Geometrical Effects on Ultrasonic Al Bump Direct Bonding for Microsystem Integration: Simulation and Experiments

Area of Science:

• Materials Science
• Mechanical Engineering
• Semiconductor Manufacturing

Background:

• Ultrasonic metal bump direct bonding is crucial for semiconductor packaging.
• Optimizing bump geometry is essential for reliable interconnections.
• Understanding stress distribution is key to enhancing joint strength.

Purpose of the Study:

• To simulate and analyze stress distribution in ultrasonic metal bump direct bonding using finite element analysis.
• To investigate the impact of geometrical parameters (shape, size, height) on bonding interface stress.
• To correlate simulation results with experimental findings on joint shear strength.

Main Methods:

• Finite element analysis (FEA) was used to simulate stress distribution in Al, Cu, and Ni/Pd/Au bumps.
• Simulations varied bump geometrical parameters under specified ultrasonic bonding conditions (35 kHz, 200 N, 200 °C, 5 s).
• Experimental validation involved creating Al bumps on Si chips via sputtering and lithography, followed by subtractive etching and Ar plasma treatment.

Main Results:

• Maximum stress was higher for square bumps compared to round bumps.
• Little square bumps exhibited at least 15% greater maximum stress than little round and big round bumps.
• Subtractive lithography etching and Ar plasma treatment enhanced joint strength, with actual shear strength correlating positively with simulated maximum stress.

Conclusions:

• Bump geometry significantly influences stress distribution and joint strength in ultrasonic bonding.
• Little aluminum square bumps achieved a maximum shear strength of 44.6 MPa under optimized conditions.
• FEA is a valuable tool for predicting and optimizing ultrasonic metal bonding processes.