Applied Surface Science 253 (2007) 6217–6221
www.elsevier.com/locate/apsusc
An XPS study of Al2Au and AlAu4 intermetallic oxidation
C. Xu a, T. Sritharan a,*, S.G. Mhaisalkar a, M. Srinivasan a, S. Zhang b
b
a
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798
Received 4 October 2006; accepted 19 January 2007
Available online 30 January 2007
Abstract
Samples of Al2Au and AlAu4 were examined using XPS after controlled oxidation in air. AlAu4 showed a strong tendency to oxidize compared
to Al2Au. The binding energies (b.e.) of Au 4f and Al 2p XPS emissions were determined for both intermetallics. Heavy oxidation of AlAu4
resulted in a unique Au 4f emission near the surface which was attributed to Au dissolved in aluminum oxide.
# 2007 Elsevier B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy; Binding energy; Intermetallic compounds; Oxidation; Gold
1. Introduction
In Au wire bonded electronic packages, the substrate pad
frequently contains Al metallization that conducts electronic
signals to the IC chip. A good Al–Au bond inevitably contains
intermetallics at the interface, the nature, thickness and
transformation of which will play a key role in the reliability
of the joint [1–5]. With increasing integration and miniaturization, the wire diameter and the metallization thickness
have become extremely small and, consequently the role of
the interface intermetallics has become an even more critical
factor that determines the reliability of the whole package.
One of the mechanisms that purportedly could cause failure is
oxidation/corrosion at the wire bond. Recently, Piao et al. [6–
9] studied the room temperature oxidation behavior of Al–Au
thin films containing Al2Au and AlAu2. They reported that Al
oxide formation depletes Al in the intermetallic leading to Au
enrichment with associated phase transformation from Alrich to Au-rich phases. This was detected by peak shift in the
X-ray photoelectron spectroscopy (XPS) spectra. Thus,
oxidation of interface intermetallics could trigger phase
transformations at the interface that may compromise the
reliability of the bond. Oxidation could take place during
usage of the package but, more importantly, accelerated
oxidation could occur when wire bonds are subjected to high
* Corresponding author. Tel.: +65 67904931; fax: +65 67909081.
E-mail address: [email protected] (T. Sritharan).
0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2007.01.068
temperature processing in the range 175–200 8C during the
manufacture.
One of the common intermetallics detected in Al–Au wire
bonds is the Au-richest AlAu4 but its oxidation behavior does
not appear to have been studied. Even information on the XPS
peak positions of Au and Al in AlAu4 is not available in the
literature. This paper reports results of an XPS study of
oxidized surfaces of bulk Al2Au and AlAu4. The investigations
show that oxidation of AlAu4 could be more severe than Al2Au
and thus precautions are necessary when AlAu4 forms at the
interface of wire bonds. Phase changes consequential to
oxidation are also discussed.
2. Experimental
Approximately 1.5 g of the intermetallics Al2Au and AlAu4
were produced by arc melting stoichiometric amounts of
99.99% pure Au and Al granules in an argon atmosphere. The
sample was repeatedly flipped over and remelted to achieve
homogeneity. A portion of the samples was ground to powder
for XRD confirmation of the phases. The peaks obtained
matched with the powder diffraction datafiles of Al2Au and
AlAu4 in ICSD file nos. 57501 and 104616, respectively. In
addition, the melting points of Al2Au and AlAu4 samples
measured by a differential scanning calorimeter (1055 and
526 8C) also matched with values quoted in the literature [10].
Thus, the samples can be considered as pure intermetallics.
Nine specimens of size 4 mm 2 mm were cut from each
intermetallic and one convenient face was polished down to
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C. Xu et al. / Applied Surface Science 253 (2007) 6217–6221
1 mm diamond paste surface finish and ultrasonically cleaned.
The polished samples were heated at 175 8C in an oven in air
for durations of 0.25, 0.5, 1, 2, 4, 8, 16 and 24 h with care to
avoid contamination and contact with the polished surface. The
XPS surface measurement was performed in a Kratos AXIS
Ultra spectrometer with a monochromatized Al Ka X-ray
source (1486.71 eV) operated at a reduced power of 150 W
(15 kV and 10 mA). The base pressure in the analysis chamber
was 2.66 107 Pa. The core-level spectra were obtained in Al
2p and Au 4f regions at a photoelectron take-off angle of 908
measured with respect to the sample surface and were recorded
in 0.1 eV steps with a pass energy of 40 eV. The binding energy
(b.e.) scale of the XPS spectrum was calibrated with the C 1s
peak (neutral C–C peak at 284.5 eV). The chemical states for
depth profiles (etching profiles) were analyzed on the same
equipment for the 24 h oxidized samples. An Ar+ ion gun with
acceleration voltage of 4 kV and filament current of 15 mA was
used to etch the samples at a gas pressure of 6.65 106 Pa.
Ion bombardment was performed at an incident angle of 458 to
the surface normal. The Al 2p and Au 4f spectra were obtained
after each etching cycle of 3 min. Depth profiling was done
until the oxygen peak became minimal. These experimental
conditions in depth profiling were kept constant for both
intermetallics.
