3D Structuration Techniques of LTCC for Microsystems Applications

This thesis aimed at developing new 3D structuration techniques for a relatively recent new ceramic technology called LTCC, which stands for Low Temperature, Co-fired Ceramic. It is a material originally developed for the microelectronic packaging industry; its chemical and thermal stabilities make it suitable to military-grade and automotive applications, such as car ignition systems and Wi-Fi antennae (GHz frequencies). In recent years however, the research in ceramic microsystems has seen a growing interest for microfluidics, packaging, MEMS and sensors. Positioned at the crossing of classical thick-film technology on alumina substrate and of high temperature ceramics, this new kind of easily structurable ceramic is filling the technological and dimensional gap between microsystems in Silicon and classical "macro microsystems", in the sense that we can now structure microdevices in the range from 150 mm to 150 mm. In effect, LTCC technology allows printing conductors and other inks from 30 mm to many mm, structuration from 150 mm to 150 mm, and suspended structures with gaps down to 30 mm thanks to sacrificial materials. Sensors and their packaging are now merged in what we can call "functional packaging". The contributions of this thesis lie both in the technological aspects we brought, and in the innovative microfluidic sensors and devices created using our developed methods. These realizations would not have been possible with the standard lamination and firing techniques used so far. Hence, we allow circumventing the problems related to microfluidics circuitry: for instance, the difficulty to control final fired dimensions, the burden to produce cavities or open structures and the associated delaminations of tapes, and the absence of "recipe" for the industrialization of fluidic devices. The achievements of the presented research can be summarized as follows: The control of final dimensions is mastered after having studied the influence of lamination parameters, proving they have a considerable impact. It is now possible to have a set of design rules for a given material, deviating from suppliers' recommendations for the manufacture of slender structures requiring reduced lamination. A new lamination method was set up, permitting the assembly of complex microfluidic circuits that would normally not sustain standard lamination. The method is based on partial pseudo-isostatic sub-laminations, with the help of a constrained rubber, subsequently consolidated together with a final standard uniaxial lamination. The conflict between well bonded tapes and acceptable output geometry is greatly attenuated. We achieved the formulation of a new class of Sacrificial Volume Materials (SVM) to allow the fabrication of open structures on LTCC and on standard alumina substrates; these are indeed screen-printable inks made by mixing together mineral compounds, a glassy phase and experimental organic binders. This is an appreciable improvement over the so-far existing SVMs for LTCC, limited to closed structures such as thin membranes. An innovative industrial-grade potentially low-cost diagnostics multisensor for the pneumatic industry was developed, allowing the measurement of compressed air pressure, flow and temperature. The device is entirely mounted by soldering onto an electro-fluidic platform, de facto making it a true electro-fluidic SMD component in itself. It comprises additionally its own integrated SMD electronics, and thanks to standard hybrid assembly techniques, gets rid of external wires and tubings – this prowess was never achieved before. This opens the way for in situ diagnostics of industrial systems through the use of low-cost integrated sensors that directly output conditioned signals. In addition to the abovementioned developments, we propose an extensive review of existing Sacrificial Volume Materials, and we present numerous applications of LTCC to sensors and microsystems, such as capacitive microforce sensors, a chemical microreactor and microthrusters. In conclusion, LTCC is a technology adapted to the industrial production of microfluidic sensors and devices: the fabrication steps are all industrializable, with an easy transition from prototyping to mass production. Nonetheless, the structuration of channels, cavities and membranes obey complex rules; it is for the moment not yet possible to choose with accuracy the right manufacturing parameters without testing. Consequently, thorough engineering and mastering of the know-how of the whole manufacturing process is still necessary to produce efficient LTCC electro-fluidic circuits, in contrast with older techniques such as classical thick-film technology on alumina substrates or PCBs in FR-4. Notwithstanding its lack of maturity, the still young LTCC technology is promising in both the microelectronics and microfluidics domains. Engineers have a better understanding of the structuration possibilities, of the implications of lamination, and of the most common problems; they have now all the tools in hand to create complex microfluidics circuits.

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