Radio Frequency Microelectromechanical System Devices Research Articles

Estimating the In-Plane Young's Modulus of Polycrystalline Films in MEMS

Polycrystalline films in microelectromechanical systems (MEMS) sometimes have a crystallographic fiber texture that causes their in-plane Young's modulus to differ from the bulk isotropic value, which influences device behavior, lifetime, and reliability. We estimate the in-plane Young's modulus of electrodeposited nickel bridges in radio-frequency MEMS devices by measuring the crystallographic texture using X-ray diffraction and then computing a texture-weighted average of the single-crystal elastic coefficients. The nickel bridges have a 001 fiber texture and are predicted to have an in-plane Young's modulus of 195-200 GPa, about 5-7% less than the bulk isotropic value of 210 GPa. The method presented here takes into account the full distribution of crystallite orientations to predict the in-plane Young's modulus. The method is rapid, general, and capable of estimating the in-plane Young's modulus of polycrystalline film components in individual MEMS devices in an array, making it ideal for MEMS design, analysis, and quality control.

Finite-Volume Method for Creep Analysis of Thin RF MEMS Devices Using the Theory of Plates

Creep is a critical physical mechanism responsible for the failure of radio-frequency (RF) capacitive micro-electro-mechanical systems (MEMS) switches, especially those operating at high RF power. Accurate modeling of creep in RF MEMS metallic membranes is necessary to estimate device lifetime and to improve their reliability. Moreover, the devices are frequently very thin, with aspect ratios as high as 1:500, and conventional three-dimensional structural modeling is onerous and unnecessary. In this article we extend a cell-centered finite-volume approach, previously developed to model thin membranes using Mindlin-Reissner plate theory, to study creep in RF MEMS devices. Results show that the present methodology can accurately predict the long-term creep behavior in thin RF MEMS devices in a computationally efficient manner.

Modeling RF MEMS Devices

The term radio frequency (RF) microelectromechanical systems (MEMS) refers to electronic devices with a moving submillimeter-sized part (beam, comb, disc, or ring), which provide RF functionality. Alternative definitions include bulk or surface micromachined devices, such as thin film bulk acoustic resonators (FBARs), which rely on energy transduction from the electrical energy domain to the acoustic energy domain and vice versa to provide RF functionality. Many introductory articles and textbooks have been written on MEMS and RF MEMS. This article focuses on electrostatically actuated RF MEMS devices, such as RF MEMS switches, switched capacitors and varactors, and vibrating RF MEMS resonators.

Full integrated process to manufacture RF-MEMS and MMICs on GaN/Si substrate

Radio Frequency Micro-Electro-Mechanical System (RF-MEMS) represents a feasible solution to obtain very low power dissipation and insertion loss, very high isolation and linearity switch with respect to “solid state” technologies. In this paper, we demonstrate the full integration of RF-MEMS switches in the GaN-HEMT (Gallium Nitride/High Electron Mobility Transistor) fabrication line to develop RF-MEMS devices and LNA-MMIC (Low Noise Amplifier/Monolithic Microwave Integrated Circuit) prototype simultaneously in the same GaN wafer. In particular, two different coplanar wave (CPW) LNAs and a series of discrete RF-MEMS in ohmic-series and capacitive-shunt configuration have been fabricated. RF-MEMS performances reveal an insertion loss and isolation better than 1 and 15 dB, respectively, in the frequency range 20–50 GHz in the case of pure capacitive shunt switches and in the frequency range 5–35 GHz for the ohmic-series switches. Moreover, the GaN HEMT device shows an Fmax of about 38 GHz and a power density of 6.5 W/mm, while for the best LNA-MMIC we have obtained gain better than 12 dB at 6–10 GHz with a noise figure of circa 4 dB, demonstrating the integration achievability.

Performance Evaluation and Equivalent Model of Silicon Interconnects for Fully-Encapsulated RF MEMS Devices

This paper aims to demonstrate the utility of silicon interconnects for radio-frequency (RF) microelectromechanical system (MEMS) devices that are packaged using a wafer-scale encapsulation process. Design and fabrication steps for the packaged interconnects are described. Measurement results show that encapsulated devices can be operated at frequencies up to 6 GHz with less than 1 dB insertion loss from the through-package silicon interconnects. This paper also describes a simple and accurate lumped-element model for simulating the performance of packaged silicon interconnects. The model is verified with S-parameter measurements from 50 MHz to 6 GHz. The modeling method and extracted values are intended to aid in the design and simulation of RF MEMS devices packaged using this technology.

The role of creep in the time-dependent resistance of Ohmic gold contacts in radio frequency microelectromechanical system devices

It is shown that measured and calculated time-dependent electrical resistances of closed gold Ohmic switches in radio frequency microelectromechanical system (rf-MEMS) devices are well described by a power law that can be derived from a single asperity creep model. The analysis reveals that the exponent and prefactor in the power law arise, respectively, from the coefficient relating creep rate to applied stress and the initial surface roughness. The analysis also shows that resistance plateaus are not, in fact, limiting resistances but rather result from the small coefficient in the power law. The model predicts that it will take a longer time for the contact resistance to attain a power law relation with each successive closing of the switch due to asperity blunting. Analysis of the first few seconds of the measured resistance for three successive openings and closings of one of the MEMS devices supports this prediction. This work thus provides guidance toward the rational design of Ohmic contacts with enhanced reliabilities by better defining variables that can be controlled through material selection, interface processing, and switch operation.

