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

在本論文中，使用漸進展開法以及微擾理論簡化得到液晶材料受到外加電壓影響的公式解。在此公式中，包含有液晶材料的各項材料參數，有別於數值解，可適用於不同的液晶材料，並可更進一步地探討液晶材料在不同幾何環境下的行為反應。 本論文包含了兩種實驗結構設計，分別為固定距離的電容(晶胞)以及可變距離的電容(微機電式平行板可變電容)，每種結構分別使用空氣以及液晶作為介電材料。在晶胞的實驗中，其結果可用於驗證本論文推導出之公式，並確認本公式可適用各項材料的可能性假設。將液晶材料充填於微機電式平行板可變電容中，並利用公式對此系統進行預測及實驗驗證。經量測後，可變電容使用空氣作為介電材料時，電容調變量之比例為54.2%，而系統之吸附電壓約為25.5伏特。在充填液晶作為介電材料後，電容調變量比例和吸附電壓都得到了顯著的改變，分別為122.4%以及13伏特。在實驗結果中，使用一般的平行板電容，在不改變幾何尺寸設計的情況下，僅將介電材料更換成液晶材料，電容調變量即可得到明顯的提昇。即使使用固定距離之電容(晶胞)，其電容調變量比例仍然可以達到2.5，約略為液晶材料之介電常數異向性比值( ），如符號表內所示。 由於高電容調變量以及低吸附電壓之特性，使得充填液晶材料的可變電容能夠在較小的電壓範圍內獲得較高的電容調變量。此特性能夠應用在微機電式開關(MEMS switches)上，以增加無線通訊中的開關元件之性能，對於無線通訊微波開關的電性品質會有很高的提升。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