PHASE-CONTRAST X-RAY PROJECTION MICROSCOPY FOR MATERIALS CHARACTERISATION (2002)

ABSTRACT We have developed a point-projection X-ray microscope based around an SEM host instrument which exploits both phase and absorption contrast imaging to provide information about a sample at resolutions down to 80nm. The resulting images can be analysed via phase retrieval methods to produce a density map of the object with a superior signal to noise ratio than that which could be obtained by absorption contrast alone. Further improvements in resolution can be obtained by applying deconvolution methods to the original images to remove the effect of the asymmetric X-ray source shape from the image. This form of microscopy is also well suited to tomography and examples are given of imaging and tomography of materials science samples. 1. INTRODUCTION Laboratory based X-ray microscopy has a long history, starting with early work with microfocus sources [1] and crossed mirrors [2] to the great interest in SEM-based systems in the 50’s and 60’s [3,4] which use the SEM electron beam focused onto a metal target to produce a microfocus X-ray source. It is in these early instruments that the instrument described in this paper, dubbed the X-ray ultra-microscope (XuM), has its roots. A major advantage of point projection microscopy is its simplicity. No optical elements are required and resolution is primarily determined by the source size. Developments in SEM technology have led to availability of high brightness FEG sources enabling the generation of very small X-ray sources. These allow imaging to better than 100nm resolution. Coupled with improvements in the source are developments in detector technology with modern CCDs offering very high X-ray sensitivity together with the ability to acquire images electronically. Together these developments make point-projection X-ray microscopy a much more attractive technique [5]. With the small X-ray source size obtainable in a SEM-based system, diffraction effects can be observed. These can be exploited to produce inline phase-contrast which enhances the visibility of edges and boundaries within an object. This closely parallels Gabor’s ideal of in-line electron holography [6]. It significantly improves the quality and information content of images and, most importantly, enables the imaging of weakly or non-absorbing objects [7]. With the addition of phase contrast effects the image intensity no longer bears a simple relationship to the projected density of the object. Consequently, while the edge-enhanced image is well suited to qualitative analysis, quantitative analysis requires further image processing. To this end phase retrieval methods are used to extract the phase and intensity distribution in the object plane (eg immediately after the sample) from the intensity distribution in the image plane. The projected density of the object may be determined from the object-plane phase (or intensity) recovered from a single image for a sample composed of a homogenous material. For an inhomogenous sample, multiple images at different energies are required. The phase retrieval process also has the effect of suppressing noise in the image and the resulting projected density (of phase) image has much improved signal to noise compared a simple absorption contrast image of the same sample.