Electrostatic Discharge and Cycling Effects on Ohmic and Capacitive RF-MEMS Switches

An extensive electrical characterization of radio frequency microelectromechanical systems (RF-MEMS) switches has been carried out to identify the dynamic response of both ohmic and capacitive devices, driven in different conditions of bias and actuation time. We have found that an optimum actuation voltage must be chosen as a tradeoff between good switch transmission and isolation properties and the need to avoid bouncing phenomena when the actuation voltage has been applied. Moreover, the degradation of shunt RF-MEMS strongly depends on the duty cycle of the actuation voltage waveform and, hence, on the time that the switch spends in the actuated state. We have also investigated the reliability to electrostatic discharge (ESD) events of the RF-MEMS switches. Using a transmission line pulser time domain reflectometer on wafer systems, we have stressed RF-MEMS switches in different conditions and found that the reliability of RF-MEMS devices could be heavily affected by ESD phenomena.

Integration of 0/1-Level Packaged RF-MEMS Devices on MCM-D at Millimeter-Wave Frequencies

This paper reports on the development and optimization of 0/1-level packaged coplanar waveguide (CPW) lines and radio-frequency microelectromechanical systems (RF-MEMS) switches up to millimeter-wave frequencies. The 0-level package consists of an on-chip cavity obtained by flip-chip mounting a capping chip over the RF-MEMS device using BenzoCyclobutene (BCB) as the bonding and sealing material. The 0-level coplanar RF feedthroughs are implemented using BCB as the dielectric; gold stud-bumps and thermocompression are used for realizing the 1-level package. The 0-level packaged switches have been flip-chip mounted on a multilayer thin-film interconnect substrate using a high-resistivity Si carrier with embedded passives and substrate cavities. The insertion loss of a single 0/1-level transition is below -0.15 dB at 50 GHz. The measured return loss of a 0/1-level packaged 50-Omega CPW line remains better than -19 dB up to 71 GHz and better than -15 dB up to 90 GHz. It is shown that the leak rate of BCB sealed cavities depends on the BCB width, and leak rates as low as 10-11 mbar.l/s are measured for large BCB widths (> 800 mum), dropping to 10-8 mbar.l/s for BCB widths of around 100 mum. Depending on the bonding conditions, shear strengths as high as 150 MPa are achieved.

Application of Au-Sn eutectic bonding in hermetic radio-frequency microelectromechanical system wafer level packaging

Development of packaging is one of the critical issues toward realizing commercialization of radio-frequency-microelectromechanical system (RF-MEMS) devices. The RF-MEMS package should be designed to have small size, hermetic protection, good RF performance, and high reliability. In addition, packaging should be conducted at sufficiently low temperature. In this paper, a low-temperature hermetic wafer level packaging scheme for the RF-MEMS devices is presented. For hermetic sealing, Au-Sn eutectic bonding technology at temperatures below 300°C is used. Au-Sn multilayer metallization with a square loop of 70 µm in width is performed. The electrical feed-through is achieved by the vertical through-hole via filling with electroplated Cu. The size of the MEMS package is 1 mm × 1 mm × 700 µm. The shear strength and hermeticity of the package satisfies the requirements of MIL-STD-883F. Any organic gases or contamination are not observed inside the package. The total insertion loss for the packaging is 0.075 dB at 2 GHz. Furthermore, the robustness of the package is demonstrated by observing no performance degradation and physical damage of the package after several reliability tests.

RF-MEMS and SiC/GaN as enabling technologies for a reconfigurable multi-band/multi-standard radio

With more and more air interfaces introduced into the wireless market, the product spectrum of different base stations by a worldwide-engaged infrastructure vendor becomes increasingly diverse. Therefore, it is desirable to shrink the product spectrum while still matching the different market needs. This paper presents key enabling technologies, such as radio frequency-microelectromechanical system (RF-MEMS) and silicon carbide/gallium nitride (SiC/GaN) transistors, which allow for a base station radio architecture that can be reconfigured. The paper first analyses the market scenario and compares the principles of a reconfigurable radio with classical software radio technology. Direct-mixing transceiver architecture is proposed for implementation, and solutions for multi-band capability are discussed. RF-MEMS devices are used to switch filters, tune capacitances and inductances, and specifically adapt impedance matching. For multi-band power amplifiers, the benefit of operating a SiC or GaN radio frequency (RF) power device at high voltage is addressed. All these technologies lead to an analog software radio (also called frequency agile radio) where it will turn out that such a radio can be realized at less complexity and cost compared to a classical digital software radio. Furthermore, it will show up that analog software radio technology based on RF-MEMS also has the potential to enter the terminal market.