Our research interest is in wafer-based and flux-driven materials science. Modern microelectronic, opto-electronic, bio-sensor, and MEMS devices are built on wafers, involving the growth or removal of mono-layers of atoms from the wafer surface or an inter-phase interface. We deal with open systems, in which the initial wafer surface is constant and the flux can be atoms, molecules, or energy beams.
Our major research areas are (1) Lead (Pb)-free solder metallurgy for electronic and optical packaging technology, (2) Advanced materials reliability problems of microelectronic devices, especially concerning the flip chip technology, and (3) Nanoscale interdiffusion and reactions. In addition, we also conduct exploratory research on (4) Dislocations and grain boundaries in Si, and (5) Kinetic theory of interdiffusion and reactions.
On Pb-free solder metallurgy, we study the applications of eutectic SnAg, SnAgCu, SnCu, SnZn as solder bump to flip chip technology. The wetting reaction and solid state aging of these Pb-free alloys with thin film under-bump-metallization are of interest. Due to the large difference in thermal expansion coefficients between the Si chip and its packaging substrate, the solder joints are stressed. In turn, the stress due to chip-packaging interaction may affect the integrity of Cu/ultra low k multi-layered interconnect structure on the chip. The diameter of the solder balls is approaching 50 μm, so electromigration is becoming a reliability issue. The advanced materials reliability problems due to a combined action among chemical, mechanical, and electrical forces in flip chip technology will be studied systematically. Electromigration induced microstructure evolution and grain rotation in solder alloys requires investigation. A unique nature of most Pb-free solders is that they are Sn-rich, hence the old topics of Sn whisker, Sn pest, and Sn cry are of interest again. We shall combine micro-diffraction in synchrotron radiation, focused ion beam imaging, and cross-sectional transmission electron microscopy to study these issue. In optical packaging, we interest in how to wet an optical fiber by molten solder and how to achieve high precision alignment by solder joints.
On interconnect tehcnology, our research emphasizes the effect of current crowding on vacancy and solute diffusion in electromigration. The nature of the electromigration force along the direction of current density gradient or normal to the current flow will be explored. Why failure tends to initiate in low current density regions will be studied. The effect of current crowding on joule heating as well as on stress concentration will be analyzed. The nature of back stress induced by electromigration and whether or not there is back stress in Cu interconnect will be investigated.
On nanoscale interdiffusion and reactions, we study the reaction of ultra thin metal films on nano Si wires and vice versa, and the nanoscale explosion in multi-layered nano-thickness thin films. Hollow nanostructures based on the Kirkendall effect will be investigated.
On extended defects in Si, we study the nucleation, growth, and ripening of dislocation loops formed by ion implantation and post-implantation annealing. The interaction of these loops with metallic atoms such as Ni and Co will be investigated. We also investigate the nano-grid of screw dislocation network, or very small angle twist-type grain boundary in Si bicrystals formed by wafer bonding. Again we examine the interaction of implanted metallic atoms with these dislocation networks.
Kinetic theory of phase transformations in open systems under the constraint of a constant surface area applies to phase changes on a wafer or on a given area of surface or interface will be developed. For example, the constraint of constant area is fundamental to the ripening of hemispherical scallops during the reaction between molten solder and Cu. It also applies to linear rate of grain growth in thin film deposition.