Gupta D. (Ed.). Diffusion Processes in Advanced Technological Materials
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352 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
depths below the polymer surface. However, traces of Ag, which are still
harmful to semiconductor devices, can be detected even at a depth of
400 nm. The annealing time did not significantly affect the silver distri-
bution, which appears to be almost stationary. Fig. 7.9 also shows the
effect of 0.08 monolayer (ML) of Cr deposited prior to
110m
Ag evapora-
tion. The penetration profile largely resembles the resolution function,
which was obtained from a thin layer of
110m
Ag deposited onto a Fe-Si-B
glass at room temperature. (The Fe-Si-B glass was chosen because the
absence of diffusion at room temperature had been checked before.) The
small differences are most likely due to the dependence of the resolution
function on the matrix material. Comparison of the profiles with and
without Cr predeposition provides clear evidence of Ag diffusion at
room temperature.
The experiments also demonstrate that a Cr layer as thin as 0.08 ML
is an excellent barrier against silver diffusion into the polymer.
[23]
The bar-
rier function of Cr, which also acts as an adhesion promoter, has been
reported before.
[90, 91]
However, much thicker continuous films well above
1 nm were used. We attribute the excellent barrier function of a 0.08 ML
film to the well-known fact that Cr is very reactive and forms strong bonds
to most polymers (see Sec. 7.3). This results in a very fine (probably
atomic) dispersion of the Cr atoms, which act as effective traps to the Ag
atoms. Results from our Monte Carlo simulations shown in Fig. 7.10 con-
firm the excellent barrier function of atomic traps on a surface well below
monolayer coverage.
Experiments carried out at 35 and 187°C show that predeposited
nonradioactive silver layers in the ML range also act as barriers.
[82]
The barriers are less effective, however, despite their much larger
thickness, because Ag forms larger clusters with a lower cluster den-
sity at the polymer surface.
[9]
Co-deposition of ≈3 ML of nonradioac-
tive silver after 3 minutes of tracer evaporation (12 minutes total
deposition time) also leads to a considerable barrier effect (Fig. 7.9).
The barrier function of the Ag clusters, which are much larger than the
clusters formed without predeposition of nonradioactive Ag, allows us
to rule out the argument that the barrier function of Cr is only a sput-
tering artifact caused by the finer dispersion of Ag in the presence of
the Cr layer. This artifact can also be excluded because the sputtering
rate of Ag is slightly higher than that of the polymer. Therefore, pref-
erential sputtering of the polymer matrix cannot occur. Additional
striking evidence for the reliability of the present ion-beam depth pro-
filing technique comes from experiments mentioned above, where the
polymer film was removed from the substrate after diffusion annealing
and sputtering was performed from the backside through the entire
film.
[23]
Ch_07.qxd 11/12/04 4:10 PM Page 352
Figure 7.11 shows a penetration profile from a sample annealed at
187°C, which is plotted on a semilogarithmic scale versus the square of
the penetration depth as suggested by the ordinary thin-film solution of
Fick’s 2
nd
law (Gaussian curve). On this scale, we detect a linear range
with a diffusivity D 8 10
15
cm
2
/s, which is very close to that
reported by Willecke and Faupel
[81]
(8.6 10
15
cm
2
/s; see Fig. 7.8). In
light of the present results, the range is attributed to diffusion of clusters.
The inset also shows additional linear ranges attributed to the reduced
mobility of larger clusters. The diffusion of trace amounts of isolated
atoms, which are orders of magnitude faster than the clusters investigated,
[81]
was obviously not detected in the experimental window chosen.
The almost stationary nature of the Ag
110m
penetration profiles
obtained in TMC polycarbonate under the conditions is described by the
empirical law exp[(zz
0
)]
1x
0
- with x
0
close to 10 (z penetration
depth). Physically, it is attributed to the immobilization of the silver
METAL DIFFUSION IN POLYMERS, FAUPEL ET AL. 353
Figure 7.10 Monte Carlo simulations showing the trapping of evaporated mobile
silver atoms by an atomic dispersion of immobile chromium traps corresponding
to about 0.01 nm Cr. The top and bottom figures are top and cross-sectional
views, respectively. (a) Without traps, silver clusters form at the surface by random
nucleation, and a small amount of silver diffuses into the bulk. (b) The chromium
atoms act as nucleation centers and lead to a much finer dispersion of silver clus-
ters. Due to trapping in the surface clusters, diffusion into the bulk is effectively
blocked. The simulations were carried out with the program described by Faupel
et al.
