Abstract:
A semiconductor device comprises: a semiconductor substrate; a gate insulating film formed on the top surface of the semiconductor substrate; a gate electrode formed on the gate insulating film; diffusion layers formed in the semiconductor substrate to be used a source layer and a drain layer; and a silicide layer formed to overlie the diffusion layers; wherein an oxygen concentration peak, where oxygen concentration is maximized, is at a level lower than said top surface in a cross-section taken along a plane perpendicular to said top surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application(s) No(s). 2002-23548, filed on Jan. 31, 2002, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a semiconductor device and a method of manufacturing a semiconductor device.  
           [0004]    2. Related Background Art  
           [0005]    Integrated circuits having MOS transistors are becoming more and more enhanced in terms of microminiaturization and operation speed. To prevent short-channeling effects such as punch-through along with the microminiaturization of MOS transistors, relatively shallow source and drain diffusion layers are formed.  
           [0006]    To ensure high-speed operations of MOS transistors, the SALCIDE (Self-Aligned Silicide) technique is frequently used, as it reduces the contact resistance between the diffusion layers and a metal by forming a silicide layer on the diffusion layers in self-alignment. In the SALICIDE technique, silicide is formed by the interaction between the deposited metal and silicon as the substrate material. Therefore, in case a metal is directly deposited on shallow source and drain diffusion layers, silicide often appears after being downwardly thrust through the diffusion layers. As a result, leakage occurs between the source and drain diffusion layers and the substrate.  
           [0007]    As a countermeasure, the Elevated Source-Drain technique has been developed. This is a technique that forms a silicide layer by depositing a metal on a silicon single-crystal layer selectively formed on the source and drain regions. Since the silicon of the silicon single-crystal layer interacts with the metal and forms the suicide, the silicide does not excessively erode the source or drain diffusion layers. Therefore, it was expected that the downward penetration of the silicide through the source or drain diffusion layer was prevented.  
           [0008]    In the Elevated Source-Drain technique, silicon is epitaxially grown on the source and drain diffusion layers, which are limited regions of the entire surface of the semiconductor substrate. In order to obtain a sufficiently thick silicon single-crystal layer by the epitaxial growth process, the vapor-phase epitaxy (VPE) technique needs annealing at a high temperature not lower than 800° C.  
           [0009]    Such high-temperature annealing, however, causes thermal diffusion of impurities in the source and drain diffusion layers. In the epitaxial growth process, excessive diffusion of these diffusion layers may invite the short-channeling effect in miniaturized MOS transistors. Therefore, high-temperature annealing of semiconductor substrates is not desirable after sources and drains are formed.  
           [0010]    Apart from this, there is the Solid Phase Epitaxy (SPE) technique that first deposits amorphous silicon on a semiconductor substrate and thereafter anneals it at approximately 600° C. to change the silicon to single crystal. Even with the Solid Phase Epitaxy, a silicon single-crystal layer can be formed on source and drain diffusion layers. When annealing is carried out at a relatively low temperature around 600° C., thermal diffusion of source and drain diffusion layers is immaterial.  
           [0011]    However, even in Solid Phase Epitaxy, if a silicon oxide exists on the semiconductor substrate, amorphous silicon deposited on the silicon oxide sometimes fails to change to single crystal. In this case, amorphous silicon on the source and drain diffusion layers can change to single crystal only partly and insufficiently for use in the Elevated Source-Drain technique. As a result, in a step of selectively etching the amorphous silicon deposited on the top surface of the semiconductor substrate, the silicon having failed to change to single crystal on source and drain regions is undesirably etched simultaneously. Therefore, this technique could not make the best use of the Elevated Source-Drain technique.  
           [0012]    Especially when the semiconductor substrate is a p-type substrate containing an impurity such as boron, because it is easily oxidized, amorphous silicon deposited on the top surface of the p-type semiconductor substrate containing boron, or the like, is difficult to single-crystallize sufficiently.  
           [0013]    These and other problems involved in the conventional techniques are discussed below with reference to the drawings.  
           [0014]    [0014]FIGS. 20 through 24 are cross-sectional views that show a semiconductor substrate in an enlarged form to demonstrate a conventional method of manufacturing a semiconductor device in the order of its procedures.  
           [0015]    As shown in FIG. 20, an isolating region  30  is formed in the semiconductor substrate  10 . The substrate  10  has formed a gate insulating film  40  on its top surface and a gate electrode  60  on the gate insulating film  40 . A sidewall protective layer  85  is formed on the sidewall of the gate electrode. The semiconductor substrate  10  further includes diffusion layers  70 ,  72  as source and drain layers.  
