Rhodium films offer excellent resistance to oxidation and other types of corrosion. These films are also highly reflective in the visible, infrared, and ultraviolet regions of the spectrum, leading to their use as mirrors in optics. [[i]]. In addition, rhodium may find uses in the microelectronics industry (as electrical contacts), as diffusion barriers, reflective coatings, catalysis, and wear-resistant coatings for extreme conditions.

CVD growth of Rh films is attractive since it can be peformed under milder conditions compared to physical vapor deposition (PVD), it does not require high vacuum systems, also CVD processes allow selective growth of films and much better step coverage compared to PVD.

    Rhodium volatile MOCVD precursors contain either traditional organometallic ligands such as CO, C2H4, C5H5, and diketonates (non-fluorinated like acac, or fluorinated like trifluoroacetylacetonate (tfac)).  For example, precursors for the CVD of rhodium films include, [RhCl2(PF3)2]2 [Rh(μ-Cl)(CO)2]2, [Rh2(CO)8], [Rh(η5-C5H5)(CO)2], [Rh(η5-C5H5)(η2-C2H4)2], RhCp(COD) (COD = 1,5-cyclooctadiene), [Rh(η3-allyl)3], Rh(η3-C3H5)(CO)2, Rh(acac)(CO)2, Rh(hfac)(CO)2, Rh(hfac)(COD), [Rh(tfac)3]. Often precursors contain PMe3 and ligands containing CF3 groups are known to increase volatility in many cases. For example, rhodium complexes using less conventional ligands bis(trifluoromethyl)pyrazolate (3,5-(CF3)2-Pz) or bis(trifluoromethyl)pyrrolyl (3,4-(CF3)2-Pyr) and PMe3 were synthesized and tested as potential Rh MOCVD precursors.[[ii]]

The deposited Rh film quality (film properties, impurity levels) is strongly dependent on the precursor used. For example, Rh layers grown using [Rh(μ-Cl)(CO)2]2 using Ar as a carrier gas at 180 oC consisted of 49% Rh, 24% C, and 27% O, whereas Rh films grown under the same conditions from [Rh(η3-allyl)3] contained 86% Rh and 14% C. Rh layer quality also depends on the substrate type and growth conditions (like carrier gas – inert or H2)

Often rhodium (III) chloride RhCl3 is used as starting material for the preparation of the rhodium complexes used as MOCVD precursors (in presence of reducing agents):

RhCl3 + CO → ½ [RhCl(CO)2]2 + … (CO insertion in Rh-Cl bonds)

RhCl3 + 3 MgBr-CH2CH=CH2 → Rh(h3-C3H5)3 + 3 “MgBrCl”

RhCl3 + 3 NaCp → RhCp2Cl + 2 NaCl

RhCp2Cl + Na → RhCp2 + NaCl

Other Rh MOCVD precursor can be synthesized from [RhCl(CO)2]2, RhCp2Cl, and RhCp2, for example by methathesis:  [RhCl(CO)2]2 + 2 TlCp → 2 RhCp(CO)2 + 2 TlCl

Fig. Comparison of vapor pressure of some Rh precursors

The comparison of some rhodium MOCVD precursors is presented in [[i]]

The highest volatility (highest vapor pressure) was observed for Rh(allyl)3 precursor (lnP(Pa) = -8476/T (K)+ 29,4), whereas much lower  vapor pressure for Rh(acac)(CO)2 and especially for [Rh(Cl)(CO)2]2 was observed: lnP(Pa) = -9887/T(K) + 31.9, and lnP(Pa) -10052/T(k) + 32.8), respectively.

[i] J.-C. Hierso, Ph. Serp, R. Feurer, Ph. Kalck, Appl. Organomet. Chem., 1998, 12(3), p.161-172, "MOCVD of rhodium, palladium and platinum complexes on fluidized divided substrates: Novel process for one-step preparation of noble-metal catalysts." , http://deepblue.lib.umich.edu/bitstream/handle/2027.42/38322/689_ftp.pdf?sequence=1

     In order to understand the decomposition mechanisms of the precursors, the decomposition products from deposits (formed in the CVD conditions) under He or H2/He mixture were compared for several Rh precursors.  The inorganic and organic residues were trapped from the gas phase after the reactor, and analysed by Gas Chromatography –Mass Spectrometry (GC-MS). In addition, certain products such as CO or HCl were identified by IR measurements. (Table ) [[i]]

      Under an inert He atmosphere, [Rh(Cl)(CO)2]2 produced CO above 125 °C and Rh/Cl ratio near 1:1 was found in the solid. However, when ~20% H2 was added to the He, rapid loss of CO above 75 °C was detected, followed by the gas phase formation of HCl. It was shown that reactive RhH2 species were formed in the gas phase (as a result of loss of CO and HCl from complex), these species gave rise to Rh anchoring by interaction with particular sites on the support. H2 plays a major role in dechlorination of [Rh(Cl)(CO)2]2  during deposition and thus increases the purity of the deposits.

      Rh(acac)(CO)2 under He conditions produced CO, CO2, acetone and butanone. However, under H2/He (85 °C) Rh(acac)(CO)2 first decomposed with loss of CO, then formation of 2,4-pentanedione (acacH) happened.

     Rh(allyl)3 precursor decomposed to propene and 1,5-hexadiene under pure He atmosphere, whereas under H2/He it gave a mixture of propene C3H6 and propane C3H8.