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Enhancing MEMS switch reliability through the reduction of dielectric charging and bouncing mitigation

By Chean Hung Lai


RF MEMS switch is becoming the preferred choice for RF switching due to its outstanding performance when compared to conventional counterparts. However, due to its electro-mechanical nature, the RF MEMS switch suffers from reliability issues such as early failure, inconsistent switching characteristics, and higher cost where a driver circuit is necessary to drive the MEMS device. Thus, the reliability and characteristic of the RF MEMS switch have to be improved significantly before it can benefit from a much wider market acceptance. Dielectric charging in MEMS switches was reported to be one of the causes that limits the lifespan of the device. This phenomenon is mainly due to the high actuation voltage required to activate the MEMS switch where a penetration and trapping of charge carriers occur within the dielectric layer present in most capacitive RF MEMS switches. Other notable causes of early failure of the MEMS switch are mechanical failure and material degradation of the contacting surfaces. Mechanical contact made in metal-tometal MEMS switches improves the insertion loss, however, accelerates the physical failure of the device. Contact bouncing occurrence during the actuation period was reported to be one of the main factors that accelerates wear and tear of the contacting surfaces. In this work, the dielectric charging was investigated for the conventional driving method and existing charge-reducing actuation schemes through numerical simulation and corroborated by experimental findings. A novel actuation voltage waveform was proposed to effectively minimize the charge accumulation during continuous actuation, thus prolonging the lifetime of the MEMS switches. Also, the contact bouncing occurrence of MEMS switch was analyzed by simulating the dynamic response of the moving component upon contact. The ability of mitigating contact bouncing of the proposed actuation voltage waveform was evaluated and compared to the existing actuation schemes. Experiments were conducted on commercially available RF MEMS switches to support the analytical findings. Apart from life-limiting issues, MEMS switches also suffer from inconsistency of switching response due to parameters variation. The micro-scaled fabrication imperfection causes the devices' parameters to vary which in turn affects the switching characteristics of the MEMS switch such as pull-in and pull-out voltage requirements. In order to examine the robustness of different actuation waveforms to the MEMS switch's parameters variation, the dynamic response of the typical RF MEMS switch was simulated with changeable parameters, and the effects on the switching response were observed. Experiments were carried out to verify the analytical findings. Beside the characteristic variation due to fabrication, the temperature effect on MEMS switches was also investigated. A novel actuation strategy was introduced to optimize the actuation voltage while the MEMS switch is working under varying temperature condition. Finally, a generic MEMS switch driver was developed to implement the proposed actuation voltage waveform. As demonstrated in the analyses, the reliability of electrostatically actuated MEMS switch is expected to improve significantly without any physical changes on the MEMS device by replacing the conventional actuation method with the proposed MEMS switch driver. In addition, the implementation of the proposed Temperature Dependent Actuation Voltage (TDAV) approach was also presented. This actuation method can also be integrated into the proposed MEMS switch driver as an add-on feature when the system is operating under changeable temperature condition

Topics: Contact bouncing, Dielectric charging, MEMS switch, Radio frequency microelectromechanical systems, Reliability
Year: 2012
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