Investigation of the Growth Behaviour of Cobalt Thin Films from Chemical Vapour Deposition, Using Directly Coupled X-ray Photoelectron Spectroscopy

Abstract

Thin films and coatings are a basis for many technological processes, including microelectronics, electrochemistry and catalysis. The successful deposition of metal films and nanoparticles by chemical vapour deposition (CVD) needs control over a number of physico-chemical processes such as precursor and substrate selection, delivery, temperature, pressure and flow conditions. Here, cobalt thin films were deposited by means of pulsed-spray evaporation chemical vapour deposition (PSE-CVD) from ethanol solutions of Co(acac)₂ and Co(acac)₃ on bare glass and silicon substrates. The physico-chemical properties of the grown films were characterised by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and HIM (helium ion microscopy). Co(acac)₂ enabled the growth of cobalt metal at lower temperatures than Co(acac)₃. The difference in deposition temperature was attributed to the ability of ethanol to reduce Co(acac)₂ better than Co(acac)₃. In addition, the film deposited from Co(acac)₂ exhibited a higher metal content and a less porous structure than that deposited from Co(acac)₃. Increasing the substrate temperature enhanced the carbon content because of the thermal decomposition of both precursors. Using a nickel seed layer improved the growth rate until a critical temperature of 360 ℃, at which the thermal decomposition of the precursor becomes predominant. A decrease in the deposition temperature when using the nickel seed layer was only observed with Co(acac)₂ precursor; the growth behaviour under these conditions was monitored with a unique UHV-compatible PSE-CVD reactor directly attached to an XPS system and ascribed to an enhancement of its catalytic reduction by ethanol.