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A growth of C/W multilayers - experiment and modelling

The artificial multilayer coatings in the nanometer range are used as optical elements for the EUV and X-ray wavelength radiation. High reflectivity mirrors require uniform layer thickness and densities, a high stack regularity, and perfectly smooth and sharp interfaces. From the viewpoint of both technology and optical properties, the tungsten/carbon combination is one of the best for the X-ray radiation, with tungsten layer properties governing the reflectivity of the mirror[1-3]. The C/W multilayers and single layers of pure carbon or tungsten were deposited by magnetron sputtering onto various substrates. Substrate temperature was used to control carbon structure/phase composition, while either working gas pressure or discharge power was used to modify the tungsten (ά/β) phases ratio. The structure and phase composition of pure tungsten films have been determined by the Wide Angle X-ray Scattering (WAXS) performed simultaneously with the Grazing Incident Small Angle X-ray Scattering (GISAXS). Topography/ morphology of the surface and multilayer interfaces - decisive multilayer property for the role of X-ray mirror - has been observed by two global (XRR, GISAXS) and two local methods (TEM, AFM). Measurement of small angle x-ray scattering under grazing incidence angle (GISAXS) with a 2D detector allows insight into the nature of the W/C multilayer growing process. The evolution of the interface morphology of a thin film during deposition is classified using a set of roughness exponents, which define the universality class of growth[4-5]. The exponents describe the change in the local roughness of the surface as a function of length scale and time, as well as the replication of the interfacial features as the film is grown. The analysis was performed for the selected set of C/W multilayers, yielding variation of critical exponents with the deposition conditions. Finally, the obtained critical exponents are used to identify the class of growth in a particular case, as well as to correlate the experimental and model values. [1] B.K. Gan, B.A. Latella, R.W. Cheary, Applied Surface Science 239, 2

Carbon/tungsten multilayers; XRR; GISAXS; AFM; TEM

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