Abstract:
A method of manufacturing a MOS transistor capable of suppressing a short channel effect by suppressing boron (B) ion diffusion in the MOS transistor. The method includes steps of: forming an impurity diffusion suppressing layer in an active region of a semiconductor substrate; forming an impurity layer containing boron ions in a lower portion of the impurity diffusion suppressing layer; and thermally treating on the substrate, wherein the impurity diffusion suppressing layer suppresses diffusion of the boron ions during the thermal treatment step.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (a) Field of the Invention  
         [0002]     The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of manufacturing a MOS transistor.  
         [0003]     (b) Description of the Related Art  
         [0004]     Recently, technologies for MOS devices have been rapidly developed, so that small-sized high-performance MOS devices can be obtained. In order to achieve the small-sized high-performance MOS devices, technologies relating to thicknesses of gate oxide layers, source/drain regions, and channel region must be improved.  
         [0005]     As the MOS transistors are highly integrated, a short channel effect (SCE) occurs. In order to suppress the short channel effect, the thickness of the gate oxide layer needs to be small. In addition, the source/drain regions need to be formed with a shallow junction to reduce a charge sharing effect.  
         [0006]     In addition, in order to suppress the short channel effect of the MOS transistor, a lightly doped drain region and a halo region may be provided in a vicinity of the channel region. Preferably, impurity ions implanted to adjust threshold voltages are distributed on or near a surface of the channel region.  
         [0007]     On the other hand, boron (B) or BF ions are used to adjust the threshold voltage in n-channel MOS transistors and form the LDD region in p-channel MOS transistors.  
         [0008]     However, B ions have a tendency to be widely diffused as subsequent processes such as thermal treatment are carried out. As a result, in the p-channel MOS transistor, B ions may be sufficiently diffused to reach an EOR (end of range) of the channel region, so that a parasitic capacitance between the gate and the drain may increase. On the other hand, in the n-channel MOS transistor, B ions may be sufficiently diffused into a lower portion of the substrate to reach a lower portion of the channel region, so that it is difficult to suppress the short channel effect.  
       SUMMARY OF THE INVENTION  
       [0009]     In order to solve the aforementioned problems, an object of the present invention is to provide a method of manufacturing a MOS transistor capable of suppressing a short channel effect by preventing boron (B) ions from unnecessarily diffusing in the MOS transistor.  
         [0010]     According to an aspect of the present invention, there is provided a method of manufacturing a MOS transistor, comprising steps of: forming an impurity diffusion suppressing layer in an active region of a semiconductor substrate; forming an impurity layer containing boron ions in a lower portion of the impurity diffusion suppressing layer; and performing a thermal treatment process on the substrate, wherein the impurity diffusion suppressing layer suppresses diffusion of the boron ions during the thermal treatment process.  
         [0011]     In the above aspect of the present invention, the impurity diffusion suppressing layer may contain germanium or nitrogen ions.  
         [0012]     According to another aspect of the present invention, there is provided method of manufacturing a MOS transistor, comprising steps of: forming a germanium implant layer in the surface of an active region defined by isolation structures in a semiconductor substrate by a first ion implanting process; forming an impurity layer containing boron ions in the active region of the substrate by a second ion implanting process to adjust a threshold voltage, the impurity layer being formed at a deeper level than the germanium implant layer; and heating the substrate.  
         [0013]     In the latter aspect of the present invention, the first ion implanting process may be performed with an implanting energy of about 5 to 50 keV and a concentration of 1×10 14  to 5×10 14  ions/cm 2 , and the second ion implanting process may be performed with an implanting energy of about 10 to 50 keV and a concentration of 1×10 13  to 1×10 14  ions/cm 2 .  
         [0014]     In addition, heating may comprise a rapid thermal annealing process at a temperature of about 800 to 1000° C. in an N 2  ambient for 10 to 30 seconds.  
         [0015]     According to still another aspect of the present invention, there is provided a method of manufacturing a MOS transistor, comprising steps of: forming nitrogen implant layers in a surface of a semiconductor substrate at both sides of a gate over an active region defined by an isolation layer in the substrate by a first ion implanting process; and forming lightly doped drain regions containing boron ions in the substrate at both sides of the gate by using a second ion implanting process. A gate insulating layer may be between the gate and the active region of the substrate.  
