Ion beam induced micro-structuring of polymeric surfaces

Abstract

The objective of this research is to present novel methods for fabricating patterns with micro and nano-scale features on homo-polymer substrates and polymer blend films. Top–down (irradiations through patterned openings over large areas) and bottom–up (local modifications of substrate properties) approaches were combined to introduce nano/micrometer length scale features spanning over large areas (few mm2) of the substrates which can be used for products like environmental sensors, micro-reactors etc. Ion projection lithography technique was used to create specific local interactions in different substrates viz. cross-linking in the case of uniaxially stretched homo-polymer substrates and pre-patterned substrates with varying surface energies for thin polymer blend films. The irradiation of polystyrene (PS) surfaces with ion beams led to pronounced chemical and physical modifications, when the ions were scattered at the polymer chains. The local mechanical properties of the PS surface layers could be tailored (as measured by Young´s modulus) by changing the ion doses and ion species (with different masses). By annealing pre-stretched irradiated PS near the glass transition temperature, surface rippling occurred only in the irradiated areas. The moduli obtained from rippling periodicities and elastic model assumptions were in the range between 8 to 800 MPa and characterized the irradiated PS as rubber-like. From that, the network density and the molar mass of entanglement were quantified. Results confirm that even for non-discrete layered systems, a consistent elastic description can be applied and relevant polymer parameters can be derived from the well-known surface rippling. In the case of guided polymer blend phase separation, the gold layer coated silicon substrates were bombarded with focused ion beams (FIB) to sputter away Au grains in the irradiated regions and expose silicon oxide surface underneath, thereby creating pre-patterns of relatively hydrophobic (Au) / hydrophilic (silicon oxide) regions. In the pre-patterned regions, the spin-coated films consisting of immisible PS and PtBA (poly-tert-butyl acrylate) blend exhibited phase separation induced by the underlying pre-patterns and formed two distinct ordered morphologies with periodicities much smaller than that of the pre-patterns. The effects of varying periodicities of the pre-patterned structures, pre-pattern geometries (viz. 1-D arrays of stripes and spaces and 2-D arrays of squares), blending ratios and spin-coating parameters on the resulting morphologies were investigated. Observed affinities of PS and PtBA towards Au and silicon oxide surfaces respectively were explained in terms of work of adhesions from the wetting analysis.