Grazing incidence synchrotron x-ray diffraction method for analyzing thin films
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M. Bellotto Universita di Brescia, 25060 Brescia, Italy
M. Hart Department of Physics, The University, Manchester M13 9PL, United Kingdom (Received 16 March 1987; accepted 11 May 1987) A method using synchrotron radiation parallel beam x-ray optics with a small incidence angle a on the specimen and 2©-detector scanning is described for depth profiling analysis of thin films. The instrumentation is the same as used for 0:2© synchrotron parallel beam powder diffractometry, except that the specimen is uncoupled from the detector. There is no profile distortion. Below the critical angle for total reflection ac, the top tens of Angstroms are sampled. Depth profiling is controlled to a few Angstroms using a small a and 0.005° steps. The penetration depth increases to several hundred Angstroms as a approaches ac. Above ac there is a rapid increase in penetration depth to a thousand Angstroms or more and the profiling cannot be sensitively controlled. At grazing incidence the peaks are shifted several tenths of a degree by the x-ray refraction and an experimental procedure for calculating the shifts is described. The method is illustrated with an analysis of iron oxide films. I. INTRODUCTION X-ray diffraction methods are widely used for the characterization of thin films.1 The increasing use of thin films with special tailored properties for electronic devices has stimulated the need for improved characterization methods. The properties are determined by the thin-film materials and its crystal structure. The microstructure is critically dependent on the various parameters used in the deposition of the film, and their effect on the required properties can be followed by x-ray diffraction patterns. The conventional x-ray methods currently used for thin-film analysis were developed for powder and bulk samples and are used in a large variety of studies. When used for thin films, the x-ray beam usually penetrates the entire film making it impossible to follow structural variations as a function of film depth. The most widely used method employs a divergent beam from an x-ray tube in a focusing geometry.2 This imposes a strict requirement on the coupling between specimen surface and detector, which must maintain a ©:20 angular relation to avoid defocusing and line broadening. In the Seemann-Bohlin focusing geometry, the specimen is set at a fixed small angle (about 5° minimum) and the detector is moved around the focusing circle by a special linkage.3 However, mechanical restriction prevents using smaller angles required for grazing incidence and any departure from the strict focusing requirements causes large geometrical aberrations in the pattern. Neither J. Mater. Res. 2 (4), Jul/Aug 1987
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method is therefore applicable to grazing incidence studies. It is well known that thin films are usually not uniform, and indeed it may be desirable to purposely vary the microstructure as a function of thickness to obtain the desired properties. Methods that obtain diffraction patterns from different film d
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