Pulsed Laser Deposition of Laterally Graded X-Ray Optical Multilayers on Substrates of Technical Relevance
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R. DIETSCH*, TH. HOLZ*, R. KRAWIETZ**, H. MAI*, B. SCHONEICH*, R. SCHOLZ***, S. VOLLMAR*, B. WEHNER** * FhG-Institut flir Werkstoffphysik und Schichttechnologie, Helmholtzstral3e 20, D-01069 Dresden, Germany ** TU Dresden, Institut flir Kristallographie und Festk6rperphysik, MommsenstraBe 13, D-01069 Dresden, Germany MPI fiir Mikrostrukturphysik, Am Weinberg 02, D-06120 Halle, Germany ABSTRACT Pulsed Laser Deposition (PLD) is used for the preparation of Ni/C, W/C, and Mo/Si multilayers having X-ray optical quality. For the synthesis of layer stacks involving a uniform or a graded thickness distribution across 4"-wafers the conventional thin film deposition equipment of PLD has been modified. This modification provides a precise spatial control of the plasma plume orientation in the deposition chamber. With this arrangement the emission characteristic of the plasma source can be computer controlled and the desired coating profile can be tailored across an extended substrate via a stepper-motor-driven target manipulator. Thus film thickness uniformity (Ats < 2%) is obtained on substrates up to 4" diameter even for smaller target-substrate distances. For laterally graded Ni and C individual layers linear thickness gradients of dts/dx = 3.2 x 10' were confirmed over the total substrate length by spectroscopic ellipsometry. The parameters deduced from single layer deposition were applied for the synthesis of laterally graded Ni/C multilayers. A mean value of the gradient of the stack period thickness dt/dx = 6.2 x 10 8 confirmed by X-ray reflectometry (nominal value: dt0 /dx = 6.4x10"8 ) characterizes precision and reproducibility of the coating process. INTRODUCTION For more than a few years PLD has been successfully used for the preparation of Ni/C, W/C and Mo/Si X-ray optical multilayers on 3" x 1" substrates [1]. At present the PLD process is upscaled for the deposition of thin films showing uniform thickness distribution across 4" diameter substrates [2]. Another goal is the synthesis of tailored film thickness profiles. Especially in X-ray optics the lateral variation of layer thickness opens further applications for multilayers in X-ray analysis [3]. In particular for the conversion of divergent beams of conventional X-ray sources into parallel ones, parabolically bent multilayers with a definite lateral gradient of the period thickness are required. For laterally graded multilayers, stack period gradient and X-ray optical properties have to be adjusted to the experimental conditions. Particularly a reflectivity > 80 % across the total substrate length for the useful spectral range, i.e. and the suppression of the second BRAGG diffraction order, are required [4]. In this paper the PLD preparation of 10 - 50 period Ni/C multilayers with nominal stack period thickness gradients dt/dx = 6.4 x 10.8 on 4" x 1" Si substrates is described. At first the experimental setup was calibrated by the deposition of Ni and C single layers. The determination of layer thicknesses and thickness gradients across substrate length has
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