Patent Publication Number: US-2011053329-A1

Title: Semiconductor device including a gate electrode of lower electrial resistance and method of manufacturing the same

Description:
PRIORITY STATEMENT 
     This application is a divisional under 35 U.S.C. §121 of U.S. application Ser. No. 11/878,096, filed on Jul. 20, 2007, which claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2006-0071375, filed on Jul. 28, 2006 in the Korean Intellectual Property Office (KIPO), the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     Example embodiments relate to a semiconductor device including a gate electrode (e.g., poly-metal gate electrode) having a lower electrical resistance and/or an improved barrier characteristic and a method of manufacturing the same. 
     2. Description of the Related Art 
     Research for reducing the electrical resistance of a gate electrode of a semiconductor device is being conducted in response to increased integration so as to operate the semiconductor device at higher speeds. A conventional gate electrode (e.g., a metal gate electrode) may include a metal layer (e.g., a higher conductivity metal layer) formed on a gate insulating layer to achieve higher conductivity. For example, an aluminum layer formed on a gate insulating layer may be used as a gate electrode for a semiconductor device. However, the higher conductivity metal layer may be sensitive to heat, thus resulting in a metal gate electrode with lower heat resistance. Consequently, the metal gate electrode may not be suitable for a subsequent self-alignment structure, because the self-alignment structure may require a thermal treatment at a relatively high temperature, wherein the gate electrode may function as a mask pattern while forming the self-alignment structure. 
     To improve the lower heat resistance of the metal gate electrode, a polysilicon layer doped with impurities may be used as a gate electrode (e.g., a polysilicon gate electrode) for a semiconductor device. Consequently, the polysilicon gate electrode may be more suitable for the self-alignment structure because of a higher heat resistance of the polysilicon layer. In addition, the polysilicon layer may be formed on a gate oxide layer at an earlier stage of a manufacturing process of the semiconductor device to reduce or prevent contamination of the gate oxide layer. However, the polysilicon gate electrode may have a relatively high electrical resistance. The polysilicon layer may have a higher sheet resistance, such that the polysilicon gate electrode may have a greater contact resistance when the integration degree of a semiconductor device is higher. As a result, the polysilicon gate electrode may not be suitable for a higher integrated semiconductor device requiring a lower electrical resistance. 
     To reduce the electrical resistance of the polysilicon gate electrode, a refractory metal silicide layer having an improved heat resistance may be formed on the polysilicon layer to thereby form a multi-layered structure including the polysilicon layer and the metal silicide layer on the polysilicon layer (e.g., a polycide gate electrode). However, the decreased electrical resistance of the polycide gate electrode may still not suffice the lower resistance requirements of a ultra-large scale integrated (ULSI) circuit. 
     To reduce the electrical resistance of the polycide gate electrode, a poly-metal gate electrode may be used in place of the polycide gate electrode. The poly-metal gate electrode may have a higher heat stability and a lower electrical resistance than the polycide gate electrode. The poly-metal gate electrode may have a multi-layered structure, wherein a conductive metal layer having a higher heat resistance may be formed on a polysilicon layer doped with impurities, such that the poly-metal gate electrode has an electrical resistance lower than the polysilicon gate electrode or the polycide gate electrode. Thus, although a line width of a circuit may be reduced to a smaller size (e.g., ULSI circuit), a semiconductor device including the poly-metal gate electrode may have a relatively stable operation speed, because of the reduced electrical resistance of the gate electrode due to the conductive metal layer of the poly-metal gate electrode. For example, a tungsten (W) layer may be used as the conductive metal layer of the poly-metal gate electrode (e.g., tungsten/polysilicon gate electrode), because the tungsten layer may have an electrical resistance of about 10 micro-ohm/cm or less. 
       FIG. 1  is a cross-sectional view illustrating a conventional semiconductor device including a tungsten/polysilicon gate electrode. Referring to  FIG. 1 , the conventional semiconductor device  90  may include a gate insulating layer  12  on a semiconductor substrate  10  (e.g., a silicon wafer) and a tungsten/polysilicon gate electrode structure  80  on the gate insulating layer  12 . The tungsten/polysilicon gate electrode structure  80  may include a polysilicon layer  14  on the gate insulating layer  12 , a tungsten silicide (WSi x ) layer  16  on the polysilicon layer  14 , a tungsten nitride (WN) layer  18  on the tungsten silicide layer  16 , and a tungsten layer  20  on the tungsten nitride layer  18 . 
     The gate insulating layer  12  may electrically insulate the gate electrode structure  80  from the substrate  10 . The gate insulating layer  12  may include silicon oxide. The polysilicon layer  14  may be doped with impurities (e.g., boron (B), phosphorus (P), arsenic (As)) and may have an electrically-sufficient conductivity. The tungsten nitride layer  18  may reduce or prevent the diffusion of impurities from the polysilicon layer  14  to the tungsten layer  20 . In addition, the tungsten nitride layer  18  may improve the adhesion between the polysilicon layer  14  and the tungsten layer  20 . The tungsten silicide layer  16  may reduce or prevent a chemical reaction (e.g., interfacial reaction) between nitrogen (N) and silicon (Si) at an interface between the tungsten nitride (WN) layer  18  and the polysilicon layer  14 , thus reducing or preventing the formation of an interfacial insulating layer (e.g., a silicon nitride layer) between the tungsten nitride layer  18  and the polysilicon layer  14 . Furthermore, the tungsten silicide layer  16  may prevent an interfacial reaction between the tungsten nitride layer  18  and the polysilicon layer  14  due to an oxidizer in a selective re-oxidation process. Accordingly, the tungsten nitride layer  18  may reduce a contact resistance at an interface between the polysilicon layer  14  and the tungsten nitride layer  18 , thus improving an ohmic characteristic of the tungsten/polysilicon gate electrode structure  80 . 
     The gate insulating layer  12 , the polysilicon layer  14 , the tungsten silicide layer  16 , the tungsten nitride layer  18 , and the tungsten layer  20  may be sequentially formed on the substrate  10  and patterned by a photolithography process using a mask layer (not shown) to achieve the conventional tungsten/polysilicon gate electrode structure  80  shown in  FIG. 1 . However, the above-mentioned method of forming the conventional tungsten/polysilicon gate electrode structure  80  may have various disadvantages. 
     During a process of depositing tungsten (W) onto a surface of the tungsten nitride layer  18 , the tungsten nitride layer  18  may react with the tungsten silicide layer  16  to form a tungsten silicon nitride (WSi x N y ) layer having a larger electrical resistance at the interface between the tungsten nitride layer  18  and the tungsten silicide layer  16 . The tungsten silicide layer  16  may be formed to reduce or prevent the generation of an interface insulating layer by a nitrogen component of the tungsten nitride layer  18 . However, during the process of forming the tungsten layer  20 , the nitrogen component of the tungsten nitride layer  18  may react with a silicon component of the tungsten silicide layer  16  to form an interfacial insulating layer having a larger electrical resistance between the tungsten silicide layer  16  and the tungsten nitride layer  18 . Furthermore, the tungsten nitride (WN) of the tungsten nitride layer  18  may dissociate into tungsten (W) and nitrogen (N), and the tungsten nitride layer  18  may be reduced in thickness because of the generation of the interfacial insulating layer. As a result, the tungsten nitride layer  18  may not sufficiently reduce or prevent the diffusion of impurities from the polysilicon layer  14  to the tungsten layer  20 . 
     Experimental results may show that the reactivity between a tungsten nitride layer and a tungsten silicide layer may depend on the mole ratio of tungsten (W) and silicon (Si) in the tungsten silicide layer. Table 1 shows tungsten (W) reactivity with respect to the mole ratio of tungsten (W) and silicon (Si) in the tungsten silicide layer. The experimental results in Table 1 were obtained by an experiment performed at a temperature of about 800° C. and a pressure of about 1 atmospheric pressure. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Reactant 
                 W 2 Si + W 
                 WSi + W 
                 WSi 2  + W 
                 WSi 3  + W 
                 WSi 4  + W 
                 WSi 5  + W 
               
