Haye-Phenomenological study of iron and lanthanum magnetron co-sputtering using two reactive gases-2016

Surface & Coatings Technology 298 (2016) 39–44
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Surface & Coatings Technology
journal homepage: www.elsevier.com/locate/surfcoat
Phenomenological study of iron and lanthanum magnetron
co-sputtering using two reactive gases
Emile Haye, Fabien Capon, Silvère Barrat, Denis Mangin, Jean-François Pierson ⁎
Institut Jean Lamour (UMR CNRS 7198), Université de Lorraine, Parc de Saurupt, 54011 Nancy Cedex, France
a r t i c l e
i n f o
Article history:
Received 31 January 2016
Revised 4 April 2016
Accepted in revised form 18 April 2016
Available online 20 April 2016
Keywords:
Oxynitride perovskite
Sputtering mechanism
Thin films
a b s t r a c t
Due to their tunability, oxynitride perovskites are mainly studied for new applications as smart devices. The miniaturization of such component has led to thin film synthesis and reactive magnetron sputtering from metallic
targets is an efficient way for such objective. However, co-sputtering in two reactive gases involves complex
mechanisms, and studies are usually dedicated to highly reactive element (Ti, Si, Nb, Al, Zr). We purpose a phenomenological study of lanthanum and iron co-sputtering in Ar/O2/N2 atmosphere. In a first step, single target
sputtering with one and two reactive gases has been studied. Elemental and compound sputtering modes have
been determined. La target exhibits a clear hysteresis behavior while Fe shows a reversible one as a function of
the reactive gas flow rate. In a second step, La and Fe targets have been co-sputtered using one or two reactive
gases. In each case, the mechanisms involved are mainly driven by La target, with a shift of the hysteresis loop
to higher gas flow rate. Then, a two dimensional diagram is purposed with delimitation of sputtering regimes,
depending on reactive flow rate (N2 and O2). Finally, two LaFeO3 − xNx multilayers have been synthesized
with two initial atmospheres (Ar/O2 ratio are 21/7 and 21/1 sccm) where nitrogen is introduced step by step.
SIMS analysis and voltage measurements during sputtering, deposition rate, electrical resistivity and UV–visible
measurements show that nitrogen incorporation can be roughly controlled choosing the atmosphere composition. Selected conditions have been determined to deposit oxynitride perovskite thin films with an optimized
nitrogen content to tune the film optical properties, with Ar/O2/N2 ratio of 21/1/6.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Magnetron sputtering is a well-known process used for oxide and
nitride thin film synthesis which has been widely studied since the
70s. Reactive sputtering with one or two targets in the presence of
one reactive gas can be easily used to deposit single metal or complex
oxides and nitrides [1–10]. A second reactive gas can be added to the
gas mixture to deposit oxynitride thin films. The control of the reactive
gas flow rates allows controlling the film stoichiometry [11–16]. Different authors have developed and upgraded models permitting a deep
understanding of the reactive sputtering process of one or two targets
[17,18]. The effect of target poisoning using one reactive gas O2 or N2
during the sputtering of one metallic target is well described. Depending on the secondary electron yield of the ceramic compared to the
metal one, an increase or a decrease of the target voltage can be noticed
during the switching between elemental and compound sputtering
mode (ESM and CSM) [19]. However, the case of two metallic targets
in the presence of two reactive gases is almost not reported yet. Highly
reactive system such as titanium, chromium, silicon, zirconium or
aluminum have been studied [20–22], but co-sputtering of metal
⁎ Corresponding author.
E-mail address: jean-francois.pierson@univ-lorraine.fr (J.-F. Pierson).
http://dx.doi.org/10.1016/j.surfcoat.2016.04.039
0257-8972/© 2016 Elsevier B.V. All rights reserved.
targets for oxynitride perovskites synthesis (rare earth or alkaline
elements associated with transition metal) is almost not reported.
Nevertheless, such oxynitrides are very promising candidates for new
applications due to the possibilities of properties tailoring by tuning
nitrogen/oxygen ratio [23–25]. Nitrogen doping of ABO3 perovskite
leads to bandgap reduction with modification of optical and electrical
properties. Such oxynitride thin films are mainly synthesized by post
annealing of oxides under NH3, or using precursor oxide target
sputtered in the presence of nitrogen [23,24,26]. For a better understanding of mechanisms encountered during the oxynitride perovskite
synthesis from co-sputtering of metallic targets using two reactive
gases, it is important to study such systems.
