Xiaobin Yuan

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

Initial observation and analysis of dielectric-charging effects on RF MEMS capacitive switches

Capacitance voltage and RF-output characteristics of electrostatically actuated MEMS switches were measured under different control and stress voltages. It was found that positive voltage stress caused negative charging of the dielectric whereas negative voltage stress caused positive charging of the dielectric. This is consistent with the amphoteric nature of traps in the silicon oxynitride dielectric used for the switches. A hypothesis of charge injection in minutes and charge migration in milliseconds was proposed to explain real-time and nonsymmetrical drift of pull-down and hold-down voltages of the 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.

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

A transient charging model to predict actuation-voltage shift in RF-MEMS capacitive switches

For state-of-the-art RF MEMS capacitive switches, a dielectric-charging model was constructed to predict the amount of charge injected into the dielectric and the corresponding shift in actuation voltage. The model was extracted from measured charging and discharging transient currents on the switch dielectric under different control voltages. The model was verified against the actuation-voltage shift under different control waveforms. Duty factor and peak voltage of the control waveform were found to be critical acceleration factors for the charging effects while actuation frequency is not an acceleration factor. The model is capable of predicting the actuation-voltage shift under complex control waveforms such as the dual-pulse waveforms. For RF MEMS capacitive switches that fail mainly due to dielectric charging, the model can be used to design control waveforms that can either prolong lifetime or accelerate failure.