               
                 (mole) 
               
               
                 Product 
                 1.33W 
                 0.33W 
                 — 
                 — 
                 — 
                 — 
               
               
                 (mole) 
                 0.33W 5 Si 3   
                 0.33W 5 Si 3   
                 0.57WSi 2   
                 1.28WSi 2   
                 2.00WSi 2   
                 2.00WSi 2   
               
               
                   
                   
                   
                 0.28W 5 Si 3   
                 0.14W 5 Si 3   
                   
                 1.00Si 
               
               
                 Reaction Ratio of 
                 0% 
                 67% 
                 100% 
                 100% 
                 100% 
                 100% 
               
               
                 Tungsten 
               
               
                   
               
            
           
         
       
     
     Table 1 shows that when a tungsten mole fraction is greater than a silicon mole fraction in a tungsten silicide layer (e.g., tungsten-rich tungsten silicide layer), tungsten (W) was not reacted with tungsten silicide (WSi). However, when the tungsten mole fraction is equal to the silicon mole fraction in a tungsten silicide layer, about 0.67 mole of tungsten (W) was reacted with tungsten silicide (WSi), and about 0.33 mole of tungsten (W) remained non-reacted in the tungsten silicide layer, so that about 67 percent of tungsten (W) was reacted with tungsten silicide (WSi). When a silicon mole fraction is greater than a tungsten mole fraction in a tungsten silicide layer (silicon-rich tungsten silicide layer), all of tungsten (W) was reacted with tungsten silicide (WSi). 
     In view of the experimental results in Table 1, a reaction between a tungsten silicide layer and a tungsten nitride layer may not occur where the tungsten silicide layer includes tungsten-rich tungsten silicide. Because the reaction may not occur, the tungsten nitride molecules of the tungsten nitride layer may not dissociate into tungsten (W) and nitrogen (N). Consequently, the thickness of the tungsten nitride layer may be maintained so as to preserve its barrier characteristics. In contrast, a reaction between a tungsten silicide layer and a tungsten nitride layer may occur where the tungsten silicide layer includes silicon-rich tungsten silicide. Because the reaction may occur, the tungsten nitride molecules of the tungsten nitride layer may dissociate into tungsten (W) and nitrogen (N) during a deposition process or a thermal treatment process. Consequently, the thickness of the tungsten nitride layer may be reduced so as to deteriorate its barrier characteristics. 
     The mole ratio of metal to silicon in the tungsten silicide layer  16  may be about 1:5 or more for the tungsten silicide layer  16  to function as an ohmic layer between the polysilicon layer  14  and the tungsten layer  20 . Thus, the tungsten silicide layer  16  includes silicon-rich tungsten silicide. Consequently, the tungsten nitride layer  18  may dissociate into tungsten (W) and nitrogen (N) during a deposition process or a thermal treatment process. Accordingly, the tungsten nitride layer  18  may not sufficiently reduce or prevent the diffusion of impurities from the polysilicon layer  14  into the tungsten layer  20 . 
     Furthermore, an agglomeration may be generated on the surface of the tungsten silicide layer  16  during a thermal treatment process, which may reduce its ohmic characteristic.  FIG. 2  is a scanning electron microscope (SEM) image showing the agglomeration generated on a conventional tungsten silicide layer as a result of a thermal treatment process at a temperature of about 815° C. As shown in  FIG. 2 , the tungsten silicide layer may be partially agglomerated to a height of about 95 Å after the thermal treatment process, thus increasing the thickness of the tungsten silicide layer  16 . Consequently, the sheet resistance of the tungsten silicide layer  16  may increase as a result of the increase in thickness. However, the increase in sheet resistance of the tungsten silicide layer  16  may deteriorate the ohmic characteristic of the tungsten/polysilicon gate electrode structure  90 , thereby decreasing reliability. 
     To reduce or prevent an increase in the thickness of the metal silicide layer, a titanium silicide (TiSi x ) layer may be used instead of the tungsten silicide layer (WSi)  16 . However, forming the titanium silicide layer to a desired thickness may be difficult. Because the titanium silicide layer may be formed by a sputtering process or a chemical vapor deposition (CVD) process at a temperature higher than about 600° C., the reactivity of titanium (Ti) with silicon (Si) may increase. As a result, controlling the final thickness of the titanium silicide layer may be difficult. Additionally, the titanium silicide layer may have surface defects (e.g., agglomerations, voids). Furthermore, because the titanium silicide layer may need to include a silicon-rich titanium silicide to improve its ohmic characteristic, a tungsten nitride layer  18  on the titanium silicide layer may be reduced in thickness due to dissociation of the tungsten nitride (WN) during a thermal treatment process. Thus, a titanium silicide layer may not improve the decreased barrier characteristic of the tungsten nitride layer  18 . 
     SUMMARY OF EXAMPLE EMBODIMENTS 
     Example embodiments provide a semiconductor device having a poly-metal gate electrode capable of reducing or preventing the dissociation of a barrier layer and/or the generation of an agglomeration at a surface of an interface reaction preventing layer. Example embodiments also provide a method of manufacturing the semiconductor device having the above poly-metal gate electrode. 
     A semiconductor device according to example embodiments may include a gate insulating layer, a polysilicon layer, an interface reaction preventing layer, a barrier layer, and a conductive metal layer. The gate insulating layer may be formed on a semiconductor substrate. The polysilicon layer may be formed on the gate insulating layer and may be doped with impurities. The interface reaction preventing layer may be formed on the polysilicon layer and may include a metal-rich metal silicide having a metal mole fraction greater than a silicon mole fraction. The barrier layer may be formed on the interface reaction preventing layer and may reduce or prevent the diffusion of impurities doped in the polysilicon layer. In addition, the interface reaction preventing layer may reduce or prevent a chemical interfacial reaction with the barrier layer. The conductive metal layer may be formed on the barrier layer and may include a refractory metal. 
     The mole ratio of metal to silicon in the metal silicide layer may be in the range of about 1:1 to about 4:1. For example, the metal silicide layer may include one of tungsten silicide, titanium silicide, and tantalum silicide. The barrier layer may include a nitride layer including a refractory metal. For example, the nitride may be one of a tungsten nitride, titanium nitride, and tantalum nitride. The conductive metal layer may include one of a tungsten layer, titanium layer, and tantalum layer. The semiconductor device may further include source/drain regions and a channel region. The source/drain regions may be doped with impurities formed at the surface of the substrate. The channel region may be formed at a surface of the substrate between the source/drain regions. 
     A method of manufacturing a semiconductor device according to example embodiments may include forming a first layer on a semiconductor substrate and forming a second layer doped with impurities on the first layer. A third layer may be formed on the second layer and may include a metal-rich metal silicide having a metal mole fraction greater than a silicon mole fraction. A fourth layer may be formed on the third layer, wherein the fourth layer may reduce or prevent the diffusion of impurities doped in the second layer. The third layer may reduce or prevent a chemical interfacial reaction with the fourth layer. A fifth layer may be formed on the fourth layer and may include a refractory metal. The fifth layer, the fourth layer, the third layer, the second layer, and the first layer may be sequentially patterned to form a gate electrode including a gate insulating layer, a polysilicon layer, an interface reaction preventing layer, a barrier layer, and a conductive metal layer on the semiconductor substrate. 
     The first layer may be formed to a thickness of about 10 Å to about 100 Å by a thermal oxidation process. The second layer may be formed to a thickness of about 500 Å to about 2000 Å by chemical vapor deposition (CVD) process using a source gas including silane (SiH 4 ) gas and an impurities gas. The third layer may be formed on the second layer to a thickness of about 10 Å to about 50 Å. The third layer may be a metal-rich metal silicide, wherein a mole ratio of metal to silicon in the metal-rich metal silicide may be in the range of about 1:1 to about 4:1. The metal silicide may include tungsten, titanium, tantalum, and other suitable metals. 
