Energy distribution of O- ions during reactive magnetron sputtering

Article abstract of DOI:10.1063/1.2266888

Low, medium, and high energy O^- ion populations were experimentally detected during magnetron sputtering of Al in an Ar/O₂ atmosphere. Based on calculations, the authors propose that nonsputtered O ^- ions originating from the target surface are accelerated in the cathode fall, while sputtered O^- ions may be excluded as a significant contribution to the high energy ion population. Furthermore, the formation of medium energy O^- ions is consistent with the notion of sputtered, in the cathode fall accelerated, and subsequently dissociated AlO^- and AlO₂^- clusters. These findings may be of importance for understanding plasma energetics and growth involving electronegative species.

Full text of DOI:10.1063/1.2266888

Energy distribution of O− ions during reactive magnetron sputtering
Stanislav Mráz and Jochen M. Schneider
Citation: Appl. Phys. Lett. 89, 051502 (2006); doi: 10.1063/1.2266888
View online: http://dx.doi.org/10.1063/1.2266888
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APPLIED PHYSICS LETTERS 89, 051502 ͑2006͒
Energy distribution of O− ions during reactive magnetron sputtering
Stanislav Mráz and Jochen M. Schneidera͒
Materials Chemistry, RWTH Aachen University, Kopernikusstr. 16, D-52074 Aachen, Germany
͑Received 12 April 2006; accepted 15 June 2006; published online 1 August 2006͒
Low, medium, and high energy O− ion populations were experimentally detected during magnetron
sputtering of Al in an Ar/O2 atmosphere. Based on calculations, the authors propose that
nonsputtered O− ions originating from the target surface are accelerated in the cathode fall, while
sputtered O− ions may be excluded as a significant contribution to the high energy ion population.
Furthermore, the formation of medium energy O− ions is consistent with the notion of sputtered, in
the cathode fall accelerated, and subsequently dissociated AlO− and AlO2− clusters. These findings
may be of importance for understanding plasma energetics and growth involving electronegative
species. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2266888͔
The significance of the ion flux and energy on the mi-
crostructure evolution of thin films grown by reactive mag-
netron sputtering was recently reviewed by Petrov et al.1
Furthermore, the authors discussed preferential growth of ni-
trides based on the differences in surface adatom ͑admol-
ecule͒ mobility. Alike altering of preferential orientation of
yttria stabilized zirconia films deposited by pulsed laser
deposition was reported by Voevodin et al.2 The film struc-
ture could be similarly controlled by the energy of the bom-
barding Zr+ ions by adjusting the negative substrate bias.
If electronegative reactive gases such as O2 are em-
ployed for film growth, the formation of negative ions is
expected. If the kinetic energy of the O− ions is large, they
may be of importance for the structure evolution even if a
negative substrate bias is applied. Tominaga et al. reported
the presence of energetic O− ions in the magnetron sputtered
Zn ͑Ref. 3͒ and Zr ͑Ref. 4͒ in an Ar/O2 atmosphere mea-
sured with time-of-flight mass spectrometry. Recently,
Zeuner et al.5 and Mišina et al.6 measured the O− energy
distribution function ͑EDF͒ for magnetron sputtered Si and
Ti in an Ar/O2 atmosphere, respectively. Zeuner et al.5 sug-
gested that the high energy O− ions with energy correspond-
ing to the target potential are generated at the target and
accelerated in the cathode fall. In contrast to the data pub-
lished by Zeuner et al.5 a report by Mišina et al.6 indicates
that the most probable energy of the O− ions is larger than
the target potential. The authors concluded that this may be
understood based on energy transfer by sputtering.6 Our re-
cent work7 is consistent with the proposal put forward by
Mišina et al.6 We have presented a simple model based on
sputtering theory, which describes the high and the medium
energy range of the measured O− EDF for magnetron sput-
tered zirconia and hafnia rather well.7 Based on comparing
our calculations with the measured O− EDF it is evident that
oxygen is sputtered prior to the acceleration over the cathode
fall.7
The presence of low, medium, and high energy O− ions is
reported and their origin is discussed.
A mass-energy analyzer ͑MEA͒ ͑PPM 422, Pfeiffer
Vacuum͒ was employed for the ion energy distribution mea-
surement. The measurement was carried out in a DN160/
100CF reducing cross pumped with a combination of a tur-
bomolecular and a rotary-vane pump with an effective
pumping speed of ϳ100 l/s down to the base pressure of
Ͻ1ϫ10−4 Pa.7 The MEA was located opposite to the target
at a distance of 70 mm, while the orifice was directed to-
wards the target racetrack resulting in a radial displacement
of the MEA of 26 mm with respect to the cathode axis. The
pulsed dc plasma was generated with an ENI RPG-100E
