Towards Sub-100 Nm X-ray Microscopy for Tomographic Applications

We have demonstrated that structures down to 150 nm can be visualized in X-ray projection images using a nano-focus X-ray source. Due to their unlimited depth of focus, they don a limit on the specimen size in the case of 3D tomographic imaging. Further simulation studies have shown that optimization of the detector response curve and switching from a reflective Xray target to a transmission target should allow us to reach sub-100 nm resolutions. INTRODUCTION A common way to achieve a spatial resolution better than a few 100 nm in X-ray projection imaging is by using X-ray optics such as Fresnel zone plates. Although the use of these components can result in extreme high resolution (Chu et al., 2008), they are not always suited for tomographic imaging due to their very limited depth of focus. This limits to size of specimen that can be imaged in focus to a few microns (Suzuki, 2008). To overcome this limitation while still retaining a nanometer scale resolution, we investigated the achievable spatial resolution using X-ray sources with a nano-focus spot size. This approach has an infinite depth of focus. One of the factors determining the smallest X-ray focal spot size attainable in an X-ray source is the type of electron gun used. Sources based on classical W filaments can achieve a focal spot diameter around <900 nm (Brunke, 2008). When the electron beam is generated using a LaB6 crystal, X-ray spot diameters between 250 nm (Hamamatsu, 2009) and 400 nm (Tohken, 2009) have been reported. However, the smallest X-ray spot sizes are currently achieved using electron guns based on Schottky field emission. These are often found in scanning electron microscopes (SEM). Using an SEM as a nano-focus X-ray source, we studied the influence of X-ray energy and geometrical set-up on the achievable spatial resolution. EXPERIMENTAL SETUP Figure 1 shows a schematic drawing of how a JEOL JSM-7000F SEM was used for X-ray imaging. A specimen holder is mounted on a rotation stage. The distance from the specimen to the target can be varied. The target is mounted at a 45 degree angle relative to the electron beam. The produced X-rays penetrating through the sample are acquired by direct detection in a cooled back-thinned CCD camera (Princeton Instruments PIXIS-XO 512). This camera has 512x512 pixels and a pixel size of 24 m. The electron gun is operated at 30 kV. The target can be coated with a thin substrate of a specific material to generate characteristic X-rays with appropriate energies. 89 Copyright ©-International Centre for Diffraction Data 2010 ISSN 1097-0002