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Failure analysis of complex 3D stacked-die IC packages using Microwave
Induced Plasma afterglow decapsulation
Article in Proceedings - Electronic Components and Technology Conference · July 2015
DOI: 10.1109/ECTC.2015.7159691
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Failure Analysis of Complex 3D Stacked-die IC Packages using
Microwave Induced Plasma Afterglow Decapsulation
J. Tang1,*, M.R. Curiel2, S.L. Furcone2, E.G.J. Reinders3, C.Th.A. Revenberg3, and C.I.M. Beenakker4
1
JIACO Instruments B.V., Mekelweg 4, 2628 CD, Delft, The Netherlands
2
Freescale Semiconductor Inc., Tempe Product Analysis Laboratory, 2100 East Elliot Road, Tempe, AZ 85284, USA
3
MASER Engineering B.V., Capitool 56, 7521 PL, Enschede, The Netherlands
4
Delft University of Technology, Laboratory of Electronic Components, Technology and Materials (ECTM)
Delft Institute of Microsystems and Nanoelectronics (Dimes), Mekelweg 4, 2628 CD, Delft, The Netherlands
*
Phone: 0031 6 8171 2138
*
Email: jiaqi@jiaco-instruments.com
Abstract
Quality control and failure analysis of IC packages require
physical access to the die during destructive analysis.
Successful analysis depends on the critical preservation of the
original state of the die, bond wire, bond pad, and original
failure sites during the package decapping process. The
currently used acid and conventional plasma decapping
techniques have their intrinsic limitations in certain analysis
cases, for example 3D stacked-die package with complex
structures, HAST stressing that causes epoxy hardening, the
use of epoxy refill materials and acid-resistant epoxy molding
compounds, the use of Cu and PdCu bond wires, preservation
of original die and bond wire surface features, preservation of
original failure sites and contaminants, etc. Such difficult
cases with conventional decapping techniques often suffers
from low analysis accuracy and low confidence level during
root cause failure analysis, and consequently poses major
threat to the quality assurance of the IC products.
In order to overcome the limitations of conventional
decapsulation techniques, a new generation decapsulation
technique is developed with an atmospheric pressure
Microwave Induced Plasma (MIP). As the MIP system
operates at atmospheric pressure and the mean free path of
ions at that pressure is extremely low, only isotropic high
selectivity etching occurs by long lived oxygen radicals.
Recipe and decapping process development is made to solve
each of the challenging cases individually.
As an ultimate evaluation on the capability of this MIP
decapping technique, we deliberately combined multiple
decapping difficulties into one single device: a 3D stacked-die
package that has deliberately-introduced contamination and
epoxy refill and went through HAST.
It appears that the O2–only MIP afterglow decapsulation
successfully
preserves
the
deliberately-introduced
contamination sites in the 3D stacked-die package. The unique
capability of MIP decapsulation in facilitating accurate failure
analysis and quality control is demonstrated.
I. Introduction
Semiconductor devices are routinely decapsulated for
failure analysis and quality control. For plastic semiconductor
packages, epoxy molding compound has to be removed
selectively in a reasonable processing time. It is crucial that all
the metal bond wires, aluminum bond pads, silicon die, and
original failure sites are not damaged by the decapsulation
978-1-4799-8609-5/15/$31.00 ©2015 IEEE
process itself, so that reliable further analysis can be
performed. However, the trend to reduce cost by adopting
copper wire bonding [1, 2] and to increase performance by
adopting 3D stacked-die packaging presents challenges to the
conventional acid [3, 4] and conventional plasma [5]
decapsulation techniques.
