, a CMOS semiconductor device in which the work functions of the gate electrodes in the n-type and p-type MOS semiconductor device are controlled in different respective values can be manufactured.  
         [0043]     Various embodiments of the invention are explained in detail below with reference to the accompanying drawings.  
       FIRST EMBODIMENT  
       [0044]     A first embodiment relates to a manufacturing method for a CMOS semiconductor device with a basic structure, that is a method for doping nitrogen into an entire Mo film used to gate electrodes before patterning the gate electrodes and then out-diffusing nitrogen from the gate electrodes of only one type to reduce the nitrogen concentration.  
         [0045]     FIGS.  1  to  5  are cross sectional views showing a manufacturing method for a MOS semiconductor device according to the first embodiment of the invention.  
         [0046]     First, as shown in  FIG. 1 , a first conductivity type, for example, p-type, semiconductor device region  11  and a second conductivity type, for example, n-type, semiconductor device region  12 , and an isolation  13  for isolating adjacent semiconductor devices from each other are formed in a silicon substrate  10 . As the silicon substrate  10 , a p-type substrate is used in many cases, but an n-type substrate can also be used. In this case, the manufacturing process is basically the same except for adequately adjusting types and doses of ions in the implantations, as required.  
         [0047]     A process for forming the isolation  13  and first and second semiconductor device regions  11 ,  12  is simply explained below. Since the structure shown in  FIG. 1  can be formed by a conventional technique, thus the process for forming the structure is not shown in the drawing.  
         [0048]     A silicon thermal oxide (SiO 2 ) film and a silicon nitride (SiN) film by, for example, CVD are formed on the silicon substrate  10 . Then, an isolation  13  is formed to isolate adjacently formed semiconductor devices electrically to each other. A region other than the isolation  13  to be formed is covered with a resist film. The SiN film and SiO 2  film are removed by anisotropic dry etching with the resist as a mask. Further, the surface of the silicon substrate  10  is etched to form a shallow trench  13   t  (outline is shown in  FIG. 1 ) for the isolation. A thick insulation film, for example, an SiO 2  film by CVD is deposited on an entire substrate  10 . The isolation trench  13   t  is completely filled with the CVD SiO 2  film. As a filling insulation film, an SiO 2  film formed by CVD or thermal oxidation, an SiN film formed by CVD or a combination of the above films can be used. Next, the surface is planarized by chemical-mechanical polishing (CMP). In this case, the SiN film acts as a stopper for the CMP, and the insulation film, for example, SiO 2  film deposited higher than the SiN film is removed. Thus, an isolation  13  with the structure called a shallow trench isolation (STI) is formed. It is preferable to form an isolation  13  with an STI structure, however another structure, such as an LOCOS or the like, can be employed.  
         [0049]     Next, a region other than a region being formed the first semiconductor device region  11  is covered with a resist film (not shown). Then, the first conductivity type dopant of p-type, for example, boron (B) is implanted at a high energy into the region being formed the first semiconductor device region  11  using the resist film as a mask. Likewise, the second conductivity type dopant of n-type, for example, phosphorus (P) is implanted into a region being formed the second semiconductor device region  12 . After that, an annealing is performed to electrically activate the implanted dopant and recover damage caused by the implantation. Each of the first and second semiconductor device regions  11 ,  12  is generally called a well. Next, the SiN and SiO 2  films initially formed on the silicon substrate  10  are removed by etching, for example. Thus, a structure having a first and second semiconductor device regions  11 ,  12  isolated by an isolation  13  is formed as shown in  FIG. 1 .  
         [0050]     Then, as shown in  FIG. 2 , a gate insulator  101 , an Mo film  102  for forming gate electrodes, and a TiN film  105  used to diffuse nitrogen into the Mo film are sequentially formed on an entire surface of the silicon substrate  10 . In this embodiment, a silicon thermal oxide film with a thickness of 2 nm is used as the gate insulator  101 , an Mo film  102  with a thickness of 50 nm formed by the sputtering is used and a TiN film  105  with a thickness of 30 nm formed by the reactive sputtering is used. The film formation methods and film thickness are not limited to above. As the gate insulator  101 , an insulator such as a thermal SiO 2  film, an SiN film formed by CVD, for example, or a silicon oxy-nitride (SiON) film formed by thermally oxidizing a CVD-SiN film or another insulator having a dielectric constant larger than that of the SiO 2  film may be used. The Mo film  102  may be formed by sputtering or CVD. The TiN film  105  may be formed by reactive sputtering or CVD. As the film  105  for diffusing nitrogen into the Mo film  102 , a metal nitride film such as a tantalum nitride (TaN) film or tungsten nitride (WN) film may be used in addition to the TiN film. When nitrogen is doped into the Mo film from the TiN films formed in various conditions, equivalent electrical performance was attained regardless of the formation conditions of the TiN films. The composition of the TiN film is analyzed, and it is found that an atomic ratio of Ti to nitrogen ranges from 1:1 to 1:1.5. Based on the findings, a nitrogen concentration in the TiN film may be acceptable equal to or larger than that of a Ti, thus the TiN film having the nitrogen concentration of wide range may be used.  
