Design and Analysis of RF MEMS Capacitive Shunt Switch and Impact of Geometric Trade-offs on RF Performance

Author Name(s): Surendra K. Waghmare, Dilip D. Shah
Author Email: surendra.waghmare@raisoni.net

Abstract

The electromagnetic and the electromechanical characteristics of the radio frequency micro-electro-mechanicalsystem (RF MEMS) switches for high-frequency applications are the critical performance metrics that need to optimize. Performance indices of the RF MEMS switches such as isolation, insertion loss, pull-in voltage, holddown voltage, reliability are dependent on types and properties of conducting and insulating materials that are used in the construction of switch. This article proposes the design and analysis of the two terminal capacitive shunt switches built on a coplanar waveguide (CPW) for applications in subsets of Ka- and V-Band frequency range. The proposed switch used a fixed-fixed gold membrane with the low-spring constant uniform single meander flexures support and achieved a low pull-in voltage of 5.1 Volts. An impact of the variation of the geometric parameter trade-offs like conducting membrane height, dielectric material height, and the air gap between the membrane and the dielectric materials like Silicon Nitride (Si3N4) and Hafnium Dioxide (HfO2) are studied to investigate RF and electromechanical performance of the switch.

Introduction

The RF MEMS switches are widely finding their use as an essential component in the RF/microwave/mm-wave devices and circuits applicable in the commercial and defense products. RF MEMS switches are favored over the semiconductor devices (PIN Diodes and MESFETs or JFETs) due to their potential performance such as high isolation, low insertion loss, broader operating bandwidth, high capacitance ratio, almost zero power consumption, 20-40 times higher cut-off frequency. The drawback of the RF MEMS switches is its higher actuation voltage which leads to use of an additional up-converters increasing device size. Also, the high electric field at the insulating layer results into charge injection or dielectric breakdown and thus limits the reliability of the switches (Tilmans, 2002; Rebeiz, 2003; M. Angira et al., 2015). While designing an electrostatic actuationbased RF MEMS switches for the high-frequency applications, it is desirable to have a low pull-in voltage for actuation and excellent RF performance for required frequency bands of operation. To optimize these performance indices, types, and properties of conducting and insulating materials play an essential role (Sharma et al., 2012). Till date, many researchers have studied and represented the design aspects of the RF MEMS capacitive shunt switches on the geometric parameter trade-offs. A RF MEMS switch was designed and simulated (P. Bahmanyar et al. 2010) based on silicon substrate coplanar waveguide (CPW) with Si3N4 insulating material and shown good RF response such as insertion loss better than -0.65 dB and high isolation of -38 dB in Ka-band and demonstrated the effect of the variation of the geometric parameters on switching behaviour. Simulation results show that varying the width of the membrane, it effects on the resonance frequency and isolation loss in the downstate performance of the switch. Also, decreasing the thickness of the insulating material (Si3N4) improved isolation.

Conclusion

The RF MEMS switch is designed and simulated for the subsets of Ka and V-Band applications using Si3N4 and HfO2 dielectrics. A novel approach of a step-in nature actuation electrode is used to ensure low-pull in voltage and uniform contact of the upper electrode to dielectric layer on the down electrode. Use of the ribs surrounding to the steps of the switch results in low Von-mises stress during the actuation of the switch. With 0.3 µm thick Si3N4 insulating material and 0.5 µm membrane height, switch optimized the RF characteristics in subsets of Ka- and V-Band frequency ranges. The on-state RF characteristics shows insertion loss of better than -0.5 dB and return loss of better than -10 dB below 37.5 GHz. The off-state RF performance shows the maximum isolation of -54.38 dB at 38.5 GHz. Varying geometric parameters like an air gap, dielectric height, and the membrane thickness affects the electromagnetic and electromechanical performance of the switch.

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