Among the various materials likely to be used as substitutes for polysilicon, TiSi2 appears to offer a greater number of advantages compared to other silicides used in VLSI devices. It has, indeed, the lowest resistivity (15-mu-OMEGA.cm) and can be selectively deposited through chemical vapor deposition techniques. Whatever the starting gaseous phase, TiCl4-H2 or TiCl4-SiH4-H2, selectivity never fails even in the case of submicronic lines. The problem, however, originates from an undesirable etching of the silicon substrate even through the use of the TiCl4-SiH4-H2 gaseous phase. Different reaction mechanisms have been recently proposed by J.L. Regolini, D. Bensahel, G. Bomchil and J. Mercier [Appl. Surf. Sci. 38 (1989) 408], which involve: (i) silicon substrate etching by chlorinated species, (ii) silicon substrate consumption to form TiSi2, (iii) TiSi2 deposition from the gaseous species, (iv) silicon deposition from SiH4 cracking, and which show that TiSi2 selectively deposited layers can be achieved at 800-degrees-C without any silicon substrate consumption through a careful control of the various gas flow ratios. In particular, they pointed out that the TiCl4 thermal diffusivity is about one order of magnitude lower than the SiH4. Hydrogen is thus used in excess allowing a faster thermallization of the gas mixture [J. Mercier, J.L. Regolini and D. Bensahel, J. Electron. Mater. 19-3 (1990) 253]. As far as we are concerned, TiSi2 selectively deposited films are obtained on 4 inch silicon wafer from the TiCl4-SiH4-H2 gaseous phase by using a RTLPCVD reactor where gas mixture is injected all around the substrate. Thickness measurements show great differences between the edge and the center of the wafer. These differences can be interpreted as variations in the gas mixture composition: TiCl4, a heavy molecule, is too slow to reach the wafer center, and mainly reacts at the edge of the substrate leading to the formation of a thick TiSi2 layer with silicon consumption and silicon etching. On the contrary, at the center of the substrate, the gas mixture becomes impoverished in TiCl4, and produces a silicon pedestal under a thin TiSi2 film. Between the center and the edge of the wafer, the gas mixture composition achieves the right balance, allowing for TiSi2 to deposit with neither silicon etching and consumption, or silicon pedestal deposition. Regarding our reactor, the solution allowing for deposition of TiSi2 films with a proper thickness homogeneity on a 4 inch diameter silicon substrate should be obtained through a special geometry of gas mixture injectors. Mention is made of recent results achieved through different gas introduction systems.
