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Die-to-wafer hybrid bonding development for HVM

By Jonathan Abdilla [BE Semiconductor Industries N. V.], Guan Huei See [Applied Materials, Inc.]

T his article addresses the key back end of line (BEOL) inorganic dielectric with the release of excessive water (H 2 O)

requirements for a successful
issues noted above, as well as adding other
die-to-wafer (D2W) hybrid and Cu, and is expected to improve over the molecules, as well as metal-metal diffusion
that will enable the electrical connection.
bonding process. The selected process benefits. Overall, the preparation needs for A successful bond can be achieved
steps from bonding pad formation to grind die bonding are simplified, Cu bump and with careful surface engineering of the
and singulation, as well as an integrated underfill processes are removed, and TCB dielectric, typically with dielectric surface
D2W bonding process will be addressed, is replaced with room temperature HB. This roughness values of <0.4nm [3]. Of the
providing key technical values for the directly translates into reduction of vertical two HB flavors, wafer-to-wafer (W2W) or
various steps involved. The integrated form factor and scaling to higher input/ D2W, the latter enables higher system-level
bonding processes include wet cleaning, output (I/O) density, thereby leveraging the yield thanks to use of known-good-dies
degassing and plasma surface treatment. mature BEOL that scaled well below the sub- only. This paper describes the necessary
Successful Cu-Cu diffusion through grain micron range. The change of pad material conditions to achieve a successful D2W HB
growth across the boundary interface will from a complex alloy (Cu-Ni-Sn-Ag) to pure suitable for high-volume manufacturing
show the efficacy of such a tool, C-mode Cu and the direct connection through HB (HVM) production.
scanning acoustic microscopy (C-SAM) at room temperature are expected to offer
results will address the topic of voids, and shorter interconnect lengths, lower resistance PVD-CMP co-optimization
electrical yield results will also be presented. and improvement in thermal diffusion. for D2W HB
Actual placement accuracy results will also As a result, an increase in system-level As elegant as it looks, HB also has many
be shared for both collective and direct D2W performance with better bandwidth and/or challenges. The bonding requirements
hybrid bonding. speed can be expected. that work for W2W HB, e.g., the Cu
HB is achieved as a two-step process: chemical mechanical polishing (CMP)
Introduction first, by leveraging on the initial forces of process that ensures good roughness
The adoption of 3D architectures in surface interaction at atomic proximity of (<0.4nm), dishing (<5nm) and erosion,
advanced packaging between chips is driven the dielectric-dielectric interface to form are not directly transferable to D2W. A
by high-performance computing (HPC) the “tacking,” which is the initiation of the D2W test vehicle and bonding flow was
and artificial intelligence (AI) requirements bonding, and finally, following up with a developed based on the illustration of the
[1, 2]. Flip chip has been the main technology fusion process by annealing at an elevated key process sequence shown in Figure 1.
of forming the die attachment in packaging. temperature (100°C-400°C) to form both Previous work done on W2W test vehicles
It requires a copper (Cu) pillar made out of a stronger dielectric-dielectric covalent bonds successfully demonstrated thick barrier and
metal alloy, namely copper-nickel-tin-silver
(Cu-Ni-Sn-Ag) that occupies an opening with
a critical dimension (CD) of 20-40µm on a
passivation polymer dielectric (e.g., polymide
or polybenzoxazole [PBO]) for each of the
contact points. Such a thickness is needed to
allow underfill material to flow reliably and to
act as a stress buffer for mechanical integrity.
The flip-chip attach is done through thermal
compression bonding (TCB) where at least
one of the dies or substrates is heated to
ensure that the Cu bump reaches the eutectic
state so that Sn-Ag can form a good electrical
contact. With this bonding technique, some
amount of inter-metallic compound that
is causing higher resistivity and weaker
reliability is inevitably generated.
The migration toward Cu hybrid bonding
(HB) will only require the use of standard
Figure 1: Selected process steps from bonding pad formation to grind and singulation.

6 6 Chip Scale Review November • December • 2023 [ChipScaleReview.com]