Abstract:
A process for forming a semiconductor device comprises the steps of providing a semiconductor substrate assembly comprising a semiconductor wafer having an active area formed therein, a plurality of transistor gates each having a TEOS cap thereon and a pair of nitride spacers along each gate, a plurality of conductive plugs each contacting the wafer, and a BPSG layer overlying the transistor gates and contacting the active area. A portion of the BPSG layer is etched thereby exposing the TEOS caps. A portion of the BPSG layer remains on the active area after completion of the etch. Subsequently, a portion of the TEOS caps are removed to expose the transistor gates and a titanium silicide layer is formed simultaneously to contact the transistor gates and the plugs. An inventive structure resulting from the inventive process is also described.

Description:
This is a division of U.S. patent Ser. No. 09/146,639 filed Sep. 3, 1998 and issued Nov. 26, 2002 as U.S. Pat. No. 6,486,060. 

   FIELD OF THE INVENTION 
   This invention relates to the field of semiconductor assembly, and more particularly to a process for forming conductive structures having enhanced conductivity, and the in-process structure resulting from the process. 
   BACKGROUND OF THE INVENTION 
   Structures such as polycrystalline silicon (poly) plugs, interconnects, and transistor gates are commonly formed during the manufacture of semiconductor devices such as microprocessors, memory devices, and logic devices. To manufacture a plug, for example, a masked dielectric layer is formed over an underlying substrate assembly and an etch is completed to form a hole in the dielectric which exposes the underlying structure in the area where contact is to be made. A blanket poly layer is deposited over the dielectric layer which fills the hole in the dielectric layer and contacts the underlying structure. The poly is then removed from a planar surface of the dielectric, typically using a chemical mechanical polishing (CMP) process which leaves the plug formed within the dielectric layer. Interconnects and gates are typically formed by depositing a blanket layer of poly over a semiconductor substrate assembly, then masking and etching the layer. 
   As the sizes of the plugs and line widths decrease with improving manufacturing technology, the doped poly structure may provide excessive resistance and insufficient conductance. To reduce the resistance of a structure, a silicide layer is often formed underneath the plug or over the top of the plug, gate or interconnect. To form the silicide layer to enhance plug conductivity the silicide layer can be formed before formation of the plug. A titanium chemical vapor deposition (CVD) process results in titanium reacting with the exposed silicon wafer to form titanium silicide. An unreacted titanium metal layer will also form over any exposed dielectric layer which is then stripped. After stripping the unreacted titanium, the poly plugs are formed over the silicide layer as described above. 
   A silicide layer can also be formed over the plug, transistor gate or other interconnect after forming the blanket poly layer which forms the gate or interconnect. During a titanium CVD process similar to that described above for forming silicide under the plug, the titanium reacts with the polysilicon to form silicide on top of the poly layer, then the poly is masked and etched to define the line or plug. 
   U.S. Pat. No. 5,381,302 by Sandhu et al. and U.S. Pat. No. 5,198,384 by Dennison, each assigned to Micron Technology, Inc. and incorporated herein by reference in their entirety, describe a process for forming a silicide layer over a poly plug. 
   While the silicide layer interposed between the silicon wafer and the poly plug provides decreased resistance and increased conductance it can also provide a path for leakage between an adjacent transistor channel region and an active area, thereby increasing junction leakage. A process and structure which provides improved conductance and reduced resistance and which does not increase junction leakage would be desirable. It would be further desirable to provide a plug having a maximized amount of silicide formed thereon. 
   SUMMARY OF THE INVENTION 
   The present invention provides a new process and structure having fewer problems associated with the formation of silicided polycrystalline silicon plugs, particularly problems resulting in excessive transistor junction leakage. In accordance with one embodiment of the invention, a semiconductor substrate assembly is provided which comprises a semiconductor wafer, a plurality of transistor gates, and a plurality of conductive plugs which contact the wafer. A silicide layer is formed which simultaneously forms to contact the plurality of gates and the plurality of plugs. In another embodiment, a plug is formed which comprises silicide on the sidewalls, thereby maximizing the amount of the desirable silicide to minimize resistance. 
