Jihperng Leu

Fracture of metal-polymer line structures. I. Semiflexible polyimide

The fracture characteristics of metal/polymer line structures formed by depositing Au/Cr lines on a semiflexible polyimide, pyromellitic dianhydride‐oxydianiline (PMDA‐ODA), substrate have been investigated using a stretch deformation technique. The delamination behavior, fracture morphology, fracture energy, and energy dissipation rate have been determined as a function of line width and thickness. The metal dimension was found to influence the crack formation mode and morphology. The experimental studies were supplemented by finite‐element analysis to evaluate the stress distribution and deformation energetics of the line structure, which takes into account the plastic deformation of the metal and the polymer. Results from this analysis show that the observed fracture characteristics can be attributed to the edge and thickness effects induced by metal confinement. Essentially, the deformation behavior is determined by the mechanical environment induced by metal confinement at the interface. Plastic deformation of both metal and polymer plays an important role in controlling the stress distributions as well as the deformation energetics. The fracture energy of the metal‐polyimide interface determined by an overall energy balance method was consistent with that obtained from energy dissipation rate. The average value is 25 J/m2 for the Au/Cr/PMDA‐ODA line structure.

Isothermal stress relaxation in electroplated Cu films. I. Mass transport measurements

Recent studies on Cu interconnects have shown that interface diffusion between Cu and the cap layer dominates mass transport for electromigration. The kinetics of mass transport by interface diffusion strongly depends on the material and processing of the cap layer. In this series of two papers, we report in Part I the interface and grain-boundary mass transport measured from isothermal stress relaxation in electroplated Cu thin films with and without a passivation layer and in Part II a kinetic model developed to analyze the stress relaxation based on the coupling of grain boundary and interface diffusion. We show that a set of isothermal stress relaxation experiments together with appropriate modeling analysis can be used to evaluate the kinetics of interface and grain-boundary diffusion that correlate to electromigration reliability of Cu interconnects. Thermal stresses in electroplated Cu films with and without passivation, subjected to thermal cycling and isothermal annealing at selected temperatures...

Fabrication and CO Sensing Properties of Mesostructured ZnO Gas Sensors

This study presents the synthesis of mesostructured ZnO using a template replication method and the fabrication of a carbon monoxide sensor using the synthesized ZnO using the dielectrophoresis process. The mesoporous carbon, CMK-3, was employed for the template and zinc nitrite was used as the precursor for synthesizing the ordered porous ZnO. The mesostructured ZnO was analyzed using X-ray diffraction XRD patterns, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and N2 adsorption–desorption isotherms. The XRD patterns indicated that the ZnO exhibited a highly ordered structure after the template was removed. The morphology of the ZnO was randomly oriented and the structure was polycrystalline. The surface area, pore volume, and pore size of the porous ZnO were 61.3 m2 g−1, 0.31 cm3 g−1, and 5.3 nm, respectively. When CO gas was injected, the optimum sensitivities at 250°C were 12.1, 14.6, 18.4, 26.0, and 60% when the CO concentrations were 10, 20, 30, 50, and 70 ppm, respectively.

Label free sub-picomole level DNA detection with Ag nanoparticle decorated Au nanotip arrays as surface enhanced Raman spectroscopy platform.

Label free optical sensing of adenine and thymine oligonucleotides has been achieved at the sub-picomole level using self assembled silver nanoparticles (AgNPs) decorated gold nanotip (AuNT) arrays. The platform consisting of the AuNTs not only aids in efficient bio-immobilization, but also packs AgNPs in a three dimensional high surface area workspace, assisting in surface enhanced Raman scattering (SERS). The use of sub-10 nm AgNPs with optimum inter-particle distance ensures amplification of the chemically specific Raman signals of the adsorbed adenine, thymine, cytosine and guanine molecules in SERS experiments. High temporal stability of the Raman signals ensured reliable and repeatable DNA detection even after three weeks of ambient desk-top conservation. This facile architecture, being three dimensional and non-lithographic, differs from conventional SERS platforms.

