Keith Aubin

Optical excitation of nanoelectromechanical oscillators

We report a method of optical excitation of nanomechanical cantilever-type oscillators. The periodic driving signal with a controlled modulation amplitude was provided by a 415 nm diode laser, wherein the laser spot was located at some distance away from the clamped end of the cantilever. The measured resonant response of the cantilever was obtained at distances in excess of 160μm with varying oscillator dimensions. The effectiveness of the driving mode is studied for different combinations of materials, namely Si–SiO2 and Si3N4–SiO2. These observations were considered within the theoretical framework of the mechanism of heat transfer. We show that measurable amplitudes of vibrations can be obtained at temperature changes much less than 1°.

Limit cycle oscillations in CW laser-driven NEMS

Limit cycle, or self-oscillations, can occur in a variety of NEMS devices illuminated within an interference field. As the device moves within the field, the quantity of light absorbed and hence the resulting thermal stresses changes, resulting in a feedback loop that can lead to limit cycle oscillations. Examples of devices that exhibit such behavior are discussed as are experimental results demonstrating the onset of limit cycle oscillations as continuous wave (CW) laser power is increased. A model describing the motion and heating of the devices is derived and analyzed. Conditions for the onset of limit cycle oscillations are computed as are conditions for these oscillations to be either hysteretic or nonhysteretic. An example simulation of a particular device is discussed and compared with experimental results.

Prion protein detection using nanomechanical resonator arrays and secondary mass labeling

Nanomechanical resonators have shown potential application for mass sensing and have been used to detect a variety of biomolecules. In this study, a dynamic resonance-based technique was used to detect prion proteins (PrP), which in conformationally altered forms are known to cause neurodegenerative diseases in animals as well as humans. Antibodies and nanoparticles were used as mass labels to increase the mass shift and thus amplify the frequency shift signal used in PrP detection. A sandwich assay was used to immobilize PrP between two monoclonal antibodies, one of which was conjugated to the resonators surface while the other was either used alone or linked to the nanoparticles as a mass label. Without additional mass labeling, PrP was not detected at concentrations below 20 microg/mL. In the presence of secondary antibodies the analytical sensitivity was improved to 2 microg/mL. With the use of functionalized nanoparticles, the sensitivity improved an additional 3 orders of magnitude to 2 ng/mL.

Frequency entrainment for micromechanical oscillator

We demonstrate synchronization of laser-induced self-sustained vibrations of radio-frequency micromechanical resonators by applying a small pilot signal either as an inertial drive at the natural frequency of the resonator or by modulating the stiffness of the oscillator at double the natural frequency. By sweeping the pilot signal frequency, we demonstrate that the entrainment zone is hysteretic and can be as wide as 4% of the natural frequency of the resonator, 400 times the 1/Q∼10−4 half-width of the resonant peak. Possible applications are discussed based on the wide range of frequency tuning and the power gain provided by the large amplitude of self-oscillations (controlled by a small pilot signal).

Shell-type micromechanical actuator and resonator

Dome-shaped radio-frequency micromechanical resonators were fabricated by utilizing the buckling of a prestressed thin polysilicon film. The enhanced rigidity of the dome structure leads to a significant increase of its resonant frequency compared to a flat plate resonator. The shell-type geometry of the structure also provides an imbedded actuation mechanism. Significant out-of plane deflections are actuated by mechanical stress introduced within the plane of the shell. We demonstrate that thermomechanical stress generated by a focused laser beam, or microfabricated resistive heater, provides an effective and fast mechanism to operate the dome as an acoustic resonator in the radio-frequency range. All-optical operation of the shell resonator and an integrated approach are discussed.

Third-order intermodulation in a micromechanical thermal mixer

A radio frequency (RF) micromechanical shell-type resonator with a resistive thermal actuator is shown to perform as a highly linear, broadband mixer and a high-quality factor post-translation (intermediate frequency) filter. The resistor is capable of frequency translation of RF carrier signals as high as 1.5 GHz to the intermediate frequency of 12.7 MHz. The thermal actuator allows electrical isolation between the input and output of the mixer-filter, dc bias independent mixing, and provides a 50-Ohm load to match the output of front-end electronics. High linearity is demonstrated in the mixer with a third-order input intercept point of +30 dBm for interferers spaced at a 50-kHz offset from the carrier frequency. A variant of the Duffing oscillator model and finite element modeling are used to analyze the origin of nonlinearities in the micromechanical system. [1503].

