Chemical vapor depositions process for depositing titanium silicide films from an organometallic compound

A process for depositing titanium suicide films via chemical vapor deposition takes place in a deposition chamber that has been evacuated to less than atmospheric pressure and utilizes, as reactants, the organometallic compound tertiary-butyltris-dimethylamido-titanium and a silicon-containing compound such as silane. The deposition temperature, which is dependent on the nitrogen source, is within a range of 400 to 800.degree. C. The low end of the temperature range utilizes a plasma-enhanced deposition process, while the higher end of the temperature range relies on thermal decomposition to initiate the reaction. The films deposited using the process have a sheet resistance of about 2 to 10 ohms per square and contain less than 5 percent carbon impurities and less than 5 percent oxygen impurities by weight. 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 TiSi.sub.X N.sub.(1-X) can be deposited. Additionally, by adding tungsten-containing organometallic compounds such as bis(2,4-dimethylpentadienyl)tungsten or tungsten halide compounds such as WF.sub.6 or WCl.sub.6 to the silicon and titanium precursors, a titanium silicide film having the general formula TiSiW can be deposited.

BACKGROUND OF THE INVENTION 
 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. Description of Related Art 
 The compound titanium silicide (TiSi.sub.2) 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 contacts 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 a 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 conformal layers of any thickness may be 
 deposited. This is especially advantageous in 
 ultra-large-scale-integration circuits, where minimum feature widths may 
 be smaller than 0.3 .mu.m. Layers as thin as 10 .ANG. may be readily 
 produced using CVD. However, TiSi.sub.2 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 .mu.m generations of integrated 
 circuits, and resistivity values and barrier qualities that more closely 
 approach those of sputtered and annealed titanium silicide films. 
 SUMMARY OF THE INVENTION 
 This invention includes various processes for depositing titanium silicide 
 (TiSi.sub.2) 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 organometallic 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(CH.sub.3).sub.3 (NR.sub.2).sub.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 400.degree. C. to 800.degree. 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 TiSi.sub.X N.sub.(1-X) can be deposited. 
 Additionally, by adding tungsten-containing organometallic compounds such 
 as bis(2,4-dimethylpentadienyl)titanium or tungsten halide compounds such 
 as WF.sub.6 or WCl.sub.6 to the silicon and titanium precursors, a 
 titanium silicide film having the general formula TiSiW can be deposited.

