Abstract:
A semiconductor device includes an insulating gate field effect transistor including a gate electrode, wherein the gate electrode includes a polycrystalline semiconductor film having a crystal defect density of about 1×10 18  cm −3  or less. In certain embodiments, the polycrystalline semiconductor film may be oxidation thermally annealed by subjecting the polycrystalline semiconductor film to thermal treatment in an oxidation atmosphere to carry out oxidization of the polycrystalline semiconductor film and activation of impurities simultaneously.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method for fabricating the same, and more particularly to a semiconductor device including a gate electrode or source/drain regions obtained by control of diffusion and activation of impurities in polycrystalline semiconductor film, and a method for fabricating the same. 
     2. Description of the Related Art 
     Recently, CMOS transistors having a dual-gate structure have been developed. The dual-gate structure is provided to prevent variation of the threshold voltage of a transistor as well as the short channel effect. This structure includes two surface-channel transistors, one of which is an NMOS transistor having a gate electrode containing an n-type impurity and the other of which is a PMOS transistor having a gate electrode containing a P-type impurity. 
     Japanese Laid-Open Publication No. 6-224380 discloses a method for doping a gate electrode of a conventional dual-gate structure CMOS transistor with an impurity and activating the impurity in the doped gate electrode.- Specifically, as shown in FIG. 1, a p − -well  106 , an n − -well  107 , a field oxide film (element-isolating region)  102 , and an inversion prevention layer  104  are provided on a semiconductor substrate  101  in a well-known way. A gate insulation film  105  (e.g., made of oxide film) is provided to cover those layers in a well-known way. A polycrystalline silicon film  103  to serve as a gate electrode is formed on the gate insulation film  105  with LPCVD. Thereafter, the polycrystalline silicon film  103  is patterned into the desired shape, and the source/drain regions and the gate electrode are doped with an impurity as a dopant by means of ion implantation. The resulting structure is then subjected to thermal treatment so as to activate the implanted dopant ions. 
     Japanese Laid-Open Publication No. 3-138930 discloses a transistor including a shallow junction which is provided in source/drain regions using a stacking structure for preventing the short channel effect which emerges as the transistor becomes smaller. FIG. 2 is a cross-sectional view showing diagrammatically a structure of the stacking-structure transistor disclosed the above-described publication. 
     In the conventional transistor having a structure as shown in FIG. 2, a gate electrode structure includes a gate insulating film  203 , a gate electrode  204 , and an insulating top layer  205 . The gate electrode structure is provided over a region which will be a channel region between regions which will be source/drain regions  207 . The gate electrode structure and those regions are positioned between field oxide films  202  (element-isolating regions) provided on a substrate  201 . Sidewalls  211  are provided on the sides of the gate structure. 
     To form the source/drain regions  207  in this structure, a polycrystalline silicon film is provided to cover the gate electrode structure. Thereafter, the polycrystalline silicon film is etched back to a level indicated reference numeral  206  shown in FIG.  2 . The polycrystalline silicon film  206  is doped with an impurity as a dopant and subjected to thermal treatment. The thermal treatment causes a solid phase diffusion so that the dopant diffuses from the polycrystalline silicon film  206  into the semiconductor substrate  201 , thereby producing the source/drain regions  207 . 
     After the formation of the source/drain regions  207 , a silicide film  208  and an inactive dielectric layer  209  are formed over the polycrystalline silicon film  206 , and a metal wire  210  is provided, resulting in the structure shown in FIG.  2 . 
     However, when attempting to fabricate the conventional dual-gate-structure CMOS transistor using the surface-channel transistor, the following problems arise. 
     Phosphorous and arsenic as n-type impurities have a lower diffusion rate and a lower activation ratio in the polycrystalline silicon than Boron as a p-type impurity. For this reason, carriers are adversely depleted from the gate insulating film side of the gate electrode. The depletion layer of the gate electrode has a capacitance which is added in series to the capacitance of the gate insulating film, resulting in a reduction in effective capacitance. This reduces a driving current for the transistor. The driving current varies depending on the degree of depletion. 
     In general, impurity implantation is simultaneously carried out both for the gate electrode and the source/drain regions to be formed in order to reduce the number of steps in formation of the dual-gate-structure CMOS transistor. In this case, preferably, the source/drain function has a shallow junction so as to prevent the short channel effect of the transistor. To this end, reduced energy of ion implantation is preferable. This is, however, likely to deplete carriers from the gate electrode. 
     As described above, there is a trade-off between prevention of depletion in the gate electrode and prevention of the short channel effect. The prevention of the short channel effect leads to the depletion in the gate electrode. On the other hand, when the gate electrode is doped under conditions for preventing the depletion, the junction in the source/drain regions become deep, resulting in an increase in the short channel effect. 
     On the other hand, the short channel effect is increased as the size of the transistor is decreased. To prevent the short channel effect in this situation, the stacking-structure transistor has a shallow junction which is formed in the source/drain region using the stacking structure. When the source/drain regions are provided with the stacking structure using polycrystalline silicon film, the following problems arise. 
     To form as shallow a junction as possible, the amount of impurities implanted into the polycrystalline silicon film is preferably reduced so as to avoid the influence of extraordinary accelerated diffusion from a high-concentration region. The reduced amount of impurities of the source/drain regions does not cause the source/drain regions to have a sufficiently lowered level of resistance, resulting in an increase in parasitic resistance. This leads to a decrease in a transistor current in operation. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a method for fabricating a semiconductor device including a polycrystalline semiconductor film containing impurities, includes the steps of introducing the impurities into the polycrystalline semiconductor film; and subjecting the polycrystalline semiconductor film to thermal treatment in an oxidization atmosphere to carry out oxidization of the polycrystalline semiconductor film and activation of the impurities simultaneously. 
     According to another aspect of the present invention, a method for fabricating a semiconductor device including a polycrystalline semiconductor film containing impurities, includes the steps of depositing the polycrystalline semiconductor film; oxidizing the polycrystalline semiconductor film; introducing the impurities into the oxidized polycrystalline semiconductor film; and subjecting the oxidized polycrystalline semiconductor film to annealing to activating the impurities. 
