Cheolgyu Kim

Control of Reversible Self-Bending Behavior in Responsive Janus Microstrips

Here, we demonstrate a simple method to systematically control the responsive self-bending behavior of Janus hydrogel microstrips consisting of a polymeric bilayer with a high modulus contrast. The Janus hydrogel microstrips could be easily fabricated by a simple micromolding technique combined with an initiated chemical vapor deposition (iCVD) coating, providing high flexibility in controlling the physical and chemical properties of the microstrips. The fabricated Janus hydrogel microstrip is composed of a soft, pH-responsive polymer hydrogel layer laminated with a highly cross-linked, rigid thin film, generating a geometric anisotropy at a micron scale. The large difference in the elastic moduli between the two layers of the Janus microstrips leads to a self-bending behavior in response to the pH change. More specifically, the impact of the physical and chemical properties of the microstrip on the self-bending phenomena was systematically investigated by changing the thickness and composition of two layers of the microstrip, which renders high controllability in bending of the microstrips. The curvature of the Janus microstrips, formed by self-bending, highly depends on the applied acidity. A reversible, responsive self-bending/unbending exhibits a perfect resilience pattern with repeated changes in pH for 5 cycles. We envision that the Janus microstrips can be engineered to form complex 3D microstructures applicable to various fields such as soft robotics, scaffolds, and drug delivery. The reliable responsive behaviors obtained from the systematic investigation will provide critical information in bridging the gap between the theoretical mechanical analysis and the chemical properties to achieve micron-scale soft robotics.

A quantitative strain analysis of a flexible single-crystalline silicon membrane

This study presents a quantitative strain analysis of a single-crystal Si membrane for high performance flexible devices. Advanced thinning and transfer methods were used to make flexible single-crystal Si devices. Two Si membrane strain gauges, each with a different stack, were fabricated on a polydimethylsiloxane/polyimide film using a silicon-on-insulator wafer. One gauge contains a 10-μm-thick handling Si layer, whereas the handling Si layer was completely removed for the other case. Although the Si membrane with the 10-μm-thick handling Si layer is flexible, the strain applied to the active Si layer (0.127%) is three times higher than the strain applied to the Si membrane without the handling Si layer (0.037%) at a bending radius of 5 mm. This leads to the more reliable electrical and mechanical performance of the device fabricated on the Si membrane without the handling Si layer. The experimental results were verified through a finite element method simulation and analytical modeling. The quantitati...

Flash-Induced Stretchable Cu Conductor via Multiscale-Interfacial Couplings

Abstract Herein, a novel stretchable Cu conductor with excellent conductivity and stretchability is reported via the flash‐induced multiscale tuning of Cu and an elastomer interface. Microscale randomly wrinkled Cu (amplitude of ≈5 µm and wavelength of ≈45 µm) is formed on a polymer substrate through a single pulse of a millisecond flash light, enabling the elongation of Cu to exceed 20% regardless of the stretching direction. The nanoscale interlocked interface between the Cu nanoparticles (NPs) and the elastomer increases the adhesion force of Cu, which contributes to a significant improvement of the Cu stability and stretchability under harsh yielding stress. Simultaneously, the flash‐induced photoreduction of CuO NPs and subsequent Cu NP welding lead to outstanding conductivity (≈37 kS cm−1) of the buckled elastic electrode. The 3D structure of randomly wrinkled Cu is modeled by finite element analysis simulations to show that the flash‐activated stretchable Cu conductors can endure strain over 20% in all directions. Finally, the wrinkled Cu is utilized for wireless near‐field communication on the skin of human wrist.

FEM simulation of warpage orientation change of FRP polymer substrate during thermal processing

Warpage of semiconductor packaging substrate during or after manufacturing processes has been a serious issue in the electronics industry. We found that the packaging substrate, which has initial warpage at room temperature, changes its warpage orientation by flipping from a concave to a convex cylindrical shape during the thermal processing. In this paper, we demonstrate that the rotation of warpage orientation is attributed to viscoelastic properties of fiber reinforced polymer (FRP) packaging substrate by using finite element method (FEM) simulations and experiments. Due to the viscoelastic properties in FRP substrate, the specimen shape was converted into a saddle shape during the warpage directional transition and changed its orientation. In conclusion, it is shown that the viscoelastic property of FRP substrate is a critical factor in analyzing warpage orientation change and its behavior.

