Abstract:
A semiconductor manufacturing process for depositing a tungsten silicide film on a substrate includes deposition of a tungsten silicide nucleation layer on the substrate using a (CVD) process with a silane source gas followed by deposition of the tungsten silicide film with a dichlorosilane source gas. This two step process allows dichlorosilane to be used as a silicon source gas for depositing a tungsten silicide film at a lower temperature than would otherwise by possible and without plasma enhancement. Tungsten silicide films deposited by this process are characterized by low impurities, good step coverage, and low stress with the silicon substrate.

Description:
TECHNICAL FIELD 
     This invention relates to semiconductor manufacture and more particularly to a novel process for the deposition of tungsten silicide (WSi x ) films. 
     BACKGROUND OF THE INVENTION 
     Refractory metals and their silicides are often used in semiconductor manufacture. One silicide that is of particular interest in the formation of MOS and CMOS integrated circuits is tungsten silicide. Tungsten silicide, generally in the form of (WSi 2 ), can be used in the formation of integrated circuits as an intermediate, barrier, or conducting film. Tungsten silicide can also be used with polysilicon as a polycide-gate in transistor formation. Among the useful properties of tungsten silicide are its low bulk resistance and low stress. 
     In the formation of a tungsten silicide film, the film quality and step coverage provided by the deposited film greatly influences the completed semiconductor devices. This is because poor step coverage and an irregular surface topography generated by multi-layer processes causes serious difficulties for subsequent processing steps such as lithography, deposition, and etching. Poor step coverage and high stress may also degrade device reliability by causing stress induced cracking. Moreover, impurities in the deposited film may degrade device reliability and cause metal migration. 
     Chemical vapor deposition (CVD) is typically utilized in the deposition of tungsten silicide. In general (CVD) is favored because the deposited film has lower impurities and better step coverage than a sputtered film. In such a (CVD) system the reaction chemistry is typically based on the reaction of a silicon source gas, such as silane (SiH 4 ), with a reactant gas, such as tungsten hexaflouride (WF 6 ). One advantage of a (CVD) deposition process of tungsten silicide using this reaction chemistry is that because of the high reactivity of silane (SiH 4 ) and tungsten hexaflouride (WF 6 ), relatively low process temperatures can be utilized. These deposition temperatures are typically in the range of 250°-400° C. A disadvantage of a (CVD) deposition process of tungsten silicide using this reaction chemistry is the very high levels of fluorine impurities in the deposited film. In addition, step coverage may be poor. 
     In order to provide a tungsten silicide film having a lower impurity content and a better step coverage, a (CVD) reaction chemistry using higher order silanes such as dislane dichlorosilane and trislane have been proposed. U.S. Pat. Nos. 4,684,542 to Jasinski et al. and U.S. Pat. No. 4,966,869 to Hillman et al. disclose such processes. 
     Such a process is also disclosed in the technical report entitled, “Properties of WSi x  using dichlorosilane in a single-wafer system” reported in the J. Vac. Sci. Technol. B 6(6), Nov/Dec 1988. This process was carried out in a (CVD) cold-wall reactor using a reaction of tungsten hexafluoride (WF 6 ) and dichlorosilane (SiH 2 Cl 2 ) gases. 
     In this prior art process plasma enhancement of the process gases using radio frequency-induced glow discharge, was required to obtain the thermal deposition. The resultant film was characterized by a much lower fluorine concentration than films deposited with a conventional WF 6 /SiH 4  chemistry. In addition, films deposited with a dichlorosilane source gas are characterized by excellent step coverage. 
     Although this process allowed the tungsten silicide film to be deposited at relatively low process temperature (450°-650° C.), the requirement of plasma enhancement for thermal deposition tended to complicate the procedure. Such a complicated procedure may not be entirely suitable for large scale semiconductor manufacture. In addition plasma can cause degradation of devices due to radiation induced damage. 
     There is then a need in the art for a process suitable for large scale semiconductor manufacture for forming high quality tungsten silicide films at relatively low process temperatures. Accordingly, it is an object of the present invention to provide a process for depositing tungsten silicide at a relatively low process temperature. It is another object of the present invention to provide a process for depositing a tungsten silicide film using dichlorosilane as a silicon source gas such that a high quality film having low impurities, low stress, and good step coverage is provided. It is yet another object of the present invention to provide a process for depositing a high quality tungsten silicide film that is adaptable to large scale semiconductor manufacture. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a novel process for depositing a tungsten silicide film is provided. The process of the invention, simply stated, includes the steps of; depositing a tungsten silicide nucleation layer on a substrate using silane as a silicon gas source and then switching to dichlorosilane as the silicon gas source to complete deposition of the tungsten silicide film. This two step process permits the dichlorosilane silicon source gas to react with a reactant gas at a relatively low temperature. In addition, this process can be performed in a cold wall (CVD) system without the need for plasma enhancement of the reactant gases. The reactant gas is preferably tungsten hexaflouride mixed with inert carrier gases. 
     Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram illustrating the basic steps of a process for depositing a tungsten silicide film in accordance with the process of the invention; and 
         FIG. 2  is a schematic diagram of an apparatus for carrying out the process of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , the process of the invention is shown and broadly stated, comprises, a method for depositing a tungsten silicide (WSi x ) film on a substrate that includes the steps of: 
     depositing a nucleation layer of tungsten silicide (WSi x ) on a substrate using a (CVD) process with silane (SiH 4 ) as a silicon source gas and tungsten hexafluoride (WF 6 ), as a reactant gas, step  10 ; and 
     depositing a tungsten silicide (WSi x ) film on the nucleation layer using a (CVD) process with dichlorosilane (SiH 2 Cl 2 ) as a silicon source gas, step  12 . 
     This process permits a high quality tungsten silicide (WSi x ) film to be deposited at relatively low temperatures in a cold walled (CVD) reaction chamber. Furthermore, the process can be carried out without a plasma enhancement of the (CVD) process. 
     Referring now to  FIG. 2  a cold wall (CVD) system  14  suitable for carrying out the method of the invention is shown. The cold wall (CVD) system  14  includes a reaction chamber  16  wherein a plurality of silicon wafers  18  may be located on a graphite boat  20 . The graphite boat  20  is heated to a desired temperature by halogen lamps  21 . The cold wall (CVD) system also includes a premix chamber  22  wherein the reactant and source gases are mixed prior to being directed through a shower head  24  into the reaction chamber  16 . The cold wall (CVD) system  14  may also include pressure control means in the form of a pressure sensor  26 , a pressure switch  28 , an air operated vacuum valve  30 , and a pressure control valve  32 . In addition, reactant gases given off by the chemical reactions in the process chamber  16  are drawn by a roots blower  34  into a particulate filter  36  and to the atmosphere. 
     A supply of carrier gases  38  are coupled through suitable valving to the premix chamber  22 . The carrier gases may include Argon, Nitrogen, and Helium or other suitable inert gases. A supply of a tungsten hexaflouride (WF 6 ) reactant gas  38  is also coupled to the premix chamber through suitable valving. A silicon source gas in the form of a silane (SiH 4 ) source gas  42  or dichlorosilane (SiH 2 Cl 2 ) source gas  44  are also coupled to the premix chamber  22  through suitable valving. 
     With this arrangement the inert carrier gases (Ar, N2, He), and the reactant gas (WF 6 ) can be combined in the premix chamber  22  with the desired silicon source gas. This silicon source gas will initially be silane (SiH 4 ). This can be used to produce an initial nucleation layer of tungsten silicide (WSi x ) on the substrate. Following formation of the nucleation layer, the source gas is switched and dichlorosilane (SiH 2 Cl 2 ) is used as the silicon source gas for deposition of the remainder of the tungsten silicide (WSi x ) film. The flow rate of the carrier gases (Ar, N 2 , He) may be as great as five to ten times the flow rate of the silicon source gas (either silane or dichlorosilane). The flow rate of the silicon source gas (either silane or dichlorosilane) in turn may be about 50-100 times the flow rate of the reactant gas. 
     During the process of the invention, the pre-mixture gas combination (silicon source, reactant and carrier gases) is directed from the premix chamber  22  through the shower head  24  and into the reaction chamber  16 . As these gases intermix inside the reaction chamber  16 , they begin to react and deposit a uniform film of tungsten silicide (WSi x ) upon the heated surface of the silicon wafers  18 . The pressure control components  26 ,  28  and  30  can be utilized to monitor and maintain a desired pressure during the process. The flowrates of the gases (silicon source, reactant and carrier) can also be controlled to achieve the desired reaction and film quality. 
     Using this arrangement it is desirable to initially deposit a nucleation layer of tungsten silicide (WSi x ) on the silicon wafers  18  using a silane (SiH 4 ) silicon source for a relatively short time such that a thin or discontinuous nucleation layer is formed on the surface of the silicon wafers  18 . By way of example and not limitation, at a temperature of about 450° C. and with a silane flow rate of about 400 sccm, this nucleation layer is deposited in about 1-25 seconds. This requires a flow of reactant gas (WF 6 ) of about 4 sccm and a flow of inert gases (Ar, N2, He) of about 2800 sccm. Following deposition of the nucleation layer the silicon source gas may be switched abruptly or gradually to dichlorosilane and the tungsten silicide film can be deposited to the desired thickness using the dichlorosilane as the source gas. As an example this thickness may be in the range of about 1000 to 3000 Å. 
     A temperature of the silicon wafers  18  during both steps of the deposition process (i.e. nucleation and deposition) will be on the order of about 450° C. or less and may be in the range of 200° C. to 500° C. This is lower than prior art deposition processes using dichlorosilane (SiH 2 Cl 2 ) as a silicon gas source. This is because the initially deposited nucleation layer deposited with a silane silicon gas source can be performed at a relatively low temperature and allows the subsequent deposition process with the dichlorosilane sourse gas to also proceed at the lower temperature. In addition, with the nucleation layer deposited with a silane source gas there is no need to plasma enhance the (CVD) process. 
     The resultant film deposited with a dichlorosilane (SiH 2 Cl 2 ) silicon gas source is characterized by a low impurity content and by a low stress with the silicon substrate. Moreover, the resultant tungsten silicide (WSi x ) film is characterized by a high quality step coverage. This permits the process to be utilized for high density applications such as in the conformal deposition of tungsten silicide (WSi x ) between closely spaced structures. 
     It is to be understood that although the present invention has been described with reference to a preferred embodiment, various modifications known to those skilled in the art, may be made to the process steps presented herein without departing from the invention as recited in the following claims.