1. Field of the Invention
The present invention generally relates to a method for forming a tungsten silicide layer, and more particularly, the present invention relates to a method for depositing a tungsten silicide layer (WSix) using a tungsten source gas and a silicon source gas containing chlorine.
2. Description of the Related Art
As semiconductor devices become highly integrated, the size of patterns formed on individual chips become smaller and an interval between the patterns becomes narrower. These ultra-fine patterns create problems with the conventional use of polysilicon as a wiring material for a gate electrode and a bit line. That is, as the size of the patterns become smaller, the specific resistivity of polysilicon is increased, which in turn can result in an RC time delay and an IR voltage drop. For this reason, polycide, which has characteristics similar to those of polysilicon but with much smaller specific resistivity (e.g., several times to several tens times smaller) has been suggested as a wiring material for the gate electrode and the bit line of a VLSI circuit. More specifically, a composite layer of polysilicon and a refractory metal silicide has been used as a polycide of wiring electrodes.
Silicides of refractory metals, such as tungsten (W), molybdenum (Mo), titanium (Ti) and tantalum (Ta), have been adapted for use as a low resistive wiring material suitable for manufacturing the VLSI circuit. Silicides are bonded to polysilicon doped with high-density impurities, whereby a gate electrode having a polycide structure is formed. Preferably, a low pressure chemical vapor deposition (LPCVD) method is used for depositing refractory metal silicides. Tungsten silicide, when bonded to polysilicon, exhibits superior characteristics with respect to self-passivation, stability against wet chemicals, surface roughness, adhesion, anti-oxidation and reproducibility.
Tungsten silicide (WSix) thin films are formed using monosilane (SiH4) and tungsten hexafluoride (WF6) as a precursor gas and are deposited on semiconductor substrates by LPCVD. However, certain drawbacks are associated with the LPCVD process. One such drawback is that tungsten silicide cannot be conformally deposited on a stepped portion of a substrate. Another drawback is that a substantial amount of fluorine remains in the deposited tungsten silicide thin film, which in turn can result in operational defects in the manufactured semiconductor device. That is, when the semiconductor wafer is later exposed to a temperature exceeding 850xc2x0 C. during an anneal process, the remaining fluorine ions move into a lower silicon oxide layer through a polysilicon layer. An effective thickness of the silicon oxide layer is thus increased, resulting in inconsistent electric characteristics of the semiconductor device having the silicon oxide layer.
For this reason, it has been suggested to deposit the tungsten silicide layer using dichlorosilane (DCS; SiH2Cl2) instead of monosilane. A tungsten silicide layer which has been deposited using dichlorosilane (hereinafter, referred to as a xe2x80x9cDCS tungsten silicide layerxe2x80x9d) exhibits superior step coverage and low fluorine content as compared with a tungsten silicide layer which has been deposited using monosilane (hereinafter, referred to as a xe2x80x9cMS tungsten silicide layerxe2x80x9d).
In view of its advantages, the DCS tungsten silicide layer has been preferred over the MS tungsten silicide layer as the wiring material for a word line (that is, gate electrode) and a bit line. However, the DCS tungsten silicide layer is not without its drawbacks. That is, various problems result from the fact that when the dichlorosilane gas is reacted with tungsten hexafluoride (WF6) gas, chlorine radicals remain on a surface and an inner portion of a thin film.
FIG. 1 is a flow chart showing a conventional method for depositing a DCS tungsten silicide layer.
Referring to FIG. 1, a silicon wafer, that is, a semiconductor substrate having a polysilicon layer at an uppermost region thereof is loaded in a process chamber of a plasma-enhanced chemical vapor deposition (PECVD) apparatus (step S1). Then, tungsten hexafluoride (WF6) gas and dichlorosilane (SiH2Cl2) gas are introduced into the process chamber to form a tungsten silicide nucleus on a surface of the polysilicon layer (step S3). Then, under reaction conditions of a pressure of about 1 to 1.5 Storr, a deposition time of 10 to 25 seconds, and a temperature of 450 to 700xc2x0 C., tungsten hexafluoride (WF6) gas and dichlorosilane (SiH2Cl2) gas are introduced into the process chamber in a ratio of about 13:180. As a result, a tungsten silicide layer is deposited on the surface of the polysilicon layer having the tungsten silicide nucleus (step S5). Then, a post-flushing process is carried out for about 10 seconds to reduce stress by introducing a monosilane (SiH4) gas into the process chamber (step S7) at flow rate of about 300 sccm.
FIG. 2A is a graph showing a chlorine profile in the tungsten silicide layer and the polysilicon layer when the DCS tungsten silicide layer has been deposited, and FIG. 2B is a graph showing a chlorine profile in the tungsten silicide layer and the polysilicon layer after an annealing process has been carried out.
