Aurelian Crunteanu

rf-microwave switches based on reversible semiconductor-metal transition of VO2 thin films synthesized by pulsed-laser deposition

Microwave switching devices based on the semiconductor-metal transition of VO2 thin films were developped on two types of substrates C-plane sapphire and SiO2 / Si, and in both shunt and series configurations. Under thermal activation, the switches achieved up to 30-40 dB average isolation of the radio-frequency rf signal on 500 MHz-35 GHz frequency band with weak insertion losses. These VO2-based switches can be electrically activated with commutation times less than 100 ns, which make them promising candidates for realizing efficient and simple rf switches.

Charging in Dielectricless Capacitive RF-MEMS Switches

This paper presents results on high lifetime RF microelectromechanical (RF-MEMS) dielectricless capacitive switches. Using air gap variation only, these RF MEMS capacitive switches achieve a capacitance ratio of 9, associated with small residual charging. The reliability of the switch has been studied, pull-in and pull-out voltages shifts have been observed, modeled and validated with good agreement between model and measurements for a switch held for one month in the down state. The dependence of charging mechanism to the biasing signal is also presented by applying unipolar and bipolar waveforms with different duty cycles. Charging can be modeled using a simple Curie-Von Schweidler equation. It is shown that the duty cycle of the bias signal is strongly accelerating switch failure, and that using bipolar signals improves the lifetime of these switches by several orders of magnitude. The projected lifetime of current RF-MEMS dielectricless switches held in the down state using bipolar signals is several tens of years.

Dielectric less capacitive MEMS switches

This paper presents the fabrication and experimental results on a capacitive switch without dielectric. Using an appropriate design of the switch, acceptable performances have been obtained with Con/Coff ratio around 10. The key advantage of this switch is that dielectric charge trapping related failure modes are eliminated. An example is shown with measured intrinsic loss less than 0.3 dB at 15 GHz and isolation better than 15 dB at the same frequency.

Voltage and current-activated metal-insulator transition in VO2-based electrical switches : a lifetime operation analysis

Abstract Vanadium dioxide is an intensively studied material that undergoes a temperature-induced metal–insulator phase transition accompanied by a large change in electrical resistivity. Electrical switches based on this material show promising properties in terms of speed and broadband operation. The exploration of the failure behavior and reliability of such devices is very important in view of their integration in practical electronic circuits. We performed systematic lifetime investigations of two-terminal switches based on the electrical activation of the metal–insulator transition in VO2 thin films. The devices were integrated in coplanar microwave waveguides (CPWs) in series configuration. We detected the evolution of a 10 GHz microwave signal transmitted through the CPW, modulated by the activation of the VO2 switches in both voltage- and current-controlled modes. We demonstrated enhanced lifetime operation of current-controlled VO2-based switching (more than 260 million cycles without failure) compared with the voltage-activated mode (breakdown at around 16 million activation cycles). The evolution of the electrical self-oscillations of a VO2-based switch induced in the current-operated mode is a subtle indicator of the material properties modification and can be used to monitor its behavior under various external stresses in sensor applications.

High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes

We report the fabrication of VO2 -based two terminal devices with 125-nm gaps between the two electrodes, using a simple, cost-effective method employing optical lithography and shadow evaporation. Current-voltage characteristics of the obtained devices show a main abrupt metal-insulator transition (MIT) in the VO2 film with voltage threshold values of several volts, followed by secondary MIT steps due to the nanostructured morphology of the layer. By applying to the two-terminal device a pulsed voltage over the MIT threshold, the measured switching time was as low as 4.5 ns and its value does not significantly change with device temperature, supporting the evidence of an electronically driven

Sub-Microsecond RF MEMS Switched Capacitors

This paper presents fast switching RF microelectromechanical systems (MEMS) capacitors with measured switching times between 150-400 ns. By introducing bent sides on a planar microbeam, it is shown experimentally that the resonance frequency of aluminum bridges is increased by a factor of 25 compared to standard RF MEMS components. In addition, this original shape can be implemented easily in post-processing of complementary metal-oxide-semiconductor circuits. Several designs are presented with measured mechanical resonance frequencies between 1-3 MHz and measured switching times under 400 ns. Using this original approach, sub-microsecond RF MEMS switched capacitors have been designed and fabricated on quartz substrate. Their resulting RF performance is presented with a measured capacitance ratio of 2.3. Reliability tests have also been performed and have demonstrated no significant mechanical behavior variation over 14 billion cycles and a moderate sensitivity to temperature variation.

