Comment on "Comparison of air breakdown and substrate injection as mechanisms to induce dielectric charging in microelectromechanical switches" [Appl. Phys. Lett. 92, 043502 (2008)]

The purpose of this comment is to provide additional insight into the reliability of microelectromechanical capacitive switches (MEMSs) investigated by Molinero and Casta er [Appl. Phys. Lett. 92, 043502 (2008)]. We show that the presence or absence of ambient humidity determines whether the shift in the capacitance-voltage (C-V) curve of oxide-based MEMS occurs as a result of voltage stress. In humid air, negative and positive shifts in the C-V curve are observed after negative and positive bias stress. In dry air no such shifts in the C-V curve are seen. These shifts are similar to those reported on oxide-based switches by Molinero and Castaner [Appl. Phys. Lett. 92, 043502 (2008)] where they show shifts occurring in room ambient pressure, but not in vacuum. This indicates that not only air pressure but also air humidity can be responsible for shifts in MEMS. (C) 2009 American Institute of Physics. (doi: 10.1063/1.3255008

A low frequency MEMS energy harvester scavenging energy from magnetic field surrounding an AC current-carrying wire

This paper reports on a low frequency piezoelectric energy harvester that scavenges energy from a wire carrying an AC current. The harvester is described, fabricated and characterized. The device consists of a silicon cantilever with integrated piezoelectric capacitor and proof-mass that incorporates a permanent magnet. When brought close to an AC current carrying wire, the magnet couples to the AC magnetic field from a wire, causing the cantilever to vibrate and generate power. The measured average power dissipated across an optimal resistive load was 1.5 μW. This was obtained by exciting the device into mechanical resonance using the electro-magnetic field from the 2 A source current. The measurements also reveal that the device has a nonlinear response that is due to a spring hardening mechanism

A simple electrical test method to isolate viscoelasticity and creep in capacitive microelectromechanical switches

A bipolar hold-down voltage was used to study mechanical degradation in radio-frequency microelectromechanical capacitive shunt switches. The bipolar signal was used to prevent the occurrence of dielectric charging and to isolate mechanical effects. The characteristics of material stress relaxation and recovery were monitored by recording the change of the pull-in voltage of a device. The creep effect in movable components was saturated by repeated actuation to the pulled-in position, while comparison with a theoretical model confirmed the presence of linear viscoelasticity in the devices. (C) 2014 AIP Publishing LLC

Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible. (C) 2012 American Institute of Physics. (http://dx.doi.org/10.1063/1.4726116

Broadening the Bandwidth of Piezoelectric Energy Harvesters Using Liquid Filled Mass

AbstractA narrow bandwidth is one of the most challenging issues that vibrational energy harvesters have to overcome. This paper demonstrates a novel method of broadening the bandwidth without significantly reducing the peak output voltage. The method uses a liquid filled mass to create a sliding mass effect in order to broaden the bandwidth. The fluid mass increased the full-width-half-maximum (FWHM) value from 1.6Hz to 4.45Hz with no significant decrease in peak-to-peak voltage when compared to an empty mass. The fluid filled mass has a non-linear mass distribution during low frequency, high acceleration applications

Identification of the transient stress-induced leakage current in silicon dioxide films for use in microelectromechanical systems capacitive switches

Dielectric charging at low electric fields is characterized on radio-frequency microelectromechanical systems (RF MEMS) capacitive switches. The dielectric under investigation is silicon dioxide deposited by plasma enhanced chemical vapor deposition. The switch membrane is fabricated using a metal alloy which is shown to be mechanically robust. In the absence of mechanical degradation, these capacitive switches are appropriate test structures for the study of dielectric charging in MEMS devices. Monitoring the shift and recovery of device capacitance-voltage characteristics revealed the presence of a charging mechanism which takes place across the bottom metal-dielectric interface. Current measurements on metal-insulator-metal devices confirmed the presence of interfacial charging and discharging transient currents. The field-and temperature-dependence of these currents is the same as the well-known transient stress-induced leakage current (SILC) observed in flash memory devices. A simple model was created based on established transient SILC theory which accurately fits the measured data and reveals that charge exchange at the bottom metal-dielectric interface is responsible for charging currents and pull-in voltage changes in these MEMS devices. (C) 2015 AIP Publishing LLC

