Abstract:
A method for fabricating semiconductor device includes the steps of: providing a substrate having a gate structure thereon; forming a silicon layer on the substrate to cover the gate structure entirely; planarizing the silicon layer; and performing a replacement metal gate (RMG) process to transform the gate structure into a metal gate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. application Ser. No. 14/873,223 filed Oct. 2, 2015, and incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of forming amorphous silicon layer on one side of the gate structure and contact plug on another side of the gate structure. 
       2. Description of the Prior Art 
       [0003]    With the trend in the industry being towards scaling down the size of the metal oxide semiconductor transistors (MOS), three-dimensional or non-planar transistor technology, such as fin field effect transistor technology (FinFET) has been developed to replace planar MOS transistors. Since the three-dimensional structure of a FinFET increases the overlapping area between the gate and the fin-shaped structure of the silicon substrate, the channel region can therefore be more effectively controlled. This way, the drain-induced barrier lowering (DIBL) effect and the short channel effect are reduced. The channel region is also longer for an equivalent gate length, thus the current between the source and the drain is increased. In addition, the threshold voltage of the FinFET can be controlled by adjusting the work function of the gate. 
         [0004]    Typically, contact areas for contact plugs decrease substantially after the fabrication of semiconductor device enters 10 nm node and results in increase of resistance. Moreover, the fabrication of contact plugs also requires more masks to be used. The increase of masks further induces an increase in resistance when even a little shift is found in active region and degrades the operation of the device. Hence, how to resolve this issue has become an important task in this field. 
       SUMMARY OF THE INVENTION 
       [0005]    According to a preferred embodiment of the present invention, a method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having a gate structure thereon; forming a silicon layer on the substrate to cover the gate structure entirely; planarizing the silicon layer; and performing a replacement metal gate (RMG) process to transform the gate structure into a metal gate. 
         [0006]    According to another aspect of the present invention, a semiconductor device is disclosed. The semiconductor device includes: a substrate; a first gate structure on the substrate; a first spacer adjacent to the first gate structure; a first contact plug adjacent to the first gate structure and contact the first spacer; and a silicon layer around the first gate structure. 
         [0007]    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 
         [0008]      FIGS. 1-6  illustrate a method for fabricating semiconductor device according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIGS. 1-6 ,  FIGS. 1-6  illustrate a method for fabricating semiconductor device according to a preferred embodiment of the present invention. As shown in  FIG. 1 , a substrate  12 , such as a silicon substrate or silicon-on-insulator (SOI) substrate is provided, and a transistor region, such as a PMOS region or a NMOS region is defined on the substrate  12 . At least a fin-shaped structure  14  and an insulating layer (not shown) are formed on the substrate  12 , in which the bottom of the fin-shapes structure  14  is preferably enclosed by the insulating layer, such as silicon oxide to form a shallow trench isolation (STI). A plurality of gate structures  16  and  18  are formed on part of the fin-shaped structure  14 . It should be noted that even though two gate structures are disclosed in this embodiment, the quantity of the gate structures is not limited to two, but could by any quantity depending on the demand of the product. 
         [0010]    The formation of the fin-shaped structure  14  could be accomplished by first forming a patterned mask (now shown) on the substrate,  12 , and an etching process is performed to transfer the pattern of the patterned mask to the substrate  12 . Next, depending on the structural difference of a tri-gate transistor or dual-gate fin-shaped transistor being fabricated, the patterned mask could be stripped selectively or retained, and deposition, chemical mechanical polishing (CMP), and etching back processes are carried out to form an insulating layer surrounding the bottom of the fin-shaped structure  14 . Alternatively, the formation of the fin-shaped structure  14  could also be accomplished by first forming a patterned hard mask (not shown) on the substrate  12 , and then performing an epitaxial process on the exposed substrate  12  through the patterned hard mask to grow a semiconductor layer. This semiconductor layer could then be used as the corresponding fin-shaped structure  14 . In another fashion, the patterned hard mask could be removed selectively or retained, and deposition, CMP, and then etching back could be used to form a STI surrounding the bottom of the fin-shaped structure  14 . Moreover, if the substrate  12  were a SOI substrate, a patterned mask could be used to etch a semiconductor layer on the substrate until reaching a bottom oxide layer underneath the semiconductor layer to form the corresponding fin-shaped structure. If this means is chosen the aforementioned steps for fabricating the STI could be eliminated. 
