Mechanistic Studies of Palladium Thin Film Growth from Palladium(II) β-Diketonates. 1. Spectroscopic Studies of the Reactions of Bis(hexafluoroacetylacetonato)palladium(II) on Copper Surfaces

Abstract

The reactions of bis(hexafluoroacetylacetonato)palladium(II), Pd(hfac)2, on copper have been studied. Whereas multilayers of Pd(hfac)2 desorb molecularly from copper surfaces between 200 and 270 K, submonolayer coverages of Pd(hfac)2 react in a multistep sequence. At temperatures below 120 K, Pd(hfac)2 transfers its hfac ligands to the copper surface and the metal center is reduced to Pd0. The hfac groups adopt a variety of molecular orientations on the surface at 120 K but undergo an apparent ordering transition near 300 K that re-orients the hfac groups to an upright geometry (perpendicular to the surface plane). At low coverages on clean surfaces, further annealing results in the decomposition of the surface-bound hfac ligands to give adsorbed Pd atoms and COCF3, CF3, and fluoride species. These intermediates ultimately yield a partial monolayer of Pd atoms, a carbon (possibly graphitic) deposit, and a variety of organic products that desorb between 500 and 650 K; the desorbing flux after ionization consists of CF3COF, COCF3, CF3, and CO fragments. The activation parameters for the decomposition of hfac groups on copper foils have been determined to be A = 1.3 × 1013 s-1 and Ea = 36.8 kcal mol-1. When Pd(hfac)2 is dosed onto copper surfaces bearing submonolayer coverages of carbon, a new reaction channel is evident: some of the hfac ligands abstract copper atoms from the surface and generate Cu(hfac)2, which desorbs when the surface is heated. The palladium atoms remain behind and diffuse into the bulk of the Cu crystal. This redox transmetalation reaction (PdII(hfac)2 + Cu0 → Pd0 + CuII(hfac)2) is the same one that is responsible for the ability of Pd(hfac)2 to effect the chemical vapor deposition of palladium selectively on copper at higher pressures. This redox transmetalation reaction is the first example of the simultaneous etching of copper and deposition of palladium.