Se Il Park

Fabrication and characterization of freestanding 3D carbon microstructures using multi-exposures and resist pyrolysis

We present a fabrication method for freestanding complex 3D carbon microstructures utilizing a lithogaphy step and a heating step. We developed two fabrication methods for multi-level 3D SU-8 microstructures, which were used as polymer precursors in a carbonization process. In one method, multiple SU-8 layers were successively coated and cross-linked. In the other method, aligned partial exposures were used to control the thickness of the freestanding SU-8 layer. Freestyle, freestanding carbon microstructures were fabricated by heating 3D SU-8 microstructures below 1000 °C in a nitrogen atmosphere. Characterization of the pyrolysis process, through measurements such as dimensional changes, roughness, hardness, elastic modulus and resistivity, was performed for positive resists AZ5214 and AZ9260 as well as SU-8. 3D carbon microstructures fabricated using our methods can be utilized for various applications such as low cost resonating microsensors and microfluidics.

Biosensor utilizing resist-derived carbon nanostructures

The authors present a biosensor using pyrolyzed electron beam resist nanostructures as an active conducting channel. Versatile, arbitrarily shaped nanostructures such as nanowires, nanodots, and suspended nanobridges are fabricated by a facile electron beam resist thermal decomposition method. The nanostructures typically show 15–21nm thickness, 100–200nm width, 0.6nm roughness, and p-type majority conduction with tailored resistivity of 5.2–0.75Ωcm. Streptavidin-biotin binding and pH dependent conductance modulation are demonstrated using pyrolyzed resist based devices.

Enhancement of Coulomb blockade and tunability by multidot coupling in a silicon-on-insulator-based single-electron transistor

A dual-gate-controlled single-electron transistor with coupled dot geometry has been fabricated on a silicon-on-insulator structure. Coupled dots are defined by tunable gates which are designed to separately control the tunneling potential barriers to compensate for disorder due to size fluctuation in quantum dots. The Coulomb-blockade phenomena observed in linear and nonlinear transport regimes were found to be enhanced by the multidot coupling. The Coulomb staircase (nonlinear effect) appears more clearly with the increasing number of coupled dots, indicating definite suppression of the inevitable cotunneling process. In the linear regime, the frequency of Coulomb oscillation was able to be tuned by changing the interdot coupling strength. These results indicate that enhancement of the Coulomb blockade and tunability can be achieved through replacing the traditional single dot by gate-controlled multidots in future single-electron devices.

The fabrication of carbon nanostructures using electron beam resist pyrolysis and nanomachining processes for biosensing applications.

We present a facile, yet versatile carbon nanofabrication method using electron beam lithography and resist pyrolysis. Various resist nanopatterns were fabricated using a negative electron beam resist, SAL-601, and they were then subjected to heat treatment in an inert atmosphere to obtain carbon nanopatterns. Suspended carbon nanostructures were fabricated by the wet-etching of an underlying sacrificial oxide layer. Free-standing carbon nanostructures, which contain 130xa0nm wide, 15xa0nm thick, and 4xa0µm long nanobridges, were fabricated by resist pyrolysis and nanomachining processes. Electron beam exposure dose effects on resist thickness and pattern widening were studied. The thickness of the carbon nanostructures was thinned down by etching with oxygen plasma. An electrical biosensor utilizing carbon nanostructures as a conducting channel was studied. Conductance modulations of the carbon device due to streptavidin-biotin binding and pH variations were observed.

Dust particle growth in rf silane plasmas using two-dimensional multi-pass laser light scattering

We measured a spatio-temporal distribution of particle size and a spatial growth rate in a capacitively coupled silane plasma using in situ multi-pass laser light scattering. The two-dimensional measurement was accomplished using a low power He–Ne laser and a set of spherical mirrors across the plasma that enables us to span multiple beam paths over the plasma region in the vertical direction from the electrode sheath to the bulk plasma. In temporal, the measurement result shows two particle growth periods in which the fast particle growth (nucleation) is followed by the slow particle growth (coagulation). In spatial, the fastest particle growth occurred at the highest vertical position that corresponds to the furthest position from the sheath. The particle coagulation modeling indicates that it is consistent with the largest proto particle creation rate in the plasma bulk.