         [0051]     A heat treatment is carried out at 800° C. for one minute in an N 2  atmosphere to diffuse nitrogen into the entire Mo film  102  from the TiN film  105  through solid phase diffusion. Thus, the Mo film doped with nitrogen therein can be formed (not shown). By considering the diffusion of nitrogen into the Mo film, the heat treatment condition is preferably at a temperature between 800° C. and 900° C. and a time being approximately one minute, and the atmosphere may be available an atmosphere which does not react with the TiN film. An inert gas atmosphere, such as argon (Ar), may be available in addition to nitrogen.  
         [0052]     The nitrogen concentration in the Mo film  103  formed by nitrogen diffusion described above was analyzed by the backside SIMS (secondary ion mass spectroscopy). It is resulted that the nitrogen concentration is 2×10 21 /cm 3  at an interface between the Mo film  103  and the SiO 2  film  101 . The backside SIMS is a method to SIMS-analyze an impurity depth profile in a film formed on a front surface from backside of the film while sputtering to remove its backside, after thinning a substrate from backside by grinding, for example, to a very thin thickness.  
         [0053]     Next, a region other than the second semiconductor device region  12  is covered with a resist film (not shown). The TiN film  105  on the second semiconductor device region  12  is removed by dry etching using the resist film as a mask. As etching gas for the dry etching above, for example, chlorine (Cl 2 ) gas or a mixed gas of Cl 2  and boron trichloride (BCl 3 ) may be used.  
         [0054]     Further, portions of Mo film being formed gate electrodes on the first and second semiconductor device regions  11 ,  12  are covered with a resist film (not shown). Then, the TiN film  105  on the first semiconductor device region  11  and the Mo film  103  doped with nitrogen on the first and second semiconductor device regions  11 ,  12  are sequentially etched by anisotropic dry etching using the resist film as a mask. Thus, gate electrode structures  110 ,  120  are formed as shown in  FIG. 3 . As the anisotropic dry etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used for the TiN film  105  and, for example, a mixed gas of Cl 2  and oxygen (O 2 ) may be used for the Mo film  103  doped with nitrogen. As is clearly seen in the drawing, the gate electrode structure  110  on the first semiconductor device region  11  is a three-layered structure including a gate insulator  111 , a Mo film  113  doped with nitrogen and a TiN film  115 . That is, the upper surface of the Mo film  113  doped with nitrogen is covered with the TiN film  115 . On the other hand, the gate electrode structure  120  on the second semiconductor device region  12  is a two-layered structure including a gate insulator  121  and a Mo film  123  doped with nitrogen. That is, the upper surface and side surfaces of the Mo film  123  doped with nitrogen are exposed.  
         [0055]     Next, a region other than the first semiconductor device region  11  is covered with a resist film (not shown) to form sources/drains  117  in the first semiconductor device region  11  as shown in  FIG. 5 . Then, a dopant of the second conductivity type (n type), such as arsenic (As), is implanted with high concentration into the surface region of the silicon substrate  10  in a self-aligned manner using the gate electrode structure  110  as a mask. Thus, implanted layers  117   i  are formed as shown in  FIG. 4 . Likewise, in order to form sources/drains  127  in the second semiconductor device region  12 , dopant of the first conductivity type (p type), such as boron (B), is implanted with high concentration into the surface region of the silicon substrate  10 . Thus, implanted layers  127   i  are formed.  
         [0056]     After that, an SiN film (not shown) is formed on an entire surface of the resultant structure. The first semiconductor device region  11  is only covered with a resist film (not shown), and then the SiN film on a region other than the first semiconductor device region  11  is removed by isotropic dry etching using the resist film as a mask. Next, a region other than the first semiconductor device region  11  is covered with a resist film (not shown), and then an anisotropic dry etching is performed to leave SiN films  118  only on the side surfaces of the gate electrode structure  110  on the first semiconductor device region  11  using the resist film as a mask. As a result, as shown in  FIG. 4 , the SiN films  118  preventing diffusion of nitrogen can be formed only on the side surfaces of the gate electrode structure  110  on the first semiconductor device region  11 . As a nitrogen diffusion preventing film, an SiO 2  film or the like may be used in addition to the SiN film. Thus, for the gate electrode structure  110  on the first semiconductor device region  11 , the upper surface of the Mo film  113  doped with nitrogen is covered with the TiN film  115  and the side surfaces are covered with the SiN films  118 . Therefore, the Mo film  113  doped with nitrogen is not exposed. On the other hand, for the gate electrode structure  120  of the second semiconductor device region  12 , none of the upper surface and side surface of the Mo film  123  doped with nitrogen is covered with any films. Thus, the Mo film  123  doped with nitrogen is exposed.  
