Comprehensive study on Chip to wafer hybrid bonding process for fine pitch high density heterogeneous applications

This paper explores the Chip-to-Wafer (C2W) hybrid bonding process for fine-pitch, high-density heterogeneous applications, specifically targeting Cu-to-Cu bonding for interconnects at 12 µm pitch and below. It focuses on critical process steps and evaluates the bonding technology, including wafer dicing, plasma cleaning, and post-bonding annealing.

Key Points:

1. Chip-to-Wafer Hybrid Bonding:

• Challenges: The need for high interconnect density, ultra-fine pitch, and low-temperature bonding. The paper highlights the difficulties of achieving strong bonding interfaces with minimal voids, especially in fine-pitch applications.

• Cu-Cu Bonding: The study investigates Cu-to-Cu hybrid bonding as a promising solution for high-density interconnects in applications like 2.5D and 3D packaging.

2. Process Flow and Methodology:

• The chip, with a 6 µm Cu pad and 12 µm pad pitch, is used in the hybrid bonding process.

• Key steps in the process include wafer dicing, plasma cleaning, bonding, and post-bonding annealing at 300°C for 2 hours.

• Bonding Parameters: Bond force (20N to 50N), bond time (30 to 90 seconds), and post-annealing temperature (200°C and 300°C) were varied to evaluate process optimization.

• Surface Activation: Plasma treatments using gases like nitrogen, oxygen, and hydrogen were used to improve surface wettability before bonding .

3. Wafer Dicing and Surface Preparation:

• Dicing Process: The paper investigates the mechanical dicing method, which aims to reduce die chipping and minimize silicon dust. Results show minimal die chipping and particle contamination after the dicing process.

• Surface Cleaning: Plasma cleaning steps ensure that die surfaces are clean and ready for bonding, which is critical to prevent bonding defects like voids.

4. Plasma Treatment:

• Plasma treatment plays a crucial role in improving surface wettability and cleaning the Cu and thermal oxide surfaces. Oxygen, nitrogen, and hydrogen plasma were tested at different power levels and times.

• The results show that higher plasma power and longer exposure times reduce contact angles, making the surfaces more hydrophilic.

5. Bonding Results:

• Cu-Cu Bonding: The hybrid bonding achieved Cu-to-Cu bonding accuracy within ±2 µm. However, some bonding defects were observed, including small gaps between the Cu pads and the thermal oxide interface, which impacted yield.

• The study also evaluated the impact of bonding force on quality. A higher force (100N) provided better bonding strength, with a shear strength of 21.5 MPa at 100N, compared to lower forces.

• The best results for bonding yield were seen in oxide-to-oxide bonding, with a yield of 87.7% for 188 dies bonded onto a 12-inch oxide wafer.

6. Summary and Conclusions:

• Plasma treatments (oxygen, nitrogen) reduce the contact angle without damaging the passivation layer, improving the bonding quality.

• Hydrogen plasma effectively removes the Cu oxide layer, crucial for strong Cu-to-Cu bonding.

• Cu dishing (~5 nm) is required for successful Cu-to-Cu interconnects.

• Bonding accuracy was achieved within ±2 µm, showing the potential for ultra-fine pitch applications.

• The paper demonstrates that hybrid bonding, particularly Cu-to-Cu bonding, is viable for fine-pitch interconnects down to 12 µm and potentially below. Key factors influencing bonding quality include surface preparation, plasma treatment, and bonding parameters.

• Key Findings:

Acknowledgment:

The study was supported by the Science and Engineering Research Council of A*STAR, Singapore, and various contributors to the project.