• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.132-132
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• 2008

Silicon carbide (SiC) is an attractive material for high-power, high-temperature, and high-frequency applications. So far, atomic force microscopy (AFM) has been extensively used to study the surface charges, dielectric constants and electrical potential distribution as well as topography in silicon-based device structures, whereas it has rarely been applied to SiC-based structures. In this work, the surface potential and topography distributions SiC with different doping levels were measured at a nanometer-scale resolution using a scanning kelvin probe force microscopy (SKPM) with a non-contact mode AFM. The measured results were calibrated using a Pt-coated tip and a metal defined electrical contacts of Au onto SiC. It is assumed that the atomically resolved surface potential difference does not originate from the intrinsic work function of the materials but reflects the local electron density on the surface. It was found that the work function of the Au deposited on SiC surface was higher than that of original SiC surface. The dependence of the surface potential on the doping levels in SiC, as well as the variation of surface potential with respect to the schottky barrier height has been investigated. The results confirm the concept of the work function and the barrier heights of metal/SiC structures.

• Journal of Electrical Engineering and Technology
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• v.5 no.4
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• pp.640-645
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• 2010

This paper describes the power generating property of hydrothermally grown ZnO nanorods on a flexible polyethersulfone (PES) substrate. The piezoelectric currents generated by the ZnO nanorods were measured when bending the ZnO nanorod by using I-AFM, and the measured piezoelectric currents ranged from 60 to 100 pA. When the PtIr coated tip bends a ZnO nanorod, piezoelectrical asymmetric potential is created on the nanorod surface. The Schottky barrier at the ZnO-metal interface accumulates elecntrons and then release very quickly generating the currents when the tip moves from tensile to compressed part of ZnO nanorod. These ZnO nanorods were grown almost vertically with the length of 300-500 nm and the diameter of 30-60 nm on the Ag/Ti/PES substrate at $90^{\circ}C$ for 6 hours by hydrothermal method. The metal-semiconductor interface property was evaluated by using a HP 4145B Semiconductor Parameter Analyzer and the piezoelectric effect of the ZnO nanorods were evaluated by using an I-AFM. From the measured I-V characteristics, it was observed that ZnO-Ag and ZnO-Au metal-semiconductor interfaces showed an ohmic and a Schottky contact characteristics, respectively. ANSYS finite element simulation was performed in order to understand the power generation mechanism of the ZnO nanorods under applied external stress theoretically.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.110-110
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• 2011

The nanomechanical properties of fully lithiated and unlithiated silicon nanowire deposited on silicon substrate have been studied with atomic force microscopy. Silicon nanowires were synthesized using the vapor-liquid-solid process on stainless steel substrates using Au catalyst. Fully lithiated silicon nanowires were obtained by using the electrochemical method, followed by drop-casting on the silicon substrate. The roughness, derived from a line profile of the surface measured in contact mode atomic force microscopy, has a smaller value for lithiated silicon nanowire and a higher value for unlithiated silicon nanowire. Force spectroscopy was utilitzed to study the influence of lithiation on the tip-surface adhesion force. Lithiated silicon nanowire revealed a smaller value than that of the Si nanowire substrate by a factor of two, while the adhesion force of the silicon nanowire is similar to that of the silicon substrate. The Young's modulus obtained from the force-distance curve, also shows that the unlithiated silicon nanowire has a relatively higher value than lithiated silicon nanowire due to the elastically soft amorphous structures. The frictional forces acting on the tip sliding on the surface of lithiated and unlithiated silicon nanowire were obtained within the range of 0.5-4.0 Hz and 0.01-200 nN for velocity and load dependency, respectively. We explain the trend of adhesion and modulus in light of the materials properties of silicon and lithiated silicon. The results suggest a useful method for chemical identification of the lithiated region during the charging and discharging process.

• International Journal of Naval Architecture and Ocean Engineering
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• v.2 no.4
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• pp.185-194
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• 2010

A pumpjet propulsor (PJP) was designed for an underwater body (UWB) with axi-symmetric configuration. Its performance was predicted through CFD study and models were manufactured. The propulsor design was evaluated for its propulsion characteristics through model tests conducted in a Wind Tunnel (WT). In the concluding part of the study, evaluation of the cavitation performance of the pumpjet was undertaken in a cavitation tunnel (CT). In order to assess the cavitation free operation speeds and depths of the body, cavitation tests of the PJP were carried out in behind condition to determine the inception cavitation numbers for rotor, stator and cowl. The model test results obtained were corrected for full scale Reynolds number and subsequently analyzed for cavitation inception speeds at different operating depths. From model tests it was also found that the cavitation inception of the rotor takes place on the tip face side at higher advance ratios and cavitation shifts towards the suction side as the RPS increases whereas the stator and cowl are free from cavitation.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.63-68
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• 2005

The nanoindentation technique is widely used to investigate the mechanical properties of nano-microscale materials. The nanoindentation method for assessing mechanical properties at low loads and shallow depths is already well established fur the characterization of thin films as well as bulk materials. In this study, we evaluated residual stress in DLC and Au thin films usign nanoindentation technique with a new stress-relaxation model. Moreover, We suggest a composite hardness equation and quantify the magnitude of hardness increase by using an equation based on the interface hardness and the interface thickness, derived by comparing results derived from this equation and those determined in nanoindentation tests. Finally, We present an indentation size effect (ISE) model that extends the available contact depth for ISE application down to several tens of nanometers by considering the tip bluntness effect.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.24 no.4
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• pp.145-150
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• 2014