Compared to traditional single-die packages, decapsulation
of complex 3D stacked-die IC packages is even more
challenging due to the stacking structure and small package to
die size ratio. Conventional acid decapsulation easily cause
over-exposure of the top layer die and bond wires whilst the
middle and bottom layer structures are still not exposed. In
addition, preserving the package perimeter to perform further
electrical tests in specific test sockets is a challenge with
conventional acid decapsulation. Conventional plasma
decapsulation faces even more severe problems to reach the
middle and bottom layer structures, the thick molding
compound layer and stepwise structure make the task
impossible. The use of carbontetrafloride (CF4) in
conventional plasma decapsulation often results in unwanted
overetching damage to the original Si3N4 passivation layer and
Si die. Such overetching damage is unavoidable due to the
fluorine-plasma etching selectivity of SiO2 to Si and Si3N4 is
limited [6-8].
Successful root cause failure analysis depends on the
critical preservation of the original contamination and original
failure site during the decapping process. The challenge of the
conventional techniques such as acid decapsulation and
conventional plasma decapsulation when looking for possible
contamination at the interface of the mold compound and die
is that they have the potential to etch or attack the contaminant
and/or the surface of the die hence destroying the evidence.
In order to overcome the limitations of conventional
decapsulation techniques, a new generation decapsulation
technique is needed. Recent work published by our group [9]
has demonstrated that a Microwave Induced Plasma (MIP)
system with a Beenakker type microwave resonant cavity [10,
11] as the plasma source has great advantage compared to the
conventional techniques mentioned before. The highly
confined plasma jet results in a high flux of neutral radicals in
the plasma afterglow, which contributes to the high molding
compound etching rate and high etching selectivity towards
Cu bond wire and Si die. The MIP system has unique
capability in decapsulation of Copper (Cu) and Palladiumcoated Copper (PdCu) wire-bonded single-die IC packages
after High Temperature Storage (HTS), Temperature Cycling
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2015 Electronic Components & Technology Conference
(TC) [12], Highly Accelerated Stress Test (HAST) [13]
stressing conditions.
In this paper we explore the applicability of the MIP
afterglow decapsulation in complex 3D structure 3-layered
stacked-die IC package with deliberately-introduced
contamination. We deliberately combined multiple decapping
difficulties into the test vehicle: a 3D stacked-die package that
has deliberately-introduced contamination and epoxy refill and
went through HAST. MIP decapping recipe development and
optimization will be investigated. The final MIP decapping
result and failure analysis of the failed device will be
discussed.
II. MIP decapsulation system
A Microwave Induced Plasma (MIP) decapsulation system
was built to solve the challenging 3D stacked-die package and
copper wire package decapsulation tasks (see Fig.1). The
system consists of a microwave generator (Sairem solid-state,
f=2450+-20 MHz, P=0~180 W), a lab-built TM010 mode
Beenakker type microwave resonant cavity, a gas discharge
tube, three mass flow controllers, a CCD camera, a
programmable XYZ-stage, and a computer to control the
components. The MIP system is able to generate a stable
plasma under atmospheric pressure. Localization control and
process monitoring during plasma etching are enabled, thus IC
package decapsulation process can be well-controlled with
high reproducibility.
The microwave power from the generator is delivered to
the Beenakker cavity via a coaxial cable. The cavity is
designed to resonate at 2.45 GHz in the TM010 mode. In this
mode the electric field amplitude inside the cavity is zero at
the periphery and maximum in the center. A quartz or alumina
gas discharge tube is inserted through the center of the cavity
to sustain the plasma. Argon (Ar) is the plasma carrier gas.
Oxygen (O2) and carbontetrafloride (CF4) can be added as
etchant gas.
As the MIP system operates at atmospheric pressure and
the mean free path of ions at that pressure is extremely low,
only isotropic high selectivity etching occurs by long lived
oxygen radicals. The prevention of ions and microwave
leakage fields on the IC package sample is crucial to avoid
damage to the device inside the package. Semiconductor
devices remain functional after their packages have been
decapsulated by this MIP system.
