Abstract:
A sidewall spacer is formed in a CMOS device by depositing a layer of silicon nitride on a wafer and anisotropically etching away the silicon nitride layer with a chorine-based plasma etchant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field Of The Invention  
           [0002]    This invention relates a semiconductor fabrication technique, and in particular to a method of forming spacers in CMOS devices.  
           [0003]    1. Description Of The Prior Art  
           [0004]    In the fabrication of CMOS devices, a polysilicon gate is typically deposited over a gate oxide grown on the silicon substrate. Ion implantation occurs through the field oxide located between the gates. In order to protect the gate region during the implantation step, the side walls of the gates are protected with vertical walls known as spacers. In the past, amorphous silicon was used for the spacers, but this created problems during subsequent etching in that it was difficult to avoid over-etching and the removal of part of the underlying layer. Over-etching occurs when etching extends into the underlying layer. Often when the spacers were removed, a polysilicon gate would be etched at the same time leaving a notch, known as a “mouse bite” in the polysilicon gate.  
           [0005]    An object of the invention is to alleviate this problem.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with one aspect of the present invention, the side wall spacers are made of silicon nitride. Silicon nitride has the advantage of offering good selectivity, making it easy to avoid over-etching when the precursor conformal layer is removed.  
           [0007]    In accordance with an important aspect of the invention, a conformal silicon nitride layer is etched using a chlorine-based plasma etch chemistry. Such etch chemistry consists of chlorine (Cl 2 ), hydrogen bromide (HB r ), and a mixture of helium and oxygen. This is in contrast to the conventional CHF 3  chemistry, which is normally used for silicon nitride etching. The applicants have found that using conventional silicon nitride etch chemistry, it is very difficult to avoid over-etching into the gate oxide because of the high plasma power and the limited oxide/nitride selectivity of 1.8 to 1.  
           [0008]    In accordance with the present invention, it has been found surprisingly that oxide/nitride selectivities in the order of 3 to 1 can be achieved, which completely protects the gate oxide. In the invention the etching is caused largely by chemical rather than the mechanical interaction associated with CHF 3  plasmas. The Cl −  radicals etch the silicon nitride chemically and the HBr component is responsible mainly for mechanical etching by the plasma. The reaction of HBr with silicon nitride also creates a protective polymer layer that protects the more vertical profiles of the etched films, resulting in a more anisoptropic etch. Some spacers are left along the polysilicon lines with a rounded shape.  
           [0009]    While the etch rate using chlorine-based etch chemistry, typically around 2000 Angstroms per minute, is somewhat slower than CHF 3 —based chemistry, typically around 5000 Angstroms per minute, this trade-off is more than offset by the improvement in selectivity obtainable.  
           [0010]    The conformal nitride layer is preferably deposited onto a thin oxide layer, typically about 200 Angstroms thick. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:  
         [0012]    FIGS.  1  to  4  show successive steps in the manufacture of a CMOS device in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    Referring now to the drawings, the CMOS device comprises a silicon substrate  10 . First, as shown in FIG. 1, a silicone nitride layer (not shown) is deposited on the substrate and then patterned to leave regions of exposed silicon. The wafer is then exposed to an oxidizing atmosphere to grow field oxide layer  12  in a conventional manner. After removal of the nitride layer, gate oxide  14  is grown between the field oxide regions  12 .  
         [0014]    After growing the gate oxide  14 , a polysilicon layer is grown and subsequently patterned to form polysilicon gates  16  on gate oxide  14  (FIG. 2).  
         [0015]    Typically, after the patterning of the polysilicon gate  16 , a first ion implantation takes place to form source and drain active regions  15 .  
         [0016]    Next, an oxidation step takes place (FIG. 3) to grow a thin silicon oxide layer  17 , which is typically about 200 Angstroms thick.  
         [0017]    In the next step, a conformal silicon nitride layer  18  is deposited over the surface of the wafer using LPCDV (Low Pressure Chemical Vapor Deposition). The objective is to remove the silicon nitride layer  18  except for the vertical side wall portions  20  protecting the polysilicon gates  16  by etching down to the thin oxide layer  17 . The remaining side wall portions  20  forming the spacers slightly rounded as shown. The removal of the horizontal portions of the silicon nitride layer without effecting the underlying layers requires good nitride/oxide selectivity and also good anisotropy to avoid lateral etching into the side walls  20 .  
         [0018]    In accordance with the principles of the invention, the anisotropic etch is performed using an etch recipe comprising chlorine (Cl 2 ), hydrogen bromide (HB r ) and a mixture of helium and oxygen. The etch is performed as an reactive ion etch using a magnetic field to ensure good uniformity on the wafer. The preferred etch variables are set out in the following table:  
                                                       Step   Main Etch   Over Etch                           Pressure    150 mTorr   100 mTorr           Power    325 Watts   150 Watts           Magnetic field    75 Gauss    75 Gauss           HBr flow    10 Sccm    10 Sccm           Cl 2  flow    30 Sccm    15 Sccm           He/O 2  flow     0 Sccm    10 Sccm           Nitride Etch rate   2063 Å/min.   480 Å/min.           Nitride Etch uniformity   0.96%   11.3%           Oxide Etch rate    680 Å/min.    30 Å/min.           Oxide Etch uniformity   3.7%   23.0%           Nitride to Oxide   3:1   16.5:1                      
 
         [0019]    The etch is formed in two steps: the main etch and the over-etch. It will be noted that in the main etch, nitride/oxide selectivities of 3 to 1 are achieved, and in the over-tech, to remove any remaining nitride layer, selectivities as high as 16.5 to 1 are achieved.  
         [0020]    The etch endpoint is detected using optical methods by detecting the chlorine spectral line at 4705 Å. At this point, the chlorine stops being consumed, so by looking at the chlorine spectral line at 470.5 nm, it is possible to determine when the silicon nitride has been completely etched.  
         [0021]    Finally, as shown in FIG. 4, a second polysilicon layer  22  is deposited over the wafer and selectively etched so that the second layer  22  lies over the polysilicon gates  18 .  
         [0022]    Next ion implantation is carried out through the thin oxide layer to form active components, such as transistors. Typically, N+ and P+ species are implanted, and in accordance with the principles of the invention, these are implanted directly through the thin oxide layer  17 , which is left in situ.  
         [0023]    The use of silicon nitride spacers has several advantages. It gives more flexibility in the etching techniques that can be used for the adjacent films. It permits good control of conformability to the wafer surface, and the good selectivity permits the avoidance of over-etching of the underlying layer.  
         [0024]    This is important because the silicon nitride layer is about 3,000-4,000 Å thick.  
         [0025]    It is indeed surprising, but highly significant, that the use of an etch recipe typically applied to polysilicon improves the selectivity and anisotropy of a silicon nitride etch, a fact that leads to a major improvement in the fabrication of CMOS devices.