Dustin W. Carr

Measurement of mechanical resonance and losses in nanometer scale silicon wires

We present data on nanofabricated suspended silicon wires driven at resonance. The wires are electrostatically driven and detected optically. We have observed wires with widths as small as 45 nm and resonant frequencies as high as 380 MHz. We see a strong dependence of the resonant quality factor on the surface to volume ratio.

Nanofabrication and electrostatic operation of single-crystal silicon paddle oscillators

We report the fabrication and characterization of paddle oscillators featuring nanometer-scale supporting rods. The devices show two resonances in the 1–10 MHz range, which we attribute to the translational and torsional modes of motion. While the frequency response of the translational motion shows evidence of nonlinear behavior, the torsional response remains symmetric throughout the range of excitation. We present a model for the electrostatic excitation of the two modes. Torsional motion is induced via asymmetries of the system, and amplified by a modulation of the effective torsional constant. The model of the translational motion predicts a nonlinear behavior for displacements as small as 15 nm. Analysis of both modes of motion consistently suggests structures softer than expected from bulk silicon. Quality factors approaching 103 are measured.

Fabrication of nanoelectromechanical systems in single crystal silicon using silicon on insulator substrates and electron beam lithography

We have demonstrated a process for fabricating nanometer-scale electromechanical structures of diverse geometries in single crystal silicon, using silicon on insulator substrates. We pattern the substrate using high resolution electron beam lithography with 100 keV electrons followed by Al evaporation and liftoff. The Al is used as an etch mask in CF4 reactive ion etching to pattern the top silicon layer. We then undercut structures using a buffered oxide etch. The structures were made from substrates having a top silicon thickness of 200 or 50 nm, and a buried oxide thickness of 400 nm. With this process we have made a variety of movable structures. We describe the performance of an electrostatically driven Fabry–Perot interferometer that consists of a μm sized pad suspended by wires that are 100–200 nm wide. We have also made much smaller mechanical structures such as suspended silicon beams as narrow as 30 nm.

Parametric amplification in a torsional microresonator

We observe parametric amplification in a torsional micron-scale mechanical resonator. An applied voltage is used to make a dynamic change to the torsional spring constant. Oscillating the spring constant at twice the resonant frequency results in a phase dependent amplification of the resonant motion. Our results agree well with the theory of parametric amplification. By taking swept frequency measurements, we observe interesting structure in the resonant response curves.

Measurement of nanomechanical resonant structures in single-crystal silicon

We have used electron beam lithography to make very small (<30 nm linewidth) mechanical structures in single-crystal silicon. These structures can be driven capacitively by applying a voltage between the suspended portion and the underlying substrate. Optical interference techniques are used to detect and measure the motion of the structures with resonant frequencies above 40 MHz. We employed a design consisting of a square mesh with a 315 nm period, which results in a low mass (∼1×10−13 g) and large relative surface area (10−6 cm2). Also, by making suboptical-wavelength features, the optical properties can be altered, leading to an improved measurement sensitivity. We measured the oscillations at small amplitudes where the detected change in the optical reflection is proportional to the drive amplitude.

Optical transmission through double-layer metallic subwavelength slit arrays

We present measurements of transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. We perform numerical simulations using rigorous coupled wave analysis to explain the strong dependence of the peak transmission on the lateral shift between the metal layers.

Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer

We experimentally demonstrate operation of a laterally deformable optical nanoelectromechanical system grating transducer. The device is fabricated in amorphous diamond with standard lithographic techniques. For small changes in the spacing of the subwavelength grating elements, lossy propagating resonant modes in the plane of the grating cause a large change in the optical reflection amplitude. An in-plane motion detection sensitivity of 160 fm/square root(Hz) was measured, exceeding that of any other optical microelectromechanical system transducer to our knowledge. Calculations predict that this sensitivity could be improved to better than 40 fm/square root(Hz) in future designs. In addition to having applications in the field of inertial sensors, this device could also be used as an optical modulator.

Fabrication of dissimilar metal electrodes with nanometer interelectrode distance for molecular electronic device characterization

We report a versatile process for the fabrication of dissimilar metal electrodes with a minimum interelectrode distance of less than 6 nm using electron beam lithography and liftoff pattern transfer. This technique provides a controllable and reproducible method for creating structures suited for the electrical characterization of asymmetric molecules for molecular electronics applications. Electrode structures employing pairs of Au electrodes and non-Au electrodes were fabricated in three different patterns. Parallel electrode structures 300 μm long with interelectrode distances as low as 10 nm, 75 nm wide electrode pairs with interelectrode distances less than 6 nm, and a multiterminal electrode structure with reproducible interelectrode distances of 8 nm were realized using this technique. The processing issues associated with the fabrication of these structures are discussed along with the intended application of these devices.

Laterally deformable nanomechanical zeroth-order gratings: anomalous diffraction studied by rigorous coupled-wave analysis

We describe a novel optomechanical device that produces strong reflectance and polarization modulation of incident light. The structure is based on a suspended nanomechanical grating with lateral deformability, and rigorous coupled-wave analysis has been used to fully model the optical properties of the device. The grating consists of two interdigitated gratings that may be moved with respect to each other with an applied force. The structures proposed here are designed to be readily manufacturable with device processing developed for surface-micromachined microelectromechanical systems and with known microelectromechanical systems materials, such as silicon, silicon nitride, and amorphous diamond. As the spacing of the grating is changed, an anomalous diffraction effect is observed, a Woods type anomaly in which there exists a resonance in propagating leaky modes within the grating, resulting in a dramatic change in the reflectance characteristics for slight changes in the grating. One of the unique features of this structure is that a reflected optical signal can be used to detect subangstrom in-plane motion of structures greater than 10 nm.

Mechanical dissipation in tetrahedral amorphous carbon

We have fabricated micromechanical oscillators from tetrahedrally coordinated amorphous carbon (ta-C) in order to study mechanical dissipation mechanisms in this material. Cantilever oscillators with either in-plane or out-of-plane dominant transverse vibrational modes and free-free beam oscillators with in-plane modes were fabricated with critical dimensions ranging from 75nm to over 1mm. The resonant frequency and quality factor were measured for all oscillators. The resonant frequencies ranged from a few kilohertz to several megahertz, while the quality factor remained nearly constant at approximately 2–4×103. Possible dissipation mechanisms were evaluated for these oscillators, and it was found that the observed dissipation was not limited by mechanical clamping losses, air damping, thermoelastic dissipation, or dissipation due to phonon-mechanical vibration interactions. However, an extrinsic dissipation mechanism in which dissipation is limited by a spectrum of defects in ta-C was found to be consis...