Abstract:
A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate having gate structure thereon, wherein the gate structure comprises a high-k dielectric layer; increasing an ambient pressure around the gate structure to a predetermined pressure by injecting a first gas; reducing the ambient pressure to a base pressure; and forming a spacer around the gate structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of increasing ambient pressure around gate structure by introducing low activity gas before spacer formation. 
         [0003]    2. Description of the Prior Art 
         [0004]    In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers. 
         [0005]    In current fabrication of high-k metal transistor, spacer formation is typically accomplished by using furnace to deposit dielectric layer such as silicon carbon nitride along with a temperature ramp approach before etching back the dielectric layer for forming spacer. Devices obtained through this approach however still reveal unsatisfactory current leakage result. Hence, how to improve the current fabrication for fabricating high-k metal gate transistor has become an important task in this field. 
       SUMMARY OF THE INVENTION 
       [0006]    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 gate structure thereon, in which the gate structure includes a high-k dielectric layer; increasing an ambient pressure around the gate structure to a predetermined pressure by injecting a first gas; reducing the ambient pressure to abase pressure; and forming a spacer around the 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. 1-5  illustrate a method for fabricating semiconductor device according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIGS. 1-5 ,  FIGS. 1-5  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 wafer or silicon-on-insulator (SOI) substrate is provided, in which a plurality of shallow trench isolations (STIs) (not shown) could be formed in the substrate  12 . 
         [0010]    Agate structure  14  is then formed on the substrate  12  by first depositing an interfacial layer (not shown), a high-k dielectric layer (not shown), a silicon layer (not shown), a first hard mask layer (not shown), and a second hard mask layer (not shown) on the substrate  12 . A patterned transfer is conducted thereafter by forming a patterned mask, such as a patterned resist (not shown) on the second hard mask layer, and a dry etching process is conducted by using the patterned resist to remove part of the second hard mask layer, part of the first hard mask layer, part of the silicon layer, part of the high-k dielectric layer, and part of the interfacial layer for forming a gate structure  14 . In other words, the gate structure  14  is preferably composed of a patterned interfacial layer  16 , a patterned high-k dielectric layer  18 , a patterned silicon layer  20 , a patterned first hard mask  22 , and a patterned second hard mask  24 . 
         [0011]    In this embodiment, the interfacial layer  16  is preferably composed of silicon material such as silicon dioxide (SiO 2 ), silicon nitride (SiN), or silicon oxynitride (SiON), or other dielectric material with high permittivity or dielectric constant. The silicon layer  20  is preferably composed of single crystal silicon, doped polysilicon, or amorphous polysilicon, the first hard mask  22  is composed of silicon nitride, and the second hard mask  24  is composed of silicon oxide. Despite the hard mask of this embodiment is a two-layered structure, the hard mask could also be a single-layered structure selected from the group consisting of SiC, SiON, SiN, SiCN and SiBN, which is also within the scope of the present invention. 
         [0012]    As the present embodiment pertains to a high-k first process from gate last process, the high-k dielectric layer  18  preferably has a “I-shaped” cross section and preferably be selected from dielectric materials having dielectric constant (k value) larger than 4. For instance, the high-k dielectric layer  20  may be selected from hafnium oxide (HfO 2 ), hafnium silicon oxide (HfSiO 4 ), hafnium silicon oxynitride (HfSiON), aluminum oxide (Al 2 O 3 ), lanthanum oxide (La 2 O 3 ), tantalum oxide (Ta 2 O 5 ), yttrium oxide (Y 2 O 3 ), zirconium oxide (ZrO 2 ), strontium titanate oxide (SrTiO 3 ), zirconium silicon oxide (ZrSiO 4 ), hafnium zirconium oxide (HfZrO 4 ), strontium bismuth tantalate (SrBi 2 Ta 2 O 9 , SBT), lead zirconate titanate (PbZr x Ti 1-x O 3 , PZT), barium strontium titanate (Ba x Sr 1-x TiO 3 , BST) or a combination thereof. 
