Reliability and failure analysis of RF-MEMS switches for space applications

The aim of this thesis is to perform an electrical characterization and several reliability tests on different kind of RF-MEMS switches in order to analyze which are the weaknesses and the strengths of this new technology. Electrical characterizations have been done using two different measurement systems. The first, based on a vector network analyzer and a power supply, has been used to test the RF performances of the devices and to extract the best actuation and deactuation voltages. The second set up, based on the internal RF signal generator of the VNA, and an 8-GHz digital signal oscilloscope, has been used to characterize the electrical performances like actuation time, release delay. Cycling stress, one of the most common test used to understand the robustness of this kind of devices, has been per- formed on different topologies of switch in order to better understand how some parameters, such as the shape of the beams or the actuation voltage, impact on the reliability of the device. Furthermore, the influence of continuous actuation stress on the reliability of dielectric-less switches has been investigated, comparing different designs and studying the variation of the main electrical parameters induced by the stress and the successive recovery phase. Other two critical issues investigated in this thesis are the sensitivity to EOS/ESD events of RF-MEMS switches under actuated and not-actuated conditions and their sensitivity to different kind of radiation, protons and X-rays. The sensitivity to EOS/ESD has been studied between RF-OUT and ground with no bias voltage applied (up-state membrane), and between RF-OUT and GND at different voltages (fully actuated / partially bended membrane). Moreover, the same devices have been characterized under HBM regime in order to study if any correlation between TLP tests and the Human Body Model exists for these devices. Concerning the radiation effects, the impact of 2MeV protons and l0keV x-rays radiation stresses has been analyzed at increasing radiation dose and during subsequent annealing at room temperature.Lo scopo di questa tesi è caratterizzare da un punto di vista elettrico ed affidabilistico diversi tipi di RF-MEMS switches al fine di analizzare punti di forza e debolezze di questa nuova tecnologia. Le caratterizzazioni elettriche sono state effettuate utilizzando due differenti set-up di misura. Il primo, composto da un network analyzer e da un generatore di tensione, è stato utilizzato per valutare le performance RF dei dispositivi ed estrarre le migliori tensioni di attuazione e disattuazione. Il secondo, basato sul generatore interno di segnale del network analyzer e da un oscilloscopio digitale è stato utilizzato per misurare i tempi di attuazione e disattuazione. Il cycling stress, uno dei metodi più comunemente utilizzati per valutare l’affidabilità di tali siwtch, è stato effettuato su diverse tipologie di dispositivi al fine di valutare quale fosse l’impatto di parametri come la forma delle struttura sospesa o la tensione di attuazione sull’affidabilità. Si è inoltre studiato l’effetto di un’attuazione continua su dispositivi privi di dielettrico, paragonando quattro diversi tipi di layout e studiando il cambiamento dei principali parametri elettrici durante lo stress e durante la fase di recupero. Altri 2 problemi critici che si sono affrontati in questa tesi sono la sensibilità ad eventi EOS/ESD di switch RF-MEMS sia attuati che disattuati e gli effetti di irragiamenti con protoni o raggi X. Test EOS/ESD sono stati condotti tra i pad RF-OUT e GND senza una tensione applicata (quindi con la membrana sospesa sollevata), e tra RF-OUT e GND a differenti tensioni (membrana a contatto e parzialmente piegata). Gli stessi dispositivi sono stati inoltre in regime HBM per vedere se esiste una correlazione tra i test condotti con TLP e lo Human Body Model. Per quanto riguarda gli irragiamenti, si è studiato l’effetto di protoni con energia pari a 2MeV e raggi X a 10KeV durante lo stress incrementando la dose e durante la fase di annealing a temperatura ambiente

