Limiting or Continuous Thermal Etching of First Row Transition Metal Oxides Using Acetylacetone and Ozone

The thermal etching of first row transition metal oxides was surveyed at 250 °C using acetylacetone (Hacac) and ozone (O3). The metal oxides include Sc2O3, V2O5, VO2, Cr2O3, Mn2O3, MnO, Fe2O3, Fe3O4, Co3O4, CoO, NiO, CuO, Cu2O, and ZnO. The key measure of etching was the formation of volatile M­(acac) x etch products in the gas phase observed by in situ quadrupole mass spectrometry (QMS). Behavior consistent with continuous, unrestricted thermal etching during multiple Hacac minidoses was observed for ZnO, Mn2O3, and MnO. Various degrees of limiting, restricted behavior with respect to number of Hacac minidoses were observed for the other metal oxides. This self-limiting behavior could be used to define a thermal atomic layer etching (ALE) process. The limiting behavior was assessed using the time-resolved ion intensities for the M­(acac) x etch products during five sequential Hacac minidoses. O3 was then used to refresh the metal oxide (1) by oxidizing the metal oxide or (2) by removing carbon species that could form during Hacac decomposition on some of the metal oxides. A final Hacac dose was then employed to check for an enhancement in ion intensity for the M­(acac) x etch products resulting from the O3 exposure. Sc2O3, Cr2O3, MnO, Fe3O4, CoO, NiO, CuO, and ZnO all formed M­(acac) x etch products with the same oxidation state as the original metal oxide. In contrast, other metal oxides including V2O5, Mn2O3, and Co3O4 displayed M­(acac) x etch products with a lower oxidation state relative to the original metal oxide. This reduction may occur by oxygen loss from the metal oxide resulting from Hacac combustion. Two metal oxides, Fe2O3 and VO2, displayed evidence for multiple M­(acac) x etch products that either stayed in the original oxidation state of the metal oxide or changed to a lower oxidation state. One metal oxide, Cu2O, formed Cu­(acac)2 through a disproportionation reaction that produced metallic Cu on the surface. No etching was observed for MnO2 and TiO2. Mn2O3 and Co3O4 both showed strong evidence of Hacac combustion and formed CO2 and H2O during the Hacac exposures. Quartz crystal microbalance experiments at 200 °C confirmed the continuous, unrestricted thermal etching of ZnO atomic layer deposition (ALD) films during Hacac exposure. For Cu2O, the observed etch product was Cu­(acac)2. Spectroscopic ellipsometry experiments on Cu2O films at 180 °C showed evidence of disproportionation where Cu metal grows on the underlying Cu film during th