IEDM2024|Hybrid Bonding Technology for 3D Chiplet AI Applications

Introduction

Hybrid bonding technology is an innovative ultra-fine pitch interconnect method, suitable for high-performance chiplet systems involving multi-chip/multi-layer stacking. This advanced interconnection technology achieves an interconnect density of approximately 106/mm², meeting the demands of next-generation AI applications.

Figure 1 illustrates the development progression of AI chiplet architectures, including: (a) 2.5D chiplet layout with logic and memory, (b) 3D interconnected logic and memory using hybrid bonding, and (c) a comparison of traditional C4 solder, copper pillar, and hybrid bonding methods.

Traditional solder connection methods have reached their limits in pitch and density. The shift from conventional C4 solder and copper pillar technologies to hybrid bonding represents a significant advancement in semiconductor packaging technology, enabling thinner packages with enhanced electrical and thermal characteristics, particularly suitable for high-performance AI applications.

Figure 2 demonstrates the relationship between I/O bandwidth (GB/s/mm²) and I/O pitch (µm), highlighting the superior bandwidth achievable with hybrid bonding compared to traditional solder connections.

System Architecture and Performance Considerations

Implementing hybrid bonding in 3D chiplet systems requires careful architectural consideration. Different stacking configurations, such as Face-to-Face (F2F) and Face-to-Back (F2B), and arrangements where logic chips are placed over memory (LoM) or memory over logic (MoL), each have distinct characteristics.

Figure 3 presents various 3D integration schemes (LoM, MoL, F2F, F2B) and their relationship with through-silicon via (TSV) requirements, comparing 40 µm solder connection with 4 µm hybrid bonding.

Thermal management is a critical challenge in stacked chiplet architectures, especially for high-power 3D applications. At identical pitches, hybrid bonding reduces the thermal resistance per layer interconnection by approximately 20 times compared to traditional solder connections.

Figure 4 compares memory chip junction temperatures in LoM configurations using solder connections and hybrid bonding, illustrating superior thermal dissipation performance of hybrid bonding.

Advanced Process and Material Considerations

The success of hybrid bonding hinges on precise process control and material selection. The first critical step involves polishing dielectric layers and copper pads to molecular-level flatness. Copper vias are typically recessed several nanometers below the interlayer dielectric (ILD) surface to ensure optimal bonding.

Figure 5 illustrates the relationship between contact area and recess depth for various pad sizes at 300°C, indicating that reducing recess depth can significantly enhance the contact area across all pad sizes.

Figure 6 shows infrared spectra comparing oxide ILDs with and without a sealing layer, demonstrating the superior moisture resistance of sealed ILDs.

The bonding process begins at room temperature with hydrogen bond formation between silanol groups, followed by thermal annealing that strengthens ILD bonding and facilitates copper-copper solid-state diffusion.

Figure 7 displays the bonding interface of sealed ILD, indicating an oxide thin layer approximately 3-5 nanometers thick.

Surface Characterization and Quality Control

Surface characterization is crucial for ensuring reliable hybrid bonding. Advanced measurement techniques such as atomic force microscopy (AFM) and electron backscatter diffraction (EBSD) assess surface quality and grain structures.

Figure 8 compares AFM measurements of 0.8µm diameter pads treated by different cleaning processes, identifying optimal copper recess values for hybrid bonding.

Figure 9 displays EBSD grain distribution maps for 0.8µm copper pads, highlighting predominant grain orientations in [220], [200], and [111] directions.

Process Optimization and Results

Successful hybrid bonding implementation requires meticulous optimization of numerous process parameters. Chip-to-Wafer (D2W) and Wafer-to-Wafer (W2W) bonding methods have achieved minimum pitches as low as 2.5µm.

Figure 10 presents a scanning electron microscope (SEM) cross-section of D2W hybrid bonding, showing well-bonded vias at a pitch of 2 µm with 0.8 µm diameter vias.

Figure 11 showcases characterization data for W2W hybrid bonding, including optical inspection, transmission electron microscopy (TEM) analysis of copper/dielectric boundaries, and cross-sectional imaging of bonding interfaces.

Conclusion

Hybrid bonding technology has significantly advanced semiconductor packaging, especially suited for AI applications requiring high-bandwidth I/O and 3D logic/memory multi-layer stacking. Achieving 2 µm pitch D2W and 2.5 µm pitch W2W hybrid bonding demonstrates the technology's capability to meet future thermal and electrical requirements for high-power logic/memory stacking applications.

Reference

[1] K. Sakuma et al., "D2W and W2W Hybrid bonding system with below 2.5 micron pitch for 3D chiplet AI applications," in 2024 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2024.