Vincent T. K. Sauer

Optical interferometric displacement calibration and thermomechanical noise detection in bulk focused ion beam-fabricated nanoelectromechanical systems

Optical interferometric techniques are used for absolute (calibrated) displacement measurements of focused ion beam (FIB)-fabricated nanoelectromechanical systems (NEMS). FIB nanomachining of bulk Si gives rapidly prototyped cantilever and doubly clamped beam devices. Ion impingement from orthogonal directions allows tailoring of deep, undercut-free gaps between the device layer and the bulk, in turn allowing large amplitude NEMS oscillatory motion, access to a nonlinear readout regime and a new calibration method for optical interferometric displacement detection. The measurements are sensitive enough to determine the thermomechanical noise floor of a bulk FIBed NEMS device with a displacement sensitivity of 166 fm Hz−½, limited by the combination of optical shot noise and detector dark current. This sensitivity, comparable to the state of the art for free-space optical interferometry of NEMS, validates the robustness of the bulk FIB fabrication technique for rapid prototyping of nanoscale mechanical devices.

Nanophotonic detection of side-coupled nanomechanical cantilevers

A silicon nanophotonic Mach-Zehnder interferometer (MZI) is used to detect the mechanical resonance of a cantilever external to a nanophotonic waveguide. Small cantilever devices, below the cut-off for waveguide supported modes, are fabricated ∼140 nm away from one MZI arm. Cantilever resonant frequencies up to 60 MHz are measured with mechanical quality factors around 20 000 and signal to noise ratios up to 1000. Phase-locked loop frequency stability measurements indicate a mass sensitivity of 2 zg in an example cantilever of 0.5 pg mass. An interferometric transduction mechanism is confirmed, and the system is shown to work effectively in all-optical operation.

Stiction-free fabrication of lithographic nanostructures on resist-supported nanomechanical resonators

The authors report a highly flexible process for nanostructure lithography to incorporate specific functions in micro- and nanomechanical devices. The unique step involves electron beam patterning on top of released, resist-supported, surface micromachined structures, hence avoiding hydrofluoric acid etching of sensitive materials during the device release. The authors demonstrate the process by creating large arrays of nanomechanical torque magnetometers on silicon-on-insulator substrates. The fabricated devices show a thermomechanical noise-limited magnetic moment sensitivity in the range of 5 × 106 μB at room temperature and can be utilized to study both magnetostatics and dynamics in nanomagnets across a wide temperature range. The fabrication process can be generalized for the deposition and patterning of a wide range of materials on micro-/nanomechanical resonators.

Confocal Scanner for Highly Sensitive Photonic Transduction of Nanomechanical Resonators

We show that a simple confocal laser scanning system can be used to couple light through grating couplers into nanophotonic circuits. The coupling efficiency is better than 15% per coupler. Our technique avoids using multi-axis fibre stages and is especially advantageous when the nanophotonic circuit is kept in vacuum, e.g., for nanomechanical resonator displacement transduction. This was demonstrated by recording the resonant response of a nanomechanical doubly clamped beam embedded in a race-track optical cavity. The nanophotonic transduction offers an increase of two orders of magnitude in transduction responsivity compared with conventional free-space optical interferometry.

Bulk focused ion beam fabrication with three-dimensional shape control of nanoelectromechanical systems

Although focused ion beam (FIB) milling has previously been used for fabrication of compliant nanostructures and devices, few instances of FIB nanomachining of such devices out of bulk materials have been reported. We use FIB to fabricate nanoelectromechanical systems (NEMS) devices out of bulk materials. Ion impingement from multiple directions allows sculpting with considerable three-dimensional control of device shape, including tapering and notching. Finite-element modeling of device frequencies agrees with optical interferometric measurements, including for the effect of a localized notch. We envision that bulk FIB fabrication will be useful for NEMS prototyping, milling of tough-to-machine materials and generalized nanostructure fabrication with three-dimensional shape control.

