Materials Science and Engineering B 109 (2004) 99–103
Electrical properties of thin rf sputtered aluminum oxide films
M. Voigt∗ , M. Sokolowski
Institut für Physikalische und Theoretische Chemie der Universität Bonn, Wegelerstraße 12, D-53115 Bonn, Germany
Abstract
Thin films of aluminum oxide (Al2 O3 ) were fabricated by rf magnetron sputtering. Different sputter conditions, e.g., composition of the
sputter gas (Ar:O2 ), sputter gas pressure, deposition rate, and preparation of the Al2 O3 sputter target before deposition were investigated
with the aim to achieve good insulating films with high electrical breakdown fields. The Al2 O3 films had a thickness of 160 nm and were
deposited on ITO covered glass. By evaporation of Au electrodes on top of the Al2 O3 films thin film capacitors were fabricated. Current
voltage (I–V) measurements were performed under high vacuum and temperatures between 4 and 300 K. Significant scattering of the I–V
curves and “burn-in” effects are observed. We find that an admixture of 1% of O2 in the sputter gas improves the electrical properties, but
higher breakdown fields and smaller leakage currents are obtained for sputtering in pure Ar, using a sputter target conditioned in an Ar:O2
mixture. Impedance spectra, revealed a dielectric constant of ∼7 for all Al2 O3 films. Atomic force microscopy experiments reveal that the
surfaces are rather rough with grain sizes in the order of 0.3–0.5 ␮m.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Aluminum oxide; rf magnetron; Sputtered films; Electrical properties
1. Introduction
Aluminum oxide (Al2 O3 ) is a widely used electrical insulating material. This is due to its high electrical breakdown field, its large bandgap, and its high dielectric constant.
In particular, Al2 O3 films with thicknesses in the range of
50–300 nm are interesting for the preparation of gate insulators in thin film field effect transistors (FETs). Thin films
of Al2 O3 can be fabricated by dc or rf magnetron sputtering
of either an Al target in Ar:O2 mixtures (reactive sputtering) [1–4], or by sputtering of an Al2 O3 target in pure Ar,
or Ar:O2 mixtures [4–12].
One aspect of the investigations in the last years has
been the optimisation of the dielectric properties, in particular the electrical resistivity and electrical breakdown fields
of Al2 O3 films fabricated by magnetron sputtering. However, the electrical properties of thin sputtered Al2 O3 films
were often only described by the dielectric constants (ε) and
breakdown fields [6,8–10,12]. Current voltage (I–V) curves
were reported only exceptionally [3–5,7,11]. Nevertheless,
these are also of high interest, since they contain information about the electrical conduction mechanisms leading to
∗ Corresponding author. Tel.: +49-228-73-2520;
fax: +49-228-73-2551.
E-mail address: [email protected] (M. Voigt).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2003.10.056
unwanted leakage currents, e.g., across the gate insulator in
a FET structure, and may contain information on the mechanism of the electrical breakdown, in addition to the statistical
analysis of the breakdown fields [9]. In the present paper, we
report I–V curves measured for sandwiched Al2 O3 films in
detail. Our aim is to demonstrate the influence of the sputter
parameters and to gain insight in the origin of the leakage
currents and their relation to the dielectric break down fields.
In addition to the electrical measurements, atomic force microscopy data of the surface morphology will be reported.
2. Experimental
The aluminum oxide films were prepared by rf magnetron
sputtering (13.56 MHz) from an Al2 O3 target (1 in. diameter)
with a nominal purity of 99.99% in a high vacuum chamber
with a base pressure of typically 2 × 10−6 mbar. The target
to sample distance was about 10 cm. Sputter powers were
between 20 and 200 W. The sputter gas pressure was varied
between 1.3 × 10−3 and 8.0 × 10−3 mbar. Either pure Ar
(99.998%), or Ar:O2 mixtures were used as sputter gases.
Further details of the sputter conditions are given in Table 1.
As substrate we used commercially available indium tin
oxide (ITO) covered glass slides (25 mm × 25 mm). Prior to
sputtering they were cleaned by organic solvents, H2 O2 , and
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M. Voigt, M. Sokolowski / Materials Science and Engineering B 109 (2004) 99–103
Table 1
Summary of sputter conditions of Al2 O3 films
Sample
Sputter gas
composition
Sputter gas
pressure (mbar)
Base pressure
(mbar)
Sputter rate
(nm min−1 )
Sputter
power (W)
Conditioning
of targeta
TS b (K)
A
B
C
D
E
F
G
H
I
Ar
Ar/O2 (10:1)
Ar/O2 (100:1)
Ar
Ar
Ar
Ar
Ar
Ar
8.0 × 10−3
8.0 × 10−3
8.0 × 10−3
8.0 × 10−3
8.0 × 10−3
1.3 × 10−3
1.3 × 10−3
1.3 × 10−3
1.3 × 10−3
1.1 × 10−6
2.1 × 10−6
1.6 × 10−6
1.6 × 10−6
2.0 × 10−6
1.9 × 10−6
2.0 × 10−6
6.0 × 10−7
8.0 × 10−7
3.4
1.4
2.3
3.5
3.5
5.1
2.3
5.0
0.5
200
200
200
200
200
200
100
200
20
Yes
Yes
Yes
Noc
Yes
Yes
Yes
Yes
Yes
307
307
307
308
265
300
302
255
303
All oxide films had a nominal thickness of 160 nm.
