Hyungjoo Na

Lithography-Free Fabrication of Large Area Subwavelength Antireflection Structures Using Thermally Dewetted Pt/Pd Alloy Etch Mask

We have demonstrated lithography-free, simple, and large area fabrication method for subwavelength antireflection structures (SAS) to achieve low reflectance of silicon (Si) surface. Thin film of Pt/Pd alloy on a Si substrate is melted and agglomerated into hemispheric nanodots by thermal dewetting process, and the array of the nanodots is used as etch mask for reactive ion etching (RIE) to form SAS on the Si surface. Two critical parameters, the temperature of thermal dewetting processes and the duration of RIE, have been experimentally studied to achieve very low reflectance from SAS. All the SAS have well-tapered shapes that the refractive index may be changed continuously and monotonously in the direction of incident light. In the wavelength range from 350 to 1800 nm, the measured reflectance of the fabricated SAS averages out to 5%. Especially in the wavelength range from 550 to 650 nm, which falls within visible light, the measured reflectance is under 0.01%.

Fabrication of a multi-walled carbon nanotube-deposited glass fiber air filter for the enhancement of nano and submicron aerosol particle filtration and additional antibacterial efficacy.

We grew multi-walled carbon nanotubes (MWCNTs) on a glass fiber air filter using thermal chemical vapor deposition (CVD) after the filter was catalytically activated with a spark discharge. After the CNT deposition, filtration and antibacterial tests were performed with the filters. Potassium chloride (KCl) particles (<1 μm) were used as the test aerosol particles, and their number concentration was measured using a scanning mobility particle sizer. Antibacterial tests were performed using the colony counting method, and Escherichia coli (E. coli) was used as the test bacteria. The results showed that the CNT deposition increased the filtration efficiency of nano and submicron-sized particles, but did not increase the pressure drop across the filter. When a pristine glass fiber filter that had no CNTs was used, the particle filtration efficiencies at particle sizes under 30 nm and near 500 nm were 48.5% and 46.8%, respectively. However, the efficiencies increased to 64.3% and 60.2%, respectively, when the CNT-deposited filter was used. The reduction in the number of viable cells was determined by counting the colony forming units (CFU) of each test filter after contact with the cells. The pristine glass fiber filter was used as a control, and 83.7% of the E. coli were inactivated on the CNT-deposited filter.

Bidirectional electrothermal electromagnetic torsional microactuators for large angular motion at dc mode and high frequency resonance mode operation

This paper presents a novel design of a bidirectional torsional micromirror utilizing vertically driven electrothermal electromagnetic silicon beam actuators to generate large angular motion in both static mode and high-frequency resonance mode with low operational voltages. The microactuators are fabricated on a silicon-on-insulator (SOI) wafer using three photo masks in order to form two different thicknesses of single crystal silicon (SCS) device layer and backside cavities. When the driving bias is applied to the device in the static mode operation, four buckle beams placed alongside the torsion bars are subjected to thermal expansion and buckle in the vertical direction generating torsional displacement of the micromirror with respect to two torsion bars, the center of rotation. The direction of buckle is controlled by the Lorentz force caused by the current flowing through the silicon beams to be buckled in the magnetic field applied, enabling the bidirectional motion of the torsional micromirror. At resonance, Lorentz force itself drives the actuator instead of thermal expansion force from the buckle beams. The maximum static angular displacement of the torsional actuator is 13.42° (26.84°, optical angle) under a driving dc voltage of 7.5 V. In the resonance mode operation, the measured angular displacement is 8.22° (16.44°, optical angle) at 10.64 kHz under sinusoidal driving voltages of 0 to 4.4 V.

