Albert T. Wu

Direct observation and kinetic analysis of grain rotation in anisotropic tin under electromigration

The resistance of β-tin decreases by 10% under electromigration because of the anisotropic properties of the material during electromigration and the consequent rotation of grains. The diameter of the grains was measured ex situ and the angle of the rotating grains was directly calculated. The angle of rotating grains increased in proportion to the time for which the current was applied. At higher temperature, at which electromigration effect was stronger, an unexpected rotation behavior was observed. The kinetics of atomic movement along the a and c axes under various test conditions is discussed.

Controlled positions and kinetic analysis of spontaneous tin whisker growth

This study achieved controlling the positions of spontaneous growth of tin whiskers. We surmounted the unpredictable growing nature of such whiskers and performed accurately quantitative analyses of the growth kinetics and yielded precise measurement of the growth rate. Furthermore, using synchrotron radiation x-ray, this study determined the stress variations in conjunction with whisker growth that fitted appropriately to the model. Accordingly, the results could address the debate held for decades and prove that forming a surface oxide layer is one of the required and necessary conditions for controlling the positions of spontaneous growth of tin whiskers.

Interfacial Reactions Between Diffusion Barriers and Thermoelectric Materials Under Current Stressing

This study investigates the stability of the interfaces between an n-type thermoelectric material, Bi2Te3, and pure tin solders in a thermoelectric module. The module was stressed by a current at elevated temperatures. A map of the failure criteria of the module was constructed. Ni was used as the diffusion barrier between the solder and the thermoelectric materials, but it reacted with these materials and formed intermetallic compound (IMC) layers. The phase transformation of the IMC layers formed voids at the interfaces and reduced the strength of the joints. Current stressing promoted the formation of IMC. The kinetics of the compound growth was studied. The results suggested that the formation of NiTe IMC was successfully inhibited, but the Bi4Te5 compound grew from an infinitely adaptive series of (Bi2)m(Bi2Te3)n because of the depletion of Te.

Disruption of crystalline structure of Sn3.5Ag induced by electric current

This study presented the disruption of the Sn and Ag3Sn lattice structures of Sn3.5Ag solder induced by electric current at 5–7 × 103 A/cm2 with a high resolution transmission electron microscope investigation and electron diffraction analysis. The electric current stressing induced a high degree of strain on the alloy, as estimated from the X-ray diffraction (XRD) peak shift of the current stressed specimen. The XRD peak intensity of the Sn matrix and the Ag3Sn intermetallic compound diminished to nearly undetectable after 2 h of current stressing. The electric current stressing gave rise to a high dislocation density of up to 1017/m2. The grain morphology of the Sn matrix became invisible after prolonged current stressing as a result of the coalescence of dislocations.

Polarity Effect in a Sn3Ag0.5Cu/Bismuth Telluride Thermoelectric System

This study investigates electromigration in Bi2Te3 thermoelectric (TE) material systems and the effectiveness of the diffusion barrier under current. The Peltier effect on the interfacial reaction was decoupled from the effect of electromigration. After connecting p- and n-type Bi2Te3 to Sn3Ag0.5Cu (SAC305) solders, different current densities were applied at varying temperatures. The Bi2Te3 samples were fabricated by the spark plasma sintering technique, and an electroless nickel-phosphorous (Ni-P) layer was deposited at the solder/TE interfaces. The experimental results confirm the importance of the Ni diffusion barrier in joint reliability. Intermetallic compound layers (Cu,Ni)6Sn5 and NiTe formed at the solder/Ni-P and Ni-P/substrate interfaces, respectively. The experimental results indicate that the mechanism of NiTe and (Cu,Ni)6Sn5 compound growth was dominated by the Peltier effect at high current density. When the current density was low, the growth of NiTe was affected by electromigration but the changes of thickness for (Cu,Ni)6Sn5 were not obvious.

Preferred orientation relationships with large misfit interfaces between Ni3Sn4 and Ni in reactive wetting of eutectic SnPb on Ni

Ni3Sn4 grains were formed on Ni by reactive wetting between molten eutectic SnPb and thermally annealed Ni foil. Using synchrotron white beam micro x-ray diffraction analysis, two kinds of preferred orientation relationships between Ni3Sn4 and Ni were found. The existence of preferred orientation with large interfacial misfit is suggested as a general mechanism of intermetallic compound formation in reactive solder wetting on metals.

Electromigration-induced back stress in critical solder length for three-dimensional integrated circuits

Because of the miniaturization of electronic devices, the reliability of electromigration has become a major concern when shrinking the solder dimensions in flip-chip joints. Fast reaction between solders and electrodes causes intermetallic compounds (IMCs) to form, which grow rapidly and occupy entire joints when solder volumes decrease. In this study, U-grooves were fabricated on Si chips as test vehicles. An electrode-solder-electrode sandwich structure was fabricated by using lithography and electroplating. Gaps exhibiting well-defined dimensions were filled with Sn3.5Ag solders. The gaps between the copper electrodes in the test sample were limited to less than 15 μm to simulate microbumps. The samples were stressed at various current densities at 100 °C, 125 °C, and 150 °C. The morphological changes of the IMCs were observed, and the dimensions of the IMCs were measured to determine the kinetic growth of IMCs. Therefore, this study focused on the influence of back stress caused by microstructural evolution in microbumps.

Elucidating the Metal-Induced Crystallization and Diffusion Behavior of Al/a-Ge Thin Films

As used in wafer-level bonding in microelectromechanical system (MEMS) devices, the eutectic Al/a-Ge bilayer is characterized by its remarkable hermetic sealing after annealing. For MEMS packaging, this study investigates metal-induced crystallization (MIC) of the amorphous Ge and the layer exchange of Al and Ge, mainly by scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) analysis, and x-ray photoelectron spectroscopy (XPS). A kinetic mechanism to explain the layer exchange of Al and Ge is developed. Experimental results indicate that round-shaped extrusions form on the upper surface of the Ge layer when the bilayer is annealed at 400°C, i.e., close to the Al-Ge eutectic temperature. The morphology and the formation of the extrusions are also discussed. Finally, the bilayer is tested by immersion in red ink, with these results indicating that no red ink penetrates the bonding area of two bonded bilayer films. Therefore, results of this study demonstrate the feasibility of applying the eutectic Al/a-Ge bilayer to MEMS as a hermetic sealing material.

High dislocation density of tin induced by electric current

A dislocation density of as high as 1017 /m2 in a tin strip, as revealed by high resolution transmission electron microscope, was induced by current stressing at 6.5 x 103 A/ cm2. The dislocations exist in terms of dislocation line, dislocation loop, and dislocation aggregates. Electron Backscattered Diffraction images reflect that the high dislocation density induced the formation of low deflection angle subgrains, high deflection angle Widmanstatten grains, and recrystallization. The recrystallization gave rise to grain refining.

Kinetic Analysis of Spontaneous Whisker Growth on Pre-treated Surfaces with Weak Oxide

This study sought to clarify the relationship between cracks in surface oxide layers and the growth behavior of tin whiskers. The number, length, and total volume of extrusions were precisely calculated and residual stress was measured using synchrotron radiation x-ray diffractometry. The aim was to elucidate the influence of stress on the driving force and flux involved in atomic diffusion. The distance between weak spots was shown to be the most significant factor involved in the growth of whiskers. The results could explain why increasing the density of the surface weak spots could reduce the number of long whiskers. Measuring the dimensions of whiskers yielded a precise kinetic model capable of describing the migration of atoms to the root of whiskers, resulting in their spontaneous growth.