Molecular structure and thermal/mechanical properties of polymer thin films

Abstract The molecular structure and chain packing morphology are shown to be the fundamental factors in determining the thermal-mechanical properties of polymer thin films. This is demonstrated for two polyimides, PMDA-ODA and BPDA-PDA, which have contrasting molecular structures with semi-flexible and rigid rod-like chain morphologies. We first examine the effect of molecular structure on the chain morphologies, then the influence of chain morphology on the materials properties. By examining the degree of anisotropy in materials properties between the lateral and vertical directions with respect to the film plane, a clear correlation between molecular structure and properties is observed. This is further manifested in the fracture behavior of metal-polyimide fine line structures, where the fracture characteristics are found to be largely controlled by the mechanical properties of the polyimide. A strong polyimide, BPDA-PDA, is found to have a fracture toughness about threefold higher than a soft polyimide, PMDA-ODA.

Numerical simulations and experimental measurements of stress relaxation by interface diffusion in a patterned copper interconnect structure

We describe a series of experiments and numerical simulations that were designed to determine the rate of stress-driven diffusion along interfaces in a damascene copper interconnect structure. Wafer curvature experiments were used to measure the rate of stress relaxation in an array of parallel damascene copper lines, which were encapsulated in a dielectric, and passivated with an overlayer of silicon nitride or silicon carbide. The stress relaxation was found to depend strongly on the choice of passivation. Three-dimensional finite element simulations were used to model the experiments, and showed that this behavior is caused by changes in the diffusivity of the interface between the copper lines and the passivation. By fitting the predicted stress relaxation rates to experimental measurements, we have identified the interfaces that contribute to stress relaxation in the structure, and have estimated values for their diffusion coefficients.

Fabrication of Mesostructured Cobalt Oxide Sensor and Its Application for CO Detector

The paper presents a method for mesostructured cobalt oxide sensor fabrication by the template replication method and immobilization using dielectrophoresis process. The method consists of enriching the mesostructured cobalt oxide particles in a microelectrode gap that drove mesostructured cobalt oxide particles to a high electric field region and then exposed to CO gas. The cobalt oxide sensor has improved sensing performance in carbon monoxide (CO) detection at concentration as low as 10 ppm and a very short recovering time, due to the higher surface area.

Fourier transform infrared studies of polyimide and poly(methyl‐methacrylate) surfaces during downstream microwave plasma etching

The change in chemical structure of the top surface layer of thin poly(methylmethacrylate) (PMMA) and polyimide (PI) films (200–1500 A) during downstream microwave NF3/CF4/O2 plasma etching are investigated by in situ Fourier transform infrared (FTIR) reflection–absorption spectroscopy. Plasma fluorination of PI by either NF3 or CF4 leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CFx), acyl fluoride, benzoyl fluoride, acyl hypofluorite, and polyfluorinated benzene. The surface fluorination process is found to be controlled by diffusion in the product layer and the depth of fluorination is estimated from infrared absorbance to be approximately 500 A. Addition of oxygen leads to a reduction in the fluoride species and the development of broad absorption bands representing oxygenated surface species. The FTIR data are supplemented by ex situ x‐ray photoelectron spectroscopy observations and mechanisms for the observed modifications of the polymer surf...

Chemistry and morphology of Cr/BPDA‐PDA interface formation

X‐ray photoelectron spectroscopy has been used to investigate the interaction between Cr and BPDA‐PDA polyimide for Cr coverages up to 18 A. Detailed core level analysis reveals that the spectral changes induced by Cr deposition are a consequence of both the chemistry and the redistribution of reaction products in the overlayer. Two distinct N bonding configurations are readily resolved; one formed at lowest coverage remains localized at the buried interface, whereas the second becomes intermixed in the overlayer. Uniform overlayer growth is observed above ∼3.5 A. The data for low Cr coverages are compared to molecular orbital calculations for two model bonding configurations. Agreement between theory and experiment for the C 1s core level is better for a model compound with an O atom removed from the polymer backbone than for one where Cr is bound to a benzene ring. However, the quantitative differences suggest that a model should be considered where Cr interacts with carbonyl groups and additional reaction sites.

Superior local conductivity in self-organized nanodots on indium-tin-oxide films induced by femtosecond laser pulses

Large-area surface ripple structures of indium-tin-oxide films, composed of self-organized nanodots, were induced by femtosecond laser pulses, without scanning. The multi-periodic spacing (~800 nm, ~400 nm and ~200 nm) was observed in the laser-induced ripple of ITO films. The local conductivity of ITO films is significantly higher, by approximately 30 times, than that of the as-deposited ITO films, due to the formation of these nanodots. Such a significant change can be ascribed to the formation of indium metal-like clusters, which appear as budges of ~5 nm height, due to an effective volume increase after breaking the In-O to form In-In bonding.