A. Olkhovets

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.

Optically pumped parametric amplification for micromechanical oscillators

Micromechanical oscillators in the rf range were fabricated in the form of silicon discs supported by a SiO2 pillar at the disk center. A low-power laser beam, (Plaser∼100 μW), focused at the periphery of the disk, causes a significant change of the effective spring constant producing a frequency shift, Δf(Δf/f∼10−4). The high quality factor, Q, of the disk oscillator (Q∼104) allows us to realize parametric amplification of the disk’s vibrations through a double frequency modulation of the laser power. An amplitude gain of up to 30 was demonstrated, with further increase limited by nonlinear behavior and self-generation. Phase dependence, inherent in degenerate parametric amplification, was also observed. Using this technique, the sensitivity of detection of a small force is greatly enhanced.

Autoparametric optical drive for micromechanical oscillators

Self-generated vibration of a disk-shaped, single-crystal silicon micromechanical oscillator was observed when the power of a continuous wave laser, focused on the periphery of the disk exceeded a threshold of a few hundred μW. With the laser power set to just below the self-generation threshold, the quality factor for driven oscillations increases by an order of magnitude from Q=10 000 to Qenh=110 000. Laser heating-induced thermal stress modulates the effective spring constant via the motion of the disk within the interference pattern of incident and reflected laser beams and provides a mechanism for parametric amplification and self-excitation. Light sources of different wavelengths facilitate both amplification and damping.

Low voltage electron beam lithography in PMMA

We have studied low voltage (1–2 kV) electron beam lithography processes in PMMA and compared them to conventional high voltage processing. We looked at the deposited metal after liftoff as well as directly imaging resist profiles by atomic force microscopy. As expected, the proximity effects were greatly reduced. The forward scattering was found to increase at low voltage. The study of developed resist profiles showed that linewidth versus dose has a single Gaussian functional form, proving that forward scattering plays the major role in line broadening. The effective Gaussian linewidth is 60 nm at 1 kV in a 50 nm resist layer. Modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.

Actuation and internal friction of torsional nanomechanical silicon resonators

We report on the actuation and mechanical properties of silicon resonators with nanometer-scale supporting rods operating in the 3–20 MHz range. The symmetrically designed paddles can be excited both in their flexural and torsional modes of motion. Fabrication imperfections as small as 10–20 nm provide enough asymmetry to allow such torsional excitation. We also report on internal friction studies in these systems. Thin Al overlayers contribute to the room temperature internal losses, as quality factor drops from 3300 to 380 for 160 A thick film. A temperature dependence of internal friction has a broad peak in the T=160–190 K range, and attributed to the Debye relaxation and thermally activated friction mechanisms. Analysis shows that the peak shifts to higher temperatures with increasing resonator frequency.

Non-degenerate nanomechanical parametric amplifier

We present a nanomechanical parametric amplifier system, which consists of two electrostatically coupled resonators, and operates using 3 frequencies (non-degenerate amplifier). The use of 3 frequencies eliminates phase sensitivity between the signal and pump voltage. We observed the amplification of small signals by up to 43 dB. The width of the response of the resonator is tuned by varying the pump voltage. The resonant frequencies of the individual oscillators can be slightly tuned by adjusting the bias voltages. To our knowledge, this is the first reported mechanical parametric amplification scheme using two separate resonators.

Charge Induced Pattern Distortion in Low Energy Electron Beam Lithography

Charge induced pattern distortions in low voltage electron beam lithography in the energy range of 1 to 5 kV were investigated. Pattern distortion on conducting substrates such as silicon was found to be small, while significant pattern placement errors and pattern distortions were observed in the case of electrically insulating substrates caused by charge trapping and deflection of the incident electron beam. The nature and magnitude of pattern distortions were found to be influenced by the incident electron energy, pattern size, electrical conductivity, and secondary electron emission coefficient of the substrate. Theoretical modeling predicts the electron beam deflection to be directly proportional to the trapped surface charge density and inversely proportional to the accelerating voltage.

Dual exposure glass layer suspended structures: A simplified fabrication process for suspended nanostructures on planar substrates

We have developed and demonstrated here a simplified flexible fabrication process for glass nanomechanical systems. This process uses a single layer of spin on glass (SOG) material with two negative tone electron beam exposures at two different exposure energies to define the suspended and support structures, respectively. After development the SOG can be converted into glass. The process is additive and can be applied to any flat substrate. We have fabricated a variety of glass nanomechanical oscillators and measured their mechanical resonances using a mechanical piezoelectric driving force and optical interferometric detection. Suspended structures were fabricated with thickness of less than 50 nm and lateral dimensions of less than 100 nm supported anywhere from 150 to 800 nm above the substrate. Resonance frequencies for glass wires with both ends fixed (cross section 110 nm×180 nm) and lengths of 4–9 μm range from 7 to 30 MHz, with quality (Q) factors of over 1000. Annealing the structures in an oxygen ambient roughly doubles both the frequencies and the Q factors.

Scanning tunneling microscope induced luminescence of lithographically prepared Au dots

We observe high contrast scanning tunneling microscope (STM)-induced luminescence maps from isolated lithographically prepared gold dots on silicon using gold and tungsten STM tips. We observe geometry-related light emission with photon intensity stronger at protrusions than in the valleys. The light intensity dependence on tunnel voltage bias agrees with theoretical predictions. A noticeable variation of onset bias with the tip material is observed.

Light-activated self-generation and parametric amplification for MEMS oscillators

High-frequency microoptoelectromechanical systems (MOEMS) are proposed as active devices for radio frequency signal processing. Parametric amplification (PA), generation, frequency modulation and frequency conversion on the micromechanical level were demonstrated at MHz range by microfabricated single-crystal silicon mechanical resonators. A focused laser beam was used to pump energy into the motion of the oscillator, to control the frequency response and to provide a carrier signal for the frequency up-conversion. Laser light interaction with the microelectromechanical system (MEMS) was realized through the stress pattern induced within the microfabricated structure by the focused laser beam. Stress-induced stiffening of the oscillator provides control over the effective spring constant and leads to a parametric mechanism for amplification of mechanical vibrations. Periodic modulation of the laser intensity synchronized with the driving force allowed us to demonstrate a degenerate (phase-sensitive) PA scheme with gain in access of 30dB. Design of the oscillator as a part of the built-in Fabry-Perot cavity provides auto-modulation of the effective spring constant as a result of the position-dependent absorption of the light by the oscillator. The auto-modulation mechanism allows a parametric self-excitation induced by continuous wave (CW) laser beam. Self-sustained generation was observed when laser power exceeded a threshold of few hundred microWatts. Nonlinear effects cause frequency dependence vs. laser power, providing a mechanism for frequency modulation of the self-generated vibrations. The same type of optical scheme can also work as an ideal frequency mixer, which combines the self-generated response with an external high-frequency modulation of the laser intensity.