RF micro-electro-mechanical devices for 0.8-2.5 GHz applications in mobile terminals

Abstract

This thesis presents a wide tuning range micro-electro-mechanical (MEM) capacitor. The two-gap MEM capacitor has a measured nominal capacitance of 1.58 pF and achieves a tuning range of 2.25:1 with parasitic capacitance. When all parasitic capacitance to the substrate are extracted the measured nominal capacitance is 1.15 pF and the tuning range is 2.71:1. The device is made of electroplated gold and has a Q of 66 at 1 GHz, and 53 at 2 GHz. In addition, a novel three-state capacitor is presented. Measured capacitance of the first, the second and the third state are 0.86 pF, 1.61 pF and 3.68 pF, respectively.

A novel temperature-compensated two-state microelectromechanical (MEM) capacitor is presented. The principle to minimize temperature dependence is based on geometrical compensation and can be extended to other devices such as continuously tunable MEM capacitors. The compensation structure eliminates the effect of intrinsic and thermal stress on the device operation. This leads to a temperature-stable device without compromising the quality factor (Q) or the voltage behavior. The compensation structure increases the robustness of the devices, but does not require any modifications to the process. Measurement results verify that the OFF and ON capacitance change is less than 6 % and the pull-in voltage is less than 5 % when the temperature is varied from −30 °C to +70 °C. In addition to the temperature stability, the charging of the dielectric layer is studied and a new continuous reliability measurement set-up is presented.

This thesis describes important design principles of electrostatically actuated MEM capacitors. Key design principles, such as temperature compensation, calculation of mechanical properties, and calculation of electrical properties of MEM capacitor are studied in detail. A new design principle that describes how pull-in and release voltage ratio is only dependent on up and down capacitance ratio and not on the mechanical properties such as a spring constant is also derived. In addition, it is shown how the RF signal affects the voltage behavior of the MEM capacitor. Two-state, three-state and continuously tunable MEM capacitors are designed and fabricated using presented design principles.

Modeling, fabrication and analysis of a truly three-dimensional high-quality-factor toroidal inductor using polymer replication processes is presented. The critical dimensions are in the micrometer range, and the applied manufacturing method is based on the polymer replication. Electrical measurements show that the inductor with an inductance of 6.0 nH exhibits a Q of 37 at 1 GHz and a peak quality factor of 50 at a frequency of 3 GHz. Furthermore, the applied manufacturing technique can be extended to become a flexible packaging platform.