Abstract:
Disclosed herein is a semiconductor device including a gate insulating film formed over a semiconductor substrate, and a gate electrode formed over the gate insulating film, wherein the gate insulating film is so provided as to protrude from both sides of the gate electrode, and the gate electrode includes a wholly silicided layer.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2007-131452 filed in the Japan Patent Office on May 17, 2007, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method of manufacturing a semiconductor device, particularly a semiconductor device including a fully silicided gate electrode and a manufacturing method therefor. 
     2. Description of the Related Art 
     In relation to the transistor structures in recent years, a further reduction in the microscopic size of insulated gate type field effect transistors using a silicon oxide film (these field effect transistors will hereinafter be referred to as MOSFET) has been advanced. However, when thinning of a gate insulating film is advanced along with the proportional reduction of the MOSFET, an increase in the gate leak current due to the tunnel current becomes a problem. In order to obviate this problem, researches of MOSFETs having a gate insulating film formed by use of a high dielectric constant material (high dielectric constant gate insulating film) have been made. 
     On the other hand, polycrystalline silicon with an impurity added thereto is used as a material for gate electrodes of MOSFETs. Since the material is a semiconductor, however, the surface of the gate electrode would undergo a little depletion, causing a lowering in the current driving force of the transistor. As a countermeasure against this problem, a Fully Silicided gate (FUSI) technology in which the gate electrode is wholly silicided has come to be investigated. 
     Besides, in recent years, from the viewpoints of reducing the gate leak current and simultaneously suppressing the depletion of the gate electrode, researches have been made of application of a gate stack structure based on a combination of the high dielectric constant gate insulating film with the FUSI technology to MOSFETs (refer to Motofumi Saitoh et al., “Strain Controlled CMOSFET with Phase Controlled Full-silicide (PC-FUSI)/HfSiON Gate Stack Structure for 45 nm-node LSTP Devices”, 2006  Symposium on VLSI Technology Digest of Technical Papers,  2006 IEEE, 2006). 
       FIGS. 8A to 8D  show an example of a method of manufacturing a semiconductor device having the above-mentioned gate stack structure. First, as shown in  FIG. 8A , a polysilicon gate electrode  103  is formed in a pattern over a silicon substrate  101 , with a high dielectric constant material gate insulating film  102  therebetween. Thereafter, as shown in  FIG. 8B , insulating side walls  104  are formed at side walls of the gate insulating film  102  and the gate electrode  103 . Next, an insulating film  105  is formed so as to bury the gate electrode  103  and the side walls  104 , and the insulating film  105  is flattened by CMP (Chemical Mechanical Polishing), thereby exposing the gate electrode  103 . Subsequently, as shown in  FIG. 8C , a nickel film  106  is built up in the state of covering the insulating film  105  and the gate electrode  103 . Thereafter, as shown in  FIG. 8D , a heat treatment is conducted to bring the polysilicon constituting the gate electrode  103  and the nickel film  106  as an upper layer into reaction with each other, whereby the gate electrode  103  as a whole is fully silicided, to form a fully silicided gate electrode  103 ′. 
     SUMMARY OF THE INVENTION 
     However, in the case of forming the fully silicided gate according to the above-mentioned procedure, the following problem would be generated. At the time of siliciding the polysilicon gate electrode by conducting a heat treatment, the gate electrode  103  is expanded in volume, as shown in  FIG. 8D . The volume expansion leads to the generation of a gap in region A between the gate insulating film  102  under the fully silicided gate electrode  103 ′ and the side wall  104 . Then, the metallic material (nickel) diffuses leakingly through the gap to the side of the silicon substrate  101 , possibly causing such problems as variations in the threshold voltage of the MOSFET, leakage, etc. 
     Thus, there is a need for a semiconductor device such that a fully silicided gate electrode can be formed while preventing leakage of a metallic material to the substrate side and that the device characteristics can be maintained notwithstanding the presence of the fully silicided gate electrode, and for a method of manufacturing the semiconductor device. 