         [0057]     Next, an annealing is performed, for example, at 900° C. for one minute in N 2  to the substrate with the structure shown in  FIG. 4 . The annealing is performed to electrically activate the implanted dopants  117   i ,  127   i  to form the sources/drains  117 ,  127  and recover damage caused by the implantation. Further, during the annealing, nitrogen doped in the Mo film  123  of the gate electrode structure  120  on the second semiconductor device region  12  is out-diffused from its exposed upper and side surfaces. As a result, a Mo film  124  with extremely reduced nitrogen can be formed.  
         [0058]     The nitrogen concentration in the Mo film  124  after nitrogen out-diffusion is analyzed by backside SIMS and, it is found that the nitrogen concentration at the interface between the Mo film  124  and the SiO 2  film  121  was 1×10 21 /cm 3 . That is, the nitrogen concentration in the Mo film  124  can be reduced by a half.  
         [0059]     As shown in  FIG. 5 , the upper surface of the gate electrode structure  110  on the first semiconductor device region  11  is covered with the TiN film  115  and the side surfaces are covered with the SiN films  118 . Therefore, nitrogen does not out-diffuse from the Mo film  113  doped with nitrogen, thus the Mo film  113  with high nitrogen concentration may be maintained as doped. For an atmosphere of the annealing, gas hardly reacting with Mo or Ti, for example, nitrogen, argon or the like, may be used. As a result, as shown in  FIG. 5 , a CMOS structure having an n-type MOS semiconductor device  1   n  and p-type MOS semiconductor device  12   p  can be formed.  
         [0060]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wiring is performed to complete a CMOS semiconductor device.  
         [0061]     As described the detail later, the semiconductor device formed according to the embodiment has different capacitance-voltage (C-V) characteristics on the n-type MOS device  11   n  and p-type MOS device  12   p  and it is confirmed that the gate electrodes thereof have different work function values.  
         [0062]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  124  of the n-type MOS device  11   n  and p-type MOS device  12   p  can be adjusted by the method according to the second embodiment of the invention. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  124  of the n-type and p-type MOS devices  11   n ,  12   p , respectively, is formed.  
       SECOND EMBODIMENT  
       [0063]     A second embodiment relates to a manufacturing method for a CMOS semiconductor device having regions with shallower junction depth formed near gate edges (hereinafter referred to as extensions) to relax the short channel effect of a MOS semiconductor device. Similar to the first embodiment, nitrogen is doped into an entire Mo film before gate electrode patterning and then nitrogen is out-diffused from the gate electrodes of only one of the MOS devices.  
         [0064]     FIGS.  6  to  8  are cross sectional views showing the steps of a manufacturing method for a MOS semiconductor device according to the second embodiment of the invention. In this embodiment, the process up to the step of forming the gate electrode structures  110 ,  120  is the same as that of the first embodiment. Therefore, the process up to the gate electrode formation is briefly explained below, but the detail explanation is omitted.  
         [0065]      FIG. 6  is the same as  FIG. 3 . First and second semiconductor device regions  11 ,  12  and an isolation  13  to isolate adjacent semiconductor devices from each other are formed on a silicon substrate  10 .  
         [0066]     On the first semiconductor device region  11 , a three-layered gate electrode structure  110  including a gate insulator  111 , a Mo film  113  doped with nitrogen and a TiN film  115  is formed. On the second semiconductor device region  12 , a two-layered gate electrode structure  120  including a gate insulator  121  and a Mo film  123  doped with nitrogen is formed. The resultant structure is shown in  FIG. 6 . As seen clearly in  FIG. 6 , an upper surface of the Mo film  113  doped with nitrogen in the gate electrode structure  110  on the first semiconductor device region  11  is covered with a TiN film  115 . On the other hand, upper and side surfaces of the Mo film  123  doped with nitrogen in the gate electrode structure  120  on the second semiconductor device region  12  are exposed.  
         [0067]     In the present embodiment, a silicon thermal oxide film with a thickness of 2 nm is used as a gate insulator  101 , a Mo film  102  with a thickness of 50 nm formed by sputtering is used, and a TiN film  105  with a thickness of 30 nm formed by the reactive sputtering is used. And a heat treatment is carried out at 800° C. for one minute in an N 2  atmosphere to diffuse nitrogen through solid phase diffusion from the TiN film  105  into the Mo film  102 . As dry etching gas used to remove the TiN film  105  on the second semiconductor device region  12 , for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. Further, as anisotropic dry etching gas to form the gate electrode structures  110 ,  120  on the first and second semiconductor device regions  11 ,  12 , for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used for the TiN film  105  on the first semiconductor device region  11  and, for example, a mixed gas of Cl 2  and oxygen (O 2 ) may be used for the Mo film doped with nitrogen on the first and second semiconductor device regions  11 ,  12 .  