   Objects and advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross section depicting a plurality of transistor gates formed over a doped semiconductor wafer, and further depicts a plurality of dielectric layers and a mask formed thereover; 
       FIG. 2  is a cross section of the  FIG. 1  structure after patterning the planar dielectric layer and forming of a blanket conductive layer; 
       FIG. 3  is a cross section of the  FIG. 2  structure after planarizing the conductive layer to form a plurality of plugs; 
       FIG. 4  is a cross section of the  FIG. 3  structure after etching the planar dielectric and after forming a refractory metal layer (which forms silicide on the conductive layer) and a protective dielectric layer over the titanium layer; 
       FIG. 5  is a cross section of the  FIG. 4  structure after removing the protective dielectric layer and any unreacted refractory metal. 
   

   It should be emphasized that the drawings herein may not be to exact scale and are schematic representations. The drawings are not intended to portray the specific parameters, materials, particular uses, or the structural details of the invention, which can be determined by one of skill in the art by examination of the information herein. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A first embodiment of an inventive process used during the formation of a semiconductor device is depicted in  FIGS. 1–5 .  FIG. 1  depicts a wafer substrate assembly  10  including a semiconductor wafer  12  having doped regions (active areas) therein  14 , at least one transistor, and preferably a plurality of transistors, with each transistor comprising gate oxide  16  and a conductive transistor control gate portion  18 . Each transistor further comprises dielectric spacers  20 , for example formed from silicon nitride, and a capping layer  22 , for example formed from tetraethyl orthosilicate (TEOS) or another undoped silicon dioxide layer. A TEOS layer from about 300 Å to about 800 Å thick would be sufficient. 
     FIG. 1  further includes a planar dielectric layer  24 , for example a borophosphosilicate glass (BPSG) layer from about 4,000 Å to about 8,000 Å thick, having a mask  26  formed thereupon. Alternatively, borosilicate glass (BSG), phosphosilicate glass (PSG), or another doped silicon dioxide layer may be used. The mask layer leaves various areas of the BPSG exposed  28 , for example regions between various transistors, and areas between a transistor array  30  and a periphery  32  of a semiconductor device as depicted. A shallow trench isolation area  34  separates the array  30  from the periphery. Next, the exposed BPSG  24  is removed using an etch selective to the TEOS caps  22  and nitride spacers  20  (i.e. an etch which minimizes etching of TEOS and nitride and maximizes etching of BPSG). The exposed BPSG is removed to expose the underlying layer (in the instant case, the silicon wafer  12 ) as depicted in  FIG. 2 . A wet etch such as a hydrofluoric acid (HF) dip, or a dry etch would remove the BPSG selective to nitride and TEOS. 
   Regarding the dry (plasma) BPSG etch selective to nitride and TEOS, one factor that affects the etch rate and the etch selectivity of the process is pressure. The total pressure has a preferred range of from about 1 millitorr to about 400 millitorr. A more preferred pressure range for a plasma etch is in a pressure range of from about 1 millitorr to about 100 millitorr. The most preferred pressure range for a plasma etch is from about 1 millitorr to about 75 millitorr. The pressure may be increased, however, above the most preferred ranges. For example the RIE etch may be performed at about 100 millitorr. Selectivity of the BPSG to the TEOS and nitride can be optimized at a pressure range between about 10 millitorr and about 75 millitorr. Pressure increases may result in a loss in selectivity. The range in selectivity, however, can be adjusted to accommodate different pressures. As such, selectivity and pressure are inversely related. Temperature is another factor that affects the selectivity of the etching process used. A preferable temperature range of the reactor cathode during the plasma etch has a range of about 10° C. to about 80° C., and more preferably about 20° C. to about 40° C. This is the temperature of a bottom electrode adjacent to the semiconductor substrate  12  during the etching process. The preferable range of the semiconductor materials is between about 40° C. and about 120° C., and more preferably between about 40° C. and about 90° C. 
   Undoped silicon dioxide layer  22  and nitride spacers  20  seen in  FIGS. 1 and 2  protect underlying layers from the fluorinated chemical etch. As illustrated in  FIG. 2 , the etch will anisotropically remove the exposed portions  28  of doped BPSG layer  24 . The etch removes material from the BPSG layer  24  at a higher material removal rate than that of undoped TEOS layer  22  and nitride spacers  20 . Preferably, the etch has a material removal rate for substantially doped silicon dioxide (BPSG  24 ) that is at least 10 times higher than that of substantially undoped silicon dioxide (TEOS  22 ) or silicon nitride  20 . 
