Arrays of Carbon Nanotubes as RF Filters in Waveguides

NASA's Jet Propulsion Laboratory, Pasadena, California

Brushlike arrays of carbon nanotubes embedded in microstrip waveguides provide highly efficient (high-Q) mechanical resonators that will enable ultra-miniature radio-frequency (RF) integrated circuits. In its basic form, this invention is an RF filter based on a carbon nanotube array embedded in a microstrip (or coplanar) waveguide, as shown in Figure 1. In addition, arrays of these nanotube-based RF filters can be used as an RF filter bank.

Applications of this new nanotube array device include a variety of communications and signal-processing technologies. High-Q resonators are essential for stable, low-noise communications, and radar applications. Mechanical oscillators can exhibit orders of magnitude higher Qs than electronic resonant circuits, which are limited by resistive losses. This has motivated the development of a variety of mechanical resonators, including bulk acoustic wave (BAW) resonators, surface acoustic wave (SAW) resonators, and Si and SiC micromachined resonators (known as "microelectromechanical systems" or MEMS). There is also a strong push to extend the resonant frequencies of these oscillators into the GHz regime of state-of-the-art electronics. Unfortunately, the BAW and SAW devices tend to be large and are not easily integrated into electronic circuits. MEMS structures have been integrated into circuits, but efforts to extend MEMS resonant frequencies into the GHz regime have been difficult because of scaling problems with the capacitively-coupled drive and readout. In contrast, the proposed devices would be much smaller and hence could be more readily incorporated into advanced RF (more specifically, microwave) integrated circuits.

During the past few years, techniques for fabricating highly-ordered, dense arrays of nearly uniform carbon-nanotube cantilevers like so many bristles of a brush (see Figure 1) have provided the essential basis for this new device. The basic principle of operation of such an array as band-pass filter is excitation of a mechanical (acoustic) deformation of the nanotubes by an incident RF wave (Figure 2). Coupling between the RF signal and the nanotubes is provided by Coulomb forces on electric charges in the nanotubes. The device functions as a narrow-band RF filter because incident waves are reflected from the metallic nanotubes, except at the mechanical resonant frequency of the array. The high-Q mechanical resonance of the uniform nanotube array filters the incoming RF signal and couples the RF wave at the resonance frequency into the output electrode.

The resonance frequency of a nanotube cantilever depends on its diameter and length. For example, it is estimated that the resonance frequency of a carbon nanotube 10 nm in diameter and 100 nm long would be about 4 GHz. By adjusting the dimensions of the nanotubes in the array, it should be possible to select resonance frequencies that range from below 100 kHz up to tens of GHz.

There have also been attempts to make mechanical resonators using silicon cantilevers. However, the silicon devices investigated thus far have been limited to operation at frequencies below 400 MHz, whereas carbon-nanotube devices with Q values of the order of 10³at a frequency of 2 GHz have been demonstrated. Moreover, there are experimental data that suggest that carbon nanotube resonators should exhibit linear response over a larger dynamic range relative to silicon mechanical resonators.

Figure 1. This Array of Carbon Nanotubes, with a diameter nonuniformity of <5 percent, was fabricated in a process that included the use of a nanopore template (J. Xu et al.).

Figure 2. A Brushlike Array of Carbon Nanotubes embedded in a microstrip waveguide would act as a band-pass filter.

This work was done by Daniel Hoppe, Brian Hunt, Michael Hoenk, and Flavio Noca of Caltech for NASA's Jet Propulsion Laboratory and by Jimmy Xu of Brown University.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to