     In one embodiment of this invention, the method for fabricating a semiconductor device includes the steps of: depositing an amorphous silicon film; and obtaining the polycrystalline silicon film by crystallizing the amorphous silicon film. 
     In one embodiment of this invention, the polycrystalline semiconductor film has a crystal defect density of about 1×10 18  cm −3  or less. 
     In one embodiment of this invention, the polycrystalline semiconductor film is a polycrystalline silicon film. 
     In one embodiment of this invention, the impurities are phosphorous, boron, arsenic, or antimony. 
     According to another aspect of the present invention, a semiconductor device includes an insulating-gate field effect transistor structure having a gate electrode. The gate electrode includes a polycrystalline semiconductor film having a crystal defect density of about 1×10 18  cm −3  or less. 
     In one embodiment of this invention, the gate electrode has a multilayer structure of the polycrystalline semiconductor film and a metal film or metal suicide film. 
     In one embodiment of this invention, the polycrystalline semiconductor film is a polycrystalline silicon film. 
     According to another aspect of the present invention, a semiconductor device includes an insulating-gate field effect transistor structure having source/drain regions stacked above a semiconductor substrate. The source/drain regions include a polycrystalline silicon film having a crystal defect density of about 1×10 18  cm −3  or less. 
     In one embodiment of this invention, the source/drain regions have a multilayer structure of the polycrystalline semiconductor film and a metal film or metal silicide film. 
     In one embodiment of this invention, the polycrystalline semiconductor film is a polycrystalline silicon film. 
     According to another aspect of the present invention, a semiconductor device includes an insulating-gate field effect transistor structure having a gate electrode. The gate electrode includes a polycrystalline semiconductor film having an average crystal grain diameter of about 50 nm or more in the thickness direction. 
     According to another aspect of the present invention, a semiconductor device includes an insulating-gate field effect transistor structure having source/drain regions stacked above a semiconductor substrate. The source/drain regions have a polycrystalline semiconductor film having an average crystal grain diameter of about 50 nm or more in the thickness direction. 
     In one embodiment of this invention, the polycrystalline semiconductor film included in the gate electrode is a polycrystalline silicon film. 
     In one embodiment of this invention, the polycrystalline semiconductor film included in the source/drain regions is a polycrystalline silicon film. 
     According to this invention, in a semiconductor device including a polycrystalline semiconductor film containing impurities, the polycrystalline semiconductor film containing the impurities is formed by the step for introducing the impurities into the polycrystalline semiconductor film, and in the step for subjecting the polycrystalline semiconductor film to thermal treatment in an oxidization atmosphere to oxidize the polycrystalline semiconductor film, the impurities are activated. The oxidization atmosphere may be any atmosphere such as oxygen and water vapor which generates an oxidization reaction. The temperature of the oxidization is about 600° C. to about 1200° C. Since the polycrystalline semiconductor film is oxidized with the oxidization atmosphere during the activation of the impurities, the polycrystalline semiconductor film is recrystallized, thereby reducing the crystal defect density in the polycrystalline semiconductor film. For this reason, the amount of the impurities which are trapped by crystal defects and are not activated can be reduced, resulting in an increase in an activation ratio. 
     According to another aspect of this invention, impurities are introduced into an oxidized polycrystalline semiconductor film before the polycrystalline semiconductor film is subjected to annealing for activation of the impurities. This leads to recrystallization of the polycrystalline semiconductor film, thereby reducing the crystal defect density of the polycrystalline semiconductor film significantly. Only a reduced portion of impurities which are implanted into the polycrystalline semiconductor film in the later ion implantation is trapped by crystal defects. An increased activation ratio can be obtained after annealing for activation. 
     The polycrystalline semiconductor film may be, for example, polycrystalline silicon film. The crystal defect density of the polycrystalline silicon film is, for example, about 1×10 18  cm −3  or less. The crystal defect density is smaller than the impurity amount by one order of magnitude or more, thereby sufficiently reducing the amount of impurities which are trapped by crystal defects and do not contribute to the activation. 
     The recrystallized polycrystalline semiconductor film is a polycrystalline film having an average crystal grain diameter of about 50 nm or more along the depth direction from the interface to the surface. The average crystal grain diameter is more preferably about 100 nm or more. 
     The polycrystalline semiconductor film may be, for example, polycrystalline silicon film. An average crystal grain diameter of the polycrystalline semiconductor film is about 50 nm or more, more preferably about 100 nm or more. The crystal defect density is therefore smaller than the impurity amount by one order of magnitude or more, thereby sufficiently reducing an amount of impurities which are trapped by crystal defects and do not contribute to the activation. 
     The above-described impurities may be phosphorous, boron, arsenic, or antimony. In particular, phosphorous has a high probability of being trapped by a defect in the polycrystalline silicon film. Therefore, reduction of crystal defects in the polycrystalline semiconductor film can increase the activation ratio significantly. 
     When the above-described polycrystalline semiconductor film is formed by the step for deposition of the amorphous silicon film and the step for crystallization of the amorphous silicon film, the resultant polycrystalline semiconductor film has a large crystal grain diameter. For this reason, the defect density in the film is decreased as compared with when direct deposition of the polycrystalline semiconductor film. 
     Further, when a gate electrode of an insulating-gate field effect transistor includes the above-described polycrystalline semiconductor film according to this invention, the following effects are obtained. 
