Alessandro Faes

Stress characterization of electroplated gold layers for low temperature surface micromachining

In this work, the stress of electroplated gold films has been analyzed versus plating current density and bath temperature. Two different plating solutions have been adopted, one being based on cyanide-gold salt, the other on sulfite-gold. Gold surface quality was investigated in the experimented range of plating temperature and current density, in order to control the limit conditions for plating: surface roughness and non-uniformity appear whenever deposition parameters are brought to the limit (typically below 2 mA/cm/sup 2/ and above 5 mA/cm/sup 2/). Plated gold stress measurement was carried out by wafer curvature comparison, before and after deposition, using Stoneys formula for thin films/sup 1,2/. A current density range between 1.5 and 6 mA/cm/sup 2/ and temperature range between 50 and 70/spl deg/C was investigated. Stress analysis was also carried out on a Cr-Au multilayer, which actually is the structural layer employed for gold microstructures: the multilayer consists of a chromium adhesion layer, a PVD gold seed layer and a plated gold layer, with thickness respectively 10 nm, 150 nm and 1500 nm. A range of stress was obtained, varying from tensile to compressive: cyanide bath yielded stress from -30 MPa to about 0 MPa, and sulfite bath showed stress between -90 MPa and 110 MPa. Stress variation induced by thermal treatments after deposition was also investigated, by examining the effect of photoresist sacrificial etching on the internal stress of chromium-gold structural layers: the final stress was about 180 MPa tensile for all samples, regardless the as-deposited stress, with a variation ranging from about 80 MPa to more than 200 MPa.

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.

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.

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.

A low-cost microsystem for noninvasive uroflowmetry

A microsystem composed of a micromachined resistive flow sensor and a signal conditioning CMOS IC is proposed for biomedical applications. The device can be adapted to noninvasively monitor urinary dysfunctions in male patients. The flow sensor, thermally simulated with ANSYS, is based on the hot-film principle: A thin film of gold laid on a suspended micromachined silicon membrane is heated while the fluid under test flows through the duct mounted above the membrane. The flow rate is sensed by measuring the temperature difference between two of the four polysilicon temperature sensors realized on the membrane. Simulations of the flow sensor with flow rates within 0.1-18 ml/s evidence a maximum temperature difference of 20degC between the temperature sensors. Characterization of the fabricated flow sensor shows temperature coefficient of resistance (TCR) values of -1930 ppm/degC for the polysilicon resistors, i.e., a resistance variation of about 4% at high flow rates. The CMOS readout designed for the flow sensor is a resistive bridge-to-duty cycle converter based on a relaxation oscillator. The digital output of the circuit is duty-cycle modulated by the change in resistance of the flow sensors elements. Experimental tests on the CMOS interface, conducted with a setup of 1% precision resistors, report a maximum nonlinearity below 0.9% and a resolution of 7 bits over the full range of 4% resistance variation. The CMOS integrated readout circuit, provided with a digital output, allows simple signal interfacing towards any standard PC for periodical data transfer and storage

Influence of Etching Potential on Convex Corner Anisotropic Etching in TMAH Solution

Anisotropic etching with tetramethylammonium hydroxide (TMAH) water solutions is a simple and CMOS-compatible way to obtain geometrical patterns in single-crystal silicon wafers. The fabrication of trenches and other features is although limited by the need to compensate convex corners which tend to be etched very fast. Such compensation produces footings at the bottom edge of the etched walls, yielding a complex and scarcely predictable geometry which might affect the performance of devices in applications such as fluidics. The etch rates for different crystal planes are affected not only by the TMAH concentration and etching temperature but also by the etching reaction potential. In this work, shallow TMAH etching of single-crystal silicon wafers in 25-wt% TMAH at 90°C is examined up to a depth of 30 m, at potentials ranging from -1 to -2 V, using etching masks to obtain compensated convex corners. Identification of the sidewalls as {311} planes is performed by angle measurement on SEM and optical images. The etch ratio of the (100) crystal plane versus both (311) and (111) planes is measured at the various potentials. Morphological differences between cathodic and anodic potentials with respect to the open-circuit potential (OCP) are examined: Cathodic etching (between OCP and -2 V) yields footing patterns and a pronounced undercut, while anodic etching (between -1 V and OCP) produces smooth sidewalls with no footing, an increased (100)/(311) etch ratio, and a decreased (100)/(111) etch ratio.

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.

Characterization of quartz-based package for RF MEMS

In the last decade Micro-Electro-Mechanical Systems (MEMS) technology experienced a significant development in various fields of Information and Communication Technology (ICT). In particular MEMS for Radio Frequency (RF) applications have emerged as a remarkable solution in order to fabricate components with outstanding performances. The encapsulation of such devices is a relevant aspect to be addressed in order to enable wide exploitation of RF-MEMS technology in commercial applications. A MEMS package must not only protect fragile mechanical parts but also provide the interface to the next level of the packaging hierarchy in a cost effective technology. Additionally, in RF applications the electromagnetic impact of the package has to be carefully considered. Given such a scenario, the focus of this work is the characterization of a chip capping solution for RF-MEMS devices. Such solution uses a quartz cap having an epoxy-based dry film sealing ring. Relevant issues affecting RF-MEMS devices once packaged, e.g. the mechanical strain induced by the cap and the hermeticity of the sealing ring, are worth investigating. This work focuses on the study of induced strain, as a function of different bonding parameters. Dimensional features of the sealing ring (i.e. the width), and process parameters, like temperature and pressure, have been considered. The package characterization is performed by using basic test vehicles, such as strain gauges, designed to be integrated inside the internal cavity of the package itself. Polysilicon piezoresistors are used as strain gauges, whereas aluminum resistors are used as thermometers to assess the impact of temperature changes on strain measurements. Experimental data are reported including calibration of the sensors as well as environmental measurements with and without cap. In addition measurements of the shear stress of the proposed packaging solution are also reported.

MEMS technology for RF passive components

In this work a wide overview on the exploitation of MEMS technology (MicroElectroMechanical-Systems) for Radio Frequency (i.e. RF-MEMS) and telecommunication systems is provided. In the last 10-15 years MEMS technology proved to be a valuable solution for the realization of passive components for RF circuits, like variable capacitors (i.e. varactors), inductors, switches (i.e. micro-relays), as well as more complex networks, like RF filters, impedance matching tuners etc., with remarkable characteristics in terms of low-loss, high Q-factor (quality factor), and large reconfigurability. The integration within RF circuits and platforms, of passive lumped elements and networks with such prominent characteristics, would lead to a significant increase of performance and reconfigurability of the whole system (e.g. mobile handset, satellite, radar, and so on).