LaFeO3 is one of a well-studied orthorhombic perovskite with potential applications as solid-oxide fuel cell (SOFC), gas sensors or pigments.
Its anionic substitution (i.e., substitution of O atoms by N ones) has been
scarcely study. However promising results (different pigment color,
bandgap and transport properties modification) have been reported
for bulk materials [27]. Among the various processes that can be used
to synthesize such oxynitride, reactive magnetron sputtering is an
efficient method to tune the oxygen to nitrogen ratio [8,11]. In order
to investigate sputtering mechanisms, in situ target voltage measurements have been performed associated with secondary ion mass
spectroscopy measurements on as-deposited films. Deposition rate
40
E. Haye et al. / Surface & Coatings Technology 298 (2016) 39–44
Table 1
Experimental conditions.
Pressure
0.5 Pa
Distance Fe target — substrate holder
Distance La target — substrate holder
Current on Fe target
Power on La target
Oxygen flow rate range
Nitrogen flow rate range
7 cm
5.5 cm
0.2 A
110 W
0–10 sccm
0–30 sccm
and electrical resistivity measurements have been also used to support
explanations due to the direct influence of nitrogen and oxygen content.
UV–visible spectrometry is also used as direct signature of the
qualitative nitrogen content on the optical band gap.
2. Experimental
LaFeO3 and LaFeO3 − xNx films were deposited by reactive magnetron co-sputtering from two metallic targets in a 50 L chamber,
equipped with turbomolecular pump backed by a mechanical pump.
Iron target (diameter: 50 mm, thickness: 0.25 mm, purity: 99.5%) was
sputtered by DC sputtering, while lanthanum (diameter: 50 mm,
thickness: 3 mm, purity: 99.9%) was sputtered with pulsed-DC
(50 kHz, toff = 4 μs). The base pressure before deposition was close to
10−5 Pa. Argon, oxygen and nitrogen flow rates were controlled by
Alphagaz mass flowmeters with a resolution of 0.5 sccm. Preliminary
trials permitted to optimize the distances between targets and substrate
rotating holder. The distance was 7 cm for iron, while that of lanthanum
was 5.5 cm. The sputtering of the iron target was performed at a
constant current while the power was fixed for the La target [28].
Table 1 summarizes the deposition conditions used within this work.
Electrical resistivity measurements at room temperature were
performed using the 4 point probe method. The deposition rate was
estimated from thickness measurements using a tactile profilometer.
UV–visible measurements were performed using a Varian Cary 5000
spectrophotometer. Secondary ion mass spectroscopy (SIMS) measurements have been performed with Cs+ ion with Cameca IMS7F.
3. Results and discussion
3.1. Instability region of Fe, La and Fe–La systems in Ar/O2, Ar/N2 and Ar/O2/
N2 mixtures
Before studying the deposition of oxynitride perovskite thin films,
preliminary experiments have been performed using one or two targets
and only one reactive gas (O2 or N2). The effect of the oxygen flow rate
on the target voltage of a single target sputtering is presented in Fig. 1a.
The behavior of iron and lanthanum targets is clearly different. Before
introducing oxygen, the voltage on the La target is approx. 325 V,
corresponding to the elemental sputtering mode (ESM). An abrupt
decrease of the voltage to approx. 115 V is noticed when the oxygen
flow rates exceeds 2 sccm and the target is working in the compound
sputtering mode (CSM) [29,30]. The hysteresis loop is attributed to
getter effect [31,32]. On the other hand, the evolution of the iron target
voltage as a function of the oxygen flow rate is quite reversible [1].