     To form the third layer, a dummy metal layer may be formed on the second layer, and a thermal treatment may be performed on the dummy metal layer under a nitrogen atmosphere. The dummy metal layer may be formed on the second layer by one of a sputtering process, a CVD process, and an atomic layer deposition (ALD) process. The thermal treatment for the dummy metal layer may be performed at a temperature of about 400° C. to about 600° C. using ammonia (NH 3 ) gas or a mixture of ammonia (NH 3 ) gas and hydrogen (H 2 ) gas. Additionally, a lower portion of the dummy metal layer contacting the second layer may be transformed into a metal silicide by the thermal treatment to thereby form the third layer, and an upper portion of the dummy metal layer may be transformed into a metal nitride by the thermal treatment to thereby form the fourth layer, so that the third layer and the fourth layer may be simultaneously formed on the second layer. 
     The third layer may also be formed by providing titanium chloride (TiCl 4 ) gas and hydrogen (H 2 ) gas to a processing chamber having a pressure of about 0.5 Torr to about 10 Torr and a temperature of about 400° C. to about 600° C. The titanium chloride (TiCl 4 ) gas and hydrogen (H 2 ) gas may be deposited onto the second layer by a CVD process to thereby form a titanium silicide layer on the second layer. 
     Alternatively, a metal silicide layer may be formed on the second layer by an ALD process using a metal precursor and hydrogen (H 2 ) gas as a reaction gas at a temperature of about 400° C. to about 600° C. to form the third layer. Alternatively, a dummy metal layer may be formed on the second layer by a CVD process using a metal precursor and a hydrogen (H 2 ) gas at a temperature of about 400° C. to about 600° C., wherein the dummy metal layer may be transformed into a metal silicide layer by a hydrogen (H Z ) plasma treatment or an ammonia (NH 3 ) plasma treatment to thereby form the third layer. Alternatively, a metal silicide layer may be formed on the second layer by an ALD process using a metal precursor and a silane (SiH 4 ) gas as a reaction gas at a temperature of about 400° C. to about 600° C. to form the third layer. 
     To form the fourth layer, an argon (Ar) gas and a nitrogen gas may be provided to a processing chamber containing a target metal and a substrate having the third layer. Plasma may be generated in the processing chamber, and a metal nitride layer may be formed on the third layer by a chemical reaction between nitrogen atoms of the nitrogen gas activated by the plasma and metal atoms sputtered from the target metal by the plasma. The target metal may include one of tungsten, titanium, and tantalum. To generate the plasma, electrical power of about 0.3 kW to about 1.2 kW may be supplied to the processing chamber. For example, the nitrogen gas may be removed from the processing chamber and the electrical power supplied to the processing chamber may be increased to about 0.5 kW to about 1.6 kW to form the fifth layer on the fourth layer. 
     A metal nitride layer may be formed on the third layer by a CVD process or an ALD process to form the fourth layer. The metal nitride layer may include one of a tungsten nitride layer, a titanium nitride layer, and a tantalum nitride layer. The fourth layer may be formed to a thickness of about 10 Å to about 100 Å. 
     To form the fifth layer, a refractory metal may be deposited on the fourth layer by one of a sputtering process, a CVD process, a PVD process, and an ALD process. For example, the refractory metal may include one of tungsten, titanium, tantalum, and alloys thereof. 
     Source/drain regions may be formed on the surface of the substrate adjacent to the gate electrode by injecting impurities onto the surface of the substrate by an ion implantation process using the gate electrode as an ion implantation mask. 
     According to example embodiments, a metal-rich metal silicide may be used as the interface reaction preventing layer, so that a chemical reaction between the interface reaction preventing layer and the barrier layer may be reduced or prevented during a thermal treatment process at a higher temperature. Therefore, the interface reaction preventing layer may improve the barrier characteristic of the barrier layer. Further, the interface reaction preventing layer may be formed relatively uniformly by a lower temperature process, so that surface agglomerations may be sufficiently reduced or prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a conventional semiconductor device including a tungsten/polysilicon gate electrode. 
         FIG. 2  is a scanning electron microscope (SEM) picture showing an agglomeration generated on the tungsten silicide layer of a conventional semiconductor device during a thermal treatment process. 
         FIG. 3  is a cross-sectional view illustrating a semiconductor device including a poly-metal gate electrode according to example embodiments. 
         FIGS. 4A to 4F  are cross-sectional views illustrating a process for manufacturing the semiconductor device of  FIG. 3  according to example embodiments. 
         FIGS. 5A to 5D  are scanning electron microscope (SEM) pictures measuring the adhesiveness of a titanium-rich titanium silicide (TiSi x ) layer formed in a lower temperature process according to example embodiments. 
         FIG. 6  is a transmission electron microscope (TEM) picture measuring the adhesiveness of a titanium-rich titanium silicide (TiSi x ) layer conformally formed on a gate insulating layer according to example embodiments. 
         FIGS. 7A to 7D  are scanning electron microscope (SEM) pictures measuring the adhesiveness of a conventional titanium-rich titanium silicide (TiSi x ) layer formed in a higher temperature process. 
         FIG. 8  is a graph estimating a voltage-current characteristic of a poly-metal gate electrode including a titanium-rich titanium silicide (TiSi x ) layer according to example embodiments. 
         FIG. 9  is a graph showing an auger electron spectroscopy (AES) analysis result of a titanium-rich titanium silicide (TiSi x ) layer formed in a lower temperature process according to example embodiments. 
         FIG. 10  is a graph showing an auger electron spectroscopy (AES) analysis result of a conventional titanium-rich titanium silicide (TiSix) layer formed in a higher temperature process. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or connected to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Example embodiments are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 3  is a cross-sectional view illustrating a semiconductor device including a poly-metal gate electrode according to example embodiments. Referring to  FIG. 3 , the semiconductor device  900  may include a gate electrode  200 , source/drain electrodes  300   a  and  300   b , and a channel region  400 . The gate electrode  200  may have a poly-metal structure formed on a substrate  100 . The source/drain electrodes  300   a  and  300   b  may be formed on the surface of the substrate  100  adjacent to the gate electrode  200 . The channel region  400  may be formed under the gate electrode  200 , and the source/drain electrodes  300   a  and  300   b  may be electrically connected to each other by the channel region  400 . 
     The substrate  100  may include a silicon substrate, a silicon-on-insulator (SOI) substrate, a germanium substrate, a germanium-on-insulator (GOI) substrate, a silicon-germanium substrate, and an epitaxial layer obtained by a selective epitaxial growth (SEG) process. For example, the substrate  100  may include a silicon wafer. The substrate  100  may include p-type well (not shown) or n-type well (not shown). 
     The substrate  100  may include an active region where a variety of conductive structures may be formed and a field region surrounding the active region. An insulating layer may be formed on the field region so that the conductive structures formed on the adjacent active regions may be electrically isolated from each other. For that reason, the insulating layer on the field region of the substrate  100  may be referred to as a device isolation layer  110 . For example, the device isolation layer  110  may be a field oxide layer or a trench isolation layer. A trench isolation layer may be used as the device isolation layer  110 , because the trench isolation layer may permit a higher degree of integration than a field oxide layer. 
     The source/drain regions  300   a  and  300   b  doped with impurities may be formed at a surface of the substrate  100 . The source/drain regions  300   a  and  300   b  may be formed at the surface of the substrate  100  adjacent to the gate electrode  200  by an ion implantation process. The channel region  400  may be formed under the gate electrode  200  between the source/drain regions  300   a  and  300   b.    