     In order to fulfill the above need, according to an embodiment of the present invention, there is provided a semiconductor device including a gate insulating film formed over a semiconductor substrate, and a gate electrode formed over the gate insulating film. In the semiconductor device, the gate insulating film is so provided as to protrude from both sides of the gate electrode, and the gate electrode includes a wholly silicided layer. 
     In the semiconductor device configured as above, the gate insulating film protruding from both sides of the gate electrode is provided between the gate electrode and the semiconductor substrate, so that there is no interface that provides rectilinear connection between the gate electrode and the semiconductor substrate. Therefore, the silicide material constituting the gate electrode, particularly, the metal is prevented from migrating by diffusion to the semiconductor substrate side. 
     Besides, according to another embodiment of the present invention, there is provided a method of manufacturing a semiconductor device configured as above. The manufacturing method includes the following steps, which are sequentially carried out. First, in a first step, a silicon film is formed over a semiconductor substrate, with a gate insulating film therebetween. Next, in a second step, the silicon film is patterned while leaving the gate insulating film over the semiconductor substrate, to form a gate electrode including the silicon film. Thereafter, in a third step, an insulating side wall is formed at a side wall of the gate electrode over the gate insulating film, and the gate insulating film is pattern etched by using the side wall and the gate electrode as a mask. Subsequently, in a fourth step, a metal film is formed in the state of covering an exposed part of the gate electrode, and the gate electrode is fully silicided by a heat treatment. 
     In another embodiment of the invention, the gate electrode is fully silicided in the condition where the gate insulating film is so provided as to protrude from both sides of the gate electrode to the positions under the side wall, i.e., in the condition where there is no interface that provides rectilinear connection between the gate electrode and the semiconductor substrate. Therefore, even in the case where the gate electrode is expanded at the time of being fully silicided and where a gap is generated due to delamination at an interface disposed in the range from the gate electrode to the semiconductor substrate, it is possible to prevent the silicide material constituting the gate electrode, particularly, the metal from migrating by diffusion to the semiconductor substrate side. 
     According to an embodiment of the present invention, the metallic material contained in the fully silicided gate electrode may be prevented from migrating by diffusion to the semiconductor substrate side, so that it is possible to obtain a semiconductor device in which variations in threshold voltage, leakage and the like troubles are prevented, notwithstanding the presence of the fully silicided gate electrode, and in which the device characteristics are maintained stably. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  are sectional step diagrams (No.  1 ) illustrating a method of manufacturing a semiconductor device according to a first embodiment of the present invention. 
         FIGS. 2A to 2D  are sectional step diagrams (No.  2 ) illustrating the method of manufacturing a semiconductor device according to the first embodiment. 
         FIGS. 3A to 3D  are sectional step diagrams (No.  3 ) illustrating the method of manufacturing a semiconductor device according to the first embodiment. 
         FIGS. 4A to 4C  are sectional step diagrams (No.  4 ) illustrating the method of manufacturing a semiconductor device according to the first embodiment. 
         FIGS. 5A to 5D  are sectional step diagrams (No.  1 ) illustrating a method of manufacturing a semiconductor device according to a second embodiment of the present invention. 
         FIGS. 6A to 6D  are sectional step diagrams (No.  2 ) illustrating the method of manufacturing a semiconductor device according to the second embodiment. 
         FIGS. 7A to 7C  are sectional step diagrams (No.  3 ) illustrating the method of manufacturing a semiconductor device according to the second embodiment. 
         FIGS. 8A to 8D  are sectional step diagrams for illustrating a manufacturing method according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, embodiments of the semiconductor device and the manufacturing method therefor according to the present invention will be described in detail below, based on the drawings. In the following description of the embodiments, the configuration will be described following the sequence of manufacturing steps. 