         [0011]    The fabrication of the gate structures  16  and  18  could be accomplished by a gate first process, a high-k first approach from gate last process, or a high-k last approach from gate last process. Since this embodiment pertains to a high-k first approach, gate structures  16  and  18  composed of high-k dielectric layer and polysilicon material  20  could be first formed on the fin-shaped structure  14  and spacers  22  and  24  are formed on the sidewall of the gate structures  16  and  18 . A source/drain region  26  and/or epitaxial layer  28  are then formed in the fin-shaped structure  14  and/or substrate  12  adjacent to two sides of the spacers  22  and  24 , and a silicide layer (not shown) could be selectively formed on the source/drain region  26  and/or epitaxial layer  28 . 
         [0012]    Next, as shown in  FIG. 2 , a liner  30  could be selectively formed on the substrate  12  gate structures  16  and  18 , and a silicon layer  32  is formed on the liner  30  thereafter. Next, a planarizing process, such as CMP is conducted to remove part of the silicon layer  32  and part of the liner  30  so that the top surfaces of the silicon layer  32 , liner  30 , and gate structures  16  and  18  are coplanar. In this embodiment, the liner  30  could be selected from the group consisting of silicon oxide and silicon nitride, the silicon layer  32  is selected from the group consisting of amorphous silicon, polysilicon, and epitaxial layer, but most preferably amorphous silicon. 
         [0013]    Next, a replacement metal gate (RMG) process is conducted to transform the gate structures  16  and  18  into metal gates. The RMG process could be accomplished by first performing a selective dry etching or wet etching process, such as using etchants including ammonium hydroxide (NH 4 OH) or tetramethylammonium hydroxide (TMAH) to remove the polysilicon material  20  from each of the gate structures  16  and  18  for forming a recess (not shown). Next, a conductive layer including at least a U-shaped work function metal layer  34  and a low resistance metal layer  36  is formed in each recess, and a planarizing process is conducted so that the surfaces of the U-shaped work function layer  34  and low resistance metal layer  36  are even with the surface of the silicon layer  32 . Depending on the high-k first approach or high-k last approach being conducted, the cross-section of high-k dielectric layer (not shown) could be either I-shaped or U-shaped. 
         [0014]    In this embodiment, the work function metal layer  34  is formed for tuning the work function of the later formed metal gates to be appropriate in an NMOS or a PMOS. For an NMOS transistor, the work function metal layer  34  having a work function ranging between 3.9 eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or titanium aluminum carbide (TiAlC), but it is not limited thereto. For a PMOS transistor, the work function metal layer  34  having a work function ranging between 4.8 eV and 5.2 eV may include titanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC), but it is not limited thereto. An optional barrier layer (not shown) could be formed between the work function metal layer  34  and the low resistance metal layer  36 , in which the material of the barrier layer may include titanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN). Furthermore, the material of the low-resistance metal layer  36  may include copper (Cu), aluminum (Al), titanium aluminum (TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof. Since the process of using RMG process to transform dummy gate into metal gate is well known to those skilled in the art, the details of which are not explained herein for the sake of brevity. According to an embodiment of the present invention, part of the work function metal layer  34  and part of the low resistance metal layer  36  of the gate structures  16  and  18  could be removed to form recess (not shown), and a hard mask (not shown) is filled into each recess so that the surfaces of the hard mask and silicon layer  32  are coplanar. Preferably, the hard mask could be selected from the group consisting of silicon oxide, silicon nitride, silicon oxynitride, and silicon carbon nitride. 
         [0015]    Next, as shown in  FIG. 4 , a patterned mask (not shown) is formed on the gate structures  16  and  18  to expose the silicon layer  32  between the gate structures  16  and  18 , and an etching process is conducted by using the patterned mask as mask to remove the silicon layer  32  adjacent to the gate structures  16  and  18 , or more specifically the silicon layer  32  between two adjacent gate structures  16  and  18  for forming a contact hole  38 . Preferably, the contact hole  38  completely exposes the spacers  22  and  24  between the two adjacent gate structures  16  and  18 . 