         [0013]    After the gate structure  14  is formed, a wet etching process or a wet clean is conducted to remove residues or particles on the substrate  12 , and at this stage, a native oxide  26  is also grown naturally on the surface of substrate  12  and gate structure  14  along with electron charges after the wet clean. 
         [0014]    After the wet clean is conducted, as shown in  FIGS. 2-3 , the substrate  12  is charged to a furnace  28  for various pressure and temperature treatments, in which  FIG. 2  illustrates a perspective view of the semiconductor device during the treatments and  FIG. 3  is a flow chart diagram illustrating the treatments conducted after charging the substrate  12  into the furnace  28 . First, in step  102 , the substrate  12  is charged to the furnace  28  for pressure and temperature treatment, in which the temperature of the furnace  28  is controlled at around 400° C., but not limited thereto. 
         [0015]    Next, in step  104 , ambient pressure around the gate structure  14  is reduced to a base pressure, in which the base pressure is preferably less than 0.1 Torr. 
         [0016]    Next, in step  106 , ambient pressure around the gate structure  14  is increased to a predetermined pressure by injecting a low activity gas. According to a preferred embodiment of the present invention, the predetermined pressure is between 10-760 Torr, and more preferably between 50-200 Torr, and the injected low activity gas is selected from the group consisting of nitrogen gas, argon and helium. 
         [0017]    Next, in step  108 , ambient temperature around the gate structure  14  is increased at a speed of 5-20° C./min from 400° C. to a deposition temperature, in which the deposition temperature is between 500-750° C. It should be noted that even though the ambient temperature is preferably increased after increasing the ambient pressure to the predetermined pressure as in step  106 , the ambient temperature around the gate structure  14  could also be increased at the same time while the ambient pressure is increased. In other words, the actions of increasing ambient temperature to a range between 500-750° C. and increasing ambient pressure to a range between 10-760 Torr or between 50-200 Torr could be carried out simultaneously, which is also within the scope of the present invention. 
         [0018]    Next, in step  110 , ambient pressure is reduced to base pressure again while low activity gas is purged out from the furnace  28 , in which the base pressure is preferably less than 0.1 Torr. 
         [0019]    Next, in step  112  and as shown in  FIG. 4 , a dielectric layer  30  is formed on the substrate  12  and gate structure  14  after returning the ambient pressure to base pressure, and in step  114 , the substrate  12  is discharged from the furnace  28 . Preferably, the formation of the dielectric layer  30  is accomplished by conducting a chemical vapor deposition (CVD) process or atomic layer deposition (ALD) process, and the dielectric layer  30  is selected from the group consisting of silicon nitride, silicon oxide, and silicon carbon nitride. 
         [0020]    As shown in  FIG. 5 , the dielectric layer  30  is then etched back for forming a spacer  32  around the gate structure  14 . Depending on the demand of the process, formation of elements including lightly doped drain, additional spacers, source/drain region, epitaxial layer, silicides, and contact etch stop layer and replacement metal gate (RMG) process could be conducted thereafter to complete the fabrication of a metal gate transistor, and as these processes are well known to those skilled in the art, the details of which are not explained in for the sake of brevity. This completes the fabrication of a semiconductor device according to a preferred embodiment of the present invention. 
         [0021]    Overall, the present invention inserts a step of increasing ambient pressure around the gate structure by introducing low activity gas between pumping down ambient pressure to base pressure after charging wafer into furnace and ramping up ambient temperature to deposition temperature. Preferably, by increasing ambient pressure around the gate structure with injection of low activity gas before deposition of dielectric layer for forming spacer, charges generated on the surface of gate structure and substrate could be neutralized significantly and exposed surfaces of the silicon layer and high-k dielectric layer from gate structure as well as substrate could also be repaired by the introduction of low activity gas. As a result, current leakage of the device is improved substantially. 
         [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.