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Polymer films on invisible substrate

Novel X-ray detector enables contrast variation in scattering experiments

PTB-News 2.2015
Especially interesting for

nano- and thin-film technologies

surface science

The inner structure of technologically relevant polymer films in the nanometer range can, in principle, be easily investi-gated with scattering of synchrotron ra-diation in the X-ray range at wavelengths below 1 nm. The sensitivity of this meth-od is, however, often considerably limit-ed by additional scattering occurring at the boundary between the film and the substrate. In the PTB laboratory at the synchrotron radiation source BESSY II, this effect has now been clearly reduced by means of contrast variation at appro-priate wavelengths or photon energies. Hereby, the use of a novel detector for scattered X-rays has been decisive in achieving the corresponding – mostly low – photon energies.

Horizontal cuts through GISAXS images before (top) and after (bottom) thermal processing. Each line represents an intensity profile (blue: low; red: high) at a specific angle of incidence of the X-rays onto the sample (i.e. at different penetration depths). The thermally processed sample shows the lack of signals of the surface-sensitive scattering at small ang-les of incidence, which suggests a loss of structure order at the surface. The order at larger angles of incidence, i.e. in the presence of scattering also coming from deeper layers close to the boundary with the substrate, remains unchanged.

X-ray small-angle scattering under grazing incidence (GISAXS: Grazing Incidence Small Angle X-ray Scattering) is an established method for the contact-free determination of the size of vertical and lateral surface structures in the range from a few nanometers up to several micrometers. In current research activities on self-organized nanostructured polymer films as organic photovoltaic systems or on functionalized surfaces for applications in medicine as well as in micro- and/or nano-electronics, GISAXS is sometimes the only method available when it comes to characterizing structures in the nanometer range. Due to the high scattering contrast at the boundary between the polymer film and the substrate, a high share of substrate scattering may occur which may overlap the – mostly weak – scat-tering observed on the nanostructures with-in the polymer film in such a way that the data can no longer be analyzed.

With the new in-vacuum hybrid-pixel detector PILATUS 1M, which was developed by PTB in collaboration with Dectris Ltd., highprecision GISAXS measurements on thinfilm polymers on a silicon substrate have been made possible for the first time at very low X-ray energies, namely at the silicon-K edge at 1.84 keV. By adjusting the contrast, the silicon substrate can be made practically “invisible” to X-rays. This allowed structure changes inside thin-film polymers to be observed with vertical (in-depth) resolution, before and after thermal processing. In both cases, the surface of the thermally processed sample displayed a strong decrease in the order within the first approx. 30 nm of the 85 nm thick film.

The results make a considerable con-tribution to a deeper understanding of the restructuring processes in self-organized polymer films. This is important for the precise control of the structure in technological applications. At the same time, this has shown very clearly that the in-vacuum PILATUS detector has allowed an extension of the measurement capabilities at PTB's four-crystal monochromator beamline of the electron storage ring BESSY II.


Michael Krumrey
Department 7.1 Radiometry with Synchrotron Radiation
Phone: +49 (0)30 3481-7110
E-mail: michael.krumrey(at)ptb.de

Scientific publication:

J. Wernecke, H. Okuda, H. Ogawa, F. Siewert, M. Krumrey: Depth-dependent structural changes in PS-b-P2VP thin films induced by annealing. Macromole-cules 47, 5719–5727 (2014)