Abstract:
A metal oxide semiconductor (MOS) transistor is disclosed. The MOS transistor includes: a semiconductor substrate; a gate disposed on the semiconductor substrate, wherein the gate comprises two sidewalls; a spacer formed on the sidewalls of the gate; a source/drain region disposed in the semiconductor substrate; a silicide layer disposed on top of the gate and the surface of the source/drain region; and a retarded interface layer disposed in the junction between the silicide layer and the gate and source/drain region.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a division of U.S. application Ser. No. 11/161,990 filed Aug. 25, 2005, and incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of fabricating semiconductor devices, and more particularly, to a method of performing salicide processes on MOS transistors.  
         [0004]     2. Description of the Prior Art  
         [0005]     Field effect transistors are important electronic devices in the fabrication of integrated circuits, and as the size of the semiconductor device becomes smaller and smaller, the fabrication of the transistors also improves and is constantly enhanced for fabricating transistors with smaller sizes and higher quality.  
         [0006]     In the conventional method of fabricating transistors, a gate structure is first formed on a substrate, and a lightly doped drain (LDD) is formed on the two corresponding sides of the gate structure. Next, a spacer is formed on the sidewall of the gate structure and an ion implantation process is performed to form a source/drain region within the substrate by utilizing the gate structure and spacer as a mask. In order to incorporate the gate, source, and drain into the circuit, contact plugs are often utilized for interconnection purposes, in which the contact plugs are composed of conducting metals such as tungsten and copper. Nevertheless, the interconnection between the contact plugs and the silicon material of the gate structure and the source/drain region is usually poor, hence a silicide material is often formed over the surface of the gate structure and the source/drain region to improve the ohmic contact between the contact plugs and the gate structure and the source/drain region. Today, the process known as self-aligned silicide (salicide) process has been widely utilized to fabricate silicide materials, in which a source/drain region is first formed, a metal layer comprised of cobalt, titanium, or nickel is disposed on the source/drain region and the gate structure, and a rapid thermal process (RTP) is performed to react the metal layer with the silicon contained within the gate structure and the source/drain region to form a metal silicide for reducing the sheet resistance of the source/drain region.  
         [0007]     However, when the suicides are being formed, the atoms within the metal layer will diffuse into the substrate and deplete the silicon within the source/drain region, thereby damaging the original lattice structure of the source/drain region and causing the PN junction between the source/drain region and the silicon substrate to react with the silicon contained within the source/drain region as a result of an overly short distance between the PN junction and the silicide layer. Ultimately, the problems become much worse in the design of ultra shallow junctions (USJ) as the suicides often come in contact directly with the substrate and result in failure of the device.  
         [0008]     Please refer to  FIG. 1  and  FIG. 2 .  FIG. 1  and  FIG. 2  are perspective diagrams showing the fabrication of a conventional field effect transistor. As shown in  FIG. 1 , a gate structure  106  having a gate dielectric layer  102  and a gate electrode  104  is first formed on a substrate  100 . Next, an ion implantation process is performed to form a lightly doped drain  110  in the substrate  100 . Next, a liner  107  and a spacer  108  are formed on the sidewall of the gate structure  106  and another ion implantation is performed to form a source/drain region  112  in the substrate  100 . Next, a sputtering process is performed to form a metal layer  114  over the surface of the gate electrode  104 , the spacer  108 , and the substrate  100 . Subsequently, as shown in  FIG. 2 , a rapid thermal process (RTP) is performed to react the contact area between the metal layer  114  and the gate electrode  104  and the source/drain region  112  into a silicide layer  116 .  
         [0009]     In order to prevent the short channel effect of the transistors and improve the interconnect resistance of the integrated circuit, the junction depth of the source and drain needs to be effectively reduced for fabricating transistors containing silicides. However, if the thickness of the suicides on the source and drain is decreased while reducing the junction depth of the source and drain, the interconnect resistance and contact resistance may increase simultaneously. On the other hand, if the depth of the suicides is kept constant, the distance between the PN junction of the source/drain region  112  and the silicon substrate and the silicide layer  116  may become overly short and result in junction leakage and a piping effect.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an objective of the present invention to provide a method of performing salicide process on a MOS transistor to solve the above-mentioned problems.  
         [0011]     According to the present invention, a method of performing salicide processes on a MOS transistor, wherein the MOS transistor comprises a gate structure and a source/drain region, the method comprising: performing a selective growth process to form a silicon layer on the top of the gate and the source/drain region; performing an ion implantation process to form a retarded interface layer between the silicon layer and the gate and source/drain region; forming a metal layer on the silicon layer; and reacting the metal layer with the silicon layer for forming a silicide layer.  
         [0012]     Additionally, the present invention discloses a metal oxide semiconductor (MOS) transistor, in which the MOS transistor comprises: a semiconductor substrate; a gate disposed on the semiconductor substrate, wherein the gate comprises two sidewalls; a spacer formed on the sidewalls of the gate; a source/drain region disposed in the semiconductor substrate; a silicide layer disposed on top of the gate and the surface of the source/drain region; and a retarded interface layer disposed in the junction between the silicide layer and the gate and source/drain region.  
         [0013]     By performing an ion implantation process before or after disposing an epitaxial layer on the top of the gate and the surface of the source/drain region, the present invention is able to utilize a retarded dopant as a retarded interface layer to stop the reaction of the salicide fabrication, thereby improving problems such as junction leakage and nickel silicide piping effect.  
         [0014]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  and  FIG. 2  are perspective diagrams showing the fabrication of conventional field effect transistor.  
