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

Abstract

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 [(η 2 ‐L)Ca(μ‐η 2 :η 2 ‐L)(μ‐η 2 :η 1 ‐L)Ca(η 2 ‐L)]. These amidinate complexes showed unprecedentedly high volatility as compared to the widely employed and commercially available Ca II precursor, [Ca 3 (tmhd) 6 ]. In CaS ALD with 1 and H 2 S, the ALD window was approximately two times wider and lower in temperature by about 150 °C than previously reported with [Ca 3 (tmhd) 6 ] and H 2 S. 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.