           [0016]    The top surface of the semiconductor substrate  10  in the regions of the diffusion layers  70 ,  72  are exposed to epitaxially grow a silicon single-crystal layer thereon. However, the top surface of the semiconductor substrate  10  is oxidized when contacting air, and a silicon oxide  90  is produced on the top surface of the semiconductor substrate.  
           [0017]    As shown in FIG. 21, an amorphous silicon layer  100  is deposited on the top surface of the semiconductor substrate and on the gate electrode  60 .  
           [0018]    As shown in FIG. 22, the amorphous silicon layer  100  is annealed. However, the silicon oxide  90  exists between the top surface of the semiconductor substrate  10  and the amorphous silicon layer  100 , and locally prevents the amorphous silicon layer  90  from direct contact with the top surface of the semiconductor substrate  10 . Since the amorphous silicon layer  100  can epitaxially grow only along the crystal on the top surface of the semiconductor substrate  10 , part of the amorphous silicon layer  100  not contacting the top surface of the semiconductor substrate  10  cannot grow epitaxially even when it is annealed. As a result, the silicon single-crystal layer  120  transformed from the amorphous silicon layer  100  by annealing does not become uniform in thickness and quality on the top surface of the semiconductor substrate  10 .  
           [0019]    As shown in FIG. 23, as a result of etching by making use of the difference in etching rate between the silicon single-crystal layer and the amorphous or polycrystalline silicon, the amorphous silicon  100  and the polycrystalline silicon transformed from the amorphous silicon  100  are etched, and the silicon single-crystal layer  120  remains.  
           [0020]    As shown in FIG. 24, a metal acts on the silicon deposited on the semiconductor substrate  10 , as a result, a silicide layer  130  is formed. In regions where the silicon single-crystal layer  120  is thin, the deposited metal acts not only on the silicon of the silicon single-crystal layer  120  but also on the silicon of the diffusion layers  70 ,  72 . Therefore, the diffusion layers  70 ,  72  are excessively encroached by the silicide layer  130 , which may grow even beyond the diffusion layers  70 ,  72 . Thus, the advantage of the Elevated Source-Drain technique is not harnessed sufficiently.  
           [0021]    Here is needed a semiconductor device manufacturing method capable of forming a silicon single-crystal layer acceptable for use with the Elevated Source-Drain technique on source and drain diffusion layers at a relatively low temperature.  
           [0022]    Additionally needed is a semiconductor device having a silicide layer formed by the Elevated Source-Drain technique and uniform in thickness and quality, keeping the contact resistance low between the source and drain diffusion layers on one part and source and drain electrodes on the other part, and available for more progressed microminiaturization than a conventional one.  
         BRIEF SUMMARY OF THE INVENTION  
         [0023]    A semiconductor device according to an embodiment of the invention comprises: a semiconductor substrate; a gate insulating film formed on the top surface of the semiconductor substrate; a gate electrode formed on the gate insulating film; diffusion layers formed in the semiconductor substrate to be used a source layer and a drain layer; and a silicide layer formed to overlie the diffusion layers;  
           [0024]    wherein an oxygen concentration peak, where oxygen concentration is maximized, is at a level lower than said top surface in a cross-section taken along a plane perpendicular to said top surface.  
