Disruption in 2.5D/3D Packaging: Hybrid Bonding Rising as New Cornerstone

The surge in AIGC and new technologies such IoT, AI, 5G, AR/VR are driving a huge demand for computational power of high-end chips. This has been even outpacing the performance increase offered by the long-standing Moore’s Law, ushering in a “post-Moore” era where revolutions in advanced chip design are crucial.

Over recent years, chiplet design has seemingly become the mainstream approach for upgrading high-end chips. The concept is to allow more transistors on a single chip, effectively increasing the production yield of high-end chips while reducing overall costs.

By the large, major IC players have all jumped on board. Even Apple has joined the game by releasing their M1 Ultra SoC using the chiplet concept, which doubles computational performance by integrating two M1 Max units in a single chip.

The CPU sector is definitely a clear demonstration of this trend:

• AMD took the leap with chiplet design in their 2nd-gen EPYC CPUs, doubling the computing cores from 32 to 64 within two years, while slashing costs by up to half. The company has extended this approach to their 4th-gen EPYC CPUs and even pioneered the GPU Navi 31, the first of its kind to use chiplets.
• Intel started incorporating chiplets into their Lakefield series SoC in 2020. Looking ahead, their upcoming CPUs like the Meteor Lake set for 2023, and Arrow Lake and Lunar Lake scheduled for 2024, will all use chiplet design.

Transition from Bumping to Hybrid Bonding

Our analysis in “Chiplet Design: A Real Game-Changer for Substrates” laid out the comprehensive impact of the evolution of chiplet technology on substrates. In fact, chiplets have already caused a significant disruption to the most advanced semiconductor packaging technologies, necessitating the transition towards advanced 2.5D and 3D packaging technologies.

The bottleneck of advanced packaging lies in the chiplets’ interconnections, with bump and microbump still being the key technology for linking chips and forming I/O joints. These connection densities are hard to enhance, thus limiting the overall chip’s transmission speed. In addition, the more chiplets being stacked, the bigger the chip volume gets. The challenge is how to limit the chip size within a specific range, considering the current technical constraints.

Therefore, copper-to-copper hybrid bonding, also known as DBI (Direct Bond Interconnect), has been emerging as the key technology route that overcomes major hurdles in chiplet integration from the bottom-up.

Unlike bumping technology, hybrid bonding significantly shrinks the I/O joint space. The future transmission demand requires the I/O joint space between chiplets to be less than 10µm. While bumping is limited to around 20µm, hybrid bonding can take this down to an impressive 1µm or even less. This also means more I/O joints can be fitted in the same chip size – even reaching up to millions on a mere 1cm2 chip.

On top of this, hybrid bonding only adds an extra 1-2µm of thickness, compared to the 10-30µm of microbump, thereby helping reduce the thickness of stacked chips.

To put it simply, hybrid bonding can boost transmission efficiency, minimize energy usage with higher density of copper joints, manage chip volume, and even cut down on material costs.

The Race for Advanced Packaging Is Kicking Off

Moving forward, hybrid bonding is set to become the key technology supporting the continuous development of chiplet design and 3D packaging. This has been exemplified by TSMC’s front-end So IC packaging technology which is based on hybrid bonding. This puts AMD, a key customer of TSMC, in a favorable position to get ahead.

From AMD’s roadmap of 3D V-Cache technology, they have stacked SRAM on top of CCX (CPU Complex), and gradually integrated it into Milan-X series, the EPYC server CPUs, and Ryzen series, the consumer-grade CPUs, over the past two years. This has significantly improved performance and power consumption as a whole.

Not to be outdone, this year Intel also launched their Foveros Direct packaging technology, which is also based on hybrid bonding route. Assuming everything proceeds smoothly, we can anticipate the release of CPUs utilizing Foveros Direct technology by 2024.

As we look at the current products, AMD’s hybrid bonding apparently focuses on stacking SRAM and computing units at the moment. However, as CPU leaders deepen their understanding of this technology, the application field is expected to further expand. In other words, the future of hybrid bonding solutions stacking multiple computation units is just around the corner.