Experimental investigation on the exploitation of an active mechanism to restore the operability of malfunctioning RF-MEMS switches

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An Active Self-Recovery Mechanism to Restore the Operability of RF-MEMS Switches Affected by Stiction

MicroElectroMechanical switches for Radio Frequency applications, namely RF-MEMS, have been demonstrated to exhibit remarkable performance, like very low-loss, high Q-Factor and good linearity, thus enabling the manufacturing of low-cost lumped elements (switches, variable capacitors and inductors) as well as of complex networks as phase shifters for antenna/radar applications, power dividers and switching units and matrices

Aspects of Mechanical Reliability for RF-MEMS Switches with Self-Recovery Mechanism to Counteract Stiction

In this work we discuss the operation of an active self-recovery mechanism, embedded within MEMS (MicroElectroMechanical Systems) switches for Radio Frequency (RF) applications, able to counteract the stiction induced by charge accumulation [1] and micro-welding formation [2]. Such a mechanism, based on the thermo-electric effect, allows for restoring the MEMS switch back to normal operation after a failure. This is done by means of two factors, namely, the entrapped charges dispersion speed-up [3] within the insulating layer between the electrodes, and the application of shear forces on the welding points, both induced by the heat. Preliminary experimental results, collected by a few fabricated MEMS switch samples, confirm the viability of the proposed approach

Heat-Based Recovery Mechanism to Counteract Stiction of RF-MEMS Switches

Stiction of MEMS (MicroElectroMechanical System) switches for RF (Radio Frequency) applications is a critical issue as it may jeopardize temporarily or permanently the operability of such devices. In this work we present a novel mechanism to enable the self-recovery of RF-MEMS switches in case of stiction. It is based on the application of a restoring force on the stuck membrane, induced by the thermal stress due to self-heating of the switch itself. The heat is generated by a current driven through a high resistivity polysilicon serpentine housed underneath the anchoring points of the suspended switch. After a detailed theoretical analysis, we will report FEM-simulation results (Finite Element Method) describing the behaviour of the structure discussed in this paper

Modeling of gold microbeams for characterizing MEMS packages

Modeling of gold microbeams for characterizing MEMS packaging solutions in terms of strains induced to the MEMS devices as well as hermetic sealing capability is presented. The proposed test structures are meant to be manufactured by the surface micromachining front-end technology available at FBK. They are based on arrays of rectangular-shaped cantilever beams as well as clamped-clamped bridges, with a width of 20 μm and a length ranging from 100 to 400 μm, to be realized by a 2 μm thick film of electroplated gold. The resonant frequency of the microbeams is modeled by FEM simulations as a function of substrate deformations, which could be induced by the package. Clamped-clamped bridges show a linear change with respect to the square of the resonant frequency up to 1800 ppm/μstrain in case of in-plane deformations. The impact of temperature excursions is also simulated, in order to use these structures for assessing thermally induced deformations. Cantilever beams are modeled as variable capacitors to detect out-of-plane deformations. Finally, both an analytical model and FEM simulations are used to study cantilever beams as resonators for detecting pressure changes, showing an impact on the quality factor in a range from 1-2 bar down to 10^-3-10^-2 mbar

A Measurement Procedure of Technology-related Model Parameters for Enhanced RF-MEMS Design

The accurate design of Micro-Electro-Mechanical- Systems (MEMS) for Radio Frequency (RF) architectures (e.g., reconfigurable transceivers) relies on suitable models describing the static and, above all, the dynamic electromechanical and electromagnetic behaviour of moveable structures. Such models usually include multiple parameters, whose values depend on the adopted manufacturing technology, as well as the uncertainty sources affecting the process itself. As a consequence, measuring the technology-related model parameters of a given class of MEMS structures is essential to estimate and to reduce, at an early design stage, possible mismatches between simulation results and device performances. In order to address this issue, in this paper we describe a procedure to measure the parameters describing the behaviour of RF-MEMS switches that are most severely affected by residual mechanical stress and surface roughness. The validity of the proposed methodology is confirmed by the good accordance between simulation and experimental results

Experimental Investigation on the Exploitation of an Active Mechanism to Restore the Operability of Malfunctioning RF-MEMS Switches

RF-MEMS (MicroElectroMechanical-Systems for Radio Frequency applications) switches and components can enable the realization of high-performance and highly-reconfigurable blocks for a variety of applications in the field of telecommunications, spanning from mobile phones to scanning radar systems and satellite communications. Nevertheless, the exploitation of MEMS technology in the RF field is still limited by the relatively poor reliability of RF-MEMS devices and networks. In this work, we discuss the exploitation of an active mechanism that was recently presented by the authors, and capable of improving the robustness of RF-MEMS switches against stiction. The mechanism exploits the heat generated by an electric current driven through a high-resistivity PolySilicon serpentine, embedded within the switch structure, to recover the normal operability of the RF-MEMS relay, and is effective both against charge entrapped in the insulating layer as well as micro-welded spots due to large RF signals. The mechanism can be added with only minimal changes to a wide variety of already existing RF-MEMS switches and components topologies. In this paper we report the first experimental results showing a successful release of a stuck switch after the heater is activated. Moreover, we discuss proper activation methods of the proposed mechanism by performing FEM simulations in order to maximize the benefits of the PolySilicon heater operation without impairing the mechanical characteristic of the MEMS switch

