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Challenges with self-assembly applied to die-to-wafer

hybrid bonding

By Emilie Bourjot, Frank Fournel, Pierre Montméat, Loic Sanchez, Thierry Enot [CEA-Leti]
T h e q u e s t f o r s m a l l e r in mass production especially for Today, direct placement D2W is

components with bet ter
performances at a competitive imaging applications. In this approach, performed by pick-and-place (P&P)
processes that make it difficult to
bonding is performed at wafer scale. With
cost has been the driving force for the latest generation of W2W bonders, reconcile high placement accuracy with
3D-IC (integrated circuit) technology suppliers claim alignment capabilities of high throughput. Indeed, to achieve
developments for several decades. less than 3σ. The main advantage of the placement accuracy of better than
Recent advances in bonding techniques technique is high throughput. However, 1µm, P&P tool throughput must be
have provided particular benefits for there is a drawback in terms of design lower than 1,000 dies per hour. Tool
applications such as memory, high- flexibility, which remains low because suppliers are examining solutions to
performance computing, and photonics, the bottom and top dies must have the increase this throughput, for example,
where performance is strongly linked same dimensions. by implementing multiple heads on
to the density of interconnections. In contrast to W2W bonding, hybrid P&P tools. However, breakthrough
For such applications, hybrid bonding D2W bonding should increase design D2W processes are needed to remain
emerged as a key booster to increase 3D flexibility. This process re-uses the competitive and to reach industrial
interconnection density. know-how developed for W2W, but requirements. In this context, CEA-
Hybrid bonding is similar to direct adds dicing and cleanliness challenges. Leti has been working for several years
bonding except that it is applied to When those challenges are managed, on the development of a self-assembly
mixed Cu/dielectric surfaces to create an it will pave the way for heterogeneous process. This promising process has
electrical interconnection between two 3D structures by creating the means to the potential to increase throughput to
parts. Direct bonding is quite distinct mix and match technologies on a single several thousand dies per hour thanks
from thermo-compression and adhesive bottom substrate with high-density to self alignment, and it is the subject of
bonding, which require temperature and interconnections. Moreover, the known- this article.
pressure, or additional materials such as good die (KGD) concept can be applied
polymer, respectively, to ensure contact to increase overall product yield by Self assembly driven by the
between the two surfaces. In contrast, selecting only good dies before assembly. capillary forces of water droplets
direct bonding is a spontaneous process, D2W technology is of great interest for The self-assembly process is based on
and no external loading is required many applications, such as edge devices the use of the capillary forces of droplets
during its thermal annealing step. because of the increasing need for real- of liquid to align a die to a target site on
However, direct bonding does have time computing of large amounts of data a wafer. Capillary forces are exerted as
stringent surface requirements in terms with a limited power budget. Other “More the liquid tries to minimize its surface
of topography, planarity, roughness, than Moore” applications are good energy by regulating its surface tension.
and particulate contamination. If candidates, e.g., photonics, imagers and From a macroscopic point of view, the
uncontrolled, irregularities in these displays, optical transceivers, and radio liquid tends to minimize its interfacial
characteristics can lead to bonding frequency (RF). contact area with the surrounding air
defects. The two critical parameters
for successful direct bonding are clean
surfaces and an appropriate topography.

Direct hybrid bonding: W2W and
D2W
Two assembly configurations are
possible with direct hybrid bonding:
wafer-to-wafer (W2W) and die-to-wafer
(D2W). W2W hybrid bonding was first
proposed by Dr. Suga in 2000, then
developed at CEA-Leti with hydrophilic
bonding in 2009 in a R&D environment,
and then Sony brought it to an industrial
level in 2016. It is now a mature process Figure 1: Water containment with both topographic and chemical contrast.

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