Materials Research (10)
Physics And Astronomy (10)
MRS Online Proceedings Library Archive (8)
Journal of Materials Research (2)
Materials Research Society (10)
Oxidation and reduction behavior of pure indium
Harry Schoeller, Junghyun Cho
Journal: Journal of Materials Research / Volume 24 / Issue 2 / February 2009
Published online by Cambridge University Press: 31 January 2011, pp. 386-393
Print publication: February 2009
Fundamental knowledge on the oxidation behavior of pure indium, commonly used as a low-temperature, fluxless soldering material in micro-electro-mechanical system (MEMS) devices, is of importance as it influences the solder joint reliability. A thermodynamic model of the oxidation and reduction behavior of indium is developed by constructing an Ellingham diagram, and by using H2(g) reactions. Partial pressure (p) of H2O was shown to be the critical parameter in creating a reducing environment in the applicable solder reflow temperature range. Verification of the thermodynamic models was then carried out through heating and melting of indium in controlled glove box environments by adjusting p(H2)/p(H2O). The nanometer scale thickness of the oxide layer grown on indium was measured by a spectroscopic ellipsometer. The growth mechanism for oxidation in air below 220 °C follows Uhlig's logarithmic law where electron transport is the rate-controlling mechanism, implying that there is an incubation period for the onset of initial oxidation. Its activation energy was found to be 0.65 eV.
Thermodynamics and Kinetics of Oxidation of Pure Indium Solders
Harry Schoeller, Jongman Kim, Seungbae Park, Junghyun Cho
Journal: MRS Online Proceedings Library Archive / Volume 968 / 2006
Published online by Cambridge University Press: 26 February 2011, 0968-V03-07
Print publication: 2006
MicroElectroMechanical System (MEMS) devices often require low-temperature, fluxless soldering techniques due to their high temperature sensitivity and performance requirements of the components. While seeking the development of a soldering technology using pure indium, the major focus of this study is to assess the thermodynamics and kinetics of indium oxidation at various solder reflow environments that will ultimately provide a processing window for solder reflow and surface oxide cleaning. With a glove box employed to generate reducing environments, oxygen, moisture, and hydrogen contents were varied to examine their effects on oxidation and reduction behavior of indium. We also explored oxidation mechanisms at different regimes of temperature and time. In particular, electron transport from indium to indium oxide is shown to be the rate controlling mechanism under specific oxidizing conditions. For accurate thickness measurements, a spectroscopic ellipsometer was employed. In addition, the effect of indium oxidation on solder joint reliability was observed via wetting angle and interfacial shear strength measurements.
Structural Evolution and Mechanical Behavior of Bio-inspired Oxide Films on Self-Assembled Organic Layers
Guangneng Zhang, Junghyun Cho
Published online by Cambridge University Press: 26 February 2011, 0975-DD10-02
A bio-inspired approach is employed to deposit the oxide films on the substrates coated with self-assembled organic layers. Particularly, titania and zirconia films are grown in aqueous precursor solutions at near room temperatures. This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. Multiscale structural evolution resulting from initial bulk nucleation, nanoparticle aggregation, and ultimate film formation are systematically studied by adjusting the precursor solution conditions. Dynamic light scattering (DLS) is utilized to characterize initial nanoparticles and their associated aggregates/clusters formed in situ in solution. Corresponding nano- and microstructure developments of the oxide films are investigated through high-resolution transmission electron microscope (TEM) and scanning electron microscope (SEM). In addition, mechanical performance is evaluated with the aid of a dynamic nanoindentation testing to establish the structure-property relationships of the bio-inspired oxide films. The goal of this study is to have a capability to tailor microstructures and mechanical behaviors by identifying the controlling mechanisms responsible for nucleation and growth of such oxide films.
Parylene-PDMS Bilayer Coatings for Microelectronic and MEMS Packaging
Hyungsuk Lee, Junghyun Cho
Current microelectronic devices and microelectromechanical systems (MEMS) require that packaging costs be reduced with more enhanced device performance. In addition, the packaging materials are often exposed to harsh environments, for which their performance is drastically degraded. Importantly, such devices become lighter and smaller, precluding the use of conventional packaging materials and schemes. Given that, surface coatings can provide an alternative solution for some of the aforementioned issues. Polydimethylsiloxane (PDMS) is a good candidate material in many encapsulating applications but its surface must be effectively protected due to its poor surface properties. In this study, the PDMS surface is coated with the parylene C film through a vapor-phase deposition. Proper surface modification of PDMS is then essential to generate desirable interfacial adhesion and performance between the parylene C and the PDMS. Effects of plasma treatment were examined in this study to evaluate their effectiveness on the surface modification of the PDMS. In order to explore mechanical performances of the bilayer coatings, dynamic nanoindentation and feedback-control nanoindentation testings were employed. In addition, extensive surface characterizations are performed with atomic force microscope (AFM) and optical microscope (OM).
Nanostructured Ceramic Film Formation on Self-Assembled Monolayers via a Biomimetic Approach
Junghyun Cho, Douglas A. Blom, Dorothy W. Coffey, Lawrence F. Allard, Junghyun Cho
Published online by Cambridge University Press: 26 February 2011, 0901-Ra21-06
A biomimetic approach is employed to deposit ceramic films on organic self-assembled monolayers (SAMs) coated substrates. Specifically, zirconia (ZrO2) films are grown in a zirconium sulfate precursor solution at near room temperatures (∼70°C). This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. The resultant zirconia films consist of nanosized particles (5-10 nm) that are precipitated out in a supersaturated precursor solution. Cross-sectional TEM and STEM works were performed to quantitatively analyze the film structure and chemistry, as well as interfacial region of the ceramic-SAM films. A stepwise deposition process was developed to avoid excessive formation of aggregation. Further, the dynamic nanoindentation testing was employed to assess the thickness and film-only intrinsic mechanical properties for direct comparison among the films processed with different processing parameters and microstructures. The films with finer particulate structure displayed higher intrinsic modulus than did those with coarser structure.