           [0025]    A method of manufacturing a semiconductor device according to an embodiment of the invention comprises: forming a gate insulating film on the top surface of a semiconductor substrate; forming a gate electrode on the gate insulating film; forming diffusion layers in a self-aligned manner in the semiconductor substrate on opposite sides of the gate electrode; forming an amorphous layer on the top surface of the semiconductor substrate above the diffusion layers; implanting ions of an injection substance into the semiconductor substrate through an interface thereof with the amorphous layer; annealing the semiconductor substrate at a relatively low temperature to partly change the amorphous layer to a single-crystal layer; and sputtering a metal onto the single-crystal layer and thereby forming a silicide layer from the single crystal and the metal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is an enlarged cross-sectional view of a semiconductor substrate under a process of a semiconductor device manufacturing method according to the first embodiment of the invention;  
         [0027]    [0027]FIG. 2 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG.  1  in the semiconductor device manufacturing method according to the first embodiment;  
         [0028]    [0028]FIG. 3 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 2 in the semiconductor device manufacturing method according to the first embodiment;  
         [0029]    [0029]FIG. 4 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 3 in the semiconductor device manufacturing method according to the first embodiment;  
         [0030]    [0030]FIG. 5 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 4 in the semiconductor device manufacturing method according to the first embodiment;  
         [0031]    [0031]FIG. 6 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 5 in the semiconductor device manufacturing method according to the first embodiment;  
         [0032]    [0032]FIG. 7 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 6 in the semiconductor device manufacturing method according to the first embodiment;  
         [0033]    [0033]FIG. 8 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 7 in the semiconductor device manufacturing method according to the first embodiment;  
         [0034]    [0034]FIG. 9 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 8 in the semiconductor device manufacturing method according to the first embodiment;  
         [0035]    [0035]FIG. 10 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 9 in the semiconductor device manufacturing method according to the first embodiment;  
         [0036]    [0036]FIG. 11A is an enlarged cross-sectional view of a semiconductor device  200  manufactured by a semiconductor device manufacturing method according to the first embodiment;  
         [0037]    [0037]FIG. 11B is a graph showing changes of oxygen and germanium concentrations with depth from the top surface  12  of the semiconductor device  200 ;  
         [0038]    [0038]FIG. 12 is an enlarged cross-sectional view of a semiconductor substrate under a process of a semiconductor device manufacturing method according to the second embodiment of the invention;  
         [0039]    [0039]FIG. 13 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 12 in the semiconductor device manufacturing method according to the second embodiment;  
         [0040]    [0040]FIG. 14 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 13 in the semiconductor device manufacturing method according to the second embodiment;  
         [0041]    [0041]FIG. 15 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 14 in the semiconductor device manufacturing method according to the second embodiment;  
         [0042]    [0042]FIG. 16 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 15 in the semiconductor device manufacturing method according to the second embodiment;  
         [0043]    [0043]FIG. 17 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 16 in the semiconductor device manufacturing method according to the second embodiment;  
         [0044]    [0044]FIG. 18 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 17 in the semiconductor device manufacturing method according to the second embodiment;  
         [0045]    [0045]FIG. 19 is an enlarged cross-sectional view of the semiconductor substrate under a process continuous from FIG. 18 in the semiconductor device manufacturing method according to the second embodiment;  
         [0046]    [0046]FIG. 20 is an enlarged cross-sectional view of a semiconductor substrate under a process of a conventional semiconductor device manufacturing process;  
         [0047]    [0047]FIG. 21 is an enlarged cross-sectional view of a semiconductor substrate under a process of a conventional semiconductor device manufacturing process;  
         [0048]    [0048]FIG. 22 is an enlarged cross-sectional view of a semiconductor substrate under a process of the conventional semiconductor device manufacturing process;  
         [0049]    [0049]FIG. 23 is an enlarged cross-sectional view of a semiconductor substrate under a process of the conventional semiconductor device manufacturing process; and  
         [0050]    [0050]FIG. 24 is an enlarged cross-sectional view of a semiconductor substrate under a process of the conventional semiconductor device manufacturing process. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    Some embodiments of the invention will now be explained below with reference to the drawings. However, these embodiments should not be construed to limit the invention, and respective components shown in the drawings may not accord with their scale.  
         [0052]    [0052]FIGS. 1 through 10 are enlarged cross-sectional views of a semiconductor substrate for sequentially showing a semiconductor device manufacturing method according to the first embodiment of the invention. In this embodiment, a p-type MOS transistor is manufactured.  
         [0053]    As shown in FIG. 1, an n-type well region  20  is formed by introducing arsenic, phosphorus or other impurity into a semiconductor substrate  10  and annealing it. In this embodiment, the depth of the n-type well region  20  from the top surface  12  of the semiconductor substrate  10  is about 1 μm.  
         [0054]    After that, an isolation  30  is formed by burying an oxide in a predetermined region. In this embodiment, the isolation  30  is made by the STI (shallow trench isolation) technique. The depth of the isolation  30  from the top surface  12  of the semiconductor substrate  10  is about 400 nm.  
         [0055]    As shown in FIG. 2, a substrate protective oxide film  48  is next formed on the top surface  12  of the semiconductor substrate  10 . The substrate protective oxide film  48  is used to protect the substrate  10  against the impulse of subsequent channel-ion injection  58 . In this embodiment, the thickness of the substrate protective oxide film  48  is about 10 nm. Thereafter, channel-ion injection  58  is carried out to adjust the threshold voltage of the MOS transistor.  