         [0016]    Next, as shown in  FIG. 5 , a contact plug formation process is conducted by depositing metal materials into the contact hole  38 , which could be accomplished by sequentially forming a barrier layer  40  and a metal layer  42  composed of low resistance material into the contact hole  38 . The barrier layer  40  is selected from the group consisting of Ti, TiN, Ta, and TaN while the metal layer  42  is selected from the group consisting of W, Cu, Al, TiAl, and CoWP. A planarizing process such as CMP is then conducted to remove part of the barrier layer  40  and part of the metal layer  42  for forming a contact plug  44  in the contact hole  38 . The contact plug  44  preferably contacts the spacers  22  and  24  directly and electrically connected to the source/drain region  26  and epitaxial layer  28  in the substrate  12 . 
         [0017]    Next, as shown in  FIG. 6 , an interlayer dielectric (ILD) layer  46  is formed on the silicon layer  32 , the gate structures  16  and  18 , and on the contact plug  44 , and a plurality of contact holes (not shown) is formed in the ILD layer  46 , in which the ILD layer  46  and silicon layer  32  are preferably composed of different material. For instance, the ILD layer  46  could be selected from the group consisting of silicon oxide and silicon nitride. Next, a contact formation is conducted to form a plurality of contact plugs  52  composed of barrier layer  48  and metal layer  50  in the ILD layer  46 , in which the contact plugs  52  are electrically connected to the gate structures  16  and and the contact plug  44  respectively. This completes the fabrication of a semiconductor device according to a preferred embodiment of the present invention. 
         [0018]    Referring again to  FIG. 6 , which further illustrates a structural view of a semiconductor device according to a preferred embodiment of the present invention. As shown in  FIG. 6 , the semiconductor device includes a substrate  12 , a gate structure  16  and a gate structure  18  on the substrate  12 , a spacer  22  adjacent to the gate structure  16 , a spacer  24  adjacent to the gate structure  18 , a contact plug  44  adjacent to the gate structures  16  and  18  and contacts the spacers  22  and  24  directly, and a silicon layer  32  surrounding the gate structures  16  and  18 . 
         [0019]    Specifically, the silicon layer  32  is disposed on the left side of gate structure  16 , the contact plug  44  is disposed on the right side of gate structure  16 , and the contact plug  44  is disposed on the left side of gate structure  18 . The contact plug  44  is disposed between the gate structures  16  and  18  while contacting the spacers  22  and  24  at the same time, the sidewalls of the contact plug  44  is totally consisting of the spacers  22  on the left and the spacer  24  on the right, or no other elements such as silicon layer or ILD layer is disposed between the gate structures  16  and  18  except the contact plug  44 . In addition, the top surfaces of the silicon layer  32 , gate structure  16 , gate structure  18 , and contact plug  44  are all coplanar. In this embodiment, the silicon layer  32  is preferably composed of amorphous silicon, the contact plug  44  is composed of a barrier layer  40  and a metal layer  42 . 
         [0020]    A ILD layer  46  is further disposed on the silicon layer  32  and gate structures  16  and  18 , and a plurality of contact plugs  52  are formed in the ILD layer  46  to electrically connect the gate structures  16  and  18  and contact plug  44 . In this embodiment, the ILD layer  46  and silicon layer  32  are composed of different material. For instance, the ILD layer  46  could be selected from the group consisting of silicon oxide and silicon nitride. 
         [0021]    Overall, the present invention first forms at least a gate structure on a substrate, forms a silicon layer preferably composed of amorphous silicon on the substrate and the gate structure, planarizes the silicon layer, uses RMG process to transform the gate structure into metal gate, removes the silicon layer on one side of the gate structure to form contact hole, and then forms a contact plug in the contact hole. This produces a device having silicon layer on one side of the gate structure and contact plug on the other side of the gate structure. By using the aforementioned fabrication process, it would be desirable to reduce the difficulty for fabricating contact plugs as the semiconductor industry enters 10 nm node and beyond and increase the contact area of the contact plug at the same time. 
         [0022]    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.