Role of the electro-thermo-mechanical multiple coupling on the operation of RF microswitch

A phenomenological approach is proposed to identify some effects occurring within the structure of the microswitch conceived for radio frequency application. This microsystem is operated via a nonlinear electromechanical action imposed by the applied voltage. Unfortunately, it can be affected by residual stress, due to the microfabrication process, therefore axial and flexural behaviors are strongly coupled. This coupling increases the actuation voltage required to achieve the so-called ‘‘pull-in'' condition. Moreover, temperature may strongly affect strain and stress distributions, respectively. Environmental temperature, internal dissipation of material, thermo-elastic and Joule effects play different roles on the microswitch flexural isplacement. Sometimes buckling phenomenon evenly occurs. Literature show that all those issues make difficult an effective computation of ‘‘pull-in'' and ‘‘pull-out'' voltages for evenly distinguishing the origin of some failures detected in operation. Analysis, numerical methods and experiments are applied to an industrial test case to investigate step by step the RF-microswitch operation. Multiple electro-hermomechanical coupling is first modeled to have a preliminary and comprehensive description of the microswitch behavior and of its reliability. ‘‘Pull-in'' and ‘‘pull-out'' tests are then performed to validate the proposed models and to find suitable criteria to design the RF-MEM

Statistical investigation of the mechanical and geometrical properties of polysilicon films through on-chip tests

In thiswork,we provide a numerical/experimental investigation of themicromechanics-induced scattered response of a polysilicon on-chip MEMS testing device, whose moving structure is constituted by a slender cantilever supporting a massive perforated plate. The geometry of the cantilever was specifically designed to emphasize the micromechanical effects, in compliance with the process constraints. To assess the effects of the variability of polysilicon morphology and of geometrical imperfections on the experimentally observed nonlinear sensor response, we adopt statistical Monte Carlo analyses resting on a coupled electromechanical finite element model of the device. For each analysis, the polysilicon morphology was digitally built through a Voronoi tessellation of the moving structure, whose geometry was in turn varied by sampling out of a uniform probability density function the value of the over-etch, considered as the main source of geometrical imperfections. The comparison between the statistics of numerical and experimental results is adopted to assess the relative significance of the uncertainties linked to variations in the micro-fabrication process, and the mechanical film properties due to the polysilicon morphology

Design Modelling and Mechanical/Acoustic Characterization of Piezoelectric Micro Ultrasound Transducers

This master thesis is realized in STMicroelectronics thanks to a collaboration between the company and Politecnico di Milano. The activity is aimed at modelling and experimentally characterizing a Piezoelectric Micro-machined Ultrasound Transducer (PMUT). This is a new generation MEMS (Micro Electro Mechanical System) able to send and receive ultrasound waves by exploiting the piezoelectric effect: here the attention will be focused on the sending mode only. Throughout all the activity a continuous comparison between numerical simulation and experimental results is proposed. This approach is the typical work flow to launch a product into the market. The design modelling is done by using the finite element software COMSOL Multiphysics 5.6 while the laboratory campaign is carried out through Polytec MSA500 and other electronic equipment. The analysis performed are mainly focused on investigating the static and dynamic mechanical behavior of the device and only as a closing section the emitted acoustic field has been considered. Concerning the mechanical characterization, the main fields of investigation are the deformed configurations both of the single membrane and the whole device, the resonance frequencies of the membranes, the dynamic oscillation and the cross-talk phenomena through which the membranes within the same die can interact. Starting from the distorted geometry of the membranes, this is due to the fabrication process which introduces residual stresses and in turn causes a non flat configuration of the membrane: it is noted that the initial upward deformation flattens by applying an increasing DC voltage. Similarly, even the deformation of the die is caused by the presence of residual stresses. Going on, the modal analysis is performed: the first six modes are evaluated to have a complete characterization but the only one exploited in applications is the first one, having a frequency equal to 140kHz. Once the frequency is known, a dynamic analysis is carried out. The membranes are activated by means of a single sinusoidal voltage signal at the resonance frequency and the oscillation ring down is analyzed. Thanks to these measurements, it has been possible to measure the damping of the device by computing the Q factor. This is carried out in presence of Air and Vacuum and the values obtained are respectively 22 and 182: in this way the fluid and mechanic contributions to the damping are divided. Furthermore, by studying the oscillation ring down it appears the need to develop a non linear hysterical piezoelectric model to simulate the dynamic behavior of PZT layer: it will be part of the future activity. Subsequently, the presence of the undesired phenomena of cross-talk has been experimentally investigated. Because of this effect, the membranes can interact each other and the oscillation of one membrane can put in motion the close ones. The analysis has been performed in vacuum and air: it is noted that the acoustic contribution to the cross-talk has a higher influence and in particular the communication occurs through the back cavities. The last part of the thesis is devoted to the acoustic measurements of the emitted field in terms of directionality and sound pressure level. The radiation pattern of the emitted acoustic field by the membrane is simulated by means of a 2D axysimmetric model. Moreover, the pressure intensity has been evaluated at 2cm over the membrane both through simulation and experimentally: a mismatch is noted and it is due to the inability of the model to consider the oscillation cross-talk of the other membranes. From here comes the second future development to be investigated: the emitted acoustic field considering the oscillation of the other membranes or avoid the cross-talk by changing the design of the device, i.e closing the cavity at the bottom of the membranes. This thesis is the starting point of future activities aimed at modelling the non linear hysterical piezoelectric behavior to better match the dynamic response and studying the cross-talk effect in the acoustic emission.Universidad de Sevilla. Máster en Ingeniería Industria

