st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Ar plasma assisted sputtering of silver nanolayers and their thermal stability
J.Siegel, E. Podkorytov, A. Řezníčková, O. Kvítek, V. Švorčík
Department of Solid State Engineering, Institute of Chemical Technology in Prague, Technická 5, 16628 Prague,
Czech Republic
Abstract: Ultra-thin Au films sputtered on glass substrate were investigated. The samples
were annealed in air at atmospheric pressure and in evacuated chamber. The dependence of
the annealing process on ambient pressure was studied. The thermally induced changes in
surface morphology were followed by atomic force microscopy. Changes induced by elevated
temperature have a great impact on dielectric, optical and physical properties of the prepared
structures. UV-Vis spectroscopy was used to investigate optical parameters of annealed layers.
It was found that reduced ambient pressure stabilizes continuous structure of the thin gold
film during annealing process.
Keywords: Gold, sputtering, annealing, surface morphology
1. Introduction
Thermal stability of ultra-thin metal layers is of great
importance in vast range of applications starting in electronics and ending with barrier packaging material, where
thermal stress induces dramatic changes of desired product properties. Potentially, this process can lead to the
rupturing of continuous metal coverage, causing formation of a discontinuous structure accompanied by significant decrease of barrier properties or conductivity.
Conductivity falls down to the values of electrical insulator is undesirable especially in electronic applications
where low resistance is needed. Thermally treated ultra-thin metal layers, however, give rise to structures
which possess entirely new properties that are very uncommon for such material. This is particularly true for
gold and silver nanostructures [1-5].
Besides interesting catalytic, electronic or bactericidal
properties noble metal nano-(structures/particles) also
exhibit distinctive shape dependent optical properties that
have attracted great technological interest. This is particularly true for gold nanostructures [1,3]. Thin gold films,
both continuous and discontinuous, have been of great
interest for „green“ nanotechnologies. Some pioneering
works related to energy efficient windows dates back to
early 1950s [6]. Particularly detailed studies were made in
the 1980s [7] and the interest in high-performance transparent conductors and infrared reflectors continues still
today not only for Au films [8] but also for films based on
other metals e.g. Ag [9-11]. Ultra-thin layers of noble
metals have been used in medicine for decades. For example silver, that is non-toxic to eukaryotic cells, is bactericidal and has been used topically to prevent infections
in clinical settings, such as burn wounds [12] and toxic
epidermal necrolysis [13].
Different methods have been used to produce gold
nano-particles or nano-islands on silicon or glass substrates. Coherent gold nano-islands were prepared on
(100) oriented Si substrates by a physical method combining thermal evaporation of thin gold layer and its successive annealing [14]. More frequently, magnetron sputtering, thermal and e-beam evaporation methods combined with post-deposition thermal annealing processes
were used to fabricate gold nano-islands and
nano-particles on substrates [1,2,15,16].
In this work, ultra-thin gold layers prepared by plasma
assisted DC sputtering on glass substrate and their changes induced by thermal treatment are studied. Effect of
ambient gas pressure during annealing process on electrical and optical properties of sputtered thin Au structures is
investigated too. AFM method is used to investigate
changes in surface morphology, which has crucial impact
on electrical and optical properties of resulting structures.
Electrical resistance of prepared structures is measured by
two-point method. UV-Vis absorption spectra are recorded with the aim to follow changes in optical properties
accompanying the surface structure transformation.
2. Experimental
Gold layers were sputtered on 1.5 × 1.5 cm2 borosilicate microscopic glass, supplied by Glassbel Ltd., CR.
Average surface roughness of the glass substrate was (in
area 1.5 × 1.5 μm2) Ra = 0.18 nm (measured at five different positions). Sputtering was accomplished on Balzers
SCD 050 device from gold target (supplied by
Goodfellow Ltd., purity 99.99 %). The deposition conditions were: DC Ar plasma, gas purity 99.995 %, gas
pressure of 4 Pa, discharge power of 7.5 W, sputtering
time ranging from 4 to 500 s. Under these experimental
conditions homogeneous distribution of gold over the
glass surface is achieved. Post-deposition annealing of
Au-coated glass was carried out at 300°C (±3°C) for 1
hour either in air using a thermostat Binder oven and or in
evacuated (p = 10-3 Pa) heated chamber. In both cases
the heating rate was 50°C min-1 and the annealed samples
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
were left to cool in air to room temperature. For electrical
resistance measurements, additional Au contacts were
sputtered over the prepared layers at the discharge power
of 15 W for 130 s.
