Mechanistic studies of CVD metallization processes: Reactions of rhodium and platinum beta-diketonate complexes on copper surfaces

The hexafluoroacetylacetonate complexes Rh(hfac)(C2H4)(2) and Pt(hfac)(2) a re known to serve as chemical vapor deposition precursors to Rh and Pt thin films. In the absence of a reducing carrier gas, the depositions ate surfa ce-selective and occur preferentially on copper, but under these conditions , the metallization processes are unexpectedly inefficient relative to the rapid deposition of Pd on Cu seen for the palladium analogue Pd(hfac)(2). M echanistic studies of the reactions of Rh(hfac)(C2H4)(2) and Pt(hfac)(2) on copper sui faces under ultrahigh vacuum conditions have now been performed in order to elucidate the factors responsible for the differences among th ese surface-selective metallization processes. The studies demonstrate that adsorption of the rhodium complex Rh(hfac)(C2H4)(2) on copper surfaces is accompanied by the loss of the coordinated ethylene groups, even at 100 K. At these low temperatures, the adsorbed Rh(hfac) fragments are oriented in several ways with respect to the surface. Healing the substrate above simil ar to 150 K causes the hfac ligands to realign to a perpendicular orientati on relative to the surface. The platinum precursor Pt(hfac)(2) adsorbs mole cularly at 100 K with the molecular planes of the hfac ligands oriented par allel to the copper surface. Heating the substrate to temperatures above 15 0 K again results in a realignment of the hfac ligands to a perpendicular o rientation. This reorientation is accompanied by a partial reduction of the Pt centers (as judged from shifts seen in X-ray photoelectron spectroscopy core level data), a result suggesting that the hfac ligands begin to disso ciate from the platinum centers near 150 K. At temperatures above 220 K, th e transfer uf the hfac ligands from both complexes to the copper surface is complete, as signaled by the reduction of the metal centers to the zero-va lent state. The copper-bound hfac ligands are further transformed upon heat ing, either reacting with copper surface atoms to yield Cu(hfac)(2) (which desorbs at temperatures above 250 K) or decomposing (with fragments desorbi ng above 350 K). The presence of platinum on the copper surface promotes th e former reaction as judged by the appearance of a new reaction-limited des orption process fur Cu(hfac)(2). The presence of rhodium on the copper surf ace does not promote the formation of Cu(hfac)(2). although autocalalysis i s noted in the steady-stale reactive scattering data. The inability of the Rh and Pt precursors to engage in a sustained transmetalation reaction with the copper surface is attributed to the slow interdiffusion of copper thro ugh the Rh/Cu and Pt/Cu alloys that are produced in the near-surface region .