**7. Acknowledgment**

262 Microelectromechanical Systems and Devices

In conclusion, this chapter has been organized describing the technological aspects for manufacturing both MIMs and RF MEMS switches, and discussing, on the base of several experimental findings, the theoretical framework for the interpretation of the measured charging effects. In particular, the theoretical approach for charging occuring in the exploited devices has been based on the Poole-Frenkel effect and it has been related to the involved polarization mechanisms. Many structures have been studied, looking for the most promising ones to be used for the actuation of RF MEMS switches, minimizing the charging

Two configurations of RF MEMS switches using electrostatic actuation, and several MIMs devices simulating the RF MEMS actuation pads, with various dielectric materials and

As experienced in the RF MEMS measurements, and well established in literature, the charge stored in the dielectric material used for the actuation pads creates an electric field that is always opposite with respect to the electric field generated by the actuation voltage. This is evident in the case of an uni-polar actuation signal, with an increase in the actuation

Mainly, charging is responsible for sticking, and it is also related to the increase of the actuation voltage, especially under uni-polar DC biasing. By using lower actuation voltages or a bi-polar scheme this effect is more under control and compliant with ground and space applications, which should not overcome 50 volt of bias to be really appealing in several

The process necessary to trap and de-trap the carriers in the uni-polar scheme can be described mainly by the Poole-Frenkel effect; it is very slow, and the initial conditions for the device should need long times to re-obtain the same actuation voltage. To accelerate the restoring mechanism, a bi-polar actuation scheme was applied to the same devices, and from the experiment it turns out that the gradient experienced by the switch under test helps a faster de-trapping mechanism, giving back the initial value of the actuation conditions. Actually, the voltage difference necessary for the successive actuations in the bipolar scheme is always constant and the absolute value of the actuation and de-actuation

For the MIM structures, a comparison has been performed between different materials and electrodes to simulate the RF MEMS actuation pads. From the measurements, it turns out that the change of interface and of the dielectric material, as well as the deposition technique used for obtaining the dielectric layer, are critical choices to activate charging mechanisms. The breakdown is not critical for structures with Poly-silicon electrodes. Usually *VB* 100 V is measured, while metal bottom electrodes have *VB* ≤ 50-60 V. On the other hand all the exploited dielectric materials look like not ideal, as a linear response of the current Vs the applied voltage is recorded already at low voltage levels, thus demonstrating a not negligible resistive contribution of the bulk of the capacitor. Another possible mechanism for conduction could be the presence of Poly-silicon: the dielectric interface can probably be considered as a sort of MOS with a poly-silicon p-doped and a thin non-ideal dielectric

Charging of the samples is obtained when successive ramps are applied, as evidenced from the shift of the I Vs V characteristics by means of the application of positive and negative voltages. Moreover, the de-charging of the MIM is very slow, and also after one day there is

voltage for the switch, and it was confirmed in our measurements.

voltages too, at least for a limited number of actuations.

**6. Conclusion** 

effects.

sub-systems.

layer.

electrodes, have been measured.

Work partially funded by the European Space Agency (ESA) Contract 20847/07/NL/GLC "High Reliability MEMS Redundancy Switch".

Adriano Cola from CNR-IMM Lecce and Luigi Mariucci from CNR-IMM Roma are kindly acknowledged for helpful discussions on charge effects in MIM structures.
