Advanced Silicon Microstructuring by Electrochemical Micromachining: Technology and Applications

Electrochemical micromachining technology, namely ECM, is a newly proposed microstructuring technique based on electrochemical etching of n-type silicon in aqueous electrolytes containing hydrofluoric acid. Among the main features of ECM there are: i) high aspect-ratio (HAR) of feasible structures, and in turn, high integration density; ii) fine control of etching anisotropy (from 0 to 1), and in turn, enhanced flexibility in fabrication; iii) reduced roughness (about 20 nm) of etched surface. This thesis is aimed at demonstrating that ECM technology can be successfully used for the fabrication of silicon (Si) based microstructures and microsystems for a wide range of applications ranging from integrated optics, to optofluidics, from biology (3D cell culture) to synthesis of nano/micro-structured conducting polymers film. As to optics applications, for the first time, we report fabrication of a novel Si-optical platform where light from standard single mode optical fiber is directly coupled in and out high order vertical silicon/air PhCs operating in the near-infrared region. The platform consists of ECM-micromachined Si-substrate integrating an array of vertical silicon/air PhCs featuring a transmission peak at λ=1.55 μm, together with U-grooves and mechanical end-stop structures for readout optical fibers thus enabling easy-to-use and plug-and-play operation mode. As to optofluidic applications, ECM-fabricated vertical HAR-PhCs were integrated into optofluidic microsystems (OFM) together with fluidic microchannels obtained by potassium hydroxide (KOH) etching. A sensitivity value of 1049 nm/RIU at 1.55 μm and limit of detection of 10-3 RIU are obtained. Moreover, we report the successful application of ECM technology to the realization of all-silicon OFMs in which HAR-PhCs are integrated by one-etching-step together with microfluidic reservoirs/channels and fiber grooves, for alignment/positioning of readout optical fibers in front of the PhC. Assessment of this OFM as refractive index sensor was also performed. High sensitivity of 670 nm/RIU at 1.55 μm and good limit of detection of about 10-3of PhC-OFMs were obtained. As to 3D cell culture applications, we have demonstrated ECM-fabrication of in vitro 3D-microincubators featuring HAR Si-microstructures for mesenchymal cells culturing. In particular, the proposed microincubator selects mesenchymal cells that show the ability to grow in the gaps between two adjacent HAR silicon walls. Finally, we have developed a novel technology for microstructuring polypyrrole (PPy) films based on light-activated PPy electrosynthesis on ECM-micromachined Si-substrates. Scanning electron microscopy highlights as light-activation allows a highly conformal polymer growth yielding a 3D-PPy structure perfectly replicating the Si microstructure.