The schematic presentation of (a) the brushlike (VANTA-based) and (b) the comblike (VACNF-based) varactor.
The generic two-dimensional NEMS varactor model. The electrostatic force is represented by an arbitrary function F .
The one-dimensional lumped model of the NEMS varactor.
The characteristic voltage-capacitance curve of NEMS varactor. The system becomes mechanically instable at the pull-in voltage and the electrodes snap into each other. This limits the maximum capacitance tunability in a standard NEMS varactor to 50%.
The equipotential lines at a cross-section level between (a) a single pair of oppositely charged VACNFs, (b) two sparsely populated rows of VACNFs, and (c) two rows of densely populated VACNFs as simulated by COMSOL Multiphysics.
Schematic illustration of the process flow for the comblike varactor. (a) The silicon substrate with 400 nm thermally grown silicon dioxide. (b) 5 nm of Cr is deposited first, and then Ti/TiN contact pads are defined on top of it in a photo-lithography step. (c) Dual-layer e-beam resist is spun and patterned. (d) The electrode pattern is transferred to the wafer in a lift-off process. (e) The second dual-layer e-beam resist is spun and patterned. (f) The catalyst seeds pattern is transferred on top of the electrodes in a second lift-off process. (g) The VACNFs are synthesized. (h) The exposed area of the Cr layer is etched in a Cl2/O2 reactive ion etching process.
A SEM micrograph of a comblike varactor taken at sample tilt of 30°. Although the size of the catalyst seeds were precisely controlled using e-beam lithography, the nanofibers uniformity is still very poor both in terms of diameter and height. The electrodes are 10 μm long. The VACNFs are about 80 nm wide and a micrometer long.
A SEM micrograph of a brushlike varactor. The VANTA walls have a width of 4 μm, a height of 135 μm, and are separated by 10 μm. Adopted with permission from N. Olofsson, J. Ek-Weis, A. Eriksson, T. Idda, and E. E. B. Campbell, Nanotechnology 20, 385710 (2009). © 2009, Institute of Physics.
SEM images of the brushlike varactor by Arun et al. The VANTA walls are 7.5 μm thick and are designed in a rectangular shape to enhance their vertical alignment. Adopted with permission from A. Arun et al., Solid State Device Research Conference ESSDERC ’09, 335 (2009). © 2009, IEEE.
(a) Equivalent circuit used to fit the experimental results. (b) S21 parameter measured for the frequency range 200 MHz – 1.5 GHz for 0 V and 27.5 V actuation voltages (black line). The equivalent circuit model fits are also shown (red dashed lines). Adopted with permission from N. Olofsson, J. Ek-Weis, A. Eriksson, T. Idda, and E. E. B. Campbell, Nanotechnology 20, 385710 (2009). © 2009, Institute of Physics.
COMSOL Multiphysics was used to simulate the parasitic capacitance from the electrodes (150 nm thick and 300 nm wide) and the tunable capacitance from the VACNFs (3 μm tall and 100 nm wide). The figure shows the equipotential lines created by (a) only the electrodes and (b) the combination of the electrodes and the VACNFs.
(Color online) The parasitic capacitance from the electrodes and the tunable capacitance from the VACNFs were computed as a function of the gap between the electrodes. The tuning ratio of a comblike varactor was then calculated using Eq. (17) .
(Color online) The maximum tunability of a comblike varactor is affected by the presence of a conductive substrate beneath the oxide on top of which the varactor is fabricated. The plot shows the deterioration in the tunability for a grounded and a floating potential substrate as a function of the oxide thickness.
(Color online) Q-factor extracted from the measured resistance and capacitance as a function of frequency. Upper (blue) line: nonactuated device (0 V), Lower (red) line: actuated device with an applied voltage of 27.5 V between the nanotube walls. Adopted with permission from N. Olofsson, J. Ek-Weis, A. Eriksson, T. Idda, and E. E. B. Campbell, Nanotechnology 20, 385710 (2009). © 2009, Institute of Physics.
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