         [0068]     Next, in order to form extensions  116  in the first semiconductor device region  11  as shown in  FIG. 8 , a region other than the first semiconductor device region  11  is covered with a resist film (not shown). Then, an n-type dopant of the second conductivity type, such as arsenic, is implanted at a low energy into the surface region of the silicon substrate  10  in a self-aligned manner using the gate electrode  110  as a mask. Thus, implanted layers  116   i  are formed as shown in  FIG. 7 . Likewise, in order to form extensions  126  in the second semiconductor device region  12 , a p-type dopant of the first conductivity type, such as boron, is implanted at a low energy into the surface region of the silicon substrate  10  to form implanted layers  126   i . The implantation for forming extensions is performed at a lower energy and with a lower concentration in comparison with an implantation for forming sources/drains as will be described later.  
         [0069]     Next, an SiN film (not shown) is deposited on an entire surface of the resultant structure, then anisotropic dry etching for etching back the SiN film is performed to form SiN films  119 ,  129  as side wall spacers on the side surfaces of the gate electrode structures  110 ,  120 . As shown in  FIG. 7 , on the gate electrode structure  110  on the first semiconductor device region  11 , the upper surface of the Mo film  113  doped with nitrogen is covered with the TiN film  115  and the side surfaces thereof are covered with the SiN films  119  of the side wall spacers. Therefore, the Mo film  113  doped with nitrogen is not exposed. On the gate electrode structure  120  on the second semiconductor device region  12 , though the side surfaces of the Mo film  123  doped with nitrogen are covered with the SiN films  129  of the side wall spacers, the upper surface thereof is not covered with any film and the Mo film  123  is exposed.  
         [0070]     Next, in order to form sources/drains  117  in the first semiconductor device region  11  as shown in  FIG. 8 , a region other than the first semiconductor device region  11  is covered with a resist film (not shown). Then, an n-type dopant of the second conductivity type, such as arsenic, is implanted into the surface region of the silicon substrate  10  in a self-aligned manner with a higher concentration and at a higher energy than those in the implantation for forming the extension  116   i  using the gate electrode  110  and side wall spacers  119  as masks. Likewise, in order to form sources/drains  127  in the second semiconductor device region  12 , a p-type dopant of the first conductivity type, such as boron, is implanted into the surface region of the silicon substrate  10  with a higher concentration and at a higher energy than those in the implantation for forming the extension  126   i.    
         [0071]     Next, an annealing is performed, for example, at 900° C. for one minute in an N 2  atmosphere. The annealing is performed to electrically activate the dopants implanted to form the extensions  116 ,  126  and sources/drains  117 ,  127  and recover damage caused by the implantations. The extensions  116 ,  126  thus formed are shallower in junction depth and lower in dopant concentration than those of in the sources/drains  117 ,  127 .  
         [0072]     Further, during the annealing, on the second semiconductor device region  12 , nitrogen doped in the Mo film  123  of the gate electrode structure  120  is out-diffused from the exposed upper surface of the Mo film  123 . As a result, a Mo film  124  with a reduced nitrogen concentration can be formed as shown in  FIG. 8 .  
         [0073]     On the other hand, on the first semiconductor device region  11 , the upper surface of the gate electrode structure  110  is covered with the TiN film  115  and the side surfaces thereof are covered with the SiN films  119  of the sidewall spacers. Therefore, nitrogen is not out-diffused from the Mo film  113  doped with nitrogen so that the nitrogen concentration in the Mo film  113  can be remained in high. For an atmosphere of the annealing, a gas hardly reacting with Mo or Ti, for example, nitrogen, argon or the like, may be used. As a result, as shown in  FIG. 8 , a CMOS structure having an n-type MOS semiconductor device  11   n  and p-type MOS semiconductor device  12   p  may be formed.  
         [0074]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wirings is performed to complete a CMOS semiconductor device.  
         [0075]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  124  of the n-type MOS device  11   n  and p-type MOS device  12   p  can be adjusted by the method according to the second embodiment of the invention. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  124  of the n-type and p-type MOS devices  11   n ,  12   p , respectively, is formed.  
       THIRD EMBODIMENT  
       [0076]     Like the first embodiment, a third embodiment relates to a manufacturing method for a CMOS semiconductor device in which no extensions are formed. However, in the third embodiment, nitrogen is doped into both gate electrodes after gate electrode structures are formed and then nitrogen is out-diffused from the gate electrodes of only one of the CMOS devices.  
         [0077]     FIGS.  9  to  12  are cross sectional views showing the steps of a manufacturing method for a MOS semiconductor device according to the third embodiment of the invention. As shown in  FIG. 9 , first and second semiconductor device regions  11 ,  12  and an isolation  13  to isolate the adjacent semiconductor devices from each other are formed on a silicon substrate  10 . Further, a gate insulator  101  and a Mo film  102  for forming gate electrodes are formed on an entire surface of the resultant structure. In this embodiment, the process up to the step of forming the Mo film  102  is the same as that of the first embodiment, and therefore, the explanation is omitted.  
