Steven A. Harfenist

Selective self-assembly at room temperature of individual freestanding Ag2Ga alloy nanoneedles

Liquid gallium drops placed on thick Ag films at room temperature spontaneously form faceted nanoneedles of Ag2Ga alloy oriented nearly normal to the surface. This observation suggests that single nanoneedles can be selectively grown by drawing silver-coated microcantilevers from gallium. Needles from 25 nm to microns in diameter and up to 33μm long were grown by this method. These metal-tipped cantilevers have been used to perform atomic force microscopy (AFM) and AFM voltage lithography.

Characterization of micromanipulator-controlled dry spinning of micro- and sub-microscale polymer fibers

No current microfabrication technique exists for producing room- temperature, high-precision, point-to-point polymer micro- and sub- microscale fibers in three dimensions. The purpose of this work is to characterize a novel method for fabricating interconnected three- dimensional (3-D) structures of micron and sub-micron feature size. Poly- (methyl methacrylate) (PMMA) micro- and sub-microscale fiber suspended bridges are fabricated at room temperature by drawing from pools of solvated polymer using a nano-tipped stylus that is precisely positioned by an ultra-high-precision micromilling machine. The fibers were drawn over a 1.8 mm silicon trench, and as the solvent in the solution bridge rapidly evaporates, a suspended, 3-D PMMA fiber remained between the two pools. The resulting fiber diameters were measured for solutions of PMMA in chlorobenzene with concentrations ranging from 15.5 to 23.0 wt% 495k g mol−1 PMMA and 13.0 to 21.0 wt% 950k g mol−1 PMMA. Fibers were found to increase in diameter from 450 nm to 50 µm, roughly corresponding to the increase in concentration of PMMA. To minimize fiber diameter variance, different stylus materials were investigated, with a Parylene®-coated stylus producing fibers with the lowest variance in diameter. Overall, the fiber diameter was found to increase significantly as the solution concentration and polymer molecular weight increased.

Formation of highly transmissive liquid metal contacts to carbon nanotubes

We have developed a method to produce liquid metal contacts to carbon nanotubes that allows direct measurement of the influence of the contact on the nanotube conductance. Gallium is deposited onto standard gold nanotube contacts, where it gradually spreads to coat the contact region. The two-terminal multiwall nanotube conductance increases by as much as 1.2e2∕h during the transition from gold to gallium contacts, and approaches 2e2∕h at room temperature, with a current density of 2×108A∕cm2. Surprisingly, the conductance is independent of the contact area or contact separation, providing evidence that transport is ballistic in multiwall nanotubes.

Gallium-driven assembly of gold nanowire networks

Nanowire networks of Au–Ga alloy are fabricated at temperatures between 220 and 300°C by application of small drops of liquid gallium to 10- to 100-nm-thick gold films. As the liquid gallium drop spreads and reacts with the gold film, lamellar segregation of gold-rich and gallium-rich regions form fractal-like networks of Au–Ga nanowires connected between gold-rich islands in specific zones concentric to the gallium droplet. The wires are subsequently suspended by wet chemical etching that undercuts the ∼10-nm-thick chromium adhesion layer and the silicon substrate. Suspended nanowires as long as 6μm and as narrow as 35nm diameter have been produced using this method.

Characterization of micromanipulator controlled dry spinning of micro- and nanoscale polymer fibers

No current microfabrication technique exists for producing room-temperature, high-precision, point-to-point polymer nanofibers in three dimensions. Therefore, the purpose of this study is to characterize a novel method for fabricating such structures. Poly-methylmethacrylate (PMMA) microand nano-fibers have been fabricated using a technique which involves drawing a solvated polymer bridge between two liquid pools with a stylus positioned by an ultra-high-precision micromill. The solvent in the solution bridge rapidly evaporates, leaving a suspended PMMA fiber between the two pools. It was observed that fibers ranging in diameter from 450 nm to 50 /spl mu/m were drawn and that fiber diameter increased significantly with increasing solution concentration and increasing polymer molecular weight.

Selective plasma nitridation and contrast reversed etching of silicon

A new method of selectively patterning a silicon substrate with silicon dioxide and silicon nitride is demonstrated. An oxide patterned silicon substrate is directly nitrided using a microwave generated nitrogen plasma. Upon subsequent selective wet chemical etching using KOH, the oxide is removed and etching proceeds into the silicon, revealing a contrast reversed pattern of the oxide. The etch resistance of the nitrided surface is maximized by increasing the microwave power, pressure, and nitridation duration. The etch rate of silicon dioxide is negligibly affected and its etch rate is nearly the same as before nitridation. Compositional analysis of the nitride and the nitrided oxide using x-ray photoelectron spectroscopy confirms that microwave plasma nitridation produces Si–N covalent bonds.

Custom fabrication of freestanding and suspended three-dimensional polymer structures

An atomic force microscope (AFM) is used as a micromanipulator to fabricate freestanding micron diameter wires and bridges in a matter of minutes by pulling polymer materials into fibers. The fabrication procedure appears to have significant application in easier and more rapid prototyping of micro-, nano- and MEMS devices. While fiber pulling technology has advanced to high degrees of perfection, our process represents the first time that a nano-positioning tool has been used to fabricate three-dimensional microstructures with a degree of flexibility and simplicity far exceeding traditional MEMS and microfabrication processing methods. Preliminary efforts at demonstrating the use of the fibers in device fabrication and applications are also presented.

High aspect ratio etching of atomic force microscope-patterned nitrided silicon

Silicon that is nitrided in a pure nitrogen plasma is patterned with voltage applied by an atomic force microscope (AFM). Wet chemical etching into AFM-patterned (110) silicon produced vertical trenches as narrow as 91 nm (for one 757 nm deep trench) and with aspect ratios as large as 8.9:1 (for a 95 nm by 849 nm trench). Compared to the ridge patterns resulting from AFM oxidation and wet etching of hydrogen-passivated silicon, a substantially higher applied voltage is required to pattern nitrided silicon.

Design, Analysis, and Testing of Electrostatically Actuated Micromembranes

This paper reports a numerical design analysis of electrostatically actuated micromembranes. We systematically compare membrane performance in terms of natural frequencies, pull-in voltage (the bias voltage at which the membrane contacts the base electrode) and the effects of variable leg lengths for a given membrane size. Some experimental data on membrane deflection profiles versus bias voltage is included along with some experimentally determined pull-in voltages. Polysilicon micromembranes were successfully fabricated using the low cost MUMPs process that limits the user to three structural layers. The devices are designed with an emphasis on the response of the membrane to applied DC bias voltage to allow for variable stiffening. Circular membranes with diameters ranging from 60 to 160 μm, suspended 2 μm over square back plates of side lengths varying from 60 to 140 μm are investigated for voltages up to 90 volts. Three-dimensional electromechanical finite element simulations have been performed. Pull-in voltage values from simulations compare favorably with the measured results. It was observed that, for maximum deflection of the membrane upon application of DC bias voltage, the optimal dimensions for back plate and top membrane should fall within the ranges 80–120 μm and 80–140 μm, respectively.Copyright