[9]
and Thran
[82]
using a ratio of 100 for the surface and bulk diffusivities.
Ch_07.qxd 11/12/04 4:10 PM Page 353
354 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
atoms by cluster formation and trapping of the remaining highly mobile
single atoms deeper in the polymer bulk. This is in accord with calcula-
tions reported by Dabral et al.
[90]
and with our Monte Carlo simulations,
which also confirm the exp[(zz
0
)]
1x
0
-law as well as the magnitude of
x
0
.
[23, 82]
Moreover, the simulations directly show how a penetration pro-
file gradually becomes stationary, starting from the surface where
immobilization by clustering sets in first (see Fig. 7.12). Hence, the
simulations strongly support the present view of aggregation-induced
immobilization.
In contrast to the numerous investigations that demonstrate the very
effective impediment of metal diffusion into polymers by metal aggrega-
tion (see also Faupel et al.,
[9, 24]
there are many publications reporting
strong diffusion from continuous metal films into polymers. For example,
the drop of the metal intensity in XPS
[60, 92, 93]
and the broadening of metal
spectra in Rutherford backscattering
[92, 94]
upon annealing have been taken
Figure 7.11 Penetration profiles obtained after
110m
Ag deposition and annealing at
185°C for 70 min. The data are plotted on a semi-logarithmic scale versus the
square of the penetration depth. Diffusivities estimated from straight lines fitted
to the data in different depths ranges according to the thin-film solution of Fick’s
second law are displayed.
Ch_07.qxd 11/12/04 4:10 PM Page 354
as evidence for metal diffusion. Furthermore, it has been concluded from
angle-resolved XPS measurements that Cu diffuses strongly into the
low-k fluoropolymers Teflon-AF1600
TM[95]
and FLARE
TM
.
[93]
We have
shown, however, that XPS results on metal diffusion into polymers can be
rather misleading without complementary microstructural observations
and careful investigation of the long-time behavior of the metal inten-
sity.
[22]
Concerning the Rutherford backscattering results, it was recently
demonstrated that metal clustering at the polymer surface might mimic
metal diffusion into the polymer bulk.
[96]
Clustering is driven by the reduc-
tion in surface energy and may even take place after annealing of contin-
uous thin metal films.
[96]
To demonstrate the absence of significant diffusion from a continuous
metal film, we have evaporated Cu, with a nominal coverage of several
monolayers, onto polyimide at room temperature. Subsequently, the sam-
ples were annealed at high temperatures, and the Cu 2p
32
C 1s peak
METAL DIFFUSION IN POLYMERS, FAUPEL ET AL. 355
Figure 7.12 Monte Carlo simulations showing how a penetration profile becomes
gradually stationary [KA1] due to immobilization of metal atoms by aggregation.
Note that the immobilization starts from the surface where clustering sets in first.
The time scale is jpa. The simulations were carried out as described by Faupel et
al.
[9]
and Thran.
[82]
Here 0.08 monolayer of silver was evaporated during the first 10
4
Ch_07.qxd 11/12/04 4:10 PM Page 355
356 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
intensity ratio was measured as a function of the annealing time. An
example is given in Fig. 7.13. Similar measurements were also performed
for Au on TMC polycarbonate.
[76]
Without further quantitative evaluation,
the occurrence of the plateau in the intensity ratio unambiguously demon-
strates Cu immobilization. Any significant copper diffusion into poly-
imide would lead to a drop of the Cu 2p
32
intensity because of the very
short inelastic mean free path of the photoelectrons. Moreover, the C 1s
intensity would increase if Cu disappeared from the surface. The inset in
Fig. 7.13 demonstrates that no deviations from the plateau value were
detected, even after prolonged annealing for 30 hours at 350°C. TEM
investigations show that the Cu immobilization is due to the formation of
Cu clusters, which form during metal deposition at room temperature.
