Molecular size effect in the chemical sputtering of a-C:H thin films by low energy H+, , and ions

Abstract

a b s t r a c tWe have experimentally determined total carbon yields per incident H atom in the energy range 36-300 eV/H for H + , H þ 2 , and H þ 3 projectiles incident normally on $60 nm thick a-C:H films, using 2-D ellipsometry determination of erosion crater volumes ex vacuo, the separately characterized thin film carbon density, and the incident beam current integration accumulated on target during the crater evolution. During each beam exposure, methane production was monitored using in situ quadrupole mass spectrometry (QMS). The present total carbon yields/H for incident H þ 3 ions obtained via ellipsometry are in agreement with total mass loss measurements for H þ 3 by Balden and Roth [1] over the investigated energy range. The observed methane production per incident H for the molecular ions exhibits molecular size effects over the entire energy range investigated, confirming the trend observed in the ellipsometrybased total C yields/H.

Hydrogenation and surface density changes in hydrocarbon films during erosion using Ar/H2 plasmas

“…A clear change can be seen between 75 and 100 V ($30-40 eV if divided by 2.5 AMU as the average ion mass), where we expect to first observe some physical sputtering ($35 eV) of the C atoms. Unlike previous beam studies 18,39 where no increase in the C atom yield is seen experimentally at these low ion energies, we observe a sharp increase in the C atom yield. With an unknown quantity of H þ in the plasma, this increase seen in yield could also be due to higher energy ions impacting the surface.…”

“…Energy per atom normalized yields for H þ 2 and H þ 3 over the yield of H þ were 1.47 6 0.22 and 2.54 6 0.28, respectively. 39 Zhang et al find a similar increase in molecular ion yields over atomic ion yields using low energy D ion beams on graphite. 40 In our plasma, with H þ 2 and H þ 3 being the predicted predominant ions in near-equal quantities, we can estimate the average ion energy per AMU by dividing by the bias potential shown on the x-axis of Fig.…”

Isotope effects on plasma species of Ar/H2/D2 plasmas

“…The etch yield for D 2 containing plasmas is much greater than H 2 containing plasmas. 24 Harris et al saw a potential molecular size effect of larger molecules advantageously having higher sputtering rates than single atomic ions with normalized energies. At higher percentages, this difference falls to $2 times higher for D 2 over H 2 containing plasmas.…”