Kyung Wook Paik

Characterization of U-2 wt% Mo and U-10 wt% Mo alloy powders prepared by centrifugal atomization

The characteristics of high density UMo alloy powder solidified rapidly by the centrifugal atomization process have been examined. The results indicate that most of the atomized UMo alloy particles have a smooth surface and near-perfect spherical shape. The atomized powder, irrespective of particle size, is found to be single phase γ-U alloy with isotropic structure and non-dendritic grain. The continuous cooling DSC trace of U-2 wt% Mo alloy shows a small, broad endothermic peak originated from the formation of α-U phase and U2Mo phase, whereas that of U-10 wt% Mo alloy shows no peak over all temperature ranges associated with the decomposition of γ-U phase. The γ-U phase of U-2 wt% Mo powder is decomposed as the α-U phase and the U2Mo phase after an annealing treatment at 400°C for 100 h. But the γ-U phase of atomized U-10 wt% Mo powder remains as it was.

The multilayer-modified Stoney’s formula for laminated polymer composites on a silicon substrate

The thermomechanical behavior of multilayer structures is a subject of perennial interest. Stoney’s formula has long been one of the most important tools for understanding thermomechanical stress for single-layered structures like spin-coated polyimides or deposited metal thin film on substrates. In today’s microelectronics, however, as multilayer substrates have become widely available, the “modified version” of Stoney’s formula for multilayer applications is not only useful but necessary. While the majority of reports in the literature have focused on single-layer analysis, in this study, we examined an extended usage of Stoney’s formula for multilayer analysis. A simple model, the multilayer-modified Stoney’s formula, which predicts the stress contribution of each individual layer is proposed and verified through experiments and numerical analysis. Using various kinds of materials employed in a typical lamination-based multichip module technology, the thermomechanical behavior of the lamination-based m...

Thermal compatibility of centrifugally atomized U–Mo powders with aluminium in a dispersion fuel

Abstract The thermal compatibility of centrifugally atomized U–Mo alloys with aluminum has been studied. Samples of extruded dispersions of 24 vol.% spherical U–2 wt.% Mo and U–10 wt.% Mo powders in an aluminum matrix were annealed for over 2000 h at 400°C. No significant dimensional changes occurred in the U–10 wt.% Mo/aluminum dispersions. The U–2 wt.% Mo/aluminum dispersion, however, increased in volume by 26% after 2000 h at 400°C. This large volume change is mainly due to the formation of voids and cracks resulting from nearly complete interdiffusion of U–Mo and aluminum. Interdiffusion between U–10 wt.% Mo and aluminum was found to be minimal. The different diffusion behavior is primarily due to the fact that U–2 wt.% Mo decomposes from an as-atomized metastable γ-phase (bcc) solid solution into the equilibrium α-U and U2Mo two-phase structure during the experiment, whereas U–10 wt.% Mo retains the metastable γ-phase structure throughout the 2000 h anneal and thereby displays superior thermal compatibility with aluminum compared to U–2 wt.% Mo.

Comparison of electroplated eutectic Sn/Bi and Pb/Sn solder bumps on various UBM systems

The effect of a reflow process and under bump metallurgy (UBM) systems on the growth of intermetallic compounds for Sn/Bi and Pb/Sn solder bump/UBM interfaces was investigated. The UBM systems were sputtered Al/Ti/Cu, Al/Ti/electroless plated Cu, Al/NiV/sputtered Cu, Al/electroless plated Ni/Au, and the solder bumps were eutectic Pb/Sn and Bi/Sn fabricated by an electroplating method. Microstructure and composition of intermetallic compound (IMC) phases and their morphology were examined using scanning electron microscopy and X-ray diffraction. The IMC compounds were confirmed to be Cu/sub 6/Sn/sub 5/, Cu/sub 3/Sn, Ni/sub 3/Sn/sub 4/, or Ni/sub 3/Sn depending on UBM and solder types. The effect of IMC growth on bump adhesion strength was also investigated using a ball shear test. It turns out that ball shear strengths of the systems depends not on processing conditions but on solder/UBM interfacial adhesion strength.

Characteristics of U3Si and U3Si2 Powders Prepared by Centrifugal Atomization

The characteristics, that is, morphology, size distribution, alloy phase and microstructure of U3Si and U3Si2 powders, solidified rapidly by a centrifugal atomization, were investigated. The atomized powders consist of spherical particles with a relatively narrow size distribution independent of the alloy composition. The particle size distribution can be controlled by adjusting the atomization parameters such as feeding rates of the melt and revolution speeds of the disk. The major phases of atomized U3Si and U3Si2 powders are α-U and U3Si2 and U3Si2, respectively. The atomized U3Si powder has a dendritic structure of very fine and non-faceted U3Si2 precipitates with less fibric and eutectic U3Si2 structure. The microstructure of U3Si2 powder shows a cellular structure with fine U3Si2 grains and finely dispersed silicon-rich precipitates. The time for complete peritectoid reaction of the atomized U3Si particles and the resulting grain size are greatly reduced, due to the refinement of primary U3Si2 preci...

