17 Interconnect Quality and Reliability of 3D Packaging
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17.5.4.2 Electromigration in TSVs
Early reliability studies of TSV started during the last decade with thermo-mechanical
simulations, since the thermal expansion mismatch between Cu TSV and Si die is
very large. Several studies evaluated, through finite element simulations, fatigue of
Cu TSV, delamination at TSV interfaces, and Si cracking due to thermo-mechanical
stress. These studies point out that the existence of the Cu TSV affects the stress distributions and the interconnection reliability [54–56]. For EM, most studies focus on
matter flux diffusion modeling at TSV interfaces, but there is no sufficient emphasis
on experimental results [57–59].
Frank and Moreau [60–62] and their group did EM experimental analysis of a
high density Cu TSV-last technology of 2–4 μm diameters, and 15 μm of length.
EM induced voids were found in the adjacent metal level, right at TSV interface.
Moreover, the expected lifetime benefit by increasing line thickness did not occur due
to increasing dispersion of voiding mechanism. They also reported EM experimental
reliability of Cu TSV-middle technology of 10 μm diameter and 80 μm thickness. EM
revealed similar degradation mechanism and kinetics as those in TSV-last approach.
Figure 17.32 shows the FIB cross-sectional image of the failure in both the thin and
thick process on TSV-last approach.
For thin metal process, current density exponent n and activation energy E A of
Black’s equation are extracted through MTTF data. Very close values are obtained
for both configurations. E A = 0.9 ± 0.1 eV is the expected activation energy for
surface diffusion of Cu on Cu damascene surfaces, ensuring by SiN capping. n =
2 ± 0.2 is acceptable regarding the theoretical value of 2 to be expected. Yet it might
be affected by local Joule heating in the TiN barrier as void becomes larger than
the TSV section. As failure is revealed to be driven by void growth, any ageing
extrapolations should be made with a consistent but conservative n = 1.
For thick metal process, activation energy and current exponent for down-stream
configuration are respectively of 0.8 ± 0.2 eV and n = 2.4 ± 0.3, which are close to
values of the thin metal level process, but it is less accurate due to the difference in
lognormal standard deviation between MTTF distributions.
With the same E A = 0.9 eV, and a conservative n = 1, the MTTF distributions
of both thin and thick metallization processes are extrapolated at same operation
conditions of current and temperature. At 50% cumulative failure, the benefit of
thick metallization process is about 6 times higher than thin metallization process.
However, since lifetime is always extrapolated at 0.1% cumulative failure, thicker
process has at this cumulative failure no significant benefit (only 1.1 times thin
process).
Another group from Samsung [63] also found the similar EM results. Electromigration induced voids were found at Cu/SiN interface between TSV bottom and
backside metal, but not at TSV itself due to thee unexpectedly strong reliability of
TSV.
559
17.5.4.2 Electromigration in TSVs
Early reliability studies of TSV started during the last decade with thermo-mechanical
simulations, since the thermal expansion mismatch between Cu TSV and Si die is
very large. Several studies evaluated, through finite element simulations, fatigue of
Cu TSV, delamination at TSV interfaces, and Si cracking due to thermo-mechanical
stress. These studies point out that the existence of the Cu TSV affects the stress distributions and the interconnection reliability [54–56]. For EM, most studies focus on
matter flux diffusion modeling at TSV interfaces, but there is no sufficient emphasis
on experimental results [57–59].
Frank and Moreau [60–62] and their group did EM experimental analysis of a
high density Cu TSV-last technology of 2–4 μm diameters, and 15 μm of length.
EM induced voids were found in the adjacent metal level, right at TSV interface.
Moreover, the expected lifetime benefit by increasing line thickness did not occur due
to increasing dispersion of voiding mechanism. They also reported EM experimental
reliability of Cu TSV-middle technology of 10 μm diameter and 80 μm thickness. EM
revealed similar degradation mechanism and kinetics as those in TSV-last approach.
Figure 17.32 shows the FIB cross-sectional image of the failure in both the thin and
thick process on TSV-last approach.
For thin metal process, current density exponent n and activation energy E A of
Black’s equation are extracted through MTTF data. Very close values are obtained
for both configurations. E A = 0.9 ± 0.1 eV is the expected activation energy for
surface diffusion of Cu on Cu damascene surfaces, ensuring by SiN capping. n =
2 ± 0.2 is acceptable regarding the theoretical value of 2 to be expected. Yet it might
be affected by local Joule heating in the TiN barrier as void becomes larger than
the TSV section. As failure is revealed to be driven by void growth, any ageing
extrapolations should be made with a consistent but conservative n = 1.
For thick metal process, activation energy and current exponent for down-stream
configuration are respectively of 0.8 ± 0.2 eV and n = 2.4 ± 0.3, which are close to
values of the thin metal level process, but it is less accurate due to the difference in
lognormal standard deviation between MTTF distributions.
With the same E A = 0.9 eV, and a conservative n = 1, the MTTF distributions
of both thin and thick metallization processes are extrapolated at same operation
conditions of current and temperature. At 50% cumulative failure, the benefit of
thick metallization process is about 6 times higher than thin metallization process.
However, since lifetime is always extrapolated at 0.1% cumulative failure, thicker
process has at this cumulative failure no significant benefit (only 1.1 times thin
process).
Another group from Samsung [63] also found the similar EM results. Electromigration induced voids were found at Cu/SiN interface between TSV bottom and
backside metal, but not at TSV itself due to thee unexpectedly strong reliability of
TSV.