P. Ekkels

An electrostatic fringing-field actuator (EFFA): application towards a low-complexity thin-film RF-MEMS technology

This paper presents a novel electrostatic actuator using fringing fields as the actuation mechanism, i.e. an electrostatic fringing-field actuator or EFFA. The novel device is produced on an insulating substrate in a simple two-mask process involving only one sacrificial layer and one metallization. To demonstrate the EFFA capabilities, we produced and characterized EFFAs in various technological implementations of remarkable simplicity. This simplicity allows a vast flexibility for the processing and as a result strongly eases the integration of the EFFAs into existing technologies. For the basic device, we report a non-de-embedded measured capacitance ratio of 1:3 and a lifetime of more than 107 cycles with 40 V bipolar actuation at 100 Hz in N2 atmosphere. Both the capacitance ratio and the C–V profile were tuned by modifying the technology, e.g. coating the substrate before processing the EFFAs and the design, e.g. switching from clamped-free to clamped–clamped devices. We finally report a complete RF-MEMS technology using the EFFAs as single actuators. Switchable and tunable LC tanks, phase shifters, series and shunt parallel-plate capacitors actuated as relays and capacitive relays are presented to demonstrate the possibilities of this technology.

Evaluation of platinum as a structural thin film material for RF-MEMS devices

Surface micromachined metal armatures are commonly used for MEMS applications of which RF-MEMS is the most well known. In most cases metals with a high conductivity, such as aluminum or gold, are used. These metals often have a low melting point and therefore have a low thermal stability and show plastic deformation of the structures at relatively low temperatures (<200 °C). High melting point metals, such as platinum, are expected to show plastic deformation only at higher temperatures which makes them interesting for use as a structural layer in RF-MEMS devices. In this paper, we present a technology to realize suspended platinum structures by means of surface micromachining. An improved lift-off process allows patterning 1 µm Pt films on a polyimide sacrificial layer. A comparison of the characteristics and armature resonance frequencies between RF-MEMS switches with Pt armatures and AlCu0.5% alloy armatures reveals an increased thermal stability for the former up to at least 250 °C. This enables zero-level packaging of switches at relative high temperatures without affecting their performances. The lower conductivity of Pt compared to AlCu0.5% does not lead to a significant increase in RF losses. Implementing AlN as a dielectric material, the Pt-based capacitive shunt switches reported in this paper showed lifetimes in excess of 5×107 cycles under standard testing conditions.

Influence of the substrate on the lifetime of capacitive RF MEMS switches

We show for the first time that the substrate can influence the lifetime of capacitive RF MEMS switches. We demonstrate that the influence of the substrate should not be ignored. The influence of the environment on the lifetime of a switch is different when it is fabricated on two different substrates. We also present that a switch actuated with a DC voltage lower than the pull-in voltage can pull-in after some time. The goal of the performed experiment was to emphasize the charging of the substrate. The presented results help to understand the substrate charging problem.

New insights into charging in capacitive RF MEMS switches

This paper discusses dielectric charging in electrostatic RF-MEMS switches. We show that more than one charging mechanism can be present and impacts their lifetime. These different mechanisms can cancel, mitigate or enhance each otherpsilas influence on the lifetime, depending on the materials used and on the test conditions. Contrarily to the common understanding of the dielectric charging, we show that charge trapping in the dielectric interposer is not always the dominant charging mechanism leading to the failure. We finally show that bipolar actuation is not a general remedy for charging in electrostatic RF MEMS switches.