         [0016]     In the aspect of the present invention described in the preceding paragraph, the first ion implanting process may be performed with an implanting energy of about 10 to 50 keV and a concentration of 1×10 14  to 5×10 14  ions/cm 2 , and the second ion implanting process may be performed with an implanting energy of about 5 to 50 keV and a concentration of 1×10 14  to 5×10 15  ions/cm 2 . In addition, the process may further comprise heating by a rapid thermal annealing process at a temperature of about 600 to 800° C. in an N 2  ambient for 10 to 60 seconds.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0018]      FIGS. 1A  to  1 C are cross sectional views for explaining a method of manufacturing an n-channel MOS transistor according to a first embodiment of the present invention;  
         [0019]      FIG. 2  is a graph showing a impurity concentration distribution of a threshold voltage adjusting layer in the n-channel MOS transistor according to the first embodiment of the present invention; and  
         [0020]      FIGS. 3A  to  3 F are cross sectional views for explaining a method of manufacturing an p-channel MOS transistor according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Now, exemplary embodiments of the present invention will be described with reference to the attached drawings. However, the present invention can be embodied in various modifications and thus is not limited to the embodiments described below.  
         [0022]     Firstly, a method of manufacturing an n-channel MOS transistor according to a first embodiment of the present invention will be described with reference to  FIGS. 1A  to  1 C.  
         [0023]     Referring to  FIG. 1A , an active region is defined by forming an isolation layer  110  (e.g., comprising a plurality of isolation structures) in a p-type semiconductor substrate  100 . The p-type semiconductor substrate  100  is a silicon substrate. The isolation layer  110  may be formed by a shallow trench isolation (STI) process. In some cases, the isolation layer  110  may be formed by a LOCOS process. In addition, instead of the p-type semiconductor substrate  100 , an n-type semiconductor substrate may be used. In this case, a p-type well region is formed on the n-type semiconductor substrate.  
         [0024]     Next, a relatively shallow germanium (Ge) implant layer  121   a  is formed in or near the surface of the active region of the substrate  100  by first implanting Ge ions ( 121 ) into the substrate  100 . The Ge implant layer  121   a  prevents and/or inhibits boron (B) ions of a subsequently-formed threshold voltage adjusting layer from diffusing into a lower portion of the substrate  100  during a thermal treatment process. Here, the first ion implanting process is typically performed with an implanting energy of about 5 to 50 keV and a concentration of 1×10 14  to 5×10 14  ions/cm 2 . When the Ge ions are implanted in the active region of the substrate  100 , oxygen atoms (not shown) may be collected or gettered in an EOR (end of range) portion of the Ge implant layer  121   a.    
         [0025]     Referring to  FIG. 1B , in order to adjust the threshold voltage of a subsequently formed transistor, a B implant layer  122   a  is formed in the active region of the substrate  100  by implanting B ions  122  as p-type impurities into the substrate  100  with a second ion implanting process. Here, the B implant layer  122   a  is formed at a deeper level than the Ge implant layer  121   a . The second ion implanting process is typically performed with an implanting energy of about 10 to 50 keV and a concentration of 1×10 13  to 1×10 14  ions/cm 2 .  
         [0026]     Referring to  FIG. 1C , the threshold voltage adjusting layer  130  is formed in the surface of the active region of the substrate  100  by performing a thermal treatment process (e.g., heating or annealing). Here, the thermal treatment process may comprise an RTP or RTA (rapid thermal processing or rapid thermal anneal), typically performed at a temperature of about 800 to 1000° C. in an N 2  ambient for 10 to 30 seconds. During the thermal treatment process, the B ions  122  are diffused closer to the surface of the substrate  100 , by the relatively shallow Ge implant layer and/or oxygen collected/gettered in the Ge implant layer  121   a . As a result, the threshold voltage adjusting layer  130  has an impurity profile in which more impurities are distributed near the surface of the substrate  100 . Consequently, the present invention effectively suppresses the short channel effect.  
         [0027]      FIG. 2  is a graph showing an impurity concentration distribution of the threshold voltage adjusting layer  130  with respect to the vertical distance from the surface of the substrate  100 . In this graph, the curve “A” represents a Ge concentration profile in the surface of the substrate  100 ; the curve “B” represents a B concentration profile in the surface of the substrate  100  implanted with the Ge ions according to the present invention; and the curve “C” represents a conventional B concentration profile in the surface of the substrate  100  not implanted with Ge ions. As shown in  FIG. 2 , it can be noted that, if the Ge ions are implanted in the surface of the substrate  100 , a larger amount of B ions are distributed in the surface of the substrate  100  than without the Ge implant layer. Thus, in this aspect of the present invention, the boron ions for a threshold voltage adjustment layer may be implanted in the active region of the substrate such that the threshold voltage adjustment layer has a greater peak concentration depth than the germanium implant layer.  