     In general, to prevent depletion of a gate electrode, impurities having a concentration of about 1×10 19  cm −3  or more are required. On the other hand, to control threshold voltage, a channel impurity concentration is preferably about 1×10 17  cm −3  to about 3×10 17  cm −3 . In a typical ion implantation step, even when it is assumed that an impurity distribution is simply a Gaussian distribution, it is at a deep position about 4.3 σ from the peak depth Rp (projected range) of the impurity distribution that the impurity concentration is decreased by about three orders of magnitude. To avoid depletion, ion implantation is generally carried out so as to provide a peak concentration of about 1×10 20  cm −3 . To prevent impurity ions from penetrating into the channel, however, the depth position about 4.3 σ from the Rp should be within the polycrystalline silicon film included in the gate electrode. When this position is deeper than the thickness of the polycrystalline silicon film included in the gate electrode, the impurity ions reach the channel region. When the energy of ion implantation is set so that the depth position about 4.3 σ from the Rp is within the polycrystalline silicon, such ion implantation causes a very shallow Rp. This makes it difficult to keep an impurity concentration of about 1×10 19  cm −3  or more in the gate electrode (the polycrystalline silicon film) in the vicinity of the gate insulating film. 
     On the other hand, in this invention, the above-described polycrystalline silicon film having a high activation ratio is used as the gate electrode. This makes it easy to keep an impurity concentration of about 1×10 19  cm −3  or more in the gate electrode (the polycrystalline silicon film) in the vicinity of the gate insulating film while preventing the impurity ions from penetrating into the channel region. 
     Furthermore, the gate electrode of the insulating-gate field effect transistor can include a multilayer film of the polycrystalline silicon film obtained by this invention and a metal film or metal silicide film, thereby obtaining a gate electrode having a further low resistivity. 
     The source/drain regions, which are stacked above the channel region in the insulating-gate field effect transistor, can include the polycrystalline silicon film of this invention. In this case, the conventional high-concentration implantation is not required, since a relatively low impurity concentration can provide sufficient activation, resulting in a low resistivity. This prevents accelerated diffusion which is a problem in impurity implantation at high concentration, thereby making it easy to generate a shallow junction. 
     In general, in fabrication of the dual-gate-structure CMOS transistor, ion implantation into the gate electrode and ion implantation into the source/drain regions are simultaneously carried out so as to reduce the number of fabrication steps. Conventionally, to achieve the above-described simultaneous implantation in the stacking-structure source/drain regions, conditions for the implantation and diffusion (activation) should satisfy the following three conditions: a condition for preventing the gate depletion; a condition for preventing the impurities penetrating from the gate electrode into the channel region; and a condition for preventing the formation of a deep source/drain junctions by impurities expanding into the semiconductor substrate, thereby reducing the effective channel length. In this case, as described above, there is a trade-off relationship between prevention of the gate depletion and prevention of penetration of the impurities into the channel region. Similarly, there is a trade-off relationship between the condition for forming the non-offset source/drain regions and the condition for prevention of penetration of the impurities into the channel region. In the conventional technology, it is difficult to provide conditions satisfying the above-described three conditions simultaneously, resulting in a small process margin. 
     On the other hand, in this invention, it is possible to improve the activation ratio in the polycrystalline silicon film. Therefore, a small amount of implantation can provide a low resistivity and a low concentration of implantation. As a result, according to this invention, the process margin can be enlarged, so that the above-described three conditions can be easily satisfied. 
     The source/drain regions, which are stacked above the channel region in the insulating-gate field effect transistor, can include a multilayer film of the polycrystalline silicon film obtained by this invention and a metal film or metal silicide film, thereby obtaining a further low resistivity. 
     Hereinafter, the functions of this invention will be described. 
     According to this invention, a reduced crystal defect density (e.g., about 1×10 18  cm −3  or less) of a polycrystalline semiconductor film (e.g., a polycrystalline silicon film) allows sufficient activation of the impurities therein. As a result, a semiconductor device (e.g., a transistor) having an excellent operating characteristic (e.g., sufficiently high transconductance). 
     Specifically, there is substantially no depletion of the gate electrode in a transistor including a gate electrode including the polycrystalline semiconductor film (e.g., the polycrystalline silicon film) of this invention. For this reason, there is substantially no variation in threshold voltage and there is a reduction in transistor current in operation. Furthermore, the step for obtaining the low resistivity of the gate and the formation of the source/drain regions can be simultaneously carried out, thereby simplifying the fabricating process. 
     In a transistor having the stacking structure, the source/drain regions, which are formed of the polycrystalline semiconductor film (e.g., the polycrystalline silicon film) of this invention, can have a low resistivity and a shallow junction simultaneously. 
     Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor device obtaining a satisfactory characteristic by reducing a crystalline defect density in the polycrystalline semiconductor film constituting the gate electrode or the source/drain regions and therefore increasing an activation ratio of impurities; and (2) providing a method for fabricating the semiconductor device by annealing which allows a significant reduction in crystalline defect density of the polycrystalline semiconductor film. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view illustrating a fabricating process of a conventional dual-gate-structure CMOS transistor. 
     FIG. 2 is a cross-sectional view illustrating a configuration of a conventional stacking-structure transistor. 
     FIG. 3 is a cross-sectional view illustrating a fabricating process of a dual-gate-structure CMOS transistor semiconductor device according to Example 1 of the present invention. 
     FIG. 4 is a diagram showing the C-V characteristics of an NMOS transistor included in the dual-gate-structure CMOS transistor semiconductor device shown in FIG. 3, and the C-V characteristics of a conventional device. 
     FIG. 5 is a diagram showing the dependence of maximum mutual conductance on the amount of implanted phosphorous with respect to the NMOS transistor included in the dual-gate-structure CMOS transistor semiconductor device shown in FIG. 3, and the dependence of maximum mutual conductance on the amount of implanted phosphorous obtained in a conventional device. 
     FIG. 6 is a cross-sectional view illustrating a fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 7 is a cross-sectional view illustrating another fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 8 is a cross-sectional view illustrating another fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 9 is a cross-sectional view illustrating another fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 10 is a cross-sectional view illustrating another fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 11 is a cross-sectional view illustrating another fabricating step of the dual-gate-structure CMOS transistor semiconductor device shown in FIG.  3 . 
     FIG. 12 is a cross-sectional view illustrating a configuration of a stacking-structure CMOS transistor semiconductor device according to Example 2 of the present invention. 
     FIG. 13 is a cross-sectional view illustrating a fabricating step of the stacking-structure CMOS transistor semiconductor device shown in FIG.  12 . 
