Impact of Thermal History on Grain Size and Grain Size Distribution of Thermally Cycled Cu-Tsvs

Through silicon vias (TSV) are copper (Cu) interconnects used in three dimensional integrated circuits (3D-IC) and other packaging to allow inter-chip communication.  Since Cu is anisotropic, understanding the evolution of microstructure in TSVs is of major importance as it can influence the Cu’s electrical properties, such as resistance and electro-migration resistance, as well as mechanical properties. These properties can affect the materials response to thermo-mechanical stresses undergone by Cu-TSVs.  Microstructural studies have focused on isothermal holding temperatures and times to imitate fabrication conditions [1-3], however, in-use conditions, which are typically of lower temperatures and higher frequencies to replicate thermal cycling over a chip’s lifetime, have not been greatly explored.  This study correlates simulated in-use cycling parameters, such as cycling temperature and number of cycles, to microstructural changes, specifically grain size, observed in the Cu of TSVs.  Silicon (Si) chips containing blind Cu-TSVs underwent 2000 thermal cycles from room temperature to maximum temperatures of 100 °C, 150 °C, or 200 °C.  Grinding, polishing, and focus ion beam (FIB) milling were used to cross-section the TSVs and provide sufficient surface quality for electron backscattering diffraction (EBSD) to evaluate microstructural changes from thermal cycling.  The average grain size remained around 3 μm and no significant differences were documented for average grain sizes along the length of TSVs before and after cycling.  Furthermore grain size distribution also, showed little change with peaks around 1 μm for samples containing minimal voiding and approximately 2.5 μm for samples with significant pre-existing voids.  While the low cycling temperatures used in this study are indicative of the conditions used in a majority of applications, they are too low to activate grain boundary migration and cause much microstructural changes in the Cu-TSVs.

References:

1. C. Okoro, R. Labie, K. Vanstreels, A. Franquet, M. Gonzalez, B. Vandevelde, E. Beyne, D. Vandepitte and B. Verlinden, Journal of Materials Science, 46 [11] 3868-3882 (2011).

2. A. Heryanto, W. N. Putra, A. Trigg, S. Gao, W. S. Kwon, F. X. Che, X. F. Ang, J. Wei, R. I Made, C. L. Gan and K. L. Pey, Journal of Electronic Materials, 41 [9] 2533-2542 (2012).

3. H. Shin, B. Kim, J. Kim, S. Hwang, A. Budiman, H. Son, K. Byun, N. Tamura, M. Kunz, D. Kim and Y. Joo, Journal of Electronic Materials, 41 [4] 712-719 (2012).