Effect of anisotropic mechanical properties of woven composite substrates on warpage orientation of printed circuit boards

In this study, a mechanism of warpage orientation determination is investigated for thin package substrates. Understanding the mechanism is important because assembly process yield will significantly increase if the warpage orientation is controllable. We found that the copper clad laminate (CCL) itself already contains considerable residual stress from its thermo-compression fabrication, due to low coefficient of thermal expansion (CTE) of the woven glass/epoxy core substrate. It was interesting that most of the specimens bent in bias direction of the glass fabric. Concentrating on thermo-mechanical properties of the composite substrate, it was verified that the warpage orientation is sensitively determined by both anisotropic moduli and CTEs of the thin and compliant core substrate.

Mechanism of warpage orientation rotation due to viscoelastic polymer substrates during thermal processing

Abstract The warpage orientation, which refers to the direction of maximum and minimum curvatures in a cylindrical warpage, was observed to have changed by flipping from a concave to a convex shape during thermal processing. In this paper, the mechanism of the warpage orientation rotation is demonstrated through analyzing the stress state and curvatures of the specimens using finite element method (FEM) simulations and experiments. It is revealed that the warpage transition temperature, where the curvature changes to other shapes, corresponds to the stationary point of the stress-temperature curve and the curvature change of the minimum direction precedes the curvature change of the maximum direction during the warpage orientation rotation. This precedence results from the stress relaxation of the fiber reinforced polymer (FRP) substrate. Because the curvature of minimum direction flips backward in advance of maximum direction, the cylindrical warpage shape converts to a saddle shape and it induces the rotation of the warpage orientation. The simulation without the viscoelastic properties of the FRP substrate is conducted and used for comparison in order to verify the stress relaxation effect of the warpage orientation rotation phenomenon. In conclusion, it is demonstrated that the viscoelastic properties of the FRP substrate are a critical factor in analyzing the warpage orientation rotation and its behavior.

Nanotransplantation Printing of Crystallographic-Orientation-Controlled Single-Crystalline Nanowire Arrays on Diverse Surfaces

The fabrication of a highly ordered array of single-crystalline nanostructures prepared from solution-phase or vapor-phase synthesis methods is extremely challenging due to multiple difficulties of spatial arrangement and control of crystallographic orientation. Herein, we introduce a nanotransplantation printing (NTPP) technique for the reliable fabrication, transfer, and arrangement of single-crystalline Si nanowires (NWs) on diverse substrates. NTPP entails (1) formation of nanoscale etch mask patterns on conventional low-cost Si via nanotransfer printing, (2) two-step combinatorial plasma etching for defining Si NWs, and (3) detachment and transfer of the NWs onto various receiver substrates using an infiltration-type polymeric transfer medium and a solvent-assisted adhesion switching mechanism. Using this approach, high-quality, highly ordered Si NWs can be formed on almost any type of surface including flexible plastic substrates, biological surfaces, and deep-trench structures. Moreover, NTPP provides controllability of the crystallographic orientation of NWs, which is confirmed by the successful generation of (100)- and (110)-oriented Si NWs with different properties. The outstanding electrical properties of the NWs were confirmed by fabricating and characterizing Schottky junction field-effect transistors. Furthermore, exploiting the highly flexible nature of the NWs, a high-performance piezoresistive strain sensor, with a high gauge factor over 200 was realized.

Methodology development of warpage analysis of polymer based packaging substrate

Warpage of packaging substrate has been at issue due to thin and flexible substrate. It is occurred during manufacturing processes. Although warpage research based on thin metal film and silicon substrate was actively studied, it has difficulty about research of polymer based electronic due to its flexibility and low stiffness compared to metal and silicon substrate. We suggest a new methodology of warpage analysis to predict the warpage behavior of polymer composite substrate based bilayer specimen during temperature rising. The warpage analysis is performed in the sequence of scanning 3D surface, calculating curvature and built-in stress of film and verifying the result using FEM simulation. Two main factors of warpage behavior are built-in stress in film layer and stress induced by misfit of coefficient of thermal expansion between film and substrate. Built-in stress, arisen from built-in strain, is generated during film lay-up process such as electro-plating of copper, curing process of polymer. It is computed from the curvature at room temperature using the strain-curvature relation. Though this analysis method, the predicted curvature through temperature cycle showed good agreement with the experiment.