IC package sample heating in conventional plasma etchers
is normally achieved by placing a hotplate beneath the IC
package. In the MIP system, sample heating is made through
direct heating by the plasma effluent. Direct heating by the
MIP effluent gas gives the advantage of maintaining a low IC
package bulk temperature, while having a localized heating on
the plasma etching site.
A programmable XYZ-stage is used as the IC package
sample stage. The movement of the stage is controlled by a
computer and programs are written to define customized scan
routes. There are basically two approaches to etch a defined
area by plasma. The conventional approach is to put the IC
sample in a chamber filled with plasma and use a mask to
define the area for etching. The alternative approach is to
make a very confined plasma and scan the plasma beam across
the area that is intended for etching. The advantage of the later
process is not only the convenience of defining the etching
area, but also the possibility to vary the scan speed at different
regions to achieve variable etching profiles across the IC
package sample.
A CCD camera is integrated into the MIP system to enable
real-time monitoring of the plasma etching process. Because
the stray field generated from the plasma is low, there is no
influence on the CCD camera due to electromagnetic
interference and clear images can be received throughout the
etching process.
III. MIP etching principle and efficiency
At atmospheric pressure, reactive neutral species will
dominate plasma chemistry while ions become relatively
insignificant. When O2 etchant gas is added into the Ar plasma
carrier gas, the free electrons that have high enough energy
will induce excitation and dissociation of ground state triplet
oxygen. Active species in the MIP plasma afterglow can be
atomic oxygen O(3P), metastable molecular oxygen, and
ozone O3. It has been reported that atomic oxygen is the active
species involved in oxygen plasma afterglow etching of
polymer materials [14].
Ar/O2 MIP afterglow at 40 W microwave power is shown
in Fig.2. Glow in the plasma effluent is observed when O2 gas
is added into Ar plasma, while the light might come from
excited state oxygen and recombination of oxygen atoms. The
long tail of glow that is observed in low power Ar/O2 MIP
afterglow demonstrates the high radical flux that is used for
high efficiency etching of epoxy molding compound.
Figure 2. Ar/O2 MIP afterglow demonstrating the high
radical flux generated for high efficiency etching.
Figure 1. Schematic representation of the MIP system.
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IV. O2/CF4 plasma decapsulation (low etching selectivity)
Conventional plasma decapsulation machines require both
CF4 and O2 to be added as etchants to remove epoxy molding
compound. It is not possible for conventional plasma to
decapsulate IC packages with O2-only plasma. This is because
the SiO2 filler residues form an agglomerate layer that is
difficult to remove, thus etching can only reach a thin surface
layer and soon comes to a stop due to the SiO2 filler residues.
To remove the SiO2 fillers in molding compound,
conventional plasma machines have to use CF4 gas in addition
to O2 gas as etchants. The epoxy in molding compound is
completely etched by oxygen plasma forming H2O, CO2. At
the same time, the SiO2 filler is etched at a low rate by
fluorine radicals forming SiF4, so that the SiO2 filler
agglomerate structure become loose and can be further
removed by blowing off by pressurized air.
Disadvantage of using CF4 in conventional plasma
decapsulation technique is the inevitable overetching damage
to Si3N4 passivation layer, Si die, and original failure sites like
surface contaminants (see Fig.3). Such process-induced
damage makes further root cause failure analysis difficult as
the original sample state or evidence has changed due to the
decapsulation process.
Figure 3. Low selectivity O2/CF4 plasma etching caused
inevitable overetching damage on Si3N4 passivation layer.[9]
V. O2–only MIP decapsulation (high etching selectivity)
In order to achieve infinite high etching selectivity and
preserve the original state of Si3N4 passivation layer, Si die,
and original failure sites, we invented an O2-only MIP
decapsulation process.
The process contains two steps, first step is selectively
remove epoxy in the molding compound by the high flux of O
neutral radical in the MIP afterglow, second step is ultrasonic
clean the etched IC package in de-ionized water to selectively
remove the SiO2 filler residues. Typical O2-only MIP
decapsulation process requires 1 hour to fully decapsulate an
IC package without causing process-induced damage. This O2only MIP etching and ultrasonic cleaning combination proved
to be a crucial process to preserve the original state of Si3N4
passivation layer, Si die, and original failure sites after MIP
decapsulation (see Fig.4).