Annealing at 350°C causes the clusters to grow. As discussed in Sec. 7.3,
Cu clustering at polymer surfaces arises from the high cohesive energy of
the metal and the low metal-polymer interaction energy. Apparently, the
cohesive energy is high enough to immobilize the Cu atoms by prevent-
ing their dissociation from the clusters at temperatures far below the metal
boiling point. Cluster growth does not appear to occur by ordinary Ostwald
ripening, but by a temperature-induced mobility of smaller clusters and
cluster coalescence, thus causing a plateau in the Cu 2p
32
C 1s peak
Figure 7.13 Evolution of the Cu 2p
32
/C 1s peak intensity ratio in XPS upon
annealing at 350°C after deposition of 1.6 nm Cu (7 ML nominal coverage) onto
polyimide at room temperature at rates of 1.6 and 0.03 nm/min. The inset shows
results from a separate experiment involving room-temperature deposition (1.6
nm of Cu[KA2] at 0.16 nm/min.) and prolonged annealing for 30 hours at 350°C.
The leveling off is a clear indication of Cu immobilization.
Ch_07.qxd 11/12/04 4:10 PM Page 356
intensity due to the decrease in the cluster mobility and the increase in the
cluster distance with increasing cluster size.
[71]
Simple calculations
[22, 97]
show that the cluster growth observed in
TEM is the main reason for the small drop of the Cu 2p
32
C 1s peak
intensity ratio of about 40% during annealing (see Fig. 7.13). Even in the
initial annealing stages prior to complete Cu immobilization, Cu diffusion
into the polymer obviously plays a minor role at best.
On the other hand, capacitance-voltage measurements involving
Cu/oxide/polymer/oxide/Si capacitors suggest drift of Cu
ions into poly
(arylene ether) and fluorinated polyimide at elevated temperatures in
strong electric fields of the order of 10
6
V/cm, although the drift of Cu
ions through the polymers was not demonstrated directly.
[98, 99]
The drift
current dropped rapidly within several minutes and was not observed after
incorporation of thin-nitride barrier layers.
[99]
Benzocyclobutene proved to
be much less prone to Cu
ion drift.
[98–104]
Mallikarjunan et al.
[100]
studied interfaces of Cu, Al, Ta, and Pt with
hybrid organosiloxane polymer (HOSP) using bias-temperature stressing.
Surprisingly, they found an increasing extent of ion transport in the order
Cu Ta Al. No ions were seen in Pt, in accord with its very high ion-
ization potential. It was demonstrated that the metal-polymer interface
chemistry, particularly the presence of interfacial oxygen, plays a key role in
aiding metal ionization and subsequent mobile ion penetration into the
polymer. While Ta and Al form stable oxide or metal-silicon oxide layers
on SiO
2
, and Ta is even used as a diffusion barrier between Cu and SiO
2
,
it was suggested that insufficient surface oxygen content of HOSP leads
to the formation of metal-rich unstable and discontinuous oxides with
unbonded metal ions. Even the conventional barrier material Ta
x
N was
shown to fail in contact with HOSP due to the incorporation of oxygen
impurity or the reaction with surface oxygen, which can lead to tantalum
ionization and drift into HSPO. This clearly shows that conventional bar-
rier materials may not be suitable for the novel low-k dielectrics. Cu can
also oxidize readily. The interfacial oxides do not form protective barriers
and were suggested to be the source for mobile metal ions.
[101]
More ion
penetration was in fact observed when copper was deposited on highly
oxidized surfaces.
[100–103]
This implies that eliminating interfacial oxygen
can suppress the formation of mobile Cu ions.
The interpretation of the drift experiments does not conflict with the
present view of aggregation-induced metal immobilization in polymers.
