Abstract:
A spot-implant method for MOS transistors. An asymmetric masking film ( 50 ) is formed on a semiconductor substrate and on a transistor gate ( 30 ) with an opening ( 45 ) adjacent to the transistor gate ( 30 ). A spot region ( 70 ) is formed adjacent to the transistor gate ( 30 ) by ion implantation ( 60 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is generally related to the field of MOSFET transistor fabrication and more specifically to a novel process for achieving a spot-implant for use in forming the pocket region in a MOSFET transistor without the use of a photo mask.  
         BACKGROUND OF THE INVENTION  
         [0002]    The threshold voltage of a MOSFET transistor is the voltage that must be applied between the gate electrode and the source electrode to switch the transistor on. This threshold voltage is determined by the gate oxide (dielectric) thickness, the transistor gate length, and the doping concentration in the silicon substrate region beneath the transistor gate. Because most integrated circuits have a fixed voltage supply, the control of the threshold voltage to very tight tolerances across the circuit is crucial to ensuring the proper operation of the circuit. As the transistor gate or channel length is reduced below about 0.4 microns the short channel effect begins to dominate the transistor threshold voltage. This short channel effect results in a threshold voltage that decreases with decreasing gate length. In order to compensate for the short channel effect, an additional implant is introduced during the transistor fabrication process to increase the doping concentration in the substrate beneath the edges of the transistor gate. This additional implant is typically known as a pocket or halo implant.  
           [0003]    As illustrated in FIG. 1, the pocket implants  31 ,  32  are performed after the transistor gate  30  and the gate dielectric  20  is formed. The pocket implants  31  and  32  are usually angled implants that result in formation of regions  33  and  34 . The pocket implants are angled to ensure that some of the implanted species end up under the transistor gate  30 . The only areas of regions  33  and  34  that are effective in reducing the short channel effect are the areas at or under the transistor gate (or gate)  30 . In addition to the angled pocket implants described above, zero tilt implants are also used to form pocket regions. In addition to pocket implants, drain and source extension implants are also performed at this time and result in the formation of the doped region  35  shown in FIG. 1. Because both the pocket implants and the drain and source extension implants are blanket implants of opposite dopant types i.e. n-type and p-type) counter doping effects will make each region less effective. In addition to the silicon substrate  10 , the gate  30  is also subjected to both implants. The counter doping effect of the pocket implant in the gate could begin to have a deleterious effect on transistor operation as the size of the transistors is reduced.  
           [0004]    To reduce the above described deleterious effects on transistor performance, a method for forming pocket regions of doping next to the edge of the gate without counter doping the drain extension and source extension regions is required. Currently such methods or fabrication process involve the use of photo masks. In such a process, photoresist would be formed and patterned to expose the silicon substrate next to the gate before the pocket implant process. Photolithographic processes however are the most expensive steps in the fabrication of MOSFET transistors and such a process would be prohibitively expensive. In addition, the alignment of a photo mask to a transistor gate on the substrate is very difficult. A low cost method that does not involve the use of a photo mask is required.  
         SUMMARY OF THE INVENTION  
         [0005]    The instant invention describes a method for forming a spot implant region in a semiconductor substrate. An embodiment of the instant invention comprises the steps of: providing a semiconductor substrate with a gate dielectric over said semiconductor substrate and a transistor gate over said gate dielectric; forming a masking film on said semiconductor substrate and said transistor gate with an opening adjacent to said transistor gate; and forming a spot implant region in said silicon substrate adjacent and adjacent to said transistor gate by implanting a first species through said opening in said masking film; the transistor gate comprises polycrystalline silicon; the first species is an element from the group consisting of arsenic, phosphorous, boron, germanium, antimony, carbon, indium, and boron containing compounds and the masking film is silicon nitride.  
           [0006]    The main advantage of the instant invention is the formation of a small area (spot) implant without the use of a photo mask. In an embodiment of the instant invention, the small area implant can be used as a pocket region of a MOS transistor. This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    In the drawings:  
         [0008]    [0008]FIG. 1 is a cross-sectional view showing the formation of pocket regions and drain and source extension regions according to the prior art.  
