We have fabricated and characterized radio frequency microelectromechanical systems (RF MEMS) ohmic switches for applications in discrete tunable filters and phase shifters over a frequency range of 0 to 20 GHz. Our previously reported cantilever switches have been redesigned for higher isolation and are now achieving 22 dB of isolation at 10 GHz. The measured insertion loss is 0.15 dB at 10 GHz. We have also fabricated and characterized new devices, designated “crab” switches, to increase isolation and contact forces relative to the cantilever design. The measured insertion loss and isolation are 0.1 dB per switch at 20 GHz and 22 dB at 10 GHz, respectively. A simple and accurate equivalent model has been developed, consisting of a transmission line segment and either a series capacitor to represent the blocking state or a series resistor to represent the passing state. Experimental analysis of the switch shows that high contact and substrate capacitive coupling degrades the isolation performance. Simulations indicate that the isolation improves to 30 dB at 10 GHz by reducing these capacitances. The crab switch design has a measured contact force of 120 μN, which represents a factor of four increase over the cantilever switch contact force and results in consistent, low-loss performance.
We have developed radio frequency microelectromechanical systems (RF MEMS) capacitive switches using amorphous diamond (a-D) as a novel tunable dielectric with controlled leakage. The switch is fabricated from sputtered and electroplated metals using surface micromachining techniques. The mechanical stress and resistivity of the a-D dielectric are controlled by the parameters of a high-temperature annealing process. These initial devices exhibit a down-state capacitance of 2.6 pF, giving an isolation of better than 18 dB at 18 GHz, and a predicted static power dissipation of 10 nW. This technology is promising for the development of reliable, low power RF MEMS switches.
Radio frequency microelectromechanical systems (RF MEMS) is an enabling technology for the miniaturization of future radar and communication systems. RF MEMS ohmic and capacitive switch performance and fabrication are discussed. Sandia National Laboratories’ program in RF MEMS is motivated by defense and national security applications not currently being met by the private sector. Examples of fabricated switches and switched circuits under investigation at Sandia are presented.
We have fabricated and tested a surface micromachined, metal-metal contacting radio frequency microelectromechanical systems (RF MEMS) switch. The switch was fabricated out of electroplated metals on semi-insulating GaAs at process temperatures below 300°C. It was anchored by folded springs to one end of a coplanar waveguide (CPW) gap, forming a cantilever. This configuration allowed us to simplify the fabrication process by eliminating mechanical dielectric films that are normally necessary to isolate the switch contact from the actuation metal. The measured insertion loss and isolation at S band were 0.21 dB and 28 dB isolation, respectively. An average switching speed of 83 μs at 55 volts was measured. This switch demonstrated >105 cold switching cycles without sticking, however rapid increase of the contact resistance was observed. A new switch was designed to increase isolation and reduce insertion loss by decreasing the coupling capacitance and increasing the contact force.
Future microwave networks require miniature high-performance tunable elements such as switches, inductors, and capacitors. We report a micro-machined high-performance tunable capacitor suitable for reconfigurable monolithic microwave integrated circuits (MMICs). The capacitor is fabricated on a GaAs substrate using low-temperature processing, making it suitable for post-process integration with MMICs, radio frequency integrated circuits (RFICs) and other miniaturized circuits. Additionally, the insulating substrate and high-conductivity metal provide low-loss operation at frequencies over 20 GHz. The device demonstrates a capacitance of 150 fF at 0 V bias, pull-in at about 15 V to 18 V, and further linear tuning from 290 fF to 350 fF over a voltage range of 7 V to 30 V. Also, the device demonstrates self-resonance frequencies over 50 GHz, and Q’s over 100 at 10 GHz. To enable integration into circuits, a simple equivalent circuit model of the device has been developed, demonstrating a good match to the measured data through 25 GHz. Initial testing to 1 billion cycles indicates that metal fatigue is the primary limitation to reliability and reproducibility, and that dielectric charging does not have a significant impact on the device. This device is promising for high-performance tunable filters, phase shifters, and other reconfigurable networks at frequencies through K-band.
