Cleaning of carbon contaminated vacuum ultraviolet-optics: Influence on surface roughness and reflectivity

The loss of transmission of optical elements by irradiation is a big problem of all synchrotron radiation laboratories. A major reason for the reduced transmission is the contamination with graphitic carbon that is due to adsorbed and cracked hydrocarbons. In accordance with earlier experiments at other storage rings, a cleaning procedure was developed at BESSY which consists of an oxygen discharge suspended by an rf cavity. The atomic oxygen that is created in the plasma reacts with the carbon to CO and CO2 and dissolves from the surface. The investigations focused on the items: (a) influence of the cleaning procedure on the reflectivity, (b) its influence on the surface roughness of the optical element, and (c) transfer of the procedure to other elements (e.g., vacuum ultraviolet detectors, diodes, different substrates). For an in-situ check of the cleaning efficiency, the reflectivity of gold-and carbon-coated test samples was measured before and after cleaning using the BESSY HE-TGM 2 beamline. The microroughness was investigated employing different surface sensitive techniques like scanning electron microscopy, scanning tunneling microscopy, and interferometry