The surface morphology and roughness, characterized
by Ra, of the gold coated glass samples were examined
using AFM (Digital Instruments CP II Veeco), working in
tapping mode with silicon P-doped probes RTESPA-CP
and with a spring constant of 20–80 Nm-1. Evaluation of
Au layer thickness (effective thickness) was carried out
by AFM-scratch method on the AFM scan done in contact
mode [17]. Thickness variations do not exceed 5%. All
scans were acquired at a scanning rate of 1 Hz. In the
present experiment, structure homogeneity was tested by
a scratch technique at five different positions.
Electrical sheet resistance (Rs) of the gold structures
was determined by a standard two-point technique (direct
Ohm’s method) using KEITHLEY 487 pico-ampermeter.
The electrical measurements were performed at a pressure
of about 10 Pa to minimize the influence of atmospheric
humidity. The typical error of the sheet resistance measurement did not exceed ±5%.
UV-Vis absorption spectra were measured using PerkinElmer’s Lambda 25 UV-Vis-NIR Spectrometer in the
spectral range 300-800 nm at scanning rate of 240 nm
min-1 and a data collection interval of 1 nm. Pristine glass
slide was used for background measurement. The typical
data uncertainty obtained under this arrangement is below
±5%.
3. Results and discussion
Thickness of sputtered layers was measured by AFM.
Dependence of the layer thickness on sputtering time is
displayed in Fig. 1. Linear dependence between sputtering
time and structure thickness is evident even in initiatory
stage of the layer growth. This finding is in contradiction
with results obtained earlier for Au sputtering on
polyethyleneterephthalate [18]. In that case, in the initial
stage of the layer growth the deposition rate was lower.
AFM images of Au coated glass samples (as-deposited,
annealed in air and annealed in evacuated chamber) are
shown in Fig. 2 (deposition time 75 s) and Fig. 3 (deposition time 200 s). For the sake of comparison appropriate
(a)
(b)
(c)
Ra = 15.5 nm
Ra = 3.6 nm
Fig. 2 AFM images of gold strucutres deposited for 75 s
showing (a) as-sputtered, (b) air annealed and (c) vacuum
annealed samples. Ra is surface roughness in mn.
vertical scale was chosen for corresponding images. Surface morphology of the as-deposited films exhibits a decrease of the surface roughness with increasing deposition
time, the decrease being a consequence of the layer
growth mechanism [19]. During initiatory stage of the
(a)
Ra = 4.0 nm
(b)
(c)
Fig. 1 Dependence of the layer thickness (effective thickness) on the sputtering time.
Ra = 4.0 nm
Ra = 15.5 nm
Ra = 3.6 nm
Fig. 3 AFM images of gold strucutres deposited for 200 s
showing (a) as-sputtered, (b) air annealed and (c) vacuum
annealed samples. Ra is surface roughness in mn.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
layer growth isolated gold islands (clusters) are formed.
Further deposition leads to formation of connections between clusters and the deposited layer exhibits worm-like
structure which with ongoing deposition turns into homogeneous and uniform gold coverage (see Fig. 3a). The
annealing has dramatic effect on surface morphology of
the thin gold films. The formerly continuous Au layer
changes during the annealing process into island-like
structure. When the layers are annealed in air, pronounced
coalescence of the deposited material occurs and originally flat surface structure changes into discontinuous
islands [2]. Reduction of ambient pressure, however,
causes stabilization of continuous coating and changes in
surface morphology are thus less significant. Observed
island-like structure is much smoother compared to samples annealed in air. The sample sputtered for 200 s and
annealed in evacuated chamber exhibits homogeneous,
and from the material point of view, continuous layer
which possesses considerable conductivity (see Fig. 4).
Fig. 4 Dependence of sheet electrical resistance on sputtering
time for as-deposited, annealed in air and annealed in vacuum samples.
The dependence of electrical sheet resistance (Rs) of the
Au structures on the sputtering time is shown in Fig. 4.
Priority was given to the dependence on sputtering time
since the accuracy of AFM thickness determination is
limited for short deposition times. It is well known that a
rapid decline of sheet resistance of the sputtered layer
indicates a transition from the electrical discontinuous to
the electrical continuous layer [20]. One can see that the
most pronounced change in the sheet resistance of
as-sputtered layer occurs between 30 and 50 s of sputtering times, corresponding to approximate 6 and 10 nm
layer thicknesses respectively. Thus, the layers with the
thickness below 6 nm can be considered as discontinuous
ones, while the layers with a thickness above 10 nm are
definitely electrically continuous. Dramatic change in the
sheet resistance occurs after the thermal annealing. The
samples annealed in air are electrically discontinuous up
to the sputtering time of 400 s when the continuous coverage is created and a percolation limit is overcome.