         [0056]    As shown in FIG. 3, after the substrate protective oxide film  48  is next removed, a gate insulating film  40  is formed on the top surface  12  of the semiconductor substrate  10 . The thickness of the gate insulating film  40  is approximately several nanometers. The gate insulating film  40  may be a silicon oxide film, but also usable is an oxynitride film made by introducing several % of nitrogen into a silicon oxide film, an high dielectric constant such as TaO 2 , ZrO x , HfO x  (where x is a positive integer), or any of their silicate film.  
         [0057]    After that, polycrystalline silicon is deposited on the gate insulating film  40  by CVD (chemical vapor deposition), for example. Thereafter, a gate electrode  60  is formed by patterning the deposited polycrystalline silicon by photolithography. In the instant embodiment, the thickness of the gate electrode  60  is around 150 nm.  
         [0058]    As shown in FIG. 4, ion injection  75  is next carried out to form a diffusion layer  70 . The diffusion layer  70  is formed in a self-aligned manner in opposite sides of the gate electrode  60  by introducing ions to the top surface  10  of the semiconductor substrate  10  through the gate insulating film  40 .  
         [0059]    The diffusion layer  70  is used as a source layer or a drain layer, and may be used as a part of a LDD (lightly doped drain) structure. In the instant embodiment, the diffusion layer  70  is used as an extension layer for making the LDD structure doubling the source or drain layer. By using the source or drain layer of the LDD structure, generation of hot electrons and a short-channel effect can be prevented.  
         [0060]    In the instant embodiment, the impurity used for extension ion injection  75  may be boron, for example. The dose of boron may be approximately 5×10 14  cm −2 , for example, and the injection energy is approximately 10 keV, for example. Therefore, the diffusion layer  70  has a p-type conductivity. The depth of the diffusion layer from the top surface  12  is around 40 nm.  
         [0061]    Thereafter, a silicon oxide film is deposited to cover the top surface  12  and the gate electrode  60 , and a silicon nitride film is deposited thereon. Both the silicon oxide film and the silicon nitride film may be deposited by LP-CVD, for example. The silicon oxide film is used as a liner layer having the role of stopping etching when the silicon nitride film is etched.  
         [0062]    As shown in FIG. 5, the silicon nitride film and the silicon oxide film are selectively removed by anisotropic etching, and they partly remain as a sidewall liner layer  80  and a sidewall protective layer  85  on the sidewall of the gate electrode  60 . The sidewall liner layer  80  and the sidewall protective layer  85  are approximately 5 nm thick and 20 nm thick, respectively, for example.  
         [0063]    The sidewall liner layer  80  and the sidewall protective layer  85  protect the sidewall of the gate electrode  60 , and also function as a spacer during ion injection for forming source and drain diffusion layer  72 . That is, the sidewall protective layer  85  makes the source and drain layer  72  implanted in a self-aligned manner. Thereby, the diffusion layers  70 ,  72  form a LDD structure. In the instant embodiment, depth of the diffusion layer  72  is approximately 50 nm.  
         [0064]    After the silicon oxide film and the silicon nitride film are removed, the diffusion layer  70  or  72  on the top surface of the semiconductor substrate  10  is exposed. The crystal surface of the top surface  12  of the semiconductor substrate  10  assists epitaxial growth of a silicon single-crystal layer on the top surface  12 .  
         [0065]    On the other hand, exposure of the crystal surface of the top surface  12  of the semiconductor substrate  10  to air causes a silicon oxide  90  to be produced by oxidation of silicon on the top surface.  
         [0066]    As shown in FIG. 6, an amorphous silicon layer  100  is then deposited over the exposed top surface  12  and the gate electrode  60 . The amorphous silicon  100  is formed by LP-CVD, for example, using silane (SiH 4 ) in an atmosphere held at approximately 600° C. In the instant embodiment, the thickness of the amorphous silicon is about 50 nm.  
         [0067]    As shown in FIG. 7, ions are injected into the semiconductor substrate  10  through its interface with the amorphous silicon  100 . Injection material for this ion injection  110  may be, for example, germanium, arsenic, boron, argon, which is an inactive substance, or any of their congener elements. In the instant embodiment, germanium is used as the injection material for the ion injection  110 . The quantity of the injection material for the ion injection  110  may be, for example, approximately 1×10 15  cm −2 , and the injection energy is about 7 keV, for example.  
