04.11.2014
Fig. 1:Top: Schematic representation of the re-orientation of the PS-b-P2VP-Bock-Copolymer-Film by thermal processing, as determined on the basis of the combined results from contrast-adapted GISAXS, X-ray reflectometry and AFM measurements.
Bottom: Horizontal section through GISAXS images before (left) and after (right) thermal processing. Each line represents an intensity profile (blue: low, red: high intensity) at a specific penetration depth of the X-radiation. The thermally processed sample clearly shows the absence of a structure at surface-sensitive scattering. This indicates the disordering or “smoothing” of the lamellas on the surface, whereas the order is preserved deeper in the film, towards the substrate interface.
X-ray small-angle scattering under grazing incidence (GISAXS: Grazing Incidence Small Angle X-ray Scattering) is a versatile method for the contact-free determination of the size of vertical and lateral surface structures in the range from a few nanometers up to micrometers. In current fields of research and application such as self-organized nanostructured polymer films as organic photovoltaics systems, for functionalized surfaces in medicine or as form templates for micro- and nanoelectronics, GISAXS is a frequently used – and sometimes the only available – method for nanodimensional structure characterization. However, during the complex analysis of the data, the problem frequently arises that a high fraction of substrate scattering, which overlaps the weak scattering of the polymer nanostructures to be investigated, occurs at the boundary between polymer film and silicon substrate and makes an assessment difficult or impossible.
With the new in-vacuum hybrid-pixel detector PILATUS 1M, which has been developed within the scope of a cooperation between PTB and Dectris Ltd., it has now been possible to carry out high-precision GISAXS measurements on the silicon-K edge at 1.84 keV and to make the silicon substrate practically "invisible" for X-ray scattering by contrast matching of silicon substrate and polymer film. This allowed structure changes inside thin-layer polymer films to be observed (within the scope of a doctoral dissertation) with vertical resolution before and after thermal processing. In both cases, a lamella structure in the form of a fingerprint with a period length of approx. 60 nm is present at the surface of the films. In contrast to the untreated polymer film, the thermally processed sample showed a strong decrease in the order only within the first approx. 30 nm of the film (85 nm in thickness, see figure) while the lamella ordering is preserved towards the substrate interface.
The results contribute to a deeper understanding of the restructuring processes in self-organized polymer films and offer a new tool for the structural analysis. This is important for a precise control of the structure in technological applications. In addition, the extended possibilities and measuring techniques of the in-vacuum PILATUS detector were demonstrated on the Four-Crystal Monochromator Beamline of PTB at the electron storage ring BESSY II.
Publication:
J. Wernecke, H. Okuda, H. Ogawa, F. Siewert, M. Krumrey, Depth-Dependent Structural Changes in PS-b-P2VP Thin Films Induced by Annealing, Macromolecules 47, 5719–5727 (2014)Contact:
J. Wernecke, 7.11, e-mail: 
Jan.Wernecke(at)ptb.de
M. Krumrey, 7.11, e-mail: Michael.Krumrey(at)ptb.de