Study of ageing of the metallization layer
of power semiconductor devices
S. Pietranico1&2, S. Pommier1, S. Lefebvre2, Z. Khatir3, S. Bontemps4, E. Cadel5
1
LMT-Cachan, 61, avenue du Président Wilson, F94235 Cachan,
2
SATIE, 61, avenue du Président Wilson, F94235 Cachan,
3
INRETS-LTN, 2 av. du Général Malleret-Joinville, F94114 Arcueil,
4
Microsemi PPG, 26 rue de Campilleau, F33520 Bruges.
5
GPM, UFR Sciences et Techniques, Av. l'Université - BP12, F76801 St Etienne du Rouvray
Abstract: In order to accelerate the ageing of the metallization layer of power semiconductor
TM
devices, repetitive short circuit operations are applied to COOLMOS Transistors. Regularly, during
repetition of short-circuit operations metallization is observed in a Scanning Electron Microscope which
allows us to observe the process of cracking at the grain boundaries of the metallization layer.
Different electrical characterizations were also done regularly and evolution of characterization results
are linked to the metallization ageing in order to better understand the ageing process of the
metallization layer and the effect of metallization layer ageing on the electrical performances.
I. INTRODUCTION
In the field of high temperature applications
(automotive,
aeronautics),
under
high
temperature swings, or under hard working
conditions (short-circuit or avalanche) ageing
of aluminum metallization layer on the dies
results in a process of aluminum reconstruction
which in terms can lead to device failure. The
ageing process of the metallization layer
results for the difference of coefficient of
thermal expansion between aluminum and
silicon and depends on temperature swing
value and on maximum temperature. We
propose in this paper to study the ageing
process of the metallization layer under
accelerated stress conditions (short-circuit
operations) for several short circuit energies
leading to different maximum temperatures in
the Aluminum layer.
In order to characterize the electrical
resistances of die metallization pad as well as
contact between bond wires and metallization,
Microsemi has realized a dedicated package.
TM
Several modules with 600 V COOLMOS
have been provided for this study. A four probe
contact design was chosen for the bond wire
connections (with judicious location of bond
contacts) in order to perform precise
measurement of the Al metallization layer. Also
metallization layer resistance is regularly
measured and other electrical parameters like
on-state
resistance,
threshold
voltage,
saturation current …
studied and different mode of failures were
identified [1-7]. Fig. 1 [7] depicts results
obtained during repetitive short circuit
operations in a previous study. It shows the
short-circuit robustness of 600 V NPT IGBT
transistors where a critical energy (EC) was
found. Ec is 0.81 J at TCASE = 25 °C and 0.62 J
at TCASE = 125 °C. Each point is the result of
repetitive tests on a given device, with on the
X-axis the dissipated energy and on the Y-axis
the number of short-circuit cycles until
destruction. The critical energy clearly defines
two failure modes depending whether the
short-circuit energy is greater or lower than this
singular value.
Fig. 1: Robustness in repetitive short-circuit
conditions of 600V NPT IGBT (effect of case
temperature)
II. TEST CONDITIONS
Similar results [8] have already been shown for
TM
COOLMOS transistors.
Behavior of power semiconductor devices
under short-circuit operations has been already
For short circuit energies below the critical
value (E < EC), failure occurs after a large
number of short circuits. These results show
that a cumulative damaging mechanism occurs
in devices ageing which leads to failure after at
4
least 10 short circuit cycles. In these
conditions, failures systematically appear at
turn-off when trying to switch off the shortcircuit current with a destruction phenomenon
which looks like dynamic latchup [6-8].
In a previous study [9] a power module
including IGBT dies was developed by
Microsemi in order to set-up test campaigns
allowing to evaluate the ageing of aluminium
metallization of emitter pad and bond wires
(especially electrical contact between wires
and metallization). It is well known that Al
reconstruction appears in the Al layer when it
is subjected to temperature cycles, especially
at high temperature. Due to the large
difference of coefficients of thermal expansion
between Al and Si, significant plastic
deformation occurs in the aluminium layer
leading to initiation of cracks and propagation
which ultimately results in severe degradations
[9-13].
