Flavio Giacomozzi

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 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.

A Novel MEMS-Tunable Hairpin Line Filter on Silicon Substrate

In this paper a novel MEMS-tunable hairpin line filter is presented. The circuit has been realized on a 525 mum high resistivity silicon substrate using a well established micromachining process at ITC-IRST in Trento, Italy. The tunability of the device is obtained by line sections added to the U-line branches of the resonators of the hairpin filter through 10 identical MEMS ohmic contact cantilever switches. Measurements of the MEMS switch show an isolation and an insertion loss better than 20 dB and 0.3 dB, respectively, in the filter frequency range. Measured S-parameters of the MEMS-filter exhibit an insertion loss and a return loss better than 4.5 dB and 17 dB respectively, with a 15% passband at 6.2 GHz and 10% tuning range

Broadband RF-MEMS Based SPDT

A broadband single pole double throw (SPDT) switch has been developed for use in the range of 0 to 30 GHz. The switch consists of a cascade of a MEMS ohmic series and a capaci-tive shunt switch with floating electrode in each branch. It is manufactured on high-resistive silicon using surface micro-machining technology. The SPDT switch provides an insertion loss better than -0.6 dB, return loss smaller than -20dB, and isolation better than -40 dB in nearly the whole band. A switching voltage around 50 V is needed. The switch is used as a building block for more complex switching networks. The fabrication process is described and the measured RF-performances are reported and discussed. A failure analysis exhibits a lifetime up to 109 actuations

A MEMS-Reconfigurable Power Divider on High Resistivity Silicon Substrate

This paper presents a MEMS-reconfigurable power divider on high resistivity silicon substrate with variable power ratio. The circuit is based on two cascaded hybrid couplers connected through a tunable phase shifter that produces the required power ratio. A 5 state prototype has been fabricated on a 525 m high resistivity silicon substrate employing two 3 dB branch line couplers and a reflection-line MEMS phase shifter. The latter is reconfigured through two MEMS-switched open ended lines, whose lengths can be varied through the actuation of eight ohmic contact MEMS switches. Measurements of the MEMS switch show an isolation and an insertion loss better than 15 dB and 0.2 dB, respectively, with a contact resistance lower than 1 Ohm in the entire power divider bandwidth. RF measurements of the power divider exhibit a return loss better than 16 dB and an isolation better than 17 dB in the bandwidth [11.8-12.2] GHz with nominal power ratios of 1:0, 6:1,1:1,1:6, and 0:1.

RF-MEMS SPDT switch on silicon substrate for space applications

The paper illustrates the activity carried out under an ESA contract for the development of a miniaturized RF-MEMS SPDT switch and switch matrix using micromachining technology on a silicon substrate for power applications. A manufacturing procedure, based on an eight masks process, has been set up. At present, a broadband single-pole-double-throw (SPDT) switch operating in the 0-30 GHz frequency range has been fabricated and measured. Isolation of about -40 dB and insertion loss better than -0.7 dB have been obtained.

Dielectric charging in microwave microelectromechanical Ohmic series and capacitive shunt switches

The charging of the dielectric used for the actuation in microelectromechanical system (MEMS) devices is one of the major failure sources for switches based on this technology. For this reason, a better understanding of such an effect is vital to improve the reliability for both ground and space applications. In this paper, the expected response of MEMS switches to unipolar and bipolar dc actuation voltages has been measured and modeled. Two configurations of MEMS switches, namely, an Ohmic series and a shunt capacitive one designed for microwave applications, have been studied as a test vehicle for charging effects related to the dc actuation pads. The recorded data have been interpreted mainly through the Poole–Frenkel effect due to charge injection when a high voltage is applied to the dielectric layer. Metal-Insulator-Metal (MIM) structures have been also considered as a complementary information for the response of the dielectric material.

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.

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.