The Advanced Photon Source
a U.S. Department of Energy Office of Science User Facility

Synchrotron X-ray Studies of Allotropic Phase Transformation and Atomic Diffusion under Electron Current Stressing

Type Of Event
Sponsoring Division
Pei-Tzu Lee, Taiwan Photon Source. National Synchrotron Radiation Research Center
Olga Antipova
Start Date
Start Time
09:00 a.m.


Some studies of the microelectronic package were conducted by synchrotron radiation, e.g., nano-X-ray fluorescence microscopy, white beam Laue nanodiffraction and X-ray computed tomography. First, with the advance of semiconductor processing technology, the number of transistors on a silicon chip increase rapidly. Accompanying this advance, the back-end process and packaging technology also progress rapidly, resulting in rapid miniaturization and increased power density. Such trend makes the current densities carried by various metal lines increase quickly. A direct consequence of a high current density is the electromigration effects. Synchrotron X-ray analysis of the Sn electromigration behavior and Sn whisker growth in a Blech structure using nano-X-ray fluorescence microscopy and white beam Laue nanodiffraction was conducted. Sn depletion at the cathode and whisker/extrusion formation at the anode were characterized in-situ, and the results obeyed the electromigration kinetics. White beam Laue nanodiffraction analysis showed that a noticeable compressive deviatoric stress in the direction of electron flow built up at the anode of Sn strips, particularly in the roots of Sn whiskers, confirming that electromigration-induced atomic accumulation occurred downstream in a strip and that Sn whiskering was closely related to internal stress resulting from atomic accumulation in confined segments. Finally, a theoretical model based on fundamental electromigration theory revealed that Sn diffused predominately through lattice and grain boundary paths. Additionally, in pursuit of the high reliability of microelectronic devices, researchers and engineers have devoted their efforts to the improvements of micro joint integrity. Cu6Sn5 and Cu3Sn are two typical intermetallic compounds nucleated in the Cu/Sn system, governing the mechanical/electrical reliability of micro joints. The allotropic phase transformation of Cu6Sn5 from a hexagonal (η) to monoclinic (η’) structure at 186 °C is scientifically interesting and technologically important. As the η’ lattice is crystallographically pseudosymmetrical with η, the identifications of η and η’ and their transformation are extremely difficult via traditional techniques, such as electron backscatter diffraction, transmission electron microscopy, or, X-ray powder diffraction. The η-to-η’ transformation and phase distribution by Laue diffraction via synchrotron white X-ray radiation is conducted. The allotropic species (η and η’) can be well distinguished, and two-stage phase transformation kinetics is proposed. This non-destructive method can further provide the crystallographic information associated with the underlying Cu3Sn nanolayer and Cu crystal and their crystallographic correlations with respect to the phase transformation, which advances fundamental understanding of Cu/Sn metallurgical reaction.

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