Mei-Lin Chan

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

Hemispherical wineglass resonators fabricated from the microcrystalline diamond

We present the development of millimeter scale 3D hemispherical shell resonators fabricated from the polycrystalline diamond, a material with low thermoelastic damping and very high stiffness. These hemispherical wineglass resonators with 1.1 mm diameter are fabricated through a combination of micro-electro discharge machining (EDM) and silicon micromachining techniques. Using piezoelectric and electrostatic excitation and optical vibration measurement, the elliptical wineglass vibration mode is determined to be at 18.321 kHz, with the two degenerate wineglass modes having a relative frequency mismatch of 0.03%. A study on the effect of the size and misalignment of the anchor and resonator’s radius variation on both the average frequency and frequency mismatch of the 2θ elliptical vibration modes is carried out. It is shown that the absolute frequency of a wineglass resonator will increase with the anchor size. It is also demonstrated that the fourth harmonic of radius variation is linearly related to the frequency mismatch. (Some figures may appear in colour only in the online journal)

Rotary Liquid Droplet Microbearing

A rotational stage with a 10-mm-diameter single-crystal silicon rotor supported by liquid droplet ball bearings is described. The 100-300-μm-thickness droplet bearings are retained on the rotor surface with a micropatterned amorphous-flouropolymer-based superhydrophobic (SHP) surface coating that yields a 156° contact angle. The droplets slide on a SHP bearing raceway that is formed from laser-roughened poly-dimethylsiloxane (PDMS) on the surface of the stator, achieving a 10°-contact-angle hysteresis that results in very low sliding friction. The stage is driven by a rotating external magnetic held that provides up to 3 μN · m torque through a permanent magnet mounted on the rotor. The liquid bearing provides a passive wear-free interface between rotor and stator with a measured drag coefficient of 0.94 · 10-3 μN · m/r/min, rotating up to a speed of 2400 r/min, and a mean minimum operating torque of 0.3 μN · m. The bearing design is stable in position and tip/tilt, with a tip mode stiffness of 5.4 μN · m/deg and measured nonrespectable rotor wobble of 0.3 mrad. The experimentally measured bearing stiffness, drag coefficient, and startup torque are shown to compare well with values predicted from analytical models based on surface tension forces on the droplet bearings.

Magnetic Scanometric DNA Microarray Detection of Methyl Tertiary Butyl Ether Degrading Bacteria for Environmental Monitoring

A magnetoresistive biosensing platform based on a single magnetic tunnel junction (MTJ) scanning probe and DNA microarrays labeled with magnetic particles has been developed to provide an inexpensive, sensitive and reliable detection of DNA. The biosensing platform was demonstrated on a DNA microarray assay for quantifying bacteria capable of degrading methyl tertiary butyl ether (MTBE), where concentrations as low as 10 pM were detectable. Synthetic probe bacterial DNA was immobilized on a microarray glass slide surface, hybridized with the 48 base pair long biotinylated target DNA and subsequently incubated with streptavidin-coated 2.8 μm diameter magnetic particles. The biosensing platform then makes use of a micron-sized MTJ sensor that was raster scanned across a 3 mm by 5 mm glass slide area to capture the stray magnetic field from the tagged DNA and extract two dimensional magnetic field images of the microarray. The magnetic field output is then averaged over each 100 μm diameter DNA array spot to extract the magnetic spot intensity, analogous to the fluorescence spot intensity used in conventional optical scanners. The magnetic scanning result is compared with results from a commercial laser scanner and particle coverage optical counting to demonstrate the dynamic range and linear sensitivity of the biosensing platform as a potentially inexpensive, sensitive and portable alternative for DNA microarray detection for field applications.

Micromachining 3D hemispherical features in silicon via micro-EDM

This paper presents an investigation of micro electrical discharge machining (μEDM) as a viable method for micromachining 3D shapes in silicon. The approach integrates a two-step μEDM process with standard silicon microfabrication techniques to create smooth and axisymmetric 3D hemispherical structures with eccentricity, ε ~ 0.11 and a radius variation <; 2%. Through the selection of ultrahard polycrystalline diamond as the μEDM electrode, the low tool wear allows for high throughput machining of 200 wells in silicon within a short total processing time of 80 min. Feasibility of the approach is demonstrated in the fabrication of millimeter scale hemispherical shell structures using the machined silicon features as a mold.

