Design and fabrication of resonators to realize wireless, wearable and totally passive sensors

During the recent years, the technology behind sensors has taken enormous steps in developing new systems able to unobtrusively monitor our lives. Indeed, thanks to flexible electronics, the brittle and rigid chips can be replaced with a new generation of devices able to conformal adapt their shape to the human body and to improve comfort. In parallel, the Internet of Things (IoT) has become one of the major markets; and the prevalence of traffic in telecommunication networks is now produced by the objects around us. Smart sensors are gaining interest even more in modern networks where a multitude of standards for their connection is expanding the umbrellas of 5G and 6G. In this scenario, a crucial step forward can be the use of wireless nodes. This trend has led to a new generation of devices and microsystems, composed by microprocessors and/or radio frequency circuitry, to send data remotely, at the cost of larger footprints and higher power consumption. Flexible radiofrequency resonators present some key features necessary for the development of wearable, wireless, and low consuming non-invasive sensing platforms. These devices combine the advantages of flexibility and wearability with remote sensing capabilities. Indeed, working in the Radio Frequencies (RF), they can be directly interrogated by travelling electromagnetic fields emitted through antennas, avoiding additional circuitry for the transmission of data. Moreover, several resonators can be integrated into a single chip, enhancing the sensing capabilities and increasing the performance of the systems. In the literature, several flexible radiofrequency resonators are reported, with microstrip and Micro-Electro Mechanical System (MEMS) technologies among the most suitable for integration on flexible materials. MEMS are one of the most interesting categories of microwave resonators. In this kind of devices, the ElectroMagnetic (EM) wave perturbations are converted into mechanical perturbations. Shortening of mechanical wavelength leads to very small resonating areas working in the Ultra High Frequency (UHF) range, with footprints of the order of tens of micrometres. The main drawback of this strategy is related to its complicated fabrication processes, which are not compatible with standard microfabrication processes when flexible substrates are taken into account. 6 Differently, microstrip resonators can be fabricated in a very straightforward manner even on flexible materials with big yields and cost-effective processes, exploiting 3D printing techniques. The performance of microstrip resonators is comparable with MEMS at the cost of larger footprints; therefore, the working frequency of these resonators has to be high enough to be miniaturized and allow the development of small sensing platforms. In a nutshell, flexible microwave resonators represent a disruptive technology in the quest for smart and wearable sensor nodes. Although their high potential, there are several challenges to be addressed. In this thesis, the design and the fabrication of flexible microwave resonators and antennas have been exploited and investigated. The theory and the fabrication techniques behind the development of a flexible RF-resonator are treated. The development of two classes of resonators has been reported. More in specific, thin-film flexible MEMS resonators have been designed through Finite Element Method (FEM) models and by an innovative approach exploiting calibration procedures. Experimental data and fabrication tolerances can be included in the estimation of the resonant frequency of the resonator using Monte Carlo simulations with a much lower computational cost than previous simulations. The design approach has been validated through the fabrication of a standard silicon thin-Film Bulk Acoustic wave Resonator (FBAR). Moreover, the same technique has been applied to the development of pass-band FBAR filters. This possibility has been validated through the reverse engineering of an existing device. Thanks to this design approch, two generations of MEMS resonators have been fabricated: the first by a more straightforward process, grown directly on a flexible polyimide substrate layer and tested as a gravimetric sensor; the second one, characterized by a suspended structure, exploiting a flexible Kapton substrate and suspended by a polymeric membrane. In this case, several resonators working in parallel have been integrated into a combined device. After the development of MEMS resonators, a microstrip-based Combined Complementary Split Ring Resonator (C-CSRR) has been produced. The device is composed of two identical CSRRs and its design has been performed using the Finite Difference Time Domain (FDTD) method. Then the fabrication has been performed using a multi-material 3D printer on a 200 µm Kapton substrate. The sensor demonstrates high sensitivity to water droplets and temperature when 7 immersed in liquids and it can be exploited for skin sensing or ingestible applications. Finally the development of flexible anntennas has been performed producing two different devices. The first is a patch multilayer antenna based on a polyehtilene naphthalate substrate whose radiation properties have been enhanced through the integration with a Split Ring Resonator (SRR) in between the top and the ground planes. The second proposed antenna has been a Planar Inverted-F Antenna (PIFA) based on the same substrate, e.g. PolyethilenE Naphthalate (PEN), but characterised by a planar geometry and a shorting pin between the positive and the negative arms, which reduce its electrical length. Both two the antennas have been designed using FDTD models and fabricated exploiting a multimaterial 3D printer