Research On High Quality Factor Technology For Radio Frequency Micro-Electro-Mechanical Systems Resonators

In the past decades,micro-electro-mechanical systems(MEMS)technologies have shown promising prospects in a large and diverse set of application regions,including the audio,sensor,wireless communication and microenergy harvest.In particular in the field of wireless communication,the MEMS device has been rapidly developed by the great promotion of radio frequency micro-electro-mechanical systems(RF-MEMS)devices(i.e.,MEMS oscillator and MEMS filter)due to their salient trait,such as small size,low power consumption and high integration.The core component of MEMS oscillator and MEMS filter is the MEMS resonator and a MEMS resonator with high quality factor(Q)can significantly improve the performance of associated MEMS device.For instance,high-Q MEMS resonator contributes to the reduction of close-to-carrier phase noise and maintaining frequency stability in the case of MEMS oscillators,high-Q MEMS resonator helps to reduce the insertion loss in the case of MEMS filters,high-Q MEMS resonator helps to improve the precision in the case of MEMS sensors.Therefore,high-Q MEMS resonators are urgently demanded in the rapid development and in-depth application of MEMS technology.This dissertation first introduces the state-of-art status and working principle of traditional acoustic resonators,and then summarizes the differences and advantages between MEMS resonators and traditional acoustic resonators.By analyzing the energy loss mechanism of MEMS resonators,the state-of-art high quality factor technology for MEMS resonators are systematically investigated in this dissertation,which provides a foundation for this dissertation.The core component of thin-film piezoelectric-on-scilion(TPoS)MEMS resonator is introduced in this research to investgate the electromechanical energy conversion principle,resonant mode characteristics and energy loss mechanism,respectively.Meanwhile,the foundry fabrication process,the test method and the approach of extracting parameters of equivalent circuit model for TPoS resonator are discussed.Based on the above theoretical analysis and technical background,the high quality factor technologies for RF-MEMS resonators are proposed in this dissertation,and which can be summarized as follows:Phononic crystals(PnC)is proposed to reduce the anchor loss of MEMS resonators and further results in the Q improvement.The basic concepts,theories of PnC and the effects of PnC for MEMS resonators are systematically explored in this dissertation.To verify that the existence of acoustic bandgaps can be formed by the associated PnC structure,the multiphysical delay lines are proposed.Through the structural optimization design,multi-stage PnC structure is proposed to reduce the anchor loss of resonators and further results in the Q improvement.The Q of the resonator with five-stage PnC structure reached 9744 at the frequency of 109.85 MHz,while the Q of conventional resonator at the same resonant mode is ~1000.The proposed spider-web like PnC(SW-PnC)structure offers lightweight and small size compared with the circle hole PnC structure,while numerical calculations are performed to take an insight into the superior acoustic isolation properties of the SW-PnC.The mechanical vibration isolation system based on suspended frame structure is proposed to reduce the energy dissipation of MEMS resonators and further results in the quality factor improvement.In order to explore the working principle of the frame structure,this dissertation investigates the effects of vibration isolation-based frame in MEMS resonators and expounds the underlying physical mechanisms.To quantify the effects of energy dissipation reduction by the frame structure,multiphysical FEA simulation models are proposed.The numerical calculation and the experimental comparison were employed to systematically investigate the energy dissipation reduction of MEMS resonators with suspended frame.The Q of resonator with 50 ?m width of frame structure reached 6779 at 30.4 MHz,which is 3.8 times higher than that of the conventional resonator.A method combining the frame and PnC is proposed to further improve the Q of MEMS resonators with frame structure only.The approach of electrode optimization for MEMS resonator is proposed to suppress spurious modes and the realization method of high-Q phononic frequency combs are presented in this dissertation.The influence of electrode optimization on surface charge density and vertical displacement current density of the resonator is researched.The numerical calculation and the experimental comparison were employed to investigate the effect of electrode optimization on quality factor improvement of ring-shaped resonators.The Q of ring-shaped resonator with optimized electrode structure is more than 10000 at 83.586 MHz,while the Q of ring-shaped resonator with conventional electrode structure is only 3712 at the same resonant mode.Since the TPoS resonator can realize a multifrequency design on a single chip,and the PnC has excellent acoustic isolation characteristics,the realization method of high-Q phononic frequency combs is successfully proposed.In summary,due to the low quality factor caused by the energy loss of RF MEMS resonators,this dissertation focused on the research work of high Q technologies for RFMEMS resonators by using theoretical analysis,numerical calculation,experimental design,test characterization and so on.Five approaches are proposed to improve the quality factor of resonators in this dissertation and then realized the high-Q MEMS resonator.