Jacopo Iannacci

A general purpose reconfigurable MEMS-based attenuator for Radio Frequency and microwave applications

In this paper we present a power attenuator for RF (RadioFrequency) and microwave signals entirely designed in MEMS (MicroElectroMechanical-System) technology. It is fabricated in the RF-MEMS technology available at Fondazione Bruno Kessler (FBK) based on a surface micromachining process. The network is realized in a low-cost manufacturing process and its dimensions are significantly compact compared to traditional implementations of RF power attenuators. More interestingly, employment of MEMS technology for such architecture enables a very large reconfigurability, making the network compatible with different standards and usable in several wireless communication systems. Electromechanical and RF behaviour of the discussed network are simulated and compared against experimental results collected by the first fabricated samples. RF measured performances are rather promising in spite a technology issue occurred during the fabrication deteriorating the attenuator low-frequency characteristic. RF modelling of such issue (already fixed in the batches being currently fabricated) is shown and discussed through this paper.

A flexible technology platform for the fabrication of RF-MEMS devices

The paper reports about the technology platform for the fabrication of RF-MEMS devices developed at FBK. The most important process features, requirements and possible applications are presented and described. The basic fabrication process, together with some of the more important process variations and its capabilities are reported. Finally, some examples of produced devices and their performances are briefly presented.

Experimental Validation of Mixed Electromechanical and Electromagnetic Modeling of RF-MEMS Devices Within a Standard IC Simulation Environment

The validity and applicability of a high-level simulation approach of radio-frequency microelectromechanical-system (RF-MEMS) devices, based on a library of analytical compact models of elementary MEMS components, are investigated through an extensive comparison between simulation results and measurements of some representative devices (variable capacitors and series ohmic switches). The in-house developed simulation tool is implemented in a standard IC simulation environment supporting behavioral description capabilities. The devices are built in a silicon substrate technology with suspended gold membranes. We analyze the mechanical, electrical, and RF response of the devices. The RF behavior is modeled by extracting a lumped element network from measured S-parameters (scattering-parameters) to account for parasitic effects and by wrapping this network around the intrinsic MEMS device simulated with the compact models. We show that an accuracy within 5% is obtained in all considered physical domains and conditions, provided that some effective parameters (including the residual air gap in the actuated state and the RF parasitic elements) are properly extracted from measurements and accounted for in the simulations. The main factors limiting the models predictive capability are due to process nonidealities, such as plate bending due to residual stress gradient, oxide charging, surface roughness, and suspended membrane thickness variations, rather than for instance in-plane geometric process variations.

Reliability of MEMS: A perspective on failure mechanisms, improvement solutions and best practices at development level

Abstract Reliability of MEMS (MicroElectroMechanical-Systems) devices is a crucial aspect as it can discriminate the successful from partially or totally missed reaching of Microsystem technology based market products. However, the topic of MEMS reliability is significantly articulated, as it comprises numerous physics of failure and diverse failure mechanisms. Thereafter, it requires a pronounced sensitivity related to the actual operation conditions (environmental and functional) of the Microsystem device within the final application. In other words, reliability of MEMS is nowadays regarded as a standalone transversal discipline that must be seriously taken into account already from the early design phase. The purpose of this paper is to provide the reader at first with basic knowledge around the concept of reliability. Thereafter, the most relevant physics of failure and failure mechanisms typical of MEMS are grouped and briefly discussed, with specific attention to their employment in the field of displays. A synthetic review of valuable solutions to improve specific reliability aspects of MEMS devices for diverse applications is then proposed to the reader. Eventually, a brief discussion focused on best practices to address properly reliability during the whole development chain of innovative MEMS based products completes the contribution. It is a belief of the author that the particular blend of topics and aspects reported in the following pages, as well as the attitude of considering reliability as a transversal discipline of science, contribute to provide this contribution with an important benefit if compared to the reviews on reliability of MEMS previously published in literature.

Multi-modal vibration based MEMS energy harvesters for ultra-low power wireless functional nodes

The aim of this contribution is to report and discuss a preliminary study and rough optimization of a novel concept of MEMS device for vibration energy harvesting, based on a multi-modal dynamic behavior. The circular-shaped device features Four-Leaf Clover-like (FLC) double spring-mass cascaded systems, kept constrained to the surrounding frame by means of four straight beams. The combination of flexural bending behavior of the slender beams plus deformable parts of the petals enable to populate the desired vibration frequency range with a number of resonant modes, and improve the energy conversion capability of the micro-transducer. The harvester device, conceived for piezoelectric mechanical into electric energy conversion, is intended to sense environmental vibrations and, thereby, its geometry is optimized to have a large concentration of resonant modes in a frequency range below 5-10 kHz. The results of FEM (Finite Element Method) based analysis performed in ANSYSTM Workbench are reported, both concerning modal and harmonic response, providing important indications related to the device geometry optimization. The analysis reported in this work is limited to the sole mechanical modeling of the proposed MEMS harvester device concept. Future developments of the study will encompass the inclusion of piezoelectric conversion in the FEM simulations, in order to have indications of the actual power levels achievable with the proposed harvester concept. Furthermore, the results of the FEM studies here discussed, will be validated against experimental data, as soon as the MEMS resonator specimens, currently under fabrication, are ready for testing.

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

Abstract 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 RF-MEMS ohmic switches, increasing their reliability.

A MEMS reconfigurable quad-band class-E power amplifier for GSM standard

In this paper we present a reconfigurable Class-E Power Amplifier (PA) whose operation frequency covers all uplink bands of GSM standard. We describe the circuit design strategy to reconfigure PA operation frequency maximizing the efficiency. Two dies, manufactured using CMOS and MEMS technologies, are assembled through bondwires in a SiP fashion. Prototypes deliver 20dBm output power with 38% and 26% drain efficiencies at lower and upper bands, respectively. MEMS technological issues degrading performance are also discussed.

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.

Evolution of electrical parameters of dielectric-less ohmic RF-MEMS switches during continuous actuation stress

The evolution of the main electrical parameters of dielectric-less ohmic RF-MEMS cantilever-based switches during continuous actuation stress was investigated in this work. Thanks to different designs, the main electrical parameters changes were attributed to a charging phenomena of the oxide over the substrate near the polysilicon actuator, leading to both narrowing and shifting of traditional hysteresis-like curves. Recovery procedures were also analyzed. Furthermore, the breakdown occurrence was also investigated, supported by both emission microscope and optical images.

A MEMS Reconfigurable Quad-Band Class-E Power Amplifier for GSM Standard

In this work we present a reconfigurable mid-power Class-E Power Amplifier (PA) [1,2] operating at ~900MHz and ~1800MHz (GSM standard [3]) realized hybridizing one chip manufactured in AMS 0.35 ?m CMOS technology and one MEMS sub-network. The CMOS chip realizes the active part of the circuit, whereas the MEMS block (realized in FBK technology) implements a reconfigurable impedance Matching Network (MN) that transforms the 50? antenna load to the 12? impedance required by the PA in order to deliver 20dBm output power in both the GSM operating frequency bands. The prototype of the MEMS/CMOS PA we realized delivers 20dBm with 38% and 26% drain efficiencies at 900MHz and 1800MHz, respectively, demonstrating to be a feasible option compared to standard commercial solutions.