Unraveling diffusion behavior in Cu-to-Cu direct bonding with metal passivation layers

Area of Science:

• Materials Science and Engineering
• Semiconductor Device Fabrication

Background:

• High-density interconnects are crucial for advanced integrated systems.
• Traditional copper bonding requires high temperatures (>400 °C), necessitating lower-temperature alternatives.
• Metal passivation bonding is explored to reduce Cu/SiO2 hybrid bonding temperatures.

Purpose of the Study:

• To investigate the diffusion behavior of various metal passivation layers (Pt, Ti, Ta, Cr) in low-temperature direct copper bonding.
• To understand the underlying bonding mechanisms and identify key factors influencing diffusion.
• To determine the role of metal passivation layer crystallinity in the bonding process.

Main Methods:

• Direct copper bonding experiments using Cu/SiO2 substrates with different metal passivation layers.
• Analysis of diffusion behavior under low-temperature bonding conditions.
• Characterization of metal passivation layers, focusing on crystallinity, surface roughness, and grain size.
• Post-bonding examinations to correlate diffusion with passivation layer properties.

Main Results:

• Diffusion behavior deviated from conventional metal-metal bonding factors like surface roughness and grain size.
• A strong correlation was observed between the crystallinity of metal passivation layers and diffusion behavior.
• Consistent diffusion was noted in Pt and Cr passivation samples despite differences in crystallinity, highlighting its critical role.

Conclusions:

• Crystallinity is the pivotal factor governing diffusion behavior in low-temperature Cu/SiO2 hybrid bonding, overriding other experimental variables.
• Understanding and controlling crystallinity enables optimized bonding characteristics at reduced temperatures.
• This research advances die-to-wafer bonding, paving the way for more highly integrated systems.