However, even for longer sputtering times up to 500 s the
sheet resistance changes slowly and it does not achieve a
saturation which is observed on the as-sputtered and vacuum annealed samples. This phenomenon results from
structural changes of the Au layer during post-deposition
thermal annealing in air atmosphere. Morphology changes
were discussed in detail above (see Figs. 2,3). Rapid decrease of the sheet resistance as observed for as-sputtered
layers for deposition times between 25 to 50 s (see Fig. 4)
was not seen in this case. Transition from electrically discontinuous to electrically continuous layer in the case of
the samples annealed in air is more gradual and occurs
between the deposition times 250–500 s. Concerning
vacuum annealed samples the situation is completely different. One can see that the intensive thermal annealing
leads to only mild shift of the resistance drop towards
longer deposition times. Transition from electrically discontinuous to continuous layer occurs in narrow interval
of deposition times ranging from 100 to 150 s. Reduction
of ambient pressure during the annealing process thus
significantly reduces the minimal gold thickness necessary to maintain electrical transport properties of ultra-thin layers from about 70 nm (air annealed samples) to
about 30 nm (vacuum annealed samples).
UV-Vis absorption spectra of as-deposited and annealed
samples (in air and in vacuum) for various deposition
times are shown in Fig. 5. As expected, the absorbance
increases with increasing deposition times so as the Au
layer becomes thicker. Ultra thin as-deposited layers as
well as gold nano-structures created as a consequence of
the thermal annealing exhibit a distinctive absorption
peak at around 550 nm related to surface plasmon resonance (SPR) [21,22]. Spectra of as-deposited samples
possess the SPR peak only at deposition times below 20 s.
Nevertheless, in this case the peak is hardly identifiable
and widespread reflecting insufficient separation of fundamental building blocks of gold layer (clusters) in initiatory stadium of the layer growth. Samples sputtered for
50 s and more have absorption spectra typical for that of
bulk gold. As we have already shown earlier, annealing of
the samples in air has significant impact on their surface
morphology. When a system of isolated islands is formed,
quantum confinement of electrons become dominant and
the samples show strong SPR peak even for relatively
long deposition times (up to 100 s). The surface plasmon
peak is shifted from 610 to 530 nm as the nominal layer
thickness decreases from 20 to 5 nm. It is well known that
optical absorption of island films of gold is a function of
island density [23]. The absorption band resulting from
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
bound plasma resonance in the particles is shifted to
longer wavelengths as the island density increases. As the
effective layer thickness becomes greater, the absorption
band width increases owing to a wider particle size distribution.
Contrary to that, after the annealing process at low
pressure, leading to surface morphology stabilization,
SPR peak is not seen in the sample spectrum deposited for
deposition times above 100 s. Broad receding peaks,
however, are still clearly identifiable for shorter deposition times (50-20 s). Even though the spectra of vacuum
annealed samples deposited for 100-200 s do not show
SPR peak, they seem to be affected by the annealing procedure and their absorbance is generally lower than that
of as-sputtered samples, reflecting the fact that localized
absorption characteristic of gold films is highly sensitive
to the particle size, surface structure, shape, and surrounding medium [2,24].
(b)
(c)
Absorbance
(a)
Fig. 5 UV-Vis absorption spectra of Au thin films sputtered
for different times (inset numbers refer to deposition time in
seconds): (a) – as-sputtered, (b) – vacuum annealed and (c) –
air annealed samples.
4. Conclusion
Ultra-thin Au films were prepared by sputtering on
glass substrate and their behaviour under thermal annealing in air and in vacuum were investigated by different
techniques. The as-deposited film thickness was found to
be linear function of the deposition time. Surface roughness of as-deposited films decreases with increasing deposition time. Thermal annealing leads to an increase of the
surface roughness due to formation of gold clusters which
is much more pronounced on the samples annealed in air.
Formation of the gold clusters leads to an increase in the
film sheet resistance, significant shift of the electrical
percolation threshold towards longer deposition times and
occurrence of specific UV-Vis absorbance peak typical for
surface plasmon oscillations in gold nanostructures.
Annealing conditions were found to have crucial impact
on the structure of gold film and its optical and electrical
properties. Annealing under vacuum leads to the formation of more homogeneous and stable gold layer. This
finding is of interest for applications, where thin film continuity and low electrical resistance are important.
5. Acknowledgment
Financial support of this work from the GACR projects
No. P108/11/P337 and P108/12/G108 is gratefully
acknowledged.
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