Characterization of Residual Stress in Microelectromechanical Systems (MEMS) Devices Using Raman Spectroscopy

Due to the small scale of MEMS devices, the inherent residual stresses during the deposition processes can affect the functionality and reliability of the fabricated devices. Residual stress often causes device failure due to curling, buckling, or fracture. Currently, few techniques are available to measure the residual stress in MEMS devices. In this dissertation, Raman spectroscopy is used to measure and monitor the residual and induced stresses in MUMPs polysilicon MEMS devices. Raman spectroscopy was selected since it is nondestructive, fast, and provides potential in situ stress monitoring. Raman spectroscopy scans on unreleased and released MEMS fixed-fixed beams, cantilevers, and micromirror flexures were performed to obtain residual stress profiles. The profiles are compared to analytical models to assess the accuracy of Raman spectroscopy. I performed post-processing thermal anneals, phosphorous diffusions and phosphorous ion implantations to characterize the residual stress changes within MEMS devices. From post-processing experiments, the Raman residual stress profiles on MUMPs structures indicate a stress reduction by over 90%, which is verified with on-chip test structures. The reduced residual stress levels can improve the performance, reliability, and yield of the MEMS devices as they become smaller. In addition, I present the first Raman stress measurements in III-V MEMS

Electrostatic diaphragm micropump electro-fluid-mechanical simulation

In this work, a fully-silicon mechanical displacement micropump is proposed and investigated. Electrostatic actuation of a flexible diaphragm is used to generate the pressure difference required to transport the fluid at the microscale. The study is carried out by exploiting the Finite Element method in a multiphysics framework, considering simplified geometries and boundary conditions. These investigations suggest the possibility to adopt the proposed device for applications in biomedical and biological fields. Achievable stroke volumes and flow rates are computed: values are in line with those obtained for similar devices presented in the literature

