engleski

Lateral continuity of ultrathin tungsten layers for X-ray mirrors

For obtaining a considerable reflectivity of hard X-rays by tungsten/carbon multilayers for angles of incidence >2° a bilayer period of less than 2.5 nm is required. A further decrease of the bilayer period is mostly limited by lateral discontinuity of the tungsten layer below its average thickness of about 1.0 nm. In this work we have examined the development of tungsten layer continuity, starting from separate W islands which by percolation process form a perforated layer upon thickening, and finally make a continuous layer. A series of W/C multilayered stacks, with different carbon layer thickness and tungsten layer thickness, have been prepared by sequential RF/DC magnetron sputtering. With a nominal 1 nm C/1 nm W bilayer, a reflectivity (for λ = 0, 154 nm) of about 10-15% has been achieved at about 2°. In order to explore the threshold of continuous tungsten layer formation, its thickness has been varied by reasonable steps in the 0, 2 - 1, 2 nm range. Special samples with a single tungsten layer in a sandwich between carbon layers have been prepared for the detailed examination by TEM which provides a direct insight into the local W-layer morphology. The „global“ information on the lateral W-layer continuity has been retrieved from GISAXS measurements. XRR measurements have supplied additional information for the onset of W-layer discontinuity. The obtained results allow an estimation of the thickness at which a continuous tungsten layer is formed.

C/W nanolaminates; magnetron sputtering; X-ray reflectivity; GISAXS

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