     The gate electrode  200  may be formed on the channel region  400  of the substrate  100 . The gate electrode  200  may include a poly-metal electrode structure  290  and a gate insulating layer  210 . The gate insulating layer  210  may be formed between the poly-metal electrode structure  290  and the channel region  400 . The gate insulating layer  210  may electrically insulate the poly-metal electrode structure  290  from the channel region  400 , thus reducing or preventing a current leakage between the electrode structure  290  and the substrate  100 . The gate insulating layer  210  may include a silicon oxide layer formed on a surface of the substrate  100  by a thermal oxidation process to a thickness of about 10 Å to about 100 Å. The gate insulating layer  210  may include a metal oxide layer or a metal silicide layer provided that the metal oxide layer or the metal silicide layer has a sufficiently small equivalent oxide thickness (EOT) so as to sufficiently reduce or prevent current leakage through the gate insulating layer  210 . 
     The poly-metal electrode structure  290  may include a multi-layer structure having a polysilicon layer  220 , an interface reaction preventing layer  230 , a barrier layer  240 , and a conductive metal layer  250  sequentially stacked on the gate insulating layer  210 . The polysilicon layer  220  may have a smaller resistivity (e.g., electrical resistance) and a higher oxidation resistance. The polysilicon layer  220  may have a higher stability so as to have sufficient mechanical stability with respect to an externally applied force. Furthermore, the polysilicon layer  220  may be doped with impurities so as to have sufficient electrical conductivity. The impurities may include a group III or group V element depending on the electrical polarity of the semiconductor device. The polysilicon layer  220  may have a thickness of about 500 Å to about 2000 Å. 
     The conductive metal layer  250  may have a lower resistivity and may be provided on the polysilicon layer  220 . The conductive metal layer  250  may include a refractory metal that is sufficiently resistive to a thermal treatment process. The refractory metal of the conductive metal layer  250  may include tungsten (W), titanium (Ti), tantalum (Ta), and alloys thereof. For example, the conductive metal layer  250  may include tungsten, which may have a lower resistivity than the other refractory metals, and may have a thickness of about 500 Å to about 1000 Å. 
     The barrier layer  240  may reduce or prevent the diffusion of impurities from the polysilicon layer  220  into the conductive metal layer  250  during a process of depositing a refractory metal onto the polysilicon layer  220 . Thus, the barrier layer  240  may reduce or prevent the silicidation of the refractory metal during the deposition process and may improve the adhesiveness of the conductive metal layer  250  and the polysilicon layer  220 . The barrier layer  240  may include a nitride including a refractory metal (e.g., a tungsten nitride (WN) layer, a titanium nitride (TiN) layer, and a tantalum nitride (TaN) layer). The barrier layer  240  may include a metal nitride having the same metal component as the conductive metal layer  250  or as that of the metal silicide of the interface reaction preventing layer  230 . For example, the barrier layer  240  may include a tungsten nitride (WN) layer, and the conductive metal layer  250  may include a tungsten (W) layer, or the barrier layer  240  may include a titanium nitride (TiN) layer, and the interface reaction preventing layer  230  may include a titanium silicide layer. The barrier layer  240  may have a thickness of about 10 Å to about 100 Å. 
     The interface reaction preventing layer  230  may be interposed between the barrier layer  240  and the polysilicon layer  220 . The interface reaction preventing layer  230  may reduce or prevent the generation of an interface insulating layer having a higher resistance (e.g., a silicon nitride layer) on a surface of the polysilicon layer  220  and may reduce or prevent an interfacial reaction by an oxidizer during a re-oxidation process. The interface reaction preventing layer  230  may include a refractory metal silicide (e.g., titanium silicide (TiSi x ), tantalum silicide (TaSi x )). Accordingly, the electrical resistance at an interface between the polysilicon layer  220  and the conductive metal layer  250  may be sufficiently reduced or prevented to thereby improve the ohmic characteristic of the poly-metal electrode structure  290 . The interface reaction preventing layer  230  may have a thickness of about 10 Å to about 50 Å. 
     The interface reaction preventing layer  230  may include a metal silicide having a mole ratio of metal to silicon (metal:Si) about equal to or greater than 1:1. Consequently, an interfacial reaction between the interface reaction prevention layer  230  and the barrier layer  240  may be reduced or prevented during a thermal treatment performed at a relatively high temperature. The interface reaction preventing layer  230  may include a metal silicide having a mole ratio of metal to silicon (metal:Si) about equal to or greater than 2:1. However, when the mole ratio of metal to silicon (metal:Si) is more than about 4:1 in the interface reaction preventing layer  230 , formation of the interface reaction preventing layer  230  may take longer, thus reducing process efficiency. Accordingly, it may be beneficial for the interface reaction preventing layer  230  to have a mole ratio of metal to silicon (metal:Si) in the range of about 2:1 to about 4:1. The interface reaction preventing layer  230  may reduce or prevent the thinning of the barrier layer  240  during a thermal treatment process, thus preserving its barrier characteristic. 
     The interface reaction preventing layer  230  may include titanium silicide, wherein the mole ratio of titanium (Ti) to silicon (Si) may be about equal to or greater than 1:1 (e.g., titanium-rich titanium silicide), so that the metallic properties may be more dominant in the interface reaction preventing layer  230 . Consequently, an interfacial reaction at the interface of the interface reaction preventing layer  230  and the barrier layer  240  may be reduced or prevented during a higher temperature thermal treatment process to thereby reduce or prevent dissociation of the barrier layer  240 . 
     Titanium-rich titanium silicide may be deposited at a lower temperature of about 400° C. to about 600° C. The deposition process may be easier to control because of the slower reaction rate between titanium and silicon. Accordingly, the titanium-rich titanium silicide layer may be formed relatively uniformly on the surface of the polysilicon layer  220  while reducing or preventing the generation of agglomerations and/or voids. As described in the following experimental results, the relative uniformity of the titanium-rich titanium silicide layer may be maintained during a thermal treatment performed at a higher temperature to thereby reduce or prevent the agglomeration of the titanium-rich titanium silicide layer and the increased sheet-resistance caused by the agglomeration. Therefore, the ohmic characteristic of the poly-metal electrode structure  290  may be improved. 
     The semiconductor device  900  may further include a spacer (not shown) at a side portion of the gate electrode  200 . For example, the source/drain electrodes  300   a  and  300   b  may have a light junction area where the impurities may be implanted by an ion implantation process using the gate electrode  200  as an ion implantation mask and a heavy junction area where the impurities may be implanted by an ion implantation process using the spacer as an ion implantation mask. 
     As mentioned above, the semiconductor device  900  may include the poly-metal gate electrode  200  having the interface reaction preventing layer  230  with the metal-rich metal silicide. Accordingly, a chemical reaction at the interface between the interface reaction preventing layer  230  and the barrier layer  240  may be reduced or prevented during a higher temperature thermal treatment process, thus preserving the barrier characteristic of the barrier layer  240 . In addition, the interface reaction preventing layer  230  may have a relatively uniform thickness because of a lower temperature process, thus reducing or preventing agglomerations at the surface of the interface reaction preventing layer  230 . 
       FIGS. 4A to 4F  are cross-sectional views illustrating processing steps for manufacturing the semiconductor device shown in  FIG. 3  according to example embodiments. Referring to  FIG. 4A , a trench isolation layer may be formed on a semiconductor substrate  100  as a device isolation layer  110 , thereby defining an active region and a field region on the substrate  100 . A pad oxide layer and a pad nitride layer may be formed on the substrate  100  and may be patterned into a pad oxide layer pattern and a pad nitride layer pattern through which the substrate  100  is partially exposed. A trench may be formed on the semiconductor substrate  100  by an etching process using the pad oxide layer pattern and the pad nitride layer pattern as etch masks. An oxide layer having an increased fill-up characteristic may be formed to a sufficient thickness on the resultant structure including the trench so as to fill the trench using a plasma-enhanced chemical vapor deposition (PECVD) process. As a result, the trench may be filled with the oxide layer. The oxide layer, the pad nitride layer pattern, and the pad oxide layer pattern may be removed from the substrate  100  until a top surface of the substrate  100  is exposed using a planarization process (e.g., a chemical mechanical polishing (CMP) process), so that the oxide layer remains in the trench to thereby form the trench isolation layer on the substrate  100  as the device isolation layer  110 . 
