RHODIUM ALLYLS

Rhodium tris(allyl) Rh(C3H5)3 (Rh(allyl)3 )

Rhodium (III) tris(allyl) Rh(allyl)3 (allyl = η3-C3H5) is one of the most investigated complex for deposition of Rh metal thin films by MOCVD.

Amorphous Rh films contaminated with carbon (~14 mass%C) were obtained by the decomposition of Rh(allyl)3 without assistance of H2, whereas in the presence of H2 the contamination in the layers were reduced (4 mass%C, <2 mass%O). Pure and crystalline Rh metal films (<3 mass%C) at growth rates of about 5 *10-3/nm/s were obtained when performing the decomposition of Rh(allyl)3 in the H2 plasma. (example growth conditions are presented in the Table). [i]

[i] J. R. Vargas Garcia, T. Goto, Mater. Trans., Vol. 44, No. 9 (2003) pp. 1717 to 1728, « Chemical Vapor Deposition of Iridium, Platinum, Rhodium and Palladium », https://www.jim.or.jp/journal/e/pdf3/44/09/1717.pdf

Rh(η-C3H5)3 vapor pressure

Fig. Vapor pressure Rh(allyl)3 vs Rh(acac)(CO)2 and [RhCl(CO)2]2 (Clausius-Clypeyron equation)

Rhodium (III) tris(allyl) Rh(allyl)3 is very volatile compared to for conventional Rh(acac)(CO)2 and [RhCl(CO)2]2 precursirs.(vapor pressure comparison is presented on Fig.). [i]

Rh(η-C3H5)3 (Rh(allyl)3) as precursor for MOCVD

Rh(η-C3H5)3 for Rh metal by low temperature MOCVD

Rh(allyl)3 was applied as precursor for the low-temperature MOCVD deposition of Rh thin films. Crystalline Rh films with >97% metal composition were obtained in the presence of H•. Amorphous layers containing significant amounts of residual C (14%) were obtained when perfroming deposition using H2; layer composition did not differ significantly from layers obtained by the vacuum thermal deposition of Rh(allyl)3.[[i]

Also oher studies reported CVD of rhodium using triallylrhodium [Rh(η3-C3H5)3] as precursor precursors [[ii], [iii]], and the CVD enhancement by plasma or lasers was demonstrated [iv],[v]

[i] D.C. Smith, S.G. Pattillo, N.E. Elliott, Th.G. Zocco, C.J. Burns, J.R. Laia, A.P. Sattelberger, MRS Proc., 1989, vol.168, « Low-temperature Chemical Vapor Deposition of Rhodium and Iridium thin Films », DOI: http://dx.doi.org/10.1557/PROC-168-369, http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8113306

[ii] W. WESTWOOD. Gmelin handbook of inorganic chemistry, Supplement Vol. Al. Technology of platinum-group metals. 8th ed. 1986. pp. 43-50

[iii] H. D. KAESZ, R.S. WILLIAMS, R.F. HICKS, Y.-J. A. CHEN, Z. XUE, D. XU, D.K. SHUH, H. TNRIDANDAM, Mat. Res. Soc. Symp. Proc. 131, 395 (1989)

[iv] K. L. KOMPA Angew. Chem. Int. Ed. Engl. 27, 1314 (1988)

[v] A. ETSPULER, H. SUHR, Appl. Phys. 48, 373 (1989).

Rh(η-C3H5)3 for Rh metal by MOCVD

[Rh(η-C3H5)3] (and for comparison [Rh(η-C3H5)(CO)2], [RhCp(CO)2], and [RhCp (cod)] ) was investigated as precursors for the MOCVD growth of Rh layers. The obtain Rh layers contain C contamination, which could be greatly reduced by carrying out CVD process in the presence of H2. The deposited films adhered well to Si substrates.

When [Rh(η-allyl)3] was used as precursor, there was a decrease in the minimum temperature required for CVD (in contrast no such effect was observed with [Rh(η-C3H5)(CO)2] or [RhCp(CO)2] as precursors. Thus, the H2 effects on film purity and CVD temperature are independent.

Pyrolysis of [Rh(η-C3H5)3] (as well as [Rh(η-C3H5)(CO)2]) gave 1,5-hexadiene as the only organic product (as was determined by GC and GCIMS), but no rhodium clusters or dimers could be detected at intermediate stages. Similar pyrolysis in the presence of H2 produced much propene as well as 1,5-hexadiene. (32% propene + 67% 1,5-hexadiene + traces of other unidentified compounds). The product l,5-hexadiene could be formed by concerted reductive elimination at Rh atom or by loss of ally1 radicals followed by radical recombination and is the only product in the absence of H2.

As for the growth in H2, hydrogen could react directly with allylrhodium molecule giving propene, though there are no precedents for reaction of H2 with rhodium(III) organometallics. Thus, partial reaction of the precursor giving lower oxidation state Rh complexes might be necessary before H2 could oxidatively add; propene could then be formed by reductive elimination. Aternatively. reaction could be catalyzed by metallic Rh, which is known as a good hydrogenation catalyst. Hydrogen is presumed to be involved directly in the mechanism of CVD, because it influences the temperature of MOCVD process, the purity of rhodium film, as well as the nature of the organic products formed from [Rh (η-C3H5)3 precursor.[i]

[i] R. Kumar, R.J. Puddephatt, Canad. J. Chem., 1991, « New precursors for organometallic chemical vapor deposition of rhodium », http://www.nrcresearchpress.com/doi/abs/10.1139/v91-017

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