1. Field of the Invention
This invention relates to integrated circuit fabrication technology and, more specifically, to processes for forming titanium silicide films via chemical vapor deposition.
2. State of the Art
The compound titanium silicide (TiSi2) is used extensively in the manufacture of integrated circuits. It is frequently used to reduce the sheet resistance of conductively-doped silicon conductors. It is also used to provide solid electrical contact between conductive plugs and an underlying conductively-doped silicon layer.
In a common application for integrated circuit manufacture, a contact opening is etched through an insulative layer down to a conductive region (which may have been formed by diffusion or a combination of implanting and diffusion) to which electrical contact is to be made. Titanium metal is deposited on the surface of the diffusion region and subsequently converted to titanium silicide, thus providing an excellent conductive interface at the surface of the diffusion region. A titanium nitride barrier layer is then deposited, coating the walls and floor of the contact opening. Chemical vapor deposition of tungsten metal or polycrystalline silicon (polysilicon) follows. Deposition of titanium metal and the subsequent conversion of the titanium metal can be replaced by the direct deposition via chemical vapor deposition of titanium silicide. The deposition step is followed by an elevated temperature anneal step which causes titanium silicide molecules to migrate into the underlying silicon layer, thus providing reliable electrical interface between the two layers.
At least three processes have been proposed for creating thin titanium silicide films: (1) reactive sputtering; (2) annealing, in an inert ambiance, a titanium layer that has been sputter-deposited on top of a silicon layer; and (3) chemical vapor deposition, using titanium tetrachloride and a silicon containing compound, such as silane or dichlorosilane, as reactants.
Both reactive sputtering and annealing of deposited titanium result in films having poor step coverage, which are of limited use in submicron manufacturing processes. Chemical vapor deposition processes have an important advantage in that a conformal layer of any thickness may be deposited. This is especially advantageous in ultra-large-scale-intrgration circuits, where minimum feature widths may be smaller than 0.3 xcexcm. Layers as thin as 1 ( )xc3x85 may be readily produced using CVD. However, TiSi2 coatings prepared using titanium tetrachloride have greater resistivity and are poor barriers to atomic migration than sputtered or annealed titanium silicide layers.
What is needed is a new chemical vapor deposition process which will provide highly conformal titanium silicide films of high purity, with step coverage that is suitable for sub-0.25 xcexcm generations of integrated circuits, and resistivity values and barrier qualities that more closely approach those of sputtered and annealed titanium silicide films.
This invention includes various processes for depositing titanium silicide (TiSi2) films containing less than five percent carbon impurities and less than five percent oxygen impurities by weight via chemical vapor deposition and the use of an organo-metallic precursor compound. Sheet resistance of the deposited films is within a range of about 2 to 10 ohms per square. The deposition process takes place in a deposition chamber that has been evacuated to less than atmospheric pressure and utilizes the organometallic compound tertiary-butyltris-dimethylamido-titanium (TBTDMAT) and a silicon containing compound as reactants. The compound tertiary-butyltris-dimethylamido-titanium has the formula TiC(CH3)3(NR2)3. FIG. 1 depicts the structural formula of tertiary-butyltris-dimethylamido-titanium. The deposition temperature, which is dependent on the silicon source, is within a range of about 400xc2x0 C. to 800xc2x0 C. The deposition reaction may be performed within approximately the lower half of the temperature range if a plasma enhanced chemical vapor deposition process is employed. If thermal decomposition is relied on to initiate a reaction between the organometallic compound and the silicon-containing compound, the reaction must be carried out in approximately the upper half of the temperature range. As a general rule, the more stable the reactants, the higher the decomposition temperature.
Titanium silicide films incorporating various other compounds may be deposited using either of the heretofore described embodiments of the process by adding other precursors to the TBTDMAT and the silicon-containing compounds. For example, by adding nitrogen-containing compounds such as amines, ammonia, and hydrazines to the silicon and titanium precursors and using the same reaction parameters, a film having the general composition TiSixN(1xe2x88x92x) can be deposited. Additionally, by adding tungsten-containing organometallic compounds such as bis(2,4-dimethylpentadienyl)titanium or tungsten halide compounds such as WF6 or WCI6 to the silicon and titanium precursors, a titanium silicide film having the general formula TiSiW can be deposited.