MEMS are rapidly emerging as critical components in the telecommunications industry. This enabling technology is currently being implemented in a variety of product and engineering applications. MEMS are currently being used as optical switches to reroute light, tunable filters, and mechanical resonators. Radio frequency (RF) MEMS must be compatible with current Gallium Arsenide (GaAs) microwave integrated circuit (MMIC) processing technologies for maximum integration levels. The RF MEMS switch discussed in this paper was fabricated using various layers of polyimide, silicon oxynitride (SiON), gold, and aluminum monolithically fabricated on a GaAs substrate. Fig. 1 shows a metal contacting series switch. This switch consists of gold signal lines (transmission lines), and contact metallization. SiON was deposited to form the fixed-fixed beam, and aluminum was deposited to form the top actuation electrode. To ensure product performance and reliability, RF MEMS switches are tested at both the wafer and package levels. Various processing irregularities may pass the visual inspection but fail electrical testing. This paper will focus on the failure mechanisms found in the first generation of RF MEMS developed at Sandia National Laboratories. Various tools and techniques such as scanning electron microscopy (SEM), resistive contrast imaging (RCI), focused ion beam (FIB), and thermally-induced voltage alteration (TIVA) have been employed to diagnose the failure mechanisms. The analysis performed using these tools and techniques led to corrective actions implemented in the next generation of RF MEMS metal contacting series switches.
A surface-micromachined two-degree-of-freedom system that was driven by parallel-plate actuation at antiresonance was demonstrated. The system consisted of an absorbing mass connected by folded springs to a drive mass. The system demonstrated substantial motion amplification at antiresonance. The absorber mass amplitudes were 0.8 - 0.85 micrometer at atmospheric pressure while the drive mass amplitudes were below 0.1 micrometer. Larger absorber mass amplitudes were not possible because of spring softening in the drive mass springs. Simple theory of the dual-mass oscillator has indicated that the absorber mass may be insensitive to limited variations in strain and damping. This needs experimental verification. Resonant and antiresonant frequencies were measured and compared to the designed values. Resonant frequency measurements were difficult to compare to the design calculations because of time-varying spring softening terms that were caused by the drive configuration. Antiresonant frequency measurements were close to the design value of 5.1 kHz. The antiresonant frequency was not dependent on spring softening. The measured absorber mass displacement at antiresonance was compared to computer simulated results. The measured value was significantly greater, possibly due to neglecting fringe fields in the force expression used in the simulation.
Thy monolithic integration of MicroElectroMechanical Systems (MEMS) with the driving, controlling, and signal processing electronics promises to improve the performance of micromechanical devices as well as lower their manufacturing, packaging, and instrumentation costs. Key to this integration is the proper interleaving, combining, and customizing of the manufacturing processes to produce functional integrated micromechanical devices with electronics. We have developed a MEMS-first monolithic integrated process that first seals the micromechanical devices in a planarized trench and then builds the electronics in a conventional CMOS process. To date, most of the research published on this technology has focused on the performance characteristics of the mechanical portion of the devices, with little information on the attributes of the accompanying electronics. This work attempts to reduce this information void by presenting the results of SPICE Level 3 and BSIM3v3.1 model parameters extracted for the CMOS portion of the MEMS-first process. Transistor-level simulations of MOSFET current, capacitance, output resistance, and transconductance versus voltage using the extracted model parameters closely match the measured data. Moreover, in model validation efforts, circuit-level simulation values for the average gate propagation delay in a 101-stage ring oscillator are within 13 - 18% of the measured data. These results establish the following: (1) the MEMS-first approach produces functional CMOS devices integrated on a single chip with MEMS devices and (2) the devices manufactured in the approach have excellent transistor characteristics. Thus, the MEMS-first approach renders a solid technology foundation for customers designing in the technology.
Results are presented supporting the use of supercritical carbon dioxide (SCCO2) drying to enhance the yield of surface-micromachined micromechanical devices following the final release etch. The equipment and extraction process of the SCCO2 system are described, and results of successfully released cantilevered beams and microengines are presented. A new system capable of drying 6 inch wafers is also described.
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