Seung-Beck Lee

Direct patterning of carbon nanotube network devices by selective vacuum filtration

We report on a method which allows direct patterning of density controlled carbon nanotube network (NTN) devices during network formation. By lithographically patterning photoresist on the filter membrane, the nanotubes in suspension were guided to the exposed pattern areas of the filter surface during vacuum filtration allowing in situ device formation. The NTNs were transferred to an elastomer substrate by directly curing polydimethylsiloxane on its surface. We compared electrical characteristics of NTN patterns fabricated simultaneously using this method and found that the different NTNs with equal nanotube density showed reproducible characteristics.

A Highly Sensitive and Reliable Strain Sensor Using a Hierarchical 3D and Ordered Network of Carbon Nanotubes

A 3D network of single-walled carbon nanotubes embedded in poly-(dimethylsiloxane) is presented as a promising route to the fabrication of a flexible film with ordered and interconnected single-walled carbon nanotubes. This is possible using a simple transfer method of as-grown hierarchical single-walled carbon nanotubes on a Si pillar substrate. This film is used as a highly sensitive strain gauge sensor. This type of network embedded in a polymer film should be applicable to many fields involving mechanically stable and reliable strain sensors.

Clogging-free microfluidics for continuous size-based separation of microparticles.

In microfluidic filtration systems, one of the leading obstacles to efficient, continuous operation is clogging of the filters. Here, we introduce a lateral flow microfluidic sieving (μ-sieving) technique to overcome clogging and to allow continuous operation of filter based microfluidic separation. A low frequency mechanical oscillation was added to the fluid flow, which made possible the release of aggregated unwanted polystyrene (PS) particles trapped between the larger target PS particles in the filter demonstrating continuous μ-sieving operation. We achieved collection of the target PS particles with 100% separation efficiency. Also, on average, more than 98% of the filtered target particles were retrieved after the filtration showing high retrieval rates. Since the oscillation was applied to the fluid but not to the microfluidic filter system, mechanical stresses to the system was minimized and no additional fabrication procedures were necessary. We also applied the μ-sieving technique to the separation of cancer cells (MDA-MB-231) from whole blood and showed that the fluidic oscillations prevented the filters from being blocked by the filtered cancer cells allowing continuous microfluidic separation with high efficiency.

Multi-step ion beam etching of sub-30 nm magnetic tunnel junctions for reducing leakage and MgO barrier damage

We have demonstrated the fabrication of sub 30 nm magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy. The multi-step ion beam etching (IBE) process performed for 18 min between 45° and 30°, at 500 V combined ion supply voltage, resulted in a 55 nm tall MTJ with 28 nm diameter. We used a negative tone electron beam resist as the hard mask, which maintained its lateral dimension during the IBE, allowing almost vertical pillar side profiles. The measurement results showed a tunnel magneto-resistance ratio of 13% at 1 kΩ junction resistance. With further optimization in IBE energy and multi-step etching process, it will be possible to fabricate perpendicularly oriented MTJs for future sub 30 nm non-volatile magnetic memory applications.

Flexible and transparent touch sensor using single-wall carbon nanotube thin-films

Here we report on the fabrication and demonstration of flexible and transparent touch sensors using carbon nanotube thin film(CNTF). We fabricated two types of touch sensors. The first type was a compressively strained CNTF pressure sensor, and the second type was a 5-wire resistive CNTF touch sensor. The change in compressively strained CNTF conductivity depended on the strength of the applied vertical pressure and with 0.2 MPa applied vertical pressure, the CNTF sheet conductance increased rapidly by 10 %. We applied cyclic pressures (0.2 MPa) at 15 second intervals and found that the tactile pressure dependence was reproducible for several hundred cycles. In the case of 5-wire resistive CNTF touch sensors, the touch activation pressure of is 23 Pa. And it showed good repeatability over 105 times.