Location dependence of a MEMS electromagnetic transducer with respect to an AC power source

A MEMS, silicon based device with a cantilever oscillationsand an integrated magnet is presented for magnetic to electrical transduction. The cantilever structure can be configured either as an energy harvester to harvest power from an AC power line or as an AC current sensor. The positioning of the transducer with respect to the AC conductor is critical in both scenarios. For the energy scavenger, correct positioning is required to optimize the harvested power. For the current sensor, it is necessary to optimise the sensitivity of the sensor. This paper considers the effect of the relative position of the transducer with respect to the wire on the resulting electromagnetic forces and torques driving the device. It is shown here that the magnetic torque acting on a cantilever beam with an integrated magnet and in the vicinity of an alternating electromagnetic field is a very significant driver of the cantilever oscillations

Design, modelling and preliminary characterisation of microneedle-based electrodes for tissue electroporation in vivo

We analysed the use of microneedle-based electrodes to enhance electroporation of mouse testis with DNA vectors for production of transgenic mice. Different microneedle formats were developed and tested, and we ultimately used electrodes based on arrays of 500 μm tall microneedles. In a series of experiments involving injection of a DNA vector expressing Green Fluorescent Protein (GFP) and electroporation using microneedle electrodes and a commercially available voltage supply, we compared the performance of flat and microneedle electrodes by measuring GFP expression at various timepoints after electroporation. Our main finding, supported by both experimental and simulated data, is that needles significantly enhanced electroporation of testis

A study on the spatial dependence of a MEMS electromagnetic transducer

A MEMS, silicon-based device with a piezoelectric layer and an integrated magnet is presented for magnetic to electrical transduction. The cantilever structure can be configured either as an energy harvester to harvest power from an AC power line or as an AC current sensor. The positioning of the transducer with respect to the conductor is critical in both scenarios. For the energy scavenger, correct positioning is required to optimize the harvested power. For the current sensor, it is necessary to optimize the sensitivity of the sensor. This paper considers the effect of the relative position of the transducer with respect to the wire on the resulting electromagnetic forces and torques driving the device. It is shown in this paper that it is the net equivalent torque at the anchor that must be considered and not just the vertical component of the magnetic force as has been widely assumed heretofore. We show that for single wire conductors, the commonly made assumption that there exists two symmetrical power peaks at 45° either side of the wire is untrue, but rather that the net driving torque on one side of the wire can be more than an order of magnitude greater than the other

Use of portable air purifiers to reduce aerosols in hospital settings and cut down the clinical backlog

SARS-CoV-2 has severely affected capacity in the NHS, and waiting lists are markedly increasing due to downtime of up to 50 minutes between patient consultations/procedures, to reduce the risk of infection. Ventilation accelerates this air cleaning, but retroactively installing built-in mechanical ventilation is often cost-prohibitive. We investigated the effect of using portable air cleaners (PAC), a low-energy and low-cost alternative, to reduce the concentration of aerosols in typical patient consultation/procedure environments. The experimental setup consisted of an aerosol generator, which mimicked the subject affected by SARS-CoV-19, and an aerosol detector, representing a subject who could potentially contract SARS-CoV-19. Experiments of aerosol dispersion and clearing were undertaken in situ in a variety of rooms with 2 different types of PAC in various combinations and positions. Correct use of PAC can reduce the clearance half-life of aerosols by 82% compared to the same indoor-environment without any ventilation, and at a broadly equivalent rate to built-in mechanical ventilation. In addition, the highest level of aerosol concentration measured when using PAC remains at least 46% lower than that when no mitigation is used, even if the PAC’s operation is impeded due to placement under a table. The use of PAC leads to significant reductions in the level of aerosol concentration, associated with transmission of droplet-based airborne diseases. This could enable NHS departments to reduce the downtime between consultations/procedures