Coplanar-waveguide embedded mechanically-bistable DC-to-RF MEMS switches

This paper reports on a novel electrostatically actuated, mechanically bi-stable metal-contact MEMS switch suitable for switching signals from DC to microwave frequencies. In contrast to conventional RF MEMS switches whose actuator is typically built on-top of the transmission lines, the switch mechanism is completely embedded in the signal line of a low-loss three-dimensional micromachined coplanar waveguide, which results in a minimum transmission line discontinuity. Furthermore, the in-line switch is mechanically bi-stable by an interlocking hook mechanism, i.e. it maintains both its on-state and its off-state without applying external actuation energy. External actuation voltage is only needed for triggering the transition between the two states. Thus, it is a true static zero-power device suitable for extremely low power applications and for applications where the switch configuration must be maintained even at a power failure. As a third major feature, the switch provides with active opening capability which potentially improves the switch reliability and makes the design suitable for soft, low-resistivity contact materials. The switches have been fabricated in a single photolithographical process step together with their three-dimensional coplanar waveguides. Including a 500 μm long 3D micromachined transmission line with less than 0.4 dB/mm loss up to 10 GHz, the total insertion loss was measured to 0.15 and 0.3 dB at 2 and 10 GHz, respectively, and the switch isolation is 45 and 25 dB at 2 and 10 GHz, respectively. The minimum transmission line discontinuity of the switch concept is demonstrated by its insertion loss of less than 0.1 dB up to 20 GHz for the switch mechanism alone, i.e. when corrected by the transmission line loss. The electrostatic actuation voltages for the different switch designs were measured to be between 23 and 39 V.

I. Introduction

MEMS (microelectromechanical systems) metal-contact switches are microrelays fabricated by semiconductor manufacturing techniques. Due to their small size and ohmic switching behavior, they offer superior signal performance as compared to solid-state relays or electromagnetic relays: low insertion loss, high isolation, excellent signal linearity, and maintaining their good properties over a very large bandwidth. MEMS metal-contact switches, in contrast to capacitive switches, can handle signals from DC up to many tens of GHz [1], [2], [3]. Electrostatic actuation is the preferred operation principle for MEMS switches because of the large actuation forces, very low power consumption (including the driving circuitry), good reliability, and the suitability for high-volume wafer-scale manufacturing techniques [4]. Most switch designs are based on vertically moving elements.