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5 1 I H I G H L I G H T S 2 0 2 3
PRINCIPAL PUBLICATION AND AUTHORS
Intragranular thermal fatigue of Cu thin films: Near-grain boundary hardening, strain localization and voiding, K. Hlushko (a,b), T. Ziegelwanger (a), M. Reisinger (c), J. Todt (a), M. Meindlhumer (a), S. Beuer (d), M. Rommel (d), I. Greving (e), S. Flenner (e), J. Kopeček (f), J. Keckes (a), C. Detlefs (g), C. Yildirim (g), Acta Mater. 253 118961 (2023); https:/doi.org/10.1016/j.actamat.2023.118961 (a) Department of Materials Science, Montanuniversität Leoben (Austria) (b) Materials Center Leoben Forschung GmbH, Leoben (Austria) (c) KAI Kompetenzzentrum Automobil- und Industrieelektronik GmbH, Villach (Austria) (d) Fraunhofer Institute for Integrated Systems and Device Technology IISB, Erlangen (Germany) (e) Institute of Materials Physics, Helmholtz-Zentrum Hereon, Hamburg (Germany) (f) FZU - Institute of Physics, Czech Academy of Science, Praha (Czech Republic) (g) ESRF
REFERENCES
[1] M. Kutsal et al., IOP Conf. Ser. Mater. Sci. Eng. 580, 012007 (2019). [2] S. Moser et al., Microelectron. Reliab. 137, 114782 (2022).
Interestingly, in certain parts of the surface, comparatively stable structures remain visible throughout many cycles, as marked by arrows in Figure 34c. This is where the correlation with results from DXFM was able to help the overall understanding greatly, as it revealed the large differences in deformation behaviour present between the Cu grain interiors and the grain boundaries (GBs). It could be shown that the residual strain and orientation gradients initially present inside Cu grains are essentially homogenised by thermo-mechanical cycling, while at the GBs, they pile up along with newly introduced lattice defects (Figure 33a-d).
In conclusion, this is interpreted as deformation- induced recovery inside the Cu grains, accompanied by hardening and embrittlement at GBs. Future recipes for Cu metallisation synthesis will be able to take these new insights into account, diminishing their impact on functional performance and boosting the lifetime of microelectronic devices.
Fig. 34: SEM images of the surface of the tested Cu film after various numbers of thermal cycles. The scale bar in (a) is 10 µm. Along with an increase in roughness starting at low cycle numbers, voids form, eventually percolating through the film s entire thickness, thereby increasing electrical sheet resistance. Arrows in (c) indicate a Cu grain where further deformation inside the grain is comparatively hindered, while the surrounding regions continue to deform.
