Low-Temperature and Fine-Pitch Wafer-to-Wafer Hybrid Bonding using Nanotwinned and Nanocrystalline Copper for Advanced Packaging

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#Hybrid bonding #advanced packaging #metrology #2.5D #3D #die stacking #Cu-Cu

(21:46 + Q&A) Dr. Wei-Lan Chiu, Industrial Technology Research Institute (ITRI)
From the First IEEE Hybrid Bonding Symposium
Summary: This topic consolidates research on low-temperature wafer-to-wafer hybrid bonding using both nanotwinned copper (nt-Cu) and nanocrystalline copper (nc-Cu), two promising materials for advanced semiconductor integration in applications such as 5G, artificial intelligence (AI), and high-performance computing (HPC). The studies emphasize the importance of reducing bonding temperatures to improve interconnect density and reliability in modern packaging technologies, as traditional solder-based methods face limitations such as electromigration, current crowding, and void formation. On a 12-inch silicon wafer, a fine-pitch redistribution layer (RDL) is first deposited to enable electrical interconnections between wafers, followed by a SiO2 dielectric layer and a 200 nm thick SiCN diffusion barrier layer. The SiCN layer effectively prevents Cu diffusion and enhances the stability of the bonding interface. Dual damascene structures are employed to form copper bonding pads and dummy pads to mitigate stress impacts on the bonding quality. Nt-Cu and Nc-Cu layers could be deposited by physical vapor deposition (PVD) or electrochemical deposition, with an average grain size of approximately 80 nm on Nc-Cu grains. The CMP process requires a specific polishing slurry to ensure selective polishing between copper and dielectric layers. After oxygen plasma treatment, the upper and lower wafers undergo pre-bonding to enhance contact between the dielectric layers. The bonding wafers are then annealed under low pressure in a vacuum environment. Due to the higher thermal expansion coefficient of copper compared to dielectric layers, such as SiCN and SiO2, increased pressure on the copper pads enhances Cu-Cu contact and surface diffusion, achieving strengthened low-temperature bonding. All bonding samples passed the reliability test with thermal shock cycling and pulling testing.
Nanotwinned copper exhibits exceptional performance due to its highly -oriented grain structure, which enhances surface diffusivity and electromigration resistance. The bonding temperature for nt-Cu-to-nt-Cu bonding has been successfully reduced to 200°C, achieving nearly 90% void-free interfaces, making it ideal for wafer-to-wafer bonding applications.
Nanocrystalline copper offers even further temperature reductions, enabling bonding at temperatures as low as 150°C. This is attributed to the high density of grain boundaries in nc-Cu, which facilitates rapid copper diffusion at low temperatures compared to regular Cu structure. The wafer-to-wafer bonding process using nc-Cu/SiO2 hybrid structures demonstrated strong bonding interfaces with high mechanical reliability, surviving over 1,000 cycles of thermal shock testing. The nc-Cu-to-nc-Cu bonding exhibited near 97% coverage in bonding quality, with negligible voids, and a bonding strength of approximately 12 J/m².
These findings highlight the potential of nt-Cu and nc-Cu in heterogeneous integration, where fine-pitch interconnects and reduced bonding temperatures are critical for scaling semiconductor technologies. By leveraging these advanced copper materials, the studies pave the way for more reliable and efficient 3D IC integration, replacing traditional solder bumps and addressing challenges in modern electronics packaging.
Bio: Dr. Wei-Lan Chiu holds a B.S. degree (2008) in physics from National Cheng Kung University (NCKU) and M.S. (2010) and Ph.D. (2015) degrees in physics and materials science and engineering, respectively, from National Chiao Tung University (NCTU). He has valuable experience as a Post-Doctoral Researcher at NCTU (2016) and currently serves as a Project Manager at the Industrial Technology Research Institute (2017~now). His research interests include thermal migration and metallurgical reactions in microbumps for three-dimensional integrated circuits (3D IC) packaging, metal electroplating of nanotwinned Cu (nt-Cu) redistribution layers (RDL), and the development of Nc-Cu or Nt-Cu/SiO2 wafer-to-wafer hybrid bonding processes at low temperatures. Dr. Chiu has received recognition for his outstanding contributions, including the Electronic and Optoelectronic System Research Laboratories Outstanding Research Award in 2021 and the Outstanding Young Engineer Award by the Chinese Society of Mechanical Engineers in 2023.

For other edited  videos from this symposium, visit  https://attend.ieee.org/hbs/?page_id=456

(21:46 + Q&A) Dr. Wei-Lan Chiu, Industrial Technology Research Institute (ITRI)
From the First IEEE Hybrid Bonding Symposium
Summary: This topic consolidates research on low-temperature wafer-to-wafer hybrid bonding using both nanotwinned copper (nt-Cu) and nanocrystalline copper (nc-Cu), two promising materials for advanced semiconductor integration in applications such as 5G, artificial intelligence (AI), and high-performance computing (HPC). The studies emphasize the importance of reducing bonding temperatures to improve interconnect density and reliability in modern packaging technologies ...

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