In order to characterize the electrical
resistances of die metallization pad as well as
contact between bond wires and metallization,
a four probe contact design was chosen for the
bond wire connections (with judicious location
of bond contacts) in order to perform precise
measurement of the Al metallization layer.
Measurements
allowed
by
Microsemi
packaging have shown the effect of the
repetition of short circuit operations on the
increase of the resistivity of the Al layer. This is
explained by Al reconstruction and cracks of
the metal layer due to ageing. Strong
degradation
of
the
metallization
(Al
reconstruction and cracks) as well as bond
wire lift offs were observed on all tested
devices. In order to better understand the
ageing process, similar experiments are being
developed for this study this time with
TM
COOLMOS
transistors seeking to relate
precisely the mechanisms of degradation of
the
metallization
layer
with
electrical
characterization results (increase in the Al
sheet resistance). For that, metallization is
regularly observed in a Scanning Electron
Microscope which allows us to observe the
process of cracking at the grain boundaries at
different step of ageing and for different
experimental conditions (dissipated energy).
Fig. 2 shows the dedicated power module with
TM
800V COOLMOS
dies. Fig.3 shows the
experimental test circuit allowing repetition of
SC operations. A circuit breaker was used in
order to protect the die of the DUT (Device
Under Test) after failure.
TM
Fig. 2: Dedicated COOLMOS power module
realized by Microsemi
Fig. 3: Test Circuit
Fig.4 shows waveforms for a dissipated energy
larger than the critical value, when failure
appears during first short-circuit operation. The
current decrease during the first part of shortcircuit is explained by heating of the die which
results in the decrease of carrier mobility in the
channel. When dissipated energy is large
enough (higher than the critical value), thermal
runaway appears leading to device failure. For
the tested dies, critical energy is about 2,4 J at
25°C ambient temperature.
Fig. 4: Failure after thermal runaway
TM
In this paper, COOLMOS transistors will be
submitted to the repetition of two different
short-circuit tests corresponding to two
different energy values, 0,47 and 1,15 J,
significantly lower than the critical value (2,4J).
Fig. 5 shows waveforms for a dissipated
energy equal to 1,15 J. Temperature, which is
the main accelerating factor of the ageing of
metallization layer, unfortunately could not be
measured in this study and the results will be
presented only in terms of dissipated energy
per cycle. The differences in behavior to be
observed, however, are mainly related to
differences in temperatures reached at the end
of short-circuit operation.
Fig. 6 gives an example of measurement
methodology for Al layer resistance evaluation
where RAL_45 is the Al layer resistance between
wires 4 and 5 (see Fig.5), RC are the contact
resistance between Al layer and bond wire and
RW are the wires resistances
Fig.7 shows effect of repetition of short-circuit
operations in the case of the lowest dissipated
energy on the metallization I(V) characteristic.
The results allow to estimate value of RAL_45
and evolution of this sheet resistance during
ageing process.
III. RESULTS
Fig.5: Waveforms in the case of a dissipated energy
equal to 1,15 J (E = 400V, TC = 25°C)
III.1
Evolution
of
Aluminum
sheet
resistance during ageing
As mentioned above, several electrical
parameters have been monitored during
ageing tests in order to point out ageing
indicators. Contrary to what was observed in
previous study [6-9] during the repetition of
short-circuit cycles with higher energy (close to
the critical value) there is no evolution of the
on-state resistance (RDSON) and of the shortcircuit current like shown on figure 8 and 9.
Fig. 6: Example of part of Al layer measurement
(a)
Fig. 8: Current during repetition of short-circuit
operations (W = 0,47J, E = 300V)
(b)
Fig. 7: I(V) characteristic of the Al metallization
during ageing, W=0,47J/cycle (a), W=1,15J/cycle (b)
Fig. 9: Current during repetition of short-circuit
operations (W = 1,15J, E = 400V)
The only electrical parameter whose evolution
has been found is the Aluminum sheet
resistance. Fig. 10 depicts the evolution of the
relative Al metallization resistance during the
repetition of the short circuit operations for the
two test conditions, corresponding to
dissipated energy of 0,47 and 1,15 J/cycle.
