Seungmin Cho

Fast Synthesis of High-Performance Graphene Films by Hydrogen-Free Rapid Thermal Chemical Vapor Deposition

The practical use of graphene in consumer electronics has not been demonstrated since the size, uniformity, and reliability problems are yet to be solved to satisfy industrial standards. Here we report mass-produced graphene films synthesized by hydrogen-free rapid thermal chemical vapor deposition (RT-CVD), roll-to-roll etching, and transfer methods, which enabled faster and larger production of homogeneous graphene films over 400 × 300 mm(2) area with a sheet resistance of 249 ± 17 Ω/sq without additional doping. The properties of RT-CVD graphene have been carefully characterized by high-resolution transmission electron microscopy, Raman spectroscopy, chemical grain boundary analysis, and various electrical device measurements, showing excellent uniformity and stability. In particular, we found no significant correlation between graphene domain sizes and electrical conductivity, unlike previous theoretical expectations for nanoscale graphene domains. Finally, the actual application of the RT-CVD films to capacitive multitouch devices installed in the most sophisticated mobile phone was demonstrated.

Wetting‐Transparent Graphene Films for Hydrophobic Water‐Harvesting Surfaces

Wetting-transparent graphene films grown in situ by chemical vapor deposition on hydrophobic (roughened) copper surfaces offer excellent resistance to copper corrosion while maintaining the intrinsic hydrophobicity of the surface, enabling superior performance for water-harvesting applications.

Issues in Assembly Process of Next-Generation Fine-Pitch Chip-On-Flex Packages for LCD Applications

Some of the current assembly issues of fine-pitch chip-on-flex (COF) packages for LCD applications are reviewed. Traditional underfill material, anisotropic conductive adhesive (ACA), and nonconductive adhesive (NCA) are considered in conjunction with two applicable bonding methods including thermal and laser bonding. Advantages and disadvantages of each material/process combination are identified. Their applicability is further investigated to identify a process most suitable to the next-generation fine-pitch packages (less than 35 mum). Numerical results and subsequent testing results indicate that the NCA/laser bonding process is advantageous for preventing both lead crack and excessive misalignment compared to the conventional bonding process

Temperature Dependent Deformation Analysis of Ceramic Ball Grid Array Package Assembly Under Accelerated Thermal Cycling Condition

A robust scheme of moireinterferometry for real-time observation is employed to study the temperature dependent thermo-mechanical behavior of a ceramic ball grid array package assembly. The scheme is implemented with a convection-type environmental chamber that provides the rapid temperature control required in accelerated thermal cycling. Thermal deformations are documented at various temperatures. Thermal-history dependent analyses of global and local deformations are presented. A significant nonlin- ear global behavior is documented due to complete stress relaxation at the maximum temperature. An analysis of solder interconnections reveals that inelastic deformation accumulates at the bottom eutectic solder fillet only at high temperatures. @DOI: 10.1115/1.1646426#

Healing Graphene Defects Using Selective Electrochemical Deposition: Toward Flexible and Stretchable Devices

Graphene produced by chemical-vapor-deposition inevitably has defects such as grain boundaries, pinholes, wrinkles, and cracks, which are the most significant obstacles for the realization of superior properties of pristine graphene. Despite efforts to reduce these defects during synthesis, significant damages are further induced during integration and operation of flexible and stretchable applications. Therefore, defect healing is required in order to recover the ideal properties of graphene. Here, the electrical and mechanical properties of graphene are healed on the basis of selective electrochemical deposition on graphene defects. By exploiting the high current density on the defects during the electrodeposition, metal ions such as silver and gold can be selectively reduced. The process is universally applicable to conductive and insulating substrates because graphene can serve as a conducting channel of electrons. The physically filled metal on the defects improves the electrical conductivity and mechanical stretchability by means of reducing contact resistance and crack density. The healing of graphene defects is enabled by the solution-based room temperature electrodeposition process, which broadens the use of graphene as an engineering material.

