Vahid Fakhfouri
Inkjet printing of functional materials, photolithography and micromolding are 3 different techniques largely used in the field of micro-electromechanical systems (MEMS) with increasing interest for the fabrication of new devices and the development of new applications. This Ph.D. thesis provides new insights into each of these fields and, by combining them, aims to enable the access to new application fields. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacturing of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices. First the piezo-acoustic inkjet printing technology is described and a theory to explain the drop generation process is developed and evaluated. The experimental work was focused on the drop-on-demand inkjet printing of viscous polymers, particularly SU-8, a negative tone photo-resist epoxy. It validates the theoretical predictions and shows the shortcomings of the existing theories. The camera-assisted high accuracy alignment (< 3 µm misalignment) technique developed in the frame of this work enables the precise positioning of microdrops on pre-structured substrates and accurate filling of high aspect ratio micromolds. Additionally, polymeric microlenses were also Inkjet-printed and characterized. The highly reproducible optical properties of such microlenses, i.e. the focal length in the range of 40-150 µm and the numerical aperture in the range of 0.25-0.75, can be adjusted for different applications in micro-optical systems. An innovative technique, based on micromolding, for the microfabrication of high aspect ratio ceramic structures using polymeric precursors is also presented. Polyureasilazane, a precursor for silicon-carbide-based ceramics was investigated. Pipettes and inkjet printers were used to fill high-aspect ratio SU-8 molds. The micromolded structures were released using a sacrificial layer. After a pyrolysis step which enables the polymer-to-ceramic transformation in a simple tube furnace, the final structures showed that this process is very reliable as the mold shapes were perfectly replicated and a smooth surface was obtained. This technique is an interesting candidate for the fabrication of ceramic devices for harsh environments and high temperatures. Incorporation of luminescent nanocrystals (NCs) in SU-8 is another contribution of this thesis. For this purpose, the selection of an appropriate solvent for both NCs and the epoxy-resist formulation is fundamental in directing the nanocomposite resist preparation. The NC-modified SU-8 was successfully patterned and high aspect ratio structures with micrometer resolution were fabricated, showing that the overall UV-structuring capability of the modified resist was not affected by the NC incorporation. The obtained microstructures were shown to retain the NC emission properties. Finally the possibility to combine inkjet printing, micromolding and photopolymerization for the fabrication of Hybrid AFM probes by a single-plane surface-micromachining technique was demonstrated. Hybrid AFM probes, with a tip made of a hybrid organic/inorganic polymer and a cantilever and a holder made of SU-8 were micro-structured, characterized and tested for AFM imaging. The results were compared with a commercial silicon probe and a SU-8 AFM cantilever. This technology can be used to convey new properties to scanning microscopy probes for new sensing applications or to improve the quality of the existing devices. Functional materials can be inkjet-printed to form the tips with a minimum material waste. This thesis is intended to contribute to the advancement of micromachining technologies in the area of micro-devices with enhanced properties. In particular, it shall contribute to the cost effective fabrication of hybrid components made of different functionalized materials.
EPFL2008