Scanning Magnetoresistance Microscopy for Imaging Magnetically Labeled DNA Microarrays

We demonstrate the use of a magnetic tunnel junction (MTJ) sensor as a probe for non-contact scanning microscopy of magnetically labeled DNA microarrays. Induced magnetic stray fields from 2.8 mum diameter magnetic particles are detected using the MTJ sensor, while two dimensional scanning generates the magnetic map of the DNA microarray with a spatial resolution of 1 mum over a large scan area exceeding 1 cm2. Current particle-sensor spacing of ~20 mum results in a detection resolution of ~30 magnetic particles within each 100 mum diameter DNA spot. Our results highlight the use of scanning magnetoresistive microscopy as a convenient and powerful technique for the accurate detection and identification of biomolecules tagged with magnetic particles.

Impact of doping and microstructure on quality factor of CVD diamond micromechanical resonators

The effect of doping and microstructure is explored on CVD diamond MEMS resonators. Hundreds of surface micromachined double ended tuning fork (DETF) resonators were fabricated in nanocrystalline diamond (NCD) and microcrystalline diamond (MCD) films deposited using hot filament CVD technique with varying levels of Boron doping. High resistivity (1926 MΩ·cm) NCD cantilevers and DETF demonstrated impressive Q-factors of 232,562 at f = 61.86 kHz and Q = 201,435 at f = 263.66 kHz, respectively. These Qs are the highest Q-factors yet reported for diamond resonators and the highest for cantilevers fabricated from any polycrystalline material. Higher boron doping resulted in reduced Q due to defect losses. Higher surface loss was observed in both MCD and NCD as doping increased. Observed Q-factors were almost the same for MCD and NCD at frequencies near 10 MHz.

Self-assembled microfabrication technology for 3D isotropic negative index material

We present a fabrication method to realize three dimensional (3D) isotropic homogeneous negative index material (3DNIMs) using a low cost and massively parallel manufacturable and self-assembly technique. The construction of self-assembled 3D-NIM array was realized through two dimensional (2-D) planar microfabrication techniques exploiting the as-deposited residual stress imbalance between a bi-layer consisting of e-beam evaporated metal (chromium) and a structural layer of low stress silicon nitride deposited by LPCVD on a p-doped silicon substrate. A periodic continuation of a single rectangular unit cell consisting of split-ring resonators (SRR) and wires were fabricated to generate a 3D assembly by orienting them along all three Cartesian axes. The thin chromium and silicon nitride bi-layer is formed as hinges. The strain mismatch between the two layers at the hinge curls the structural layer containing the SRR upwards. The self-assembled out-of-plane angular position depends on the thickness and material composing the bi-layer. This built-in stress-actuated assembly method is suitable for applications requiring a thin dielectric layer for the SRR and/or active devices.

An electrohydrodynamically driven microfabricated actuator for the study of miniature ion propulsion engine and electric wind devices

For the study of miniature ion propulsion engine and electric wind devices, we have developed an electrohydrodynamically (EHD) driven microfabricated actuator. It consumes a maximum power of 100 mW and has a maximum resultant driving force of 0.45 /spl mu/N in the first observed driving mode. The actuator consists of a mass/spring configuration fabricated with dual ion drives for propulsion. DC partial electrical discharge produces and accelerates the ions. Electric wind is generated by the momentum transfer from the ions to the air. Momentum is also transferred by virtue of the formation of intermittent space charge near the ionization zone. A selection between single and dual ion drives allows observation of various oscillation modes beginning at 896 Hz. The maximum out-of-plane oscillation amplitude measured was approximately 2 /spl mu/m.

Low-power magnetically-actuated microvalves for highly parallel microfluidic automation

This paper presents the design and actuation of low-power, magnetically-actuated microvalves fabricated with CMOS-compatible technology. A prototype chip comprised of 64 microvalves within 2.2 × 2.2 mm2 and requiring <; 50 mW to actuate is demonstrated. Each valve consists of a microcoil and a paramagnetic bead as the valve seat, and is used to address a 50 picoliter well to enable highly parallel biochemical sensing and analysis operations.