Density measurement of particles in rf silane plasmas by the multipass laser extinction method

Measurement of the time evolution of the particle number density was investigated in rf silane plasmas by using the multipass laser extinction method. A He–Ne laser beam underwent multiple reflections on one horizontal plane of the plasma. The extinction signal increased in proportion to the beam pass numbers. A 1011cm−3 density of 8nm radius particles was measured at 10s in a 32mTorr and 50W discharge using nine passes. The primary particle density was obtained by comparing the measured particle sizes with the calculated sizes from the light extinction signals and the Brownian free molecule coagulation model.

High frequency carbon nanomechanical resonators embedded with carbon nanotube stiffening layers

We present batch-fabricated carbon-based nanomechanical resonators which are laminated by a pyrolyzed carbon layer and single-walled carbon-nanotube network (C/CNT resonators). The embedded CNT layers simultaneously enhance the electrical conductivity (∼160-fold) and mechanical stiffness (10% higher Young’s modulus) compared to nonstiffened carbon-only resonators. Dynamic behaviors of the fabricated C/CNT and carbon-only resonators, including fundamental frequency, Q-factor, and frequency tuning characteristics show comparable performance to the silicon based resonators.

Fabrication of carbon nanomechanical resonators with embedded single walled carbon nanotube stiffening layers

This study presents a new fabrication method for high frequency nanomechanical resonators by utilizing carbon layers with embedded single walled carbon-nanotube layers. The carbon layers, fabricated by pyrolysis of photo- or electron beam resists, showed low-density and moderate Youngs modulus which is suitable to sensitive nanodevices. The carbon-nanotube layers underneath carbon layers enhanced conductivity and Youngs modulus while maintaining low-density of carbon nanoresonators. These two layers are fashioned into doubly clamped nanomechanical beams by electron beam lithography and carbon dry etching processes. Dynamic behaviors such as resonant frequency and Q-factor are investigated using magnetomotive detection method.

Electron temperature and pressure dependences of nonlinear phenomena in dust particle oscillation in dc plasmas

Experiments and calculations were performed to study the dependence of the nonlinear dust oscillatory motion on electron temperature and gas pressure in direct current plasmas. The frequency spectra of the dust particle oscillations induced by an excitation wire were measured in different discharge conditions. At higher electron temperature in accordance with the higher cathode voltage, the nonlinear phenomena of the oscillation spectra became more prominent. The amplitude of the subharmonic resonance peak was large and the frequency shift of the primary resonance peak was observed to be more significant in the case of high electron temperature. The force profile near the particle trap position was calculated in order to understand the dependence of the oscillation spectra on the electron temperature and the electron density. The electron temperature dependence of the particle oscillation was well explained from the calculated force profile. In addition, it was experimentally shown that the amplitudes of both the subharmonic resonance and the primary resonance became large as the pressure was decreased, which was consistent with the calculation.

Pyrolyzed carbon biosenosor for aptamer-protein interactions using electrochemical impedance spectroscopy

We present an electrochemical biosensor utilizing pyrolyzed carbon as a working electrode for the aptamer-based thrombin detection. The pyrolyzed carbon is fabricated by photolithography and thermal decomposition of photoresist in an N2 atmosphere. Physical and electrical properties of carbon thin film pyrolyzed with a positive photoresist, AZ9260, were studied for the biosensor application. Electron transfer resistance changes due to thrombin binding onto the carbon surface modified with thrombin aptamer were measured using electrochemical impedance spectroscopy techniques. Thrombin concentrations between 0.5 nM and 500 nM were detected by the electrochemical measurements.