Abstract:
A method for fabricating a metal-oxide semiconductor (MOS) transistor is disclosed. The method includes the steps of: providing a semiconductor substrate; forming a silicon layer on the semiconductor substrate; performing a first photo-etching process on the silicon layer for forming a gate pattern; forming an epitaxial layer in the semiconductor substrate adjacent to two sides of the gate pattern; and performing a second photo-etching process on the gate pattern to form a slot in the gate pattern while using the gate pattern to physically separate the gate pattern into two gates.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 12/837,475 filed Jul. 15, 2010, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for fabricating MOS transistor, and more particularly, to a method of defining polysilicon slot before formation of epitaxial layer. 
     2. Description of the Prior Art 
     In the field of semiconductor fabrication, the use of polysilicon material is diverse. Having a strong resistance for heat, polysilicon materials are commonly used to fabricate gate electrodes for metal-oxide semiconductor transistors. The gate pattern fabricated by polysilicon materials is also used to form self-aligned source/drain regions as polysilicon readily blocks ions from entering the channel region. 
     As the dimension of semiconductor devices decreases, the fabrication of transistors also improves substantially for fabricating small size and high quality transistors. Conventional approach of fabricating the gate of metal-oxide semiconductor (MOS) transistors typically forms a polysilicon layer on a semiconductor substrate and a hard mask on the polysilicon layer before using two photo-etching processes (PEP) to pattern the polysilicon layer and the hard mask into a gate of the transistor. Preferably, the first photo-etching process is conducted to pattern the hard mask and the polysilicon layer into a plurality of rectangular polysilicon gate pattern as the second photo-etching process forms a polysilicon slot in each of the rectangular gate pattern for separating each gate pattern into two gates. Thereafter, elements including spacers are formed on the sidewall of the gate and lightly doped drains and epitaxial layer are formed in the semiconductor substrate adjacent to two sides of the spacer. 
     However, as the polysilicon slot is preferably formed before the formation of epitaxial layer, the etching ratio involved during the formation of the polysilicon slot typically affects the process thereafter. For instance, if the etching ratio of the second photo-etching process is low, the gate pattern would not be etched through completely to form the polysilicon slot and phenomenon such as polysilicon residue and line end bridge would result, whereas if the etching ratio of the second photo-etching process is high, the hard mask disposed on top of the polysilicon gate pattern would be consumed, which would further induce consumption of the spacer formed on the sidewall of the gate thereafter. As some of the spacer on the sidewall is consumed away, a portion of the gate is exposed and un-wanted epitaxial layer would be formed on the exposed portion of the gate. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a method for fabricating a MOS transistor for resolving the aforementioned issue caused by conventional approach. 
     According to a preferred embodiment of the present invention, a method for fabricating a metal-oxide semiconductor (MOS) transistor is disclosed. The method includes the steps of: providing a semiconductor substrate; forming a silicon layer on the semiconductor substrate; performing a first photo-etching process on the silicon layer for forming a gate pattern; forming an epitaxial layer in the semiconductor substrate adjacent to two sides of the gate pattern; and performing a second photo-etching process on the gate pattern to form a slot in the gate pattern while using the gate pattern to physically separate the gate pattern into two gates. 
     According to another aspect of the present invention, 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 four sidewalls, and two of the four sidewalls opposite to each other comprise a spacer thereon while the other two sidewalls opposite to each other comprise no spacer; and an epitaxial layer disposed in the semiconductor substrate adjacent to two sides of the spacer. 
     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 
         FIGS. 1-6  illustrate a method for fabricating a MOS transistor according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-6 ,  FIGS. 1-6  illustrate a method for fabricating a MOS transistor according to a preferred embodiment of the present invention. As shown in  FIG. 1 , a semiconductor substrate  12 , such as a silicon substrate or a silicon-on-insulator (SOI) substrate is provided. At least an active region  14  is defined on the semiconductor substrate  12  and a plurality of shallow trench isolations (STI)  16  are formed for separating the active region  14  from adjacent regions or devices. 
