w
w
w.
m
r
c
y.
c
o
m
WHITE
PAPER
TSV Composition
The basic structure of a TSV is comprised of a
copper (Cu) via/pillar with a very thin (typically
0.5-1 um) layer of silicon dioxide (SiO
2
) submerged
in silicon (Si). Figure 1 is an example of a passive
TSV interposer.
The Need For Silicon Dioxide (SiO
2
)
The SiO
2
layer acts as a DC-leakage barrier, as
the silicon effectively behaves like a conductor up
to its respective relaxation frequency (fe). At the
relaxation frequency the silicon transitions to an
effectively pure dielectric, as the dopants are no
longer able to respond.
The relaxation frequency of silicon can be calculated
by Equation (1).
Equation (1)
The dielectric relaxation frequency marks the end of the first three primary modes a TSV may exhibit, the slow-wave mode (SWM).
The SWM exists from ~DC-f
e
, it is induced by the very small wave impedance (|η|) the signal "sees" at the Si/SiO
2
boundary.
The wave impedance can be calculated by Equation (2).
Equation (2)
Since SiO
2
is effectively a pure dielectric, and the silicon behaves like a conductor at frequencies below the dielectric relaxation
frequency (f
e
), the vast majority of the signal's respective electric field (E) is "captured" in the SiO
2
layer. Further exacerbating the
SWM is the relatively thin (~0.5 um) SiO
2
layer thickness (t
sio
₂
). Due to this very small thickness, the signal experiences a very large
capacitance (C).
The capacitance can be roughly approximated by Equation (3).
Equation (3)
1
Figure 1. Example of a passive TSV interposer
Where:
•
C
= capacitance
• = ~3.9
•
Area = area of TSV Cu and Si/SiO
2
boundary (um
2
)
•
=
0.5
um (thickness of SiO
2
)
2
2
