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Artifact-Free Decapsulation of Silver Wire Bonded Semiconductor Devices
Using Microwave Induced Plasma
J. Tang, J. Wang, W. van den Hoek
JIACO Instruments, Feldmannweg 17, 2628 CT, Delft, The Netherlands
jiaqi@jiaco-instruments.com
D. Lohier, B. Forgerit, G. Gabaston
HIREX Engineering, Toulouse, France
C.I.M. Beenakker
Delft University of Technology, Mekelweg 4, 2628 CD, Delft, The Netherlands
Abstract
Decapsulation of silver wire bonded packages with known
techniques often results in damaged silver wires. The chemical
properties of silver and silver compounds make silver bond
wire inherently susceptible to etching damage by acid,
conventional plasma, and oxygen-based Microwave Induced
Plasma (MIP). In this paper we solve this problem by
developing a specific decapsulation chemistry, based on a
hydrogen-containing MIP, for artifact-free decapsulation of
silver wire bonded packages.
Introduction
Silver and silver-alloys exhibit attractive material properties for
wire bonding applications. For example, silver is softer than
copper while lower in cost than gold, making silver a good
candidate to replace copper and gold wire as an alternative low-
cost bonding wire for certain applications like Flash memory
or DRAM stack-die. Semiconductor industry has been
investigating different silver and silver-alloy wires in R&D
during the past years [1], while mass production of commercial
products with silver wires are already delivered to the market.
However, due to the chemical properties of silver, artifact-free
decapsulation of devices containing silver bond wires is
generally considered to be challenging, even more challenging
than decapsulation of devices containing copper bond wires.
Improvements to conventional acid decapsulation have been
proposed by adding iodine to the solution [2], saturation
etching [3], and optimizing acid etching parameters. However,
wire pitting and wire thinning still occur, which influence the
wire mechanical test accuracy during reliability test. The
artifacts introduced also jeopardize true root cause failure
analysis. In cases where high Tg mold compound is used in
combination with silver wire, acid decapsulation becomes even
more challenging since a more aggressive acid etching recipe
is needed to remove the high Tg mold compound.
Conventional plasma decapsulation uses carbon tetrafluoride
(CF4) and oxygen (O2) as precursors [4]. In case of silver these
cause a problem as silver oxides or silver fluorides may be
formed during the plasma decapsulation process. Both silver
oxides and silver fluorides decompose at comparatively low
temperature, leading to undesired damaging to the silver bond
wires during plasma decapsulation.
Even in case of an oxygen-based MIP that has been proven to
provide artifact-free decapsulation for copper wire devices [5],
it is found that decapsulation of silver wire devices is difficult
and often results in damaged silver wires.
In this paper we present a study on the cause of etching damage
to silver wire, as well as the development of MIP with a new
etching chemistry to achieve artifact-free decapsulation of
silver wire bonded package.
Development of Alternative Etching Chemistry
Initial development focused on understanding the reaction
mechanisms and optimizing the process conditions. Applying
an oxygen-based MIP recipe as used for copper wire (oxygen
radicals as the etchant) to silver wire results in apparent etching
damage on silver. In severe cases even dendrite growth is
observed (see Figure 1). Attempts were made to optimize
oxygen-based MIP processing conditions. Reduction of etching
damage is achieved but the silver wires are still damaged when
exposed for extended durations to the etching process (see
Figure 2).
Investigation on etching product material composition unveiled
the cause of plasma etching induced damage. Silver can form
silver oxides during oxygen-based plasma etching, similar to
the formation of copper oxides when exposing copper to
oxygen-based plasma etching. However, the material stability
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440
of silver compounds greatly differs from copper compounds
(see Table 1). Copper oxides and copper fluorides have
comparatively high melting points, thus plasma etching with O2
and CF4 gases creates a stable copper compound layer
surrounding the surface of copper wire so that etching does not
reach deeper into the bulk copper. On the contrary, silver
oxides and silver fluorides have comparatively low melting
points as well as low decomposition temperatures, thus plasma
etching with O2 and CF4 gases creates a volatile silver
compound layer that decomposes simultaneously, enabling
continuous etching of the silver wire. The volatility of the metal
compounds accounts for the major difference found in O2 and
CF4 plasma etching behavior on silver and copper wires.
Further process optimization can only bring limited
improvements. Clearly, a fundamentally different chemistry is
needed.
Figure 1. Under aggressive oxygen-based MIP etching, silver
wires are rapidly etched while showing dendrite growth
Figure 2. Under optimized oxygen-based MIP etching, silver
wires are still damaged showing roughened surface
Table 1. Melting point of Ag, Cu, and their compounds [6]
Melting Point (°C)
Melting Point (°C)
Ag
961
Cu
1083
Ag2O
d 230
Cu2O
1235
Ag2O2
d >100
CuO
1326
AgF
435
CuF
908
AgF2
690
CuF2
d 950
Ag2F
d 90
After intensive investigations we found that the above-
mentioned drawbacks can be overcome by using a plasma
comprising hydrogen. Such a hydrogen-containing plasma is
suitable for silver wire package decapsulation, without
undesirably damaging the silver bonding wires. Hydrogen-
containing plasmas are used in other applications. For instance,
thin organic polyimide films or photoresist layers of up to 1 µm
can be etched away from silicon wafer surfaces with hydrogen-
containing plasma [7]. The etch rate for this application is less
than 30 nm/min. This rate may be acceptable for the thin
polyimide films, but a much higher etch rate is required for
etching away package encapsulants comprising molding
compound in order to expose the die and the bond wires within
an acceptable time period. Such packages are normally much
thicker, typically 50 – 200 µm.