     A gate insulating layer (not shown) composed of dielectric material such as oxides or nitrides is deposited on surface of the semiconductor substrate  12 , and a polysilicon layer preferably with a depth of approximately 1000 Angstroms and a hard mask are formed sequentially on the gate insulating layer thereafter. In this embodiment, the hard mask could be selected from a material consisting of SiO 2 , silicon nitride, and SiON, and the polysilicon layer could be composed of undoped polysilicon material or polysilicon with N+ dopants, which are all within the scope of the present invention. 
     Next, a photo-etching process is performed on the hard mask and the polysilicon layer by first forming a patterned photoresist (not shown) on the hard mask and then using the photoresist as mask to carryout a patterning process. The patterning process preferably removes a portion of the hard mask, the polysilicon layer and the gate insulating layer through a single or multiple etching to form a gate pattern  24  composed of patterned gate insulating layer  18 , patterned polysilicon layer  20 , and patterned hard mask  22  in the active region  14 . The patterned photoresist is removed subsequent to the patterning process. 
     Next, as shown in  FIG. 2 ,  FIG. 2  illustrates a top view of the gate formed after the first photo-etching process. As shown in the figure, a plurality of rectangular gate patterns  24  are formed on the semiconductor substrate  12  after the aforementioned first photo-etching process, in which each gate pattern  24  is composed of a patterned gate insulating layer  18 , a patterned gate polysilicon layer  20 , and a patterned hard mask  22 . 
     As shown in  FIG. 3 , a first stage spacer formation is conducted by first depositing a silicon oxide layer (not shown) and a silicon nitride layer (not shown) on the semiconductor substrate  12 . An etching back is carried thereafter to remove a portion of the silicon oxide layer and silicon nitride layer to form a first spacer  30  composed of silicon oxide layer  26  and silicon nitride layer  28  on the sidewall of the gate pattern  24 . 
     Next, a selective epitaxial growth (SEG) process is performed to form a strained silicon in the semiconductor substrate  12 . For instance, a patterned photoresist (not shown) could be formed on the semiconductor substrate, and an etching process is conducted to form two recesses  34  in the semiconductor substrate  12  adjacent to two sides of the gate pattern  24 . A surface clean is carried out thereafter to completely remove native oxides or other impurities from the surface of the recesses  34 . Next, a selective epitaxial growth process is performed to substantially fill the two recesses  34  for forming an epitaxial layer  36 . Preferably, a light ion implantation could be conducted before the formation of the first spacer  30  and the epitaxial layer  36  to implant n-type or p-type dopants into the semiconductor substrate  12  adjacent to two sides of the gate pattern  24  for forming a lightly doped drain  32 , and the material of the epitaxial layer  36  could be selected according to the type of the transistor or demand of the product. 
     For instance, if the transistor fabricated were to be a PMOS transistor, an epitaxial layer  36  composed of silicon germanium is preferably formed in the recesses  34  to provide a compressive strain to the channel region of the PMOS transistor thereby increasing the hole mobility of the transistor. Conversely, if the transistor fabricated were to be a NMOS transistor, an epitaxial layer composed of silicon carbide (SiC) is preferably formed in the recesses  34  to provide a tensile strain to the channel region of the NMOS transistor for increasing the electron mobility of the transistor. 
     Referring now to  FIGS. 4 and 5 ,  FIG. 4  illustrates a cross-sectional view of the gate pattern after  FIG. 3  and  FIG. 5  illustrates a top view of the gate according to this embodiment. As shown in the figures, the hard mask  22  disposed on top of the polysilicon layer  20  is removed, and a spacer material layer, such as a silicon oxide layer (not shown) and a silicon nitride layer (not shown) are deposited sequentially on the semiconductor substrate  12 . A photo-etching process is then carried out by first forming a patterned photoresist (not shown) on the polysilicon layer  20  and performing an etching process by using the patterned photoresist as mask to remove the polysilicon layer  20  on top of the shallow trench isolation  16 , such as a part of the two ends and central portion of the polysilicon layer  20  for forming at least a polysilicon slot  38  in the rectangular gate pattern  24 . The polysilicon slot  38  preferably separates the gate pattern  24  into two independent gates  46 . After stripping the patterned photoresist and cleaning off remaining particles from the surface of the semiconductor substrate  12 , an etching back is conducted on the deposited silicon oxide layer (not shown) and silicon nitride layer (not shown) for forming a second spacer  44  composed of silicon oxide layer  40  and silicon nitride layer  42  on the sidewall of the gate  46 . 
