Cristiano Palego

Robustness of RF MEMS Capacitive Switches With Molybdenum Membranes

This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

Anelastic Stress Relaxation in Gold Films and Its Impact on Restoring Forces in MEMS Devices

In order to evaluate the importance of stress relaxation on device performance of capacitive RF MEMS switches, stress relaxation has been measured in 1.2-mum-thick Au films using a membrane bulge technique. When the residual stress in the films is small, the stress relaxation is fully recoverable and is well described by linear anelasticity (viscoelasticity) theory. A 27% reduction in the effective elastic modulus occurs over a three-day period under constant strain conditions at room temperature. The time dependence of the relaxation can be represented by a series of time constants with values extending from seconds to days. Linear superposition of the anelastic response can be used to accurately predict the stress under any time dependence of the strain. The prediction is accurate even during cyclic loading and unloading, and even when the strain is cycled at rates that are fast compared with any of the relaxation times. The restoring force available to open a capacitive RF MEMS switch is modeled for two different switch designs. The restoring force is shown to drop by approximately 7% or 20% at room temperature for the two cases presented.

Impact of Humidity on Dielectric Charging in RF MEMS Capacitive Switches

A novel technique is used to distinguish the charging of the surface from that of the bulk of the dielectrics of different types of RF MEMS capacitive switches under different electric fields and humidity levels. In general, bulk charging dominates in dry air, while surface charging increases linearly with increasing humidity. Under comparable electric fields and humidity levels, switches made of silicon dioxide are less susceptible to surface charging than switches made of silicon nitride. These quantitative results not only underscore the importance of packaging the switches in a dry ambient atmosphere, but also validate the novel technique for evaluating the effectiveness of dielectric preparation and packaging.

Broadband Electrical Detection of Individual Biological Cells

To resolve the dilemma of cell clogging or solution parasitics encountered by Coulter counters and to evolve a general-purpose electrical detection technique, we used broadband microwave measurements to overcome electrode polarization, ac dielectrophoresis to precisely place cells between narrowly spaced electrodes, and relatively wide microfluidic channels to prevent cell clogging. This unique combination of approaches resulted in reproducible sensing of single Jurkat and HEK cells, both live and dead, of different cultures at different times.

A Two-Pole Lumped-Element Programmable Filter With MEMS Pseudodigital Capacitor Banks

This paper presents a novel two-pole reconfigurable bandpass filter on alumina substrate for applications in X- and S-bands. An analytical approach was followed for synthesis of multiple filtering characteristics. A microstrip network on alumina was then optimized to implement a set of switched filtering functions. Finally a reconfigurable filter was fabricated and tested in order to validate the proposed approach. This filter exploits five 3-bit capacitor banks controlled with microelectromechanical systems ohmic switches to achieve 12 states with 37.5% tuning range between 1.51-2.26 GHz. Several tuning mechanisms are demonstrated including frequency, bandwidth tuning, and frequency + bandwidth tuning. Good agreement with theoretical results has been obtained.

Compact RF Model for Transient Characteristics of MEMS Capacitive Switches

A compact model is proposed to facilitate the design and simulation of the control waveform of RF microelectromechanical systems (MEMS) capacitive switches with electrostatically actuated membranes. Following conventional approaches, the pull-in motion of a membrane is simulated by an L-R-C network. However, the present model deviates from conventional approaches by adding another capacitor and a diode to simulate the gradual contact of the membrane with the stationary electrode. After contact, variable mass, spring constant, and damping factor are used to simulate the release process of the membrane. By smoothly bridging the model between pull-in, contact, and release processes, the model can efficiently simulate static and transient S-parameters of the switches up to 50 GHz. The model can be readily installed in popular computer-aided circuit design environments to analyze in the time domain the behavior of the switches and the operation of MEMS-based circuits.

Assessment of Cytoplasm Conductivity by Nanosecond Pulsed Electric Fields

The aim of this paper is to propose a new method for the better assessment of cytoplasm conductivity, which is critical to the development of electroporation protocols as well as insight into fundamental mechanisms underlying electroporation. For this goal, we propose to use nanosecond electrical pulses to bypass the complication of membrane polarization and a single cell to avoid the complication of the application of the “mixing formulas.” Further, by suspending the cell in a low-conductivity medium, it is possible to force most of the sensing current through the cytoplasm for a more direct assessment of its conductivity. For proof of principle, the proposed technique was successfully demonstrated on a Jurkat cell by comparing the measured and modeled currents. The cytoplasm conductivity was best assessed at 0.32 S/m and it is in line with the literature. The cytoplasm conductivity plays a key role in the understanding of the basis mechanism of the electroporation phenomenon, and in particular, a large error in the cytoplasm conductivity determination could result in a correspondingly large error in predicting electroporation. Methods for a good estimation of such parameter become fundamental.

Broadband microchamber for electrical detection of live and dead biological cells

A novel broadband microchamber for electrical detection of live and dead biological cells was designed, fabricated and tested. The microchamber was formed between a gold coplanar waveguide fabricated on a quartz slide and the microfluidic channels fabricated in a polydimethylsiloxane cover. The coplanar waveguide allowed broadband impedance matching and efficient cell trapping. The microfluidic channels delivered single cells precisely. Tests on Jurkat cells in both time and frequency domains showed that live cells had lower resistance but higher capacitance than that of dead cells.

RF-MEMS Switched Varactor for High Power Applications

A new kind of MEMS switched varactor has been developed to handle high power RF signals, (P>1W), at S and X-band. The design principle is presented, as well as measurements results featuring an analysis of the reliability tests. The fabricated varactors have shown a capacitance ratio of 7-8 at 5 GHz and good performances reproducibility while undergoing 1 billion cycle tests under an input RF power of 1W @ 10GHz, and 250 million cycle tests under 5W @ 3GHz, both in hot switching conditions, without visible degradation

Performance of molybdenum as a mechanical membrane for RF MEMS switches

This article details the construction and measurement of RF MEMS capacitive switches using molybdenum as the mechanical material. The resulting switches exhibit a significantly reduced rate of change in actuation voltage over temperature, with rates less than 0.03 V/°C up to 150°C. Resistivity of the molybdenum membranes averaged 10–11 μΩ-cm, yielding an effective shunt resistance of less than 0.25 Ω. Initial cycling measurements were made which show that the resulting membranes were capable of at least 20 billion cycles without failure, indicating that molybdenum is a promising mechanical material for constructing RF MEMS switches.