Rf Mems: Switches and Tunable Capacitors

RF MEMS is a key enabling device technology with the potential for significant performance, cost, and integration benefits to communications and radar applications. An overview of MEMS RF switches and tunable capacitors is presented, with examples of device technologies developed at RSC. Examples of MEMS-based circuits, including tunable filters and phase shifter circuits, are provided to illustrate the uses of these devices. Finally, implementation issues such as reliability and packaging are discussed. INTRODUCTION RF MEMS represents a valuable device technology with the potential for significant impacts in a broad range of application areas. These devices exploit the ability of small mechanical motions to generate large excursions in RF properties, enabling the tuning and reconfiguration of circuit characteristics. This can result in substantial performance enhancements in a number of communications, radar, and high-frequency signal routing circuits spanning a broad frequency range (HF to W-band). A range of different device types comprise the general category of RF MEMS, and include • RF Switches / Relays: MEMS equivalents of semiconductor PIN diodes or FET switches • Tunable Capacitors: MEMS equivalents of varactor diodes • Micromachined inductors: static components enabling greater on-chip integration • Micromechanical resonators: micromechanical elements for filtering and oscillator applications These devices represent basic building blocks for high-performance miniaturized RF subsystems. The discussions in the present paper will focus on RF MEMS switches and tunable capacitors. MEMS RF Switch A broad range of different switch concepts have been reported, each with different operational characteristics and appropriate circuit applications. A comprehensive review of the range of switch devices and constructions is beyond the scope of this paper, and thorough treatments of this diversity may be found in [1,2]. Briefly, though, switch types may be categorized in general by the following elements: • Contact mechanism: ohmic (metal-metal) vs. capacitive (mem brane) • Actuation mechanism: electrostatic, thermal, electromagnetic, piezoelectric, or combinations • Mechanical construction: torsional, lateral, or vertical flexures • Process methodology: Surface micromachined, bulk micromachined, thick film (plated), bonded wafer and hybrid process techniques To illustrate the characteristics and circuit applications of the MEMS RF switch, we use as example the Rockwell Scientific (RSC) device [3]. The structure of the RSC microrelay is a surface-micromachined device fabricated from thin films deposited atop the supporting substrate (Figure 1). The relay consists of a metal bridge bar that is suspended over a broken RF signal line on the substrate. This bridge bar is mechanically linked to two electrostatic actuator plates, and the entire suspended structure is anchored to the substrate via four compliant flexures. The device is fabricated using low-temperature (< 250°C) thin films, which offers versatility in substrate selection and permits monolithic integration with active electronics. Fig. 1: Micrograph and schematic operation of RSC ohmic-contact switch Relay functionality is controlled by the application of a bias voltage, which electrostatically actuates the bridge-supporting mechanical structure. In its unbiased state, the structure is suspended so the bridge bar is separated from the signal line via an airgap and the relay is non-conducting (OFF). The effective capacitance for this switch state, set by coupling across a micron-sized airgap, is ~1.75 fF. When a threshold bias is applied, the structure is pulled down so the bridge bar contacts the signal line and the relay is conducting (ON). The effective resistance for this state, formed by contacting metal surfaces, is ~1 Ω. The relay is switched ON to OFF by reducing the control voltage to allow the elastically deflected springs of the mechanical structure to pull the structure and bridge bar upward. The specific actuation methodology and mechanical design to great extent determine the microrelay actuation characteristics. For the electrostatic device above, actuation voltages are typically set around 70 V, although power consumption is very low. Switching times are ~10 μs, facilitated by drive capacitor flow holes for the OFF-ON transition and by stiff mechanical springs for the ON-OFF transition. While there is considerable flexibility to reduce actuation voltage in electrostatic switches by use of softer flexures, the implications on robustness to contact stiction must be considered. One of the parameters that has driven intense interest in MEMS switches is the low insertion loss. Typical S-parameter data for the RSC switch on a semi-insulating GaAs substrate are shown in Figure 2. Relay isolation in its OFF state is very good (particularly at lower frequencies), with >60dB isolation at low frequencies and ~30dB isolation at 40 GHz. This isolation is mostly limited by electrical coupling through the substrate. Insertion loss for the relay ON state is low over the entire band due to the use of metal contacts. Total device insertion loss is ~0.2-0.3dB from DC to 40 GHz, with ~0.1dB due to the relay contact itself and the remainder due to the conductor losses of the signal line. Return loss (not shown) is also very good, ranging from -40dB at 1 GHz to –25dB at 40 GHz. In addition the MEMS switch can offer excellent RF linearity and low intermodulation products with IP3 of +80dBm measured. 0 20 40 60 Frequency (GHz) -0.6 -0.4 -0.2 0.0 Sw it ch in se rt io n lo ss ( d B) Switched Line (as measured) Switch Only (extracted) 0 20 40 60 Frequency (GHz) -60.0 -40.0 -20.0 0.0 O ff -s w it ch i so la ti on ( d B ) Fig. 2: MEMS switch S-Parameter data for device on ON (left) and OFF (right) states . (cross-section through shunt) UnbiasedOFF