Investigation of polycrystalline structure of CVD diamond using white-beam x-ray diffraction

Abstract

ABSTRACT Polycrystalline structure of a chemical vapor deposited (CVD) diamond vacuum window was investigated by means of white-beam diffraction. Forward Laue diffraction patterns were recorded using two-dimensional flat panel sensor. A computer aided approximate indexation of the Laue spots was performed based on maximum correlation between observed and directly simulated patterns. Different sets of the diffraction patterns were attributed to the different diamond crystalline grains. Retrieved orientations of the illuminated grains were rather random and average linear dimension of the single grain was about 56-64 µ m. Keywords: CVD diamond, white beam Laue diffraction, polycrystalline structure 1. INTRODUCTION High flux and high degree of coherence of x-ray beams at modern synchrotrons and x-ray free electron lasers (XFEL) impose tough requirements on the properties and quality of x-ray optical elements. Nowadays basic x-ray optical elements, like vacuum windows, mirrors, monochromators etc., must withstand high heat load and radiation damage, and must be perfect enough to preserve radiation coherence. Di amond was long known as a very perspective material for x-ray optics due to its thermal and mechanical properties. However, beside an enormous price on a market, natural diamonds suits poorly to contemporary needs of the x-ray optics. Although fabrication of large enough synthetic single crystal diamonds is a very complicated and not completely st udied process, significant breakthroughs have already been done (see e.g. [1]). Contrary to single crystal diamonds, polycrystalline CVD diamonds can be fabricated faster with fewer efforts. Commercially available since recently, CVD diamond windows may become an option to the traditionally used Be vacuum windows at the hot-beam synchrotron and XFEL facilities [2]. However, quite few investigations of the scattering properties of the CVD diamond and its impact on the transmitted x-ray beam have been done [3-5]. In [5] intensity uniformity of monochromatic x-ray beam transmitted through the CVD diamond window was investigated by means of high resolution phase-contrast imaging technique. Although polished CVD diamond window performed much better than conventional Be window, some non-uniformity in the images still could be observed. This non-uniformity was attributed to diffraction from small crystalline grains inside the polycrystalline CVD diamond plate. To evaluate performance of the CVD diamond window under general conditions one needs to know an average diffraction volume of the single grain and distribution of grain orientations in its polycrystalline structure. White-beam Laue diffraction is a standard crystallographic technique used to determine crystal orientation without rotation of the sample (see e.g. [6]). This technique particularly suited our needs since the sample under investigation was a commercially available CVD diamond plate which was mounted on ICF70 vacuum flange and could not be rotated around the incident x-ray beam. In this paper we present experimental data of white-beam forward Laue diffraction on the CVD diamond window along with their numerical analysis and evaluations.