Mahmoud Khater

Dynamic actuation methods for capacitive MEMS shunt switches

We develop dynamic actuation methods for capacitive MEMS shunt switches. We show that the dynamic actuation voltage is significantly less than the static actuation voltage and demonstrate 60% reduction in the actuation voltage. We also show that this reduction in the actuation voltage depends on the specific dynamic switching technique adopted. For a given operating condition, the minimum realizable switching time is that obtained using static switching. However, we developed a dynamic switching method that yields comparable switching time to that minimum. We also found that squeeze-film damping is the dominant damping mechanism for a shunt switch with a relatively slender bridge (aspect ratio of 11:1).

A Mass Sensing Technique for Electrostatically-Actuated MEMS

We propose a technique to increase the sensitivity and simplify the process of measuring minute masses using electrostatically-actuated MEMS. The sensor is composed of a cantilever beam connected to a rigid plate at its free end and coupled to an electrode underneath it. The method depends on the observation that the sensitivity of an electrostatically-actuated MEMS is highly enhanced when the driving voltage is close to the pull-in limit. We study two cases: the device actuated by a static force (DC voltage) and a dynamic force (combined AC and DC voltage). Sensitivity analysis is used to estimate the minimum detectable mass near static pull-in and near a dynamic pull-in point due to a cyclic-fold bifurcation.Copyright

Measuring the Quality Factor in MEMS Devices

This paper demonstrates and compares different experimental techniques utilized to estimate the quality factor (Q) and natural frequency from non-contact measurements of Microelectromechanical Systems (MEMS) motions. The relative merits of those techniques are contrasted in Q factor estimation for a cantilever beam MEMS actuator, operated in three configurations: free standing, arc-shaped, and s-shaped. It is found that damping estimation techniques that seek to minimize the deviation between the response of an “assumed” linear oscillator and the measured time-history of the motions are superior to those traditional techniques, such as logarithmic decrement and half-power bandwidth. Further, it is found that Q increases three-fold as the actuator contact with the substrate evolves from a line to an area.

Binary MEMS gas sensors

A novel sensing mechanism for electrostatic MEMS that employs static bifurcation-based sensing and binary detection is demonstrated. It is implemented as an ethanol vapour sensor that exploits the static pull-in bifurcation. Sensor detection of 5 ppm of ethanol vapour in dry nitrogen, equivalent to a detectable mass of 165 pg, is experimentally demonstrated. Sensor robustness to external disturbances is also demonstrated. A closed-form expression for the sensitivity of statically detected electrostatic MEMS sensors is derived. It is shown that the sensitivity of static bifurcation-based binary electrostatic MEMS sensors represents an upper bound on the sensitivity of static detection for given sensor dimensions and material properties.

A Tunable MEMS Magnetic Sensor

This paper introduces a tunable MEMS magnetic field sensor. It uses torsional vibrations excited by Lorentz force to measure the strength of external magnetic fields. The sensor sensitivity and dynamic range can be tuned on-the-fly by varying its dc bias. Experimental demonstration shows that the sensor sensitivity can be tuned in the range of 0.139–0.283V/mT as the bias voltage varies from 0 to 6V in air and in the range of 0.038–0.955V/mT as the bias voltage varies from 0 to 5V in vacuum. While the sensor can operate in either a forced or a resonant mode, it achieves higher sensitivity and bandwidth when operating near its first torsional resonance. [2015-0285]

Nonlinear Parameter Identification of a Resonant Electrostatic MEMS Actuator

We experimentally investigate the primary superharmonic of order two and subharmonic of order one-half resonances of an electrostatic MEMS actuator under direct excitation. We identify the parameters of a one degree of freedom (1-DOF) generalized Duffing oscillator model representing it. The experiments were conducted in soft vacuum to reduce squeeze-film damping, and the actuator response was measured optically using a laser vibrometer. The predictions of the identified model were found to be in close agreement with the experimental results. We also identified the noise spectral density of process (actuation voltage) and measurement noise.

Contact damping in microelectromechanical actuators

We examine the significance of the energy loss mechanisms active in electrostatic MEMS actuators. We find that the dominant loss mechanism changes depending on the actuator mode of operation. We find that the active mechanisms in the order of their significance are: fluid-structure interactions dominant for actuators operating in air, actuator-substrate interactions dominant for actuators in contact with a substrate under vacuum, and intrinsic loss mechanisms dominant for actuators in-flight under vacuum. Further, experimental results show that the quality factor of an electrostatic MEMS actuator drops drastically as the actuator first comes into line contact with a substrate. As the contact area expands along the actuator length, the quality factor increases. Measurements under 1 Torr vacuum show a three-fold increase in the quality factor as the contact area expands from a line to 30% of the actuator area. This increase in the quality factor is attributed to the drop in the contribution of friction forces into energy losses as contact expands and adhesion forces increase.

Low Voltage Electrostatic Actuation for MEMS Actuator Using Frequency Modulation

In this paper, we derive and present an analytical model of the parallel-plate electrostatic actuator coupled with the resonant drive circuit when it is driven by a frequency-modulated input signal. Using this analytical model, we determine the stability of the fixed points of the actuator without linearization process. Furthermore, we study the effect of parasitic capacitance and the quality factor of the resonant drive circuit on the operation range and stability of the electrostatic actuator.© 2013 ASME

Techniques for dynamic analysis of bonding wire

Abstract This study describes new experimental techniques for dynamic analysis of bonding wire. The techniques employ a laser Doppler vibrometer (LDV) for non-contact measurement of wire response to transient, impact, and steady-state (harmonic) excitations. The first technique determines the transients and response time of the wire to current pulse excitations. The second technique, employs impacts delivered by a solenoid actuator to perform modal analysis on bonding wire and obtain their natural frequencies. Steady-state experimental techniques are also developed to obtain the mode shapes, nodal points, and frequency-response curves of bonding wire under thermal (current) excitation. These techniques are deployed to study the response of 300 μ m diameter Aluminum and Aluminum coated Copper bonding wires to DC and AC currents. The experimental results are interpreted and verified by comparing them to numerical results obtained from finite element analysis. This study experimentally measures and reports, for the first time, the second and fourth in-plane and the second out-of-plane bending mode shapes of bonding wire.

Modal response of bonding wires under thermal loading

This paper reports on the influence of thermal loading on the mechanical behavior of bonding wires. First, an experimental technique is developed to measure the quasi-static displacement of bonding wire. It is then deployed to measure the displacement, as well as peak temperature, of three different types of bonding wires to identify their loading conditions under DC current. An experimental technique is also developed and deployed to study of the modal response of bonding wires under thermal loads. Experimental results show a drop in the natural frequency of bonding wires with increased thermal loads. The experimental procedures developed here and applied to thick bonding wires offer a template for quasi-static and modal analysis of thin bonding wires under thermal loads at the micro- and nano-scales.