84
P. Kumar et al.
Fig. 4.11 Distribution of the
von Mises stress in a
Cu-TSV structure from a
linear elastic mechanical
model with Cu grain
microstructure [45]
Fig. 4.12 Examples of Cu pumping. (a) Uniform pumping, showing a cross-sectional image displaying curvature of the top of a TSV, resulting in deformation of the RDL structure above [50];
(b) non-uniform pumping showing a few Cu grains protruding out of the Si surface [54]. The sample
in (a) was annealed at 400 °C for 60 min, and the sample in (b) was annealed at 425 °C for 90 min
part of the via, with strain concentrations adjacent to the interface near the TSV-end
(Fig. 4.13b, c). This accrual of plastic and creep strain at the top of the TSV leads to
copper pumping, or protrusion of the Cu from the top. This is particularly acute for
blind vias, where any expansion of the Cu must necessarily occur at the open end of
the via.
EBSD studies show that Cu grains in a TSV typically have random grain boundary
misorientations, and the via has little crystallographic texture, and no correlation
between orientation and extruded grains has been noted [44]. This is evident from
Fig. 4.14, which clearly shows a random texture in the TSV. It has been suggested
that the protrusion observed is uniform when the grain size near the TSV ends is