Rhodium cyclopentadienyl dicarbonyl Rh(η5-C5H5)(CO)2 (RhCp(CO)2) is yellow-orange or deep orange oil.(distillable under high vacuum).
RhCp(CO)2 was synthesized from [Rh(CO)2Cl]2 and TlCp (reaction performed in dry toluene) and purified by high vacuum freeze distillation conditions (yield 35% after 3rd purification).
Chemical analyses resulted: %C 37.4 (calc. 37.5), %H, 2.2 (calc. 2.2), %O, 14.4 (calc. 14.3), and %Rh 46.1 (calc. 45.9). RhCp(CO)2 was characterised by FT-IR[i]
[i] P. Vest, J. Anhaus, H. C. Bajaj, Rudi. Van Eldik, Organometallics, 1991, 10(3), pp.818–819, DOI: 10.1021/om00049a053, « Pressure dependence of associative substitution reactions of (dicarbonyl)(cyclopentadienyl)rhodium » and refs therein.
In a typical decomposition experiment, [RhCp(CO)2) (10 mg) in an evacuated vessel was heated slowly to 180°C, until metallic Rh was visible; the gases evolved
were analyzed by GC/MS. The solid residue was extracted with CH2Cl2, concentrated and separated by preparative thin layer chromatography, producing red [Rh2Cp2(CO)3] (IR:v(CO) = 1986 cm-1 (lit. 1989 cm-1), 1841 cm-' in cyclohexane solution; 1H NMR: δ
= 5.50 ppm), and green [Rh3Cp3(µ-CO)3] (IR: v(CO) = 1850 cm-1, 1791 cm-1 (lit. 1849 cm-1, 1793 cm-1) in CH2C12). Pyrolysis in the presence of H2 (0.2 atm) produced similar results.[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
η-cyclopentadienyl (Cp) complex [RhCp(CO)2] was tested as precursor for the deposition of Rh metal films on Si substrates by MOCVD (and for comparison [RhCp(cod)] (cod=1,5-cyclooctadiene), [Rh(η-C3H5)(CO)2], [Rh(η-C3H5)3] and [Rh2(µ-Cl)2(CO)4] were tested as precursors) . The obtained Rh layers contained carbon impurities but these could be greatly reduced if CVD was carried out in the presence of H2. Rh films adhered well to a silicon substrates. The pyrolysis of [RhCp(CO)2] precursor gave CO and [Rh2Cp2(CO)2(μ-CO)] and [Rh3Cp3(μ-CO)3] at intermediate stages.
CVD growth using [RhCp(CO)2] were carried out in a vertical reactor at substrate temperatures 130-270°C (Si wafers used as substrates)and 10-2-10-3 Torr pressures. Highly reflective Rh metal films were formed at the growth rate 2 µm/h. The composition of the Rh layers was analyzed by XPS.
As compared to [RhCp(CO)2] and [Rh(allyl)(CO)2] as precursors, lower growth temperatures were required for MOCVD growth of Rh films using [RhCp(cod)] and [Rh(η-allyl)3 precursors. The effects of H2 and optimal CVD temperature on film purity are independent.
Pyrolysis of [RhCp(CO)2] gave CO as the only volatile product and metallic Rh as final decomposiiton product (the complexes [Rh2Cp2(CO)2(µ-CO)] and [Rh3Cp3(µ-CO)3] were formed as intermediate products in the early stages of pyrolysis (these complexes were isolated in pure form and identified by their characteristic NMR and IR spectra). The thermolysis of gaseous [RhCp(CO)2] occurs probably in similar way to photolysis of solution of [RhCp(CO)2], which is known to the initial loss of CO followed by association resulting in the compounds [Rh2Cp2(CO)2(µ-CO)] and [Rh3Cp3(µ-CO)3]. The concentration of rhodium complexes in the gas phase during CVD may be too low to allow associative reactions but it is such processes might occur in surface-bound organometallic complexes.