a Conditioning of target: sputtering in Ar:O (1:1) for 30 min at 200 W. For B and C, the target was conditioned in an Ar:O gas mixture of the same
2
2
composition, as used during the sputter process, for 30 min.
b T denotes the substrate temperature.
S
c Sputtering of the target in pure Ar for 30 min before starting deposition.
Fig. 1. Current density vs. electric field strength of four thin Al2 O3 films prepared with different sputter conditions (a–d). The films correspond to the
samples D (a), B (b), C (c) and A (d) of Table 1. For each film I–V curves measured on different contacts are shown in order to demonstrate the
scattering of the data.
M. Voigt, M. Sokolowski / Materials Science and Engineering B 109 (2004) 99–103
deionized H2 O. ITO was chosen, because of its optical transparency. Typically the substrate was water cooled and held at
temperatures between 300 and 308 K during the sputtering.
The Al2 O3 film thickness (d = 160 nm) was monitored by a
quartz microbalance, calibrated by additional measurements
with a surface profilometer. Finally, eight Au top contacts
(area: 5.5–6.7 mm2 , thickness 30–50 nm) were deposited on
top of the Al2 O3 films in the same vacuum chamber using
a thermal Au evaporation source and a shadow mask. I–V
measurements were performed in a He cryostat under high
vacuum using a Keithley 485 picoamperemeter. The voltage increase was 0.2–0.4 V s−1 . E was calculated from the
applied voltage U as E = U/d. All reported I–V curves are
original data, without any smoothing or averaging over several scans. The I–V curves were independent of the polarity.
For the determination of the dielectric constant from the capacitance, impedance spectra were recorded by a Schlumberger impedance analyser for f = 1 − 106 Hz. Atomic
force microscopy (AFM) images were obtained at ambient
conditions.
3. Results and discussion
3.1. Variation of the sputter conditions
Fig. 1 displays I–V curves which were measured for
Al2 O3 films prepared under four different sputter conditions (a)–(d). Irrespectively of the preparation conditions,
we observe significant burn-in effects at field above ca.
0.5 MV cm−1 , which cause a systematic decrease in the
current during each voltage ramp. We speculate that the
effect is partially related to the burn out of metallic shorts
due to nanopores in the Al2 O3 films. We also find significant variations of the I–V curves (up to several orders of
magnitude in current) measured for different contacts on
the same Al2 O3 film (see, e.g., Fig. 1(c)). We think that this
is due to a local variation of the Al2 O3 film quality, including the local thickness variations, in combination with the
statistical occurrence of the dielectric breakdown [9].
If the sputtering was performed in pure Ar (Fig. 1(a)),
the electrical film quality was very bad, and high, approximately ohmic, currents were observed already at low fields.
After sputtering, the Al2 O3 target exhibited a grey colour,
likely due to the enrichment of Al. This result is in agreement with the finding of Segda et al. [12], who observed
that sputtering in pure Ar, yields Al2 O3 films which are
under-stoichiometric in oxygen and thus exhibit lower electrical quality.
Better films could be prepared, if 10% (Fig. 1(b)) or 1%
of O2 (Fig. 1(c)) was added to the sputter gas. In this case
the Al2 O3 sputter target maintains its white colour during
the sputter process. For these films the dielectric breakdown
can be clearly observed as a sharp increase of the current by
about four to five orders in magnitude at a critical breakdown
field (Eb ) (see Fig. 1(b) and (c)), leading to an irreversible
101
change of the I–V curve (not illustrated). As can be seen from
Fig. 1(b) and (c), the reduction of the O2 admixture from
10 to 1%, leads to an increase in Eb by about a factor of 3,
i.e. from about 0.3–0.9 MV cm−1 . After the breakdown, we
could observe small spots (diameter about 10–150 ␮m) on
the Au contacts, where the metal was evaporated from the
sample, likely due to a local heating at breakdown channels.
Films of even higher electrical quality could be fabricated,
if the sputter target was sputtered in an Ar:O2 mixture prior
to the sputtering of the film (“conditioning of the target”),
and the sputtering of the film was then performed in pure
Ar (see Table 1). For such films, a step-like increase of the
current, indicating the dielectric breakdown (as seen, e.g., in
Fig. 1(b)), could not be observed. Instead, the current more
gradually increased with the electric field (see Fig. 1(d)). A
typical nominal resistivity (ρ = E/j) at E = 2.5 MV cm−1
and room temperature is 2.5 × 1011 cm, which is in the
range observed by others [3].
