Page 30 - Chip Scale Review_September-October_2023-digital

mechanisms. Therefore, it emerged
that a weakly hydrophilic surface with
a water contact angle (WCA) >60°
optimizes the collection and alignment
of the dies. With 8x8mm² dies meeting
this condition, excellent alignment yield
with the mean alignment of x = 46nm
and y = 132nm; 3σx = 439nm and 3σy
= 341nm, along with a high throughput
were obtained.
In comparison, currently available die
bonder P&P tools have achieved 1µm
3σ, and the next generation of die bonder
should reach 500nm 3σ. In addition to
its very good alignment capability, the
self-assembly process was demonstrated
to behave like a “collective” process.
After dicing, all dies were placed in a
Figure 4: Self-assembly bonding of dies with a water droplet for collective or direct placement D2W bonding. silicon holder for collective handling
during cleaning and surface preparation
University [1], Aalto University [2], and Improved alignment and higher before direct, simultaneous bonding of
imec [3]. Among them, they explored throughput dies to the target by self assembly. It
the very good alignment capability for The first outcome of our collaboration is interesting to note that the error in
dies with different shapes and sizes was the discussion on the key process placement of the die on the silicon holder
(millimeter to micron scales). However, parameters driving self alignment, was quantified at around +/-200µm,
integration with Cu has not been as presented at IEEE ECTC in 2022 resulting in a final misalignment of
extensively studied for hybrid bonding [7]. The water-dispensing technique 500nm 3σ. Based on these data, the self-
applications. Moreover, all of the and the surface preparation to tune assembly technique has a real potential to
reported results remain at the academic, the surface’s hydrophilicity emerged improve D2W processes in terms of both
proof-of-concept, level. as critical for the behavior of the self- throughput and alignment. Moreover, the
At CEA-Leti, initial investigations assembly process. After correctly self-assembly process was demonstrated
into this process were performed as managing the surface energies of both to be compatible with a wide range of
part of two PhD studies in 2010 and the hydrophilic and hydrophobic areas, die dimensions, since at least 70% of
2015 [4-5]. The main results were excellent alignment performances were assemblies were completed below 500nm
the demonstration of the alignment achieved on a homemade collective 3σ for the different die sizes tested
capability through experiments and self-assembly bonding bench. The (8x8mm², 2.7x2.7mm², 1.3x11.8mm² and
modeling, and the first successful degree of hydrophilicity of the bonding 2.2x11.8mm²) (Figure 5).
integration of Cu with a positive site receiving the droplet was found
ele c t r ic a l r e a d ou t . T h e s t u d ie s to play a major role in self-assembly
undertaken indicated that the process
could be viable as a proof-of-concept.
The method was subsequently matured
through continuous developments with
homemade tools, which were published
at IEEE ECTC 2019 [6].
I n 20 20 , C E A- L e t i a n d I n t e l
Components Research launched a
collaboration to evaluate the potential
of this technology from an industrial
perspective. The objectives were
to understand the key parameters
controlling self alignment and the
physics behind the process, but also
to compare the potential of this
technology to that of direct-placement
P&P. To ensure good reproducibility
of experiments and go a step further
toward industrialization, collaborations
with tool suppliers were engaged to
embed water dispensing on an industrial
die bonder. Figure 5: Self-assembly process applied to high aspect ratio dies.

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