         [0016]      FIG. 3  through  FIG. 5  are perspective diagrams showing the means of fabricating a transistor containing suicides according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     Please refer to  FIG. 3  through  FIG. 5 .  FIG. 3  through  FIG. 5  are perspective diagrams showing the means of fabricating a transistor containing suicides according to the present invention. As shown in  FIG. 3 , a substrate  200  is first provided and a gate structure having a gate dielectric layer  202  and a gate  204  is formed over the surface of the substrate  200 , in which the gate  204  is composed of conductive materials such as doped polysilicon.  
         [0018]     Next, a lightly doped ion implantation is performed to implant a light dopant (not shown) into two sides of the substrate  200  corresponding to the gate  204  to form a source/drain extension region  210  by utilizing the gate  204  as a mask. Next, a liner  206 , such as a silicon oxide layer, is deposited around the gate structure and a spacer  208  is formed over the surface of the liner  206 , in which the spacer  208  is composed of a silicon oxide offset spacer and a silicon nitride spacer. Next, a heavily doped ion implantation is performed to implant a heavy dopant (not shown) into the substrate  200  to form a source/drain region  212  with heavier dopant concentration by utilizing the gate  204  and the spacer  208  as a mask. Next, a thermal annealing process utilizing a temperature ranging from 1000° C. to 1020° C. is performed to activate the dopants within the substrate  200  and repair the damage of the crystal lattice structure of the substrate  200  during the ion implantation process.  
         [0019]     Next, a low temperature selective epitaxial growth (SEG) is performed to form an epitaxial layer  216  over the surface of the source/drain extension region  210  and the source/drain region  212 , in which the epitaxial layer  216  is composed of silicon germanium, as shown in  FIG. 4 . Next, another ion implantation process is performed to implant a retarded dopant  214 , such as fluoride ions, nitrogen, and oxygen, into the junction area between the epitaxial layer  216  and the source/drain region  212  and the gate  204  to form a retarded interface layer. Alternatively, depending on the retarded property of the dopants, a surface treatment, such as an ion implantation process, a plasma treatment, or a gas or liquid treatment containing high concentration dopants with retarded property can be performed on the surface of the gate  204 , the source/drain extension region  210 , and the source/drain region  212  to form the retarded interface layer containing dopants with retarded effects before performing the low temperature selective epitaxial growth to form the epitaxial layer  216 .  
         [0020]     Next, a surface cleaning process is performed to completely remove the native oxides and other impure materials remaining on the surface of the epitaxial layer  216  and a sputtering or deposition process is performed to form a metal layer (not shown) on the epitaxial layer  216 , in which the metal layer is composed of cobalt, titanium, nickel, platinum, palladium, and molybdenum. Subsequently, as shown in  FIG. 5 , a rapid thermal process (RTP) is performed to react the metal layer with the epitaxial layer  216  deposited earlier to form a silicide layer  220  over the top of the gate  204  and the source/drain region  212  and the non-reacted portion of the metal layer is removed afterwards.  
         [0021]     Since the dopants are implanted into the surface of the gate  204  and the source/drain region  212  via the ion implantation process performed earlier, the reaction between the metal layer and the epitaxial layer  216  will ideally stop at the retarded interface layer. In other words, the present invention is able to effectively utilize the location of the retarded interface layer and the thickness of the epitaxial layer  216  to accurately control the thickness and depth of the silicide layer  220 , thereby adjusting the contact resistance and improving conventional problems such as junction leakage, which is caused by an overly short distance between the source, drain, and substrate of the PN junction and the silicides, and nickel silicide piping effect, which is caused by the approach of suicides into the channel area during silicide reactions. Additionally, since the epitaxial layer  216  will be reacted completely to form the silicide layer  220 , the present invention is able to replace the selectively epitaxial growth (SEG) process described earlier with a low temperature selective polysilicon growth process to form a polysilicon layer over the surface of the source/drain extension region  210  and the source/drain region  212 .  
         [0022]     As shown in  FIG. 5 , after the fabrication process is completed, a transistor  222  having silicide structure is obtained, in which the transistor  222  includes a substrate  200 , a gate  204  formed on the substrate  200 , a gate dielectric layer  202  formed under the gate  204 , a spacer  208  formed over the surface of the sidewall of the gate  204 , a liner  206  formed between the sidewall of the gate  204  and the spacer  208 , and a suicide layer  220  formed on top of the gate  204  and two sides of the substrate  200  corresponding to the spacer  208 . Additionally, a source/drain region  212  and a source/drain extension region  210  are formed within the substrate  200 . Preferably, a retarded dopant  214  is included between the source/drain region  212  and the silicide layer  220  to form a retarded interface layer, in which the dopant is implanted by an ion implantation process and fluoride ions, nitrogen, and oxygen are utilized as the ion source.  
         [0023]     Overall, the advantage of the present invention is to perform an ion implantation process before or after disposing an epitaxial layer on the top of the gate and the surface of the source/drain region. Eventually, the retarded dopant injected is to be utilized as a retarded interface layer to stop the reaction of the salicide fabrication, thereby improving problems such as junction leakage, which is caused by an overly short distance between the source, drain, and substrate of the PN junction and the silicides, and nickel silicide piping effect, which is caused by the approach of suicides into the channel area during silicide reactions. Additionally, a thicker epitaxial is formed over the surface of the gate and the source/drain region as the epitaxial layer will be reacted into a silicide layer in the final stage of the process, thereby reducing the sheet resistance of the source/drain region. Consequently, the present invention is able to obtain a field effect transistor with much better ultra shallow junction structure and source/drain region with lower sheet resistance.  
         [0024]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.