Hyoung J. Cho

Graphene-P(VDF-TrFE) Multilayer Film for Flexible Applications

A flexible, transparent acoustic actuator and nanogenerator based on graphene/P(VDF-TrFE)/graphene multilayer film is demonstrated. P(VDF-TrFE) is used as an effective doping layer for graphene and contributes significantly to decreasing the sheet resistance of graphene to 188 ohm/sq. The potentiality of graphene/P(VDF-TrFE)/graphene multilayer film is realized in fabricating transparent, flexible acoustic devices and nanogenerators to represent its functionality. The acoustic actuator shows good performance and sensitivity over a broad range of frequency. The output voltage and the current density of the nanogenerator are estimated to be ∼3 V and ∼0.37 μAcm(-2), respectively, upon the application of pressure. These values are comparable to those reported earlier for ZnO- and PZT-based nanogenerators. Finally, the possibility of rollable devices based on graphene/P(VDF-TrFE)/graphene structure is also demonstrated under a dynamic mechanical loading condition.

Graphene-Based Bimorph Microactuators

A novel graphene-on-organic film fabrication method that is compatible with a batch microfabrication process was developed and used for electromechanically driven microactuators. A very thin layer of graphene sheets was monolithically integrated and the unique material characteristics of graphene including negative thermal expansion and high electrical conductivity were exploited to produce a bimorph actuation. A large displacement with rapid response was observed while maintaining the low power consumption. This enabled the successful demonstration of transparent graphene-based organic microactuators.

Electroplated thick permanent magnet arrays with controlled direction of magnetization for MEMS application

We have successfully developed a technique to electroplate thick CoNiMnP-based permanent magnet arrays with controlled direction of magnetization and improved magnetic properties by applying external magnetic fields during electroplating. The magnet arrays with individual magnet shapes and sizes were fabricated on a Si substrate using micromachining and electroplating techniques. The magnetic properties of these magnets have been characterized with the vibrating sample magnetometer. The optimized processing conditions with external magnetic fields during electroplating have improved the coercivity and the retentivity of the magnets by more than 200% and 350%, respectively, comparing with those without external magnetic fields. This paper describes the process for fabricating the magnets and the effect of external magnetic fields in controlling and improving the properties of the magnets. In addition to the capability of full scale integration, the high vertical anisotropy of thick magnet arrays can be use...

A Plastic Micro Injection Molding Technique Using Replaceable Mold-Disks for Disposable Microfluidic Systems and Biochips

This paper presents an innovative plastic micro injection molding technique using replaceable disk-mold for applications to microfluidic systems and biochips. Precisely patterned and microfabricated wafer-type mold inserts can be easily loaded into the injection molding machine. Processing time for one chip was as fast as 10 seconds while hot embossing techniques require at least several minutes. Less than a few urn of precise patterns were also achieved. A promising new material has also been introduced and characterized using the developed injection molding technique as well as demonstrated for a disposable biochip.

Effect of ultraviolet radiation exposure on room-temperature hydrogen sensitivity of nanocrystalline doped tin oxide sensor incorporated into microelectromechanical systems device

The effect of ultraviolet (UV) radiation exposure on the room-temperature hydrogen (H2) sensitivity of nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin-film gas sensor is investigated in this article. The present sensor is incorporated into microelectromechanical systems device using sol-gel dip-coating technique. The present sensor exhibits a very high sensitivity, as high as 65 000–110 000, at room temperature, for 900ppm of H2 under the dynamic test condition without UV exposure. The H2 sensitivity is, however, observed to reduce to 200 under UV radiation, which is contrary to the literature data, where an enhanced room-temperature gas sensitivity has been reported under UV radiation. The observed phenomenon is attributed to the reduced surface coverage by the chemisorbed oxygen ions under UV radiation, which is in consonance with the prediction of the constitutive equation, proposed recently by the authors, for the gas sensitivity of nanocrystalline semiconductor oxide thin-film sensors.

Microscale resin-bonded permanent magnets for magnetic micro-electro-mechanical systems applications

A micromachining technique has been developed for the fabrication of microscale resin-bonded permanent magnets. Magnetic paste has been prepared from Sr-ferrite powder and an epoxy resin, filled into lithographically defined molds, and formed into resin-bonded magnets after room temperature curing. Coercivity of 356 kA/m (4480 Oe), retentivity of 33 mT (330 G), and energy density of 2.7 kJ/m3 have been achieved in 65-μm-thick disk arrays with lateral dimensions ranging from 50 to 200 μm. Based on the developed magnet, a magnetic MEMS actuator has been designed, fabricated, and characterized. Actuation current up to ±60 mA operated the actuator up to 70 μm in attractive and repulsive motion. This work can be used for producing thick-film type permanent magnets, which can be scaled from a few tens of micrometers to millimeters on various substrates.