     FIG. 14 is a cross-sectional view illustrating another fabricating step of the stacking-structure CMOS transistor semiconductor device shown in FIG.  12 . 
     FIG. 15 is a cross-sectional view illustrating another fabricating step of the stacking-structure CMOS transistor semiconductor device shown in FIG.  12 . 
     FIG. 16 is a cross-sectional view illustrating another fabricating step of the stacking-structure CMOS transistor semiconductor device shown in FIG.  12 . 
     FIG. 17A is a cross-sectional TEM photograph of a conventional polycrystalline silicon film. 
     FIG. 17B is a cross-sectional TEM photograph of a polycrystalline silicon film of this invention into which phosphorous is implanted as a dopant. 
     FIG. 18A is a diagram illustrating a polycrystalline crystal grain diameter based on the TEM photograph shown in FIG.  17 B. 
     FIG. 18B is a diagram illustrating a polycrystalline crystal grain diameter based on the TEM photograph shown in FIG.  17 A. 
     FIG.  19 ( a ) is a side plan view illustrating the crystal structure/orientation of certain example embodiments of the instant invention. 
     FIG.  19 ( b ) is a side plan view illustrating the crystal structure/orientation of certain prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Example 1 
     FIG. 3 is a cross-sectional view illustrating a configuration of a dual-gate-structure CMOS transistor semiconductor device according to Example  1  of the present invention. 
     In FIG. 3, a p − well  302 , an n − -well  303 , and a field oxide film (element-isolating region)  304  are provided on a semiconductor substrate  301  such as a silicon substrate. 
     The p − -well  302  provides a region where an NMOS transistor element is formed. The NMOS transistor includes a gate insulating film  305  (such as an oxide film), an n + -polycrystalline gate electrode  306   a , a silicon nitride film  307  formed on both sides of the n + -polycrystalline gate electrode  306   a , a shallow n-type diffusion layer  308  (LDD region), a sidewall spacer  310 , a deep n + -diffusion layer  311 , a silicide film  313 , an interlayer insulating film  314 , and a metal wire  315 . The n − -well  303  provides a region where a PMOS transistor element is formed. The PMOS transistor includes a gate insulating film  305  (such as an oxide film), a p + -polycrystalline gate electrode  306   b , a silicon nitride film  307 , a shallow p-type diffusion layer  309  (LDD region), a sidewall spacer  310 , a deep p + -diffusion layer  312 , a silicide film  313 , an interlayer insulating film  314 , and a metal wire  315 . 
     In the dual-gate-structure CMOS transistor semiconductor device thus constructed as described above, the polycrystalline silicon films included in the gate electrodes  306   a  and  306   b  have an average crystal grain diameter of about 50 nm or more, more preferably about 100 nm or more. This size prevents the crystal defect density of the polycrystalline silicon film from exceeding about 1×10 18  cm −3 . With the polycrystalline silicon film having a crystal defect density of about 1×10 18  cm −3 , the amount of impurities which are trapped by the crystal defect and not activated can be reduced in the gate electrodes  306   a  and  306   b , thereby increasing the activation ratio of impurities. For this reason, the concentration of activated impurities in the vicinity of the gate insulating film  305  in the gate electrodes  306   a  and  306   b  can be easily set to about 1×10 19  cm −3  or more. This prevents occurrence of the depletion layer in the gate electrodes  306   a  and  306   b . Further, the low crystal defect density prevents diffusion of impurities. The impurities implanted in the gate electrodes  306   a  and  306   b  (polycrystalline silicon film) do not penetrate through the gate insulating film  305 , so that the characteristics of a transistor are not degraded. 
     FIG. 4 shows low frequency C-V characteristics of the dual-gate-structure CMOS transistor semiconductor device of Example 1 in which the NMOS transistor includes the gate electrode  306   a  made of the polycrystalline silicon film according to this invention and the gate electrode  306   a  is fabricated with phosphorous implantation. The low frequency C-V characteristics are indicated by a continuous line. For comparison, FIG. 4 also shows, by a dashed line, the characteristics of the conventional semiconductor device obtained by annealing in nitrogen atmosphere. 
     As shown in FIG. 4, in the conventional device indicated by the dashed line, as a voltage V applied to the gate is increased, the gate capacitance (normalized capacitance) C is decreased, resulting in the depletion in the gate electrode. In Example 1, such depletion is reduced, thereby obtaining satisfactory C-V characteristics. 
     FIG. 5 shows, by a continuous line, the dependence of maximum mutual conductance on the amount of implanted phosphorous with respect to the NMOS transistor included in the dual-gate-structure CMOS transistor semiconductor device. For comparison, FIG. 5 also shows, by a dashed line, the characteristics of the conventional semiconductor device obtained by annealing in nitrogen atmosphere. 
     When oxidization annealing is carried out, the maximum mutual conductance is increased at any amount of implanted impurities. The oxidization annealing can improve the characteristics of the transistor. 
     Moreover, when phosphorous is used as a dopant, a greater improvement of the characteristics obtained by the oxidization annealing according to this invention is obtained as compared with when other impurity ions are used. The effect of the oxidization annealing according to this invention is the most significant when phosphorous is implanted. 
     FIG. 17B shows a cross-sectional TEM photograph of the polycrystalline silicon film which is doped with phosphorous as a dopant and which is used in the gate electrode  306   a  of the NMOS transistor in the dual-gate-structure CMOS transistor semiconductor device of Example 1. For comparison, FIG. 17A also shows a cross-sectional TEM photograph of polycrystalline silicon film used in the conventional semiconductor device obtained annealing in nitrogen atmosphere. 
     As indicated by arrows in FIG. 17A, when the oxidization annealing is not carried out, there are a number of polycrystalline silicon grains having a small diameter in the vicinity of the interface with the gate insulating film. On the other hand, as shown in FIG. 17B, the oxidization annealing leads to an increase in the grain diameter of the polycrystalline silicon in the vicinity of the interface with the gate insulating film. 