Hysteresis is too thin to be clearly observed, and the behavior of the
target is considered to be almost reversible. Determination of elemental
and compound mode is not easy due to continuous voltage variation:
films of FexOy have been synthesized at 2 and 4 sccm, and are partially
transparent, indicating that CSM is reached from 2 sccm. However, we
assume that fully sputtering mode is reached when voltage stays constant, indicating stabilization of secondary electron emission coefficient
and complete target poisoning, even if compound sputtering mode is
observed with lower reactive flow rate. The co-sputtering of these two
targets in Ar–O2 mixtures does not modify the shape of the target
voltage variation: La still exhibits an instability while a reversible process is evident for Fe (Fig. 1b). However, the transition regime on La target is shifted to higher oxygen flow rate (+0.5 sccm) when two targets
are sputtered. Such an increase can be explained considering the increase of the sputtered atoms flux and the oxygen consumption by
the Fe target. Then, a higher amount of oxygen is required to fully
oxidize the La target. Such a shift is hardly observed on the less reactive
iron target.
Since metallic atoms are less reactive with nitrogen than oxygen, a
wider flow rate range has been studied for the sputtering of La and Fe
targets in Ar/N2 reactive mixtures. The La target voltage evolution as a
function of the nitrogen flow rate (Fig. 2a) is different to that previously
obtained using oxygen. Within the 0–2.5 sccm range, a slight increase
from 325 to 345 V is noticed. After reaching a maximum value, the La
target voltage decreases slightly. This behavior can be attributed to the
change of the secondary electron emission coefficient, due to
chemisorption of reactive species [33]. Further increase of the nitrogen
flow rate comes with an abrupt decrease of the voltage, suggesting a
transition in the sputtering mode at 7 sccm from ESM to CSM. A change
of the plasma color is also noticed during this transition due to change of
excited species. However, the hysteresis width is low, i.e. approx. the
nitrogen flow rate step used to plot this figure (0.5 sccm). In the CSM,
a continuous decrease of the target voltage is still evident. It may be
attributed to the gradual increase of the target poisoning and to the
related significant increase of deposition pressure due to the rather
low pumping speed. The shape of the iron target voltage as a function
of the nitrogen flow rate (Fig. 2a) strongly differs from that obtained
using oxygen as reactive gas (Fig. 1a). However a reversible behavior
is still noticed. After a slight increase of the voltage, a stabilization occurs
when the flow rate exceeds 10 sccm, indicating a fully compound
sputtering mode. The reversible behavior of the Fe target alone in Ar/
O2 and Ar/N2 is in accordance with previous study from Petitjean et al.
[1]. Due to lower reactivity of La and Fe with nitrogen compared to
Fig. 1. Evolution of the iron and lanthanum target voltage as a function of the oxygen flow rate. (a) Fe and La targets are sputtered separately; and (b) Fe and La targets are co-sputtered.
E. Haye et al. / Surface & Coatings Technology 298 (2016) 39–44
41
Fig. 2. Evolution of the iron and lanthanum target voltage as a function of the nitrogen flow rate. (a) Fe and La targets are sputtered separately, and (b) Fe and La targets are co-sputtered.
oxygen, metallic regime is stabilized until higher nitrogen flow rate. The
co-sputtering of La and Fe targets does not modify the shape of the voltage — flow rate curve (Fig. 2b). A slight shift to higher nitrogen flow rate
is evident to the higher consumption of nitrogen resulting from the increase of the sputtered atom amount. Whatever the reactive gas, the
sputtering mode is driven by La target, while Fe target seems to exhibit
continuous evolution of mechanisms, acting like buffer target.
When two reactive gases are introduced, mechanisms are more
complex. Atmosphere with argon and low oxygen content (1 sccm) is
initially introduced, with progressive addition of N2. When the targets
are sputtered separately, the iron one exhibits a behavior intermediate
between that observed with oxygen and with nitrogen (Fig. 3a). A
maximum value of the voltage is obtained at approx. 2.5 sccm. Further
increase of the nitrogen flow rate induces a progressive decrease of
the target voltage of approx. 20 V. Considering the low value of oxygen
introduced in the chamber (1 sccm), and results of Fig. 1, all oxygen is
consumed by getter effect. Thus, the nitrogen flow rate value necessary
to get the maximum of the iron target voltage is shifted to lower value
compared to that obtained using only nitrogen as reactive gas. The La
target also exhibits an intermediate behavior when the two reactive
gases are introduced. In the 0–2 sccm range, a maximum of the target
voltage is evident as for sputtering of lanthanum in Ar/N2 reactive mixture. At 3 sccm of nitrogen, an abrupt decrease of the voltage is noticed
indicating the transition from ESM to CSM. However, the target voltage
does not reach its minimal value at 3.5 sccm; a progressive decrease of
the voltage is observed in the 3.5–10 sccm range to reach a quite
constant value of 120 V. The hysteresis width is about the same as
that measured in Ar/O2 mixture. Thus, even using a low oxygen flow
rate that does not change the sputtering mode when used as the sole
reactive gas, the behavior of iron and lanthanum targets in Ar/O2/N2
mixtures is mainly driven by the presence of oxygen.