GaN nanorods were grown on the apex of GaN stripes by three dimensional selective growth method. $SiO_2$ mask was partially removed only on the apex area of the GaN stripes by an optimized photolithography for the selective growth. Metallic Au was deposited only on the apex of the GaN stripes and a selective growth of GaN nanorods was followed by a metal organic vapor phase epitaxy (MOVPE). We confirmed that the shape and size of the GaN nanorods depend on growth temperature and flow rates of group III precursor. GaN nanorods were grown having a taper shape which have sharp tip and triangle-shaped cross section. From the TEM result, we confirmed that threading dislocations were rarely observed in GaN nanorods because of the very small contact area for the selective growth. Stacking faults which might be originated from a difference of the crystal facet directions between the GaN stripe and the GaN nanorods were observed in the center area of the GaN nanorods.

• Clean Technology
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• v.11 no.2
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• pp.69-74
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• 2005

A metal ion detector with a submicron size electrode was fabricated by field-induced AFM oxidation. The square frame of the mesa pattern was functionalized by APTES for the metal ion detection, and the remaining portion was used as an electrode by the self-assembly of MPTMS for Au metal deposition. The conductance changed with the quantity of adsorbed copper ions, due to electron tunneling between the mobile and surface electrodes. The smaller electrode has a lower limit of detection due to the enhancement in electron tunneling through metal ions that are adsorbed between the conductive-tip (mobile) and the surface (fixed) electrode. This two-electrode system immobilized with different functional groups was successfully used in the selective adsorption and detection of target materials.

• Journal of the Microelectronics and Packaging Society
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• v.20 no.4
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• pp.7-11
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• 2013

As thermosonic ball bonding is developed for more and more advanced applications in the electronic packaging industry, the control of process stresses induced on the integrated circuits becomes more important. If Cu bonding wire is used instead of Au wire, larger ultrasonic levels are common during bonding. For advanced microchips the use of Cu based wire is risky because the ultrasonic stresses can cause chip damage. This risk needs to be managed by e.g. the use of ultrasound during the impact stage of the ball on the pad ("pre-bleed") as it can reduce the strain hardening effect, which leads to a softer deformed ball that can be bonded with less ultrasound. To find the best profiles of ultrasound during impact, a numerical model is reported for ultrasonic bonding with capillary dynamics combined with a geometrical model describing ball deformation based on volume conservation and stress balance. This leads to an efficient procedure of ball bond modelling bypassing plasticity and contact pairs. The ultrasonic force and average stress at the bond zone are extracted from the numerical experiments for a $50{\mu}m$ diameter free air ball deformed by a capillary with a hole diameter of $35{\mu}m$ at the tip, a chamfer diameter of $51{\mu}m$, a chamfer angle of $90^{\circ}$, and a face angle of $1^{\circ}$. An upper limit of the ultrasonic amplitude during impact is derived below which the ultrasonic shear stress at the interface is not higher than 120 MPa, which can be recommended for low stress bonding.

• Korean Journal of Metals and Materials
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• v.49 no.5
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• pp.411-418
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• 2011

The current study investigates the electroplating characteristics of Sn-Cu eutectic micro-bumps electroplated on a Si chip for flip chip application. Under bump metallization (UBM) layers consisting of Cr, Cu, Ni and Au sequentially from bottom to top with the aim of achieving Sn-Cu bumps $10\times10\times6$ ${\mu}m$ in size, with 20${\mu}m$ pitch. In order to determine optimal plating parameters, the polarization curve, current density and plating time were analyzed. Experimental results showed the equilibrium potential from the Sn-Cu polarization curve is -0.465 V, which is attained when Sn-Cu electro-deposition occurred. The thickness of the electroplated bumps increased with rising current density and plating time up to 20 mA/$cm^2$ and 30 min respectively. The near eutectic composition of the Sn-0.72wt%Cu bump was obtained by plating at 10 mA/$cm^2$ for 20 min, and the bump size at these conditions was $10\times10\times6$ ${\mu}m$. The shear strength of the eutectic Sn-Cu bump was 9.0 gf when the shearing tip height was 50% of the bump height.

• Journal of the Microelectronics and Packaging Society
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• v.16 no.2
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• pp.33-42
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• 2009

A polymer-based flexible neural probe was fabricated for monitoring of neural activities from a brain. To improve the insertion stiffness, a 5 ${\mu}m$ thick biocompatible Au layer was electroplated between the top and bottom polymer layers. The developed neural probe penetrated a gel whose elastic modulus is similar to that of a live brain tissue without any fracture, To minimize mechanical residual stress and bending from the probe, two new methods were employed: (1) use of a thermal annealing process after completing the device and (2) incorporation of multiple different layers to compensate the residual stress between top and bottom layers. Mechanical bending around the probe tip was clearly removed after employing the two processes. In electrical test, the developed probe showed a proper impedance value to record neural signals from a brain and the result remained the same for 72 hours. In simple in-vitro probe characterization, the probe showed a great removal of residual stress and an excellent recording performance. The in-vitro recording results did not change even after 1 week, suggesting that this electrode has the potential for great recording from neuron firing and long-term implant performance.