David I. Forehand

Modeling and characterization of dielectric-charging effects in RF MEMS capacitive switches

For the first time, charging and discharging of traps in the dielectric of state-of-the-art RF MEMS capacitive switches were characterized in detail. Densities and time constants of different trap species were extracted under different control voltages. It was found that, while charging and discharging time constants are relatively independent of control voltage, steady-state charge densities increase exponentially with control voltage. A simple charge model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage. Good agreement was obtained between the model prediction and experimental data.

Acceleration of Dielectric Charging in RF MEMS Capacitive Switches

To design and validate accelerated life tests of RF MEMS capacitive switches, acceleration factors of charging effects in switch dielectric were quantitatively characterized. From measured charging and discharging transient currents at different temperatures and control voltages, densities and time constants of dielectric traps were extracted. A charging model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage under different acceleration factors such as temperature, peak voltage, duty factor, and frequency of the control waveform. Agreement was obtained between the model prediction and experimental data. It was found that temperature, peak voltage, and duty factor were critical acceleration factors for dielectric-charging effects whereas frequency had little effect on charging

High-Cycle Life Testing of RF MEMS Switches

RF MEMS capacitive switches capable of order-of-magnitude impedance changes have demonstrated operating lifetimes exceeding 100 billion switching cycles without failure. In situ monitoring of switch characteristics demonstrates no significant degradation in performance and quantifies the charging properties of the switch silicon dioxide film. This demonstration leads credence to the mechanical robustness of RF MEMS switches.

Temperature variation of actuation voltage in capacitive MEMS switches

A simple theoretical model for temperature variation of actuation voltage for fixed-fixed beam switches is developed and applied to model capacitive radio frequency microelectromechanical systems switches. Measured data from these switches fit very well with the model over a 120/spl deg/C operating temperature range, demonstrating an average 0.127V//spl deg/C to 0.132V//spl deg/C with less than 8% deviation between measured and modeled results. The proposed model provides valuable insight into the factors impacting variability of actuation voltage over broad temperature ranges.

A transient SPICE model for dielectric-charging effects in RF MEMS capacitive switches

A transient simulation program with integrated circuit emphasis (SPICE) model for dielectric-charging effects in RF microelectromechanical system (MEMS) capacitive switches was developed and implemented in a popular microwave circuit simulator. In this implementation, the dielectric-charging effects are represented by R-C subcircuits with the subcircuit parameters extracted from directly measured charging and discharging currents in the picoampere range. The resulted model was used to simulate the actuation-voltage shift in switches due to repeated operation and dielectric charging. Agreement was obtained between the simulated and measured actuation-voltage shift under various control waveforms. For RF MEMS capacitive switches that fail mainly due to dielectric charging, the present SPICE model can be used to design control waveforms that can either prolong lifetime or accelerate failure

Robustness of RF MEMS Capacitive Switches With Molybdenum Membranes

This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

Superposition Model for Dielectric Charging of RF MEMS Capacitive Switches Under Bipolar Control-Voltage Waveforms

Bipolar control-voltage waveforms, under which the control voltage alternates between positive and negative after each cycle, have been proposed to mitigate dielectric charging in electrostatically actuated RF microelectromechanical system capacitive switches. In this study, dielectric charging under bipolar waveforms is modeled and characterized quantitatively. In general, the experimental results agree with predictions based on the superposition of unipolar charging models that are extracted under positive and negative voltages, respectively. The basic assumptions for such a superposition model are examined in detail and validated experimentally. The current analysis indicates that, while bipolar waveforms can reduce charging, it is difficult to fine tune the waveforms to completely eliminate charging.

Dielectric Charging of RF MEMS Capacitive Switches under Bipolar Control-Voltage Waveforms

Bipolar control-voltage waveforms have been proposed to mitigate dielectric charging in RF MEMS capacitive switches. In this work, for the first time, dielectric charging under bipolar waveforms is modeled and characterized quantitatively. In general, the experimental results agree with predictions based on the superposition of charging models that are extracted under either positive or negative voltage only. The present analysis indicates that, while bipolar waveforms can reduce charging, it is difficult to fine tune the waveforms to completely eliminate charging.

Top vs. bottom charging of the dielectric in RF MEMS capacitive switches

Using a movable top electrode, for the first time, top vs. bottom charging of the dielectric in metal/insulator/metal capacitors is delineated. For the Al/SiO2/Cr structure used in RF MEMS capacitive switches, charge injection from Al into the top of SiO2 was found to have a higher threshold voltage, faster charging time, and slower discharging time than charge injection from Cr into the bottom of SiO2. The higher threshold is attributed to non-ideal contact geometry and chemistry. The faster charging time is attributed to the exponential voltage dependence. The slower discharging time is attributed to diffusion across SiO2. Since top charging is more critical to switch performance and reliability than bottom charging, understanding the trade off of top vs. bottom charging can help minimize their undesirable effects.

Temperature Acceleration of Dielectric Charging in RF MEMS Capacitive Switches

Temperature acceleration of dielectric charging effects in state-of-the-art RF MEMS capacitive switches was characterized and modeled. From the measured charging and discharging transient currents at different temperatures, densities and time constants of traps in the dielectric were extracted. It was found that, while charging and discharging time constants are relatively independent of temperature, steady-state charge densities increase with temperature. A charging model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage under different temperatures. Agreement was obtained between the model prediction and experimental data