     FIGS. 18A and 18B are diagrams illustrating the cross-sectional TEM photographs shown in FIGS. 17A and 17B, respectively, in such a way to make it easy to see the diameter of the polycrystalline grains. Here the oxidization annealing oxidizes a polycrystalline silicon film having a thickness of about 250 nm by about 50 nm. As can be seen from FIGS. 18A and 18B, the crystal grain diameter of the polycrystalline silicon film subjected to the oxidization annealing is larger than when the oxidization is not carried out. In particular, the polycrystalline silicon grains smaller than about 50 nm above the interface grow to a large diameter which is almost the same as that of the grains near the interface. As a result, the oxidization annealing allows the polycrystalline silicon film having the crystal grains having uniform diameters along the cross section or the depth direction. 
     The crystal grain diameter grows to a large size in the entire polycrystalline silicon film. The average grain diameter is greater than or equal to about 50 nm. Here the average grain diameter is obtained by taking the diameter of grains along the depth direction of the polycrystalline silicon film having a thickness of greater than or equal to about 200 nm. The thickness spans from the interface with the gate insulating film to the upper surface of the polycrystalline silicon film. When the oxidization annealing is not carried out in the polycrystalline silicon film, there are a number of polycrystalline silicon grains having a small diameter, smaller than about 50 nm, above the interface. On the other hand, when the oxidization annealing is carried out, the polycrystalline silicon film has an average grain diameter greater than or equal to about 50 nm. The grain diameter of the polycrystalline silicon film is preferably greater than or equal to about 100 nm. As a result, the crystal defects in the vicinity of the interface are reduced to a crystal defect density of about 1×10 18  cm −3  or less, thereby increasing the activation ratio of impurities in the polycrystal. 
     In the dual-gate-structure CMOS transistor semiconductor device of Example 1, the polycrystalline silicon film which is included in the gate electrodes  306   a  and  306   b  has a low crystal defect density (specifically, about 1×10 18  cm −3 or less). For this reason, carriers are not depleted in the gate electrodes  306   a  and  306   b  and impurity implantation can be carried out with a low level of energy, thereby preventing the short channel effect. Further, the low crystal defect density prevents diffusion of impurities, whereby implanted impurity ions do not penetrate through the gate insulating film  305 , so that the characteristics of the transistor are not degraded. Moreover, the semiconductor device of Example 2 does not need a particular processing device which grows a phosphorous-implanted polycrystalline silicon film, as in the conventional technology disclosed in Japanese Laid-Open Publication No. 6-275788. This can improve the throughput of fabricating the device and reduce the fabricating cost. 
     Next, a method for fabricating the dual-gate-structure CMOS transistor semiconductor device of Example 1 will be described with reference to FIGS. 6 through 11. FIGS. 6 through 11 are cross-sectional views illustrating structures obtained in steps in the fabricating method. 
     As shown in FIG. 6, a p − -well  302 , an n − -well  303 , and a field oxide film (element-isolating region)  304  are formed on a silicon substrate  301  in a known way in the art. 
     To control the threshold voltage and prevent the short channel effect, a region in which an NMOS transistor element is to be formed (the p − -well  302 ) is doped with boron and a region in which a PMOS transistor element is to be formed (the n − -well  303 ) is doped with phosphorous. Those elements are implanted as impurity ions. A gate insulating film  305  (e.g., an oxide film) having a thickness of about 5 nm is then formed on the wells  302  and  303 . An amorphous silicon film  306  is then deposited to cover the field oxide film  304  and the gate insulating film  305  with LPCVD at a temperature of about 550° C. The thickness of the amorphous silicon film  306  is between about 100 nm and about 300 nm, more preferably about 150 nm. 
     Subsequently, the amorphous silicon film  306  is crystal-grown (crystallized) in an atmosphere of nitrogen gas at a temperature of about 650° C., resulting in a polycrystalline silicon film. This polycrystalline silicon film is referred to with the same reference number  306  as used for the amorphous silicon film. The polycrystalline silicon film  306  may be directly formed by LPCVD. The crystal growth of the amorphous silicon film can obtain a polycrystalline silicon film having a greater grain diameter. 
     The polycrystalline silicon film  306  is then subjected to oxidization annealing so as to reduce the crystal defect density thereof (specifically, to about 1×10 18  cm −3  or less). In Example 1, a gate electrode will be formed using the polycrystalline silicon film  306 . Instead of the polycrystalline silicon film, another material such as polycrystalline silicon germanium film can be provided. Alternatively, a multilayer structure including the above described polycrystalline semiconductor film and a metal film such as tungsten or a metal silicide film may be used as the polycrystalline silicon film  306 . 
     FIG. 6 shows a cross-sectional view of the structure which has been so far fabricated. 
     The polycrystalline silicon film  306  is then patterned to the desired shape with well-known photolithography and etching techniques, thereby obtaining the gate electrodes  306   a  and  306   b . A natural oxidization film existing on surfaces of the gate electrodes  306   a  and  306   b  made of the polycrystalline silicon film and the oxide film  305  existing on the wells  302  and  303  (activated region=the source/drain region) which are not covered with the gate electrodes  306   a  and  306   b  are completely removed with a solution of hydrofluoric acid or the like. Silicon nitride film is deposited to cover the gate electrodes  306   a  and  306   b , the wells  302  and  303 , and the field oxide film (element-isolating film)  304 . This silicon nitride film serves as an impurity implantation protecting film  307 , the thickness of which is about 3 nm to about 30 nm, more preferably about 5 nm. For the implantation protecting film  307 , silicon oxide film may be used instead of the silicon nitride film. In this case, for the knock-on effect, oxygen atoms are knocked on by the implanted ions so as to move from the silicon oxide film to the wells. These oxygen atoms prevent salicidation in a subsequent step. For this reason, the silicon nitride film is used as the implantation protecting film  307 . 
     Alternatively, impurity ions may be implanted without the implantation protecting film  307 . 