         [0078]     In this embodiment, a silicon thermal oxide film with a thickness of 2 nm is formed by a thermal oxidation as the gate insulator  101  and a Mo film  102  with a thickness of 50 nm formed by sputtering is used.  
         [0079]     Next, in order to form gate electrode structures  110 ,  120  shown in  FIG. 10 , portions of Mo film  102  being formed gate electrodes on the first and second semiconductor device regions  11 ,  12  are covered with a resist film (not shown). An etching is performed to the Mo film  102  by anisotropic dry etching using the resist film as a mask. As the anisotropic dry etching gas, for example, a mixed gas of Cl 2  and O 2  may be used. Each of the gate electrode structures  110 ,  120  has a two-layered structure including the gate insulator  111  or  121  and the Mo film  112  or  122 .  
         [0080]     After that, in order to form sources/drains  117  in the first semiconductor device region  11  as shown in  FIG. 11 , a region other than the semiconductor device region  11  is covered with a resist film (not shown). Then, an n-type dopant of the second conductivity type, such as arsenic, is implanted in a self-aligned manner with a high impurity concentration into the surface region of the silicon substrate  10  using the gate electrode structure  110  as a mask. Thus, implanted layers  117   i  are formed as shown in  FIG. 10 . Likewise, in order to form sources/drains  127  in the second semiconductor device region  12 , a p-type dopant of the first conductivity type, such as boron, is implanted with a high impurity concentration into the surface region of the silicon substrate  10 . Thus, implanted layers  127   i  are formed. As a result, the structure shown in  FIG. 10  is formed.  
         [0081]     After that, an TiN film  105  is formed on an entire surface of the substrate with the above structure as shown in  FIG. 11 . The TiN film  105  is used to diffuse nitrogen into the Mo film  112 ,  122  in the next step. As the TiN film  105  used in this embodiment, a film with 30 nm thick is formed by reactive sputtering. Next, an annealing is performed, for example, at 900° C. for one minute in an N 2  atmosphere. The annealing is performed to serve the following two purposes. The first purpose is to electrically activate the dopants implanted to form the sources/drains  117 ,  127  and recover damage caused by the implantations. The second purpose is to diffuse nitrogen from the TiN film  105  into the Mo films  112 ,  122  through solid phase diffusion. As a result, the sources/drains  117 ,  127  and Mo films  113 ,  123  doped with nitrogen are formed as shown in  FIG. 11 .  
         [0082]     Next, all of the TiN film is removed by isotropic dry etching. As isotropic dry etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. After that, an SiN film  108  is formed only on the first semiconductor device region  11  to prevent out-diffusion of nitrogen from the Mo film  113  doped with nitrogen, as shown in  FIG. 12 . That is, an SiN film  108  is formed on an entire surface of the substrate, then only the first semiconductor device region  11  is covered with a resist film (not shown). Then, the SiN film  108  on a region other than the first semiconductor device region  11  is removed by isotropic dry etching using the resist film as a mask. As a result, as shown in  FIG. 12 , the upper surface and side surfaces of the gate electrode structure  110  on the first semiconductor device region  11  are covered with the SiN film  108  for preventing out-diffusion of nitrogen. On the other hand, none of the upper surface and side surfaces of the gate electrode structure  120  on the second semiconductor device region  12  is covered with a film and, as shown in  FIG. 11 , the Mo film  123  doped with nitrogen is exposed.  
         [0083]     In this state, an annealing is performed to out-diffuse nitrogen from the exposed upper surface and side surfaces of the Mo film  123  doped with nitrogen of the gate electrode structure  120 . As a result, the nitrogen concentration in the Mo film  124  in the gate electrode structure  120  is reduced. On the other hand, on the gate electrode structure  110  on the first semiconductor device region  11 , the surfaces of the Mo film  113  doped with nitrogen are covered with the SiN film  108  for preventing out-diffusion of nitrogen, so that the nitrogen concentration in the Mo film  113  can be remained in the high concentration. In this embodiment, the annealing is performed at 900° C. for one minute in an N 2  atmosphere. For the atmosphere of the annealing, a gas hardly reacting with Mo or Ti, for example, nitrogen, argon or the like, may be used. Then, the SiN film  108  on the sources/drains  117  in the first semiconductor region  11  is removed.  
         [0084]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wirings is performed to complete a CMOS semiconductor device.  
         [0085]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  124  of the n-type MOS device  11   n  and p-type MOS device  12   p  can be adjusted by the method according to the third embodiment of the invention. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  124  of the n-type and p-type MOS devices  11   n ,  12   p , respectively, is formed.  
         [0086]     This embodiment may be modified as follows.  