     A first layer  210   a  may be formed on the substrate  100  including the device isolation layer  110 . The first layer  210   a  may be formed into a gate insulating layer  210  of the gate electrode  200  by a subsequent process. The first layer  210   a  may include a silicon oxide layer on the substrate  100 . A thermal treatment may be performed on the substrate  100  including the device isolation layer  110  at a temperature of about 850° C. for about 4 hours under an atmosphere having an oxygen gas or vapor to thereby form the silicon oxide layer on the substrate  100  to a thickness of about 10 Å to about 100 Å. The first layer  210   a  may include a metal oxide layer and a metal silicate layer in accordance with characteristics of the semiconductor device  900 . The metal oxide layer and the metal silicate layer may be formed on the substrate  100  by a CVD process, a sputtering process, or ALD process. 
     Referring to  FIG. 4B , a second layer  220   a  may be formed on the first layer  210   a . The second layer  220   a  may be formed into a polysilicon layer  220  of the poly-metal electrode structure  290  by a subsequent process. The second layer  220   a  may be formed by thermal dissociation of a source gas (e.g., silane (SiH 4 ) gas) during a CVD process. When a mixture of silane (SiH 4 ) gas and phosphine (PH 3 ) gas is provided into a processing chamber having a temperature of about 500° C. to about 650° C. and a pressure of about 25 Pa to about 150 Pa and the substrate  100  including the first layer  210   a  is provided in the processing chamber, the silane (SiH 4 ) gas may be deposited onto a surface of the first layer  210   a  to thereby form a polysilicon layer on the first layer  210   a , and phosphorus (P) may be doped into the polysilicon layer as an impurity. Thus, the second layer  220   a  including the polysilicon layer doped with phosphorus (P) may be formed on the surface of the first layer  210   a . While example embodiments disclose phosphorus (P) as the impurity doped into the polysilicon layer, various elements from Group III or Group V of the periodic table (e.g., boron (B), arsenic (As)) may be used as impurities in the polysilicon layer depending on the usage and characteristics of the semiconductor device. The second layer  220   a  may be formed to a thickness of about 500 Å to about 2000 Å on the first layer  210   a.    
     Referring to  FIG. 4C , a third layer  230   a  may be formed on the second layer  220   a . The third layer  230   a  may be formed into an interface reaction preventing layer  230  of the poly-metal electrode structure  290  by a subsequent process. The interface reaction preventing layer  230  may be formed between the barrier layer  240  and the polysilicon layer  220  and may reduce or prevent the generation of an interface insulating layer having a higher resistance based on silicon nitride of the polysilicon layer  220  and the interfacial reaction caused by an oxidizer during a re-oxidation process. Accordingly, the third layer  230   a  may include metal silicide layer having a refractory metal. For example, the metal silicide layer may include tungsten silicide (WSi x ), titanium silicide (TiSi x ), and tantalum silicide (TaSi x ). 
     Hereinafter, a formation process of the third layer  230   a  will be described. A titanium silicide layer may be formed on the second layer  220   a  as the third layer  230   a . However, a tantalum silicide layer may be formed on the second layer  220   a  by the same lower temperature process that may be used to form the titanium silicide layer although some processing conditions for the tantalum silicide layer may differ from those for the titanium silicide layer. For that reason, the tantalum silicide layer may be suitable as the third layer  230   a.    
     The third layer  230   a  including titanium silicide may be formed by a thermal treatment process. Titanium may be deposited onto a surface of the second layer  220   a  by a CVD process, a sputtering process, or an ALD process. The third layer  230   a  may be formed to a thickness of about 10 Å to about 50 Å. A thermal treatment may be performed on the titanium layer at a relatively low temperature of about 400° C. to about 600° C. in a nitrogen atmosphere. The titanium layer on the second layer  220   a  may be transformed into a titanium silicide (TiSi x ) layer due to heat applied during the thermal treatment. Accordingly, the titanium silicide layer may be formed on the second layer  220   a  as the third layer  230   a . The titanium silicide layer may include titanium-rich titanium silicide, wherein a mole ratio of titanium (Ti) to silicon (Si) may be equal to or greater than 1:1. For example, the titanium silicide layer may include titanium-rich titanium silicide, wherein a mole ratio of titanium (Ti) to silicon (Si) may be equal to or greater than 2:1. However, when the mole ratio of titanium (Ti) to silicon (Si) is more than 4:1 in the titanium-rich titanium silicide, it may take longer to form the titanium silicide layer, thus reducing process efficiency. Accordingly, the titanium silicide layer may have a mole ratio of titanium (Ti) to silicon (Si) in the range of about 2:1 to about 4:1. 
     The third layer  230   a  including titanium silicide may be formed on the second layer  220   a  by a deposition process (e.g., a CVD process, an ALD process) rather than a thermal treatment. The substrate  100  including the second layer  220   a  may be provided in a processing chamber having a temperature in the range of about 400° C. to about 600° C. and a pressure in the range of about 0.5 Torr to about 10 Torr. Titanium chloride (TiCl 4 ) gas and hydrogen (H 2 ) gas may be provided into the processing chamber. Titanium chloride (TiCl 4 ) may chemically react with the polysilicon of the second layer  220   a  to thereby form the titanium silicide layer on the second layer  220   a . The titanium chloride (TiCl 4 ) gas and the hydrogen (H 2 ) gas may be provided into the processing chamber in an amount such that a mole ratio of titanium (Ti) to silicon (Si) may be in the range of about 1:1 to about 4:1 in the resulting titanium silicide layer. 
     Where an ALD process is used for forming the third layer  230   a  including titanium silicide, a titanium precursor and monosilane (SiH 4 ) gas may be provided into a processing chamber as a reaction gas. Hydrogen (H 2 ) gas may be provided into the processing chamber in place of the monosilane (SiH 4 ) gas as the reaction gas. The processing conditions of the ALD process may be controlled in such a manner that a mole ratio of titanium (Ti) to silicon (Si) may be in the range of about 1:1 to about 4:1 in the resulting titanium silicide layer. 
     The third layer  230   a  including titanium silicide may be formed by a cyclic chemical vapor deposition (CVD) process using a titanium precursor and hydrogen (H 2 ) gas. The substrate  100  including the second layer  220   a  may be provided into a processing chamber having a temperature in the range of about 400° C. to about 600° C. along with a titanium (Ti) precursor and hydrogen (H 2 ) gas. A dummy titanium layer may be formed on the second layer  220   a  by the CVD process. The dummy titanium layer may be transformed into a titanium silicide layer by a hydrogen (H 2 ) plasma treatment process or an ammonium (NH 3 ) plasma treatment process. Accordingly, the titanium silicide layer may be formed on the second layer  220   a  as the third layer  230   a  by the cyclic CVD process, wherein a unit cycle may include a CVD process and a plasma treatment. 
     The formation process of the titanium silicide layer may be performed at a lower temperature than a conventional process, thus rendering the formation process easier to control. Consequently, the third layer  230   a  may be formed on the second layer  220   a  at a relatively uniform thickness so as to sufficiently reduce or prevent processing defects (e.g., local agglomeration, void). The third layer  230   a  may include titanium-rich titanium silicide, wherein a mole ratio of titanium (Ti) with respect to silicon (Si) may be larger than 1:1 because of the lower temperature process. Accordingly, a chemical reaction on an interface between the third layer  230   a  and a fourth layer  240   a  may be reduced or prevented during a higher temperature thermal treatment process. Thus, the dissociation of the fourth layer  240   a  may be reduced or prevented during a thermal treatment process to thereby reduce or prevent thinning of the fourth layer  240   a  during the thermal treatment process. 
