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High Quality Factor Nanostring Resonators

Scott S. Verbridge and Harold G. Craighead

Abstract

We have fabricated nanomechanical flexural resonators with room temperature quality factors as high as 207,000 at a frequency of 4.5 MHz, representing the highest quality factor yet obtained for devices with cross-sectional dimensions on the order of 100 nm. We demonstrate that the high quality factors are a result of the high tensile stress present in these doubly-clamped devices, and demonstrate an approach to within an order of magnitude of the thermoelastic limit for mechanical quality factor for devices of this scale. These devices, with the highest ever reported product of quality factor and surface-to-volume ratio, should prove useful as sensitive probes of the environments in which they are operated.

Research Summary

Figure 1: SEM images of a suspended silicon nitride beam. A 300 nm scale bar is shown.

Nanomechanical flexural resonators have been fabricated and studied, using different fabrication techniques, and from different materials. Doubly clamped nanoresonators made in high stress silicon nitride exhibited quality factors as high as 207,000 at a frequency of 4.5 MHz, and at room temperature, the highest Q ever reported for flexural resonators with cross-sectional dimensions on the order of 100 nm. These high quality factor devices were fabricated from electron-beam defined masks, as well as electrospun masks. Resulting Qs were shown to be independent of the fabrication steps, implying the high Qs are a result of the high stress silicon nitride material. Lower stress resonators have also been fabricated, including doubly clamped beams made from a lower stress film, and stress free cantilever beams, yielding for devices of either the same size or the same frequency, quality factors significantly lower than the high stress devices. We have demonstrated that our lower frequency devices, up to 50 MHz, exhibit a variation of Q with frequency consistent with what would be expected from considerations of thermoelastic dissipation. At higher frequency, this trend begins to deviate from the thermoelastic limit, possibly due to the increased importance of clamping losses for the higher frequency, shorter devices. Devices with such small cross-sectional dimensions, and hence large surface-to-volume ratio, with such high quality factor, should prove useful for the sensing of small biological molecules, as well as the probing of the physics of the interaction of such small mechanical devices with their surroundings.

Figure 2: Optically measured response of a 200 nm wide, 105 nm thick, 60 mm long resonator, with a Q of 207,000 at room temperature.

Figure 3: Quality factor data for doubly clamped high stress beams and stress-free cantilevers, plotted with the limit given by thermoelastic dissipation.

Reference

  1. "High Quality Factor Resonance at Room Temperature with Nanostrings Under High Tensile Stress", Scott S. Verbridge, Jeevak M. Parpia, Robert B. Reichenbach, Leon M. Bellan, H. G. Craighead, Journal of Applied Physics, 99, 124304-124304-8 (2006). Abstract