Tungsten silicides

Experimental evidence shows, however, that there is very little HF found as a byproduct when WSi2(s) is deposited from WF6 and SiH4.6 From thermodynamic considerations, we could anticipate reaction products such as SiF4, SiHF3, SiH2F2, SiH3F, SiF2, HF and H2 as well as possibly others. Therefore, a more accurate representation would be 

The as-deposited WSix exhibits high resistivity, which drops to acceptable 

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Tungsten silicide gates interconnections of cold wall 1 Torr... 

Kember, P. N., and Astell-Burt, P. J., Plasma-Enhanced CVD of Tungsten and Tungsten Silicide, Vide, Couche Minces, 42(236) 167-173 (Mar.-Apr., 1987)... 

Kara, T., Miyamato, T., Hagiwara, H., Bromley, E., and Harshbarger, W., Composition of Tungsten Silicide Films Deposited by Dichlorosilane Reduction of Tungsten Hexafluoride, /. Electrochem. Soc., 137(9) 2955-2959 (Sept. 1990)... 

Conductor Copper Silver Gold Tungsten Silicides Free to move 10-5 to 10-6... 

Fig. 4.14 (a) A few atomically resolved images of surfaces of WSi2 where Si atoms arc invisible. In tungsten silicide, only the W sublatticc is seen in the field ion image. Sec Section 4.4.2 for discussions on silicide formation. (b) Atomic structure of WSi2 crystal. 

Fig. 4.51 35 K helium field ion image of polycrystallinc tungsten silicide layers grown on a W emitter surface. Atom-probe analysis shows that the stoichiometry is WSi2. Images of a few crystal planes of WSi2 have also already been shown in Fig. 4.14. The dark area is the hole of the 45° mirror of the atom-probe.

Using a cold-wall CVD reactor similar to the internally-heated barrel described in Figure 22 of Chapter 1, tungsten silicide was deposited from WF6 and SiH4,5 which is often described by the overall reaction... 

The tungsten silicide films, when deposited by CVD as contrasted to sputtering, are very conformal. This behavior is shown in Figure 9, where a 5000 A polycide film (2500 A WSi2 + 2500 A poly is deposited over a small step. On steps with vertical walls, the thickness of the polycide on the vertical wall was at least 75% of its thickness on the horizontal surface. 

If SiH4 is added to the reactive gas mixture, then tungsten silicide can be deposited.18 In this case. He was used as a diluent, along with the WF6 and SiH4, and deposition was carried out in a parallel-plate, cold-wall plasma... 

A second approach to a cold wall system is the single-wafer CVD reactor developed by Varian-Torrex. A schematic of the reaction chamber is shown in Figure 25. Again, tungsten silicide is deposited in this cold-wall reactor. Other conducting films such as blanket and selective tungsten can also be deposited. 

In the fabrication of semiconductors, CVD is an important technique for the thin-film formations with an increase in device intensity performance. Tungsten hexafluoride has been widely used as a source gas of tungsten silicide (WSi,v) and tungsten metal for electrodes and interconnects. 

Tungsten hexafluoride is readily reduced to the metal by CVD as seen from Eqn. (27). Accordingly, it has been widely used for the formation of tungsten metal and tungsten silicide films. Besides Eqn. (27), the basic chemical reactions for... 

Tungsten silicide has been widely used as a gate electrode for the following reasons ... 

At present, tungsten silicide is usually used on the top of gate polysilicon as polycide structure which is an abbreviated name for polysilicon-silicide as shown in Fig. 6. 

Tungsten silicide can be also deposited by dichlorosilane (SiH2Cl2, or DCS) reduction at higher temperatures of 500-600°C [73]. 

Tungsten silicide deposited by DCS reduction contains much less fluorine than that by monosilane reduction, and the chlorine content is also low. Consequently, DCS is replacing monosilane in the CVD-WSix process because of good step coverage, good adhesion to polysilicon and Si02 and low resistivity. 

Oligosilanyl-Tungsten Compounds-Precursors for Tungsten-Silicide CVD ... 

Improvements in the performance of integrated circuits and the trend towards VLSI-technology require the replacement of polycrystalline silicon by materials with a lower resistivity for use as gate electrodes. Transition metal silicides appear to be valuable possibilities for these applications. Timgsten-silicon compounds could be suitable precursors for the precipitation of tungsten-silicide thin films. Moreover tungsten-silicon compounds are nearly unknown and of scientific interest. 

Schmitz JEJ. Chemical Vapor Deposition of Tungsten and Tungsten Silicides for VLSI/ULSI Applications. Noyes Publications 1992. p 15. 

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