X-Ray Diffraction as a Probe for Elastic Strain: Micro- and Nanoscale Investigation of Thin Metal Films
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X-Ray Diffraction as a Probe for Elastic Strain: Micro- and Nanoscale Investigation of Thin Metal Films
R. Spolenak Lab. for Nanometallurgy, Dept. of Materials, ETH Zurich, Switzerland
ABSTRACT
In the past years the concept of measuring strain by x-rays has changed significantly. The combination of 3rd generation synchrotron sources, advanced focusing techniques and large area detectors has made it possible to probe volumes smaller than a cubic micron. This devolopment has made it possible to probe microstrains directly without having to rely on highly sophisticated models to evaluate peak broadening effects. This paper will provide a review of the state of art of local strain measurements by x-rays, discuss their limitations, provide an outlook of where the field may be going within the next years and address the most important issues to be solved. Examples will be given for the current limits in terms of resolution in time, space, strain and intensity.
INTRODUCTION
Microdiffraction has become more and more popular over the past years as focusing devices became more sophisticated. This paper will focus on spot sizes smaller than five microns in diameter. The reason is that highly developed laboratory sources can nowadays also achieve spot sizes of 100 microns. However, this paper will focus on applications, in which the spot size of the beam is on the order of the grain size of the material investigated. In this case the diffraction process corresponds to single crystal diffraction. This requires the development of new methods to gain information of the sample. In the following two different ways of addressing this issue will be described. The first is a very sophisticated combination of sample movement and rotation, 2D detectors and the application of annular apertures to discern small volumes in a three dimensional grain structure [1-5]. This method relies on a monochromatic x-
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ray beam and is currently limited to several microns of spatial resolution. Highly significant results on the interaction of single grains in three dimensions have lately been obtained. For smaller structures than one micron currently a change in methodology is necessary. Current limitations in goniometer precision make it necessary to do without sample rotation. The only other way to fulfill Bragg’s law is then to vary the x-ray wavelength or to illuminate the samples with all wavelengths at the same time. This corresponds to using the classical Laue technique at lateral dimensions of one micron and with the low divergence of a synchrotron beam. As will be shown in the following this technique, though in principal two dimensional, can also be extended into three dimensions. The new methods provide the experimenter with a variety of information that can be accessed simultaneously. These are: •
Grain orientation
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Grain shape
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Details of the strain tensor (in some cases up to 6 components)
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Density of geometrically necessary dislocations
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Spatial arrangement of dislocations
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Burgers vector of dislocatio
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