Amorphous carbon thin films: Mechanisms of hydrogen incorporationduring magnetron sputtering and consequences for the secondary electronemission

Amorphous carbon (a-C) films, having low secondary electron yield (SEY), are usedat CERN to suppress electron multipacting in the beam pipes of particleaccelerators. It was already demonstrated that hydrogen impurities increase theSEY of a-C films. In this work, a systematic characterization of a set of a-Ccoatings, deliberately contaminated by deuterium during the magnetron sputteringdeposition, by scanning electron microscopy, ion beam analysis, secondary ionmass spectrometry, and optical absorption spectroscopy was performed toestablish a correlation between the hydrogen content and the secondary electronemission properties. In parallel, the mechanisms of contamination were alsoinvestigated. Adding deuterium allows resolving the contributions of intentionaland natural contamination. The results enabled us to quantify the relativedeuterium/hydrogen (D/H) amounts and relate them with the maximum SEY(SEYmax). The first step of incorporationappears to be formation of D/H atoms in the discharge. An increase in both theflux of deposited carbon atoms and the discharge current with a D2 fraction in the gas discharge can be explained by target poisoning withdeuterium species followed by etching of CxDy clusters,mainly by physical sputtering. For overall relative D/H amounts between 11% and47% in the discharge gas, the SEYmax increasesalmost linearly from 0.99 to 1.38. An abrupt growth of SEYmax from 1.38 to 2.12 takes place in thenarrow range of D/H relative content of 47%–54%, for which the nature of thedeposited films changes to a polymer-like layer.