   Preferably, etching as conducted according to this invention involves an anisotropic plasma etch with a fluorinated chemistry that etches through BPSG (or BSG or PSG or doped silicon dioxide in general). The etch is preferably selective to undoped silicon dioxide, silicon, and silicon nitride. The fluorinated chemical etch uses a type of carbon fluorine gas that is preferably selected from the group consisting of C 2 F 6 , CF 4 , C 3 F 8 , C 4 F 10 , C 2 F 8 , CH 2 F 2 , CHF 3 , C 2 HF 5 , CH 3 F and combinations thereof. There are other fluorinated etchants in a substantially gas phase that can be employed during the etching of the structure. An inert gas is often used in combination with the fluorinated etchant. Argon, nitrogen, and helium are examples of such an inert gas. The preferred gases, however, are CF 4 , CH 2 F 2 , CHF 3  and Ar. Alternatively, CH 3 F may be used in place of CH 2 F 2 . In particular, the preferred etchant is a fluorine deficient gas which is defined as a gas where there are not enough fluorine atoms to saturate the bonding for the carbon atoms. 
   Next, as depicted in  FIG. 2 , a blanket layer of polycrystalline silicon  40  (poly) from about 1,000 Å to about 4,000 Å thick is formed over the wafer substrate assembly  10  to contact the silicon wafer  12 . The poly  40  and, optionally, the BPSG  24  are planarized to remove the poly  40  from horizontal surfaces of the BPSG layer  24 , for example using chemical mechanical planarization (CMP) to result in the structure of  FIG. 3  having poly plugs  44  which contact the silicon wafer  12 . The plugs as depicted comprise a horizontal top and a substantially vertical sidewall which intersect at about 90°. 
   Subsequently, the BPSG  24  is removed, for example using an HF dip, to expose the TEOS caps  22  from the transistor gates  18 , but a portion of the BPSG  50  remains over the areas where the BPSG contacts the wafer as depicted in  FIG. 4 . Further, the exposed TEOS  22  is etched from the transistor gates  18  to expose the poly gates  18 , for example using tetramethylammonium hydroxide (TMAH) and HF. Alternately, the BPSG and TEOS can be removed using a single etch which is selective to nitride and poly. A wet etch, for example using TMAH/HF, SuperQ (3% phosphoric acid, 37% ammonium fluoride), QEtch II (1% phosphoric acid, 39% ammonium fluoride), or a dry etch, for example using CHF 3 , CF 4 , or Argon would be sufficient. A portion of the TEOS  52  may remain, which is dependent on the alignment of the mask  26  in  FIG. 1 . The removal of the BPSG  24  between  FIGS. 3 and 4  requires no mask, but the etch must be timed to clear the BPSG  24  from the TEOS caps  22  but to leave a portion of the BPSG  50  where the BPSG contacts the semiconductor wafer  12 . Further, the TEOS  22  must be cleared to expose the gates  18 . 
   A blanket refractory metal layer such as titanium, chromium, tantalum, platinum, tungsten, zirconium, and molybdenum, and preferably a titanium layer  54  from about  100  Å to about  400  Å thick is formed over the exposed surfaces of the assembly as depicted in  FIG. 4 . This includes forming the titanium over poly  18  and wafer  12  and over various dielectrics  20 ,  50  as depicted. A CVD deposition of titanium can be performed at a temperature between about 400° C. and 700° C. using TiCl 4  as a source. Using this process the titanium reacts with exposed silicon structures and remains unreacted over the dielectric structures. Next, a titanium nitride layer  56  can be formed using a CVD or PVD process to a thickness of between about 100 Å to about 500 Å. A rapid thermal anneal (RTA) step is performed to decrease the resistance of the titanium silicide, for example using a temperature between about 650° C. and about 800° C. for between about 20 seconds to about 90 seconds in a nitrogen ambient. During this step, the TiN protects the silicide from exposure to, and possible reaction with, oxygen. 
   Finally, the titanium nitride  56  and the unreacted titanium  54  overlying the dielectric is removed, for example using a solution of NH 4 OH, H 2 O 2 , and water. The remaining exposed BPSG  50  over the active areas are removed using an HF dip to result in the structure of  FIG. 5 . 
   As depicted in  FIG. 5 , the instant process results in poly plugs  44  or other poly structures having a silicided sidewall. This results in a structure having reduced resistance compared to structures of equal size and shape having silicide only over the upper surface. Further, silicide over two or more types of structures, for example a transistor gate  18  and a poly plug  44 , can be formed simultaneously which reduces manufacturing steps compared to processes which form silicide over the gate and over the plug using two or more separate steps. 
   While this invention has been described with reference to illustrative embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.