Group IV Transition Metal (M = Zr, Hf) Precursors for High-κ Metal Oxide Thin Films

This paper describes the synthesis of eight novel zirconium and hafnium complexes containing N-alkoxy carboxamidate-type ligands, as potential precursors for metal oxides and atomic layer deposition (ALD) for HfO₂. A series of ligands, viz., N-ethoxy-2,2-dimethylpropanamide (edpaH), N-ethoxy-2-methylpropanamide (empaH), and N-methoxy-2,2-dimethylpropanamide (mdpaH), were used to afford complexes Zr(edpa)₄ (1), Hf(edpa)₄ (2), Zr(empa)₄ (3), Hf(empa)₄ (4), Zr(mdpa)₄ (5), Hf(mdpa)₄ (6), ZrCp(edpa)₃ (7), and HfCp(edpa)₃ (8). Thermogravimetric analysis curves assessed for the evaporation characteristics of complexes 1-8 revealed single-step weight losses with low residues, except for the mdpa-containing complexes. Single-crystal X-ray diffraction studies of 1, 2, 5, and 6 revealed that all the complexes have monomeric molecular structures, with the central metal ion surrounded by eight oxygen atoms from the four bidentate alkoxyalkoxide ligands. Among the complexes prepared, 8 exhibited a low melting point (64 °C), good volatility (1 Torr at 112 °C), high thermal stability, and excellent endurance over 6 weeks at 120 °C. Therefore, an ALD process for the growth of HfO₂ was developed using HfCp(edpa)₃ (8) as a novel precursor. Furthermore, the HfO₂ film exhibited a low capacitance equivalent oxide thickness of ∼1.5 nm, with J_g as low as ∼3 × 10^-4 A/cm² at V_g -1 V in a metal-insulator-semiconductor capacitor (Au/HfO₂/p-Si).