Reliability of RF MEMS switches at cryogenic (liquid He) temperatures

Abstract

This paper reports on the reliability of RF MEMS switches operating in a cryogenic (<6 K) environment while monitoring the repeatability of their contact resistance (R_c) over time. Series DC-contact switches were actuated with a bipolar waveform then checked for stiction every 100 thousand contacts, and after every million cycles R_c was measured 100 times. Device lifetimes were limited to under 10 million contacts. The dominant failure mechanism is believed to be charging of the substrate underneath the electrostatic MEMS switch leading to permanent hold down of the device.

Introduction

To the microwave engineer the true excitement over Radio Frequency Microelectromechanical Systems (RF MEMS) stems from their ability to offer low insertion loss and high isolation while requiring virtually no DC power. This is especially true in the field of Radio Astronomy, where the ability of MEMS to deliver high performance and low power is useful for balloon/space based applications where power is limited by payload size restrictions. Often such applications require harsh operating environments with temperatures reaching close to absolute zero. One such application where MEMS can benefit a scientific mission is the Beyond Einstein Inflation Probe currently under development at NASA [1]. It's mission is to study the early universe by detecting a very faint signature in the polarization of the Cosmic Microwave Background (CMB). In this experiment, RF MEMS phase switches can be used to modulate the incoming signal to extract the Stokes polarization parameters of the CMB however it is currently unknown whether MEMS devices are reliable enough to measure the CMB to the desired sensitivity. RF MEMS switches have been developed to operate at very low temperatures [[2], [3], [4], [5]] and research presented by Gong et al. on cryogenic RF MEMS switches showing good performance from DC to W-band at temperatures as low as 6 K [6]. Attar et al. show excellent performance of Au RF MEMS switches from DC to 20 GHz on superconducting Nb transmission lines as well as important considerations for proper thermal grounding of such devices during testing [7]. The reliability of RF MEMS switches is well studied [8], however all of this work was conducted at room temperature, or in the case of [9] at 77 K. This work investigates for the first the feasibility of using RF MEMS to handle the switching, performance, and environmental requirements for successful long-term operation in a liquid Helium (~4 K) cryogenic environment as proposed in [1]. Building MEMS devices to operate at such low temperatures can be quite challenging. There are many phenomena to account for and obstacles to overcome. As temperatures decrease, structures undergo thermal contraction (as defined by their coefficient of thermal expansion, or CTE), which can lead to unwanted stresses in fixed-fixed beams and increase the pull-down voltage of devices [6,10]. Dielectrics that cover bias pads to prevent shorting are more likely to hold trapped charge which can cause shifts in pull down voltage and ultimately lead to device failure [11]. Using cantilever beams in DC-contact switches can alleviate the CTE mismatch problem as well as the need to have a dielectric layer over the bias pad. Changes in material hardness as devices are cooled can increase causing a subsequent increase in the contact resistance of DC-Contact switches, which ultimately leads to more loss than expected within each switch [6,12]. Contact resistance is hard to model as it is very dependent on the surface roughness of the underside of the cantilever and the region of transmission line under the cantilever contacts [12]. This is because most of the current flow occurs through asperities in the beam and transmission line that are in contact with one another. Often the contact resistance will decrease as the switch is operated through its first tens of cycles because contact forces smooth out the asperities [12]. At cryogenic temperatures, materials such as gold can harden which makes it difficult to flatten out the contacts and could result in more RF loss than expected. Loss in sensitive detection systems is important to control so that faint signals are not attenuated to the point where they can't be detected, and errors resulting from variations in contact resistance (R_C) are kept to a minimum or at the very least, well understood. Thus it is not only important to measure the ability of a MEMS switch to cycle for an extended period of time, but it is important to monitor changes in R_C over a switch's lifetime.