Radio-frequency microplasmas with energies suited to in situ selective cleaning of surface adsorbates in ion microtraps

Abstract

We have demonstrated a capacitively-coupled, RF microplasma inside the 3D electrode structure of an ion microtrap device. For this work, devices with an inter-electrode distance of 340 µm were used. The microplasmas were operated at Ω RF /2π = 23 MHz, in both He and He:N 2 gas mixtures, over a range of RF amplitudes (140 V to 220 V) and pressures (250 mbar to 910 mbar). Spectroscopic analysis of the He I 667 nm and Hα 656 nm emission lines yielded the gas temperature and electron density, which enabled calculation of the mean ion bombardment energy. For the range of operating parameters studied, we calculated mean He + energies to be between 0.3 eV and 4.1 eV. While these energies are less than the threshold for He sputtering of hydrocarbon adsorbates on Au, we calculate that the high energy tail of the distribution should remove adsorbate monolayers in as little as 1 minute of processing. We also calculate that the distribution is insufficiently energetic to have any significant effect on the Au electrode surface within that duration. Our results suggest that the microplasma technique is suited to in situ selective removal of surface adsorbates from ion microtrap electrodes.