Derek Scarbrough

Robustness of RF MEMS Capacitive Switches With Molybdenum Membranes

This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

Inkjet printing of passive microwave circuitry

Inkjet printing technology was utilized to fabricate transmission lines on a glass substrate. 50 micron resolution was realized using 10 pL drop volumes on a Corning 7740 glass substrate. This can be further improved by applying other methods as described in this paper. The conductivity of the sintered silver structures were 1/6 that of bulk silver after sintering at a temperature much lower than the melting point of bulk silver. A comparison of the DC resistance of the sintered silver shows that it can be a match for electroplated and etched copper. Printed Coplanar lines demonstrated losses of 1.62 dB/cm at 10 GHZ and 2.65 dB/cm at 20 GHz.

Understanding and Improving Longevity in RF MEMS Capacitive Switches

This paper discusses issues relating to the reliability and methods for employing high-cycle life testing in capacitive RF MEMS switches. In order to investigate dielectric charging, transient current spectroscopy is used to characterize and model the ingress and egress of charges within the switch insulating layer providing an efficient, powerful tool to investigate various insulating materials without constructing actual MEMS switches. Additionally, an in-situ monitoring scheme has been developed to observe the dynamic evolution of switch characteristics during life testing. As an alternative to high-cycle life testing, which may require days or weeks of testing, a method for performing accelerated life tests is presented. Various methods for mitigating dielectric charging are presented including: reduced operating voltage, reduced dielectric area, and improved control waveforms. Charging models, accelerated life test results, and high-cycle life test results for state-of-the-art capacitive RF MEMS switches aid in the better understanding of MEMS switch reliability providing direction for improving materials and mechanical designs to increase the operation lifetime of MEMS capacitive switches.

Performance of molybdenum as a mechanical membrane for RF MEMS switches

This article details the construction and measurement of RF MEMS capacitive switches using molybdenum as the mechanical material. The resulting switches exhibit a significantly reduced rate of change in actuation voltage over temperature, with rates less than 0.03 V/°C up to 150°C. Resistivity of the molybdenum membranes averaged 10–11 μΩ-cm, yielding an effective shunt resistance of less than 0.25 Ω. Initial cycling measurements were made which show that the resulting membranes were capable of at least 20 billion cycles without failure, indicating that molybdenum is a promising mechanical material for constructing RF MEMS switches.

Low-loss, broadly-tunable cavity filter operating at UHF frequencies

A four-pole tunable filter has been demonstrated at UHF frequencies which tunes 300 MHz to 700 MHz and exhibits mid-band losses between 0.5 dB and 0.9 dB. This filter incorporates piezo-electric tuning of capacitively-loaded cavities and frequency-tailored input/output coupling networks to maintain good impedance match across more than an octave tuning band. The unloaded quality factor of the filter cavities range from 340-550. The filter has an output intercept point greater than +54 dBm and can handle power levels of +26 dBm.

Design of high-Q absorptive bandstop filters with static and reconfigurable attenuation

This paper focuses on the design of highly selective and high quality factor (Q) absorptive bandstop filters (ABSFs) that are based on a mixed implementation scheme using acoustic wave (AW) resonators and lumped-element components. The proposed approach enables mobile form-factor notch filters with theoretically infinite attenuation even for stopbands showing 3-dB fractional bandwidths (FBWs) as narrow as 0.013-0.02% (Q=10,000-15,000). Proof-of-concept filter prototypes using commercially-available surface acoustic wave (SAW) resonators and surface-mount components are designed, built, and characterized for an example frequency of 418 MHz. Various stopbands with measured FBWs between 0.017 and 0.078% (i.e., 0.07 and 0.327 MHz in absolute terms) and attenuation levels as high as 83 dB are demonstrated. Absorptive notches with reconfigurable levels of isolation in the range of 10-83 dB are also shown through a fabricated ABSF with controllable attenuation.

Novel Ka-band phase shifters using MEMS capacitive switches

This paper presents the design of a Ka-band phase shifter comprising a slow-wave structure that tightly wraps around three closely spaced MEMS capacitive switches. The switches are of proven design and reliability, except some switches have a gap in their stationary electrodes. This novel feature has negligible effect on electromechanical operation but provides another degree of freedom for simultaneous optimization of phase shift and impedance match. The design principle is validated through specially designed thin-film test structures. The results suggest that the present design is applicable to phase shifters of different sizes and resolutions with high performance, yield and reliability, but low cost and power consumption.

Low-dispersion 180° phase shifter using two synchronized MEMS switches

Previously, the design of our novel low-dispersion phase-shifter unit cell with two asynchronous MEMS switches was limited to 90°, so that four unit cells (90°, 90°, 90° and 45°) with a total of eight MEMS switches were required for a 3-bit phase shifter. This not only increased the size and loss of the phase shifter, but also decreased its yield and reliability. Recently, we hypothesized that by using two synchronized MEMS switches, the phase shift of a unit cell could be extended to 180° so that only three unit cells (180°, 90° and 45°) with a total of six MEMS switches would be required for a 3-bit phase shifter. This paper confirms the hypothesis with measured characteristics of a fabricated 180° phase-shifter unit cell. The measured characteristics compare well with both equivalent-circuit model prediction and three-dimensional finite-element electromagnetic simulation, provided the worse-than-expected loss from individual MEMS switches is taken into account. The results provide the proof of the design principle, so that with improved MEMS fabrication process such as thicker metallization, the switch loss can be reduced and low-dispersion phase shifters can be realized with compact size, low loss, low cost, and high reliability.

Low-dispersion metamaterial-based phase shifters with reduced size and number of MEMS switches

This paper reports a low-dispersion metamaterial-based 3-bit phase shifter which occupies an area of approximately 5 mm2 and uses only six microelectromechanical systems (MEMS) switches. The phase shifter is based on a coplanar slow-wave structure with defected ground and comprises three unit cells of 180°, 90° and 45° phase shifts, respectively. Each unit cell uses two single-pole-single-throw MEMS capacitive switches in series and parallel configurations, respectively, to switch between right-handed (low-pass) and left-handed (high-pass) states for the specified phase shift. Three-dimensional finite-element electromagnetic simulation was used to help optimize the compact layout. The worst-case performance across the band of 24-28 GHz was simulated to have less than 9° root-mean-square phase error, less than 1.7 dB insertion loss, and greater than 13 dB return loss.