Chemically synthesized (a.k.a., "bottom-up") nanowires are attractive material candidates for nanoelectromechanical resonators because of their ultimately miniaturized sizes and atomically smooth surfaces. We have been interested in both fundamental and engineering aspects of such bottom-up materials and structures. In particular, we have made initial and original efforts in the development of high-performance nanoelectromechanical systems (NEMS) based on various bottom-up nanowires, in close collaborations with leading groups in the fields of materials science and nanowires syntheses.

I. Very High Frequency (VHF) Silicon Nanowire (SiNW) NEMS
We have developed the first silicon nanowire (SiNW) NEMS resonators operating at very high frequencies (VHF). The VHF NEMS resonators are based upon bottom-up epitaxially-grown SiNWs with well-terminated surfaces. Metallized SiNW resonators operating near 200 MHz are realized with quality factor Q2000~2500. Pristine SiNWs, with fundamental resonances as high as 215 MHz, are measured using a VHF readout technique that is optimized for these high resistance devices. The pristine resonators provide the highest Q's, as high as Q13100 for an 80 MHz device. SiNWs excel at mass sensing; characterization of their mass responsivity and frequency stability demonstrates sensitivities approaching 10 zeptograms. We develop SiNW-NEMS-based phase-locking techniques to perform such real-time measurements.

Figure 1. Synopsis demonstration of the VHF SiNW NEMS resonators. (left) SEM images showing typical SiNWs suspended in microtrenches, very straight wires with very well-faceted surfaces and a nice hexagonal cross-section. (middle) Measured resonance responses of a 188MHz SiN NEMS resonator, with very large signal (up to 12.5dB at B=8Tesla in a magnetomotive transduction). (right) Measured resonance responses from the 188MHz device with increasing RF drive levels, clearly showing nonlinear operations at high drive levels. Signals referred to the input of the pre-amplifiers in all the plots.

A principal advantage of the suspended SiNW resonators developed in this work is their ease of fabrication and high yield. By pushing the dimensions of the microtrenches downward and simultaneously optimizing the NW growth conditions, we expect that smaller, even molecular-size, suspended SiNWs should be achievable. These will enable scaling fundamental resonance frequencies into the extreme UHF and low microwave range. These SiNW resonators offer significant potential for applications in resonant sensing, quantum electromechanical systems, and high frequency signal processing.

We have found that VHF SiNW resonators vibrating at ~200MHz typically have displacement sensitivity of ~5fm/Hz and force sensitivity of 50~250aN/Hz, set by thermomechanical fluctuations. They have ~1nm critical amplitude and intrinsic dynamic range of 90~110dB. These SiNW resonators offer significant potential for applications in resonant sensing, quantum electromechanical systems, and high frequency signal processing.

II. Piezoresistive Silicon Nanowire NEMS with On-Chip Electronic Transduction
The above demonstration of high-performance SiNW NEMS has stimulated a lot of interesting thoughts and potential applications. From a technologically viewpoint, fully-integrated, on-chip electronic transduction schemes are highly desirable, in order to realize the full potential of the nanowire resonators. For very thin SiNWs, the key challenge is to devise novel schemes to read out their strain/displacement signals at resonances, with low noise and high efficiency. Historically, in the development of compact solid-state transducers, ingeniously engineering the strain in miniscule mechanical structures has played important roles; and it is now becoming a key for resonant NEMS.

Figure 2. Very high frequency Si nanowire resonators with fully integrated electrostatic actuation and piezoresistive self-detection. (a) Schematic diagram of bias and drive circuitry. (b) Device performance of a 40 nm thick, 96 MHz nanowire resonator with a quality factor Q of ~550. It is 1.8 µm long and has a dc resistance of 80 kΩ. The AC drive is set at V_d,AC= 0.50, 0.63, 0.71, 0.79 V for the curves respectively, with the DC voltage fixed at V_d,DC=0.2 V. The left inset shows the SEM image of the device, and the right inset shows the drive dependence of the resonance frequency. (c) Performance of a 30 nm thick, 75 MHz Si nanowire resonator with Q~700. Its length is 1.8 µm and its dc resistance is 300 kΩ. The curves are taken at different bias voltages with the same drive. The inset shows the SEM image and the linear dependence on bias voltage. (d) Data taken at varied drive with the same bias. Inset shows the quadratic dependence on drive voltage.

In this work, we have demonstrated that for very thin Si nanowires, their time-varying strain can be exploited for self-transducing the devices’ resonant motions at the VHF/UHF bands. The strain that is only second-order in doubly clamped structures, enables efficient displacement transducers due to the enhanced piezoresistance effect in these Si nanowires. This self-integrated transduction scheme requires minimum device complexity and operates readily at room temperature and vacuum conditions provided by standard packaging technologies. These devices represent a unique group of nanowire resonators that are as thin as 30 nm, with intrinsically embedded strain transducers and monolithically integrated actuation. Such prototypes should facilitate their direct integration with electronics toward functional systems for various applications.

By combining Si nanowire’s piezoresistive self-transducing with off-chip piezoelectric-disk actuation and on-chip electrostatic excitation, we have demonstrated VHF Si nanowire resonators. For devices with widths ranging from 90nm to 30nm, and lengths of 1.8µm to 5µm, the resonators operate at frequencies from 20MHz to 100MHz, with quality factors (Q's) in the range of 5501200, at room temperature and non-stringent vacuum conditions (~mTorr range). The measured Q's do not drop until pressure is raised to ~1Torr and are still appreciable at ~100Torr (e.g., Q~300 for the 30nm thick Si nanowire operating at 75MHz). The down-mixing technique developed for readout of the second-order piezoresistive effects in doubly clamped structures readily provides VHF transduction for devices with progressively shrinking dimensions and high impedances in the range from ~1k to ~1M, without the need for extra patterning or metallization. Our simple and practical demonstration of integrated Si nanowire NEMS should engender many new applications of integrated nanowire resonators and arrays.

Personnel
Caltech: Philip Feng, Michael Roukes
UC Berkeley: Rongrui He, Peidong Yang

References

1. Feng XL, He RR, Yang PD, Roukes ML, "Very High Frequency Silicon Nanowire Electromechanical Resonators", Nano Letters,Vol. 7, No. 7, 1953-1959 (2007).
2. He RR, Feng XL, Roukes ML, Yang PD, "Self-Transducing Silicon Nanowire Electromechanical Systems at Room Temperature", Nano Letters, Vol. 8, No. 6, 1756-1761 (2008).