Microstructures prepared via inkjet printing and embossing techniques

The goal of the work presented in this thesis is the combined use of inkjet printing and embossing techniques to fabricate microstructures. The thesis is divided into two sections. The first part (Chapters 1 to 4) describes a bottom-up procedure using inkjet printing to fabricate microstructures onto polymeric substrates, whereas the second part (Chapters 5 and 6) describes hot-embossing and a new technique called photo-embossing for the creation of surface relief structures, that are filled using an inkjet printer to improve resolution. The introduction offers a literature survey that describes the history and recent achievements in the inkjet printing field. Inkjet printing has developed from only producing text and graphics into a major topic in current research and development. Chapter 2 describes the behaviour of ink droplets both in-flight as well as upon impinging the substrate, which are discussed in order to understand the basics of successful inkjet printing. For equally-sized droplets of a dilute polystyrene solution, a linear relationship, which decreased, was observed between the dried droplet diameter and the printing height. Whereas, increased concentrations revealed an initial exponential decay in the dried droplet diameter, which stabilized at greater heights. At higher concentrations and heights, the polymer is believed to form a skin on the surface of the inkjet printed droplet, which causes inhibition of the in-flight evaporation of the solvent. The size-selective segregation of monodisperse silica particles in drying droplets was also studied. It was observed that the particles sediment as close as possible towards the periphery; the actual distance between the location of segregated particles and the contact line increases with increasing particle size. The third chapter describes inkjet printing of functional materials. A polyurethane dispersion was inkjet printed to fabricate three-dimensional structure with single layers having a height of approximately 10 µm. Secondly, an aqueous TiO2 nanoparticle ink was printed that gels above a certain temperature. Droplets and lines with improved morphological control and resolution were achieved using this thermal gelation effect. Defect-free straight lines could be printed on hydrophobic surfaces, which is impossible with regular inks due to the dewetting nature of these substrates. Finally, the resolution of directly inkjet printed lines of silver nanoparticles on polymer substrates that have a lower surface energy than common polymer substrates was improved: lines with a resolution down to 40 µm were printed. After depositing silver nanoparticle inks, a thermal sintering step is required in order to render the particles conductive, which is discussed in Chapter 4. Polymer substrates can usually not withstand high temperatures and, therefore, require a low temperature during the sintering process. Two new techniques are discussed that sinter in a selective way so that the polymer substrate is not affected. These techniques are exposure to microwave radiation and argon plasma. With the former technique, the sintering time was shortened by a factor of 20 and three minutes were sufficient for sintering. Furthermore, the presence of conductive antennae further promotes sintering and times of 1 second are sufficient to obtain pronounced nanoparticle sintering. The second technique uses exposure to argon plasma; a process that shows a clear evolution starting from a sintered top layer into bulk material. Chapter 5 describes a new technique called photo-embossing. This method represents a photolithographic technique for the generation of polymer relief structures that are created by a deformation of the surface of the film, which is caused by a material flux induced by a local polymerization. One of the main problems in photo-embossing is the low aspect ratio of the formed features. Chapter 5 describes the improvement of the aspect ratio by adding compounds that interfere with the reaction kinetics, such as t-butyl hydroquinone (TBHQ) and reversible addition-fragmentation chain transfer (RAFT) agents. By adding these compounds the aspect ratios were significantly improved up to a factor of almost 10. The last chapter describes the inkjet printing of a silver nanoparticle ink onto a predefined topography pattern in a thermoplastic surface. These topographical structures have been embossed into the polymer surface. Subsequently, droplets of a silver nanoparticle ink were dispensed over the asformed micro-channels. As a consequence of capillary forces the micro-channels were filled with the ink and tracks with an improved resolution were formed. The silver tracks had widths ranging from 5 to 25 µm. In general, it can be concluded that inkjet printing represents a highly suitable technique for systematic and statistical studies, since multiple equally sized droplets can be dispensed into a matrix, which subsequently can be analysed. Furthermore, inkjet printing is capable of preparing highresolution conductive features on or into polymer substrates. Together with inkjet printing, (ink) materials can be saved, since the ink is only dispensed on demand. It is, however, necessary to tune the polymer substrate as well the conductive inks properties. Alternative and selective sintering methods open new routes to produce conductive features on common polymer foils that have a relatively low glass transition temperature. In combination with different embossing techniques inkjet printing could lead to a higher resolution and more continuous overall processing of electronic devices on flexible substrates in the future. The improved photo-embossed structures offer a simple and versatile alternative for the production of large-scale/large-area relief structures in thin polymer films.