Copper oxidation and surface copper oxide stability investigated by pulsed field desorption mass spectrometry

Pulsed field desorption mass spectrometry (PFDMS) has been used to examine several types of oxide layers formed by the in situ oxidation of clean copper surfaces. Films of Cu2O, CuO/Cu2O and CuO were produced by direct oxidation of Cu field emitters and examined for their behavior at different temperatures and for the effects of high fields. The high positive field pulses applied to the Cu2O overlayers result in field-enhanced ion injection at the copper/oxide interface, field-enhanced Cu+ ion desorption or cation vacancy injection at the oxide/vacuum interface and produce only Cu+ ions in the mass spectrum. CuO produces only Cu+, CuO+ and CuO2+ ions and CuO/Cu2O bilayers produce these ions until the CuO overlayer is removed from the Cu2O layer by field desorption. Removal of the CuO overlayer re-establishes the Cu2O field ionization process, namely Cu+ ions which occur at approximately one half the field strength necessary for CuO field desorption at the same temperature. The low field strength required to produce the Cu+ ions from a Cu2O overlayer reflects its ion conduction character and the ionic binding state of the Cu+. Heating the tip covered with Cu2O overlayers to higher than 470 K causes the overlayer to decompose and results in loss of its characteristic mass spectrum. It is concluded that PFDMS can be used to study the nature of the oxide overlayers formed during the early stages of oxidation and is projected to be a useful tool to examine electrostatic field effects during oxide growth.