     For simplification purpose, only one gate pattern  24  is revealed in  FIG. 5  and other doping regions including lightly doped drain and epitaxial layers are also omitted. As shown in the figure, the polysilicon slot  38  preferably divides the gate pattern  24  into two independent portions, and as part of the silicon oxide layer  40  and silicon nitride layer  42  is removed for forming the polysilicon slot  38  and separating the gate pattern  24 , no spacer is formed on at least two opposite sidewalls of the gate  46  after the separation. In other words, a second spacer  44  composed of silicon oxide layer  40  and silicon nitride layer  42  is formed on two opposite sidewalls of the polysilicon gate  46 , whereas the other two remaining opposite sidewall contain no spacer. 
     Preferably, the polysilicon slot  38  is formed after the removal of the hard mask to facilitate a rework process conducted afterwards. For instance, a rework is typically carried out during a lithography for forming the polysilicon slot  38 , and as the hard mask  22  is removed from the active region  14  of the semiconductor substrate  12  before rework is carried out, the exposed silicon substrate surface becomes unprotected. Unfortunately, reacting gas such as oxygen used to remove photoresist material during rework typically accumulates native oxides on the surface of the substrate or forms recesses on the substrate. Hence, the present embodiment preferably removes the hard mask  22  from the polysilicon layer  20  and then deposits the aforementioned silicon oxide and silicon nitride layer on the substrate  12 . These deposited silicon oxide and silicon nitride layer could not only be used as material layers for forming the second spacer, but also be used as etching mask for forming the polysilicon slot and protecting the active region. 
     In addition to forming the polysilicon slot after removing the hard mask, as addressed in the above embodiment, the polysilicon slot  38  could also be formed at any point after the epitaxial layer  36  is formed, which is within the scope of the present invention. 
     Moreover, the above embodiment of forming the second spacer preferably forms a silicon oxide layer and a silicon nitride layer before the etching back process, and then using one single etching back to simultaneously remove a portion of the silicon oxide layer and silicon nitride layer for forming the second spacer. However, the present invention could also deposit a single silicon oxide layer before the polysilicon slot is formed, and then deposit a silicon nitride layer after the formation of the polysilicon slot to form different MOS transistor structures. 
     For instance, a silicon oxide layer  40  could be deposited on the semiconductor substrate  12  after removing the hard mask, and after following the aforementioned step for forming the polysilicon slot  38 , a silicon nitride layer  42  is deposited on the substrate  12 , and a portion of the silicon oxide layer  40  and silicon nitride layer  42  are removed through etching back process to form the second spacer  44 . As shown in  FIG. 6 , as part of the silicon oxide layer  40  is removed during the formation of the polysilicon slot  38 , the silicon oxide layer  40  of the second spacer  44  would only be disposed on two opposite sidewall of the gate, and as the silicon nitride layer  42  of the second spacer  44  is deposited after the formation of the polysilicon slot  38 , the silicon nitride layer  42  is preferably formed on four sidewalls of the gate  46 . 
     According to another embodiment of the present invention, a silicon oxide layer  40  could be deposited on the semiconductor substrate  12  after removing the hard mask, and after following the aforementioned approach for forming the polysilicon slot  38 , an etching back is carried out to remove a portion of the silicon oxide layer  40  for forming a second spacer, and then depositing a silicon nitride layer  42  on the substrate  12 , and then performing another etching back to remove part of the silicon nitride layer  42  for forming a third spacer. Despite the fabrication sequence of this embodiment is slightly different from the above approach, the same transistor structure as disclosed in  FIG. 6  could be fabricated. 
     In contrast to the conventional approach of forming polysilicon slot before the epitaxial layer, the present invention uses a first photo-etching process to define a rectangular polysilicon gate pattern, forms an epitaxial layer adjacent to two sides of the gate pattern, and then uses a second photo-etching process to define the polysilicon slot while separating the gate pattern into two gates. As the definition of the polysilicon slot is carried after the formation of the epitaxial layer, issues such as line end bridge of epitaxial layer and growth of epitaxial layer on sidewall of the gate could be prevented substantially. 
     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.