Analysis of Frequency Locking in Optically Driven MEMS Resonators

Thin, planar, radio frequency microelectromechanical systems (MEMS) resonators have been shown to self-oscillate in the absence of external forcing when illuminated by a direct current (dc) laser of sufficient amplitude. In the presence of external forcing of sufficient strength and close enough in frequency to that of the unforced oscillation, the device will become frequency locked, or entrained, by the forcing. In other words, it will vibrate at the frequency of the external forcing. Experimental results demonstrating entrainment for a disk-shaped oscillator under optical and mechanical excitation are reviewed. A thermomechanical model of the system is developed and its predictions explored to explain and predict the entrainment phenomenon. The validity of the model is demonstrated by the good agreement between the predicted and experimental results. The model equations could also be used to analyze MEMS limit-cycle oscillators designed to achieve specific performance objectives

HOPF BIFURCATION IN A DISK-SHAPED NEMS

Self-sustained mechanical vibrations of a disc-type microfabricated resonator were experimentally observed when a continuous wave (CW) laser beam was focused on the periphery of the disc (for a 40 μm diameter resonator, natural frequency 0.89MHz, the laser power above a 250 W threshold was required). A theoretical model for self-oscillatory behavior has been developed based on FEM analysis of a stress pattern created within the resonator by the focused laser beam. This model accounts for the fact that the amount of absorbed laser light is modulated due to the motion of the resonator through the optical interferometric pattern. Analytical study reveals the presence of a Hopf-type bifurcation with a critical laser power close to the experimentally observed value. Harmonic balance analysis indicates the existence of a stable limit cycle in the phase plane determining the amplitude of self-oscillations.Copyright

Laser annealing for high-Q MEMS resonators

High frequency and high quality factor, Q, (defined as a half-width of the resonant peak) are the key factors that determine applications of microelectromechanical (MEMS) oscillators for supersensitive force detection or as elements for radio frequency signal processing. By shrinking the dimensions of MEMS resonators to the sub-micron range one increases the resonant frequency of the devices. Shrinking the devices, however, also increases the surface-to-volume ratio leading to a significant degradation of the quality factor (to below 5,000) due to the increased contribution of surface-related losses. We demonstrate that local annealing performed by focused low-power laser beams can improve the quality factor of MEMS resonators by more than an order of magnitude, which we attribute to the alteration of the surface state. Quality factors over 150,000 were achieved after laser annealing 3.1 MHz disc-type oscillators (radius R=10 micrometers, thickness h=0.25 micrometer) compared with a Q=6,000 for the as-fabricated device. The mushroom-type design of our resonator (a single-crystal silicon disc supported by a thin silicon dioxide pillar at the center) provides low heat loss and also confines the electron-hole gas created by laser excitation, enhancing light absorption. The combined power of a red HeNe laser (Pred=4mW) and a blue Ar+ ion laser (Pblue=5mW) focused on the periphery of the mushroom provides enough energy for surface modification. The post-treatment quality factor, exceeding 100,000 for MHz-range resonators, boosts the performance of MEMS to be comparable to that of lower frequency single-crystal quartz devices. The local nature of laser annealing, safe for surrounding electronics, is a crucial element for integration of MEMS resonators into an integrated circuit environment.

Resistively actuated micromechanical dome resonators

We demonstrate dome-shaped, radio frequency, micromechanical resonators with integrated thermo-elastic actuators. Such resonators can be used as the frequency-determining element of a local oscillator or as a combination of a mixer and IF filter in a superheterodyne transceiver. The dome resonators (shallow shell segments clamped on the periphery) are fabricated utilizing pre-stressed thin polysilicon film over sacrificial silicon dioxide. The shell geometry enhances the rigidity of the structure, providing a resonant frequency several times higher than a flat membrane of the same dimensions. The finite curvature of the shell also couples out-of-plane deflection with in-plane stress, providing an actuation mechanism. Out-of-plane motion is induced by employing non-homogeneous, thermomechanical stress, generated in plane by local heating. A metal resistor, lithographically defined on the surface of the dome, provides thermal stress by dissipating 4 μW of Joule heat. The diminished heat capacity of the MEMS device enables a heating/cooling rate comparable to the frequency of mechanical resonance and allows operation of the resonator by applying AC current through the microheater. Resistive actuation can be readily incorporated into integrated circuit processing and provides significant advantages over traditional electrostatic actuation, such as low driving voltages, matched 50-ohm impedance, and reduced cross talk between drive and detection. We show that when a superposition of two AC signals is applied to the resistive heater, the driving force can be detected at combinatory frequencies, due to the fact that the driving thermomechanical stress is determined by the square of the heating current. Thus the thermoelastic actuator provides frequency mixing while the resonator itself performs as a high quality (Q~10,000) intermediate frequency filter for the combinatory frequencies. A frequency generator is built by closing a positive feedback loop between the optical detection of the mechanical motion of the dome and the resistive drive. We demonstrate self-sustained oscillation of the dome resonator with frequency stability of 1.5 ppm and discuss the phase noise of the oscillator.