Diamine Adduct of Cobalt(II) Chloride as a Precursor for Atomic Layer Deposition of Stoichiometric Cobalt(II) Oxide and Reduction Thereof to Cobalt Metal Thin Films

In this paper, we introduce a new Co precursor for the atomic layer deposition (ALD) of Co metal and other Co containing materials. CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) is a diamine adduct of cobalt­(II) chloride that is inexpensive and easy to synthesize, making it an industrially viable precursor. Furthermore, CoCl2(TMEDA) shows good volatility at reasonably low temperatures and is thermally stable up to a temperature of ∼300 °C. We also present a full ALD study for the deposition of CoO thin films using CoCl2(TMEDA) and water as precursors. The process was investigated within a temperature range of 225–300 °C. Saturation of the film growth with respect to both precursor pulse lengths was verified. According to X-ray diffraction, the films were a mixture of hexagonal and cubic CoO. No reflections corresponding to Co3O4 were detected. The hexagonal phase is characteristic to nanomaterials only and is not seen in bulk CoO. The crystal structure of the films could be tuned by temperature, water pulse lengths, and type of substrate material. The films deposited at 275 °C exhibited 1:1 Co:O stoichiometry and very high purity. The CoO films could be reduced to Co metal at an exceptionally low temperature of 250 °C in 10% forming gas. Continuity of the reduced Co films was improved when the CoO film was deposited on TiN instead of native oxide terminated Si. The Co content of a 50 nm reduced metal film was as high as 95 at. %, with negligible amounts of oxygen and hydrogen.