Low-Temperature Selective Growth of Tungsten Oxide Nanowires by Controlled Nanoscale Stress Induction

We report a unique approach for the patterned growth of single-crystalline tungsten oxide (WOx) nanowires based on localized stress-induction. Ions implanted into the desired growth area of WOx thin films lead to a local increase in the compressive stress, leading to the growth of nanowire at lower temperatures (600 °C vs. 750–900 °C) than for equivalent non-implanted samples. Nanowires were successfully grown on the microscale patterns using wafer-level ion implantation and on the nanometer scale patterns using a focused ion beam (FIB). Experimental results show that nanowire growth is influenced by a number of factors including the dose of the implanted ions and their atomic radius. The implanted-ion-assisted, stress-induced method proposed here for the patterned growth of WOx nanowires is simpler than alternative approaches and enhances the compatibility of the process by reducing the growth temperature.

Bidirectional Electrothermal Electromagnetic Torsional Microactuators

This paper presents a novel design of bidirectional torsional micromirror utilizing vertically driven electrothermal electromagnetic silicon beam actuators to generate large static angular motion. The microactuators are fabricated on silicon-on-insulator (SOI) wafer using three photo masks in order to form two different thicknesses of single crystal silicon (SCS) device layer and backside cavities. When the driving bias is applied to the device, four buckle beams placed alongside of the torsion bars are subjected to thermal expansion and buckle in vertical direction generating torsional displacement of the micromirror with respect to two torsion bars placed at the center. The direction of buckle is controlled by Lorentz force caused by the magnetic field applied and the amount of current flowing through the micro beam to be buckled, enabling the bidirectional motion of the torsional micromirror. The maximum static angular displacement of the torsional actuator is up to 14.35° (28.7°, optical angle) under driving DC voltage of 7.5V. For resonance mode operation, the measured angular displacement is 8.46° (16.92°, optical angle) at 10.94 kHz under sinusoidal driving voltage of 0 to 3V.

Site-specific growth and density control of carbon nanotubes by direct deposition of catalytic nanoparticles generated by spark discharge

Catalytic iron nanoparticles generated by spark discharge were used to site-selectively grow carbon nanotubes (CNTs) and control their density. The generated aerosol nanoparticles were deposited on a cooled substrate by thermophoresis. The shadow mask on top of the cooled substrate enabled patterning of the catalytic nanoparticles and, thereby, patterning of CNTs synthesized by chemical vapor deposition. The density of CNTs could be controlled by varying the catalytic nanoparticle deposition time. It was also demonstrated that the density could be adjusted by changing the gap between the shadow mask and the substrate, taking advantage of the blurring effect of the deposited nanoparticles, for an identical deposition time. As all the processing steps for the patterned growth and density control of CNTs can be performed under dry conditions, we also demonstrated the integration of CNTs on fully processed, movable silicon microelectromechanical system (MEMS) structures.

Reversible and continuous latching using a carbon internanotube interface.

Mechanical multistability is greatly beneficial in microelectromechanical systems because it offers multiple stable positioning of movable microstructures without a continuous energy supply. Although mechanical latching components based on multistability have been widely used in microsystems, their latching positions are inherently discrete and the number of stable positions is quite limited because of the lithographical minimum feature size limit of microstructures. We report a novel use of aligned carbon nanotube (CNT) arrays as latching elements in a movable micromechanical device. This CNT-array-based latching mechanism allows stable latching at multiple latching positions, together with reversible and bidirectional latching capabilities. The latching element with integrated CNTs on the sidewalls of microstructures can be adopted for diverse microelectromechanical systems that need precise positioning of movable structures without the necessity of continuous power consumption.

Continuously latchable shuttle using carbon nanotubes on sidewall surfaces

We demonstrated a novel usage of self-adjusted, vertically aligned carbon nanotube (CNT) arrays integrated on the sidewalls of microstructures as latching components. The CNT array-based latching mechanism showed stable latching at multiple latching positions, together with reversible and bidirectional latching capabilities. The latchable shuttle using CNT latch could be adopted for diverse microelectromechanical systems (MEMS) that need precise positioning of movable structures without the necessity of continuous power consumptions to hold the displaced position.