     Referring to  FIG. 4D , the fourth layer  240   a  may be formed on the third layer  230   a . The fourth layer  240   a  may be formed into a barrier layer  240  of the poly-metal electrode structure  290  in a subsequent process. The fourth layer  240   a  may reduce or prevent the diffusion of impurities from the second layer  220   a  into the fifth layer during a process for forming the fifth layer  250   a  in  FIG. 4E . For example, the fourth layer  240   a  may include a nitride layer including a refractory metal (e.g., a tungsten nitride (WN) layer, a titanium nitride (TiN) layer, a tantalum nitride (TaN) layer). The fourth layer  240   a  may include a metal nitride having the same metal as the fifth layer  250   a  or the metal silicide of the third layer  230   a . Thus, when the fifth layer  250   a  includes a tungsten layer, the fourth layer  240   a  may include a tungsten nitride (WN) layer. When the third layer  230   a  includes a titanium silicide layer, the fourth layer  240   a  may include a titanium nitride layer. When the metal of the fourth layer  240   a  is substantially identical to a metal of the fifth layer  250   a , the fourth layer  240   a  and the fifth layer  250   a  may be consecutively formed by a successive process. In addition, when the metal of the fourth layer  240   a  is substantially identical to a metal in a metal silicide of the third layer  230   a , the third layer  230   a  and the fourth layer  240   a  may also be consecutively formed by a successive process. 
     A deposition process (e.g., a CVD process, an ALD process, a sputtering process) may be used where the fourth layer  240   a  includes tungsten nitride (WN). A processing chamber may be controlled to a temperature of about 200° C., and a mixture of argon (Ar) gas and nitrogen (N 2 ) gas may be provided into the processing chamber. A power of about 0.3 kW to about 1.2 kW may be supplied to the processing chamber at a pressure of about 1300 Pa. For example, a power of about 0.8 kW may be supplied to the processing chamber. As a result, plasma for sputtering a tungsten target may be generated in the processing chamber. Tungsten atoms sputtered from a tungsten target may react with activated nitrogen atoms because of the plasma on a surface of the third layer  230   a , thereby forming the fourth layer  240   a  including tungsten nitride (WN). The fourth layer  240   a  may be formed to a thickness of about 10 Å to about 100 Å. 
     The supply of nitrogen gas may be stopped, such that only argon gas is provided into the processing chamber with a simultaneous power increase to the processing chamber. For example, electrical power of about 0.5 kW to about 1.6 kW (e.g., about 1.5 kW) may be supplied to the processing chamber while the argon gas is being provided. As shown in  FIG. 4E , the fifth layer  250   a  including tungsten may be formed on the fourth layer  240   a . The fifth layer  250   a  may be formed to a thickness of about 100 Å to about 1,000 Å. 
     The third layer  230   a  and the fourth layer  240   a  may be simultaneously formed in the same process where the fourth layer  240   a  includes titanium nitride (TiN). When a thermal treatment is performed on a titanium metal layer formed on the second layer  220   a  at a lower temperature, the titanium metal layer adjacent to the second layer  220   a  may be transformed into a titanium silicide layer by the thermal treatment, and the rest of the titanium metal layer may react with nitrogen (N 2 ) and, thus, become transformed into a titanium nitride layer. The heat-treatment performed at a lower temperature may accelerate the nitrification process, because ammonium (NH 3 ) gas or a mixture of ammonium (NH 3 ) gas and hydrogen (H 2 ) gas may be supplied to a thermal treatment conducted at a lower temperature. Thus, a lower portion of the titanium metal layer on the second layer  120   a  may be transformed into the titanium silicide layer, thereby forming the third layer  230   a  including titanium silicide. An upper portion of the titanium metal layer may be transformed into the titanium nitride layer by a nitrification process, thereby forming the fourth layer  240   a  including titanium nitride. 
     The fifth layer  250   a  may be formed on the fourth layer  240   a  by depositing a refractory metal using a sputtering process, a CVD process, a physical vapor deposition (PVD) process, or an ALD process. The refractory metal may include tungsten (W), titanium (Ti), tantalum (Ta), and other suitable metals. For example, the fifth layer  250   a  may include a tungsten layer formed by sputtering at a temperature of about 150° C., a pressure of about 4 mTorr, and a power of about 2 kW. The fifth layer  250   a  may be formed into a conductive metal layer  250  of the poly-metal electrode structure  290  by a subsequent process. The conductive metal layer  250  may include a refractory metal having a lower resistivity. The fifth layer  250   a  may be formed to a thickness of about 500 Å to about 1000 Å. 
     Referring to  FIG. 4F , the fifth layer  250   a , the fourth layer  240   a , the third layer  230   a , the second layer  220   a , and the first layer  210   a  may be subsequently removed from the substrate  100  by a patterning process (e.g., a photolithography process) using a photoresist pattern (not shown) as an etching mask. As a result, the gate insulating layer  210 , the polysilicon layer  220 , the interface reaction preventing layer  230 , the barrier layer  240 , and the conductive metal layer  250  may be sequentially stacked on the substrate  100 , thereby forming the gate electrode  200  on the substrate  100 . 
     An ion implantation process may be performed on a surface of the substrate  100  using the gate electrode  200  as an ion implantation mask so as to form source/drain electrodes  300   a  and  300   b  doped with impurities at the surface of the substrate  100  adjacent to the gate electrode  200  as illustrated in  FIG. 3 . A channel region  400  may be formed under the gate electrode  200  between the source/drain electrodes  300   a  and  300   b.    
     A spacer (not shown) may be further formed at a sidewall of the gate electrode  200  after forming the source/drain electrode  300   a  and  300   b . When the spacers are formed at the sidewall of the gate electrode  200 , an additional ion implantation process may be further performed on the surface of the substrate  100  using the spacer and the gate electrode  200  as an ion mask. As a result, the source/drain electrodes  300   a  and  300   b  may be formed into a lighter doped drain (LDD) structure having a lighter junction area and a heavier junction area. 
     Where the interface reaction preventing layer  230  includes a metal-rich metal silicide, an interfacial reaction between the interface reaction preventing layer  230  and the barrier layer  240  may be reduced or prevented during an etching process for forming the gate electrode  200 , thus reducing or preventing the dissociation of the barrier layer  240 . Accordingly, the thinning of the barrier layer  240  may be reduced or prevented despite the etching process to thereby sufficiently preserve the barrier characteristics of the barrier layer  240 . In addition, the interface reaction preventing layer  230  may be formed at a relatively low temperature, and the formation process may be controlled with relative accuracy. As a result, the interface reaction preventing layer  230  may be formed on the polysilicon layer  220  to a relatively uniform thickness, and agglomeration and/or voids may be sufficiently reduced or prevented in the interface reaction preventing layer  230  to thereby improve the ohmic characteristic of the poly-metal electrode structure  290 . 
     Tests on Characteristics of a Titanium-Rich Titanium Silicide Layer 
     For a titanium-rich titanium silicide (TiSi x ) layer to function as an interface reaction preventing layer  230  of a poly-metal electrode structure  290  according to example embodiments, it may be beneficial for the titanium-rich titanium silicide (TiSi x ) layer to be relatively non-reactive with respect to the barrier layer  240  during a thermal treatment. It may also be beneficial for there to be a sufficiently small interface resistance between the barrier layer  240  and the titanium-rich titanium silicide (TiSi x ) layer. Consequently, experiments were carried out to examine the characteristics of the titanium-rich titanium silicide (TiSi x ) layer according to example embodiments. 
     1) Test on the Interface Resistance of the Titanium-Rich Titanium Silicide Layer 
     The interface reaction preventing layer  230  may be partially agglomerated when higher temperatures are applied during a thermal treatment process, thereby resulting in a higher interface resistance. The interface resistance of the interface reaction preventing layer  230  may affect the ohmic characteristic of the poly-metal electrode structure  290 . Accordingly, experimental tests were conducted on the density of agglomerations to determine whether ohmic characteristics of the poly-metal electrode structure  290  may be improved by the interface reaction preventing layer  230  according to example embodiments. 
