Page 29 - Chip Scale Review_September-October_2023-digital

to reach an equilibrium state with the containment is to prevent liquid overflow Therefore, if the boundary is created by
lowest surface energy. This mechanism outside the bonding area, which would lithographic means, the final alignment
results in self alignment of the die with cause misalignment. Containment can will depend on the capabilities of the
its bonding site. Water was chosen as be achieved through physical means lithographic process
the working medium because of its high and/or chemical surface contrast. After self alignment, the water
surface tension, which produces strong Physical contrast is achieved by creating evaporates and direct bonding occurs
capillary forces that can be harnessed a border around the bonding site, due to the spontaneous adhesion of
to drive self alignment. Moreover, whereas chemical contrast is obtained the specially-prepared die and wafer
water is environmentally friendly and by depositing a hydrophobic material su r faces, w it hout requ i r i ng a ny
its evaporation is easily controlled in a around the periphery of the hydrophilic intermediate layer. For good bonding
monitored environment. Finally, water bonding area. Water containment is quality, both the die and wafer must
has the additional advantage of being maximized by combining both chemical meet strict bow, nanotopography and
compatible with direct bonding and is and physical contrasts, resulting in the surface roughness requirements. Any
already used in hydrophilic bonding best possible self-alignment outcome organic or particulate contamination
mechanisms. (Figures 2 and 3). The extent of of the bonding surfaces may impede
To ensure self alignment, good alignment is linked to the accuracy of bonding. Those parameters, therefore,
containment of water on the bonding site the step and by the definition of the must be critically controlled to ensure
is critical (Figure 1). The aim of this hydrophilic/hydrophobic boundary. good quality bonding.

Collective vs. direct-placement
P&P
To d ay, d i r e c t- pla c e me nt D2W
relies on robotics and optics for the
die handling, alignment and bonding
steps. A major advantage of our self-
assembly process is that it avoids the
need for mechanical movements during
the alignment and bonding steps by
exploiting the physical phenomenon
of surface tension, provided by the
water droplet, to induce self alignment
Figure 2: Water contact angle of hydrophilic bonding site and hydrophobic surrounding area. and bonding with the wafer (Table 1).
Precise alignment is, therefore, achieved
at a high-throughput rate. The handling
step can, therefore, be faster because
it requires lower accuracy (<200µm)
compared to direct-placement P&P.
Consequently, the estimated throughput
for the overall process should reach
more than 2,000 dies per hour.
Another possibility consists in using
the direct-placement D2W process for
collective assembly (Figure 4). All
dies, therefore, can be positioned on
a holder or a tape with low placement
accuracy and at high speed. Then, the
dies can be prepared as a group and
placed roughly opposite their bonding
sites. A technique is then necessary to
release the dies onto the water droplets.
The water droplet completes the fine
alignment of the dies to less than 400nm
Figure 3: Hydrophilic bonding pad surrounded by hydrophobic area for water containment. 3σ. This collective approach to assembly
could drastically increase throughput.

A short history of self-assembly
development
Self-assembly processes have been
investigated by many groups in centers
all over the world, such as: Tohoku
Table 1: Impact of self-assembly on the die to wafer process.

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