We report a detailed experimental investigation of the adhesive clamping instability in CNT nanoresonators fabricated on silicon wafers with palladium electrodes and suspended CNT channels. The nanotube is clamped down onto the palladium electrodes adhesively by van der Waals forces and operates in the string regime. We observe a decrease in the nanotube tension when the device is operated in large amplitude regime. This mechanical stress relaxation, or decrease in internal stress of the nanotube, was observed as a frequency downshift resulting from weak clamping behavior between the nanotube and the underlying palladium surface. Frequency downshifts from 97.5 MHz to 39 MHz with 60 % stress relaxation and from 72.7 MHz to 60.5 MHz (17 % relaxation) were observed for two devices. Q-factors show no change due to decrease in internal stress. Our temperature measurements in the range of 298-420 K suggest that Q-factors might arise from the interplay between adhesive clamping associated dissipation mechanisms and spectral broadening due to thermal fluctuations.

The use of selective atomic layer deposition (ALD) of platinum and alumina was investigated for topclamping and passivation of contacts of suspended pristine single-walled carbon nanotubes (SWNTs). This wet-chemical-free process offers an interesting alternative to lithography-based methods as it avoids degradation of the transfer characteristics by resist residues or amorphous carbon deposition on the carbon nanotubes (CNTs) in field-effect transistor (FET) configuration. It was observed that the annealing and clamping effect can be used not only to obtain lower contact resistances and higher signal to noise ratio (SNR), but also to induce, what appears to be, band gap opening in metallic nanotubes. On average, a current increase of more than one order of magnitude and SNRs as high as 200 could be observed for metallized and passivated small-gap semiconducting CNTs.

Abstract:This work describes an on-chip integrated micro-actuator device for slack-free carbon nanotube (CNT) resonators, improving frequency tunability and Q factor and to study non-linear mode interaction. The device fabricated on SOI wafer with low thermal budget (<600 K) encompasses a restricted symmetrical out-of-plane vibration and a stiff in-plane electro-thermal actuator with a displacement of ~112 nm at 2.7 mW. This corresponds to 5.6% of strain for a 2 μm long suspended carbon nanotube. The in-plane mechanical resonance designed to be far from CNT resonances in MHz regime is measured at 209 kHz. The design is optimized for low power consumption, electrical and thermal isolation and is customized for dry transfer of CNTs.

Electron-beam lithography (EBL) possesses tremendous capabilities in terms of patterning ultra-small features, with very low line edge roughness and high overlay accuracy. In order to reduce write times and processing steps, hybrid lithography is an interesting alternative. Here, an approach combining EBL and ultraviolet lithography (UVL) was pursued in order to pattern multi-level 3D-structures by inductively coupled plasma deep reactive ion etching (ICP-DRIE). The substrate etched hereby consisted of a silicon nitride thin film and the underlying silicon. The correlation of UVL under-and overexposures with the resist layer height and sidewall taper after development, in combination with the optimization of the ICP-DRIE selectivity in dependence of the biasing conditions and the aspect ratio dependent etch lag for small features, allowed for the accurate prediction and therefore programming of the desired structures. Further, the selectivity was tailored by introducing an oxygen pulse after the breakthrough step. This facilitated the simultaneous etching of structures with an in-plane resolution of down to 250 nm (aspect ratio 1:22) and trench depths ranging from 200 nm to 25 µm.

Here, we report the intermixing of piezoresistive and conduction modulation current in a carbon nanotube field effect transistor (CNT-FET) based resonator. We show that due to static displacement of the nanotube, as a result of electrostatic actuation, the motional current at the resonance frequency consist of both current components. For instance at a DC gate bias of 1.3 V, 3/4 of the motional current is conduction modulation current while the rest arises from piezoresistive effects. The intermixing effect due to asymmetry influences the fundamental harmonic response as well as the physical nature of the electrical signal being sensed; both of which are important for understanding frequency harmonics in nanoresonators and developing efficient readout schemes for nanoscale sensors.