         [0068]    Germanium ions accelerated by the ion injection  110  break through to the silicon oxide  90  through the amorphous silicon layer  100 , and push oxygen contained in the silicon oxide  90  from the interface between the semiconductor substrate  10  and the amorphous silicon layer  100  to below the top surface  12  of the semiconductor substrate  10 . That is, injected germanium ions knock against interfacial oxygen existing along the interface between the semiconductor substrate  10  and the amorphous silicon layer  100 , pushing it below the top surface  12 .  
         [0069]    The dose of germanium is determined by the quantity of the silicon oxide  90  or interfacial oxygen. The quantity of the silicon oxide  90  or interfacial oxygen depends on various conditions upon exposing the top surface  12  of the semiconductor substrate  10  to air, such as, the duration of time of exposure of the top surface  12  to air, the temperature, the oxygen concentration in the ambient air, and so on. Normally, these conditions are maintained constant throughout the manufacturing process of the semiconductor device. Therefore, the dose of germanium may be determined in accordance with the conditions in the manufacturing process of the semiconductor device.  
         [0070]    In the instant embodiment, the quantity of interfacial oxygen between the semiconductor substrate  10  and the amorphous silicon layer  100  is deemed to be about 1×10 15  cm −2 . Thus the dose of germanium is 1×10 15  cm −2 , equal to the quantity of the interfacial oxygen. For the purpose of reliably knocking more interfacial oxygen below the top surface  12 , the dose of germanium is preferably equal to or more than the interfacial oxygen existing along the interface between the semiconductor substrate  10  and the amorphous silicon layer  100 .  
         [0071]    In contrast, for the purpose of preventing the semiconductor substrate  10  from excessive damage, the dose of germanium may be less than the quantity of the interfacial oxygen.  
         [0072]    Injection energy of germanium must be large enough for germanium to penetrate the amorphous silicon layer  100 . On the other hand, germanium and oxygen may cause a leakage at the junction between the diffusion layer  72  and the well region  20  if they are injected or knocked deeper than the depth of the diffusion layer  72 . Therefore, injection energy of germanium is preferably limited to a level prohibiting germanium from penetrating the diffusion layer  72 .  
         [0073]    In the instant embodiment, germanium or oxygen is preferably injected or knocked shallower than the depth of the diffusion layer  72 . However, in case the device does not include the diffusion layer  72  as the source and drain layer and only includes the diffusion layer  70  as the extension layer, germanium and oxygen are preferably injected or knocked shallower than the depth of the diffusion layer  70 . In this case, since the diffusion is shallower than the diffusion layer  72 , injection energy of germanium is adjusted to be lower than the injection energy used in this embodiment.  
         [0074]    As shown in FIG. 8, the amorphous silicon layer  100  is annealed. As a result of this annealing, the amorphous silicon layer  100  on the diffusion layers  70 ,  72  is epitaxially grown to form a silicon single-crystal layer  120 . That is, in the instant embodiment, to obtain the silicon single-crystal layer  120 , the SPE technique is used. In the instant embodiment, the annealing is carried out in a hydrogen atmosphere held at approximately 600° C. in an LP-CVD apparatus.  
         [0075]    At the time of annealing, interfacial oxygen is already knocked below the top surface of the semiconductor substrate  10 , and the silicon oxide  90  no longer exists between the semiconductor substrate  10  and the amorphous silicon layer  100 . Therefore, the entirety of the amorphous silicon layer  100  is in contact with silicon crystals on the top surface  12  of the source and drain diffusion layers  70 ,  72 . As a result, the amorphous silicon layer  100  can epitaxially grow with sufficient thickness and uniform quality on the diffusion layers  70 ,  72  and can change to the silicon single-crystal layer  120 .  
         [0076]    On the other hand, top surfaces of the device-isolating portion  30 , gate electrode  60  and sidewall protective layer  85  are made of a silicon oxide, polycrystalline silicon and silicon nitride, respectively. Therefore, the amorphous silicon layer  100  does not epitaxially grow on the device-isolating portion  30 , gate electrode and sidewall protective layer  85 , and remains as the amorphous silicon layer or changes to a polycrystalline silicon layer.  
         [0077]    As shown in FIG. 9, the layer  100 ′ of amorphous silicon and polycrystalline silicon is selectively etched relative to the silicon single-crystal layer  120 . In this embodiment, this etching is carried out by LP-CVD using chlorine gas diluted to approximately 10% by hydrogen within the same chamber as that used for deposition of the amorphous silicon layer  100 . Etching selectivity of amorphous silicon relative to single-crystal silicon is 10 or more.  