An active heat-based restoring mechanism for improving the reliability of RF-MEMS switches

We propose an active mechanism to retrieve the functionality of RF-MEMS ohmic switches after stiction occurs. The mechanism exploits a micro-heater, embedded within the switch topology, to induce restoring forces on the stuck membrane (thermal expansion) when a current is driven through it. Our experimental investigations prove that driving a pulsed rather than a DC current into the heater, enables a successful release of the tested RF-MEMS stuck devices. The release of stuck RF-MEMS ohmic switches is demonstrated for a cantilever-type micro relay. The mechanism is suitable for a large variety of switch topologies, and it can be embedded with small changes and effort within most of the already existing RFMEMS ohmic switches, increasing their reliability

Enhancement of RF-MEMS switch reliability through an active anti-stiction heat-based mechanism

MicroElectroMechanical Systems for Radio Frequency applications (i.e. RF-MEMS) show very good performance and characteristics. However, their employment within large-scale commercial applications is still limited by issues related to the reliability of such components. In this work we present the Finite Element Method (FEM) modelling and preliminary experimental results concerning an active restoring mechanism, embedded within conventional MEMS/RF-MEMS ohmic (and capacitive) relays, capable of retrieving the normal operation of the switch if stiction occurs (i.e. the missed release of an actuated switch when the controlling bias is removed). The mechanism exploits the heat generated by an electric current flowing through an high-resistivity poly-silicon serpentine (Joule effect), to induce deformations in the suspended MEMS structures. Such changes in the mechanical structure result in shear and vertical restoring forces, helping the membrane release. The FEM-based thermo-electromechanical simulations discussed in this work include the coupling between different physical domains, starting from the imposed current, to the MEMS deformation. The preliminary experimental data reported in this paper show a speed-up of the dielectric discharge time due to the generated heat, as well as a change in the S-parameters, due to the membrane expansion, compatible with an upward bending of the central contact (i.e. restoring force), useful to counteracting stiction due to micro-welding

Experimental Investigation of an Embedded Heating Mechanism to Improve RF-MEMS Switches Reliability

Up to date, the remarkable performances and characteristics of MEMS switches and lumped components for Radio Frequency applications (i.e. RF-MEMS) have been demonstrated by several Authors [1, 2]. On the other hand, the reliability of such a technology still has to be fully addressed in order to enable a successful penetration of RF-MEMS technology into the market [3]. Reliability of MEMS/RF-MEMS involves several physical phenomena that can jeopardize their normal operation as well as the stability of their characteristic vs. time [4]. Among such different effects, the authors believe that one of the most important source of malfunctioning is the stiction (i.e. the switch remains stuck in the actuated position when the controlling bias is removed) due to the charge entrapment into the insulator layer and/or the formation of micro-welding [5]. In order to counteract stiction, the authors already presented an innovative RF-MEMS switch design employing an active restoring mechanism, based on an high-resistivity serpentine heater (see Figure 1) to bring it back to its normal operability when stiction occurs [6]. In this work we report on the experimental testing recently performed on such test structures employing the active mechanism, fabricated in the FBK RF-MEMS technology. Firstly we used the Laser Doppler Vibrometer (LDV) integrated into a Polytec MSA-500 optical profilometer to verify the effectiveness of the heating mechanism to induce a movement of the suspended bridge. Figure 2 shows the vibration velocities as a function of the frequency of the bridge central part when a periodic chirp signal (iheater = 2 mA) is applied to the heater. We have also verified the transient behavior over time, measuring the bridge displacement induced by the heating (not by the actuator), when a square pulsed signal is applied to the heater pads, see Figure 3. Once we checked the good response of the bridge to the heating mechanism, we tested the effectiveness of the heating on the possibility to release the switch whether stiction occurred. Being very difficult to predict if a switch is stuck, we applied the following procedure, controlled by a LabView interface: (1) actuate the switch at VBIAS = 80 V; (2) decrease VBIAS at a value slightly higher than the release voltage; (3) switch-on the heating mechanism for a user selectable time; (4) switch-off the heater. Monitoring continuously the Sparameters over time it was possible to study the bridge behavior. Figure 4 shows an example of such procedure, as well as the release of the bridge induced by the embedded heater. In order to better understand the behavior, and the limits, of such restoring mechanism, we acquired thermal images of two pieces of wafer with different area (“small” ~ 1 cm2, “big” ~ 2.4 cm2) with a FLIR A20 infrared camera. Figure 5 shows the comparison of the two dies temperature images, taken at fixed intervals, when IHEATER = 4 mA (DC). It is clear the impact on heating of the bigger die, limiting the increase of the substrate temperature because of the higher thermal dissipation. This is confirmed also by Figures 6 and 7, that show the substrate temperature increase over time at different heater currents on small and big die respectively (slower increase on bigger die)