Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
C. Effect of copper vias
In the IMEC approach (figure 8), a 3D-die stack is built
by using through-silicon Cu vias (TSVs) and a polymer
material to fill the gap between the dies [4, 7, 9].
Interconnections between dies are made using the vias
(nails) which have electrical functionality but also a thermal
advantage since the copper thermal conductivity is three
orders of magnitude higher than the thermal conductivity of
the polymer glue.
Fig. 8. Right: schematic view of IMEC`s 3D-SIC chip stacking approach
with hybrid Cu/dielectric bonding. In this figure and in the study reported,
BCB has been assumed as the bonding dielectric. Left: SEM image of a TSV
in a IMEC`s 3D stack.
Fig. 6. Interaction of two hot spots located on the same level (die 3). (a) The
graph reports the temperature measured in the center of hotspot 1, when the
ratio (r) between the hotspots separation and their diameter is varied. (b)
Temperature as a function of the distance from the hotspot 1 center, for
different values of r.
The meaning of the interaction distance can be better
understood when looking at the flux plots in figure 7. For r =
7 the hot spots spreading angles - visualized through the heat
flux lines - almost do not intercept. On the other hand, an
interaction between the respective temperature fields start to
be visible when the power sources are so closely spaced (r =
3) that their spreading angles overlap.
Fig. 7. Heat flux emanating from two hot spots active in die 3, for two
significant values of r.
The presence of TSVs in the polymer layer alters its
thermal properties: a decrease in thermal resistance of the
polymer, and thus in the overall temperature, is expected in
this case. It has been previously demonstrated [6] that, for
the case of a homogeneous power dissipation in stacked dies,
the presence of copper bumps in the polymer accounts for
the major part of the reduction in the thermal resistance in a
stack. This suggests the use of dummy Cu studs in the
polymer as a thermal management aid.
In order to quantify the impact of dummy copper on the
stack temperature a series of FEM simulations have been
performed. We have used a 2D axisymmetric model of a 2.5
x 2.5 mm 2 wide structure with 5 μm diameter Cu studs
placed in the interface layer - BCB in this case (figure 9 a).
A package resistance R1 = 1.3 K/W has been considered for
all simulations.
At first, a parametric study has been performed in which
the polymer thickness has been varied while keeping the Cu
studs pitch fixed. Figure 9b reports the results and
demonstrates that the presence of Cu studs with a
sufficiently small pitch affects significantly the thermal
resistance of the polymer layer. The copper provides for an
effective heat conduction path (fig. 10), and reduces
considerably the overall stack temperature. The maximum
temperature in a stack with dummy Cu studs is found to be
almost independent on the polymer thicknesses when the
studs pitch is small, thus the copper thermal impact is more
significant for thicker BCB layers. A similar conclusion can
be drawn from figure 9c, where the copper impact as a
function of bumps pitch is explored for two different BCB
thicknesses.
©EDA Publishing/THERMINIC 2009 59
ISBN: 978-2-35500-010-2