     First Embodiment 
     First, as shown in  FIG. 1A , device isolation regions (not shown) are formed on the face side of a semiconductor substrate  1  including a p-type single crystal silicon, and then a gate insulating film  3  including a high dielectric constant material is formed. As the high dielectric constant material, for example, a material which contains a metal oxide such as HfSiON and has a dielectric constant higher than that of silicon oxide is used, and a film thereof is formed in a thickness of about 5 nm. Incidentally, in the case of forming a p-type MOS transistor together with an n-type MOS transistor over the semiconductor substrate  1 , an n-well diffusion layer is preliminarily formed in the region for forming the p-type MOS transistor, of the n-type semiconductor substrate  1 . 
     Next, a polysilicon film  5  for forming a gate electrode is built up in a thickness of, for example, about 200-300 nm by CVD. Thereafter, as occasion demands, an impurity is introduced into the polysilicon film in the region for forming the n-type MOS transistor, or into the polysilicon film in the region for forming the p-type MOS transistor. Subsequently, a stopper layer  7  including a silicon nitride film is formed over the polysilicon film  5 . 
     Next, as shown in  FIG. 1B , the stopper layer  7  and the polysilicon film  5  are patterned by reactive ion etching (RIE) in which a resist pattern (not shown) is used as a mask. By this step, a polysilicon gate electrode  5   a  is formed over the gate insulating film  3 . Here, it is particularly important to leave the gate insulating film  3  over the whole surface of the semiconductor substrate  1 . It is to be noted, however, that part of the gate insulating film  3  which is by the side of the gate electrode  5  may be reduced in material thickness by the RIE, resulting in the formation of a step in the gate insulating film  3 . Besides, after such etching, the resist pattern used as the mask in the RIE is removed. 
     Subsequently, as shown in  FIG. 1C , a silicon nitride (Si 3 N 4 ) film  9  is built up by CVD in the state of covering the gate electrode  5   a  and in a thickness of about 20-100 nm, for example. 
     Thereafter, as shown in  FIG. 1D , the silicon nitride film  9  is subjected to whole-area etch-back by RIE, whereby the silicon nitride film  9  is left on the side surfaces of the gate electrode  5   a  inclusive of the stopper layer  7 , to form first side walls  9   a . Subsequently, the gate insulating film  3  as the under layer is also etched back and patterned, to expose the semiconductor substrate  1 . As a result, a gate electrode  5   a  having a width W 1  smaller than the width W 0  of the gate insulating film  3  is provided in the center of the patterned gate insulating film  3 . In other words, the gate insulating film  3  under the gate electrode  5   a  is protruding into the areas under the first side walls  9   a , and the interface between the gate electrode  5   a  and the first side walls  9   a  are blocked by the gate insulating film  3  and, hence, prevented from rectilinearly reaching the semiconductor substrate  1 . Incidentally, the width W 0  of the gate insulating film  3  is equal to the sum of the width W 1  of the gate electrode  3  and the widths of the first side walls  9   a  on both sides of the gate electrode  3 . 
     Next, as shown in  FIG. 2A , second side walls  11   a  including silicon nitride are formed at both sides of the first side walls  9   a  and the gate insulating film  3 . The second side walls  11   a  are formed in the same manner as the first side walls  9   a . As a result, side walls of the gate insulating film  3  including the high dielectric constant material are covered with the second side walls  11   a . Therefore, even when oxidation or a heat treatment in an oxidizing gas-containing atmosphere or building-up of an insulating film is conducted in the subsequent step(s), it is possible to prevent metal contamination of the semiconductor substrate  1  through the interface of the gate insulating film  3 , or a lowering in the relative permittivity of the gate insulating film  3  due to formation of an extremely thin silicon oxide film into the interface of the semiconductor substrate  1 . 