Rutherford Backscattering Spectrometry Studies on Copper and Chromium Diffusion in Polyimide

Diffusion of Cu in polyimide(PI) film was observed after Cu evaporation on PI at room temperature. Annealing treatment significantly enhanced the diffusion of Cu. The diffusion coefficient measured at 200 and 400 C using the Fickian erfc solution are 3 × 10 −14 and 1 × 10 −13 cm 2 /sec, respectively. Cu shows the Fickian diffusion behavior. In contrast to Cu, no clearly measurable diffusion had occurred for an as-deposited Cr specimen and a specimen annealed at 400 C for 1 hour. However, for longer annealing times a slight amount of Cr diffused into PI because of the growth of the intermixed region. The diffusion coefficients of Cr at 200 and 400 C for less than 20 hours are 2 × 10 −15 and 7 × 10 −15 cm 2 /sec, respectively. The diffusion of Cr in PI shows non-Fickian behavior.

Effects of the polymer residues on via contact resistance after reactive ion etching

The effects of CF4 addition to O2 reactive ion etching (RIE) of a polyetherimide (Ultem®) surface and resultant via contact resistance were investigated using scanning electron microscopy (SEM), x-ray electron spectroscopy (XPS), and cross bridge Kelvin resistor (CBKR). In pure O2 RIE, the SEM micrograph showed that the originally smooth Ultem surface became rough and the rough features coarsened with increasing RIE time, resulting in residual particles on the metal pads. However, in the case of O2+CF4 RIE, the Ultem surface remained smooth throughout the etching process, and residue free metal pads were obtained. The XPS experiments identified that the origin of the Ultem surface roughening was organo-Si compound, which is easily converted to silicon oxide during O2 RIE. In contrast, the atomic fluorine in O2+CF4 RIE process etched out the silicon and the silicon oxide, resulting in smooth film surface and residue free metal pads. The metal to metal via contact resistance was measured by CBKR, and the pa...

The finite element analysis of internal stresses during sequential build-up of lamination-based thick-film multilayer substrates

The thermomechanical behavior of multilayer structures is a subject of perennial interest. Among various multilayer structures, we provided a thorough investigation on the lamination-based thick-film multilayer substrates, or the multichip module substrates. Firstly, we offered an in-depth look at the composite bean analysis (CBA) recently suggested for thick-film multilayer structures. The calculated substrate bow values using the CBA model were compared with the measured values and calculated values from numerical analysis. The magnitude and distribution of biaxial, shear, and peel stresses in thick-film multilayer substrates during sequential build-up of thick-film polymer dielectrics were calculated and discussed. Secondly, for users’ convenience, the most widely-used analytical methods for microelectronics applications were summarized along with the CBA model, and their possible applications were suggested.

Studies on the high-temperature superconductor (HTS)/metal/polymer dielectric interconnect structure for packaging applications

A HTS/metal/low dielectric constant polymer dielectric hybrid interconnect structure was fabricated using high density interconnect (HDI) copper/polyimide processing techniques. Nondegraded superconducting properties, J/sub c/ of over 1 MAcm/sup -2/ and T/sub c/ of 88 K, were obtained using optimum processing conditions. A 0.6 /spl mu/m-thick YBCO film was coevaporated on LaAlO/sub 3/ substrate, annealed, patterned, and Au contact pads were deposited. A Kapton polyimide film was laminated on HTS parts in an oxygen environment. Using a siloxane polyimide (SPI) base adhesive at 170-210/spl deg/C or a polyester base adhesive at 150/spl deg/C vias were drilled on the polymer layer using a 351 nm Ar laser followed by subsequent via cleaning with O/sub 2/+CF/sub 4/ plasma. To complete test parts Ti/Cu/Ti metallization and patterning with a laser adaptive lithography were followed. It was found that the laminated polymer film on the HTS protected HTS film properties from heating and water exposure. The polyimide laminated HTS film maintains its superconductivity up to 210/spl deg/C of lamination temperature. The hybrid structure also showed excellent reliability performances: 6% J/sub c/ decrease at 150/spl deg/C/65 h thermal bake, 12% J/sub c/ decrease after 100 thermal cycles from -96 to 150/spl deg/C, and maintain superconductivity in water immersion up to more than 10 h. T/sub c/ of samples remained always the same 88 K. Coplanar HTS lines, microstrip HTS/metal lines, and HTS power strip line structures were demonstrated on a 2-in-diameter HTS wafer coupon. >

Studies on the Surface Modification of Benzocyclobutene(BCB) Film By Plasma Ions

The etch rates of BCB film in a reactive ion etcher(RIE) were measured using Ar, O 2 , O 2 +CF 4 , and O 2 +SF 6 gas mixtures. Faster etch rates were obtained when CF 4 and SF 6 were added to oxygen, since the presence of atomic fluorine enhances the etch rate of organics, while also etching Si and SiO 2 formed by exposure to oxygen gas. Surface compositional changes on the BCB film were observed by XPS after plasma modification. Pure O 2 and O 2 +CF 4 plasma oxidized the carbo-siloxane linkage (C-Si-O) of the BCB, resulting in the formation of SiO 2 on the surface. The O 2 +SF 6 plasma, however, did not produce the surface SiO 2 , because of its faster Si and SiO 2 etch rates. Ar ion sputtering following the plasma modification, restored the surface chemical composition to a state similar to the initial BCB surface.