Generally, the DCS tungsten silicide layer is deposited at a temperature of 620xc2x0 C., which is about 200xc2x0 C. higher than a deposition temperature of the MS tungsten silicide layer. Also, the DCS tungsten silicide layer exhibits a columnar structure in which a hexagonal phase and a tetrahedral site co-exist. In addition, the DCS tungsten silicide layer has a body-centered cubic structure in which a covalent bond is formed between one tungsten atom and eight silicon atoms. Excess silicon, which is not involved in the covalent bond, acts as a stacking fault. Silicon, which acts as the stacking fault, is bonded to chlorine dissociated from dichlorosilane gas, thereby forming a silicon chloride crystal. Chlorine breaks Sixe2x80x94H and Sixe2x80x94Si bonds so as to be bonded to silicon and excess silicon is re-bonded to chlorine because of the bond strength differences. Therefore, as shown in FIG. 2A, chlorine is diffused from the tungsten silicide layer into the underlying polysilicon layer as the result of a domino phenomenon of continuous dissociation and crystal-bonding, so that chlorine in the form of SiClx crystals is concentrated on an interfacial surface between the tungsten silicide layer and the polysilicon layer.
In addition, after the anneal process has been carried out at a temperature of about 800xc2x0 C., the tungsten silicide layer has a stable stoichiometry due to the tetrahedral site crystalline structure, whereby an amount of chlorine considered as SiClx crystals increases in a region of the polysilicon layer which has a relatively sufficient amount of silicon (referred to FIG. 2B).
As described above, according to the conventional DCS tungsten deposition method, a substantial amount of SiClx crystal remains in the thin film and a surface of the polysilicon layer, whereby a Milky Way phenomenon (that is, a collection of twinkling points viewed in an optical microscope) occurs which is caused by a scattering of light due to the SiClx crystal, thereby resulting a visual defect. The visual defect affects subsequent photolithography processes, making it difficult to carry out such photolithography processes.
In addition, the chlorine atoms contained in the DCS tungsten silicide layer and the polysilicon layer diffuse into an inside of the silicide layer in the form of SiClx crystals, and as a result, the polysilicon layer is abnormally grown.
That is, while the crystallization temperature of the polysilicon layer is about 530 to 550xc2x0 C., the polysilicon layer is initially deposited in an amorphous state and crystallization of the polysilicon layer is carried out in the process chamber at a temperature of about 620xc2x0 C. when the DCS tungsten silicide layer is deposited. At this time, excess silicon contained in the DCS tungsten silicide layer is bonded to chlorine to form the SiClx crystal. In this state, the SiClx crystal diffuses from the tungsten silicide layer into the underlying polysilicon layer as a result of the domino phenomenon of continuous dissociation and crystal-bonding. Due to the SiClx crystals, silicon dissociated from Sixe2x80x94Si bond or Sixe2x80x94H bond diffuses towards the polysilicon layer, thereby causing growth and crystallization of the polysilicon layer. In particular, the monosilane post-flushing process, which is carried out after the DCS tungsten silicide layer is deposited, continuously supplies excess silicon into the tungsten silicide layer. Therefore, after the DCS tungsten silicide layer deposition process and the monosilane post-flushing process have been finished, the growth and crystallization of the polysilicon layer in the amorphous state occurs locally from the diffusion of excess silicon. Accordingly, the polysilicon layer protrudes through the tungsten silicide layer formed thereon, thereby resulting a crack known as a xe2x80x9chaze phenomenonxe2x80x9d.
According to one aspect of the present invention, there is provided a method of forming a tungsten silicide layer, which includes loading a semiconductor substrate having a polysilicon layer into a process chamber of a plasma enhanced chemical vapor deposition device, and introducing a silicon source gas, a tungsten source gas, and a hydrogen compound gas for reducing a chlorine radical into the process chamber, to thereby deposit the tungsten silicide layer on the polysilicon layer.
According to another aspect of the present invention, there is provided a method for forming a tungsten silicide layer, which includes loading a semiconductor substrate having a polysilicon layer into a process chamber of a plasma enhanced chemical vapor deposition device, pre-flushing a surface of the polysilicon layer by introducing a hydrogen compound gas for reducing a chlorine radical into the process chamber, introducing a silicon source gas, a tungsten source gas and the hydrogen compound gas into the process chamber so as to form a tungsten silicide nucleus on the surface of the polysilicon layer, and introducing the silicon source gas and the tungsten source gas into the process chamber so as to deposit the tungsten silicide layer on the polysilicon layer.
According to still another aspect of the present invention, there is provided a method for forming a tungsten silicide layer, which includes loading a semiconductor substrate having a polysilicon layer into a process chamber of a plasma enhanced chemical vapor deposition device, pre-flushing a surface of the polysilicon layer by introducing a hydrogen compound gas for reducing a chlorine radical into the process chamber, introducing a silicon source gas, a tungsten source gas and the hydrogen compound gas into the process chamber so as to form a tungsten silicide nucleus on the surface of the polysilicon layer, and introducing the silicon source gas, the tungsten source gas, and the hydrogen compound gas into the process chamber so as to deposit the tungsten silicide layer on the polysilicon layer.
According to the present invention, when the tungsten silicide layer is deposited on the polysilicon layer by introducing, for example, a tungsten hexafluoride (WF6) gas and a dichlorosilane (Si2HCl2) gas into the process chamber, a hydrogen compound gas such as B2H6, PH3, AsH3 and/or NH3 is also introduced into the process chamber. As a result, a chlorine radical dissociated from the dichlorosilane gas by the hydrogen compound is reduced into hydrogen chloride (HCl) and is removed together with an exhaust gas. Accordingly, visual defects caused by SiClx crystals and abnormal growth of the polysilicon layer may be avoided.