     A shallow junction is then provided in the vicinity of a channel of the region in which an NMOS transistor element is to be formed (the p − -well  302 ). To this end, as shown in FIG. 7, the region in which a PMOS transistor element is to be formed (the n − -well  303 ) is covered with a photoresist film  401  by a photolithography step. Arsenic ions  408  are then implanted into the region in which an NMOS transistor element is to be formed (the p − -well  302 ). This ion implantation is carried out with an accelerating energy of about 2 keV to about 30 keV and an implantation of about 0.5×10 14  cm −2  to about 5×10 14  cm −2 . The arsenic ions are impurity ions which serve as donors in the silicon semiconductor. Thus, an impurity diffusion region  308  is formed in the p − -well  302  as shown in FIG. 8, which will be a shallow n-type diffusion layer  308 . 
     Alternatively, antimony ions may be used as the above-described impurity ions for the NMOS transistor element. Antimony-ion implantation is carried out with an accelerating energy of about 3 keV to about 35 keV to obtain an implantation amount of about 0.5×10 14  cm −2  to about 5×10 14  cm −2 . 
     Next, after removing the photoresist film  401 , another shallow junction is then provided in the vicinity of a channel of the region in which a PMOS transistor element is to be formed (the n − -well  303 ). To this end, as shown in FIG. 8, the region in which an NMOS transistor element is to be formed (the p − -well  302 ) is covered with a photoresist film  402  by a photolithography step. Boron ions  410  are then implanted into the region in which a PMOS transistor element is to be formed (the n − -well  303 ). This ion implantation is carried out with an accelerating energy of about 5 keV to about 40 keV to obtain an implantation amount of about 0.5×10 14  cm −2  to about 5×10 14  cm −2 . The Boron ions are impurity ions which serve as acceptors in the silicon semiconductor. Thus, an impurity diffusion region  309  is formed in the n − -well  303  as shown in FIG. 9, which will be a shallow p-type diffusion layer  309 . 
     Alternatively, In ions may be used as the above-described impurity ions for the PMOS transistor element. 
     After removing the photoresist film  402 , a sidewall spacer  310  is formed on the sides of the gate electrodes  306   a  and  306   b . Specifically, silicon nitride film is deposited to have a thickness of about 100 nm to about 200 nm. This silicon nitride film is etched back until the upper surface of the silicon oxide film on the element-isolating film  304  is exposed, thereby obtaining the sidewall spacer  310 . This etching is carried out by reactive ion etching (RIE) using a mixture of C 4 F 8  and CO gas as etchant. The gas mixture has a selection ratio of silicon nitride film to silicon oxide film which is about 50 to about 100 to 1. The sidewall spacer  310  is preferably made of the silicon nitride film in order that bird&#39;s beak can be reduced in a subsequent oxidization step. The sidewall spacer  310  may have a two-layer structure including a silicon oxide film and a silicon nitride film. 
     Thereafter, source/drain diffusion layers having a deep junction (deep diffusion layers  311  and  312 ) are provided. 
     Specifically, as shown in FIG. 9, the region in which a PMOS transistor element is to be formed on the n − -well  303  is covered with a photoresist film  403  by a photolithography step. Arsenic ions  413  are then implanted into the region in which an NMOS transistor element is to be formed on the p − -well  302 . This ion implantation is carried out with an accelerating energy of about 15 keV to about 50 keV to obtain an implantation amount of about 1×10 15  cm −2  to about 5×10 15  cm −2 , more specifically an accelerating energy of about 30 keV to obtain an implantation amount of about 3×10 15  cm −2 . The arsenic ions are impurity ions which serve as donors in the silicon semiconductor. 
     Next, after removing the photoresist film  403 , the shallow n − -diffusion layer  308  and the deep n-type diffusion layer  311  are formed in the region in which an NMOS transistor element is to be formed on the p − -well  302 . This is achieved by activating the implanted impurities by annealing in an atmosphere of nitrogen gas at a temperature of about 850° C. to about 900° C. Meanwhile, in the region in which a PMOS transistor element is to be formed on the n − -well  303 , the boron atoms which have been previously implanted are activated, so that the shallow p-type diffusion layer  309  is formed. 
     Thereafter, a deep junction is provided in the vicinity of a channel of the region in which a PMOS transistor element is to be formed on the n − -well  303 . To this end, as shown in FIG. 10, the region in which an NMOS transistor element is to be formed on the p − -well  302  is covered with a photoresist film  404  by a photolithography step. Silicon ions are then implanted into the region in which a PMOS transistor element is to be formed on the n − -well  303 . This ion implantation is carried out with an accelerating energy of about 30 keV to obtain an implantation amount of about 1×10 15  cm −2 . The silicon ions play a role in preventing a channeling effect. The arsenic ions are impurity ions which serve as donors in the silicon semiconductor. Boron ions  415  are then implanted into the region in which a PMOS transistor element is to be formed on the n − -well  303 . This ion implantation is carried out with an accelerating energy of about 10 keV to about 30 keV to obtain an implantation amount of about 1×10 15  cm −2  to about 5×10 15  cm −2 . The boron ions are impurity ions which serve as acceptors in the silicon semiconductor. 
     Next, after removing the photoresist film  404 , the deep p-type diffusion layer  312  is formed in the region in which a PMOS transistor element is to be formed on the n − -well  303 . This is achieved by activating the implanted impurities by rapid thermal annealing (RTA) at a temperature of about 1000° C. for about 10 seconds. 
     Thereafter, formation of a silicide film  313  by a salicidation step, deposition of an interlayer insulating film  314 , formation of a metal wire  315  and the like are carried out in a well-known way. As a result, a dual-gate-structure CMOS transistor semiconductor device having the desired structure as shown in FIG. 11 is obtained. 
     In the description of Example 1, the polycrystalline silicon film  306  which is to be the gate electrodes  306   a  and  306   b  is oxidized before the patterning. The oxidization annealing may be carried out after the patterning. That is, after the gate electrodes  306   a  and  306   b  are patterned, ion implantation is carried out followed by annealing for activating the ions. In this case, activation of impurities and reduction of crystal defects are simultaneously carried out, resulting in an increase in the activation ratio. 