         [0087]     The TiN film  105  shown in  FIG. 11  may be removed from the second semiconductor device region  12  after the TiN film  105  is formed. That is, the first semiconductor device region  11  is covered with a resist film (not shown), then the TiN film  105  (including the TiN films on the side surfaces of the gate electrode structure  120 ) on the second semiconductor device region  12  is removed by isotropic dry etching using the resist film as a mask. As etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. After that, an annealing is performed at 900° C. for one minute in an N 2  atmosphere, for example. One of the purposes of the annealing is to electrically activate the dopants implanted to form the sources/drains  117 ,  127  shown in  FIG. 11  and recover damage caused by the implantations. Another purpose is to diffuse nitrogen from the TiN film  105  only into the Mo film  112  of the gate electrode structure  110  on the first semiconductor device region  11  shown in  FIG. 10  to form a Mo film  113  doped with nitrogen. Thus, the Mo gate electrode  113  doped with nitrogen is formed on the first semiconductor device region  11 , the Mo gate electrode  122  without doping nitrogen is formed on the second semiconductor device region  12 , and the sources/drains  117 ,  127  are formed.  
         [0088]     After the annealing, the TiN film  105  left only on the first semiconductor device region  11  is removed by isotropic dry etching. As etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used.  
         [0089]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wirings is performed to complete a CMOS semiconductor device.  
         [0090]     With the method of the modification, the following process may be omitted and the process may be simplified in comparison with that of the third embodiment. That is, it is possible to omit both of the steps of forming a film  108  to prevent out-diffusion of nitrogen from the Mo film  113  doped with nitrogen of the gate electrode structure  110  on the first semiconductor device region  11 , and annealing to out-diffuse nitrogen from the Mo gate electrode doped with nitrogen on the second semiconductor device region  12 .  
         [0091]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  122  of the n-type MOS semiconductor device  11   n  and p-type MOS semiconductor device  12   p  can be adjusted by the method according to the modification of the third embodiment. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  122  of n-type and p-type MOS devices  11   n ,  12   p , respectively, is formed.  
       FOURTH EMBODIMENT  
       [0092]     Like the second embodiment, a fourth embodiment relates to a manufacturing method for a CMOS semiconductor device having extensions formed near gate edges to relax a short channel effect of a MOS semiconductor device. Further, like the third embodiment, the fourth embodiment relates to a case wherein nitrogen is doped into both of the gate electrodes after the gate electrode structures are formed and then nitrogen is out-diffused only from the gate electrodes one of the CMOS devices.  
         [0093]     FIGS.  13  to  15  are cross sectional views showing the steps of a manufacturing method for a MOS semiconductor device according to the fourth embodiment of the invention. In this embodiment, the process up to the step of forming the gate electrode structures is the same as that of the third embodiment, and therefore, the detail explanation for the process up to the gate electrode structure forming step is omitted.  
         [0094]     As shown in  FIG. 13 , first and second semiconductor device regions  11 ,  12  and an isolation  13  to isolate the adjacent semiconductor devices from each other are formed in a silicon substrate  10 . Like the case of  FIG. 9 , a gate insulator  101  and a Mo film  102  for forming gate electrodes are formed on the resultant structure. Then, gate electrode structures having a two-layered structure including gate insulators  111 ,  121  and Mo films  112 ,  122  are formed on the first and second semiconductor device regions  11 ,  12  by anisotropic dry etching.  
         [0095]     In this embodiment, a silicon thermal oxide film with a thickness of 2 nm is formed as the gate insulator  101  and a Mo film  102  with a thickness of 50 nm is formed by sputtering. Further, as the anisotropic dry etching gas for the Mo film  102 , for example, a mixed gas of Cl 2  and O 2  may be used to form the gate electrode structures  110 ,  120  on the first and second semiconductor device regions  11 ,  12 .  
         [0096]     Next, in order to form extensions  116  in the first semiconductor device region  11  as shown in  FIG. 14 , a region other than the first semiconductor device region  11  is covered with a resist film (not shown). Then, an n-type dopant of second conductivity type, such as arsenic, is implanted at a low energy into a surface of the silicon substrate  10  in a self-aligned manner using the gate electrode  110  as a mask so that to form implanted layers (not shown). Likewise, in order to form extensions  126  in the second semiconductor device region  12 , a p-type dopant of first conductivity type, such as boron, is implanted at a low energy into the surface region of the silicon substrate  10  to form implanted layers (not shown). The implantation for forming extension is performed at a lower energy and with a lower concentration in comparison with the implantation for forming sources/drains as will be described later.  
         [0097]     Next, a TiN film  105  is formed on an entire surface of the substrate with the above structure as shown in  FIG. 14 . The TiN film  105  is used to diffuse nitrogen into the Mo films  112 ,  122  in the next step. The TiN film  105  with 30 nm thick formed by the reactive sputtering is used in this embodiment. After that, an annealing is performed, for example, at 900° C. for one minute in an N 2  atmosphere. The annealing is performed to serve the following two purposes. The first purpose is to electrically activate the dopants implanted to form the extensions  116 ,  126  and recover damage caused by the implantations. The second purpose is to dope nitrogen into the Mo films  112 ,  122  from the TiN film  105 . As a result, the extensions  116 ,  126  and Mo films  113 ,  123  doped with nitrogen are formed as shown in  FIG. 14 .  
