Thermal atomic layer etching of CoO using acetylacetone and ozone: Evidence for changes in oxidation state and crystal structure during sequential exposures

Abstract

Thermal atomic layer etching (ALE) of CoO was demonstrated using sequential exposures of acetylacetone (Hacac) and ozone (O3). During the surface reactions, Hacac can remove CoO according to: CoO + 2Hacac → Co(acac)2 + H2O. Ozone was employed to eliminate carbon residue resulting from Hacac adsorption. In situ spectroscopic ellipsometry observed a linear decrease in CoO film thickness versus Hacac and O3 exposures with an etch rate of 0.43 Å/cycle at 250 °C. The surface of the CoO film was also observed to undergo changes in oxidation state and crystal structure with each reactant exposure. Ex situ grazing incidence X-ray diffraction (GIXRD) studies revealed that the initial h-CoO thin films were oxidized to c-Co3O4 by O3. Subsequently, the GIXRD analysis showed that the c-Co3O4 thin films were reduced to c-CoO by Hacac. X-ray photoelectron spectroscopy investigations confirmed the oxidation state of the various cobalt oxides. Quadrupole mass spectrometry measurements observed Co(acac)2 etch products during Hacac exposures on Co3O4 and CoO powder. Atomic force microscopy measurements also monitored a reduction in surface roughness during CoO ALE. These studies reveal that alternating changes in oxidation state and crystal structure occur during the sequential Hacac and O3 exposures that define CoO thermal ALE.