Abstract:
A method for fabricating metal gate transistor is disclosed. The method includes the steps of: providing a substrate, wherein the substrate comprises a transistor region defined thereon; forming a gate insulating layer on the substrate; forming a stacked film on the gate insulating layer, wherein the stacked film comprises at least one etching stop layer, a polysilicon layer, and a hard mask; patterning the gate insulating layer and the stacked film for forming a dummy gate on the substrate; forming a dielectric layer on the dummy gate; performing a planarizing process for partially removing the dielectric layer until reaching the top of the dummy gate; removing the polysilicon layer of the dummy gate; removing the etching stop layer of the dummy gate for forming an opening; and forming a conductive layer in the opening for forming a gate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a method for fabricating transistor, and more particularly, to a method for fabricating metal gate transistor. 
         [0003]    2. Description of the Prior Art 
         [0004]    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. 
         [0005]    However, devices fabricated by polysilicon still have many drawbacks. In contrast to most metal, polysilicon gates are fabricated by semiconductor materials having high resistance, which causes the polysilicon gate to work under a much lower rate than other metal gates. In order to compensate for slightly lowered rate of performance, a significant amount of silicides is applied during the fabrication of polysilicon processes, such that the performance of the device could be increased to an acceptable level. 
         [0006]    Gate electrodes fabricated by polysilicon also causes a depletion effect. In most circumstances, the optimum doping concentration for polysilicon is between about 2×20 20 /cm 3  and 3×10 20 /cm 3 . As most gate electrodes have a doping concentration of at least 5×10 21 /cm 3 , the limited doping concentration of polysilicon gates often results in a depletion region at the interface between the gate and the gate dielectric layer. This depletion region not only thickens the gate dielectric layer, but also lowers the capacitance of the gate and ultimately reduces the driving ability of the device. 
         [0007]    In order to resolve this issue, work function metal gates have been developed to replace conventional polysilicon gates. The conventional approach for fabricating metal gates typically forms a dummy gate composed primarily of polysilicon on a substrate, removes the polysilicon material of the dummy gate through dry etching or wet etching, and then deposits a metal into the depleted dummy gate for forming a metal gate. 
         [0008]    However, the conventional approach of depleting the polysilicon material from the dummy gate often damages the gate insulating layer underneath. As a result, another thermal oxidation has to be carried out to form another gate insulating layer afterwards. This not only extends the overall fabrication time but also disrupts the distribution of the dopants within the lightly doped drain or source/drain region. Hence, how to effectively resolve the above issue has become an important task. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an objective of the present invention to provide a method for fabricating metal gate transistor for solving the aforementioned problem. 
         [0010]    According to a preferred embodiment of the present invention, a method for fabricating metal gate transistor is disclosed. The method includes the steps of: providing a substrate, wherein the substrate comprises a transistor region defined thereon; forming a gate insulating layer on the substrate; forming a stacked film on the gate insulating layer, wherein the stacked film comprises at least one etching stop layer, a polysilicon layer, and a hard mask; patterning the gate insulating layer and the stacked film for forming a dummy gate on the substrate; forming a dielectric layer on the dummy gate; performing a planarizing process for partially removing the dielectric layer until reaching the top of the dummy gate; removing the polysilicon layer of the dummy gate; removing the etching stop layer of the dummy gate for forming an opening; and forming a conductive layer in the opening for forming a gate. 
         [0011]    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 
         [0012]      FIGS. 1-5  illustrate a method for fabricating a metal gate transistor according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIGS. 1-5 ,  FIGS. 1-5  illustrate a method for fabricating a metal gate transistor according to a preferred embodiment of the present invention. As shown in  FIG. 1 , a substrate  12 , such as a silicon substrate or a silicon-on-insulator substrate is provided, in which at least one transistor region  14  is defined on the substrate  12 . 