         [0028]     Next, although not shown in  FIGS. 1A-1C , subsequent processes for forming a gate and gate oxide, LDD implant regions, halo implant regions, source/drain regions, and silicide layers are carried out.  
         [0029]     Now, a method of manufacturing a p-channel MOS transistor according to a second embodiment of the present invention will be described with reference to  FIGS. 3A  to  3 F.  
         [0030]     Referring to  FIG. 3A , an active region is defined by forming an isolation layer  310  (e.g., a plurality of isolation structures) in an n-type semiconductor substrate  300 . Here, the n-type semiconductor substrate  300  may comprise a silicon substrate. The isolation layer  310  may be formed by an STI process. In some cases, the isolation layer  310  may be formed by a LOCOS process. In addition, instead of the n-type semiconductor substrate  300 , a p-type semiconductor substrate may be used. In this case, an n-type well region is formed on the p-type semiconductor substrate. Next, a gate insulating layer  320  and a gate  330  are subsequently stacked and conventionally formed on the active region of the substrate  300 . Here, the gate insulating layer  320  is an oxide layer formed by conventional thermal oxidation, having a thickness of 20 to 100 Å. The gate  330  comprises a polysilicon layer, typically having a thickness of 1500 to 3000 Å.  
         [0031]     Referring to  FIG. 3B , a nitrogen implant layer  341   a  is formed on a surface of the substrate  300  at both sides of the gate  330  by implanting nitrogen (N) ions into the substrate  300  in a first ion implanting process. The N ions  341  facilitate Si atoms of the substrate  300  to recombine with defects in the substrate  300  such as interstitial sites, so that diffusion of B ions of the subsequently-formed LDD region into the channel region during the thermal treatment process can be reduced or suppressed. The first ion implanting process is performed with an implanting energy of about 10 to 50 keV and a concentration of 1×10 14  to 5×10 14  ions/cm 2 .  
         [0032]     Referring to  FIG. 3C , in order to reduce damage to the gate  330 , gate insulating layer  320  and/or substrate  300 , an oxide layer  350  may be formed on the entire surface of the substrate  300  by thermal oxidation or a deposition process (e.g., PE-CVD, HDP-CVD, etc.). Next, the LDD regions  360  are formed in the substrate  300  at both sides of the gate  330  by implanting B or BF2 ions as lightly doped p-type impurities into the substrate  300  with a second ion implanting process. The second ion implanting process is typically performed with an implanting energy of about 5 to 50 keV and a concentration of 1×10 14  to 5×10 15  ions/cm2.  
         [0033]     Referring to  FIG. 3D , halo regions  370  are formed in a lower portion of the LDD region  360  by implanting As ions as n-type impurities into the substrate  300  in a tilted direction with a third ion implanting process. Here, the third ion implanting process is typically performed at a tilt angle of 10° to 40° with an implanting energy of about 10 to 60 keV and a concentration of 1×10 14  to 1×10 15  ions/cm 2 .  
         [0034]     Referring to  FIG. 3E , spacers  380  are formed on side walls of the gate  330  by using a general and/or conventional spacer forming process. Here, the spacers  380  comprise an insulating layer such as silicon nitride. Next, source/drain regions  390  are formed in the substrate  300  at both sides of the spacers  380  by implanting B or BF2 ions as highly doped p-type impurities into the substrate  300  with a fourth ion implanting process. The fourth ion implanting process is typically performed with an implanting energy of about 3 to 20 keV and a concentration of 1×10 15  to 5×10 15  ions/cm 2 .  
         [0035]     Next, the impurities implanted in the substrate  300  are activated by performing a thermal treatment process. Here, the thermal treatment process may comprise an RTP or RTA (rapid thermal processing or rapid thermal anneal) process, typically performed at a temperature of about 600 to 800° C. in an N 2  ambient for 10 to 60 seconds. The nitrogen implant layer  341   a  in the surface of the substrate  300  prevents or reduces diffusion of the B ions of the LDD regions  360  beyond an EOR (end of range) of the channel region. As a result, a parasitic capacitance between the gate  330  and the drain region  390  can be reduced or minimized.  
         [0036]     Referring to  FIG. 3F , the oxide layer  350  is etched by using the spacer  380  as an etching barrier to expose the upper portions of the gate  330  and the source/drain regions  390 . Next, a general and/or conventional silicide process is performed to form metal silicide layers  400  on the gate  330  and the source/drain regions  390 .  
         [0037]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.