The reason, why the conditioning of the target is better
than the admixture of O2 , is still under investigation. A very
plausible mechanism has been suggested in by Schneider
et al. [2], namely that activated O species from the O2 containing sputter gas react with residual water from the rest
gas and form OH− ions. These then lead to the formation
of aluminium hydroxide in the film and thus to a lower film
quality [2]. Presumably, the OH− formation is much smaller,
Fig. 2. Current density vs. electric field strength curves of one Al2 O3 film
(sample I of Table 1) on a log log scale at different temperatures. The
curves were measured on different contacts starting at low temperatures.
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M. Voigt, M. Sokolowski / Materials Science and Engineering B 109 (2004) 99–103
if pure Ar is used as the sputter gas. Thus conditioning of
the target in Ar:O2 mixtures prior to the sputtering appears
to be a good compromise which allows to achieve high stoichiometries of oxygen to aluminium in the film, but avoids
the detrimental formation of aluminium hydroxide.
In addition to the variation of the sputter gas composition, we have also tested the influence of lower substrate
temperatures (sample E and H, Table 1), smaller base pressures (samples H–I), and smaller sputter powers (samples G
and I). However, for all these conditions we find I–V curves
which are comparable with those of sample A (Fig. 1(d)).
We thus conclude that within the accessible parameter space,
we have reached an optimum, and that further optimisation
to higher values of Eb or ρ will require other strategies, e.g.
reduction of the film thickness [3,9] or using ultra high vacuum conditions.
Very recently Lee et al. [5] have reported I–V curves for
Al2 O3 films on ITO-glass which were prepared by the same
technique we used. Within the variation of the sputter conditions tested by Lee et al., their “best” films are comparable with ours. However, details of the I–V curves and of the
“best” sputter parameters differ from those we found. This
may possibly be due to small differences in the sputter con-
ditions, in particular the preparation of the sputter target. It
points to the importance of a very accurate control of these
parameters for reproducible results.
3.2. Dielectric constant and temperature dependence of
the I–V curves
According to impedance measurements (not shown) the
dielectric constants ε of our Al2 O3 films were typically
7.0 ± 0.2 with no systematic variations with the sputter conditions. The range of reported ε values is 7–10 [5,6,8,12,13].
Our low ε values could be related to the low densities of
the amorphous films, as suggested in [13]. Fig. 2 shows I–V
curves on a double log plot at different temperatures for a
film of good electrical quality (I of Table 1). We note that
these curves also exhibit strong variations for the different
contacts and different I–V measurements. For clarity, Fig. 2
shows only some typical curves. Principally, the I–V curves
were first taken at 4 K, and than at 100 and 300 K. As one can
see from Fig. 2, there is a considerable scattering in the I–V
curve for 300 K above 0.25 MV cm−1 , which points to the
statistical character of the charge transport across the films.
Ref. [7] discusses Poole–Frenkel emission, field ionisation
Fig. 3. AFM image of a thin magnetron sputtered alumina film (sample C of Table 1) for three different magnifications: 1000 nm × 1000 nm (a) and (b),
5 ␮m × 5 ␮m (c), and 10 ␮m × 10 ␮m (d).
M. Voigt, M. Sokolowski / Materials Science and Engineering B 109 (2004) 99–103
and trap hopping in this context. From about 0.25 MV cm−1
onward, the current increases with high exponents of the order of 10. However, this increase of the current is not an
electrical breakdown, because the curves are still reversible,
allowing to measure several comparable curves in sequence.
As expected for temperature assisted processes [7], the currents at 4 K are considerably smaller compared to 300 K by
a factor of up to six orders in magnitude (see Fig. 2).
3.3. AFM investigation of the surface morphology
Fig. 3 displays AFM images of a sputtered Al2 O3 film.
One can clearly observe a rough surface with small grains
of about 0.3–0.5 ␮m diameter. The rms surface roughness
of the oxide films is about 4 nm. This type of surface morphology was observed for the samples A to D and F, H, I of
Table 1, irrespectively of the preparation conditions. Possibly, it is due to the roughness of the underlying ITO surface,
which was measured to be the same. Evidently this surface
morphology needs to be improved considerably, if the Al2 O3
films should be used as substrates for further growth of films.
4. Conclusions
Al2 O3 films with the smallest leakage currents (nominal
resistance about 2.5×1011 cm at 2.5 MV cm−1 ) and highest dielectric break down fields were obtained, if the films
were grown in pure Ar gas after the sputter target has been
conditioned in an Ar:O2 mixture. This preparation was more
successful than sputtering in pure Ar or Ar:O2 mixtures. For
103
reproducible results, the preparation of the target prior to
sputtering turned out to be essential.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft. We thank L. Knoth, A. Schmidt, M. Schneider,
S. Schmitt and E. Umbach for performing the AFM measurements, and D. Gauer for technical assistance.
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