No diffusion was observed at zero and low electric fields. Moreover, the
oxide layer between Cu and the polymer in the drift experiments appar-
ently acts as a catalyst for the formation of Cu
ions, which are injected
into the polymer in the electric field. These small Cu
ions are expected
to be highly mobile and, unlike neutral Cu atoms, unable to form immobile
METAL DIFFUSION IN POLYMERS, FAUPEL ET AL. 357
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358 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS
clusters because of their repulsion. This interpretation is supported by the
strong diffusion of K
and Cs
ions into the polyimid PMDA-ODA
[28, 29]
and of In
ions into PTCDA,
[30]
as mentioned earlier.
Reactive metals like Cr and Ti generally form strong bonds to the
polymer chains (see Sec. 7.3). Consequently, long-range diffusion appears
to be blocked completely. As mentioned earlier in this section, no indica-
tions of Cr diffusion into polyimide were found in XPS studies, even after
very slow evaporation near the glass transition temperature.
[70]
Interfaces
of polymers with Cr and Ti proved to be sharp and thermally stable,
[17, 60,69, 90]
and these metals were shown to act as diffusion barriers. In the case of Cr,
a coverage as low as 0.08 monolayer proved to be very effective.
[23]
Al
showed features of some mobility.
[17, 65]
The mobility of Ni turned out to
be somewhere in between that of Al and Cu. The substantial mobility of
Ni atoms was also seen in the formation of Ni clusters containing
nanocomposites with a matrix of polyimide
[105]
or Teflon AF
®[106]
pro-
duced by vapor phase co-deposition of the organic and metallic compo-
nent and subsequent annealing.
7.5 Summary and Conclusions
This chapter reviews the diffusion behavior of metals on polymer
surfaces and in the bulk. We focus on work carried out by our group in
Kiel involving radiotracer measurements, surface spectroscopy, electron
microscopy, Monte Carlo simulations, and other techniques.
The adsorption energy of noble metals on polymers is much smaller
than the metal-metal interaction energy, although it can exceed 0.5 eV, for
example, for Cu on polyimide and SiLK
®
. The activation energy for sur-
face diffusion is considerably lower. This reflects undirected bonding and
gives rise to a high surface mobility down to temperatures well below
room temperature. The high metal cohesive energy compared to the rela-
tively weak interaction energy of noble metals with polymers and to the
much lower polymer cohesive energy leads to a very strong aggregation
tendency. Therefore, noble metals exhibit a Volmer-Weber surface growth
mode (cluster formation) with a critical nucleus consisting of only one
atom. The same holds for other metals of low reactivity.
The strong aggregation tendency effectively impedes metal diffusion
into polymers. In particular, no significant diffusion of noble metal into
polymers is expected from continuous metal films unless interfacial reac-
tions, particularly the presence of interfacial oxygen, promote the forma-
tion of metal ions. These cations are highly mobile and repel each other,
thus preventing aggregation. In some cases, substantial ion drift has been
observed under bias temperature stress conditions of microelectronics
Ch_07.qxd 11/12/04 4:10 PM Page 358
devices where low voltages may already give rise to very strong electrical
fields due to the small feature sizes.
Isolated neutral metal atoms of low reactivity are also highly mobile
and diffuse into polymers at very low deposition rates, where immobiliza-
tion by aggregation is less effective. In the initial deposition stage, isolated
metal atoms impinge on the polymer surface. Therefore, trace amounts of
noble metal atoms always diffuse into polymers and can penetrate several
hundreds of nm even at room temperature. This calls for using barriers in
microelectronic chip applications. Reactive metals like Cr and Ti form rel-
atively sharp interfaces with polymers and act as diffusion barriers. We
have demonstrated excellent barrier function for a Cr layer as thin as 0.08
monolayer. The behavior of Al and Ni is somewhere between that of noble
metals and reactive metals like Cr. Both metals exhibit some mobility on
and in most polymers, but Al reacts strongly with fluoropolymers. The lat-
ter case shows that general trends must be treated with caution since the
diffusion behavior for a given metal-polymer combination is strongly gov-
erned by the individual chemical interactions.
Acknowledgments
We would like to thank Jörn Kanzow and Jörn Erichsen for stimulat-
ing discussions and critical reading of the manuscript, and Haile Takele
for technical assistance. Financial support by the Volkswagen Foundation
and German Science Foundation (DFG) is gratefully acknowledged.
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