Metastable Nanocrystalline Copper for Effective Copper-to-Copper Bonding

Area of Science:

• Materials Science
• Nanotechnology
• Electronic Packaging

Background:

• Direct copper-to-copper (Cu-Cu) bonding is essential for advanced electronic packaging.
• High bonding temperatures and long durations pose challenges for heterogeneous integration.
• Existing grain engineering approaches like nanotwinned Cu (NT-Cu) and nanocrystalline Cu (NC-Cu) have limitations regarding thermal budget and processing stability.

Purpose of the Study:

• To develop a novel copper material for low-thermal-budget Cu-Cu bonding.
• To engineer a copper structure that maintains stability at room temperature but allows controlled grain growth at bonding temperatures.
• To overcome the self-annealing limitations of nanocrystalline copper for practical manufacturing.

Main Methods:

• Fabrication of metastable nanocrystalline copper (MS-Cu) with engineered grain boundaries, stacking faults, and 9R phase structures.
• Characterization of MS-Cu's structural stability during room-temperature processing.
• Evaluation of MS-Cu's bonding performance at reduced temperatures and times.

Main Results:

• MS-Cu exhibits stabilized grain boundaries, ensuring structural integrity during room-temperature storage for over 14 days.
• The engineered material facilitates substantial grain growth at elevated bonding temperatures.
• High-quality Cu-Cu bonding was achieved at a lower temperature (200 °C) and shorter time (30 min).

Conclusions:

• Metastable nanocrystalline copper (MS-Cu) offers a promising solution for advanced electronic packaging.
• MS-Cu enables low-thermal-budget Cu-Cu bonding with improved stability and processing efficiency.
• This approach provides a practical and effective method for heterogeneous integration in electronic devices.