Development of a contactless capacitive immunosensor

In the present work, a label-free, contactless and capacitive immunosensor is developed using impedance spectroscopy, in the aim to perform low-cost immunoassays. Chapter 1 puts this work in perspective with some existing techniques, while a presentation of impedance theory used in this work is carried out in chapter 2. In Chapter 3, numerical simulations using a commercial finite element method software is carried out. The response of coplanar and face-to-face designs using an insulating layer is studied with respect to a frequency ranging from 100 Hz to 10 MHz . Two levels of capacitance were observed across the frequency range : the low frequency capacitance created by the insulating layer and the high frequency capacitance created by the solution. These capacitances depend on parameters like the solution conductivity, the distance between the electrodes, the electrodes width or the insulating layer thickness. A dimensionless parameter is defined to evaluate the quality of the geometry at high frequencies. Microchips using a coplanar design are developed in Chapter 4. They are composed of two silver electrodes drilled in a PET sheet by laser photoablation. The design of both holder and of the microchips is optimized to increase as much as possible the signal-to-noise ratio. Bovine Serum Albumin is detected by a variation of the channel conductivity. Chapter 5 introduces the design of a sensor using electrodes made of a mass market aluminium foil. The study of the frequency response of the electrodes led to the creation of a discrete analytical model. The electrodes are then mounted into a holder using a face-to-face or coplanar design. The system is characterized through the variation of several geometrical parameters (height of fluid in the reservoir, electrode surface area, solution conductivity, ...). The coplanar design is also optimized to be able to work in a holder equipped with a fluidic channel. Finally, the ability of the aluminium electrodes based sensor to monitor an adsorption is studied in Chapter 6. The resonance is used to detect the adsorption of proteins like BSA on the electrodes using coplanar and face-to-face designs. The adsorption is found to follow a Langmuir isotherm and an adsorption equilibrium constant is extracted. The second adsorbate layer is detected using a coplanar design, enabling the achievement of a immunoassay.

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.

EPFL2017

Frequency-Domain Control Design in Power Systems

Christoph Manuel Kammer

The scope of this thesis encompasses two main subjects: fixed-structure data-driven control design on one side, and control design in power systems on the other. The overall goal is to identify challenging and relevant problems in power systems, to express them as rigorous specifications from the viewpoint of control systems, and to solve them by developing and applying advanced methods in robust control. This work aims to combine expertise from both fields to open up a holistic perspective and bridge the gap between control and power systems. First, the derivation of a novel fixed-structure, data-driven frequency-domain control design method for multivariable systems is described. A key feature of the method is that only the frequency response of the plant is required for the design, and no parametric model is required. The designed controllers are fully parametrized in terms of matrix polynomial functions and can take a centralized, decentralized or distributed structure. The controller performance is formulated as H_2 and H_infinity constraints on any loop transfer function. A convex formulation of the optimization problem is derived, and it is shown that the solution converges to a locally optimal solution of the original problem. The versatility of the design method is demonstrated in various simulation examples, as well as in experiments on two electromechanical setups. Next, a frequency-domain modeling approach for power grids is discussed. A model based on dynamic phasors is developed that represents the electromagnetic and electromechanic dynamics of lines, inverters, synchronous machines and constant power loads. It also offers a modular structure that makes it straightforward to combine white-, grey- and blackbox models in a single framework. Then, the control design method and dynamic phasor model are applied in two relevant power systems case studies. First, the design of a decentralized current controller for parallel, grid-connected voltage source inverters in a typical distribution grid is considered. It is shown how performance specifications can be formulated as frequency-domain constraints in order to attenuate the resonances introduced by the output filters and coupling effects, and to provide robustness against model uncertainties and grid topology changes. The controllers for all VSIs are designed in a single step, and stability and performance is guaranteed by design. Furthermore, an approach for plug-and-play control design is presented. The results are validated in numerical simulation as well as in power-hardware-in-the-loop experiments. The second study concerns the design of a distributed controller that combines primary and secondary frequency and voltage control for an islanded, meshed low-voltage grid with any number of voltage source inverters and synchronous generators in a single framework. No assumption on the R/X-ratio of the lines is made, and it is shown how advanced control specifications such as proportional active power sharing, zero frequency steady-state error and decoupling can be formulated as constraints on the norm of weighted sensitivity functions. Furthermore, the communication delays of the distributed controller are considered exactly during the design. The controller is implemented in numerical simulation, and results show significantly improved performance as compared to the classical hierarchical structure.