         [0014]    A gate insulating layer (not shown) composed of dielectric material such as oxides or nitrides is then formed on the surface of the substrate  12 . According to an embodiment of the present invention, the gate insulating layer could also be composed of pad oxide or a high-k dielectric layer consisting of HfSiO, HfSiON, HfO, LaO, LaAlO, ZrO, ZrSiO, HfZrO, or combination thereof. 
         [0015]    A stacked film (not shown) composed of an etching stop layer, a polysilicon layer, and a hard mask is formed on the gate insulating layer. Preferably, the etching stop layer is composed of a silicon nitride layer having a thickness of less than 100 Angstroms, and the polysilicon layer serving as dummy gate layer preferably has a thickness of about 1000 Angstroms. The polysilicon layer could be composed of undoped polysilicon or polysilicon having N+ dopants therein, and the hard mask could be composed of SiO 2 , silicon nitride, or SiON, which are all within the scope of the present invention. 
         [0016]    Next, a patterned photoresist (not shown) is formed on the hard mask, and a pattern transfer is conducted by using the patterned photoresist as mask through single or multiple etching processes to remove a portion of the hard mask, the polysilicon layer, the etching stop layer, and the gate insulating layer. After stripping the patterned photoresist, a dummy gate  24  composed of patterned gate insulating layer  16 , patterned etching stop layer  18 , patterned polysilicon layer  20 , and patterned hard mask  22  is formed on the substrate  12  of the transistor region  14 . 
         [0017]    As shown in  FIG. 2 , a first stage of spacer formation is carried out by first depositing a dielectric layer composed of silicon nitride or both silicon oxide and silicon nitride on the dummy gate  24 , and then etching back the deposited dielectric layer to form a first spacer  26  on the sidewall of the dummy gate  24 . 
         [0018]    Next, a light doping process is performed to form a lightly doped drain. For instance, a patterned photoresist (not shown) is formed on the region outside the transistor region  14 , and an ion implantation is carried out by using the patterned photoresist as mask to implant n-type or p-type dopants into the substrate  12  adjacent to two sides of the dummy gate  24  to form a lightly doped drain  28 . 
         [0019]    Next, a second stage of spacer formation is conducted by sequentially depositing a silicon oxide layer  30  and a silicon nitride layer  32  on the substrate  12  and the dummy gate  24 , and then etching back the deposited silicon oxide layer  30  and silicon nitride layer  32  to form a second spacer  34  around the first spacer  26 . 
         [0020]    Next, a heavy doping process is performed to form a source/drain region. Similar the above approach of forming the lightly doped drain  28 , a patterned photoresist (not shown) could be formed on regions outside the transistor region  14 , and an ion implantation is carried out by using the patterned photoresist as mask to implant n-type or p-type dopants into the substrate  12  adjacent to two sides of the second spacer  34 . After thermally diffusing the implanted dopants, a source/drain region  36  is formed in the substrate  12  adjacent to two sides of the second spacer  34 , and the patterned photoresist is stripped thereafter. 
         [0021]    Next, a silicon substrate etching back process accompanying a selective epitaxial growth (SEG) process could be performed before or after the formation of the aforementioned source/drain region to form an epitaxial layer (not shown) comprising of silicon and other materials partially in the source/drain region. A salicide process is carried out thereafter to forma silicide layer on the source/drain region  36 . As the selective epitaxial growth process and the salicide process are commonly known to those skilled in the art in this field, the details of which are omitted herein for the sake of brevity. Moreover, despite the first spacer, the lightly doped drain, the second spacer, and the source/drain region are formed sequentially in the above embodiment, the order for fabricating the spacers and doping regions could also be adjusted according to the demand of the product, which are all within the scope of the present invention. 
         [0022]    Next, an interlayer dielectric layer  38  primarily composed of oxides is formed to cover the entire dummy gate  24 . The interlayer dielectric layer  38  could include nitrides, oxides, carbides, low-k dielectric materials, or combination thereof. 