     Subsequently, as shown in  FIG. 2B , an impurity is introduced into an exposed surface layer of the semiconductor substrate  1  by ion implantation conducted by using the gate electrode  5   a , the first side walls  9   a  and the second side walls  11   a  as a mask. Next, an activating heat treatment for the thus introduced impurity is conducted, to form a low dose diffusion layer (LDD) at a surface layer of the semiconductor substrate  1 . Incidentally, the ion implantation here is conducted by using a resist pattern as a mask, so as to introduce an n-type impurity into a region for forming an n-type MOS transistor and to introduce a p-type impurity into a region for forming a p-type MOS transistor. For example, arsenic as an n-type impurity is introduced into the region for forming the n-type MOS transistor by ion implantation at an implantation energy of 1 keV and in a dose of 1E14 atoms/cm 2 , and the activating heat treatment for the impurity is carried out by a rapid heat treatment (Rapid Thermal Anneal: RTA) at 950° C. for 5 sec. 
     Next, as shown in  FIG. 2C , third side walls  15   a  including silicon oxide are formed on the outside of the second side walls  9   a . Thereafter, an impurity is introduced into an exposed surface layer of the semiconductor substrate  1  by ion implantation conducted by using the third side walls  15   a  and the like as a mask. Subsequently, an activating heat treatment for the impurity thus introduced is conducted, to form source/drain regions  17  in the surface layer of the semiconductor substrate  1 . Incidentally, the ion implantation here is conducted by use of a resist pattern as a mask so as to introduce an n-type impurity into a region for forming an n-type MOS transistor and to introduce a p-type impurity into a region for forming a p-type MOS transistor. For example, arsenic as an n-type impurity is introduced into the region for forming the n-type MOS transistor by ion implantation at an implantation energy of 20 keV and in a dose of 1E15 atoms/cm 2 , and the activating heat treatment for the impurity is carried out by RTA at 1000° C. for 10 sec. 
     Next, as shown in  FIG. 2D , the third side walls  15   a  including silicon oxide are removed by washing with a chemical liquid including hydrofluoric acid. Subsequently, a silicide layer  19  is formed at an exposed surface layer of the semiconductor substrate  1 , i.e., the surface layers of the source/drain regions  17  and the low-dose diffusion layer  13 . In this case, for example, a metal film of nickel (Ni) or the like is formed in a thickness of about 10 nm by PVD, and then a high temperature treatment such as RTA is conducted to silicide the surface layer of the semiconductor substrate  1 . After the siliciding is over, the metal film left upon the siliciding is removed. 
     Subsequently, as shown in  FIG. 3A , a stopper film  21  including silicon nitride is built up in a thickness of about 30 nm in the state of covering the gate electrode  5   a , the second side walls  11   a  and the silicide layers  19  by CVD. Next, an interlayer insulating film  23  including silicon oxide is built up by CVD in such a thickness (e.g., 500 nm) as to bury the gate electrode  5   a . Thereafter, the layer insulation film  23  is subjected to CMP, to expose the stopper film  21  and to flatten the interlayer insulating film  23 . 
     Subsequently, as shown in  FIG. 3B , RIE of the stopper film  21  and the stopper layer  7  which include silicon nitride, as contrasted to the interlayer insulating film  23  including silicon oxide, is conducted to expose a surface of the gate electrode  5   a . As a result, upper parts of the side walls  9   a ,  11   a  including silicon nitride are also removed, so that side walls of an upper part of the gate electrode  5   a  are also exposed. 
     Then, as shown in  FIG. 3C , a metal film  25  of nickel (Ni) or the like is formed in a thickness of about 10 nm in the state of making contact with the gate electrode  5   a  by PVD. 
     Thereafter, as shown in  FIG. 3D , a heat treatment such as RTA is conducted, to silicide the gate electrode  5   a  which has been formed by patterning a polysilicon film. Here, it is important to fully (wholly) silicide the gate electrode  5   a , thereby forming the fully silicided gate electrode  5   a ′. Consequently, a n-type and a p-type MOS transistor Tr having the fully silicided gate electrode  5   a ′ are obtained. 