3. Results and discussion
Fig. 1 shows the evolution of Al 2p spectra during depth
profiling in the 24 h oxidized Al2Au sample. The Al 2p
spectrum of a single species has the spin–orbit doublet structure
of Al 2p3/2 and Al 2p1/2 but the splitting of these components is
not resolved in these spectra. A shoulder at the low b.e. end is
evident at the surface indicating that peaks from two species are
overlapping. The smaller peak with lower b.e. gradually
becomes dominant with increasing depth while the peak that
was dominant at the surface diminishes. Deconvolution of the
experimental peak into two single-component peaks was done
using CasaXPS software (Version 2.3.10) and good matching
was obtained with two component peaks at b.e. of 75.8 and
73.5 eV as shown in the figure. The former corresponds well to
the Al 2p b.e. in Al2O3 [11] while the latter is consistent with
reports of Al 2p b.e. in Al2Au [6–9]. The evolution of the
relative intensities of the component peaks agrees well with the
presence of an Al2O3 layer on Al2Au.
When the oxide and metal photoelectron peaks are both
obtained, the remaining oxide thickness d could be estimated
from the ratio of the peak intensities using the Strohmeier
equation [12] given in Eq. (1)
Io
d ðnmÞ ¼ 2:8 ln 1:4 þ 1
Im
Fig. 1. Al 2p emission for depth profiling in Al2Au. The component peaks are
shown as dotted lines while the continuous curve is constructed using the
components to get good fit to the experimental data. The etching time and the
estimated thickness of remaining oxide are also given for each spectrum.
(1)
where Io and Im are the intensities (peak areas) of the oxide and
metal peaks, respectively. The remaining oxide thickness estimated for successive spectra using Eq. (1) are given in Fig. 1.
The Au 4f spectrum obtained on the surface of this sample is
shown in Fig. 2 where two spin–orbit split peaks are clear at b.e.
of 89.1 eV (Au 4f5/2 component) and 85.4 eV (Au 4f7/2
component), both within acceptable range of the b.e. values
reported by Fuggle et al. for Al2Au which are 89.4 and 85.7 eV,
respectively [13]. There was no change to the relative peak
intensities or b.e. values of Au 4f peaks on depth profiling. This
implies that the Al2O3 layer does not change the electronic
C. Xu et al. / Applied Surface Science 253 (2007) 6217–6221
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Fig. 2. Au 4f doublet obtained on the surface of 24 h oxidized Al2Au sample
before ion etching and the curve is fitted to the data.
environment of the Au atoms significantly because the surface
and subsurface signals are identical. Therefore, the oxide layer
is likely to be thin in this compound.
An example of the Al 2p emission for AlAu4 sample
oxidized for 4 h is shown in Fig. 3. All samples oxidized up to
24 h gave similar data. Attempts to deconvolute into component
peaks indicated that a single peak at b.e. of 75.7 eV will give a
good fit to the data for all oxidizing times. Since this b.e. is
equivalent to that of oxidic Al(III), we infer that only Al2O3 is
detected and that the Al 2p component of AlAu4 is absent on the
surface. This implies a thicker oxide layer on AlAu4 compared
to that on Al2Au.
The Au 4f surface spectra for the AlAu4 samples are shown
in Fig. 4. In the as-polished state two distinct peaks at b.e. of
87.8 and 84.2 eV, corresponding to Au 4f5/2 and Au 4f7/2,
respectively, fit the experimental data very well. Since the
reported values of b.e. in AlAu4 are not available for
comparison, and further since the Al 2p spectrum clearly
indicated a thick oxide layer on these samples, we were not able
to reliably attribute the b.e. values obtained above to the Au 4f
doublet in AlAu4. With increasing oxidation time, it is clear that
Fig. 4. Au 4f doublet surface scans for AlAu4 samples oxidized up to 24 h. The
dotted lines in the curves for 4 and 24 h are component peaks, while the
continuous curves are fitted using the components.
Fig. 3. Al 2p emission for AlAu4 sample oxidized for 4 h with the fitted curve
for data.
additional, overlapping peaks become necessary to fit to data.
Deconvolution shows that each of the Au 4f doublets could be
resolved into two overlapping components; one pair with b.e. of
88.8 and 87.8 and another pair at 85.2 and 84.2 eV. Thus, the
two additional peaks that appear on increasing oxidation are at
the higher energy side of the Au 4f doublet observed in the aspolished sample. There is no evidence for a high b.e.
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component in the as-polished Al 2p spectra which might
indicate differential charging in surface regions. This additional
Au 4f doublet appears to represent an emission coming from a
different chemical environment to that in the as-polished
sample [14]. Since, the Al 2p shows a thick oxide layer in the
24 h oxidized sample, the new peaks with b.e. 88.8 and 85.2 eV
must be emitted by Au atoms trapped in Al2O3. Depth profiling
of the 24 h oxidized sample is shown in Fig. 5. It is clear that the
intensity of the new peaks at 88.8 and 85.2 eV rapidly diminish
in a short etching time of only 6 min. In the 33 and 108 min
etched spectra, fitting with peaks at 87.8 and 84.2 eV in
combination with very small intensity peaks at 88.8 and
85.2 eV would make no difference to the experimental data.