     Alternatively, ion implantation to the polycrystalline silicon film  306  may be carried out before patterning, followed by oxidization annealing. Thereafter, the polycrystalline silicon film  306  may be patterned to obtain the gate electrodes  306   a  and  306   b , and then a step for forming the source/drain regions may be carried out. 
     Example 2 
     FIG. 12 is a cross-sectional view illustrating a configuration of a dual-gate-structure CMOS transistor semiconductor device according to Example 2 of the present invention. 
     In FIG. 12, a p − -well  502 , an n − -well  503 , and a field oxide film (element-isolating region)  504  are provided on a semiconductor substrate  501  such as a silicon substrate. The p − -well  502  provides a region where an NMOS transistor element is formed. The NMOS transistor includes a gate insulating film  505  (such as an oxide film), an n +  polycrystalline gate electrode  506   a , a pair of sidewalls  507 , a shallow n-type diffusion layer  508 , an n +  polycrystalline source/drain regions  511   a , a silicide film  513 , an interlayer insulating film  514 , and a metal wire  515 . The n − -well  503  provides a region where a PMOS transistor element is formed. The PMOS transistor includes a gate insulating film  505  (such as an oxide film), a p +  polycrystalline gate electrode  506   b , a pair of sidewalls  507 , a shallow p-type diffusion layer  509 , a p + -polycrystalline source/drain regions  511   b,  a silicide film  513 , an interlayer insulating film  514 , and a metal wire  515 . 
     Next, a method for fabricating the dual-gate-structure CMOS transistor semiconductor device of Example 2 will be described with reference to FIGS. 13 through 16. FIGS. 13 through 16 are cross-sectional views illustrating structures obtained in steps in the fabricating method. 
     As shown in FIG. 13, a p − -well  502 , an n − -well  503 , and a field oxide film (element-isolating region)  504  are formed on a silicon substrate  501  in a known way in the art. 
     To control the threshold voltage and prevent the short channel effect, a region in which an NMOS transistor element is to be formed on the p − -well  502  is doped with boron and a region in which a PMOS transistor element is to be formed on the n − -well  503  is doped with phosphorous. Those elements are implanted as impurity ions. A gate insulating film  505  (e.g., an oxide film) having a thickness of about 5 nm is then formed on the wells  502  and  503 , and a polycrystalline silicon film  506  is deposited to cover the field oxide film  504  and the gate insulating film  505 . The thickness of the polycrystalline silicon film  506  is about 200 nm. A silicon oxide film  605  having a thickness of about 200 nm is deposited on the polycrystalline silicon film  506  with LPCVD. FIG. 13 shows a cross-sectional view of the structure which has been so far fabricated. 
     The polycrystalline silicon film  506  is then patterned to the desired shape with well-known photolithography and etching techniques, thereby obtaining the gate electrodes  506   a  and  506   b . In this step, the silicon oxide film  605  on the polycrystalline silicon film  506  is also patterned, so that a mask oxide film  607  is formed on the gate electrodes  506   a  and  506   b . FIG. 14 shows a cross-sectional view of the structure which has been so far fabricated. 
     A silicon nitride film  520  having a thickness of about 50 nm is formed on the structure shown in FIG. 14 with LPCVD. The desired part of the resulting structure is then subjected to patterning in a photolithography step. A pair of sidewalls  507  are formed on the sides of the gate electrodes  506   a  and  506   b  with dry etching in an etchback step. In this step, the silicon nitride film  520  is left on the field oxide film (element-isolating film)  504 . Thereafter, the oxide film  505  formed on the wells  502  and  503  (active region =source/drain regions) which are not covered with the gate electrodes  506   a  and  506   b  is completely removed with a solution of hydrofluoric acid or the like. FIG. 15 shows a cross-sectional view of the structure which has been so far fabricated. 
     Thereafter, an amorphous silicon film having a thickness of about 300 nm is formed on the structure shown in FIG.  15  and is subjected to annealing in an atmosphere of nitrogen gas at a temperature of about 650° C. The annealing causes the crystal growth (crystallization) in the amorphous silicon film, resulting in a polycrystalline silicon film. The polycrystalline silicon film may be directly formed by LPCVD or like. In Example 2, the polycrystalline silicon film is obtained by the crystal growth of the amorphous silicon film. The amorphous silicon film or the polycrystalline silicon film may be formed in a low pressure CVD (LPCVD) apparatus having a preliminary exhaust chamber, a nitrogen purging chamber in which a dew point is constantly −100° C., and a deposition furnace. This device allows deposition of amorphous silicon film or polycrystalline silicon film without growing a natural oxidization film at an interface between a surface of an active region of the semiconductor substrate  501  and the deposited amorphous silicon film or polycrystalline silicon film. 
     In this case, immediately before the deposition of the polycrystalline silicon film on the wafer, the wafer is washed with a solution of hydrofluoric acid so as to remove a natural oxidization film. The wafer is then transferred into the preliminary vacuum exhaust chamber in atmospheric air. The atmospheric air is then exhausted to vacuum and is replaced with nitrogen gas. The wafer is then transferred to the nitrogen purging chamber in which the dew point is constantly −100° C. In the nitrogen purging chamber, the nitrogen purge removes water molecules adsorbed on the wafer surface completely. The water molecules adsorbed on the wafer surface cannot be removed completely in vacuum. The experiments conducted by the inventors have demonstrated that the nitrogen purge can perform such complete removal. In a typical LPCVD apparatus, the wafer is transferred to the deposition furnace along with the water molecules adsorbed on the wafer surface. The deposition of the amorphous silicon film is typically conducted at a temperature of about 500° C. to about 550° C. On the other hand, the deposition of the polycrystalline silicon film is conducted at a temperature of about 550° C. to about 700° C. Oxygen of the adsorbed water molecules reacts with the silicon wafer in the high-temperature deposition furnace. For this reason, natural oxidization film is formed on the silicon wafer surface before the polycrystalline silicon film is deposited. The natural oxidization film is formed on the interface between a surface of an active region of a semiconductor substrate and the deposited polycrystalline silicon film. On the other hand, in the LPCVD used in Example 2, water molecules are removed completely from the wafer in the nitrogen purging chamber before the wafer is transferred to the deposition furnace. Therefore, a natural oxidization film is not formed on the interface, thereby making it possible to deposit amorphous silicon film or polycrystalline silicon film. 