VI. 3D Stacked-die IC package MIP decapsulation
Due to the 3D stacking structure, the rigorous requirement
on the decapsulation process is to create a deep, narrow, and
damage-free opening to cleanly expose all layers of die and
bond wires.
Conventional acid decapping has difficulties when dealing
with 3D stacked-die IC packages. Acid etching often results in
unwanted removal of molding compound package sidewalls,
and acid flows outside the laser-ablation pre-defined cavity to
cause unwanted corrosion. For electrical tests of the device
after decapsulation, the outer-frame of IC package has to be
maintained in order to fit into a specific test socket, in which
case the acid removal of package sidewalls is unwanted.
Conventional plasma decapping faces even greater
obstacle, it is often found the plasma cannot reach the deep 3D
structure and it becomes impossible to expose the middle and
bottom layer die.
Both conventional acid and plasma decapping techniques
have extremely small processing window and many times
results in unsatisfactory decapping results, let alone when the
stacked-die package has a failure site inside and root cause
analysis has to be performed after decapsulation.
The focused plasma generated by MIP could be a solution
to 3D stacked-die IC package decapsulation due to its
extremely high etching selectivity and high aspect-ratio of
etching profile. A test vehicle is selected to investigate the
possibility of MIP in decapping stacked die packages. The
sample is a 6mm x 6mm Quad-flat no-leads (QFN) package
with 16 pins, 3 layer stacked die, 1 mil (25.4 μm) Au wire (see
Fig.5).
Figure 4. High selectivity O2-only MIP decapsulation ensures
Si3N4 passivation layer, Si die, and original failure sites are
well-preserved after MIP decapping.[9]
Figure 5. X-ray image of the lateral view of the QFN package
showing 3 layers of die and 3 layers of bond wires.
847
Figure 6. Schematic representation of the MIP decapsulation
Option 1. (White area indicates molding compound on IC
package to be removed by MIP.)
Figure 7. Schematic representation of the MIP decapsulation
Option 2. (White area indicates molding compound on IC
package to be removed by MIP.)
Two different options in MIP decapsulation are evaluated
and compared. Option 1 maintains all 4 sides of the package
frame, which aims to optimize for package completeness to fit
in electrical test sockets after decapsulation (see Fig.6).
Option 2 maintains only 3 sides of the package frame, which
aims to optimize for overall speed of package decapsulation
(see Fig.7).
Option 1: Optimize for package completeness to fit in
electrical test sockets (maintain 4-side package frame)
The MIP decapsulation process is fully compatible with
the current industry standard sample pre-treatment by laserablation. A laser cavity is first opened on the IC package to
remove the bulk layer of surface molding compound. Laserablation is stopped once the top of bond wires appear in order
to avoid laser-damage to the bond wire and die. After laserablation, a step-wised cavity is opened on the sample, marking
the intended area for further MIP decapsulation (see Fig.8.a).
O2-only MIP decapsulation processing is then conducted
by repeating the cycle of O2-only MIP etching and ultrasonic
cleaning. At Time = 10, 40, 70 minutes, the top layer, middle
layer, and bottom layer of die is exposed, respectively (see
Fig.8.b, c, d). The exposure of the middle and bottom layer of
die takes longer time because the deep and narrow trench
cavity structure creates obstacle for gas stream to pass
through. The O neutral radical in the MIP afterglow is carried
by the plasma gas to reach intended surface for etching. The
reduced gas flow rate at trench region is the major factor that
limits the overall MIP decapsulation speed.
