Research On RF MEMS Resonator

Due to the advantage of small size, low power consumption, high Q and thecompatibility to IC fabrication process, RF MEMS resonator becomes a potentialcandidate of Quartz resonator. Although with so many merits, MEMS resonator,especially capacitive MEMS resonator is mostly in lab research, there are still severalkey issues such as high motional impedance needing to be solved before its massapplication.The factors that influence the performance of capacitive MEMS resonator arediscussed in this paper. With improving the supporting structure and designing a novelmovable electrode, the performance of capacitive MEMS resonators have beenimproved. The main contents of this paper are as follows:1.To solve the problem of the large motional impedance in MEMS resonator, wedesigned a novel movable electrode with a lock-up structure, with which we can obtaina50nm electrode-to-resonator gap in a20μm thick resonator using conventionalSOI(silicon on insulator) process. A70MHz square resonator is employed and thetransmission line is simulated to show the improvement of this movable electrode. Themotional impedance reduced from44M to280after the actuation of the movableelectrode due to the calculation from the simulation result. What’s more, with thelock-up structure, the actuation voltage need to be only applied once, after the electrodeis locked up, we don’t need to apply this DC voltage in later use.2. To further improve the Q of lateral vibration MEMS resonators, a cross-shapedsupporting structure is designed. This cross-shaped supporting structure is obtained byadding a transverse beam at the static point of the straight beam. By the simulation of aface-shear mode square resonator and a flexural ring resonator each with threesupporting structures, namely straight beam support, T-shaped support and cross-shapedsupport, we find that the highest Q both appear in cross-shaped support, i.e. with thesmallest anchor loss.3. Researched the circuit equivalent model of capacitive MEMS resonator, used two methods to establish the circuit equivalent model, one is based on equivalent massand another is based on equivalent stiffness. With the established circuit model and thestudy of nonlinearity of MEMS resonator, a process to simulate the nonlinearity ofcapacitive MEMS resonator is established and the mechanical nonlinearity of aClumped-Clumped beam MEMS resonator are simulated by this method.4. Researched the fabrication process and layout designing, finally designedseveral lateral vibration MEMS resonator and a surface micromachining technologybased fabrication process, also the layouts of these resonators are designed.