Morphology, microstructure, stress and damage properties of thin film coatings for the LCLS x-ray mirrors

Abstract

The development and properties of reflective coatings for the x-ray offset mirror systems of the Linac Coherent Light Source (LCLS) free-electron laser (FEL) are discussed in this manuscript. The uniquely high instantaneous dose of the LCLS FEL beam translates to strict limits in terms of materials choice, thus leading to an x-ray mirror design consisting of a reflective coating deposited on a silicon substrate. Coherent wavefront preservation requirements for these mirrors result in stringent surface figure and finish specifications. DC-magnetron sputtered B 4 C and SiC thin film coatings with optimized stress, roughness and figure properties for the LCLS x-ray mirrors are presented. The evolution of microstructure, morphology, and stress of these thin films versus deposition conditions is discussed. Experimental results on the performance of these coatings with respect to FEL damage are also presented.

Fluence thresholds for grazing incidence hard x-ray mirrors

“…12 This is potentially due to a related, but different, optical damage mechanism, such as carbon contamination of the optical surface. 14 A rough estimate of the dose required to damage a material is to absorb enough energy to raise the material's temperature from room temperature to its melting point. 15 For ruthenium, this absorbed energy dose is 1.01 eV/atom 16 and for B 4 C this dose threshold is 3.7 eV/atom.…”

Interaction of short x-ray pulses with low-Z x-ray optics materials at the LCLS free-electron laser

“…They were deposited onto silicon wafer substrates using DC-magnetron sputtering, with the same deposition conditions as those used for the 50-nm-thick SiC reflective coatings on the LCLS hard x-ray mirrors. The deposition conditions are aimed towards SiC coatings with lower stress, at the expense of a slight increase in roughness [8]. The samples were exposed in rows of 20 to 30 single-shot exposures.…”

Boron carbide coatings for neutron detection probed by x-rays, ions, and neutrons to determine thin film quality

“…Typically, sputtered boron carbide films with thicknesses above a few hundred nm tend to delaminate from the substrate because of compressive stress induces by the sputtering process. 39,40 Fabrication of boron carbide coatings by evaporation methods was demonstrated and film thicknesses up to 20 lm were reported, but the deposited areas were of comparatively small size (several cm 2 ). [41][42][43][44] The present report describes the preparation of B 4 C coatings by DC magnetron sputtering on Si and Al substrates.…”