PREFERRED EMBODIMENT OF THE INVENTION 
 Several embodiments of a new low-pressure chemical vapor deposition process
 for depositing highly conformal titanium silicide (TiSi.sub.2) films 
 having carbon impurities of less than five percent by weight and oxygen 
 impurities of less than five percent by weight and resistivity of 
 approximately 2 ohms per square will be described in reference to the 
 cold-wall, plasma-enhanced, low-pressure chemical vapor deposition reactor
 system depicted in FIG. 2. Although the following description of the 
 process represents what the inventor believes is the preferred embodiment 
 of the process, the process may be practiced in either cold-wall or 
 hot-wall plasma-enhanced chemical vapor deposition chambers, with or 
 without premixing of the reactants. Furthermore, although the invention is
 directed to a technique for depositing conformal titanium silicide layers 
 for use in the manufacture of integrated circuits, the process is also 
 applicable to the deposition of titanium silicide on substrates other than
 semiconductor wafers. 
 A first major embodiment of the process will be described with reference to
 FIG. 2. A tertiary-butyltris-dimethylamido-titanium organometallic source 
 gas is produced by heating liquid 
 tertiary-butyltris-dimethylamido-titanium (the organometallic precursor 
 compound). The gas phase organometallic compound, transported by a carrier
 gas, is admitted into a premixing chamber 3 through control valve 1 and a 
 silicon-containing gas such as silane, chlorinated or fluorinated silanes,
 organometallic silicon compounds (or some combination thereof) is admitted
 into the premixing chamber 3 along with a carrier gas through control 
 valve 2. The carrier gases employed may be H.sub.2, N.sub.2, He or Ar. 
 Following the premixing of the gas phase reactants in premixing chamber 3,
 the premixed gases are admitted to the deposition chamber 4. Optionally, 
 the organometallic compound may be mixed with an inert carrier gas by 
 bubbling the carrier gas through the heated organometallic compound to 
 further enhance the complete gasification of that reactant. As a further 
 option, the liquid organometallic compound may be converted to a fine 
 spray or mist by a liquid injector (not shown). The mist is then passed 
 through a vaporizer chamber (also not shown) en route to the deposition 
 chamber. Within the deposition chamber 4, a semiconductor wafer 5 is 
 heated to a temperature within a range of about 400.degree. C. to 
 600.degree. C. by convection from substrate holder 6 (such as a graphite 
 or alumina boat) that, in turn, is heated via halogen lamps 7. The walls 
 of the chamber are maintained at a temperature which is sufficiently high 
 to prevent condensation of TBTDMAT molecules thereon, yet not so high that
 decomposition of TBTDMAT molecules will occur. In a cold-wall reactor, the
 wafer is maintained at a temperature that is considerably higher than that
 of the chamber walls, thereby minimizing deposition of titanium nitride on
 the chamber walls. As a general rule, the chamber walls should be 
 maintained within a range of about 50 to 400.degree. C., and optimally 
 within a range of about 100 to 200.degree. C. 
 Still referring to FIG. 2, the premixed combination of the vaporized 
 organometallic compound, one or more carrier gases and the gas-phase 
 silicon-containing compound enter reaction chamber 4 through shower head 
 15. A radio-frequency (RF) voltage, supplied by radio-frequency generator 
 8, is applied between substrate holder 6 and reaction chamber wall 16, 
 thus forming a plasma in which some of the organometallic molecules and 
 some of the silicon-containing molecules are converted to radicals, ions 
 and metastables. An RF power density of about 1-2 watts/cm.sup.2 is 
 applied to the wafer in order to generate the plasma. Organometallic 
 compound molecules are adsorbed on the surface of the semiconductor wafer 
 5, and silicon-containing radicals react with the adsorbed organometallic 
 molecules to form a uniformly thick titanium silicide layer on the surface
 of the wafer. 
 As an alternative to the procedure employed above for the first embodiment 
 of the process, a remote source PECVD reactor may be employed with similar
 results. In such a case, only the silicon-containing compound need be 
 passed through the plasma generator; the organometallic compound may 
 bypass the plasma generator en route to the deposition chamber 4. 
 Although the desired reaction may be effected at a pressure within a range 
 of about 1 to 100 torr, a preferred range is deemed to be about 2 to 10 
 torr. A constant deposition pressure within that preferred range is 
 monitored and maintained by conventional pressure control components 
 consisting of pressure sensor 9, pressure switch 10, air operating vacuum 
 valve 11 and pressure control valve 12. The byproducts of the reaction and
 the carrier gases (if carrier gases are used) pass through particulate 
 filter 13 and escape through exhaust vent 14 with the aid of a blower 17 
 to complete the process. 
 The resistivity of the TiSi.sub.2 films may be further reduced using anneal
 steps after the deposition of the films in accordance with the procedures 
 heretofore disclosed. For example, a typical rapid thermal anneal can be 
 performed at about 780.degree. C. in a nitrogen ambiance for about 20 
 seconds. Lower anneal temperatures will, of course, require longer 
 exposure periods. 
 A second major embodiment of the process will also be described with 
 reference to the diagrammatic representation of the cold-wall plasma 
 enhanced chemical vapor deposition (PECVD) chamber of FIG. 2. However, for
 this embodiment, the reactor is not operated in the plasma-enhanced mode. 
 Thus, no RF power is applied between the substrate holder 6 and reaction 
 the chamber wall 16. Instead, increased wafer temperature is relied on to 
 effect a reaction between the tertiary-butyltris-dimethylamido-titanium 
 and the silicon-containing compound. Except for the use of higher 
 temperatures within a range of about 600.degree. C. to 800.degree. C. and 
 the lack of a plasma-generating RF power, other features of the process 
 remain the same. 
 Other materials may be simultaneously incorporated in the titanium silicide
 films during either embodiment of the deposition process as heretofore 
 described. For example, a titanium silicide film incorporating nitrogen 
 and having the general formula TiSi.sub.X Ni.sub.(1-X) may be deposited by
 introducing nitrogen-containing compounds such as amines, ammonia and 
 hydrazines along with the TBTDMAT and silicon-containing compounds. 
 Additionally, a titanium silicide film incorporating tungsten and having 
 the general formula TiSiW may be deposited by introducing organometallic 
 compounds such as bis(2,4-dimethylpentadienyl)tungsten or tungsten halide 
 compounds such as WF.sub.6 or WCl.sub.6 along with the TBTDMAT and 
 silicon-containing compounds. The nitrogen-containing compounds or 
 tungsten-containing compounds may be introduced in a manner similar to 
 that of the other reactants using the temperature guidelines heretofore 
 provided for each embodiment of the invention. 
 While several embodiments of the process for depositing titanium silicide 
 using tertiary-butyltris-dimethylamido-titanium and a silicon-containing 
 compound as reactants have been disclosed herein, it will be obvious to 
 those having ordinary skill in the art that modifications and changes may 
 be made thereto without affecting the scope and spirit of the invention as
 claimed.