Characterising thin films using a microfocused beam

X-ray reflectivity (XRR) is a nondestructive technique widely used for thin layered materials characterisation. The brilliance achieved by third generation synchrotron sources, such as Diamond, has opened new opportunities for the study of materials at the micro and nanoscales and has driven the development of new X-ray micro-focusing optical devices, such as kinoform lenses. A group of researchers from Diamond has demonstrated the first application for in-house fabricated kinoform lens for XRR analysis on Diamond’s Test beamline B16.

Thin films and multi-layer structures deposited on substrate surfaces offer unique structural, magnetic and electronic properties leading to a wide range of applications, from microelectronics to optical devices or decorative coatings. For many years the XRR technique has enabled scientists to measure non-destructively the thickness, the roughness and the density of these structures. However, the glancing incidence angle required for the incoming X-ray beam means that the vertical size of the beam must be limited. It has also barred so far the application of XRR for sample scanning analysis because of the large footprint of the X-ray beam generated at sample surface. A group of researchers from Diamond has worked on the design, fabrication (in collaboration with their colleagues at the STFC Micro and Nanotechnology Centre on the Harwell Campus) and testing of germanium and silicon kinoform lenses for real synchrotron applications. These saw-tooth shaped refractive optical devices generate a line-focused X-ray beam of vertical size typically ~0.5 to 3μm. Although they work in a similar way to Compound Refractive Lenses (CRLs), kinoform lenses present much lower absorption losses. It is also possible to use a single kinoform lens for short focal length application instead of a stack of several refractive lenses. And, while kinoform lenses are chromatic, i.e. each lens is designed for a fixed wavelength.

Dr Lucia Alianelli is from the Diamond Optics and Metrology Group.

The Diamond group has used the Test beamline to perform XRR measurements, for incidence angles ranging from 0 to 3°, on Nb/C/Nb trilayer (at 12 keV) and Mo/ Si multilayer (at 19 keV) structures. The kinoform lens structures were fabricated side by side on a silicon substrate. They generated a well defined line-focus X-ray beam of nominal size ~ 50 μm x 2 μm.

Diamond scientists were able to measure thickness and roughness for both samples. They observed a more or less constant reflected intensity below the critical angle for the Mo/Si multilayer whereas they tested sample scanning mode on the trilayer sample by probing different locations 2 mm apart. Their results show that kinoform lens focused X-ray beams can be easily employed for microstructural characterization of thin layered materials using XRR measurements, in particular in sample scanning mode.

Top row: Details of silicon focusing kinoform lenses for X-ray energy E = 8 – 20 keV Bottom row: Germanium lenses for E = 20 – 150 keV

Dr Kawal Sawhney is Principal Beamline Scientist for the Test beamline and head of the Optics and Metrology Group.

He adds, “Our work demonstrates a real science application for kinoform lenses and shows that they work and will be working in the future, spectroscopy aside.” The Diamond group plans to take the kinoform lenses to other Diamond beamlines in the next couple of months to test for their performance across a range of synchrotron techniques.

For more information on the Test beamline, please contact kawal.sawhney@diamond.ac.uk

Application of kinoform lens for X-ray reflectivity analysis, M. K. Tiwari, L. Alianelli, I. P. Dolbnya and K. J. S. Sawhney, J. Synchrotron Rad. (2010). 17, 232-242
DOI: 10.1107/S0909049509055009