Page 22 - ChipScale_Nov-Dec_2020-digital

silicon vias (TSVs) and microbump pitches.
Implementing such fine-pitch microbumps
for multi-die stacking requires a careful
assembly process to prevent any damage to
the tiny microbumps. In this study, a glass
carrier is bonded to the bump side of the
wafer followed by mechanically debonding
the silicon carrier and mounting the front
side on tape. At the end of the assembly
process, the glass carrier is removed
with laser debonding, cleaning and
dicing (Figure 3).
A conventional mechanical debonding
process in this assembly will have a
negative impact on the microbumps
because this method can damage and
deform the bumps (Figures 4a-b) [3].
Therefore, a laser debonding scheme,
followed by solvent cleaning to remove
the adhesive, was chosen. In this process,
there was no damage on the bumps after
laser debonding. The measured electrical
yield of daisy chains and kelvins for
different pitches of microbumps together
with cross section scanning electron
microscope (SEM) images of the 7µm
pitch area are shown in Figure 4. It
can be seen that the electrical yield
is close to 100% for large pitches and
is around 50% for the 7µm pitch for
all three stacked interfaces. The SEM
image shown in Figure 4b shows good
thermocompression bonding alignment
and intermetallic compound formation
for the 7µm pitch structure at all three
die interfaces. This study demonstrated
that when using a glass carrier bonding
Figure 2: FOWLP flow and the full reconstructed substrate on tape before die-singulation. and laser debonding process, a good

As mentioned in the introduction, the damage or residues on the 200µm thin bonding performance for <10µm pitch
selectivity competition between a set of reconstituted substrate. microbumps can be obtained without
TBMs can be partially replaced by the damaging the microbumps.
introduction of a LRL layer for laser Die-to-wafer stacking via
debonding. We therefore opted to use a microbumps Collective die-to-wafer hybrid bonding
laser debonding scheme for the second High-density interconnects increase the For a detailed description on this
wafer flip operation. bandwidth of logic devices and the capacity process we refer to [1,4], but in short:
Because of the use of an LRL, TBM of memory stacks by decreasing through- this process aims to enable the vertical
2 can be chosen based on its adhesive stacking of devices built in different
properties, the capability to efficiently
strip it after laser debonding, and of
course its compatibility with the chosen
LRL. A TBM with a very high adhesion
to the LRL and the mold was chosen.
During a peel debond process, the
debonding of the second carrier with such
a TBM would be very difficult, certainly
with a mold on tape frame, which
typically suffers from poor adhesion.
The final result after laser debonding
and stripping of the remaining adhesive,
shown in Figure 2, does not indicate any
Figure 3: Assembly process of fine-pitch microbumps using a laser debonding process.

20
20 Chip Scale Review November • December • 2020 [ChipScaleReview.com]