The stacked-die package is decapsulated in 70 minutes by
O2-only MIP decapsulation with high reproducibility. All 3
layers of die and all 3 layers of bond wires are cleanly
exposed without process-induced damage. All 4 sides of the
package frame are maintained so the device can fit into
electrical test sockets after MIP decapsulation.
848
Figure 8. QFN stacked-die package processed by O2-only
MIP decapsulation. (Option 1. optimize for electrical test
sockets.)
Option 2: Optimize for overall decapsulation speed
(maintain 3-side package frame)
In order to increase the overall O2-only MIP decapsulation
speed, the gas stream flow pattern has to be optimized. Laserablation cavity is made wider and removed 1 side of the
package frame (see Fig.9.a). The same O2-only MIP
decapsulation processing is then conducted. At Time = 10, 20,
30 minutes, the top layer, middle layer, and bottom layer of
die is exposed, respectively (see Fig.9.b, c, d).
The different laser-ablation cavity increased the following
O2-only MIP decapsulation speed by 100%. The gas flow
pattern proved to be the rate-limiting factor in MIP
decapsulation of 3D stacked-die IC packages.
Both Option 1 and Option 2 provide the same good
decapsulation quality as the die and bond wires in all 3 layers
remains in their original state (see Fig.10). Depends on the
specific aim of analysis, one can choose the best suitable
Option that fits the next step electrical, mechanical, and
physical analysis after MIP decapsulation.
Figure 9. QFN stacked-die package processed by O2-only
MIP decapsulation. (Option 2. optimize for speed.)
Figure 10. QFN stacked-die package after O2-only MIP
decapsulation, ball bonds on top, middle, bottom layer die
remains undamaged.
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VII. Case study: Failed 3D stacked-die device
To evaluate the capability of MIP decapsulation to
preserve
deliberately-introduced
contamination,
we
deliberately added contamination on the middle layer of die in
the failed sample (see Fig.11).
The challenge of the other techniques such as acid and
conventional plasma decapsulation when looking for possible
contamination at the interface of the mold compound and die
is that they have the potential to etch or attack the contaminant
and/or the surface of the die hence destroying the evidence.
The 3D stacked-die structure makes such analysis using
conventional decapsulation techniques even more difficult.
A. Sample preparation
The failed sample is prepared in several steps, firstly the
package is partly decapsulated by laser and acid to expose the
bond shelf on the middle layer die, secondly 2% saline (NaCl)
solution contamination is deliberately-introduced and allowed
to dry, thirdly the package is refilled with LORD Thermoset
EP-937 that encapsulates the deliberately-introduced
contamination inside (see Fig.12), fourthly the refilled device
was stressed at 48hrs UHAST (85 °C/85%RH) and verified to
show leakage. A reference sample is also prepared with the
same preparation steps as the failed sample, except there is no
deliberately-introduced contamination.
B. O2-only MIP decapsulation process
The MIP decapsulation was conducted with the Option 1
developed in section VI in this paper. Firstly, laser ablation
was conducted to remove the top encapsulation material on
the IC package. It is discovered that the laser removal rate of
the epoxy molding compound and refill epoxy material are
different. The refill epoxy appears to be more difficult to
remove by laser and it remains in the center (see Fig.13).
In order to achieve a uniform surface, mechanical grinding
was further conducted until the surface of top layer die is
exposed (see Fig.14). However, it is later discovered that this
grinding step is unnecessary as the O2-only MIP decapsulation
readily removes the refill epoxy layer.
The final step is using O2-only MIP decapsulation to
expose the middle layer of die and bond wires. It is found that
the removal rate of the refill epoxy material is faster than the
surrounding epoxy molding compound during O2-only MIP
decapsulation. The middle layer die and bond wires are all
cleanly exposed for further analysis (see Fig.15).
Figure 11. Location (marked) of the deliberately-introduced
contamination on the QFN stacked-die sample.
Figure 12. The QFN sample after deliberately-introducing
contamination, the package surface is covered by refill epoxy.