When both targets are sputtered in Ar/O2/N2 mixtures (Fig. 3b), the
results slightly differ from those obtained during the sputtering of a single target. The iron target exhibits a voltage — flow rate curve very close
to that obtained during the co-sputtering in Ar/N2. Such a result
indicates that the oxygen atoms are mainly consumed by lanthanum
ones. Thus, the amount of oxygen that is gettered on the iron target is
not high enough to influence the target voltage. On the other hand,
the shape of the lanthanum voltage — nitrogen flow rate curve differs
from that obtained during the single sputtering of the rare earth. The
main difference is noticed on the decrease of the nitrogen flow rate. A
progressive increase of the voltage is observed during the cosputtering due to the less consumption of oxygen atoms. This synergetic
effect allows reaching the CSM for both La and Fe targets at lower
nitrogen flow rate for La than for the use of an Ar/N2 mixture.
From previous studied cases, it is possible to identify instability
regions, elemental and compound sputtering modes of Fe and La system
in Ar/O2/N2 system (Fig. 4). This diagram was plotted using the target
voltage — gas flow rate curves as discussed before. Upper limit of La
and Fe instability region follows a linear behavior when two reactive
gases are introduced indicating a synergic effect of O2 and N2.
3.2. Relation between atmosphere and film composition: multilayer
synthesis
From diagram of Fe–La system in Ar–O2–N2 atmosphere, we purpose
to investigate evolution of oxygen and nitrogen contents of two
LaFeO3 − xNx films. Two multilayered samples have been synthesized
using two different Ar/O2 mixtures: 21/1 and 21/7 sccm. For both
atmospheres, nitrogen has been progressively added to deposit the
multilayers. For each nitrogen flow rate, the deposition duration has
been fixed to 30 min (Table 2). The different atmospheres used are
Fig. 3. Evolution of the iron and lanthanum target voltage as a function of the nitrogen flow rate in Ar/O2/N2 atmosphere with DO2 = 1 sccm. (a) Fe and La targets are sputtered separately;
and (b) Fe and La targets are co-sputtered.
42
E. Haye et al. / Surface & Coatings Technology 298 (2016) 39–44
Fig. 4. 2D diagram of La–Fe system in Ar/O2/N2 atmosphere.
Table 2
Gas flow rate conditions for multilayers synthesis.
Multilayer A
(high oxygen content)
Multilayer B
(low oxygen content)
Time
(min)
qAr
(sccm)
qO2
(sccm)
qN2
(sccm)
qAr
(sccm)
qO2
(sccm)
qN2
(sccm)
0
30
60
90
120
150
180
210
240
270
300
330
360
21
21
21
21
21
21
21
21
21
21
21
21
21
7
7
7
7
7
7
7
7
7
7
7
7
7
0
1
3
5
7
9
11
15
19
21
23
27
29
21
21
21
21
21
21
21
21
21
21
21
21
1
1
1
1
1
1
1
1
1
1
1
1
0
1
3
5
7
9
11
15
19
21
25
29
represented with green symbols in Fig. 4. Evolution of electrical
parameters at different N2 flow rates is measured and reported for
both multilayers A and B. To ensure a stabilization of the target voltage,
this parameter has been systematically measured 20 min after the
change of the nitrogen flow rate.