Performance of Ar/sup +/-milled Ti:sapphire rib waveguides as single transverse-mode broadband fluorescence sources

Rib waveguides have been fabricated in pulsed-laser-deposited Ti:sapphire layers using photolithographic patterning and subsequent Ar/sup +/-beam milling. Fluorescence output powers up to 300 /spl mu/W have been observed from the ribs following excitation by a 3-W multiline argon laser. Mode intensity profiles show high optical confinement and the measured beam propagation factors M/sub x//sup 2/ and M/sub y//sup 2/ of 1.12 and 1.16, respectively, indicate single transverse-mode fluorescence emission. Loss measurements using the self-pumped phase conjugation technique have yielded comparable values (1.7 dB/cm) for the ribs and the unstructured planar waveguide counterparts. The combination of optimum modal properties and strong optical confinement, together with sufficient levels of fluorescence output, make the single-moded Ti:sapphire rib waveguides a very interesting candidate as a fluorescence source for optical coherence tomography applications.

Split Ring Resonators (SRRs) Based on Micro-Electro-Mechanical Deflectable Cantilever-Type Rings: Application to Tunable Stopband Filters

A new principle for the implementation of tunable split ring resonators (SRRs) is presented. The rings forming the SRRs are partly fixed to the substrate (anchor) and partly suspended (up-curved cantilever). Through electrostatic actuation, the suspended parts are deflected down, the distributed capacitance between the pair of coupled rings is modified, and hence the resonance frequency of the SRR can be electrically tuned. To obtain electrically movable rings, a cantilever-type micro-electro-mechanical-system (MEMS) design and fabrication process is applied. The resonance frequencies at the different switching states are measured by coupling the tunable SRR to a host microstrip line, and reveal that significant tuning ranges can be achieved. This novel tuning concept is applied to the implementation of tunable stopband filters at Ku band as proof-of-concept demonstrators.

Current-induced electrical self-oscillations across out-of-plane threshold switches based on VO2 layers integrated in crossbars geometry

Electrically activated metal-insulator transition (MIT) in vanadium dioxide (VO2) is widely studied from both fundamental and practical points of view. It can give valuable insights on the currently controversial phase transition mechanism in this material and, at the same time, allows the development of original MIT-based electronic devices. Electrically triggered insulator-metal transitions are demonstrated in novel out-of-plane, metal-oxide-metal type devices integrating a VO2 thin film, upon applying moderate threshold voltages. It is shown that the current-voltage characteristics of such devices present clear negative differential resistance effects supporting the onset of continuous, current-driven phase oscillations across the vanadium dioxide material. The frequencies of these self-sustained oscillations are ranging from 90 to 300 kHz and they may be tuned by adjusting the injected current. A phenomenological model of the device and its command circuit is developed, and allows to extract the analytical expressions of the oscillation frequencies and to simulate the electrical oscillatory phenomena developed across the VO2 material. Such out-of-plane devices may further contribute to the general understanding of the driving mechanism in metal-insulator transition materials and devices, a prerequisite to promising applications in high speed/high frequency networks of oscillatory or resistive memories circuits.

Microwave Power Limiting Devices Based on the Semiconductor–Metal Transition in Vanadium–Dioxide Thin Films

We present a novel concept of microwave (MW) power-limiting devices based on reversible semiconductor-to-metal transition (SMT) of vanadium-dioxide thin films integrated on coplanar waveguides. We designed, simulated, and fabricated devices, which can be reversibly driven from a low-loss (<; 0.7 dB) transmission state into an attenuating state (> 20 dB) as the VO2 material is changing from semiconductor to the metal state when the incident MW power exceeds a threshold value. These devices are broadband and present a tunable threshold power value. They could be easily integrated as protection circuits from excess power in a large variety of MW components.