Research On RF MEMS Switch

In the RF communication system,a variety of switches are used to control the signal. RF MEMS switch has low power consumption, low insertion loss, high isolation,small volume and other many excellent properties compared to conventional semiconductor switch. It has the potential to replace the PIN diodes and GaAs FET in a number of areas. Although the RF MEMS switch’s structure is simple and easy to understand, but high reliability and excellent performance is not easy to achieve. The material selection, structure design and manufacture process needs to consider all-round.This paper summarizes and analyzes the major reliability issues of ohmic contact RF MEMS switch. On this basis, we focus on the effect of residual stress on the switch and seek ways to reduce the residual stress effect through simulations and experiments.Finally we design and fabricate a Au-Si3N4-Au multilayer MEMS switch based on the stress compensation mechanism.The main contents are as follows:1.Establish the mechanical model of the switch and derive the elastic coefficient,pull-in voltage and the resonant frequency of this three key performance parameters.Build up the equivalent circuit model of ohmic contact switch at up state and down state. Summarize and analyze the causes and solutions of reliability issues that the contact switch exists currently such as contact damage and adhesion.2.Analyze the mathematical relationship among the deflection of the cantilever and residual stress and stress gradient. Analyze the residual stress effect on switch’s shape, pull-in voltage and the resonant frequency. Derive the pull-in voltage expression under residual stress and analyzes the effect of residual stress on the pull-in voltage when considering the distribution position of drive electrodes. Study the effect of residual stress on the resonant frequency of the clamped beam. Using Ashby method,we propose the material selection scheme with low residual stresses based on a silicon substrate. Summarize the commonly used method to reduce the residual stress from the structural design of switch.3.Derive of the principle of stress gradient compensation on multilayer beam.Using stress gradient compensation technique, we design a Au-Si3N4-Au structureMEMS switch. When the switch gap is 5μm, within 2~40GHz frequency band, the up state S21 is greater than-0.18 dB, S11 is less than-23 dB, the down state S21 is less than-23 dB. We also study the impact of switch gap on insertion loss in the up state and impact of switch inductance and resistance on isolation in the down state. Switch’s pull-in voltage is between 103 V and 104 V according to COMSOL simulation. When the operation voltage is 140 V with a 14μs operation voltage, the switching time is about19μs. The operation frequency should be less than 37 KHz. Reliability analysis is conducted on the switch structure and thermal stability. We have designed the production processes and fabricated the switch.