Microelectronic Interconnect Materials Division

●Pb-free Solders

　Pb-Sn solder alloys have been used for chip attachment for many years. However, environmental and health concerns over Pb have prompted the recent action by the European Union to ban the use of Pb-bearing solders in electronic products. In response to those concerns, a number of Pb-free alternatives have been developed, including Sn-Ag, Sn-Cu, Sn-Bi, Sn-Zn, Sn-In, Sn-Sb, and Sn-Ag-Cu. While these alloys cover a similar range of melting temperatures to those of Pb-bearing soldering alloys, their differences with Pb-bearing solders in electric, mechanical, chemical, and thermal properties have created new challenges in designing and building reliable Pb-free interconnect structures. Our research is intended to address the major challenges in the following areas: thermomechanical reliability, electromigration, interfacial reaction, reactive wetting, solute redistribution, atomic segregation, microstructural development and long-term stability.

�?/strong> Wettable Surfaces and Materials

　Wetting by liquid solders is an essential step in creating direct contact between liquid solders and solid metallization in reflow soldering of electronic components. Despite many years of large scale manufacturing, the number of easily wettable materials remains very small. For nearly all of them, such as Cu and Au, the excellent wetting comes with a rapid interfacial reaction. The reaction may result in loss of the metallization or development of a thick intermetallic compound layer at the interface, both of which degrade the reliability of the solder joint, especially at the interface. Therefore, a major challenge is to find a surface and/or material which is highly wettable by liquid solders but reacts very slowly with liquid and solid solder alloys. We attempt to address this challenge by examining the wetting processes and mechanisms on multilayer thin films and by designing new wettable alloys.

�?strong>Thermal Interface Materials

　As the power density in the chip continues to rise, thermal management has become a major problem. Currently the heat dissipation is limited by the thermal resistance of the interface material between Si and heat spreader. Our research attempts to address this problem by designing new thermal interface materials with high thermal conductivity, synthesizing new stable metallization to minimize interfacial resistance between the metallization and the thermal interface materials, and investigating damage mechanisms in thermal interface materials produced by thermal cycling.

�?strong>Oxide Semiconductors

　Oxide semiconductors offer a series of interesting electric, optical and chemical properties which have made them useful in many engineering applications. We are interested in wide bandgap oxide semiconductors for gas sensing and environmental applications. Our recent work has been focused on the light-enhanced gas sensing and the removal of impurity species from water by doped titanium oxides.