Reconstruction of energy and angle distribution function of surface-emitted negative ions in hydrogen plasmas using mass spectrometry

TL;DR

This paper introduces a novel mass spectrometry-based method to reconstruct the energy and angle distribution functions of negative ions emitted from surfaces in hydrogen plasmas, enabling insights into their production mechanisms.

Contribution

The proposed reconstruction algorithm is surface-agnostic and can determine the energy and angle distributions of negative ions from measured data at different tilt angles.

Findings

Reconstructed NIEADFs for HOPG and Gd show distinct production mechanisms.

Gd mainly produces negative ions via backscattering with a peak at 36 eV.

HOPG's negative ions mainly originate from sputtering with a peak at 5 eV.

Abstract

A new method involving mass spectrometry and modelling is described in this work, which may highlight the production mechanisms of negative ions on surface in low pressure plasmas. Positive hydrogen ions from plasma impact a sample which is biased negatively with respect to the plasma potential. Negative ions (NI) are produced on the surface through the ionization of sputtered and backscattered particles and detected according to their energy and mass by a mass spectrometer placed in front of the sample. The shape of the measured negative-ion energy distribution function (NIEDF) strongly differs from the NIEDF of the ions emitted by the sample because of the limited acceptance angle of the mass spectrometer. The reconstruction method proposed here allows to compute the distribution function in energy and angle (NIEADF) of the negative-ions emitted by the sample based on the NIEDF measurements at different tilt angles of the sample. The reconstruction algorithm does not depend on the NI surface production mechanism, so it can be applied to any type of surface and/or NI. The NIEADFs for HOPG (Highly Oriented Pyrolitic Graphite) and Gadolinium (low work-function metal) are presented and compared with the SRIM modelling. HOPG and Gd show comparable integrated NI yields, however the key differences in mechanisms of NI production can be identified. While for Gd the major process is backscattering of ions with the peak of NIEDF at 36 eV, in case of HOPG the sputtering contribution due to adsorbed H on the surface is also important and the NIEDF peak is found at 5 eV.