A Volatile Dialane Complex from Ring-Expansion of an N-Heterocyclic Carbene and its Use in Atomic Layer Deposition of Aluminum Metal Films

Treatment of the stable N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazolin-2-ylidene with two equivalents of AlH₃(NMe₃) afforded the structurally unusual ring expanded dialane complex 1 in 72% yield after sublimation. Complex 1 has a distorted norbornane-like C₃N₂Al₂ core with two pseudo-tetrahedral Al dihydride sites. Treatment of 1 with Cp₂TiCl₂ as a model for metal thin film precursors produced the hydride-bridged Ti(III)-Al heterobimetallic complex 2 in 45% crystalline yield. Complex 1 shows good volatility and thermal stability, subliming at 90-100 °C and 50 mTorr and decomposing in the solid state at ~200 °C. The vapor pressure of 1 is 0.75 Torr at 120 °C. These physical properties are promising for a potential atomic layer deposition (ALD) precursor. Aluminum metal films were deposited by thermal ALD using AlCl₃ and 1 as precursors with a growth rate of ~3.5 Å/cycle after 100 cycles within an ALD window between 120-140 °C. The films are crystalline aluminum metal by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis showed aluminum metal with 7.0 at.% C, 3.6 at.% N, and 0.9 at.% Cl impurities. The aluminum metal films had an electrically discontinuous morphology. Conductive aluminum metal films have been deposited under similar conditions using a different aluminum hydride reducing co-reactant, which highlights the impact that small precursor differences can have on film characteristics.