Abstract:
A method for forming tungsten plugs and layers is disclosed. A thin layer of polysilicon or amorphous silicon is formed within a contact opening. The silicon is exposed to WF 6 , thereby forming a tungsten plug.

Description:
This application is a continuation of application Ser. No. 08/141,780, filed on Oct. 22, 1993, abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to methods of semiconductor integrated circuit processing in general, and more particularly to integrated circuit processing methods which involve the use of tungsten conductors. 
     BACKGROUND OF THE INVENTION 
     Tungsten has become an increasingly popular material for semiconductor integrated circuit metallization. Tungsten has often been found superior to aluminum for the filling of vias and/or windows. There are two commonly used methods for forming tungsten. The first method, often termed “blanket tungsten” involves the reaction of tungsten hexaflouride (WF 6 ) and silane on an underlying nucleating substrate layer of TiN or TiW. The reaction produces a blanket layer of tungsten upon a substrate surface. The second method for forming tungsten, often termed “selective tungsten” involves the reaction of tungsten hexaflouride with silicon—typically a crystalline silicon substrate upon which integrated circuits are formed. 
     Those who employ selective tungsten to form a tungsten contact to a source/drain region (or other active silicon semiconductor region), often discover the subsequent formation of “worm holes.” A worm hole is a microscopic tunnel formed in the silicon. The worm hole usually contains a small amount of tungsten material at its remote end. The presence of worm holes can destroy semiconductor junction integrity. 
     In an attempt to alleviate the worm hole problem, designers frequently deposit one or more barrier or glue layers within a window. The barrier or glue layer covers the exposed silicon of the source/drain region and helps to avoid the formation of worm holes. However, the presence of the barrier or glue layer precludes the use of selective tungsten because the crystalline silicon substrate is covered by the barrier or glue layer and therefore unavailable for reaction with WF 6 . Consequently, a blanket tungsten reaction must be employed if a barrier/glue layer is used in a window. 
     A typically used barrier/glue layer is a bilayer of titanium/titanium nitride. Titanium is deposited within the window first. Then a layer of titanium nitride is formed atop the titanium. Next, a blanket tungsten layer is formed in contact with the titanium nitride layer. However, the first-deposited titanium layer forms a silicide with the underlying silicon substrate. The silicide formation may be undesirable if the underlying junction is extremely shallow. 
     Designers have consistently sought better methods of tungsten deposition. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of forming tungsten which alleviates the problems mentioned above. Illustratively, the invention includes: 
     forming a dielectric upon a substrate; and forming an opening in the dielectric exposing the substrate. 
     Then a layer of material chosen from the group consisting of polysilicon and amorphous silicon is formed within the opening and overlying the substrate and dielectric. 
     The layer of material is exposed to WF 6 , thereby forming a tungsten plug within the opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-6 are cross-sectional views useful in understanding an illustrative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, reference numeral  11  denotes a substrate. In general, the term “substrate” refers to a material upon which other materials may be formed or deposited. Substrate  11  may, for example, be silicon, epitaxial silicon, or doped silicon. Substrate  11  may contain a source or drain region. Alternatively, substrate  11  may represent the upper portion of a gate and, therefore, substrate  11  may be polysilicon or a silicide. Furthermore, substrate  11  may represent an aluminum, copper or tungsten conductor, such as the conductors typically found in upper levels of integrated circuits. 
     Reference numeral  13  denotes a dielectric. For example, dielectric  13  may be an oxide of silicon, such as a form of silicon dioxide formed by the decomposition of TEOS or another precursor. 
     Material layer  15  which is, illustratively, amorphous silicon or a polysilicon, is deposited with an opening  19  in dielectric  13 . Layer  15  overlies substrate  11 . The thickness of layer  15  may be 100-900 Å. Applicant has found that if opening  19  is one micron wide and 0.7 microns deep, the thickness of layer  15  is desirably 500 Å. 
     In FIG. 2, layer  15  has been exposed to a gas, such as WF 6 . The reaction between layer  15  and WF 6  produces tungsten material layer denoted by reference numeral  17 . It will be noted that a tungsten plug  21  which fills opening  19  and overlies substrate  11  is formed together with a blanket tungsten layer  23  overlying dielectric  13 . If desired, the blanket portion  23  of tungsten layer  17  may be etched or polished away, leaving only plug  21 . Alternatively, the blanket portion  23  of layer  17  may be patterned to form a conductive runner. 
     Another embodiment of the invention is depicted in FIGS. 3 and 4. In FIG. 3, reference numeral  25  denotes a refractory metal formed within opening  19  and the contacting substrate  11 . Illustratively, reference numeral  25  may be a layer of titanium. Reference numeral  27  denotes a barrier/glue layer material such as titanium nitride (TiN), titanium tungsten (TiW) or zirconium nitride (ZrN). Layer  15  containing amorphous or polysilicon is deposited within opening  19  upon layer  27 . 
     In FIG. 4, layer  15  has been exposed to WF 6 , creating tungsten material layer  17  having plug  29  which fills opening  19  and layer portion  30  which overlies dielectric  13 . Again, as mentioned before, tungsten material  17  may be etched back to form a plug or may be patterned to form conductive runners integral with the plug. Appropriate heating may cause that portion  33  of refractory metal  25  in contact with substrate  11  to form a silicide. 
     In another embodiment, only a single material layer, such as layer  25  or  27  comprised of either the disclosed materials or other materials having barrier or glue layer functions may be formed between layer  15  and substrate  11  within opening  19 . 
     Another embodiment is illustrated in FIGS. 5 and 6. In FIG. 5, reference numeral  37  denotes a silicide which has been formed within opening  19  by a “self-aligned silicide process,” i.e., a “silicide process.” After silicide  37  is formed, layer  15  is deposited within opening  19 . It will be noted that silicide  37  contacts substrate  11  (when substrate  11  contains a sufficient quantity of silicon) but does not extend along the vertical walls of opening  19 . 
     Layer  15  is exposed to WF 6 , creating tungsten material layer  17 , depicted in FIG.  6 . As mentioned before, the tungsten material layer  17  may be polished or patterned according to the designer&#39;s wishes. The present inventive process permits the use of selective tungsten despite the presence of silicide  37  in FIGS. 5-6. 
     Illustratively, in the above-described processes, the reaction between WF 6  and material layer  15  is performed using parameters known to those skilled in the art, such as a pressure of 0-15 mT and a temperature of 400-450° C.