       FIGS. 5A to 5D  are scanning electron microscope (SEM) images showing the agglomeration density of a titanium-rich titanium silicide (TiSi x ) layer formed in a lower temperature process.  FIG. 6  is a transmission electron microscope (TEM) image showing a poly-metal electrode structure including the titanium-rich titanium silicide (TiSi x ) layer.  FIGS. 7A to 7D  are SEM images showing the agglomeration density of a conventional titanium-rich titanium silicide (TiSi x ) layer formed in a higher temperature process. The titanium-rich titanium silicide (TiSi x ) layer in  FIGS. 5A to 5D  and  FIG. 6  was formed at a lower temperature of about 530° C., while the titanium-rich titanium silicide (TiSi x ) layer in  FIGS. 7A to 7D  was formed at a higher temperature of about 650° C.  FIGS. 5A ,  5 B,  6 ,  7 A, and  7 B were taken by an SEM after formation of the titanium-rich titanium silicide (TiSi x ) layer, and the images in  FIGS. 5C ,  5 D,  7 C, and  7 D were taken by an SEM after a rapid thermal treatment (RTP) in a nitrogen atmosphere.  FIGS. 5A ,  5 C,  7 A, and  7 C are enlarged images of a surface of the titanium-rich titanium silicide (TiSi x ) layer at a magnification of about one hundred thousand times, and  FIGS. 5B ,  5 D,  76 , and  7 D are enlarged images of a cross-section of the titanium-rich titanium silicide (TiSi x ) layer. 
     A comparison of the images in  FIGS. 5A to 5D  and the images in  FIGS. 7A to 7D  shows that the agglomeration density of the lower-temperature titanium-rich titanium silicide (TiSi x ) layer is lower than that of the higher-temperature titanium-rich titanium silicide (TiSi x ) layer, thereby showing that agglomerations of a titanium-rich titanium silicide layer may be reduced by a lower temperature process.  FIG. 5A  shows an improvement over  FIG. 7A  as to agglomerations of the titanium-rich titanium silicide (TiSi x ) layer after depositing the titanium-rich titanium silicide (TiSi x ) layer. The higher-temperature titanium-rich titanium silicide layer had a more agglomerated and irregular surface. In contrast, the lower-temperature titanium-rich titanium silicide (TiSi x ) layer was less agglomerated and had a relatively flat surface. 
     Furthermore, a comparison of the images in  FIGS. 5C and 7C  indicate that the lower-temperature titanium-rich titanium silicide layer was less agglomerated than the higher-temperature titanium-rich titanium silicide layer after a thermal treatment was performed. The improvement on the degree of agglomeration may be observed in the comparison of the SEM images in  FIGS. 5B and 7B  and the SEM images in  FIGS. 5D and 7D , which show the cross-sectional surface of the titanium-rich titanium silicide (TiSi x ) layer. Referring to  FIG. 5B , a top portion of the cross-sectional surface of the lower-temperature titanium-rich titanium silicide (TiSi x ) layer appears relatively linear, thus showing that fewer agglomerations were on the surface of the lower-temperature titanium-rich titanium silicide (TiSi x ) layer. In contrast, referring to  FIG. 5D , a top portion of the cross-sectional surface of the higher-temperature titanium-rich titanium silicide (TiSi x ) layer appears to have a plurality of concave and convex portions, thus showing that more agglomerations were on the surface of the higher-temperature titanium-rich titanium silicide (TiSi x ) layer. As a result, the comparison of  FIGS. 5B and 5D  indicates that the uniformity of the titanium-rich titanium silicide layer may be improved by the lower temperature process. 
     A comparison of  FIGS. 7B and 7D  indicates that a higher temperature thermal treatment may have a lesser effect on the uniformity of a lower-temperature titanium-rich titanium silicide layer, while a higher temperature thermal treatment may have more of an effect on the uniformity of a higher-temperature titanium-rich titanium silicide layer. Accordingly, the experimental results show that the agglomeration degree of the lower-temperature titanium-rich titanium silicide layer was smaller than that of the higher-temperature titanium-rich titanium silicide layer. Thus, the uniformity of the lower-temperature titanium-rich titanium silicide layer is improved compared to that of the higher-temperature titanium-rich titanium silicide layer. Smaller agglomerations and/or increased uniformity of the lower-temperature titanium-rich titanium silicide layer may lead to an improvement of the interface resistance of the lower-temperature titanium-rich titanium silicide layer, thereby improving the ohmic characteristic of the poly-metal electrode structure  290  including the lower-temperature titanium-rich titanium silicide layer as the interface reaction preventing layer  230 . The TEM image in  FIG. 6  shows that the boundary portion between the polysilicon layer  220  and the interface reaction preventing layer  230  and the boundary portion of the interface reaction preventing  230  and the barrier layer  240  is relatively linear. 
     Further to the above experimental results, processing defects (e.g., agglomerations, voids) may be reduced or prevented in the lower-temperature titanium-rich titanium silicide (TiSi x ) layer according to example embodiments, thereby improving the omhic characteristic of the electrode structure including the lower-temperature titanium-rich titanium silicide layer. 
       FIG. 8  is a graph illustrating the relationship between voltage and current of a poly-metal gate electrode including a titanium-rich titanium silicide (TiSi x ) layer according to example embodiments. The graph in  FIG. 8  results from a linear regression analysis using a least squares method to show the relationship between the currents independently measured at different voltages. Referring to  FIG. 8 , the current passing through the poly-metal gate electrode is linearly proportional to the applied voltage, and a coefficient of determination R 2  is about 1. Accordingly, there may be a stronger linearity at a higher reliability between the voltage and the current of the poly-metal gate electrode. Thus, the poly-metal gate electrode including the titanium-rich titanium silicide as the interface reaction preventing layer may have a stronger linearity at a higher reliability between the voltage and current thereof. As a result, the experimental results confirm that the lower-temperature titanium-rich titanium silicide according to example embodiments may improve the ohmic characteristic of the poly-metal gate electrode. 
     2) Test on the Reactivity of the Titanium-Rich Titanium Silicide Layer with the Barrier Layer 
       FIG. 9  is a graph showing analysis results of the lower-temperature titanium-rich titanium silicide (TiSi x ) layer by an auger electron spectroscopy (AES).  FIG. 10  is a graph showing analysis results of a conventional higher-temperature titanium-rich titanium silicide (TiSi x ) layer by an auger electron spectroscopy (AES). In  FIG. 9 , the AES analysis was performed with respect to the titanium-rich titanium silicide (TiSi x ) layer formed by a sputtering process at a lower temperature of about 530° C. In  FIG. 10 , the AES analysis was performed with respect to the titanium-rich titanium silicide (TiSi x ) layer formed by a sputtering process at a higher temperature of about 650° C. In  FIGS. 9 and 10 , a horizontal axis represents a reaction time in the sputtering process, and a vertical axis represents a peak intensity of an auger electron (hereinafter, referred to as “an auger peak intensity”) of each element in the sputtering process. Graph I shows the auger peak intensity of silicon, and Graph II shows the auger peak intensity of titanium. 
     Referring to  FIGS. 9 and 10 , in the lower-temperature titanium-rich titanium silicide (TiSi x ) layer, the auger peak intensity of the titanium is higher than the auger peak intensity of the silicon up to an initial reaction time of about 750 seconds after the sputtering process is performed. In contrast, in the conventional higher-temperature titanium silicide (TiSi x ) layer, the auger peak intensity of the silicon is higher than the auger peak intensity of the titanium during the same initial reaction time. Accordingly, the graphs in  FIGS. 9 and 10  show that the lower-temperature titanium-rich titanium silicide (TiSi x ) layer may have a larger amount of titanium with respect to the silicon than the conventional higher-temperature titanium silicide (TiSi x ) layer. 
     Table 2 shows thermodynamic simulation results on the reactivity of a titanium silicide (TiSi x ) layer with a tungsten nitride (WN) layer in accordance with the mole ratio of titanium (Ti) and silicon (Si) in the titanium silicide (TiSi x ) layer. The tungsten nitride (WN) layer may be provided on the titanium silicide (TiSi x ) layer and may function as a barrier layer  240  in the poly-metal gate electrode  200 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Reactant (mole) 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Ti 2 Si + W 
                 TiSi + W 
                 TiSi 2  + W 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Product 
                 1.00W 
                 0.33W 
                 0.57WSi 2   
               