We have redesigned our plasma source to achieve a high-
density hydrogen-containing plasma at atmospheric pressure.
The mold compound etching rate of the hydrogen-containing
MIP reached one third to half of oxygen-based MIP. The
introduction of hydrogen-containing gas suppressed the
formation and decomposition of volatile silver compounds, as
a result the silver wires remain undamaged after hydrogen-
containing MIP decapsulation (see Figure 3). Similar to
oxygen-based MIP decapsulation, the bond pads, passivation,
die, and electrical functionality of the device remain
undamaged after hydrogen-containing MIP decapsulation.
Figure 3. Silver wires remain undamaged after package
decapsulation by hydrogen-containing MIP
441
Case Studies
The results of this study are the basis of the development of a
fully automatic atmospheric pressure MIP decapsulation
machine. Various silver wire bonded devices in R&D phase
and in mass production phase have been decapsulated with
hydrogen-containing MIP process at customer sites.
A) Silver Wire Bonded MCU
The following case study is performed in the frame of
ANADEF “Copper wire bonding” working group. Laser
ablation followed by hydrogen-containing MIP is used to
decapsulate the silver wire bonded package (0.8mil 96.5% Ag-
alloy wire, Sumitomo G631SHQ mold compound) (see Figure
4). After MIP decapsulation the silver bond wires, silver ball
bonds, aluminum bond pads, and passivation layer on the die
are cleanly exposed without artifacts (see Figure 5 and 6). In
comparison, the same sample after acid decapsulation shows
corrosion damage on the silver wires due to acid etching (see
Figure 7).
Figure 4. Silver wire bonded MCU after laser ablation and
hydrogen-containing MIP decapsulation
Figure 5. SEM image showing artifact-free silver ball bond and
aluminum bond pad after hydrogen-containing MIP
decapsulation
Figure 6. SEM image showing artifact-free die surface after
hydrogen-containing MIP decapsulation
Figure 7. SEM image showing corroded silver wire after acid
decapsulation
442
B) Silver Wire Bonded Stacked-die Memory Device with
High-Tg Mold Compound
In extreme cases the device is a combination of multiple
challenging structures, for example silver wire with high Tg
mold compound in a multi-chip stacked-die package.
Hydrogen-containing MIP is capable of selectively removing
the high Tg mold compound to expose multiple layers of
stacked dies without artifacts (see Figure 8). The silver bond
wires can be exposed at full wire length from ball bond to stitch
bond without damage (see Figure 9 and 10).
Figure 8. Silver wire bonded stacked-die package after laser
ablation and hydrogen-containing MIP decapsulation (image
courtesy of iST)
Figure 9. SEM image showing artifact-free silver wires and
ball bonds after hydrogen-containing MIP decapsulation
(image courtesy of iST)
Figure 10. SEM image showing artifact-free silver wires and
stitch bonds after hydrogen-containing MIP decapsulation
(image courtesy of iST)
C) Other Application Cases
Since hydrogen-containing MIP can preserve silver-containing
structures during decapsulation, the application range is broad,
covering silver wire bonded ASICs, discrete devices, power
devices, DRAMs, and Flash memories, etc. During the past few
years users of MIP have developed various new applications
based on the hydrogen-containing MIP process. A few
applications that worth mentioning are Film Over Wire devices
with silver wires; smart cards with transparent mold compound
and silver wires; stacked-die NAND flash modules with silver
wires; leadframe with silver plating at down bonds or stitch
bonds; high frequency and power devices with silver sintering
layers.
Conclusions
A new etching chemistry has led to artifact-free decapsulation
of silver wire bonded package using atmospheric pressure
Microwave Induced Plasma (MIP). This new hydrogen-
containing plasma etching process has been proven on various
sample types containing silver bond wires, including R&D
prototype samples and commercially available devices. A fully
automatic MIP machine has been developed based on this
research to enable more reliable failure analysis and reliability
tests to be performed on silver wire bonded devices.
Acknowledgments
The authors would like to thank users of MIP who provided
valuable feedback on optimizing the hydrogen-containing MIP
decapsulation process and developing new applications with
this new patented etching technology.
443
References
[1] C. Liu, ‘Review on silver wire bonding’, IMPACT (2013).
[2] M. Lefevre, E. Noraz, D. Veychard, ‘Repeatable Method
for Automated Decapsulation of Silver Alloy Wire Packages,
ISTFA (2015).
[3] S. Suzuki, M. Yamaguchi, ‘Acid decapsulation for silver
wire bonded package’, ISTFA (2015).
[4] F. Kerisit, M. Lefevre, B. Domenges, W. Prellier, M. Obein,
‘Decapsulation of silver-alloy wire-bonded devices’, IPFA
(2014).
[5] C. Odegard, A. Burnett, J. Tang, J. Wang, ‘Preserving
Evidence for Root Cause Investigations with Halogen-Free
Microwave Induced Plasma Decapsulation’, ISTFA (2018).
[6] CRC Handbook of Chemistry and Physics.
[7] F.Y. Robb, ‘Hydrogen Plasma Etching of Organics’, J.
Electrochem. Soc. 1984, Volume 133, Issue 7, 1670-1674.
444