RF-MEMS switches for reconfigurable antennas

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Reconfigurable antennas are attractive for many military and commercial applications where it is required to have a single antenna that can be dynamically reconfigured to transmit or receive on multiple frequency bands and patterns. RF-MEMS is a promising technology that has the potential to revolutionize RF and microwave system implementation for next generation telecommunication applications. Despite the efforts of top industrial and academic labs, commercialization of RFMEMS switches has lagged expectations. These problems are connected with switch design (high actuation voltage, low restoring force, low power handling), packaging (contamination layers) and actuation control (high impact force, wear, fatique). This Thesis focuses on the design and control of a novel ohmic RF-MEMS switch specified for reconfigurable antennas applications. This new switch design focuses on the failure mechanisms restriction, the simplicity in fabrication, the power handling and consumption, as well as controllability. Finally, significant attention has been paid in the switch’s electromagnetic characteristics. Efficient switch control implies increased reliability. Towards this target three novel control modes are presented. 1) Optimization of a tailored pulse under Taguchi’s statistical method, which produces promising results but is also sensitive to fabrication tolerances. 2) Quantification of resistive damping control mode, which produces better results only during the pull-down phase of the switch while it is possible to be implemented successfully in very stiff devices. 3) The “Hybrid” control mode, which includes both aforementioned techniques, offering outstanding switching control, as well as immunity to fabrication tolerances, allowing an ensemble of switches rendering an antenna reconfigurable, to be used. Another issue that has been addressed throughout this work is the design and optimization of a reconfigurable, in pattern and frequency, three element Yagi-Uda antenna. The optimization of the antenna’s dimensions has been accomplished through the implementation of a novel technique based on Taguchi’s method, capable of systematically searching wider areas, named as “Grid-Taguchi” method

Review on the Modeling of Electrostatic MEMS

Electrostatic-driven microelectromechanical systems devices, in most cases, consist of couplings of such energy domains as electromechanics, optical electricity, thermoelectricity, and electromagnetism. Their nonlinear working state makes their analysis complex and complicated. This article introduces the physical model of pull-in voltage, dynamic characteristic analysis, air damping effect, reliability, numerical modeling method, and application of electrostatic-driven MEMS devices

Multiphysics modeling approach for micro electro-thermo-mechanical actuator: failure mechanisms coupled analysis

The lifetime of micro electro-thermo-mechanical actuators with complex electro-thermo-mechanical coupling mechanisms can be decreased significantly due to unexpected failure events. Even more serious is the fact that various failures are tightly coupled due to micro-size and multi-physics effects. Interrelation between performance and potential failures should be established to predict reliability of actuators and improve their design. Thus, a multiphysics modeling approach is proposed to evaluate such interactive effects of failure mechanisms on actuators, where potential failures are pre-analyzed via FMMEA (Failure Modes, Mechanisms, and Effects Analysis) tool for guiding the electro-thermo-mechanical-reliability modeling process. Peak values of temperature, thermal stresses/strains and tip deflection are estimated as indicators for various failure modes and factors (e.g. residual stresses, thermal fatigue, electrical overstress, plastic deformation and parameter variations). Compared with analytical solutions and experimental data, the obtained simulation results were found suitable for coupled performance and reliability analysis of micro actuators and assessment of their design

A micromachined zipping variable capacitor

Micro-electro-mechanical systems (MEMS) have become ubiquitous in recent years and are found in a wide range of consumer products. At present, MEMS technology for radio-frequency (RF) applications is maturing steadily, and significant improvements have been demonstrated over solid-state components. A wide range of RF MEMS varactors have been fabricated in the last fifteen years. Despite demonstrating tuning ranges and quality factors that far surpass solid-state varactors, certain challenges remain. Firstly, it is difficult to scale up capacitance values while preserving a small device footprint. Secondly, many highly-tunable MEMS varactors include complex designs or process flows. In this dissertation, a new micromachined zipping variable capacitor suitable for application at 0.1 to 5 GHz is reported. The varactor features a tapered cantilever that zips incrementally onto a dielectric surface when actuated electrostatically by a pulldown electrode. Shaping the cantilever using a width function allows stable actuation and continuous capacitance tuning. Compared to existing MEMS varactors, this device has a simple design that can be implemented using a straightforward process flow. In addition, the zipping varactor is particularly suited for incorporating a highpermittivity dielectric, allowing the capacitance values and tuning range to be scaled up. This is important for portable consumer electronics where a small device footprint is attractive. Three different modelling approaches have been developed for zipping varactor design. A repeatable fabrication process has also been developed for varactors with a silicon dioxide dielectric. In proof-of-concept devices, the highest continuous tuning range is 400% (24 to 121 fF) and the measured quality factors are 123 and 69 (0.1 and 0.7 pF capacitance, respectively) at 2 GHz. The varactors have a compact design and fit within an area of 500 by 100 μm