EPFL2018

Development of Lamb wave resonators in thin film X-cut lithium niobate

Muhammad Faizan

Microelectromechanical systems (MEMS) is among the most revolutionary technologies of 21st century, with the applications ranging from industrial systems to consumer electronics. Using MEMS in battery-powered wireless devices has long been seen as the evolutionary step in achieving a highly miniaturized and efficient RF frontend (RFFE) for the next generation of communication systems. Piezoelectric MEMS resonators are the essential component of every RFFE as they do a phenomenal job of frequency generation and filtering while meeting the stringent specifications regarding power budget and form factor. This thesis focuses on the design, fabrication and characterization of Lamb wave resonators on thin film LN for RF filters and oscillators. The goal of the project is to improve the electromechanical coupling (k_t^2) and quality factor (Q) of the resonators by first selecting an appropriate LN cut and in-plane device orientation for the intended mode and secondly, by optimizing different geometrical parameters such as electrode coverage (duty factor), gap, bus and anchor dimensions. With the goal to improve k_t^2 of the resonator, we fabricate devices with varying electrode coverages from 20% to 70% for Al and Pt electrodes. Our results show that low electrode coverages (20% to 40%) do not have significant effect on k_t^2, but for higher electrode coverages (> 40%), k_t^2 reduces by ~35% due to the non-linear increase in static capacitance (C_o). The electrode material does not show any significant dependence on k_t^2 but devices with Al electrodes show larger Q (in our case x2) than devices with Pt electrodes due to higher resistive losses. We demonstrate devices with the highest reported k_t^2 of 31% and 40% for S0 and SH0 mode resonators with Q of 720 and 590, operating at around 500 MHz and 300 MHz, respectively, in X-cut LN. In our next effort to further enhance Q, we fabricate devices by varying the geometrical dimensions of different inactive region elements such as anchor, bus and gap of the resonator. Each element plays a very important role in containing energy within the resonator cavity resulting in Q improvement. Our measurements suggest that a square anchor (L_a=W_a), a wider bus (W_b>0.3Î») and a longer gap (g=2Î») helps to improve Q of the resonator. The highest Q of 1900 and k_t^2 of 41% is achieved for the design parameters of g=2Î», W_b= 3Î»/4, W_a= 3Î»/4 and L_a= 3Î»/4, resulting in the highest-ever achieved Figure-of-Merit (FoM) of 780 for SH0 mode resonators in X-cut LN. We also demonstrated the fabrication of high performance A1 mode resonator operating in 3-6 GHz range in Z-cut LN. Our fabricated devices exhibit k_t^2 and Q up to 28% and 300, respectively. This performance is in agreement with the FEM simulations and is suitable for the 5G RF-filter technology. Lastly, we use 2-port SH0 mode resonators to build oscillators and measure phase noise (PN) as it directly relates to its frequency stability. We investigate the dependence of PN on carrier power, Q and k_t^2 by measuring the PN for 60 oscillators using different SH0 mode resonators. Our results show that a higher carrier power and Q significantly reduces PN and improves frequency stability of the oscillator, whereas k_t^2 does not show significant influence on PN. We achieve a low phase noise of -96 dBc/Hz and -129 dBc/Hz, at the offset frequency of 1 kHz and 10 kHz, respectively, for the resonator with Q of 2500.

EPFL2021