     After the above steps, as shown in  FIG. 4A , the metal film  25  left upon the siliciding is etched away, then the interlayer insulating film  23  including silicon oxide is etched away, and, further, the stopper film  21  including silicon nitride is removed, to expose the silicide layer  19  at the surface of the semiconductor substrate  1 . In this case, the side walls  9   a ,  11   a  including silicon nitride are also reduced in film amount by the etching, and side walls  21   a  including silicon nitride are left at the lower side of side walls of the fully silicided gate electrode  5   a′.    
     Thereafter, as shown in  FIG. 4B , a stress film  27  including silicon nitride and a stopper film  29  are formed in the state of covering the fully silicided gate electrode  5   a ′ and the side walls  21   a . The stress film  27  is a film for exerting a stress on a channel region  1   a  under the fully silicided gate electrode  5   a ′. In this case, a dual stress liner (DSL) film is formed so that a tensile stress is exerted on the region where the n-type MOS transistor Tr is formed and that a compressive stress is exerted on the region where the p-type MOS transistor Tr is formed. 
     In this case, first, a silicon nitride film having a tensile stress (Tensile Si 3 N 4 ) as the stress film  27  is formed in a thickness of about 30 nm by CVD or the like. Next, as the stopper film  29  at the time of processing the stress film  27 , a silicon oxide film is built up in a thickness of about 10 nm by CVD or the like. Then, etching is conducted using a resist pattern as a mask, to remove the stopper film  29  and the stress film  27  in the region where the p-type MOS transistor Tr is formed. 
     By the above-mentioned steps, the region where the n-type MOS transistor Tr is formed is covered with the stress film  27  for exerting a tensile stress on the channel region  1   a.    
     Next, a silicon nitride film having a compressive stress (Compressive Si 3 N 4 ) as the stress film  27  is built up in a thickness of about 30 nm by CVD or the like. Then, as the stopper film  29  at the time of processing the stress film  27 , a silicon oxide film is built up in a thickness of about 10 nm by CVD or the like. Thereafter, etching is conducted by use of a resist pattern as a mask, to remove the stopper film  29  and the stress film  27  in the region where the n-type MOS transistor Tr is formed. 
     By the above-mentioned steps, the region where the p-type MOS transistor Tr is formed is covered with the stress film  27  for exerting a compressive stress on the channel region  1   a.    
     Thereafter, as shown in  FIG. 4C , an interlayer insulating film  31  including silicon oxide is formed over the stopper film  29 , then flattening by CMP is conducted if required, and connection holes  31   a  reaching the silicide layer  19  are formed in the interlayer insulating film  31 , the stopper film  29  and the stress film  27 . Then, wiring conductors  33  connected to the source/drain regions  17  through the connection hole  31  and the silicide layer  19  are formed, to complete a semiconductor device  40 . 
     The semiconductor device  40  thus completed has the gate insulating film  3 , which is broader than the fully silicided gate electrode  5   a ′, between the fully silicided gate electrode  5   a ′ and the semiconductor substrate  1 . 
     In the first embodiment as above, as has been described using  FIG. 3D , full siliciding of the gate electrode  5   a  formed by patterning a polysilicon film is conducted in the condition where the continuous gate insulating film  33  is present beneath the gate electrode  5   a  and the first side walls  9   a  at both sides thereof, i.e., in the absence of an interface that rectilinearly connects the gate electrode  5   a  and the semiconductor substrate  1  to each other. Therefore, even in the case where a gap is generated due to delamination at an interface disposed in the range from the gate electrode  5   a  to the semiconductor substrate  1  as a result of expansion attendant on full siliciding of the gate electrode  5   a , it is possible to prevent a silicide material constituting the gate electrode  5   a , particularly the metal, from migrating by diffusion to the semiconductor substrate  1  side. 
     As a result, the metallic material contained in the fully silicided gate electrode  5   a ′ can be prevented from migrating by diffusion to the semiconductor substrate  1  side, so that it is possible to obtain a semiconductor device  10  in which variations in threshold voltage, leakage and the like troubles are prevented, notwithstanding the presence of the fully silicided gate electrode  5   a ′, and in which the device characteristics are maintained stably. 