         [0023]    As shown in  FIG. 3 , a chemical mechanical polishing (CMP) process or a dry etching process is performed to remove a portion of the interlayer dielectric layer  38 , part of the first spacer  26 , part of the second spacer  34 , and the hard mask  22  such that the top of the polysilicon layer  20  is exposed and substantially even with the surface of the interlayer dielectric layer  38 . 
         [0024]    As shown in  FIG. 4 , a selective dry etching or wet etching process is carried out by using etchant such as NH 4 OH or TMAH to empty the polysilicon layer  20  of the dummy gate  24  while stopping on the etching stop layer  18 . Next, a wet etching process is performed by using phosphoric acid to remove the etching stop layer  18  composed of silicon nitride. The removal of the etching stop layer  18  preferably forms an opening  40  in the dummy gate  24  while exposing the gate insulating layer  16  underneath. 
         [0025]    It should be noted that the present invention preferably forms an etching stop layer  18  composed of silicon nitride between the gate insulating layer  16  and the polysilicon layer  20 . This etching stop layer  18  could be used to protect the gate insulating layer  16  by preventing plasma or etchant used during the removal of polysilicon layer  20  from damaging the gate insulating layer  16  underneath as the polysilicon layer  20  is emptied 
         [0026]    Also, as part of the second spacer  34  is composed of silicon nitride, the present embodiment preferably removes part of the silicon nitride layer  32  of the second spacer  34  while the etching stop layer  18  is removed by the wet etching process, as shown in  FIG. 4 . 
         [0027]    Moreover, as the etching stop layer  18  is preferably composed of silicon nitride, the first spacer  26  is preferably to be fabricated with a material having different etching selectivity from the etching stop layer  18 . By doing so, the wet etching process carried out to remove the silicon nitride etching stop layer  18  would not be used to damage the first spacer  26  adjacent to the etching stop layer  18  simultaneously. 
         [0028]    Next, as shown in  FIG. 5 , a high-k dielectric layer  42  is formed in the opening  40  to cover the gate insulating layer  16 , the first spacer  26 , the second spacer  34 , and the interlayer dielectric layer  38 . In this embodiment, the high-k dielectric layer  42  is selected from HfSiO, HfSiON, HfO, LaO, LaAlO, ZrO, ZrSiO, HfZrO, or combination thereof. 
         [0029]    Next, a work function metal layer (not shown) could be deposited selectively on the surface of the high-k dielectric layer  42  according to the nature of the transistor. If the transistor fabricated were to be a NMOS transistor, a n-type metal layer could be deposited on the high-k dielectric layer  42 , in which the n-type metal layer is selected from a group consisting of TiN, TaC, TaN, TaSiN, and Al. If the transistor fabricated were to be a PMOS transistor, a p-type metal layer could be deposited on the high-k dielectric layer  42 , in which the p-type metal layer is selected from a group consisting of TiN, W, WN, Pt, Ni, Ru, TaCN, and TaCNO. 
         [0030]    Next, a conductive layer  44  composed of low resistance material is deposited on the high-k dielectric layer  42  and into the opening  40 . In this embodiment, the conductive layer  44  is selected from Al, W, TiAl, CoWP, or combination thereof. A chemical mechanical polishing process is conducted thereafter to remove a portion of the conductive layer  44  and high-k dielectric layer  42  on the first spacer  26 , second spacer  34 , and interlayer dielectric layer  38  to form a metal gate transistor in the transistor region  14 . 
         [0031]    Overall, the present invention preferably forms an etching stop layer composed of silicon nitride between the gate insulating layer and the dummy polysilicon layer. By doing so, the etching stop layer could be used to protect the gate insulating layer from plasma or etchant used during the removal of polysilicon layer. As the gate insulating layer is protected from damage caused by the removal of the polysilicon layer, the present invention also eliminates the need of conducting an additional thermal oxidation to form another gate insulating layer, thereby reducing overall fabrication time and cost substantially. 
         [0032]    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.