Hydrogen-discriminating nanocrystalline doped-tin-oxide room-temperature microsensor

Highly hydrogen (H2)-selective [relative to carbon monoxide (CO)] sensor, operating at room temperature, has been fabricated using the micronanointegration approach involving the deposition of the nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin film on microelectromechanical systems device. The present microsensor exhibits high room-temperature sensitivity towards H2 (S=12700); however, it is insensitive to CO at room temperature. In view of the different gas selectivity mechanisms proposed in the literature, it is deduced that the In2O3 doping, the presence of InSn4 phase, the low operating temperature (room temperature), the mesostructure, the small sizes of H2 and H2O molecules, the bulky intermediate and final reaction products for CO, and the electrode placement at the bottom are the critical parameters, which significantly contribute to the high room-temperature H2 selectivity of the present microsensor over CO. The constitutive equation for the gas sensitivity of the semiconductor ...

Magnetically-driven bi-directional optical microscanner

A magnetically-driven bi-directional optical microscanner has been designed, fabricated and characterized. Magnetic and structural modeling and analysis has been applied to the design of the scanner. The micromachined scanner can be operated bi-directionally under the condition of static operation. Under dynamic operation, the prototype scanner has shown stable bi-directional scanning performance at the operating frequency of 30 Hz, corresponding to 60 Hz in the regular uni-directional scanner. Compared with a conventional scanner, the microscanner has the advantages of low power consumption due to its small size and high scanning efficiency as a result of unique bi-directional actuation.

Droplet actuation on a liquid layer due to thermocapillary motion: Shape effect

Fluid Dynamics Video: In the recent years, there has been a growing interest in droplet-based (digital) microfluidics for which, reliable means of droplet manipulation are required. In this study we demonstrate thermal actuation of droplets on liquid platforms, which is ideal for biochemical microsystems and lab-on-chip applications because droplets can be transported with high speed, good control and minimal thermal loading as compared to using conventional solid substrates. In addition, other disadvantages of using solid surfaces such as evaporation, contamination, pinning, hysteresis and irreversibility of droplet motion are avoided. Based on the theoretical development and measurements, a silicon-based droplet transportation platform was developed with embedded Titanium micro heaters. A shallow liquid pool of inert liquid (FC-43) served as the carrier liquid. Heaters were interfaced with control electronics and driven through a computer graphical user interface. By creating appropriate spatio-temporal thermal gradient maps, transport of droplets on predetermined pathways was successfully demonstrated with high level of robustness, speed and reliability. The video shows normal imaging of droplet manipulation accompanied by the corresponding infrared thermal imaging showing the spatio-temporal temperature maps and the outline of the drop as it moves towards hot spots.In the thermocapillary migration of droplets on the free surface of immiscible liquids, we observe that the lens-shaped drops move from warm toward cooler regions while spherical drops move in the opposite direction. We explain this dual behavior using an analysis of surface deformation and velocity profiles of thin liquid layers subject to a lateral thermal gradient. Liquid platforms allow thermocapillary transport of drops with higher migration speeds than solid substrates and lower internal temperature fluctuation. Such conditions may be exploited in biochemical microsystems where droplet evaporation, contamination, and surface pinning need to be avoided.

Stress analysis of silicon membranes with electroplated permalloy films using Raman scattering

We have measured the stress profile on a silicon membrane electroplated with a permalloy film using Raman scattering. The effect of silicon membrane thickness and permalloy film thickness on stress distribution was studied. Depending upon the nature of stress, the optic phonon in silicon at 520 cm/sup -1/ either shifts upward (compressive) or downward (tensile). The phonon frequency shift is proportional to the magnitude of stress. A microscope X-Y stage was used to map the stress distribution over the silicon membrane that was covered and uncovered by the permalloy film. Silicon membranes in the thickness range, 9 /spl mu/m <t/sub m/<12 /spl mu/m, and permalloy films in the thickness range, 6 /spl mu/m <t/sub p/<13 /spl mu/m showed evidence of compressive stress. Based on the present results, membrane type microvalve design is optimized to prevent leakage, originating from stressed membranes. Such a nondestructive and noncontact microscopic stress analysis technique can be applied for design optimization in various magnetic MEMS devices.