Considering our 2D diagram of La–Fe system in Ar/O2/N2
atmosphere no significant change must be observed on the deposition
parameter of the multilayer A, in contrary to the multilayer B. For the
multilayer A (deposited with 7 sccm of oxygen), the lanthanum target
voltage is low (approx. 125 V) indicating that this target is working in
the compound sputtering mode (Fig. 5a). Addition of nitrogen into the
chamber leads to a slight decrease of Fe voltage and a slight increase is
noticed on the lanthanum target voltage. These variations are attributed
to increase of the total pressure inside the chamber that modifies the
plasma conditions on both targets. Nevertheless, La target is less
affected due to higher mass and lower secondary electron emission coefficient. These results clearly indicate that addition of nitrogen into the
21/7 Ar/O2 reactive mixture does not induce a change of the sputtering
regime for both targets. SIMS analysis shows a constant concentration of
La, Fe and O elements (Fig. 5b). Signal of Cs+ ions exhibits discontinuity
between each layer (not shown here) and has been used to identify
each layer and to link analysis time and different layers (each layer for
different nitrogen contents, top and bottom axis in Figs. 5b and 6b).
Nitrogen concentration increases progressively until stagnation of
concentration (at 2000 s of measurements), corresponding to a flow
rate of 7 sccm. At higher N2 flow rate, saturation occurs, without
evolution of the atomic N/O ratio. The drop of N signal at approx.
3000 s (i.e., near the substrate), corresponds to the transition from the
layer deposited without N2 to the layer deposited using 1 sccm of
nitrogen. SEM cross section analysis of the multilayer A only shows a
dense microstructure with no evolution with the nitrogen flow rate
(not shown here). Such a behavior can be explained considering that
this multilayer is deposited in the CSM. Due to initial fully compound
mode, this 21/7/x atmosphere can be used to obtain films with very
low nitrogen content. UV–visible measurements performed on 21/7/7
film exhibit a similar transmittance than LaFeO3 films, with low shift
of transmission edge, which confirms low nitrogen content.
The decrease of the oxygen flow rate to 1 sccm (multilayer
B) changes drastically the sputtering conditions (Fig. 6a). At low nitrogen flow rate the lanthanum target exhibits a voltage of approx. 300 V
that is characteristic of a target working in ESM. The same behavior is
true for the iron target. The progressive increase of the nitrogen flow
rate induces a decrease of the lanthanum target voltage as previously
reported in Fig. 3b suggesting a change of the plasma conditions and
then of the film composition. SIMS depth profiles support this hypothesis (Fig. 6b). Contrary to the multilayer A, the SIMS depth profiles of Fe
and La tend to decrease with the increase of the nitrogen flow rate.
Such a behavior can be explained considering the decrease of the film
deposition rate as confirmed on the film cross section (Fig. 7). As a
function of the nitrogen rate, different layer morphologies are evident.
Close to the silicon substrate (i.e., low nitrogen flow rate) a columnar
morphology is observed, in accordance with metallic coatings. The
increase of the nitrogen partial pressure induces a vanishing of the
columnar microstructure and the layers close to the top surface exhibit
a dense morphology that is more usually encountered for oxide or
Fig. 5. (a) Evolution of the Fe and La target voltage as a function of the nitrogen flow rate during the deposition of the multilayer A (QO2 = 7 sccm). (b) Evolution of the La, Fe, O and N depth
profile as a function of the SIMS analysis time. The corresponding nitrogen flow rate is indicated on the top axis.
E. Haye et al. / Surface & Coatings Technology 298 (2016) 39–44
43
Fig. 6. (a) Evolution of the Fe and La target voltage as a function of the nitrogen flow rate during the deposition of the multilayer B (QO2 = 1 sccm). (b) Evolution of the La, Fe, O and N depth
profile as a function of the SIMS analysis time. The corresponding nitrogen flow rate is indicated on the top axis.
nitride films deposited in similar conditions (same morphology than
multilayer A).