         [0098]     Next, all of the TiN film  105  is removed by isotropic dry etching. As etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. After that, an SiN film (not shown) is deposited by CVD on an entire surface of the substrate of the resultant structure. Then, the SiN film is etched back by anisotropic dry etching. As a result, SiN films  119 ,  129  are formed as sidewall spacers on the side surfaces of the gate electrode structures  110 ,  120  as shown in  FIG. 15 .  
         [0099]     Next, a region other than the first semiconductor device region  11  is covered with a resist film (not shown) to form sources/drains  117  in the first semiconductor device region  11  as shown in  FIG. 15 . Then, an n-type dopant of the second conductivity type, such as arsenic, is implanted in a self-aligned manner at a higher energy and with a high concentration into the surface region of the silicon substrate  10  than those in the implantations for forming the extensions  116   i  using the gate electrode structure  110  and side wall spacers  119  as a mask. Likewise, in order to form sources/drains  127  in the second semiconductor device region  12 , a p-type dopant of the first conductivity type, such as boron, is implanted into the surface region of the silicon substrate  10  at a higher energy with a higher concentration than those in the implantation for forming the extensions  126   i.    
         [0100]     After that, an SiN film  108  to prevent out-diffusion of nitrogen from the Mo film  113  doped with nitrogen is formed only on the first semiconductor device region  11 . That is, the SiN film  108  is formed on an entire surface of the substrate, then only the first semiconductor device region  11  is covered with a resist film (not shown) and the SiN film  108  on a region other than the first semiconductor device region  11  is removed by isotropic dry etching using the resist film as a mask. As a result, as shown in  FIG. 15 , the upper surface of the gate electrode  120  of the second semiconductor device region  12  is not covered with the film and the Mo film  123  doped with nitrogen is exposed. The upper surface and side surfaces of the gate electrode  110  of the first semiconductor device region  11  are covered with the SiN films  108 ,  119  to prevent diffusion of nitrogen.  
         [0101]     Next, an annealing is performed, for example, at 900° C. for one minute in an N 2  atmosphere. The annealing is performed to electrically activate the dopants implanted to form sources/drains  117 ,  127  and recover damage caused by the implantation. The sources/drains  117 ,  127  thus formed have deeper junction depth and higher dopant concentration than those of the extensions  116 ,  126 .  
         [0102]     Further, during the annealing, in the gate electrode structure  120  on the second semiconductor device region  12 , the upper surface of the Mo film  123  doped with nitrogen is exposed, while the side surfaces are covered with the sidewall spacers  129 . Therefore, nitrogen doped in the Mo film  123  is out-diffused from the exposed upper surface and a Mo film  124  having reduced nitrogen concentration can be formed. On the other hand, on the gate electrode structure  110  on the first semiconductor device region  11 , since the upper surface and side surfaces of the Mo film  113  doped with nitrogen are covered with the SiN films  108 ,  119  to prevent diffusion of nitrogen during the annealing, the nitrogen concentration in the Mo film  113  doped with nitrogen can be remained in high. As atmospheric gas used in the annealing, gas which is difficult to react with Mo may be used and, for example, N 2 , Ar or the like may be used. As a result, as shown in  FIG. 15 , a CMOS structure having an n-type MOS semiconductor device  1  in and p-type MOS semiconductor device  12   p  can be formed. Then, the SiN film  108  on the sources/drains  117  in the first semiconductor region  11  is removed.  
         [0103]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wirings is performed to complete a CMOS semiconductor device.  
         [0104]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  124  of the n-type MOS semiconductor device  11   n  and p-type MOS semiconductor device  12   p  can be adjusted by the method according to the fourth embodiment of the invention. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  124  of n-type and p-type MOS devices  11   n ,  12   p , respectively, is formed.  
         [0105]     This embodiment may be modified as follows.  
         [0106]     After the gate electrodes are formed as shown in  FIG. 13 , an implantation for forming the extensions  116 ,  126  shown in  FIG. 14  and a process for forming the TiN film  105  are performed in the same manner as in the fourth embodiment. After that, the process may be modified to remove the TiN film  105  formed on the second semiconductor device region  12 . That is, the first semiconductor device region  11  is covered with a resist film (not shown) and the TiN film  105  formed on the second semiconductor device region  12  (including the TiN film on the side surfaces of the gate electrode structure  120 ) is removed by isotropic dry etching using the resist film as a mask. As etching gas for the isotropic dry etching, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. After that, an annealing is performed at 900° C. for one minute in an N 2  atmosphere. The annealing is performed to serve the following two purposes. The first purpose is to electrically activate dopants implanted to form the extensions  116 ,  126  and recover damage caused by the implantation. The second purpose is to diffuse nitrogen from the TiN film  105  into the Mo film  112  of the gate electrode structure  110  on the first semiconductor device region  11 . As a result, the extensions  116 ,  126  are formed, and nitrogen is diffused into the Mo film  112  of the gate electrode structure  110  on the first semiconductor device region  11  to form a Mo film  113  doped with nitrogen. A Mo film  122  without doping nitrogen is formed on the gate electrode structure  120  on the second semiconductor device region  12 .  
