Synthesis of Calcium(II) Amidinate Precursors for Atomic Layer Deposition through a Redox Reaction between Calcium and Amidines

摘要

We have prepared two new Ca^II amidinates, which comprise a new class of ALD precursors. The syntheses proceed by a direct reaction between Ca metal and the amidine ligands in the presence of ammonia. Bis(N,N′‐diisopropylformamidinato)calcium(II) (>1) and bis(N,N′‐diisopropylacetamidinato)calcium(II) (>2) adopt dimeric structures in solution and in the solid state. X‐ray crystallography revealed asymmetry in one of the bridging ligands to afford the structure [(η²‐L)Ca(μ‐η²:η²‐L)(μ‐η²:η¹‐L)Ca(η²‐L)]. These amidinate complexes showed unprecedentedly high volatility as compared to the widely employed and commercially available Ca^II precursor, [Ca3(tmhd)6]. In CaS ALD with >1 and H2S, the ALD window was approximately two times wider and lower in temperature by about 150 °C than previously reported with [Ca3(tmhd)6] and H2S. Complexes >1 and >2, with their excellent volatility and thermal stability (up to at least 350 °C), are the first homoleptic Ca^II amidinates suitable for use as ALD precursors.