Addition of nitrogen into the chamber modifies sputtering
mechanisms with switch between elemental to compound sputtering
mode. Between 0 and 1 sccm of N2, La and Fe targets voltages
correspond to elemental mode, with low N and O content into the
film (drop of N signal at 9000 s, near the substrate). Within our
deposition conditions, the reactive gases are fully consumed by the surfaces due to the getter effect. Between 3 and 5 sccm, voltage on target
exhibits a large modification, indicating switching of sputtering mode
(at 3 sccm on La target and 5 sccm on Fe target). The switching between
sputtering mode affects drastically the deposition rate, which drops
from 3000 nm/h at 21/1/0 to 900 nm/h at 21/1/5. Beyond 5 sccm, getter
effect is fully completed, leading to target poisoning by oxygen and
nitrogen. Due to constant oxygen amount in the chamber with drop of
deposition rate, the relative oxygen content into the film increases
with nitrogen flow rate as evident in Fig. 6b. The increase of the nitrogen
flow rate from 5 to 25 sccm leads to an increase of the total pressure into
the chamber. Thus, the deposition rate exhibits a slight decrease (from
900 nm/h at 21/1/5 to 650 nm/h at 21/1/25) due to the decrease of
the mean free path species. However from 7 sccm, the absolute nitrogen
content slightly decreases. In the same time, oxygen content slightly increases. Since iron and lanthanum atoms react with oxygen rather than
with nitrogen, the decrease of the film deposition rate associated with a
constant oxygen flow rate induces that the film nitrogen content does
not evolve monotonously with the nitrogen flow rate. Thus, an
optimum nitrogen flow rate window (approx. 5–10 sccm) allows the
deposition of a film with the highest nitrogen content.
Resistivity measurements performed on films deposited at 21/1/0,
21/1/2, 21/1/4, 21/1/6, 21/1/8, 21/1/15 and 21/1/25 gas mixtures exhibit an increase of resistivity until 6 sccm, due to formation of oxynitride,
in accordance with deposition rate measurements. It confirms the high
nitrogen content obtained between 5 and 10 sccm. To confirm the
beneficial effect of nitrogen doping of LaFeO3, UV–visible spectrophotometry measurements have been performed (Fig. 8). Pure oxide film
exhibits a high transmittance with an optical band gap of approx.
2.5 eV. The film deposited using the 21/7/7 mixture is still highly
transparent and a slight shift of the transmittance edge is noticed.
Thus, the film deposited with high oxygen flow rate exhibits properties
very close to those of pure oxide films because of the low nitrogen
content. On the other hand, the use of a 21/1/6 gas mixture to deposit
an oxynitride implies a large shift of the absorption threshold.
Furthermore, a decrease of the transmittance is evident.
4. Conclusion
To deposit oxynitride perovskite thin films, a study of the cosputtering process of lanthanum and iron targets using two reactive
gases has been performed. The target voltage variation during evolution
of the reactive gas flow rates has been used to define different
sputtering modes (elemental, instability and compound). La target exhibits hysteresis effect with large transition regime and drives the
Fig. 7. SEM observation of sample B cross section.
44
E. Haye et al. / Surface & Coatings Technology 298 (2016) 39–44
Fig. 8. Evolution of transmittance as a function of wavelength for films synthesized in
different Ar/O2/N2 atmospheres.
system in the presence of two targets. However, the iron target presents
a reversible behavior, without hysteresis and act as a buffer target in the
presence of two targets. Addition of small oxygen amount into the
chamber leads to stabilization of compound regime at low nitrogen
content. A synergic effect between nitrogen and oxygen is purposed to
explain this result. Then, two multilayer LaFeO3 − xNx have been synthesized from two different Ar/O2 atmospheres (21/1 and 21/7 sccm) with
addition of nitrogen in the chamber. SIMS analyses have been carried
out to investigate nitrogen and oxygen contents in the film, supported
by deposition rate, resistivity and UV–visible measurements. In case of
high initial oxygen content (7 sccm), nitrogen incorporation is limited,
with “saturation” of its concentration. A nitrogen flow rate higher than
7 sccm is useless, and such conditions lead to limited nitrogen doping.
When a low oxygen amount is used (1 sccm), sputtering mechanisms
are more complex, with switching from elemental to compound
sputtering mode, with decrease of deposition rate. Competitive
behavior between deposition rate decrease and increase of nitrogen
content in the chamber leads to an optimal window where N/O ratio
is maximized. This study shown that LaFeO3 − xNx can be synthesized
by in situ co-sputtering from two metallic targets and the nitrogen
content can be roughly controlled (low or medium doping) choosing
the deposition atmosphere.
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