Additive Manufacturing and Integration of 3D MEMS using Ultrafast Lasers and Magnetic Assembly

Sammanfattning: The geometry of MEMS devices is limited by the technologies used to fabricate them. Today, microsystems are manufactured with patterning technologies that allow only for 2D and 2.5D geometries. These miniaturized devices are widely used in industry, including the automotive, electronics, and biomedical sectors, and their adoption in our society is expected to increaseeven further with the advance of the Internet of Things. 3D MEMS can contribute to this development enabling novel applications and improvedperformances, by exploiting more complex device geometries, and reducing device footprint, by integrating more functionalities onto smaller areas. In recent years, new technologies have been proposed to realize 3D microdevices by directly patterning 3D microstructures and by integrating together microchips manufactured with standard technologies. In this thesis, we develop 3D MEMS devices and fabrication technologies based on both paradigms using femtosecond laser micromachining and the magnetic assembly of tinychips.The first part of the thesis describes how laser micromachining with ultrashort pulses can be leveraged to achieve both additive and subtractive MEMS manufacturing. Two-photon polymerization of photosensitive resins enables additive manufacturing of 3D microstructures with sub-micron resolution. However, the kinds of devices, geometries, and materials that can be currently printed by two-photon polymerization are still limited, thus we set out to address some of these limitations. In the first work, we fabricate functional 3D printed accelerometers combining self-shadow masking features with directional metallization. In the second work, we demonstrate the realization of long overhanging structures (∼ 1mm) using the consecutive printing of short sections. In the third work, we 3D print polyimide, a high-performing polymer that can be used in harsh environments, where typical 3D printedpolymers are not suitable. Subtractive manufacturing by laser micromachining is demonstrated in the fourth work, where through-silicon-holes with high quality are formed using water-assisted drilling in a simple fabrication setup ,where the laser is focused on the front side of a silicon substrate and water is in contact with the backside.The second part of the thesis describes the integration of fragile and tiny MEMS devices coated with ferromagnetic thin films into silicon and polymeric substrates. The micromachined magnetized chips are integrated into receiving structures using permanent magnets. Magnetic interactions allow the non-contact handling and the vertical placement of chips at a scale and speed that is challenging for industry standard pick-&-place tools. In the fifth work, thin silicon chips for electrochemical sensing are magnetically assembled in vertical position and laterally wire bonded. In the sixth work, silicon micromachined spray nozzle chips with a diameter below 300 μm are magnetically assembled and sealed on acrylic sheets, to be used in portable soft mist inhalers.