EOS/ESD Sensitivity of Functional rf-MEMS Switches

The sensitivity to ESD events of electrostatically driven ohmic RF-MEMS switches under actuated and not-actuated conditions is here investigated. We have found that stiction and charge-trapping phenomena can be induced by EOS/ESD events. Preliminary results on HBM robustness with a good correlation with TLP tests are also reported. Electro-mechanical simulations have been carried out to study how the suspended membrane reacts to electrical overstress. Furthermore, we report on a new stiction mechanism, induced by the dielectric breakdown that can occur between the suspensions and the actuator electrodes, furnishing a possible guide line to a more robust design

Suspensions Shape Impact on the Reliability of RF-MEMS Redundancy Switches

The influence of the suspension shape on the electrical parameters and on the reliability of micro-machined ohmic series and shunt RF switches has been studied in this work. We have investigated how different spring constants influence the electrical parameters of RF-MEMS switches, in terms of the pull-in voltage, the pull-out voltage, and the evolution of scattering parameters during the DC sweep. The robustness to cycling stress has also been studied, considering different movable structures and bias voltages. We have also analyzed the behavior of RF-MEMS switches submitted to long continuous actuation, finding that meander based devices could suffer from extremely long release times, after an actuation period of some hours. However, straight beam based switches have not exhibited this problem. This unwanted behavior has been analyzed in terms of actuation time and RF-power level. The presence of bounces in the release phase has also been investigated on various types of switches with different topologies and suspended membrane thickness

Readout drain current dependence of programming window in nanocrystal memory cells

The different behaviour of nanocrystal memory cells in linear and subthreshold region was studied. It was found that the programming window reduces with increasing readout drain current. This peculiar behaviour derives from the presence of the discrete nanodots and it has not been observed in conventional floating gate memories

Radiation Sensitivity of Ohmic RF-MEMS Switches for Spatial Applications

The impact of 2 MeV protons and 10 keV X-rays radiation stresses on electrostatically actuated ohmic RF-MEMS switches has been analyzed at increasing radiation dose and during subsequent annealing at room temperature. Small variations of electrical parameters (actuation and release voltages) have been identified, accompanied by a strong rf-performances degradation. Monte Carlo TRIM simulations have been carried out to understand the mechanisms responsible of such degradations, finding that both NIEL and ionizing damages appear to play an important role

Polarization sensitive optical coherence tomography for zebrafish imaging

Preliminary results about the application of fibre-optic polarization-sensitive optical coherence tomography to zebrafish imaging are reported. Polarization properties of tissues are analyzed exploiting a data analysis algorithm borrowed from distributed measurements of birefringence in optical fibres. Results show the difference in polarization properties between birefringent tissues (as muscles) and non-birefringent ones

Impact of Continuous Actuation on the Reliability of Dielectric-less Ohmic RF-MEMS Switches

The influence of continuous actuation stress on the reliability of dielectric-less ohmic RF-MEMS cantilever-based switches was investigated in this work. Comparing different designs, the changing of the main electrical parameters during the stress were attributed to oxide-around-actuator charging phenomena, leading to both narrowing and shifting of traditional hysteresis-like curves. Recovery procedures were also analyzed

Rockfall precursor detection based on rock fracturing monitoring by means of optical fibre sensors

The monitoring of the acoustic emissions (AEs) due to rock fracturing allows the detection of the rockfall precursor signals, leading to a strong improvement of the real time assessment of the induced risk. A network of piezoelectric sensors (PZTs) provides reliable AE data, as testified by a series of applications in non-destructive testing, but PZTs are strongly affected by lightings and electromagnetic interference. In order to avoid such PZT drawbacks and limitations and therefore to allow the rock fracturing monitoring in unstable slopes, two FOS architectures (referred to as fibre coil sensor and ferrule top cantilever) have been recently proposed in previous works from the same authors. In this paper, the two sensors are tested in a more realistic scenario, by monitoring AE in a rock block in which crack is induced by highly expansive mortar; complementary activities, mainly aimed at the recognition of the type of expected signals and optimisation of the sensor array in the framework of the monitoring system, are also described here