         [0078]    In the instant embodiment, a common chamber is used both for epitaxial growth of the silicon single-crystal layer  120  and for selective etching of the amorphous silicon layer and the polycrystalline silicon layer  100 ′. This contributes to shortening the manufacturing process of the semiconductor device, enhancing the productivity and reducing the manufacturing cost. Additionally, the quality of the silicon single-crystal layer  120  is improved.  
         [0079]    Even when different chambers are used for those steps, substantially the same effect is obtained by using a so-called cluster tool and carrying out a series of epitaxial growth, selective etching, and so on.  
         [0080]    When the amorphous silicon layer and the polycrystalline silicon layer  100 ′ are selectively etched, the sidewall of the gate electrode  60  is protected by the sidewall liner layer  80  and the sidewall protective layer  85 . Therefore, the sidewall of the gate electrode  60  is not etched. The top surface of the gate electrode  60  is in direct contact with the polycrystalline silicon layer  100 ′. Since the gate electrode  60  is made of polycrystalline silicon which is same as the polycrystalline layer  100 ′, it is immaterial that the polycrystalline layer  100 ′ is not removed completely but partly remains. On the other hand, since the gate electrode  60  is sufficiently thick relative to the amorphous silicon layer and the polycrystalline silicon layer  100 ′, it is acceptable that the top surface of the gate electrode  60  is over-etched slightly.  
         [0081]    As shown in FIG. 10, a metal is next deposited on the silicon single-crystal layer  120 . This metal may be, for example, cobalt, nickel, titanium, or the like. The deposited metal acts on silicon of the silicon single-crystal layer  120  and forms a silicide layer  130  used for reducing the contact resistance.  
         [0082]    Since the metal interacts with silicon of the silicon single-crystal layer  120 , it does not erode silicon in the diffusion layers  70 ,  72  underlying the top surface  12  of the semiconductor substrate  10 . Even if the metal erodes the diffusion layers  70 ,  72 , the quantity of the eroded silicon in the diffusion layers  70 ,  72  is quite small. Therefore, the silicide layer  130  does not protrude through the bottom of the diffusion layers  70 ,  72 . Thus leakage does not occur between the source and drain diffusion layers  70 ,  72  and the substrate  10  or well region  20 . That is, this embodiment can attain sufficient effects of the Elevated Source-Drain technique.  
         [0083]    Through some subsequent steps (not shown), including the step of forming a contact and a step of forming a interconnections, the semiconductor device according to the instant embodiment is completed.  
         [0084]    As explained above, the semiconductor device manufacturing method according to this embodiment does not anneal the semiconductor substrate  10  at 600° C. or higher temperatures after forming the diffusion layers  70 ,  72 . Therefore, the embodiment can form the diffusion layers  70 ,  72  relatively shallow from the top surface  12  of the semiconductor substrate  10 , and can prevent punch-through or other short channel effect even when the semiconductor substrate is downsized extremely.  
         [0085]    Next explained is the configuration of the semiconductor substrate  200  made by the manufacturing method according to the first embodiment.  
         [0086]    [0086]FIG. 11A is an enlarged cross-sectional view of the semiconductor device  200  manufactured by the semiconductor device manufacturing method according to the first embodiment. The semiconductor device  200  according to this embodiment includes the semiconductor substrate  10 ; gate insulating film  40  formed on the top surface  12  of the semiconductor substrate  10 ; and gate electrode  60  formed on the gate insulating film  40 . In a part of the semiconductor substrate  10  on one side of the gate electrode  60 , the source-side extension layer  70   a  connected to the source electrode (not shown) is formed in a self-aligned manner making use of the sidewall of the gate electrode  60 . Similarly, in another part of the semiconductor substrate  10  on the other side of the gate electrode  60 , the drain-side extension layer  70   b  connected to the drain electrode (not shown) is formed in a self-aligned manner making use of the sidewall of the gate electrode  60 .  
         [0087]    On the gate electrode  60 , the sidewall protective layer  85  lies via the liner layer  80  to protect the gate electrode  60 . In a region of the semiconductor substrate on one side of the gate electrode  60 , the source layer  72   a  is formed in a self-aligned manner using the sidewall protective layer  85  as a spacer. Similarly, in another region of the semiconductor substrate  10  on the other part of the gate electrode  60 , the drain layer  72   b  is formed in a self-aligned manner using the sidewall protective layer  85  as a spacer.  