The minimum CVD deposition temperature was 180°C, the composition of Rh layers was following: 89-75% Rh, 7-20% C, 4-5% O.[i]
[i] R. Kumar, R.J. Puddephatt, Canad. J. Chem., 1991, « New precursors for organometallic chemical vapor deposition of rhodium », www.nrcresearchpress.com/doi/abs/10.1139/v91-017
Dicarbonylcyclopentadienyl rhodium Rh(Cp)(CO)2 was applied for the growth of Rh films. Rh layers grown in vacuum (0.14–1.4 Pa) were inevitably contaminated with carbon (~20 mass%C). However, in the presence of H2 the level of C decreased to ~3–7 mass% and reduced the temperature required for deposition from 543 to 503 K (270°C to 230°C).
[RhCp(CO)2] was applied as precursor (with ozone O3 as the oxidant/ co-precursor) for atomic layer deposition (ALD) preparation of conductive,
oxidation-resistant RhO2 films as well as RhO2 films alloyed with metals, such as Pt, targeting their application for the memory. As deposited RhO2 films had 1780 ohm/sq sheet resistance, which dropped to 650 ohm/sq after annealing in O2 at 700°
C for 1 minute. Resistivity of ~1140 µΩ×cm after O2 anneal was estimated using layer thicknes measured by profilometry. The composition of the deposited RhO2
films was analysed by XPS; the XPS depth profile (in the region of the Si 2 p peak) of RhO2 film after 30s annealing in O2 at 850°C indicated no any Si present at the film surface, confirming that grown RhO2 layers behaved as excellent Si diffusion barriers.[i]
[i] E.P.Marsh, S. Uhlenbrock, US 6881260 B2, « Process for direct deposition of ALD RhO2 », http://www.google.com/patents/US6881260
Rhodium methylcyclopentadienyl dicarbonyl Rh(η5-MeCp)(CO)2 and other alkylcyclopentadienyl-carbonyls of rhodium (Et, vinyl etc. substituted), were reported to be particularly advantageous as Rh precursors, because they are highly volatile compounds suitable for MOCVD applications like deposition of RhPt or RhRu layers. MeCpRh(CO)2) precursor was combined with Ru(cyclohexadiene)(CO)3 precursor for the growth of Rh/Ru alloy; the precursorswere kept in separate bubblers or vaporizers at ~30-33°C, the system was cold wall-type CVD reactor, pressures preferably from ~10 torr to ~0.1 torr were used.
The advantages of co-depositing Rh with Pt as oxygen barrier were revealed by XPS. After annealing Rh-Pt layer (contanining ca. 6% Rh) in O2 at 750° C/ 60 s, no O was present at the Pt—Rh interface, and no Si was found at the surface. Pure Rh layer (after RTO annealing in O2) contained RhOx on the surface; the interface with Si was composed of RhOx, SiO2 and RhSix according to XPS (composition of the layer at the rhodium/silicon interface was primarily RhOx, and Si was present as atomic Si (58%) and SiO2 (42%)). Thus, pure Rh layers are at least partially permeable to O, and the substrate is not passivated by the native oxide.
The CVD Pt—Rh layer co-deposited in a 0.35 um contact was
studied by SEM, which demonstrated reasonable step coverage and conformality (~35% step coverage on a 0.35 diameter by 2.4 micrometer contact). The Pt—Rh alloy barrier layer and electrode materials demonstrated excellent conductivity (therefore reducing
depletion effects and enhancing frequency response). The co-deposited Pt—Rh alloy layers grown by MOCVD using Rh(η5-MeCp)(CO)2 as precursor are beneficial for a wide variety of thin film applications in integrated circuit structures (especially those
using high dielectric materials, like capacitors - planar cells, trench cells (e.g., double sidewall trench capacitors), stacked cells (e.g., crown, V-cell, delta cell, multi-fingered, or cylindrical container stacked capacitors).[i]
[i] S. Uhlenbrock, E. P. Marsh, « Methods and apparatus for forming rhodium-containing layers », US 7393785 B2, http://www.google.com/patents/US7393785