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Figure 5: a) Patented sensor-indenter system; b) Crack detection by AE test method; c) Acoustic signal in time and frequency domains; and d) Typical contact cycle.
determine the crack probability during wafer
probing under conditions that occur during
manufacturing, and to identify the most
robust pad stack design in order to avoid
oxide cracks. The first chip variant (option A)
has a Cu-pad on top with a “checkerboard”
routing pattern in the upper oxide layer and
another metal fill in the oxide layer below.
In contrast, the second variant (option B) Figure 6: Pad stack options A and B.
has the same pad metal design, but a double
oxide layer without metal routing in the
upper-most isolation layers (Figure 6).
In order to assess the risk of generating
probing-related cracks, the tolerances of
the probe geometries are considered during
the study. Two different tip diameters, 5µm
and 10µm, were used for the contact cycles.
The tips are flat with a small edge radius
representing the typical dimensions and
geometry of a manufacturing probe card for
a similar range of contact force.
In total, 150 contact cycles with a
maximum contact force of 250mN for each
pad stack/tip combination were performed
to achieve nearly 100% crack probability.
During contacting, the AE signals were
cumulatively recorded and later graphically Figure 7: Scatter plot of clustered AE signals.
visualized in a scatter plot (Figure 7).
The burst signal energy in units of eu decreasing during penetration of the pad inspections. For a meaningful data
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(1eu=10 V s) is used here as a characteristic metal up to a critical contact force of around clustering of those hits, when “first
feature of an AE event. 100mN—meaning no AE crack events were oxide cracks” appear, it is important to
Figure 7 shows the burst signal energy observed. For a contact force higher than correctly set the filter limits of contact
in logarithmic scale as a function of contact 100mN, the burst signal energy of the hits is force and burst energy. Therefore, only
force for all hits derived during 150 contact increasing again. hits with energy higher than 5eu of each
cycles for pad option A and a tip diameter of In the case of a crack, the burst signal contact cycle, occurring at a certain
5µm. Here, one can see that the burst signal energy is assumed to be higher than contact force, are included and utilized
energies are high at the beginning and 5eu, which was correlated by optical for further data clustering. Hits with a

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