From Elastic Expansion to Cu Bulge-Out: Mechanisms, Measurements, and Modeling for Sub-Micron Hybrid Bonding

Presented at the 2026 IEEE Hybrid Bonding Symposium, Jan 22-23, 2026 in Silicon Valley.  More information below.
(28:40 + Q&A) Joke De Messemaeker, imec
Summary: Hybrid bonding at sub-micron pitch demands tight post-CMP topography control to ensure dielectric pre-bonding and reliable Cu–Cu contact after annealing. At larger pitch, that contact is mainly driven by elastic expansion of the Cu pads. We show how this expansion depends on pad geometry (size and depth) and anneal conditions, defining the practical limits for the bonding process window and pitch scaling. With scaling to sub-micron pitch and with reduced anneal temperature, elastic expansion drops well below one nanometer, and surface-diffusion-driven bulge-out becomes the dominant mechanism for contacting and bonding. In unequal-sized pads, this effect is mainly driven by thermal stress gradients around the Cu-dielectric interface, whereas in equal-sized pads, surface energy minimization provides the driving force. Using a dedicated 300 mm test vehicle, we collected bulge-out data from thousands of pads, revealing how the ratio of top to bottom pad size is key to maximizing the effect. Like elastic expansion, bulge-out decreases with temperature, but at 230 °C it still produces roughly ten times larger pad extension, significantly relaxing CMP constraints. Experiments with intentional Cu oxidation before bonding show its adverse effect on bulge-out, underlining the importance of surface cleanliness. The wide statistical distribution of bulge-out poses a potential yield risk. By correlating bulge-out amplitude with local crystal orientation, we show that a single texture should be avoided, and that microstructure engineering can mitigate this variability. A first data point on larger pad sizes – representative of die-to-wafer hybrid bonding for chiplet integration – further illustrates the scaling trend and its implications for process design. A discrete axisymmetric surface-diffusion model reproduces the experimentally observed bulge-out for equal-sized pads and confirms that curvature gradients dominate over thermal stress as the driving force. The model identifies the contact angle between Cu and the barrier or liner as a key parameter and predicts that diffusion kinetics scale approximately with the fourth power of pad size, defining the practical upper limit of this mechanism and the transition toward the elastic expansion regime at larger pad dimensions.
Bio: Joke De Messemaeker is an R&D group leader at imec, a renowned research center based in Leuven, Belgium, specializing in advanced microelectronics and nanotechnology innovations. She has published research on topics such as stress relaxation, 3D technologies, and hybrid bonding for electronic devices, reflecting her deep expertise in semiconductor reliability and process engineering. De Messemaeker completed her education at Katholieke Universiteit Leuven and is active in international technical conferences and publications related to wafer-scale technologies and 3D packaging. Her collaborative research often involves working with leading scientists at imec on breakthrough projects in advanced electronics. Currently, her role focuses on managing research teams and driving innovations in microelectronics fabrication, with a significant influence on the reliability and scalability of next-generation integrated circuits.

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