Simple and Robust Air Gap-Based MEMS Switch Technology for RF-Applications

This paper presents a simple and robust process for fabrication of functional electrostatic RF-MEMS switching devices with lifetimes easily exceeding 108 cycles with unipolar actuation at 100Hz. The device implements a switchable air gap capacitor and is therefore not limited in lifetime by dielectric charging as opposed to contact-type capacitive switches implementing high-k dielectrics. It is shown how these switched capacitors, even though having a capacitance ratio of only 2.8, can still form adequate switching devices and RF-circuits by proper design and combining of these devices with high-Q inductors and transmission lines. The novelty of the proposed process is that it combines a sacrificial layer consisting of a single layer with a single dry etching step for the dimples which define the air gap in the down-state. This airgap is switched by electrostatic actuation of a thick electroplated Nickel bridge-structure. The device is realized in a 4-lithographic steps process with low complexity and high robustness.

Electrostatic fringing-field actuator (EFFA): Application towards a low-complexity RF-MEMS technology

This paper presents a novel electrostatic actuator using fringing fields as actuation mechanism, ie electrostatic fringing-field actuator or EFFA. The novel device is produced on an insulating substrate in a simple 2-mask process involving only one sacrificial layer and one metallisation. As a proof of concept, we produced and characterized EFFA-based tuneable RF devices and circuits in various technological implementations of remarkable simplicity. This simplicity allows a vast flexibility for the processing and as a result strongly eases the integration of the EFFA s into existing technologies. We report a non-deembedded measured capacitance ratio of 1:3 and a lifetime of more than 10 cycles with 40 V bipolar actuation at 100 Hz in N2 atmosphere. Both the capacitance ratio and the C-V profile were tuned by modifying the technology, e.g. coating the substrate before processing the EFFA s, and the design, e.g. switching from clamped-free to clamped-clamped devices. Finally, note that the fringing-field actuation principle we demonstrated by means of RF-MEMS components can be used also as an electrostatic sensing mechanism.

A MEMS variable Faraday cage as tuning element for integrated silicon micromachined cavity resonators

This paper reports the design and the realization of a variable-sized MEMS-based Faraday cage, used to tune the resonant frequency of a silicon micromachined cavity resonator. The Faraday cage is defined by a 2D-array of electrostatically controllable metal cantilevers. The cantilever array effectively shields an enclosed (air) volume (forming the Faraday cage) located inside the micromachined cavity resonator. The Faraday cage perturbs the electromagnetic field distribution inside the cavity resonator and provides a means to electrically tune the resonant frequency of the resonator. In this realization, the cavity resonator was micromachined using bulk KOH etching in silicon and assembled in a low temperature die-to-wafer soldering process. The electrical feeding was realized in thin-film metallizations and the resonator is formed as a two-chip assembly. We measured a maximal frequency tuning of 1.2GHz at 60GHz with a tuning voltage of 16VDC.

Robustness of electrostatic MEMS actuators against electrical overstress

The response of electrostatic actuators to electrical overstress is studied in this paper. More specifically, the use of passive components (resistors and capacitors) to diminish the harmful effects of electrical overstress is demonstrated. The influence of these passives on the normal operation of the devices is also studied.

Novel Effa-Based Thin-Film RF-MEMS Technology

This paper presents applications of a novel electrostatic actuator using fringing fields as actuation mechanism, i.e. Electrostatic Fringing-Field Actuator or EFFA. The novel device is produced on an insulating substrate in a simple 2-mask process involving only one sacrificial layer and one metallisation. For the basic device, we report a non-deembedded capacitance ratio of 1:3 and a lifetime better than 107 cycles with 40 V bipolar actuation at 100 Hz in N2 atmosphere. The capacitance ratio and the C-V profile are tuned by modifying the technology and the design of the EFFAs. We present a complete RF-MEMS technology using the EFFAs as single actuators. We report for the first time on EFFA-based RF-circuits, e.g. phase shifter, as well as on tuneable parallel-plate capacitors and switches actuated by EFFAs as relays, key elements to solve the problems of bias distributions throughout complex circuits.

45 degrees loaded-line phase shifter using switchable slow wave transmission lines

The phase shifter design proves the RF principle of the EFFA based switchable slow-wave line. The EFFA can however be used as switch or tunable device in many other high-frequency applications in which a low capacitance value and moderate capacitance ratio suffices.