Wolfgang A. Vitale

Steep slope VO 2 switches for wide-band (DC-40 GHz) reconfigurable electronics

This work proves the feasibility of electrically actuated, CMOS compatible, microwave VO₂ switches on SiO₂/Si substrates with low variability, 100% yield, better than 109 cycles lifetime, ultra-steep OFF-ON transition and better RF performance than previously reported VO₂ switches on Al₂O₃ substrates (flat -0.6 dB S_21-ON with -10 dB S_21-OFF at 40 GHz). The extensive characterization of the fabricated switches has led to an optimum design with maximized S_21-ON/S_21-OFF ratio and validation as a promising solution for wideband reconfigurable electronics.

Steep-slope Metal-Insulator-Transition VO2 Switches with Temperature-Stable High ION

This letter reports a detailed experimental investigation of the slope of the current switching between OFF and ON states exploiting the metal-insulator-transition (MIT) in vanadium dioxide devices. The reported devices are CMOS compatible two-terminal switches. We experimentally demonstrate for the first time the very little dependence on temperature of the steep slope of these switches, ranging from 0.24 mV/decade at room temperature, to 0.38 mV/decade at 50 °C. The fabricated devices show excellent ON-state conduction, with ION > 1.8 mA/μm or RON <; 3 mΩ/μm, for the whole range of investigated temperatures (from room temperature to the MIT transition temperature), which recommends them as future candidates for steep-slope, highly conductive, and temperature-stable switches.

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Tunable capacitors and microwave filters based on vanadium dioxide metal-insulator transition

We report the fabrication, modeling and characterization of novel microwave tunable capacitors based on the metal-insulator transition (MIT) of Vanadium Dioxide (VO2). We present the advantages of VO2-based capacitors over alternative technologies for microwave reconfigurable electronics in terms of ease of integration, design and performance at high frequency. We show the potential of the proposed devices for RF reconfigurable electronics by fabricating a tunable bandstop filter (22.5-19.8 GHz) with insertion loss <; 2 dB up to 40 GHz.

Fine pitch 3D-TSV based high frequency components for RF MEMS applications

The development of interconnections suitable for radio-frequency (RF) and millimeter-wave (mm-wave) applications is of foremost importance for the feasibility of high-quality substrate-integrated devices. For this purpose, we introduce and validate the technology to implement fine-pitch high-aspect ratio tungsten-filled through-silicon vias (W-TSVs) adapted for high-frequency applications. The presented technology is optimized for integration with RF MEMS, for which we propose a compatible fabrication process flow. We designed and characterized RF test structures to assess the quality of the W-TSVs and their suitability for radio-frequency integrated circuits (RFIC) applications, showing low insertion loss for TSV in coplanar waveguides (CPW) and high-performance wideband mm-wave antennas.

Graphene Quantum Capacitors for High Frequency Tunable Analog Applications

Graphene quantum capacitors (GQC) are demonstrated to be enablers of radio-frequency (RF) functions through voltage-tuning of their capacitance. We show that GQC complements MEMS and MOSFETs in terms of performance for high frequency analog applications and tunability. We propose a CMOS compatible fabrication process and report the first experimental assessment of their performance at microwaves frequencies (up to 10 GHz), demonstrating experimental GQCs in the pF range with a tuning ratio of 1.34:1 within 1.25 V, and Q-factors up to 12 at 1 GHz. The figures of merit of graphene variable capacitors are studied in detail from 150 to 350 K. Furthermore, we describe a systematic, graphene specific approach to optimize their performance and predict the figures of merit achieved if such a methodology is applied.

Modulated scattering technique in the terahertz domain enabled by current actuated vanadium dioxide switches

The modulated scattering technique is based on the use of reconfigurable electromagnetic scatterers, structures able to scatter and modulate an impinging electromagnetic field in function of a control signal. The modulated scattering technique is used in a wide range of frequencies up to millimeter waves for various applications, such as field mapping of circuits or antennas, radio-frequency identification devices and imaging applications. However, its implementation in the terahertz domain remains challenging. Here, we describe the design and experimental demonstration of the modulated scattering technique at terahertz frequencies. We characterize a modulated scatterer consisting in a bowtie antenna loaded with a vanadium dioxide switch, actuated using a continuous current. The modulated scatterer behavior is demonstrated using a time domain terahertz spectroscopy setup and shows significant signal strength well above 0.5 THz, which makes this device a promising candidate for the development of fast and energy-efficient THz communication devices and imaging systems. Moreover, our experiments allowed us to verify the operation of a single micro-meter sized VO2 switch at terahertz frequencies, thanks to the coupling provided by the antenna.

CMOS-compatible abrupt switches based on VO 2 metal-insulator transition

We report an extensive experimental study of CMOS-compatible switches based on vanadium dioxide abrupt metal-insulator transition. We perform scaling studies to provide guidelines for optimization of the geometry of the VO2 device for integration on CMOS circuits, discussing the trade-off between the design parameters and the effect on performance for DC and RF applications. A VO2 thin film with resistivity varying from 5·10-2 Ω·m in the insulating state to 1.5-10-5 Ω·m in the conductive state allowed us to fabricate devices with actuation current ranging from 1.43 to 2.46 mA and resistance in the insulating state ranging from 5.8 kΩ to 35.9 kΩ. We identified a minimum actuation current of 5.75 μA for our technology, in a device miniaturized up to the limits of standard optical lithography. We show through RF measurements and simulations that the performance at high frequency of the switch can be improved using VO2 thin films with higher conductivity values, even if the contrast between the two states is lower.

Self-assembled nano-electro-mechanical tri-state carbon nanotube switches for reconfigurable integrated circuits

We report, for the first time, self-assembled cantilever and clamped-clamped tri-state carbon nanotube (T-CNT) nano-electro-mechanical (NEM) switches with sub-100 nm air-gap dual lateral gates. Unlike conventional bi-state CNT switches, the T-CNT NEM switches operate in three states: CNT in the center (OFF), CNT attracted to the left gate (ON-1) or to the right gate (ON-2). They demonstrate excellent sensing current windows (Ion/Ioff~107), ultra-low Ioff (~10-14 A), good isolation and high endurance (cycle>;102). The proposed hysteretic switches offer a complementary metal-oxide-semiconductor (CMOS)-compatible bottom-up approach for various potential applications: logic devices, memories, etc., with higher circuit density and novel ultra-scaled configurability functions.

Investigation of the metal-insulator transition in VO 2 for electronic switches with sub-1mV/decade steep subthreshold slope

We report a thorough investigation of the electrically-induced metal-insulator transition in vanadium dioxide (VO2) for abrupt switching in 2-terminal devices (0.24 mV/dec at 25 °C, 0.38 mV/dec at 50 °C). We exploit the electrothermal actuation model based on Joule heating to model and predict the low temperature dependence of the slope in VO2 switches.