Page 8 - Chip Scale Review Sep Oct_2022-digital

Direct bonding: a key enabler for 3D technologies

By Emilie Bourjot, Frank Fournel [CEA-Leti]

D irect bonding techniques (SiP) and 3D integration schemes enable Direct bonding processes

are constantly in evolution
Among bonding techniques, direct
and/or substrate materials together,
to address “More Moore” the possibility to mix different technologies bonding is of particular interest for
and “More than Moore” challenges. CEA- addressing not only high-performance 3D applications. Direct bonding is a
Leti has developed expertise in direct computing (HPC) applications requiring spontaneous bonding of two surfaces
bonding since the ‘90s with the emergence high-density interconnections, but also without liquid adhesive material. Among
of silicon-on-insulator (SOI) technology. cost-sensitive applications (e.g., edge the different direct bonding technologies,
Since then, CEA-Leti teams have continued artificial intelligence and Internet of molecular bonding – which implies
to actively innovate in direct bonding Things [IoT]). Breakthrough innovations hydrophilic surfaces – is the most
to widen the application field. This are necessary to meet all specifications in common one. When two hydrophilic
technique is based on the cohesion of two terms of substrate materials and/or high- surfaces are intimately in contact at
surfaces put intimately in contact at room density interconnections. Direct bonding room temperature, van der Waals forces,
temperature. Then, van der Waals forces represents a group of technologies that hydrogen bonds and capillarity bridges
(hydrogen bonds) and capillarity bridges enable the bonding of two substrates create the needed adhesion energy to start
create the needed adhesion energy. A post- vertically. This attribute matches perfectly and propagate this spontaneous bonding.
bonding annealing changes weak bonds with 3D requirements because it can be Just after the bonding, even if some
into covalent bonds to make one piece of applied to high-density interconnections covalent bondings are already present
material at the end. Direct bonding now through hybrid bonding and can mix at room temperature, a post-bonding
addresses not only substrate fabrication, but different materials with tiling. After a brief annealing is necessary to increase their
also the 3D interconnections domain with introduction on our expertise in direct density. The adherence energy, which
the emergence of hybrid bonding. bonding processes, a discussion of state- is also called “the bonding energy,”
This article presents the different direct of-the-art processes for hybrid bonding increases during the annealing using
bonding techniques and their application and heterogeneous III-V D2W bonding is different physical-chemical mechanisms
in the microelectronics industry and presented. Finally, we highlight the latest depending on the joining materials.
R&D as developed by CEA-Leti. In the hybrid bonding D2W improvements and Definitively, the direct bonding
first part of the article, direct bonding the critical role of TSVs in 3D integration. phenomenon is different from thermo-
physics are succinctly presented. Then, compression and adhesive bonding, which
a summary of state-of-the-art bonding More than 30 years of expertise in require, respectively, both temperature and
technologies is depicted, including hybrid bonding pressure, or an additional material such as
bonding wafer-to-wafer (WTW), hybrid CEA-Leti has developed deep expertise polymer to ensure contact between the two
bonding die-to-wafer (DTW) and III-V in direct bonding since the ‘90s with the surfaces. Direct bonding is a spontaneous
heterogeneous bonding. Advantages, emergence of SOI technology. From first bonding and its thermal evolution does
challenges, applications and stakes of each lab work on oxide/silicon bonding, the not require an external load. It is a self-
technique are compared with respect to technique was developed and maturated made bonding. Nevertheless, the price for
the suitable application domain. A third at CEA-Leti after the Unibond SOI wafer this phenomenon is the stringent surface
®
part is focused on the latest hybrid bonding was obtained. After more than 30 years, we requirements of topography, planarity,
D2W results presented by CEA-Leti at have developed an international reputation roughness, and particulate contamination
ECTC 2022 and ESTC 2022. Integration in bonding development, including direct, responsible for bonding defects. Surface
challenges are discussed as well as the role polymer, thermocompression, eutectic cleanliness and topography are critical to
of dedicated equipment development. The and anodic bonding. Our role is to have a successful direct bond.
last section presents the potential markets maturate bonding processes from proof- Obv iou sly, t he d i rect bond i ng
and associated products, with an example of-concept to industrialization. More than phenomenon is not limited to hydrophilic
of a chiplet with through-silicon vias (TSVs) 50 scientists and engineers are developing surfaces. Hydrophobic surfaces can also
and multi-layer stacking. new solutions on direct bonding processes be bonded where only weak van der
and process integration to products Waals forces are involved for adhesion.
Introduction and we have more than 135 patents in The stakes are to manage all surface
The digitalization of the world is this field. As such, wide knowledge on characteristics to be compatible with the
accelerating, which brings new useful bonding mechanisms was built especially bond type involved. Therefore, in-depth
services that increase the need for in direct bonding, on which fundamental knowledge of bonding physics is required
electronic components through a wide mechanism models for adhesion [1] and to integrate the most suitable bonding
range of applications. System-in-package adherence [2] have been proposed. technique to the targeted application.

6 6 Chip Scale Review September • October • 2022 [ChipScaleReview.com]