Nesrine Jaziri

Low-Temperature Co-Fired Ceramic (LTCC)-based mechanical sensors are inherently limited by the thickness and rigidity of multilayer ceramic stacks, which restrict miniaturization and mechanical compliance. To overcome these constraints, this work presents a hybrid LTCC/Kapton^® platform enabling high-sensitivity mechanical sensing through mechanically tunable RF passive components. The proposed approach integrates a flexible polyimide membrane, bonded onto an LTCC substrate at low temperatures using selectively electroplated indium pillars that simultaneously define the air gap and provide mechanical fixation. Inductance tuning is achieved via metal-shielding proximity effects, whereas capacitance tuning relies on force-controlled air-gap modulation in a metal–insulator–metal configuration. The fabrication process ensures precise gap control, high compliance, and structural robustness without requiring deformable ceramic membranes. Experimental characterization, including three-dimensional surface profiling and impedance measurements, demonstrates a 48% inductance tuning range with a sensitivity of 0.715 nH/mN and a 36% capacitance tuning range with a sensitivity of 47.3 fF/mN at 1 MHz. The proposed hybrid platform provides a compact and scalable solution for high-sensitivity sensors and mechanically reconfigurable RF components suitable for harsh-environment and adaptive electronics applications.

Conventional soldering processes in microsystem packaging often induce critical thermo-mechanical stresses, caused by mismatches in the coefficients of thermal expansion (CTEs) of materials such as silicon dies, molding compounds, and substrates. Reactive multilayer systems (RMS) offer a localized heat source that can significantly reduce such stresses, thereby enabling the bonding of heat-sensitive components. To expand the technological possibilities in fabricating heterogeneous microsystems, this study investigates RMS-based bonding of low temperature co-fired ceramics (LTCC) substrates. Two experimental series were conducted. In the first, both reactive foils and deposited RMS structures were used to bond relatively large chips measuring 10 mm x 10 mm under a bonding pressure of 1.5 MPa, with ignition triggered either thermally (up to 350 °C) or resistively, supported by a temperature of 200 °C. In the second series, smaller chips measuring 3 mm x 3 mm were bonded using a reactive foil to achieve a bonding pressure of 100 MPa. Here, ignition was triggered by an electrical spark, supported by a temperature of 160 °C. The bonding interfaces and metallurgical reactions were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The results reveal significant differences between bonding LTCC substrates using prefabricated reactive foils and deposited RMS structures: prefabricated foils generate higher peak temperatures, often exceeding the thermal limits of the metallization system, whereas deposited RMS structures produce lower peak temperatures and offer better compatibility due to the ability to tailor specific parameters.

Reactive bonding can overcome the issues associated with conventional soldering processes, such as potential damage to heat-sensitive components and the creation of thermomechanical stress due to differing coefficients of thermal expansion. The risk of such damage can be reduced by using localized heat sources like reactive multilayer systems (RMS), which is already a well-established option in the field of silicon or metal bonding. Adapting this process to other materials, such as low temperature co-fired ceramics (LTCC), is difficult due to their differing properties, but it would open new technological possibilities. One aspect that significantly affects the quality of the bonding joints is the pressure applied during the bonding process. To investigate its influence more closely, various LTCC samples were manufactured, and cross-sections were prepared. The microscopical analysis reveals that there is an optimum range for the bonding pressure. While too little pressure results in the formation of lots of voids and gaps, most likely in poor mechanical and electrical properties, too high pressure seems to cause a detachment of the metallization from the base material.

In vivo biomedical devices are one of the most studied applications for vibrational energy harvesting. In this paper, we investigated a novel high-displacement device for harvesting heartbeats to power leadless implantable pacemakers. Due to the location peculiarities, certain constraints must be respected for the design of such devices. Indeed, the total dimension of the system must not exceed 5.9 mm to be usable within the leadless pacemakers and it must be able to generate accelerations lower than 0.25 m/s2 at frequencies of less than 50 Hz. The proposed design is an electrostatic system based on a square electret of dimension 4.5 mm. It is based on the Quasi-Concertina structure, which has a very low resonant frequency of 26.02 Hz and a low stiffness of 0.492 N/m, allowing it to be very useful in such an application. Using a Teflon electret charged at 1000 V, the device was able to generate an average power of 10.06 μW at a vibration rate of 0.25 m/s2 at the resonant frequency.

Thermoelectric generators (TEGs) are essential devices that convert thermal energy into electrical energy. TEGs are well known for their reliability, environmental friendliness, and maintenance-free operation. The fabricated low-temperature co-fired ceramic (LTCC)-based silver/silver–palladium Y-type thermoelectric generator is designed to convert a vertical distribution of heat flow into voltage through 333 laterally oriented thermocouples. It is intended to be placed between the heat exchanger and the coolant channels in the exhaust system to harvest waste thermal energy. The novel design helps to minimize the overall internal electrical resistance of the generator, which leads to higher generated electrical power. We investigated the use of aluminum-plates with pillars as heat guides combined with LTCC built-in cavities to minimize heat losses and improve the thermal yield. This technique allows increasing the electrical power by 67% compared to flat heat guides. The paper presents both simulations, based on the finite element method, and experimental measurements to validate the proposed model's accuracy. Experiments gave an output voltage of 87.3 mV and a delivered power of 260 μW (equivalent to 1.51 μW K⁻¹) at ΔT = 172 °C for the generator TEG without cavities, while these results were 94.1 mV and 306.3 μW (equivalent to 1.78 μW K⁻¹) at the same temperature difference when cavities were added.