Dissociation reactions of CuI(hfac)L compounds relevant to the chemical vapor deposition of copper

Abstract

Density functional theory (DFT) calculations have been performed for ligand copper bond energies of typical copper β-diketonate compounds used in chemical vapor deposition (CVD) of copper films. The molecules have the general formula Cu^I(hfac)L, where hfac is hexafluoroacetylacetonate, and L represents vinyltrimethylsilane (VTMS), trimethylphosphine (PMe₃), 2-butyne (2-butyne), or 1,5-cyclooctadiene (COD). The DFT method is used with the three-parameter Becke exchange and the Lee–Yang–Parr correlation functionals (B3LYP) with different basis sets. The optimized structures correspond to the crystal structures determined using crystal X-ray diffraction. Two different structures, Cu^I(hfac)(η²-COD) and Cu^I(hfac)(η⁴-COD), are determined for the Cu^I(hfac)(COD) complex, the latter being more stable by ∼3 kcal mol⁻¹. The strength of the ligand–copper interaction is studied for the reaction Cu^I(β-diketonate)L → Cu^I(β-diketonate) + L. Bond energies of 32.1, 35.6, 33.6 and 38.4 kcal mol⁻¹ are calculated for typical Cu CVD precursors, Cu^I(hfac)(butyne), Cu^I(hfac)(COD), Cu^I(hfac)(VTMS) and Cu^I(hfac)(PMe₃), respectively. The similarity between these bond energies and reported experimental activation energies for CVD suggests that the dissociation of the ligand L could be the rate determining step for the film growth under certain conditions. The rate parameters for the dissociation reaction of Cu^I(hfac)(VTMS) are evaluated based upon the results of the DFT calculations. A simple reaction mechanism for Cu CVD is proposed and combined with transport phenomena simulations of two reported reactors configurations. Good agreement with experimental observations is obtained with a Cu^I(hfac)(VTMS) dissociation rate constant of 1.5 × 10¹⁴exp(−13.5/T), which is consistent with the computed rate constant.