After stopping the test, removing the dies from
the package and cleaning the dies, images
with a better quality have been obtained.
Fig. 13 and 14 compare the degradation of
metallization under different magnifications.
(a)
Fig.10: Evolution of relative Al sheet resistance
during repetition of short-circuit operations
During the first 5,000 short circuit operations
no significant evolution of the Al. sheet
resistance has been measured. After about
5,000 short circuits, the resistance of the Al
layer significantly and regularly increases, with
an increase strongly dependant on the
dissipated energy, of about 8 percent for a
dissipated energy equal to 0,47J/cycle and
about 700 percent in the case of a dissipated
energy equal to 1,15J/cycle.
III.2 SEM analysis
Regularly during short-circuit operation Al
metallization has been observed in a Scan
Electron Microscopy (Silicone gel was
previously
removed),
Fig 11
and
12
respectively for 0,47 J/cycle and 1,15 J/cycle.
After the first few thousand cycles, we clearly
observe after plastic deformation of the Al.
layer the initiation of fractures at grain
boundaries. However, the resistance rising
only very slightly during the first 5,000 cycles,
we can assume that the fractures are observed
as fracture initiation on the upper surface of the
metallization layer.
Then, we observe a very significant
degradation of the metallization layers for the
test to 1,15 J after 29000 short-circuits, while
the damages remain localized to the surface
and grain boundary for the test to lower
energy. However, the increase of resistance
tends to show that the fractures were
propagated in the volume of the metallization
layer during the repetition of cycles, and much
more significantly for cycles performed at high
energy than those made for lower dissipated
energy.
(b)
(c)
(d)
Fig.11: Ageing of the metallization layer at
0,47 J/cycle, (a) : initial state, after 7000 (b), 11000
(c) and 29000 (d) short-circuit operations
(a)
(a)
(b)
(b)
Fig. 13 : Metallization (magnification 8000×)
after 29000 cycles, 0,47J/cycles (a)
and 1,15J/cycle (b)
(a)
(c)
(b)
(d)
Fig.14 : Metallization (magnification 3000×)
after 29000 cycles, 0,47J/cycles (a)
and 1,15J/cycle (b)
(e)
Fig.12: Ageing of the metallization layer at
1,15 J/cycle, (a) : initial state, after 100 (b), 1000(c),
6000 (d), and 29000 (e) short-circuit operations
With high magnification, we clearly observe the
fracture between the contacts of source cells,
with a much greater ageing for the test with
higher energy
However, differences in degradations and
fractures appear more clearly with a lower
magnification. Images clearly show significant
fractures between contacts of source cells in
the case of tests with higher energy while only
initiations of fracture are visible in the case of a
lower dissipated energy.
These results have been confirmed (Fig. 14)
using Focus Ion Beam Microscopy performed
in the GPM laboratory. Sections were
positioned on fractures among the most
important.
resistance of this layer may explain some
failure modes. We will initially assess or
estimate temperatures achieved after each
cycle.
V. REFERENCES
(a)
(b)
Fig.15 : Fracture in the metallization layers after
29000 cycles, 0,47 J/cycles (a) and 1,15 J/cycle (b)
IV.
CONCLUSION
Accelerated
ageing
tests
(short-circuit
operations) were performed on modules
developed by Microsemi allowing (with
judicious location of bond contacts) to measure
the evolution of the metallization layer
resistance during the repetition of the shortcircuit cycles. Tests were conducted under
different energy dissipated per cycle. The
observation in a scanning electron microscope
showed plastic deformation of the metallization
layer from the beginning of the repetition of
cycles leading to the initiation of fractures.
We must wait for these fractures propagate
through the metallization layer to detect a
significant change in resistance of the
metallization layer. Ageing depends strongly
on the energy dissipated per cycle through the
temperature reached at the end of short-circuit.
FIB microscopy has allowed highlighting the
spread of cracks in the volume of the
metallization layer, and showing that under
high energy, micro-cracks developed across
the metallization layer which helps to explain
the significant change in resistance of this
layer.
In future work, we seek to better characterize
the mechanical behavior of the metallization
layer, and we seek to show how evolution of
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