     Subsequently, the wafer is subjected to oxidization annealing in an atmosphere of oxygen gas at a temperature of about 700° C. to about 900° C. This reduces the crystal defect density of the polycrystalline silicon film included in the gate electrodes  506   a  and  506   b  (specifically, the density is about 1×10 18  cm −2  or less). Oxidized silicon film formed by the oxidization annealing is removed with wet etching. The polycrystalline silicon film is then etched back so as to form source/drain regions  511   a  and  511   b  on the sides of the sidewall  507 . The source/drain regions  511   a  and  511   b  are made of the polycrystalline silicon film. 
     The region in which a PMOS transistor element is to be formed is then covered with photoresist film in a photolithography step. Thereafter, arsenic ions are implanted into the region in which an NMOS transistor element is to be formed. This ion implantation is carried out with an accelerating energy of about 2 keV to about 30 keV to otain an implantation amount of about 1×10 15  cm −2  to about 5×10 15  cm −2 . The arsenic ions are impurity ions which serve as donors in the silicon semiconductor. Alternatively, phosphorous ions may be used as the above-described impurity ions for the NMOS transistor element. Phosphorous-ion implantation is carried out with an accelerating energy of about 3 keV to about 35 keV to obtain an implantation amount of about 1×10 15  cm −2  to about 1×10 16  cm −2 . 
     Next, after removing the photoresist film, the region in which an NMOS transistor element is to be formed is covered with another photoresist film in a photolithography step. Boron ions are then implanted into the region in which a PMOS transistor element is to be formed. This ion implantation is carried out with an accelerating energy of about 10 keV to about 30 keV to obtain an implantation amount of about 1×10 15  cm −2 to about 1×10 16  cm −2 . The boron ions are impurity ions which serve as acceptors in the silicon semiconductor. 
     In the description of Example 2, the polycrystalline silicon film which has been doped with impurities is subjected to the oxidization annealing before being etched back to form the polycrystalline source/drain regions  511   a  and  511   b . This reduces the crystal defect density of the polycrystalline silicon film. Alternatively, the ion implantation may be carried out after the etchback of the polycrystalline silicon film, followed by the oxidization annealing, thereby reducing crystal defects. In this case, activation of impurities and reduction of crystal defects are simultaneously carried out, resulting in an increase in the activation ratio. 
     Next, after removing the photoresist film, the above-described structure is subjected to RTA at a temperature of about 1000° C. to about 1100° C. for about 10 seconds. In the RTA, the impurities implanted in the polycrystalline silicon film which is included in the polycrystalline source/drain regions  511   a  and  511   b  are activated. The impurities are diffused into the semiconductor substrate from the polycrystalline silicon film (the polycrystalline source/drain regions  511   a  and  511   b ). The RTA may be replaced with annealing which is carried out at a temperature of about 850° C. to about 950° C. for about 10 minutes to about 30 minutes. For that reason, the shallow p-type diffusion layer  509  is formed in the region in which a PMOS transistor element is to be formed on the n − -well  503 . The shallow n-type diffusion layer  508  is formed in the region in which an NMOS transistor element is to be formed on the p-well  502 . 
     Thereafter, formation of a silicide film  513  by a salicidation step, deposition of an interlayer insulating film  514 , formation of a metal wire  515  and the like are carried out in a well-known way. As a result, a dual-gate-structure CMOS transistor semiconductor device having the desired structure as shown in FIG. 16 is obtained. 
     As described above, according to this invention, a reduced crystal defect density (e.g., about 1×10 18  cm −3  or less) of a polycrystalline semiconductor film (e.g., a polycrystalline silicon film) allows sufficient activation of the impurities therein. As a result, a semiconductor device (e.g., a transistor) having an excellent operating characteristic (e.g., sufficiently high transconductance). 
     Specifically, this invention can be applied to a surface-channel CMOS transistor having a dual-gate structure. Even when energy used in implanting ions such as phosphorous or arsenic as an n-type impurity is reduced so as to prevent the short channel effect, carriers are not depleted from a gate electrode, thereby obtaining sufficient driving current. 
     This invention can be also applied to a transistor having a shallow junction formed in source/drain regions using a stacking structure for preventing the short channel effect of a transistor. Even when polycrystalline semiconductor film (e.g., polycrystalline silicon film) is used as the stacked source/drain regions, the crystal defect density of the polycrystalline semiconductor film can be about 1×10 18  cm −3  or less according to this invention. For this reason, the activation ratio of impurities in the polycrystalline semiconductor film is high, thereby obtaining sufficiently low resistivity. As a result, the resistance of the source/drain regions stacked above a gate electrode and a channel region can be sufficiently reduced. 
     Thus, according to this invention, a gate electrode and source/drain regions having a low resistance can be obtained. 
     The polycrystalline semiconductor film (e.g., polycrystalline silicon film) according to this invention can be applied to a gate electrode portion. Impurities are prevented from penetrating into the channel region during impurity implantation. Depletion of the gate electrode in the vicinity of a gate insulating film also can be prevented. For this reason, a stable gate electrode can be formed in a large range of impurity amount in the ion implantation for the gate electrode, so that control of variation in threshold voltage and driving current can be improved. 
     This invention can be applied to source/drain regions of a stacking-structure transistor. The source/drain regions can constantly have a shallow junction having a low resistance. When impurity ions are simultaneously implanted into the source/drain regions and the gate electrode so that the number of steps can be reduced, margin of the process conditions are enlarged, thereby obtaining a stable characteristic of a transistor. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.