         [0009]    FIGS.  2 A- 2 D are cross-sectional views illustrating an embodiment of the instant invention.  
         [0010]    [0010]FIG. 3 is a method of determining the incident angle for the particle flux. 
     
    
       [0011]    Common reference numerals are used throughout the figures to represent like or similar features. The figures are not drawn to scale and are merely provided for illustrative purposes.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    While the following description of the instant invention revolves around FIGS.  2 A- 2 D, the instant invention can be utilized in any semiconductor device structure. The methodology of the instant invention provides a solution to obtaining a spot implant without the use of a photo mask.  
         [0013]    The following description of the instant invention will be related to FIGS.  2 A- 2 D. Referring to FIG. 1A, a semiconductor substrate  10  is provided. A gate dielectric  20  is formed on the surface of the substrate  10 . This gate dielectric  20  may be comprised of an oxide, thermally grown SiO2, a nitride, an oxynitride, a silicate, a high-k dielectric, or any combination thereof, and is preferably on the order of 1 to 10 nm thick. A blanket layer of silicon containing material (which will be patterned and etched to form the transistor gate structure) is formed on gate dielectric  20 . Preferably, this silicon-containing material is comprised of polycrystalline silicon(“poly” or “polysilicon”), but it may be comprised of epitaxial si icon or any other semiconducting material. In forming the gate  30  from the blanket layer of silicon containing material, a layer of photoresist is formed and patterned to define the transistor gate electrode. Next, a polysilicon gaze etch process is used to remove portions of the blanket film leaving the polysilicon gate  30  shown in FIG. 2A. This polysilicon gate etch process may comprise a dry anisotropic process. After removal of the patterned photoresist film, the structure is subjected to a asymmetrical deposition process. In this process, a masking film  50  is deposited on the surfaces of the substrate  10  and the gate  30 . In an embodiment of the instant invention, the masking film  50  is deposited by exposing the substrate  10  and the gate  30  to a flux of particles  40  at an angle φ 1    55  as illustrated in FIG. 2A. The particle flux  40  will be shadowed by the gate  30  which will produce an opening  45  in the masking film  50  on the silicon substrate next to the gate  30 . In an embodiment of the instant invention, the masking film  50  can comprise silicon nitride, silicon oxide, or any suitable material.  
         [0014]    Following the deposition of the masking film  50 , an angled pocket implant  60  is performed as shown in FIG. 2B. The pocket region  70  is formed in the substrate through the opening  45  in the masking film  50 . The masking film  50  prevents the pocket implant  60  from entering other regions of the substrate  10 . Following the formation of the pocket region  70 , the masking film  50  is removed and a second masking film  80  is deposited using a second particle flux  100  at a new angle φ 2    90 . The shadowing effect of the gate  30  produces an opening  85  in the second masking film  80 . A second pocket implant  110  is performed to produce a second pocket region  120 . In general, for a MOSFET transistor pocket regions are required at both the source and drain end of the transistor. After removal of the second masking film  80 , the MOSFET transistor can be completed using standard semiconductor processing techniques.  
         [0015]    Although the instant invention has been described with respected to the formation of symmetric pocket regions it is not limited to this application. In analog applications, it might be desirable to have a asymmetric transistor and in this case only a single pocket region would be formed. In addition, in certain bipolar/MOSFET applications, the pocket region could be used as the emitter of a bipolar transistor.  
         [0016]    In the instant invention, the opening  85  in the masking film  80  will determine the size of the pocket region  120  formed after the pocket implant. It is important therefore to be able to determine the size of the opening formed during the masking film deposition process. One way of determining the size of the opening is illustrated in FIG. 3. In the Figure, the height of the transistor gate is given by α 120  and desired opening is given by β 130 . It should be noted that the height of the transistor gate  120  includes that thickness of the gate dielectric film which in most cases will be negligible. In this case the angle φ of the incident particle flux that will be used to form the masking film is given by the relation, tanφ=β/α. Using this relation, the incident angle of the flux of particles can be determined for any combination of transistor gate height  120  and desired opening  130 .  
         [0017]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encomass any such modifications or embodiments.