Spatially digitized tactile pressure sensors with tunable sensitivity and sensing range

When developing an electronic skin with touch sensation, an array of tactile pressure sensors with various ranges of pressure detection need to be integrated. This requires low noise, highly reliable sensors with tunable sensing characteristics. We demonstrate the operation of tactile pressure sensors that utilize the spatial distribution of contact electrodes to detect various ranges of tactile pressures. The device consists of a suspended elastomer diaphragm, with a carbon nanotube thin-film on the bottom, which makes contact with the electrodes on the substrate with applied pressure. The electrodes separated by set distances become connected in sequence with tactile pressure, enabling consecutive electrodes to produce a signal. Thus, the pressure is detected not by how much of a signal is produced but by which of the electrodes is registering an output. By modulating the diaphragm diameter, and suspension height, it was possible to tune the pressure sensitivity and sensing range. Also, adding a fingerprint ridge structure enabled the sensor to detect the periodicity of sub-millimeter grating patterns on a silicon wafer.

Spray-coated carbon nanotube thin-film transistors with striped transport channels

We present results for the transfer characteristics of carbon nanotube thin-film transistors (CNT-TFTs) that utilize single-walled carbon nanotube thin-films prepared by direct spray-coating on the substrate. By varying the number of spray-coatings (N(sp)) and the concentration of nanotubes in solution (C(NT)), it was possible to control the conductivity of the spray-coated nanotube thin-film from 129 to 0.1 kΩ/□. Also, by introducing stripes into the channel of the CNT-TFT, and thereby reducing the number of metallic percolation paths between source and drain, it was possible to enhance the on/off current ratio 1000-fold, from 10 to 10(4), demonstrating that it may be possible to utilize spray-coating as a method to fabricate CNT-TFTs for large area switching array applications.

Superconducting nanotransistor based digital logic gates

We describe the fabrication and digital logic operation of superconducting nanotransistors. The nanotransistor is a superconducting weak-link device that has integrated hot-phonon injector heaters to control its switching critical current. The fabrication process utilizes a self-aligned method, where the heater acts as a mask for reactive ion etching to define the device pattern. This device is much simpler to fabricate than previously reported controllable superconducting transistors and the principle of operation makes it possible to use a single nanotransistor connected to a load resistor as a NOT logic gate, allowing lower power consumption and improved levels of integration.

Negative electron-beam resist hard mask ion beam etching process for the fabrication of nanoscale magnetic tunnel junctions

The authors have demonstrated fabrication of 30 nm diameter perpendicular anisotropy magnetic tunnel junctions (MTJs) using negative electron-beam resist (NER) as the ion beam etching (IBE) hard mask. The NER pillar of 30 nm diameter and 105 nm thickness was fabricated by electron-beam lithography. The redeposition of the MTJ etching debris generated during the IBE on the outer surface of the NER pillar increased the lateral etch resistance of the resist polymer, allowing the edge profile to remain constant for the duration of the MTJ etching, resulting in a vertical MTJ sidewall profile. A multistep IBE (repetition of 45° primary etching and 30° secondary etching) was conducted to reduce the MTJ sidewall redeposition while reducing mechanical damage. The measurement results showed a tunnel magneto-resistance ratio of 22% at 30 nm junction diameter.

Real-time detection of airborne dust particles using paddle-type silicon cantilevers

The authors report on the fabrication and operation of a real-time dust particle sensor using a paddle-type silicon cantilever. Electrostatic field applied to the cantilever electrode attracts dust particles, and as they attach on the paddle surface the oscillation phase changes, reflecting the particle mass. The kinetic energy of the oscillating cantilever limits the size of the dust particle landing on its surface at the given electric field, controlling the maximum mass, and therefore the size, of the dust particles being detected. The measurement of a single dust particle of ∼1.2pg attaching and detaching from the oscillator surface was demonstrated. The results show that the paddle-type cantilever sensor may be developed into a real-time monitoring sensor of airborne submicron dust particles.