Page 38 - Chip Scale Review_January February_2023-digital

penetrates the couplant and then reaches the
sensor. The AE sensor converts the dynamic
surface-motion, which is caused by the elastic
wave, into an electrical (AC) voltage signal.
The AE signal is subsequently amplified
and commonly filtered by the preamplifier
and transmitted to the AE system where
the signal processing takes place. Finally,
the generated data set is transmitted to a
computer where it is stored, evaluated, and
displayed [2; p. 81].
A specially developed and patented sensor-
indenter system (see Figure 5a) [4] is able to
precisely generate cracks in brittle isolation
layers of thin-layer pad stacks of all kinds of
chip designs (see Figure 5b) and measure the
released acoustic waves by using a diamond
Figure 3: Stress in a layer stack during contacting causing oxide cracks. tip indenter similar to the probes of a probe
card in production. Over a certain period of
probe tip is causing a high mechanical Pad stack crack assessment time, the contact force is increased, held, and
stress that can generate an oxide crack The acoustic emission (AE) test method is then unloaded (see Figure 5d). During this
if the material-dependent fractural one of the most efficient techniques used for process, AE burst signals, so-called hits, can
stress is reached. In the case of a pure the non-destructive testing (NDT) of material be visualized in the time- and/or frequency-
vertical force vector of the probe, the characterization. It can detect, localize, domains (see Figure 5c).
contact-related mechanical stress in the and monitor material defects without any The hits are triggered above the noise
thin layers during probing is mostly destruction of the sample. For this purpose, threshold voltage, recorded, and plotted
compressive within the contact area. AE sensors are glued or pressed to the surface synchronized with the contact force as a
Figure 3a shows a half-symmetrical of the body of the sample to detect elastic function of the time in order to identify
finite-element model (FEM) with a acoustic shock waves that are expanding signals that are classified as cracks or other
simulated contour plot of the first spherically from the location of an acoustic acoustic events like plastic deformation or
principle stress for a flat cylindrical tip event like a crack or plastic deformation. friction. For a better statistically-proven data
contacting a multi-layer stack with an This advanced method of AE crack detection analysis, the contact cycle is repeated from
Al-Cu pad on top of a silicon oxide layer during probing has been developed by Dr.- 100 up to 1,000 times and the recorded AE
(SiO 2 ) and additional thin layers below. Ing. Marianne Unterreitmeier during her data are cumulated and saved chronologically
For a flat tip of 10µm diameter and at a PhD work [2] at Infineon Technologies, and in an ASCII file.
maximum contact force of 300mN, the is based on state-of-the-art AE testing. The
highest stress values are approximately methodology has been modified in a way
±8.02N/m² (=8.02GPa) and are located to apply it for the qualification of the wafer CMOS BEOL design options A and B
below the center of the indenter tip in the probing process, in order to detect low- To demonstrate the advantage of the
region of the Si 3 N 4 layer below the upper energetic acoustic signals of brittle layer new method for determining the crack
SiO 2 layer. cracks in nanometer dimensions. probability during probing, we present an
With increasing contact force, the SiO 2 Figure 4 shows a schematic view of application example using a CMOS chip
layer is elastically deflected downwards the AE measurement chain. An AE event with an evaluation of different pad stack
at the same time the Al-Cu pad metal releases an elastic wave propagating within design variants. The objective of this study,
and the Cu layer below are plastically the solid body. After the induced AE wave which was presented at SWTest conference
deformed under vertical load. This has reached the location of the AE sensor, it 2022 by Dr.-Ing. Oliver Nagler [5], was to
creates a U-shaped deformation of the
uppermost layers in the region of the
tip contact area. The upper SiO 2 layer
and the Si 3 N 4 layer of the structure are
subjected to alternating tension and
compression during the indentation. The
Cu layer and the area below the indenter
tip are predominantly under compressive
st ress i n t he ve r t ical d i re ct ion.
Immediately beneath the center of the tip
is a region of high stress concentration.
The optical inspections (see Figure 3b)
of the probe mark and oxide cracks are
highly correlated with the simulation
results [4]. Figure 4: Schematic view of AE measurement chain [2].

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36 Chip Scale Review January • February • 2023 [ChipScaleReview.com]