Thin-film encapsulation of high frequency MEMS resonator for RF applications [electronic resource]
Keywords: Mechanical Engineering

Wireless communication has greatly impacted our daily life since the first radio system was invented. Applications, such as cellular phone, satellite television, GPS navigation, and wireless Internet network, are driving the development of RF components to the direction of being smaller, cheaper and more power saving and therefore this topic has been one of the hottest research areas in MEMS field. MEMS resonators have a great potential for replacing conventional resonators used in portable wireless applications because of their merits of small size, high quality factor (Q), and low power consumption. There are also great interests in using coupled micro-resonators as band-pass filters and many research groups have already got exciting results. However, high motional impedance still remains a big obstacle for commercialization of MEMS resonators in RF applications. Despite the advance of device performance, packaging for MEMS resonators remains a critical challenge. Because of their extreme sensitivity to the environment, MEMS resonators need a vacuum packaging to achieve high quality factors (Q) and enable post-MEMS CMOS integration. The promising on-chip application also requires a CMOS compatible packaging process. Due to the stringent RF requirement, electrical properties and hermiticity of packaging are also very important. This work aims to provide a solution for a practical RF MEMS resonator that has low impedance as well as a reliable packaging. First, this work presents a thorough study of a wafer-level epitaxial silicon encapsulation process in making RF MEMS resonators. The epitaxial silicon encapsulation process developed at Stanford University has been proven to have high mechanical robustness and it provides a low-pressure environment to resonating structures. The transmission loss of silicon interconnect was measured at RF ranges in this work. The transmission loss was also modeled for device designers to simulate the interconnect properties at the design phase. Secondly, a 200 MHz width-extensional mode dielectrically-driven resonator is presented. High-k dielectric material was used to enhance the transduction and reduce the motional impedance. A modified encapsulation process was developed to package the resonator. The resonator was demonstrated to have high Q in the package. In addition, this work presents an integrated solution for wafer-level packaging and electrostatic actuation of out-of-plane RF MEMS resonators. By integrating the electrodes into the epitaxial-grown silicon layer, both the encapsulation and the out-of-plane actuation can be built in one process step, which results in an ultra-compact and robust packaging. First, designs and fabrication processes of the out-of-plane electrode are described. The mechanical and electrical properties of the electrode are discussed, modeled and characterized. A 200 kHz torsional mode beam resonator and a 12 MHz transverse-mode differential square plate resonator were fabricated using this packaging method and their performances are presented and discussed. This work also presents a 13 MHz mechanically coupled filter that is encapsulated using the same integration process.