         [0107]     After that, the same process as that of the third embodiment is performed to complete a MOS semiconductor device. The process is roughly explained below. After the annealing, the TiN film  105  left only on the first semiconductor device region  11  is removed by isotropic dry etching. As etching gas, for example, Cl 2  or a mixed gas of Cl 2  and BCl 3  may be used. After that, side wall spacers  119 ,  129  of SiN films are formed on the side walls of the gate electrode structures  110 ,  120  on the first and second semiconductor device regions  11 ,  12 . Then, implantations are performed with a high dopant concentration to form sources/drains  117 ,  127  in the first and second semiconductor device regions  11 ,  12 . Next, an SiN film  108  for preventing out-diffusion of nitrogen is formed on the upper surface of the gate electrode structure  110  on the first semiconductor device region  11 . Then, an annealing is performed to electrically activate the dopant implanted to form the sources/drains  117 ,  127  and recover damage caused by the implantation. While, in the annealing, the nitrogen concentration in the Mo film  113  can be remained high, since the surfaces of the Mo film  113  doped with nitrogen of the gate electrode structure  110  are covered with the SiN films  108 ,  119 .  
         [0108]     After that, a process required for forming a MOS semiconductor device, for example, formation of multilevel wirings is performed to complete a CMOS semiconductor device.  
         [0109]     As described above, the nitrogen concentrations in the Mo gate electrodes  113 ,  122  of the n-type MOS semiconductor device  11   n  and p-type MOS semiconductor device  12   p  can be adjusted by the method according to the modification of the fourth embodiment. That is, a CMOS semiconductor device having different work function values in the gate electrodes  113 ,  122  of n-type and p-type MOS semiconductor devices  11   n ,  12   p , respectively, is formed.  
         [0110]      FIG. 16  shows capacitance-voltage (C-V) characteristics of CMOS capacitors formed by a process as in the first embodiment in order to prove effects of the invention. In this case, a thermal oxide film of 10 nm thick is used as a gate insulator. In  FIG. 16 , a solid line shows a characteristic of an nMOS capacitor corresponding to an n-type MOS semiconductor device in which the Mo film of the gate electrode is doped with nitrogen at a high concentration. A broken line shows a characteristic of a pMOS capacitor corresponding to a p-type MOS semiconductor device in which nitrogen is out-diffused from the Mo film doped with nitrogen of the gate electrode to reduce the nitrogen therein. Further, as a reference, a dotted line shows a characteristic of a conventional capacitor using a Mo film without doping nitrogen as a gate electrode. The effect provided by doping nitrogen into the Mo film of the gate electrode can be proved by comparing the characteristics indicated by the solid line and dotted line. It is proved that, by doping nitrogen into the Mo film of the gate electrode by the method of the invention, the C-V characteristic of the nMOS capacitor indicated by the solid line is shifted by approximately −0.5 V with respect to that of the conventional MOS capacitor indicated by the dotted line. That is, the work function of Mo gate is shifted by approximately −0.5 eV by doping nitrogen into the Mo film of the gate electrode. Further, the effect provided by out-diffusing nitrogen from the Mo film of the gate electrode can be proved by comparing the characteristics indicated by the solid line and broken line. By out-diffusing nitrogen from the Mo film of the gate electrode by the method of the invention, it can be confirmed that the C-V characteristic of the pMOS capacitor indicated by the broken line is shifted by approximately +0.4 V with respect to that of the nMOS capacitor indicated by the solid line and is recovered to be sufficiently closer to that of the conventional MOS capacitor indicated by the dotted line, which is a characteristic of the MOS capacitor without doping nitrogen in Mo film. That is, the work function of Mo is shifted by approximately +0.4 eV by out-diffusing nitrogen from the Mo film of the gate electrode. In other words, the effect of nitrogen remained in the Mo film to the work function is only −0.1 eV. Therefore, it is proved that a CMOS semiconductor device having different work function values in the gate electrodes for the n-type and p-type MOS semiconductor devices can be formed by the method of the invention.  
         [0111]     As described above, the nitrogen concentrations in the Mo gate electrodes of the n-type and p-type MOS semiconductor devices can be adjusted by the method of the invention. That is, a CMOS semiconductor device having different work function values in the gate electrodes of the n-type and p-type MOS semiconductor devices, respectively, can be formed.