High Capacitance Tuning Ratio and Low Pull-in Voltage MEMS Tunable Capacitors Filled with Nematic Liquid Crystal Materials

In this thesis, the effective dielectric constant of liquid crystal materials controlled by the applied voltage is derived and verified by experiments. The effective dielectric constant is derived by asymptotic expansion and perturbation method, and the parameters of materials are contained in the function. The asymptotic solution thus can be used to describe different types of liquid crystal materials controlled by applied voltage.
Furthermore, two types of experiments are taken, which are fixed-gap capacitor (LC cells), and MEMS tunable capacitor (parallel plates). Both types of capacitors are filled with air and liquid crystal materials as the dielectric materials. The asymptotic solution derived in this thesis is verified by the experiment of LC cells, and then the liquid crystal material is filled into the MEMS tunable capacitor to discuss the behavior controlled by applied voltage.
The devices are measured by Agilent E4980A Precision LCR meter, and the results shows that the tuning ratio and the pull-in voltage can both be significantly improved. For the tunable capacitor filled with air, the tuning ratio and pull-in voltage are about 54.2% and 25.5V. Once the device is filled with the liquid crystal materials, the values are improved to 122.4% and 13V. The results shows that for the tunable capacitors with same geometric designs, the tuning ratio and pull-in voltage can be significantly improved by changing the dielectric materials from air to liquid crystal materials. Even for the fixed gap capacitor filled with liquid crystal materials, the tuning ratio can still reached 2.5 to 2.6, which is similar to the dielectric anisotropy factor defined in the nomenclature.
High tuning ratio and low pull-in voltage MEMS tunable capacitors are designed and demonstrated in this thesis. Because of these characteristics, the devices can further be used as MEMS switches in wireless communication. With the significant improvements, the MEMS switches can be operated within small actuation voltage and large capacitance tuning ratio.