Mechanical Behavior of Ceramic/SAM Bilayer Coatings
Quan Yang, Guangneng Zhang, Kaustubh Chitre, Junghyun Cho
Published online by Cambridge University Press: 01 February 2011, Y9.9
Ceramic/self-assembled monolayer (SAM) bilayer films can provide adequate protection and/or act as a multipurpose coating for microelectronics and MEMS applications, due to synergistic effects of the hybrid film structure. The organic SAM acts as a "template" for the growth of the ceramic film while the hard ceramic can provide protection from environmental and mechanical impact. To process the bilayer films, a low-temperature solution deposition technique (biomimetic process) is employed using phosphonate-based SAM and zirconium oxide precursors. A particulate zirconium oxide film is formed by enhanced hydrolysis of zirconium sulfate solutions in the presence of HCl at about 80°C, and its particle size and thickness effects are discussed. In addition, microstructure and micromechanics involved in the synthesis of both zirconia films and SAM are systematically assessed. Especially, mechanical properties such as Young's modulus and hardness are analyzed using a nanoindenter, as well as with the aid of theoretical models. Further, the substrate effects resulting from a large indentation depth relative to the film thickness are eliminated to obtain the "film-only" properties. This study also highlights the role of compliant SAM layer in forming a strain-tolerant bilayer film.
A Nanoindentation Study of Thermally-Grown-Oxide Films on Silicon
Fatih Helvaci, Junghyun Cho
Published online by Cambridge University Press: 01 February 2011, R12.10
We explore the effect of the substrate on mechanical behavior of thin films using a depth-sensing indentation. For this purpose, nanoindentation has been performed on thermally grown silicon oxide films on the silicon substrate. One primary goal of this study is to extract 'film-only' mechanical properties from the nanoindentation data by subtracting the substrate effect. It is also shown that elastic modulus of the film is more influenced by the substrate than hardness due to a larger elastic extension beyond a plastically deformed region, as well as a larger elastic mismatch between the SiO2 film and the Si substrate. Further, an inverse analysis of indentation data is proposed to estimate a thickness of thin films. Consequently, this study provides a fundamental understanding in mechanical phenomena of thin films occurring at nanoscales.
Interface Adhesion and Reliability of Microsystem Packaging
Marvin I. Francis, Kellen Wadach, Satyajit Walwadkar, Junghyun Cho
Published online by Cambridge University Press: 01 February 2011, A5.64
Flip-chip technology is becoming one of the most promising packaging techniques for high performance packages. Solder balls are used as the connection technique in the flip-chip method and the connections are reinforced by filling in the spacing between the chip and substrate with underfill. The function of the underfill is to reduce the stresses in the solder joints caused by a coefficient of thermal expansion (CTE) mismatch. The presence of polymeric underfill material will, however, make the flip-chip packaging system susceptible to interfacial failure. Thus, the purpose of this study is to examine the interfacial delamination between the dissimilar materials in order to increase the reliability of the flip-chip interconnection method, and to understand the effect of underfill curing conditions on the interface adhesion. In particular, we use a linear elastic fracture mechanics (LEFM) approach to assess interfacial toughness. For this purpose, four-point bending testing is performed to determine a critical strain energy release rate, Gc. In addition, nano-indentation testing equipped with atomic force microscope (AFM) is employed to determine structure and properties of the underfill layer.
Tailoring of Stress Development in MEMS Packaging Systems
Satyajit S. Walwadkar, Junghyun Cho, P.W. Farrell, Lawrence E. Felton
Published online by Cambridge University Press: 11 February 2011, J5.22
A better understanding of the origin and evolution of the stresses is a crucial step in improving reliability of packaging systems for microelectromechanical systems (MEMS). Given its importance, we examine the stresses developed in hermetically packaged MEMS inertial sensors. For this purpose, an optical surface profilometer is employed to assess the stresses by measuring the curvature of dummy silicon dies (3.5×3.5 mm2) assembled in different types of packages and die attach adhesives. We also explore a temporal evolution of stresses during thermal exposure of the test packages in an effort to emulate actual packaging processes and device operation conditions. The result shows different levels of stresses generated from various adhesives and package types, and also a stress evolution during packaging processes. The mechanical stress data also show a good agreement with MEMS performance data obtained from actual accelerometers. Therefore, the stress data will not only be useful in better understanding performance of MEMS packages, but the testing protocol can also provide a diagnostic tool for very small packaging systems.
Improved tensile creep properties of yttrium- and lanthanum-doped alumina: a solid solution effect
Junghyun Cho, Chong Min Wang, Helen M. Chan, J. M. Rickman, Martin P. Harmer
The tensile creep behavior of yttrium- and lanthanum-doped alumina (at dopant levels below the solubility limit) was examined. Both compositions (100 ppm yttrium, 100 ppm lanthanum) exhibited a uniform microstructure consisting of fine, equiaxed grains. The creep resistance of both doped aluminas was enhanced, compared with undoped alumina, by about two orders of magnitude, which was almost the same degree of improvement as for materials with higher dopant levels (in excess of the solubility limit). In addition, measured creep rupture curves exhibited predominantly steady-state creep behavior. Our results, therefore, verified that the creep improvement in these rare-earth doped aluminas was primarily a solid-solution effect.