     In addition, as above-mentioned, the metallic material contained in the fully silicided gate electrode  5   a  (can be prevented from diffusing to the semiconductor substrate  1  side, so that there is no need for thinning of the fully silicided gate electrode  5   a ′ in order to suppress volume expansion of the fully silicided gate electrode  5   a ′ in the full siliciding step. This makes it possible to maintain the height of the fully silicided gate electrode  5   a ′, and to augment the stress exerted on the channel region  1   a  from the stress film  27  covering the fully silicided gate electrode  5   a ′. Therefore, it becomes possible to obtain a semiconductor device  40  in which an increased stress can be exerted on the channel region  1   a  from the stress film  27 , thereby securing an enhanced carrier mobility, and which is capable of high-speed operations. 
     Furthermore, since the height of the fully silicided gate electrode  5   a ′ can be maintained, the width of the side walls  21   a  can be kept large. This ensures that the side periphery of the fully silicided gate electrode  5   a ′ is sufficiently protected by the side walls  21   a . Therefore, the side walls  21   a  serve as a stopper in forming the connection holes  31   a , and short-circuiting between the connection hole  31   a  and the semiconductor substrate  1  can be restrained. 
     Second Embodiment 
     First, the same procedure as described using  FIGS. 1A to 1D  in the first embodiment above is carried out, whereby a gate insulating film  3  is patterned over a semiconductor substrate  1 , a gate electrode  5   a  having a width W 1  smaller than the width W 0  of the gate insulating film  3  is provided over a central part of the gate insulating film  3 , and first side walls  9   a  are provided at both side walls of the gate electrode  5   a . In other words, a structure is formed in which the gate insulating film  3  under the gate electrode  5   a  is protruding into the areas under the first side walls  9   a.    
     Thereafter, as shown in  FIG. 5A , disposable side walls  41   a  including silicon oxide are formed on the outside of the first side walls  9   a . The disposable side walls  41   a  are formed by first building up a silicon oxide film in a thickness of about 20-100 nm and then subjecting the silicon oxide film to RIE. As a result, side walls of the gate insulating film  3  including a high dielectric constant material are covered with the disposable side walls  41   a . Therefore, even when oxidation or a heat treatment or building-up of an insulating film in a oxidizing gas-containing atmosphere is carried out in the subsequent step(s), it is possible to prevent contamination of the semiconductor substrate  1  with a metal from the interface of the gate insulating film  3  or a lowering in relative permittivity of the gate insulating film  3  due to formation of a very thin silicon oxide film onto the interface of the semiconductor substrate  1 . 
     Next, as shown in  FIG. 5B , an impurity is introduced into an exposed surface layer of the semiconductor substrate  1  by ion implantation conducted by use of the disposable side walls  41   a  and the like as a mask. Then, an activating heat treatment for the thus introduced impurity is conducted, to form source/drain regions  17  in a surface layer of the semiconductor substrate  1 . Incidentally, the ion implantation here is conducted by using a resist pattern as a mask, to introduce an n-type impurity into a region for forming an n-type MOS transistor and to introduce a p-type impurity into a region for forming a p-type MOS transistor. For example, arsenic as an n-type impurity is introduced into the region for forming the n-type MOS transistor by ion implantation at an implantation energy of 20 keV and in a dose of 1E15 atoms/cm 2 , and the activating heat treatment for the impurity is conducted by RTA at 1000° C. for 10 sec. 
     Subsequently, as shown in  FIG. 5C , the disposable side walls  41   a  are removed by isotropic etching. Thereafter, an impurity is introduced into the exposed surface layer of the semiconductor substrate  1  by ion implantation conducted by using the gate electrode  5   a  and the first side walls  9   a  as a mask. Next, an activating heat treatment for the thus introduced impurity is conducted, to form a low-dose diffusion layer (LDD)  13  at the surface layer of the semiconductor substrate  1 . Incidentally, the ion implantation here is conducted by using a resist pattern as a mask, to introduce an n-type impurity into a region for forming an n-type MOS transistor and to introduce a p-type impurity into a region for forming a p-type MOS transistor. For example, arsenic as an n-type impurity is introduced into the region for forming the n-type MOS transistor by ion implantation at an implantation energy of 1 keV and in a dose of 1E14 atoms/cm 2 , and the activating heat treatment for the impurity is conducted by RTA at 950° C. for 5 sec. 
