Fabrication of low temperature co-fired ceramic (LTCC)-based sensor and micro-fluidic structures

Abstract

Low temperature co-fired ceramic (LTCC) technology has attracted remarkable attention in fabrication of sensors and micro-fluidic devices in the last decade. Although structuring planar LTCC foils, which are processed and screen-printed with thick-films, seems straight forward, major problems are encountered once basic sensor features such as cavities and micro-channels are desired within the sensor module. Therefore, this study aims to accomplish two major tasks: development of an advanced technique for fabrication of well-defined and integrated cavities in LTCC and production of novel sensors by using new design parameters and compatible materials systems. Concerning the former objective, we developed graphite-based sacrificial (GB) pastes, which was screen-printed on LTCC, stacked with additional layers and fired. Cavities were formed by removal of sacrificial pastes during different stages of firing: GB was oxidized during sintering of the LTCC module. We selected membrane with inlet and outlet channels as our model device to study the oxidation kinetics of the GB paste as a function of graphite particle size, paste formulation, heating rate, channel dimensions and LTCC open-pore closure behavior and its effects on the membrane features. We found that higher heating rates (> 2.5 °C/min) shifted the oxidation of the paste to higher temperature as expected, whereas lower rates resulted in burnout and LTCC densification at lower temperatures, changing the cavity from a swollen to sagged structure, respectively. Channel dimensions were also found to have a direct influence on membrane swelling such that wider (400 µm) and thicker (> 70 µm of printed paste) channels permitted increased oxygen intake and played the major role in oxidation of the paste, after the closure of LTCC open porosity. We produced membranes within a diameter range of 7-18 mm and well-defined spacing of 13 to 100 µm. Displacement of largest membranes, which can be used as sensitive pressure sensors at sub-20 mbar range, were characterized in terms of applied pressure and membrane swelling. A hysteretic behavior of membrane displacement was observed as a function of swelling, which was absent in case of relatively flat membranes. In the second part of the thesis, fabrication of novel, LTCC-based sensors as an alternative to alumina-based ones was covered. Promising device applications were realized using distinctive properties of LTCC such as low thermal conductivity (an order of magnitude lower than alumina), design flexibility, fine substrate thicknesses (40 µm), lower elastic modulus, etc. Thus, we demonstrated three LTCC-based sensors detecting gas viscosity, force and inclination. Physical principle, materials selection, fabrication procedure and performance of these sensors were explained in detail. The materials compatibility, which is one of the utmost important parameters to assure the quality and the performance of the sensors, was discussed by using various materials characterization techniques and instruments. Differential shrinkage between LTCC tape and thick-film components and the process-dependent-tape shrinkage were found to be the major challenges, which also present the main boundaries of this technology today. In order to avoid the former problem, we modified thick-film terminations by mixing with network-forming additives such as quartz so that its shrinkage behavior was matched to LTCC. This facilitated utilization of thinner LTCC substrates, which is one of the figure of merits, for improved force sensing. The shrinkage of the sensor module, on the other hand, was compensated by manipulating design parameters in light of observed dimensional changes. Finally, future perspectives for structuring LTCC are presented, where an alternative technique to GB pastes is presented: mineral-based sacrificial pastes. In spite of similarity to GB pastes in processing conditions and parameters, these new pastes are formulated by a mixture of high and low melting temperature mineral (CaO) and glass (B2O3), respectively, where a consolidated state is formed that is etched by acid after firing. It was observed that the technique results in well-defined spacing and efficient removal characteristics. On the other hand, shrinkage mismatch to that of LTCC remains a problem.