Thus, the peaks with b.e. of 87.8 and 84.2 eV could be
attributed to the Au 4f doublet in AlAu4 while the additional
peaks at b.e. of 88.8 and 85.2 eV could be attributed to emission
from Au atoms trapped in the thick Al2O3 layer. The formation
of known (bulk) oxidised species of Au appears to be unlikely.
Au2O3 has a Au 4f7/2 b.e. near 85.9 eV but is normally formed
only under exceptional conditions like exposure of Au to highly
reactive ozone [15], atomic oxygen [16] or oxygen plasma [17].
Oxidation of AlAu4 to Al2O3 is more likely to result in the
emergence of elemental Au as there is no other known
intermetallic that has higher Au than AlAu4. The reported b.e.
of elemental Au 4f peaks are 87.5 and 83.8 eV [18]. The peaks
observed at 88.8 and 85.2 eV in this investigation are
significantly higher than the b.e. of elemental Au 4f peaks
and thus must be emitted from a different, strongly electronwithdrawing chemical environment. This b.e. shift may result
from isolated atoms or nano-clusters of gold surrounded by
oxygen ions in the oxidized layer.
The evolution of the Al 2p spectra in 24 h oxidized AlAu4
sample during depth profiling is shown in Fig. 6. The surface
scan shows only the oxidic Al 2p peak at a b.e. of 75.7 eV, in
agreement with Fig. 3. With increasing depth a second peak
becomes evident as a shoulder in 15 min, and subsequently as a
doublet in 33 min. After 15 min ion etching, the oxide layer
thickness estimated using Eq. (1) is reduced to 5.1 nm. Oxide
layer thickness at the surface could not be estimated because a
measurable Al 2p spectrum of AlAu4 was not detected.
Comparison with data in Fig. 1 shows that the oxide layer in
AlAu4 must be higher than the 10.8 nm obtained for Al2Au. In
108 min etching, the oxidic peak is minor while the second
peak becomes predominant. Deconvolution shows that the
second peak should be at a b.e. of 73.7 eV for good fitting to the
data. Thus, all Al atoms have then been oxidized to Al2O3 layer
near the surface. After 108 min etching, the signal from the
subsurface AlAu4 is clear. Thus, we attribute the b.e. of Al 2p
peak in AlAu4 to 73.7 eV.
The b.e. shifts are characteristic of the physical and chemical
environment of the analyzed species while the electronegativity
determines the type of interaction between the various atoms in
the system. Garcia-Serrano et al. [19] found in an XPS study
that elemental Au gets incorporated into Al2O3 when Al2Au
disintegrates. This results in a solid solution of Au in Al2O3
where the Au atoms could get clustered into nanoparticles upon
heating. Atsuko et al. [20] proved that Au diffused through
Fig. 5. Au 4f emissions during depth profiling in AlAu4 oxidized for 24 h. The
component peaks are dotted lines and the continuous curves are fitted to data
using the components. The etching time for each spectrum is given.
Al2O3 in a Au–Al2O3–Al sandwich to form AlAu2 intermetallics at the Al interface. Thus, dissolution of Au in Al2O3
has been hypothesized previously but its characteristic
emissions in XPS are not fully understood. This study, while
giving a strong indication of dissolution of Au in Al2O3, also
measures the Au 4f b.e. of such species. This value will be
affected by electronic interaction between Au and the
C. Xu et al. / Applied Surface Science 253 (2007) 6217–6221
6221
exist to stabilize the Au atoms although no explicit evidence for
any Au oxidic species is available. The Au 4f7/2 b.e. of 85.2 eV
is less than the reported value of 85.9 eV for Au2O3 suggesting
that any such bonds are not as ionic as the bulk oxide.
4. Conclusions
In conclusion, the b.e. of Au 4f doublet (87.8, 84.2 eV) and
Al 2p (73.7 eV) XPS emissions in AlAu4 have been
determined. It is shown that the oxide layer thickness in
AlAu4 is considerably higher than that in Al2Au. Au in AlAu4 is
reduced to elemental state during oxidation and this species of
Au in Al2O3 gives Au 4f emissions at higher b.e. values (88.8,
85.2 eV) than the elemental Au or AlAu4.
From an engineering point of view, the present study proves
that Al–Au intermetallics are prone to oxidation when heated in
air, among which AlAu4 oxidizes most readily. Since AlAu4 is
frequently the major intermetallic phase in wire bond interfaces
due to the excess of Au, package failure due to oxidation in
elevated temperatures could become a significant problem in
microelectronics.
Acknowledgements
The authors acknowledge Mr. Li Yibin for performing the
XPS measurements and Prof. Roger Smart for his invaluable
suggestions on the manuscript.
References
Fig. 6. Al 2p emissions during depth profiling in AlAu4 oxidized for 24 h. The
component peaks are dotted lines and the continuous curves are fitted to data
using the components. The etching time and the estimated thickness of
remaining oxide are also given.
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electrons to the neighbouring atoms. Some Au–O bonds may
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