[0001]    The present application claims the benefit of priority under 35 U.S.C. 119 to Korean Patent Application No. 10-2006-0116778, filed on Nov. 24, 2006, the entire contents of which are incorporated herewith by reference. 
       BACKGROUND 
       [0002]    The present invention relates to a semiconductor device, such as a positive channel metal-oxide-semiconductor (PMOS) transistor, and a fabricating method thereof. 
         [0003]      FIG. 1A  is a cross-sectional view of a positive channel metal-oxide-semiconductor (PMOS) transistor illustrating a source/drain impurity ion implantation process according to the related art.  FIG. 1B  is a cross-sectional view of a PMOS transistor illustrating an annealing process after the source/drain impurity ion implantation process according to the related art. 
         [0004]    The PMOS transistor according to the related art is formed as follows. At first, a shallow trench isolation (STI) layer  101  is formed on a semiconductor substrate  100 , such as a silicon substrate, in order to isolate semiconductor devices from one another, as shown in  FIG. 1A . Then, an implantation process is performed to form an N-well  121  on semiconductor substrate  100 . 
         [0005]    After a gate insulation layer  123 , made of a silicon oxide layer, is formed on semiconductor substrate  100 , a poly silicon layer is deposited on gate insulation layer  123  to form a gate electrode  125 . The poly silicon layer and the silicon oxide layer formed on a gate area of semiconductor substrate  100  are patterned through a photolithography process and an etching process by forming a patterned photoresist layer (not shown) on the poly silicon layer. That is, gate electrode  125  and gate insulation layer  123  are formed by etching, except the gate area, the poly silicon layer and the silicon oxide layer formed on semiconductor substrate  100  using the patterned photoresist layer as an etching mask. 
         [0006]    A lightly doped drain (LDD) area  127  is formed in active areas at both sides of gate electrode  125  on semiconductor substrate  100  by implanting low-density impurities. 
         [0007]    An insulation layer is formed on the entire surface of semiconductor substrate  100  to cover gate electrode  125 . A spacer  131  is formed on side walls of gate electrode  125  and gate insulation layer  123  by etching the insulation layer. A source/drain area  133  is formed at both sides of spacer  131  on semiconductor substrate  100  by ion-implanting high density impurities. 
         [0008]    Spacer  131  may be made of a nitride layer. In addition, a tetra ethyl ortho silicate (TEOS)  129  may be formed at the bottom of the nitride layer. 
         [0009]    In order to form source/drain area  133  in the PMOS transistor, high density impurities, such as Boron ions, may be used. 
         [0010]    Recently, the junction depth of source/drain area  133  has been reduced from about 90 nm to about 20 nm in the impurity ion implantation process for forming source/drain area  133  of the PMOS transistor. In order to satisfy such a reduced junction depth, the implantation process needs to be performed at a low energy of about 1 KeV to 5 KeV. 
         [0011]    Since Boron (B) ions have a high diffusivity, in an annealing process for activating the high density Boron ions in source/drain area  133 , Boron ions may be out-diffused even if the ion implantation process is performed with a low ion implantation energy. The speed of the PMOS transistor thus becomes slower, because the out-diffused Boron ions prevent the impurities to be ion-implanted in source/drain area  133  from reaching a desired implantation level. 
       SUMMARY 
       [0012]    Embodiments consistent with the present invention provide a semiconductor device with improved characteristics by preventing impurities from out-diffused in the junction area of, for example, a PMOS transistor, and a fabricating method thereof. 
         [0013]    In one embodiment a semiconductor device includes: a gate insulation layer formed on a predetermined area of a semiconductor substrate; a gate electrode formed on the gate insulation layer; a lightly doped drain (LDD) area formed on the semiconductor substrate at both sides of the gate electrode by implanting low density impurities in the semiconductor substrate using the gate electrode as a mask; a spacer formed on side walls of the gate insulation layer and the gate electrode; a source/drain area formed on the semiconductor substrate at both sides of the gate electrode by implanting high density impurities in the semiconductor substrate using the gate electrode and the spacer as a mask; and an oxide-nitride layer formed on the gate electrode and the source/drain area. 