Optical Effects of Graphene Electrodes on Organic Light-Emitting Diodes

We performed optical simulations and experiments to investigate the internal optics of graphene anode organic light-emitting diodes (OLEDs). The efficiencies, emission distribution, and spectral characteristics of four-layered graphene anode OLEDs were extracted and compared to those of ITO anode OLEDs. Unlike the case of the ITO anode OLED, the efficiencies and emission distributions of graphene anode OLEDs showed a weak dependency on the thickness of the organic layers. Furthermore, marginal changes in the emission spectra were observed. These results were ascribed to the negligible presence of a microcavity effect in the graphene anode OLED.

Graphene-coated meshes for electroactive flow control devices utilizing two antagonistic functions of repellency and permeability.

The wettability of graphene on various substrates has been intensively investigated for practical applications including surgical and medical tools, textiles, water harvesting, self-cleaning, oil spill removal and microfluidic devices. However, most previous studies have been limited to investigating the intrinsic and passive wettability of graphene and graphene hybrid composites. Here, we report the electrowetting of graphene-coated metal meshes for use as electroactive flow control devices, utilizing two antagonistic functions, hydrophobic repellency versus liquid permeability. Graphene coating was able to prevent the thermal oxidation and corrosion problems that plague unprotected metal meshes, while also maintaining its hydrophobicity. The shapes of liquid droplets and the degree of water penetration through the graphene-coated meshes were controlled by electrical stimuli based on the functional control of hydrophobic repellency and liquid permeability. Furthermore, using the graphene-coated metal meshes, we developed two active flow devices demonstrating the dynamic locomotion of water droplets and electroactive flow switching.

Development of predictive modeling scheme for flip-chip on fine pitch flex substrate

A verified/predictive finite element modeling scheme for flip-chip on a fine pitch flex substrate is presented. The scheme is based on the conventional local/global modeling approach to handle complicated three dimensional structures with fine features of interest, but a new formulation is introduced to be able to evaluate time-dependent non-linear stresses induced by hygroscopic as well as thermal expansion mismatches; the proposed model allows design assessment of the packages subjected to a combined temperature and moisture environment. The properties of critical materials are first determined experimentally and their performance in the model is subsequently verified by an interferometric displacement measurement technique. By combining numerical modeling and experimental verification until the results merge, the numerical model becomes more accurate and dependable. Then, the model is applied extensively to optimize the package designs for enhanced reliability.

Non-linear finite element analysis for electronic packages subjected to combined hygroscopic and thermo-mechanical stresses

A finite element modeling scheme to analyze the combined effect of thermal and hygroscopic deformation is presented. The scheme employs the conventional moisture diffusion and heat transfer analysis capabilities available in the commercial FEM packages, but a new formulation is introduced to allow the time-dependent non-linear analysis of package deformations induced by hygroscopic as well as thermal expansion mismatches. Validity of modeling scheme is verified using time-dependent displacement fields of a bi-material specimen documented by real-time moire interferometry

Hydrazine Doped Graphene and Its Stability

The electronic property of graphene was investigated by hydrazine treatment. Hydrazine (N₂H₄) highly increases electron concentrations and up-shifts Fermi level of graphene based on significant shift of Dirac point to the negative gate voltage. We have observed contact resistance and channel length dependent mobility of graphene in the back-gated device after hydrazine monohydrate treatment and continuously monitored electrical characteristics under Nitrogen or air exposure. The contact resistance increases with hydrazine-treated and subsequent Nitrogen-exposed devices and reduces down in successive Air-exposed device to the similar level of pristine one. The channel conductance curve as a function of gate voltage in hole conduction regime keeps analogous value and shape even after Nitrogen/Air exposure specially whereas, in electron conduction regime change rate of conductance along with the level of conductance with gate voltage are decreased. Hydrazine could be utilized as the highly effective donor without degradation of mobility but the stability issue to be solved for future application.