Viviana Mulloni

An equivalent-circuit model for shunt-connected coplanar microelectromechanical system switches for high frequency applications

In this paper, a circuit model to predict the microwave response of a shunt-connected capacitive microelectromechanical coplanar switch is proposed. The numerical values of the lumped elements composing the equivalent circuit are computed by means of a fully analytic approach. In particular, the contribution of resistive and inductive parasitic elements has been evaluated by using closed-form expressions. Configurations characterized by different technological solutions have been obtained and modeled. Simulations performed with the proposed approach correlate very well with actual measurements.

A simple analytical method for residual stress measurement on suspended MEM structures using surface profilometry

This paper presents an analytical method to calculate residual stress and Youngs modulus in clamped–clamped beams. These types of structures are a typical building block of many MEMS devices, and this guarantees accurate transferability of the measured parameters. The method is based on the determination of beam bending as a function of applied load by means of a surface profiler, and as a function of beam length. By modeling analytically both the elastic and the stress contribution to beam bending, it is possible to obtain both the stress value and Youngs modulus by a simple fitting of the experimental data. Results are presented for electrodeposited gold beam arrays of different widths, but the method is in principle exploitable for every type of suspended film where the residual stress strongly influences the material properties. Accuracy and limitations of the method are also discussed.

Clear evidence of mechanical deformation in RF-MEMS switches during prolonged actuation

Dielectric charging is normally considered one of the most important problems when dealing with RF-MEMS switch reliability, especially for applications which require long-term operation. Other important effects are therefore often neglected. In this paper we demonstrate that, for the case of long-term actuation in dielectric-less switches, the most important issue for switch reliability is not dielectric charging but viscoelastic deformation and creep of the mobile membrane. The measurements and the analysis are performed both for a cantilever and for a clamped–clamped switch configuration, evidencing that in the first case the mechanical deformations are more pronounced, and that they can justify almost completely the variation of actuation and release voltage experimentally measured. Mechanical deformation is also detected in a clamped–clamped switch, but it is less evident than that in the previous case. Nonetheless, even in this case they are responsible for most of the actuation and de-actuation voltage change experimentally detected.

Transient Evolution of Mechanical and Electrical Effects in Microelectromechanical Switches Subjected to Long-Term Stresses

Application of two different biasing waveforms in long-term stresses in RF MEMS switches is used to separate mechanical and electrical effects. Three different effects are shown: 1) permanent mechanical degradation (creep effect) after the first stress and relaxation period; 2) transient mechanical degradation (viscoelastic recoverable mechanism); and 3) transient electrical degradation (recoverable charge trapping). Such effects are extracted by monitoring the evolution of the actuation and release voltages in different RF MEMS switches subjected to dc biasing and recovery tests. This paper highlights the mechanical and the electrical degradation components in long-term stress tests with the aim of quantifying the weights of the different contributions.

Preconditioning Procedure for the Better Estimation of the Long-Term Lifetime in Microelectromechanical Switches

The study of long-term reliability of RF-MEMS switches subjected to continuous biasing involves different failure mechanisms: 1) short-to-medium-term stress inducing permanent phenomena, such as permanent mechanical degradation, and 2) long-term stress inducing recoverable phenomena related to mechanical viscoelastic mechanism. The lifetime prediction should consider all the long-term effects after the saturation of all the short-to-medium-term degradation effects. In fact, the lifetime estimation is affected by both the different short-term and long-term degradation mechanisms and not separating the different effects may lead to wrong lifetime prediction. In this paper, we define a preconditioning technique to be applied to each RF-MEMS switch before any stress test. The main purpose is to eliminate all the short-term permanent effects to obtain a lifetime estimation only related to the long-term phenomena.

Design and characterization of an active recovering mechanism for high-performance RF MEMS redundancy switches

This paper reports on the design, manufacturing and characterization of an active push/pull toggle RF MEMS switch for satellite redundancy networks. The actively controlled pull-up mechanism allows for extended restoring capabilities of the switch in case of down-state stiction due to long time continuous bias voltage. As a proof of concept an active push/pull MEMS capacitive switch was modeled, designed and manufactured in shunt configuration on a 50 Ω coplanar transmission line. RF measurement results show a return loss better than 20 dB in the 0.1 – 40 GHz range and an insertion loss better than 0.2 dB up to 20 GHz. Long term stress characterization is performed, proving the lifetime of the proposed device for over 50 million cycles. Finally the capability of restoring the membrane OFF state position after down-state stiction has been demonstrated.

Electro-thermal analysis of RF MEM capacitive switches for high-power applications

Self heating in electrostatically actuated RF MEM capacitive shunt switches is analyzed by coupled electrical and thermal simulations using three-dimensional finite-element analysis. The result shows that despite highly nonuniform current and temperature distributions, the self-heating effect can be approximated by lumped thermal resistances of the switch membrane and the substrate. Additionally, since the thermal resistance of thermally insulating substrates such as quartz is significant compared to that of the membrane, it is important to consider the heat transfer across both the membrane and the substrate.

Cycling reliability of RF-MEMS switches with gold-platinum multilayers as contact material

Low contact resistance and high cycling reliability are two important parameters for evaluating the quality of RF micro-electromechanical (RF-MEMS) switches. In this paper the use of a modified contact material is tested and compared to pure gold in cycling experiments performed on a RF MEMS switch in shunt capacitive configuration. The modified contact material is a gold-based multilayer with a thin layer of platinum sandwiched between two layers of gold. The experiment consists in comparing devices with the same layout but with different contact material. While the two types of switch start with similar RF performances, the device with the modified material shows a marked improvement in cycling stability and a lower series resistance up to 106 cycles when compared to gold contact devices.

Influence of fabrication tolerances on the reliability of RF-MEMS capacitive switches

In this paper the comparison between ideal mechanical and electrical performances and measured ones is presented for one of the most common type of switch typology, the RF-MEMS clamped-clamped capacitive switch. The main sources of deviation from design assumptions and ideal behavior occurring during fabrication are found and examined in detail. They can be summarized in a measurable deviation from planarity and thickness non-uniformity of the membrane, a low contact force even for a capacitive switch and a mismatch in the thicknesses of the structural layers, which might be considered an advantage in the design phase, but is an intrinsic weakness in the real device.

Reliability of capacitive RF MEMS switches subjected to repetitive impact cycles at different temperatures

The analysis of contact degradation in a not controlled atmosphere (air) at different temperatures in microstructures with electrostatic actuation is the main topic of this study. Different types of devices are subjected to 1 million impact cycles at three different temperatures (25 °C, 40 °C and 55 °C). The electrical properties are shown and the results are explained: a major operating temperature lead to a more reliable contact because the membrane internal stress decreases with the temperature, lowering the restoring force of the switch. The use of modified floating metal in the fabrication of the devices can improves the reliability of the contact producing a significant improvement in the lifetime.