Juil Yoon

Delamination of stiff islands patterned on stretchable substrates

Abstract In one design of flexible electronics, thin-film islands of a stiff material are fabricated on a polymeric substrate, and functional materials are grown on these islands. When the substrate is stretched, the deformation is mainly accommodated by the substrate, and the islands and functional materials experience relatively small strains. Experiments have shown that, however, for a given amount of stretch, the islands exceeding a certain size may delaminate from the substrate. We calculate the energy release rate using a combination of finite element and complex variable methods. Our results show that the energy release rate diminishes as the island size or substrate stiffness decreases. Consequently, the critical island size is large when the substrate is compliant. We also obtain an analytical expression for the energy release rate of debonding islands from a very compliant substrate.

Channel cracks in a hermetic coating consisting of organic and inorganic layers

Flexible electronic devices often require hermetic coatings that can withstand applied strains. This letter calculates the critical strains for various configurations of channel cracks in a coating consisting of organic and inorganic layers. The authors show that the coating can sustain the largest strain when the organic layer is of some intermediate thicknesses.

Debonding and fracture of ceramic islands on polymer substrates

We perform in-situ uniaxial tensile tests on polyimide substrates with patterned ceramic islands. The islands fail by either channel cracking or debonding from the substrate, depending on island size and thickness. To understand why different failure modes occur, we have analyzed the fracture and debonding of stiff islands on deformable substrates. Using finite element simulations, we find that the maximum tensile strain in the islands increases with island size, but decreases with island thickness. The maximum energy release rate for island/substrate debonding, in contrast, increases with both island size and thickness. Assuming that the islands do not fracture if the maximum tensile strain in the islands is lower than a critical value and that no debonding occurs if the maximum energy release rate is smaller than the interfacial toughness, the critical substrate strains for island fracture and debonding can be calculated. If the islands are thick and small, the critical debonding strain is small, and th...

Method to analyze dislocation injection from sharp features in strained silicon structures

A microelectronic device usually contains sharp features (e.g., edges and corners) that may intensify stresses, inject dislocations into silicon, and fail the device. The authors describe a method to analyze dislocation injection on the basis of singular stress fields near the sharp features, and apply the method to interpret available experiments of nitride pads on silicon substrates.

Gelation dynamics of ionically crosslinked alginate gel with various cations

Alginates can be crosslinked with multivalent cations, leading eventually to hydrogel formation. The properties of alginate gel depend on its lock structure, monomeric composition, concentration of polymer and cross linker. Among these, the properties of ionically crosslinked alginate gel can be greatly affected by multivalent cations as cross-linker. Knowledge of gelation dynamics by multivalent cations allows control over gelation characteristics, such as modulus of gel and the time required for equilibrium state, and healing properties. We have studied gelation dynamics of ionically crosslinked alginate gel. According to different types of anions bound with cations, gelation time and equilibrium viscosity was changed due to the solubility kinetics of the cation. The equilibrium viscosity is increased as the size of the cations increased even though the cations have same valency. A theoretical model is introduced to interpret dynamic change of viscosity during gelation.

The effect of coating in increasing the critical size of islands on a compliant substrate

A flexible electronic system may consist of a polymeric substrate and an array of stiff islands, on which devices are fabricated. When the substrate is stretched, the devices on the islands experience small strains but the islands may debond if they exceed a critical size. The authors show that a thin layer of polymer coating, covering the islands and the substrate, can markedly increase the critical island size.

젖음성 차이와 무전해도금을 이용한 연성 구리 회로패턴 형성

Cu circuits were successfully fabricated on flexible PET(polyethylene terephthalate) substrates using wettability difference and electroless plating without an etching process. The wettability of Cu plating solution on PET was controlled by oxygen plasma treatment and SiOx-DLC(silicon oxide containing diamond like carbon) coating by HMDSO(hexamethyldisiloxane) plasma. With an increase of the height of the nanostructures on the PET surface with the oxygen plasma treatment time, the wettability difference between the hydrophilicity and hydrophobicity increased, which allowed the etchless formation of a Cu pattern with high peel strength by selective Cu plating. When the height of the nanostructure was more than 1400 nm (60 min oxygen plasma treatment), the reduction of the critical impalement pressure with the decreasing density of the nanostructure caused the precipitation of copper in the hydrophobic region.

A Study on the Fabrication of STS 316L Films by Ion Beam Deposition with Ion Source

The thin films of 316L stainless steel were made on glass and S45C substrate by Ion beam assisted deposition with reactive atmosphere of argon and nitrogen. The films were deposited at the various conditions of ion beam power and the ratios of Ar/gas. Properties of these films were analyzed by glancing x-ray diffraction method(GXRD), AES, potentiodynamic test, and salt spray test. The results of GXRD showed that austenite phase could be appeared by ion beam treatment and the amount of austenite phase increased with the amount of nitrogen gas. The films without plasma ion source treatment had the weak diffraction peak of ferrite phase. But under the Ar plasma ion beam treatment, the strong diffraction peaks of ferrite phase were appeared and the grain size was increased from 12 to 16 nm. Potentiodynamic polarization test and salt spray test indicated that the corrosion properties of the STS 316L films with nitrogen ion source treatment were better than bulk STS 316L steel and STS 316L films with Ar ion source treatment.