Page 10 - Chip Scale Review_November-December_2023-digital

is free of organic residue and without
excessive oxidation (Figure 5e-h).
Cleanliness is paramount to the
performance of the HB. Any particle
on the bonding surface can lead to poor
adhesion, weak bonds, or complete failure
of the bond (Figure 6). To ensure the best
bonding performance, free particle control
is a high priority for any pre-treatment
system design. Particles should be
controlled to meet the device specification
in the activation chambers. In addition,
a highly efficient wet clean process is
essential to ensure the cleanliness before
the wafer and the component dies enter the
bonders. Efficient cleaning is particularly
challenging for diced wafers on flexible
tapes because the dicing processes could
Figure 4: Plot showing the plasma activation effect on the initial contact angle on an SiO 2 surface and the introduce additional particles or/and
queue time effect on the contact angle and bonding performance after activation. Bonding degrades with contaminants. Insufficient post-dicing
excessive queue time between activation and bonding as shown in the C-SAM images of the delamination
around the die edges. cleaning could leave particles on the tape or
die sidewalls (Figure 7a-b). In some cases,
process. Macroscopic observations of the chemistry is essential for sufficient further cleaning, if not done correctly, can
hydrophilicity of the SiO 2 surface are seen activation without causing physical or create more particles when stirred up from
with water contact angle measurements. chemical damage to other materials, the tape or die sidewalls and land on die
When the SiO 2 dielectric surfaces are such as Cu and plastic tapes. As shown surfaces, causing delamination near die
treated with the right plasma conditions, in Figure 5, when the ion energy is too edges (Figure 7c-f).
superhydrophilicity with a contact angle high, the dielectric surfaces are roughened, After the whole integration, bonding
of <5º can be achieved (Figure 4). It is which creates voids and diminishes the process optimization and alignment
also shown that the contact angle degrades bonding performance. In addition, the were done, a 300mm substrate wafer
over time if the wafers are exposed to sputtered materials from the dielectric was bonded (~230 dies) to validate the
air after activation. The degradation or surface and organic adhesive can further bonding performance. Figure 8a shows
the hydrophilicity leads to poor bonding redeposit on the die surface under strong the post-bond C-mode scanning electron
performance, as shown in delamination at plasma conditions. The redeposition microscopy (C-SAM) result where no gross
the die edges from C-SAM analysis (see creates an undesired Cu diffusion barrier random void was observed, indicating
insets of Figure 4). Therefore, controlling during the post-bonding annealing stage. high cleanliness efficiency. This is further
the queue time between activation and Therefore, it is important to optimize the confirmed with an electrical continuity test
bonding is crucial to achieving the best plasma activation conditions to achieve on a 10,000-daisy chain (DC) connectivity
bonding performance. surface roughness of <0.5nm and etching occupying an area of 1mm x 1mm (of
Proper selection of plasma conditions, of SiO 2 of <1nm while preserving the Cu a 6mm x 6mm die size) shown in the
including ion energy, density, and dishing profile and ensuring the surface Figure 8b wafer contour plot and in the

Figure 5: Surface and topography comparison between strong and soft plasma-treated patterned samples: a–d) TEM cross-sectional study of the Cu pad and dielectric
oxide surface, revealing surface contamination from redeposition caused by a strong plasma; e–h) Atomic force microscope measurements showing that a strong plasma
alters Cu pad profile, and an increased dishing amount.

8 8 Chip Scale Review November • December • 2023 [ChipScaleReview.com]