Real-time and post-plasma studies of influence of low levels of tungsten on carbon erosion and surface evolution behaviour in D₂ plasma

Abstract

A profound influence of monolayer tungsten coverage of hard carbon films on the evolution of carbon surface erosion behaviour, surface chemistry and morphology in D2 plasma has been established by real-time ellipsometry, x-ray photoelectron spectroscopy and atomic force microscopy measurements. The erosion of tungsten-covered carbon showed two distinct stages of plasma material interactions: rapid tungsten removal during the initial erosion period and steady-state amorphous carbon removal accompanied by large-scale surface roughness development. The initial removal of tungsten takes place at a rate that significantly exceeds typical sputter yields at the ion energies used here and is attributed to elimination of weakly bonded tungsten from the surface. The tungsten remaining on the a-C : H film surface causes surface roughness development of the eroding carbon surface by a masking effect, and simultaneously leads to a seven fold reduction of the steady-state carbon erosion rate for long plasma surface interaction times (∼100 s). Results presented are of direct relevance for material transport and re-deposition, and the interaction of those films with plasma in the divertor region and on mirror surfaces of fusion devices.

Stages in the interaction of deuterium atoms with amorphous hydrogenated carbon films: Isotope exchange, soft-layer formation, and steady-state erosion

“…surface roughening. Surface roughness typically increases with the amount of material removed (or deposited), as has been seen in other studies of D 2 plasma-based a-C:H film erosion [34] or plasma-polymer interactions by members of this group [35,36]. The behavior seen here is completely different.…”

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

“…We recently reported on the sensitivity in this region of w-D space to changes in an a-C:H film's optical density. 28 From the MD simulations (Sec. III B), we predict there to be a hard, H-depleted layer close to the surface.…”

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

“…For typical photoresist materials, such as PR193, the presence of ion bombardment during plasma exposure has a significant correlation with both the etch resistance and roughness. Sufficiently energetic ion bombardment forms a dense amorphous carbon (DAC) layer on the sample surface, as a result of the depletion of oxygen, hydrogen, and volatile‐product‐forming species . This depletion results in the surface layer becoming significantly denser than the underlying bulk material, resulting in an enhancement of the etch resistance .…”

“…Sufficiently energetic ion bombardment forms a dense amorphous carbon (DAC) layer on the sample surface, as a result of the depletion of oxygen, hydrogen, and volatile-product-forming species. [1][2][3] This depletion results in the surface layer becoming significantly denser than the underlying bulk material, resulting in an enhancement of the etch resistance. [4][5][6] However, as the DAC layer grows from the underlying bulk layer, the increase in density introduces compressive stresses within the surface layers, causing a buckling instability that can lead to the enhancement of surface roughness.…”