Production of 70-nm Cr dots by laser-induced forward transfer

The effect of donor film thickness and laser beam fluence on the size of laser-induced forward transfer (LIFT) spots is studied to achieve sub-100 nm features. A 130 fs, 800 nm laser is focused on ultrathin Cr films, and the transfer and ablation thresholds of these films at various thicknesses are determined. The minimum transfer spot size decreases with decreasing donor film thickness and incident laser fluence. Minimum LIFT spots of 70-450 nm diameter are obtained from films of 20-80 nm thickness, respectively. The 70 nm diameter transfer spots obtained from sputtered continuous films are the smallest to date.

Pulsed laser deposition of Si nanodots for photonic applications

Several growths of Si nanodots on Si and GaAs substrates were conducted by pulsed laser deposition (PLD) using a KrF laser of 248nm, 15ns, 12Hz and a Ti-sapphire laser of 800nm, 130fs, 1kHz at 1x10-5mbar vacuum. The laser fluencies on a Si target were varied from 3 to 32J/cm2 for the nanosecond (ns) PLD growths and 1-2.75J/cm2 for the femtosecond (fs) PLD. Wide range of nanodots from 20nm to a few micron size droplets were observed from both the ns and fs PLD. Auger electron spectroscopy of the nanodots was conducted and which indicated that the nanodots were without contamination. A technique using a mask consisting of an array of small holes was used to obtain high density nanodots with uniform size. The array of 100nm diameter holes was created by E-beam lithography. With this technique we have achieved 100nm Si dots with 300nm spacing between them, with few defects. We have observed that laser fluences closer to the ablation threshold work better for deposition using the EBL mask. In summary, we have demonstrated the growth of 100nm Si nanodots in an array with very few defects using the EBL masking technique.

Even nanomechanical modes transduced by integrated photonics

We demonstrate the actuation and detection of even flexural vibrational modes of a doubly clamped nanomechanical resonator using an integrated photonics transduction scheme. The doubly clamped beam is formed by releasing a straight section of an optical racetrack resonator from the underlying silicon dioxide layer, and a step is fabricated in the substrate beneath the beam. The step causes uneven force and responsivity distribution along the device length, permitting excitation and detection of even modes of vibration. This is achieved while retaining transduction capability for odd modes. The devices are actuated via optical force applied with a pump laser. The displacement sensitivities of the first through third modes, as obtained from the thermomechanical noise floor, are 228 fm Hz−1/2, 153 fm Hz−1/2, and 112 fm Hz−1/2, respectively. The excitation efficiency for these modes is compared and modeled based on integration of the uneven forces over the mode shapes. While the excitation efficiency for the ...

Wavelength-division multiplexing of nano-optomechanical doubly clamped beam systems.

Wavelength-division multiplexing is demonstrated for a set of two doubly clamped beams. Using a single input/output waveguide in a nanophotonic detection system, the two mechanical beams are independently addressable using different wavelength channels as determined by their respective racetrack resonator detection cavities. The two cavities slightly overlap, which also enables the mechanical frequency of both beams to be detected simultaneously with a single wavelength. Finally, to physically map which wavelength channel corresponds to which specific device, a heating laser is targeted individually on each beam to create a reversible mechanical frequency shift. This multiplexing method would allow for the simpler detection of large arrays of nanomechanical devices in a sensor system.

Device overshield for mass-sensing enhancement (DOME) structure fabrication

Nanoelectromechanical systems (NEMS) have demonstrated excellent sensitivity in their ability to measure small particle masses even to the point of being able to differentiate between different chemical species based on their mass. NEMS mass responsivity, however, depends upon mechanical mode profile and adsorption location, a fact which considerably complicates mass-sensing analysis and reduces overall sensitivity. We introduce a fabrication scheme-termed device overshield for mass-sensing enhancement (DOME) involving structures which physically limit the position at which a flux of material is deposited onto a NEMS resonating sensor. This surface nanomachining process uses silicon-on-insulator, silicon dioxide and silicon nitride layers to produce multiple, independent structural levels. It could be used to create MEMS over NEMS structures, to fabricate integrated shadow-masks resistant to high temperature processing, or for enhancing the mass-sensing performance of underlying nanomechanical devices. The DOME structures do not appear to significantly affect the resonator response and are shown to successfully block incoming mass from being deposited on specified portions of a NEMS beam.