     Thereafter, the same procedure as described using  FIG. 2D  and the latter figures in the first embodiment above is conducted. 
     Specifically, first, as shown in  FIG. 5D , a silicide layer  19  is formed at an exposed surface layer of the semiconductor substrate  1 . 
     Next, as shown in  FIG. 6A , a stopper film  21  including silicon nitride is built up so as to cover the gate electrode  5   a , the first side walls  9   a  and the silicide layers  19 . Further, an interlayer insulating film  23  including silicon oxide is built up, followed by CMP, to expose the stopper film  21  and flatten the interlayer insulating film  23 . 
     Subsequently, as shown in  FIG. 6B , RIE of the stopper film  21  and the side walls  9   a ,  11   a  which include silicon nitride is conducted to expose a surface of the gate electrode  5   a . Then, as shown in  FIG. 6C , a metal film  25  of nickel (Ni) or the like is formed in contact with the gate electrode  5   a.    
     Thereafter, as shown in  FIG. 6D , the gate electrode  5   a  formed by patterning a polysilicon film is fully silicided by a heat treatment such as RTA, to obtain an n-type and a p-type MOS transistor Tr having the fully silicided gate electrode  5   a′.    
     Next, as shown in  FIG. 7A , the metal film  25  and the interlayer insulating film  23  are etched away, and, further, the stopper film  21  including silicon nitride is removed, to expose the silicide layer  19  at the surface of the semiconductor substrate  1 . In this case, the side walls  9   a  including silicon nitride are also reduced in film amount, so that the side walls  21   a  including silicon nitride are left at the lower side of side walls of the fully silicided gate electrode  5   a′.    
     Thereafter, as shown in  FIG. 7B , a stress film  27  (dual stress liner film) is formed over the semiconductor substrate  1  so as to cover the fully silicided gate electrode  5   a ′ and the side walls  21   a , and, further, a stopper film  29  including silicon oxide is formed. 
     After the above steps, as shown in  FIG. 7C , an interlayer insulating film  31  including silicon oxide is formed over the stopper film  29 , flattening by CMP is conducted if required, and then connection holes  31   a  reaching the silicide layer  19  are formed in the interlayer insulating film  31 , the stopper film  29  and the stress film  27 . Then, wiring conductors  33  connected to the source/drain regions  17  through the connection hole  31  and the silicide layer  19  are formed, to complete a semiconductor device  43 . 
     The semiconductor device  43  thus completed has the gate insulating film  3 , which is broader than the fully silicided gate electrode  5   a ′, between the fully silicided gate electrode  5   a ′ and the semiconductor substrate  1 . 
     Also in the above-described second embodiment, as has been described using  FIG. 6D  above, the full siliciding of the gate electrode  5   a  is conducted in the condition where the continuous gate insulating film  3  is present beneath the gate electrode  5   a  formed by patterning a polysilicon film and the first side walls  9   a  at both sides of the gate electrode  5   a , i.e., in the absence of an interface that rectilinearly connects the gate electrode  5   a  and the semiconductor substrate  1  to each other. Therefore, the same effects as in the first embodiment can be obtained. 
     Besides, in the second embodiment, as has been described using  FIGS. 5B and 5C , the low-dose diffusion layer  13  is formed after the formation of the source/drain regions  17  needing a heat treatment at a higher temperature for activation of an impurity, by use of the disposable side walls  41   a . This ensures that the low-dose diffusion layer  13  is not affected by the heat treatment for forming the source/drain regions  17 , so that the low-dose diffusion layer  13  can be formed with high accuracy. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.