Low-Temperature (70°C) Cu-to-Cu Direct Bonding by Capping Metal Layers

• Published in: IEEE electron device letters Vol. 42; no. 10; pp. 1524 - 1527
• Main Authors: Liu, Demin, Chen, Po-Chih, Liu, Yu-Wei, Hu, Han-Wen, Chen, Kuan-Neng
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.10.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: 3D integration; Bonding; Budgets; Capping; Chemical bonds; Chromium; Copper; Cu bonding; Diffusion; Gold; Grain size; Low temperature; Low vacuum; metal passivation; Oxidation; Passivation; Recrystallization; Rough surfaces; Surface roughness; Temperature measurement; Wetting
• ISSN: 0741-3106; 1558-0563
• DOI: 10.1109/LED.2021.3105434

Cu-Cu direct bonding at low temperature with Cr wetting layer and Au passivation has been developed, and the bonding mechanism has been investigated. A chip-level Cu-to-Cu direct bonding can be achieved under a very low thermal budget condition at 70 °C for 90 s or 150 °C for 15 s, while wafer level bonding can be achieved at 100 °C under a low vacuum environment (<inline-formula> <tex-math notation="LaTeX">10^{-{2}} </tex-math></inline-formula> Torr) by deposition of capping metal layers, which can protect Cu from oxidation, as well as reduce surface roughness and grain size of Cu. During the bonding process, Cu atoms diffuse through Cr/Au layers into the bonding interface, forming a new inter-layer without voids to achieve high quality bonding. The phenomenon of recrystallization and effects of grain size have been validated by TEM analyses. This Cu-Cu bonding with capping metal layers scheme can enable bonding with ultra-low thermal budget, excellent bonding quality, good electrical performance and high reliability, showing the great feasibility for the 3D integration applications.