Page 28 - ChipScale_Mar-Apr_2020-digital

Figure 5: Copper thickness in the area of fracture for deposits obtained a) without an adhesion promoter, and b) with AP2. The insets show SEM micrographs of the area of fracture.
terms of adhesion and ductility. As promoter. The copper in the
expected from the considerations above, long delamination zone of
a correlation between the ductility of the sample (a) is thinned out and
system and the adhesion between the exhibits significant necking in
individual materials could be observed an area between the onset of
(Figure 4). The peel strength was used delamination and the fracture
to characterize the adhesion and could point. In contrast, sample (b)
be obtained by measuring the required exhibits much more uniform
force per unit area to separate two bonded copper thickness close to and
materials, whereat the angle of separation behind the delamination point.
was 180°. Adhesion promoter one (AP1) These observations indicate
resulted in a decrease of the adhesion that the beneficial effect of
between the materials accompanied by increased adhesion on the
a decrease of the ductility. In contrast, a composite ductility is caused
strong increase of adhesion and ductility by suppression of the necking
of the stack could be obtained with instability in the copper. As
adhesion promoter two (AP2). soon as the adhesion fails at
Scanning electron microscopy (SEM) any location, this effect is
cross sections were employed to generate lost, leaving the copper free
a mechanistic understanding of the to undergo necking in the
improvement of the mechanical properties delaminated zone.
upon formation of the composite. F i g u r e 6 s ho w s S E M
Figure 5 shows the failure mode of i m a g e s a t c o m p a r a b l e
Cu-dielectric stacks in tensile test scales of focused ion beam
experiments in which the copper-polymer (FIB) cross sections of the
interface was either left untreated (a), untreated interface between
or treated with AP2 (b). In both cases, electrochemical deposition
the interface underwent delamination (ECD) of copper and dielectric
around the fracture point and the (Figure 6a), the interface
shape of the metal tip shows ductile treated with AP2 (Figure 6b),
failure of the copper. Additionally, both and for comparison, a Cu-
samples exhibited significant curling dielectric interface treated
after failure, indicating that the copper with a roughening adhesion
layer was permanently elongated by promoter (Figure 6c). The
plastic deformation, while the more non-roughening ad hesion
elastic polyimide reverted to its initial promoter works by inserting
dimensions. At the untreated interface, a thin, nonconductive layer
the delamination zone is significantly b e t we e n t h e c o p p e r a n d Figure 6: SEM micrographs of high-purity copper a) without
larger than at the interface with adhesion polymer. Because it is non- adhesion promoter, and b) treated with AP2, as well as c) copper
treated with a roughening adhesion promoter for comparison.

26 Chip Scale Review March • April • 2020 [ChipScaleReview.com]
26