X-Ray Optical Properties of C/C-Multilayers Prepared by Pulsed Laser Deposition (PLD)
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observed up to 700'C for an appropriate heat treatment. The limiting factor of thermal stability is caused by SiC-formation from reactions between layer stack and silicon substrate. In-situ ellipsometry, grazing incidence x-ray diffraction, high resolution electron microscopy (HREM) and image processing were applied for specimen characterization. Introduction X-ray optical multilayers are widely used for spectroscopic analysis, monochromatization and beam handling of soft X-rays in spectrometers and at synchrotron beam lines. The quality of the multilayer structures is determined by geometrical parameters (period number, reproducibility of period thickness, ratio of single layer thicknesses, interface roughnesses) and by structural parameters (refractive index, mass density, concentration gradients). The constructive interference of the rays that are reflected at each of the up to 100 periods of a stack yields to high intensity BRAGG-peaks when x-ray wavelength X, gracing incidence angle E and double layer thickness d solve BRAGG's equation (NX=2d sin 0 , Ninteger). The peparation of amorphous carbon with different mass densities by PLD offers the chance for a synthesis of X-ray mirrors consisting of only one chemical element so that driving forces for interdiffusion and reaction are reduced to a minimum level. 345 Mat. Res. Soc. Symp. Proc. Vol. 382 c 1995 Materials Research Society
Experimental / Results
The samples were prepared by PLD in an UHV environment. Our thin film growth technology is characterized by high kinetic energies of the condensing particles (leV..1000 eV), high deposition rates ( carbon: 1.0-1.5nm/min) and the pulsed character of the ablation and deposition process. The experimental arrangement and the basic processes of the interaction between target material and pulsed laser beam are described elsewhere [1]. The deposition process is controlled by in-situ film thickness monitoring with a quartz oscillator and by in-situ ellipsometry (X=632.8nm; He-Ne-laser). Carbon single films and multilayers were deposited on silicon wafers (75*25mm, 2 ) covered with native oxide at room temperature. Carbon single films were prepared at high (2.8-107 W/cm 2) and low (0.7-107 W/cm 2) levels of laser power density (measured in the unfocussed beam of the Nd-YAG-laser, X=1064nm). A periodic change between these two levels of laser power density yields mass density modulated C/C-multilayer structures. Grazing incidence x-ray diffraction is a very sensitive and widely used tool for non-destructive specimen characterization. For the detailed description of the experimental arrangement see [2,3]. A small horizontal divergence of the incident Cu Kcz 1 - radiation ( FWHM< 0.03") is provided by collimation with a silicon (111) crystal collimator and an arrangement of several slits. The reflected intensities are measured by a scintillation counter. The simulation of reflectivity versus grazing angle E is based on the matrix model by Kr6l et al. [4] where the influence of the interface roughnesses is taken into accou
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