         [0014]    In another embodiment, a method for fabricating a semiconductor device includes: forming a gate insulation layer and a gate electrode at predetermined areas of a semiconductor substrate; forming low density impurity areas at both sides of the gate electrode on the semiconductor substrate; forming a spacer at side walls of the gate insulation layer and the gate electrode; forming a thermal oxide layer on an exposed portion of the semiconductor substrate and on the gate electrode; forming a source/drain area on the semiconductor substrate by implanting high density impurities at both sides of the gate electrode using the gate electrode and the spacer as an implantation mask; forming an oxide/nitride layer on the semiconductor substrate by processing the thermal oxide layer with an nitrogen plasma process; and performing an annealing process for activating the impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1A  is a cross-sectional view of a positive channel metal-oxide-semiconductor (PMOS) transistor illustrating a source/drain impurity ion implantation process according to the related art. 
           [0016]      FIG. 1B  is a cross-sectional view of a PMOS transistor illustrating an annealing process after the source/drain impurity ion implantation process according to the related art. 
           [0017]      FIG. 2  is a cross-sectional view of a semiconductor device including a PMOS transistor, according to an embodiment consistent with the present invention. 
           [0018]      FIGS. 3A to 3I  are cross-sectional views illustrating a method for fabricating a semiconductor device, according to an embodiment consistent with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 2  is a cross-sectional view of a semiconductor device, according to an embodiment consistent with the present invention. In one embodiment, the semiconductor device may be a PMOS transistor. As shown in  FIG. 2 , the PMOS transistor includes a device isolation layer  201  formed on a semiconductor substrate  200  to define an active area. 
         [0020]    Semiconductor substrate  200  may be a silicon substrate  200 , and device isolation layer  201  may be formed using a shallow trench isolation (STI) process. It is also possible to form device isolation layer  201  by a local oxidation of silicon (LOCOS) process. 
         [0021]    An N-well  221  is formed on semiconductor substrate  200  by implanting N type impurities in the active area. 
         [0022]    A gate insulation layer  223  and a gate electrode  225  are formed in the active area on semiconductor substrate  200 . Gate insulation layer  223  may comprise silicon oxide, and gate electrode  225  may comprise poly silicon. 
         [0023]    Spacers  229  and  231  are formed at side walls of gate electrode  225  and gate insulation layer  223 . Spacers  229  and  231  may comprise an insulation material, such as tetra ethyl ortho silicate (TEOS) and silicon nitride (SiN x ). 
         [0024]    A P type lightly doped drain (P-LDD) area  227  is formed by implanting P type impurities in the active area at both sides of gate electrode  225  on semiconductor substrate  200 . 
         [0025]    Also, a source/drain area  233  is formed in semiconductor substrate  200  by implanting high density P type impurities using spacers  231  and  229  as a mask. 
         [0026]    A silicon oxide nitride (SiON) layer  234  is formed on gate electrode  225  and source/drain area  233 . SiON layer  234  prevents the P type impurities, which may comprise, for example, Boron ions, from out-diffused in an annealing process for activating ions after implanting high density P type impurities for forming source/drain area  233 . Boron ions having 11 AMU (atomic mass unit) may be implanted in semiconductor substrate  200  to form source/drain area  233 . 
         [0027]      FIGS. 3A to 3I  are cross-sectional views illustrating a method for fabricating a semiconductor device, according to an embodiment consistent with the present invention. 
         [0028]    A device isolation layer  201  is formed on a semiconductor substrate  200 , such as a silicon substrate  200 , through a shallow trench isolation (STI) process, as shown in  FIG. 3A , so as to define an active area between device isolation layers  201 . The semiconductor device may be formed in the active area isolated and defined by device isolation layer  201 . 
         [0029]    Then, an N-well  221  is formed by implanting N type impurities in semiconductor substrate  200 , as shown in  FIG. 3B . 
         [0030]    A silicon oxide layer is formed on semiconductor substrate  200  through a thermal oxidation process, and a polysilicon layer is deposited on the silicon oxidation layer. 