               
                   
                 (mole) 
                 — 
                 0.13W 5 Si 3   
                 0.08W 5 Si 3   
               
               
                   
                   
                 0.33Ti 5 Si 3   
                 0.20Ti 5 Si 3   
                 0.20Ti 5 Si 3   
               
               
                   
                   
                 0.33Ti 
               
               
                   
                 Reaction Ratio of 
                 0% 
                 67% 
                 100% 
               
               
                   
                 Tungsten 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 2, when a mole fraction of titanium is greater than the mole fraction of silicon in the titanium silicide (TiSi x ) layer, the reaction of tungsten (W) in the tungsten nitride (WN) layer with silicon (Si) in the titanium silicide (TiSi x ) layer is relatively low (if not nil), thereby reducing or preventing the generation of tungsten silicide (WSi x ). In contrast, when a mole fraction of titanium is smaller than the mole fraction of silicon in the titanium silicide (TiSi x ) layer, all of the tungsten (W) in the tungsten nitride (WN) layer was reacted with silicon (Si) in the titanium silicide (TiSi x ) layer, thereby generating tungsten silicide (WSi x ). The simulation results also show that when the mole ratio of titanium to silicon is equal to or greater than about 2:1, the reaction of the titanium silicide (TiSi x ) layer with the tungsten is relatively low (if not nil). 
     The titanium silicide layer according to example embodiments may include titanium-rich titanium silicide (TiSi x ), wherein a mole fraction of titanium may be greater than the mole fraction of silicon. Thus, the reactivity of the titanium silicide (TiSi x ) with the barrier layer  240  may be decreased, thereby reducing or preventing the dissociation of the barrier layer during a higher temperature thermal treatment. Therefore, the barrier characteristics of the barrier layer  240  may be preserved during a higher temperature thermal treatment. 
     The interface reaction preventing layer  230  of the poly-metal electrode structure  290  may include a metal-rich metal silicide, wherein the mole ratio of metal to silicon equal to or greater than about 1:1. Thus, the chemical reaction between the interface reaction preventing layer  230  and the barrier layer  240  may be reduced or prevented during a higher temperature thermal treatment to thereby reduce or prevent the dissociation of the barrier layer  240 . Accordingly, the barrier characteristic of the barrier layer  240  may be preserved. In addition, the metal-rich metal silicide layer may be formed at a relatively low temperature of about 400° C. to about 600° C., thus enabling the formation process to be more efficiently controlled. Accordingly, the metal-rich metal silicide layer may be formed relatively uniformly on the surface of the polysilicon layer  220  with fewer processing defects (e.g., agglomerations, voids). Furthermore, the relative uniformity of the metal-rich metal silicide layer may be preserved not only after the deposition process but also after a higher temperature thermal treatment process. Thus, increased sheet resistance caused by agglomerations may be reduced or prevented. Therefore, the ohmic characteristics of the poly-metal electrode structure  290  may be improved. 
     While example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of example embodiments of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.