Figure 13. The failed QFN sample after laser ablation, note
the refill epoxy in the center remains as a hill because it is
removed slower than the surrounding molding compound.
Figure 14. Failed sample after further mechanical grinding
Figure 15. Failed QFN sample after further O2-only MIP
decapping, the middle layer die and bond wires are exposed.
850
C. Failure analysis
Reference sample
The reference sample showed no apparent abnormities on
the middle layer of die under optical microscopy and SEM
(see Fig.16) after O2-only MIP decapsulation. Energydispersive X-ray Spectroscopy (EDS) was performed on
several sites on the middle layer die surface and spectra data
on all sites are the same, confirming the reference sample does
not have any contamination (see Fig.17).
Failed sample
The failed sample showed apparent abnormities on the
middle layer of die under optical microscopy and SEM (see
Fig.18 a. and b.) after O2-only MIP decapsulation. Magnified
view under SEM further revealed the abnormal structures
were a thin layer of foreign materials on the die and bond pad
surface (see Fig.18 c). EDS was performed on several sites on
the middle layer die surface, spectra data on Site 1and Site 2
(see Fig.19) shows traces of sodium (Na) and aluminum (Al)
elements on the surface contaminant. Na was deliberatelyintroduced by the saline solution contamination. Al was
possibly due to the corrosion of Al bond pad by the saline
contamination.
The O2-only MIP decapsulation process proved its unique
capability to preserve deliberately-introduced contamination
in a 3D stacked-die package. The fragile original surface
contaminant is not removed during the MIP decapsulation
process, thus the following failure analysis on the device can
be conducted accurately and with high confidence level.
Figure 18. The failed QFN sample after MIP decapsulation,
middle layer die & bond wires are exposed. (a): optical, (b):
SEM (c): magnified SEM with EDS Site 1 and Site 2 marked.
Figure 16. The reference QFN sample after O2-only MIP
decapsulation, middle layer die and bond wires are exposed.
(a): optical, (b): SEM with EDS measurement Site 1 marked.
Figure 19. EDS spectra of failed sample Site 1 and Site 2
show traces of N, O, Na, Al, Si
Figure 17. EDS of reference sample Site 1 show N, O, Si
851
Conclusions
An O2–only MIP decapsulation process is developed for
3D stacked-die IC packages. It appears that the MIP system
has major advantage in 3D stacked-die package decapsulation
due to its focused plasma beam etching approach. As the MIP
system operates at atmospheric pressure and the mean free
path of ions at that pressure is extremely low, only isotropic
high selectivity etching occurs by long lived oxygen radicals.
Through precisely directing the flow of radicals in the
afterglow, high selectivity etching on localized area can be
achieved. Process optimization is conducted and the effects of
different package materials and laser-ablation cavity on the
MIP decapping speed are investigated. MIP decapsulation
process cleanly exposes the die and bond wires in the top,
middle, and bottom layer without causing any process induced
damage. Processing time for 3-layered stacked-die QFN
device is 30 minutes (optimize for speed) to 70 minutes
(optimize for electrical test sockets) with high repeatability.
To evaluate the capability of MIP decapsulation to
preserve
deliberately-introduced
contamination,
we
deliberately added contamination on a lower layer of die on
the 3D stacked-die QFN package and combined multiple
decapping difficulties into the same device. It appears that the
O2–only MIP decapsulation process successfully preserves the
deliberately-introduced contamination sites, enabling further
failure analysis with high accuracy.
Based on the high etching selectivity, speed, repeatability,
and superior performance on preserving deliberatelyintroduced surface contamination features in complex 3D IC
packages, we conclude that MIP afterglow decapsulation is a
unique solution to complex IC package failure analysis and
quality control.
Acknowledgments
The authors would like to thank Delft University of
Technology for access to the cleanroom lab facility, and
Dimes colleagues A. van den Bogaard, C. C. G. Visser, and R.
P. van Viersen for their help on experiments.
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