Carbon contamination of soft X-ray beamlines: dramatic anti-reflection coating effects observed in the 1 keV photon energy region

Abstract

Carbon contamination is a general problem of under-vacuum optics submitted to high fluence. In soft X-ray beamlines carbon deposit on optics is known to absorb and scatter radiation close to the C K-edge (280 eV), forbidding effective measurements in this spectral region. Here the observation of strong reflectivity losses is reported related to carbon deposition at much higher energies around 1000 eV, where carbon absorptivity is small. It is shown that the observed effect can be modelled as a destructive interference from a homogeneous carbon thin film.

DESIRS: a state-of-the-art VUV beamline featuring high resolution and variable polarization for spectroscopy and dichroism at SOLEIL

“…Indeed, at a time when the beamline was strongly C-contaminated, we observed at some photon energies (especially in the 8-14 eV range) a high level of unpolarized light with S 4 coefficients reaching up to 20%. We attribute this effect to a variable phase shift between the s and p components of light, induced by the nonhomogeneous variable thickness of the C-film deposited mainly on the cryogenically cooled first mirror (Chauvet et al, 2011) which cannot be compensated for by a single phase shift of the undulator. When the same polarimetry measurement was performed by selecting a very narrow solid angle around the main axis (about 20% of the central cone) with the fourblade aperture chamber, the S 4 parameter decreased to a few % because of the much more homogeneous sampled C-film thickness coating the beamline optics.…”

Secondary electron yield of Cu technical surfaces: Dependence on electron irradiation

“…This process, which is certainly more relevant in low vacuum environments, might occur even in ultrahigh vacuum regimes (10 À9 -10 À10 mbar) due to the dissociation of molecules such as CO and CO 2 that are usual components of the residual gas. In fact, the growth of thin carbon layers is routinely observed on surfaces exposed to high-energy radiation as in electron microscopy [29], extreme ultraviolet lithography [3,30], or synchrotron radiation beam lines [31]. Both the graphitization of the preexisting contaminating layer and the growth of a graphitic film due to the cracking of the residual gas molecules occur with a different efficiency depending on the kinetic energy of the electrons used to scrub the surface.…”

Ex-situ characterization of synchrotron radiation induced carbon contamination on LiF window

“…This non-uniform carbon layer creates serious problems in vacuum ultra violet/soft x-ray beam lines. The main issues are (i) loss of photon flux near C K-edge (≈284 eV) (ii) non uniformity in carbon layer, creates destructive interference and scattering losses (iii) non uniformity in carbon layer also varies the ratio of reflectivity of S-polarized light (R s ) to P-polarized light (R p ) [8,10]. In EUV lithography technique, the main contaminant on optics is carbon.…”

“…Generally synchrotron radiation beam lines are operated in 10 −8 -10 −10 mbar pressure, in this pressure regime the residual gases like H 2 , CO, CO 2 , CH 4 and H 2 O (in vacuum fired systems H 2 O partial pressure is very less) are mainly observed [6][7][8]. Even in ultra high vacuum conditions, the carbon atoms get deposited onto the surfaces when residual hydrocarbons dissociate during the interaction with UV/EUV photons and/or by the secondary electrons.…”