         [0031]    As shown in  FIG. 3C , a gate stack formed of gate electrode  225  and gate insulation layer  223  is formed by performing a lithography process and an etching process on the silicon oxidation layer formed on semiconductor substrate  200  and the poly silicon layer formed on the silicon oxidation layer. 
         [0032]    As shown in  FIG. 3D , low density P type impurities are implanted using gate electrode  225  as an implantation mask. As a result, a P-LDD area  227  is formed at both sides of gate electrode  225  on semiconductor substrate  200 . 
         [0033]    As shown in  FIG. 3E , an insulation layer, such as a nitride layer or an oxide/nitride layer, is formed on the entire surface of semiconductor substrate  200  to cover gate electrode  225  as the material of spacers  229  and  231 . Spacers  229  and  231  are formed on side walls of gate electrode  225  and gate insulation layer  223  by an etching process, such as an anisotropic etching process. In one embodiment, the oxide/nitride layer of spacers  229  and  231  may comprise an oxide layer made of tetra ethyl ortho silicate (TEOS) layer  229 , and a nitride layer made of a silicon nitride layer  231 . 
         [0034]    As shown in  FIG. 3F , a thermal oxidation process may be performed on the entire surface of semiconductor substrate  200 , so as to form a thermal oxide layer  232 . In one embodiment, the thermal oxidation process may be a rapid thermal process (RTP). The RTP may be performed in an oxygen atmosphere at a temperature of about 700° C. to 800° C. As a result of the RTP, thermal oxide layer  232  is formed on the exposed gate electrode  225  and the exposed semiconductor substrate  200 . 
         [0035]    As shown in  FIG. 3G , after forming thermal oxide layer  232  on semiconductor substrate  200 , high density P type impurities are implanted in semiconductor substrate  200  using gate electrode  225  and spacers  229  and  231  as an implantation mask. As a result, a source/drain area  233  having a predetermined junction depth is formed at both sides of gate electrode  225  on semiconductor substrate  200 . In one embodiment, the high density P type impurities may be 11 AMU Boron ions (11B+). 
         [0036]    As shown in  FIG. 3H , after source/drain area  233  is formed, an oxide-nitride layer  234  is formed by processing thermal oxide layer  232  through a nitrogen (N 2 ) plasma process on the entire surface of semiconductor substrate  200 . 
         [0037]    The nitrogen plasma process may be performed with predetermined conditions, such as a radio frequency (RF) power of about 150 W to 200 W, a nitride flow of about 350 sccm to 450 sccm, a pressure of about 9 torr to 22 torr, and a process time of about 100 sec to 120 sec. 
         [0038]    As shown in  FIG. 3I , a spike annealing process is performed to activate impurities implanted in semiconductor substrate  200  to form source/drain area  233 . The spike annealing process may be performed at a temperature of about 900˜1100° C. 
         [0039]    Since Boron (B) ions implanted in semiconductor substrate  200  has a high diffusivity, Boron ions may be out-diffused even if the ion implantation process is performed with a low ion implantation energy. However, oxide/nitride layer  234  formed on source/drain area  233  may operate as a barrier that prevents the Boron ions from out-diffused. 
         [0040]    Accordingly, by preventing the impurities implanted in source/drain area  233  from being out-diffused, the PMOS transistor fabricated according to a method consistent with the present invention can sustain the impurity density of source/drain area  233  after the annealing process. Therefore, the PMOS transistor consistent with the present invention can retain its operation speed, and prevent its extension resistance from increasing, thereby improving the performance of the semiconductor device. 
         [0041]    Although embodiments consistent with the present invention have been described, it should be understood that numerous other embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the appended claims. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
A semiconductor device, such as a positive channel metal-oxide semiconductor (PMOS) transistor, and a fabricating method thereof are provided. The semiconductor device includes: a gate insulation layer and a gate electrode, a semiconductor substrate, a spacer formed on side walls of the gate insulation layer and the gate electrode, a lightly doped drain (LDD) area formed on the semiconductor substrate at both sides of the gate electrode, a source/drain area formed on the semiconductor substrate at both sides of the gate electrode, and an oxide-nitride layer formed on the gate electrode and on the source/drain area.