         [0088]    The instant embodiment includes both the source-side extension layer  70   a  plus the drain-side extension layer  70   b  (hereinbelow collectively called diffusion layer  70  as well) and the source layer  72   a  plus the drain layer  72   b  (hereinbelow collectively called diffusion layer  72  as well). However, even when the semiconductor device has only one of diffusion layer  70  or  72 , the effects of the embodiment of the invention will be maintained.  
         [0089]    The semiconductor device  200  further includes a silicide layer  130  overlying the diffusion layer  70  or  72 . The silicide layer  130  is preferably connected directly to the diffusion layers  70 ,  72  to reduce the contact resistance between the diffusion layers  70 ,  72  and the source or drain electrode.  
         [0090]    However, for the purpose of completely preventing silicon in the diffusion layers  70 ,  72  from erosion in the process of forming the silicide layer  130 , a silicon single-crystal layer  120  may reside between the silicide layer  130  and the diffusion layers  70 ,  72 . In this case, the silicon single-crystal layer interposed between the silicide layer  130  and the diffusion layers  70 ,  72  are doped with an impurity.  
         [0091]    [0091]FIG. 11B is a graph showing changes of oxygen and germanium concentrations with depth from the top surface  12  of the semiconductor device  200 . Let the depth of the top surface  12  be 0 (zero). Then the depth of the oxygen concentration peak, where the oxygen concentration is maximized, and the depth of the germanium concentration peak, where the germanium concentration is maximized, is denoted by d 1 , and the depth of the diffusion layer  72  is denoted by d 2 .  
         [0092]    According to the graph of FIG. 11B, the oxygen concentration peak and the germanium concentration peak are in a level lower than the top surface  12 . Germanium and interfacial oxygen are injected or knocked to substantially the same depth d 1  from the top surface  12 . Therefore, the depth of the oxygen concentration peak from the top surface  12  of the semiconductor device  10  is approximately equal to the depth of the germanium concentration peak from the top surface  12  of the semiconductor substrate  10 .  
         [0093]    Energy for injection of germanium is adjusted to prohibit germanium and oxygen from penetrating the diffusion layer  72  and reaching the n well  20 . Therefore, according to the instant embodiment, both the depth d 1  of the oxygen concentration peak and the depth d 1  of the germanium concentration peak are shallower than the depth d 2  of the diffusion layer  72 .  
         [0094]    As explained above, the dose of germanium is determined by the quantity of interfacial oxygen. If a larger quantity of germanium than interfacial oxygen is injected, then the concentration of germanium contained in each unit surface area of the semiconductor substrate  10  is equal to or larger than the concentration of oxygen contained in each unit surface area of the semiconductor substrate  10 . That is, the value of the germanium concentration peak is equal to or larger than the value of the oxygen concentration peak.  
         [0095]    In the instant embodiment, the dose of germanium is substantially equal to the quantity of interfacial oxygen. Therefore, In FIG. 11B, the peak value of germanium concentration is approximately equal to the peak value of oxygen concentration. As a result, germanium can knock substantially all interfacial oxygen without damaging the top surface  12  excessively.  
         [0096]    The oxygen concentration being substantially zero on the top surface  12  demonstrates that the silicon oxide does not exist on the top surface  12 . Therefore, the silicon single-crystal layer grows with a sufficient thickness and uniform quality on the diffusion layers  70 ,  72 . The sufficiently thick and uniform-quality silicon single-crystal layer contributes to forming a sufficiently thick and uniform silicide layer  130  without eroding silicon in the diffusion layers  70 ,  72  excessively.  
         [0097]    In case the semiconductor device  200  is downsized, it needs diffusion layers  70 ,  72  higher in impurity concentration and shallower in structure. In such a case, the instant embodiment can fabricate a silicide layer  130  that maintains a low contact resistance without eroding the shallow diffusion layers  70 ,  72 .  
         [0098]    Thus the semiconductor device according to the embodiment can overcome the short-channel effect, an increase of the contact resistance and other problems caused by microminiaturization.  
         [0099]    [0099]FIGS. 12 through 19 are enlarged cross-sectional views of a semiconductor substrate under different, sequential processes of a semiconductor device manufacturing method according to the second embodiment of the invention. The same components as those of the semiconductor substrate according to the first embodiment are labeled with the same reference numerals.  
         [0100]    The second embodiment has a difference from the first embodiment in forming a top surface protective layer  88  on the top surface of the gate electrode  60  (FIGS. 13 through 19).  
         [0101]    As shown in FIG. 12, the n-well region  20 , device-isolating portion  30  and gate insulating film  40  are formed in the same manner as the first embodiment, and a polycrystalline silicon layer  65  is formed on the gate insulating film  40 .  
         [0102]    As shown in FIG. 13, a silicon nitride film  88  is next formed by depositing a silicon nitride and next patterning it by using photolithography. In the instant embodiment, the thickness of the silicon nitride film  88  is approximately 50 nm.  
         [0103]    As shown in FIG. 14, next using the silicon nitride film  88  as a mask, the polycrystalline silicon layer  65  is etched to form the gate electrode.  
         [0104]    As shown in FIG. 15, the liner layer  80 , sidewall protective layer  85  and diffusion layers  70 ,  72  are formed in the same manner as the first embodiment. Additionally, the amorphous silicon layer  100  is formed on the top surface  12  of the silicon substrate  10  and the gate electrode  60 . Here again, the silicon oxide  90  is produced between the top surface  12  and the amorphous silicon layer  100 .  
         [0105]    As shown in FIG. 16, germanium ions are next injected into the semiconductor substrate through its interface with the amorphous silicon  100 . Thereby, interfacial oxygen is knocked downward of the top surface  12  of the semiconductor substrate  10 .  
         [0106]    As shown in FIG. 17, the semiconductor substrate  10  is annealed at a temperature around 600° C. Since the interfacial oxygen is already knocked below the top surface of the semiconductor substrate  10 , the amorphous silicon layer  100  can change to the silicon single-crystal layer with sufficient thickness and uniform quality on the diffusion layers  70 ,  72 .  
         [0107]    On the other hand, the amorphous silicon layer  100  does not epitaxially grow on the device-isolating portion  30 , sidewall protective layer  85  and top surface protective layer  88 , and it remains in the amorphous phase, or changes to a polycrystalline silicon layer.  
         [0108]    As shown in FIG. 18, the amorphous silicon layer or polycrystalline silicon layer  100 ′ is next etched selectively relative to the silicon single-crystal layer  120 .  
         [0109]    In this embodiment, the top surface protective layer  88  prevents the gate electrode  60  from being etched. That is, the top surface protective layer  88  functions as an etching-stopper. As a result, while the gate electrode  60  is not etched, the amorphous silicon layer  100 ′ is sufficiently etched. Therefore, the instant embodiment reliably prevents over-etching of the gate electrode  60  even when the gate electrode  60  is relatively thin.  
         [0110]    In the instant embodiment, the etching of the amorphous silicon layer  100  or polycrystalline silicon layer  100 ′ may be carried out at 700° C. or a higher temperature. This contributes to increasing the etching speed, and reducing the time for the etching step of the amorphous silicon layer or polycrystalline silicon layer  100 ′ than that in the first embodiment. Thus the second embodiment enhances the productivity of the semiconductor device and reduces its manufacturing cost.  
         [0111]    As shown in FIG. 19, a metal is deposited on the silicon single-crystal layer  120  to form the silicide layer  130  in the same manner as the first embodiment.  
         [0112]    Through further steps, including the step of forming the contact and the step of forming the interconnections (not shown), the semiconductor device according to the instant embodiment is completed.  
         [0113]    The second embodiment also has the same effects as those of the first embodiment. The second embodiment, which protects both the sidewall and the top surface of the gate electrode, need not take account of over-etching of the gate electrode  60 . Additionally, the second embodiment has another effect, namely, shortening the time required for etching the amorphous silicon layer or polycrystalline silicon layer  100 ′ than that of the first embodiment.  
         [0114]    Even when replacing n-type semiconductors with p-type semiconductors and replacing p-type semiconductors with n-type semiconductors, effects of the second embodiment remain.  
         [0115]    The semiconductor manufacturing method according to any of the foregoing embodiments can form the silicon single-crystal layer available for use with the Elevated Source-Drain technique on the source and drain diffusion layers at a relatively low temperature.  
         [0116]    The semiconductor device according to any of the foregoing embodiments has the silicide layer made by the Elevated Source-Drain technique to be uniform in thickness and quality, and it is available for more enhanced microminiaturization than existing semiconductor devices while maintaining low contact resistance between the source and drain diffusion layers and the source and drain electrodes.