power supply ͑MKS Instruments͒ with the frequency of
50 kHz and a duty cycle of 90%. The total current was kept
constant at 900 mA corresponding to the current density of
14 mA/cm2. The total pressure and the p͑O2͒ were 0.8 and
0.07 Pa, respectively.
The EDF of O− ions is shown in Fig. 1 ͑solid line͒.
Three major in part overlapping ion populations can be ob-
served: low ͑from 40 to 120 eV͒, medium ͑from
80 to 220 eV͒, and high energy ͑from 240 to 340 eV͒ O−
ions with energy corresponding approximately to the target
potential within the negative half cycle. The origin of the
different ion populations may be as follows. According to
Refs. 3 and 6–8 the high energy ions may be sputtered from
From this short review it is evident that no consistent
picture of the origin of the O− EDF during magnetron sput-
tering emerges from studying the literature. Therefore we
have chosen to investigate the energy distribution of O− ion
for pulsed dc reactive magnetron sputtered alumina thin
films. The target surface was covered with an oxide layer.
FIG. 1. Measured energy distribution function ͑EDF͒ of O− ions ͑solid line͒
and theoretical EDF of sputtered O− ions ͑open square͒ accelerated in the
cathode fall. The distribution does not fit the measured high energy O− ions
well.
a͒
Electronic mail: schneider@mch.rwth-aachen.de
0003-6951/2006/89͑5͒/051502/3/$23.00
89, 051502-1
© 2006 American Institute of Physics
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051502-2
S. Mráz, and J. M. Schneider
Appl. Phys. Lett. 89, 051502 ͑2006͒
FIG. 2. Time dependence of the target potential with the frequency and the
duty cycle of 50 kHz and 90%, respectively.
the target surface and be accelerated by the full target
potential.
The theoretical energy distribution function of high en-
ergy ions derived for sputtered species in a dc plasma is
given by7
FIG. 3. ͑a͒ Measured EDF of O− ions ͑solid line͒ and target potential dis-
tribution ͑open square͒. The distribution fits the measured high energy O−
ions well. ͑b͒ Measured EDF of O− ions ͑solid line͒ and theoretical distri-
bution function for medium energy O− ions ͑filled square͒.
E
2
1
͑E0 − Emean
͒
f͑E͒ ϳ
exp −
ͩ
ͪ
ͪ
͵
ͩ
2
␴ͱ2␲
2␴
E
min
⌬U
2
⌬U
2
1
for U͑t͒ ෈ U −
,Ui +
ͪ
ͳ
E − E0
i
ϫ
3dE0,
͑1͒
x͑t͒ =
͓͑E − E0͒ + U0͔
⌬U
2
⌬U
Ά
·
0 for U͑t͒ ꢀ U −
,Ui +
,
ͪ
ͳ
i
where E, E0, U0, Emean, and ␴ are the energy of the sputtered
particle, the energy gained through the acceleration by the
target potential, the surface binding energy, the mean target
potential, and the associated standard deviation, respectively.
The integration is carried out from the minimal target poten-
tial Emin to E. In the case of pulsed dc plasma, where the
target potential is a function of time, the energy distribution
function can be modified to
2
where Ei, Ui, Umin, Umax, and ⌬U are the energy acquired by
acceleration, target potential corresponding to the energy Ei,
minimal and maximal absolute values of the target potential,
and a step size of 5 V used to divide the voltage range
͗Umin,Umax͘ into a discrete number of intervals, respectively.
As can be seen in Fig. 3͑a͒, the target potential distribu-
tion function ͓Eq. ͑3͔͒ corresponding to the U͑t͒ ͑Fig. 2͒ is
consistent with the high energy range of the EDF. Hence, we
propose that O− ions are generated at the cathode surface
through desorption of O− or O followed by subsequent elec-
tron attachment.
E − qeU͑t͒
͓͑E − qeU͑t͒͒ + U0͔
f͑E͒ ϳ
g͑t͒
3dt,
͵
pulse
1
for qeU͑t͒ ഛ E
It may be speculated that negative ion formation by de-
sorption may also be relevant for other electronegative spe-
cies that may form at the target during sputtering. The EDF
is hence defined by the voltage-time characteristic of the
pulsed dc generator used. One of the implications of this
work is therefore that different pulsed dc generators with
different voltage-time characteristics result in different
EDF’s of the electronegative species. In the thin film growth
literature, experiments are often compared at similar average
voltages or power densities. Our results indicate that this
strategy may be of limited use since the voltage-time char-
acteristic determines the EDF.
Analog discussion can be made for the medium energy
O− ions. According to Ref. 5 the dissociation of O−2 ions and
AlO−x clusters may be expected. Good agreement between
the measured EDF and the theoretical distribution function
͓Eq. ͑2͔͒ is achieved ͓Fig. 3͑b͔͒ based on the notion of sput-
tered AlO− and AlO−2 clusters, their acceleration over the
cathode fall, and subsequent dissociation. The surface bind-
ing energy was assumed to be U0=8.6 eV ͑Ref. 7͒ ͓an aver-
age value of the heat of formation of ␣ ͑17.37 eV͒ and ␥
g͑t͒ =
͑2͒
ͭ
ͮ
0 for qeU͑t͒ Ͼ E,
where q, e, and U͑t͒ are the particle charge state, elementary
charge, and the time-dependent target potential, respectively,
and the integration carried out over the pulse.
From the comparison between the theoretical ͑Fig. 1,
open square͒ and the measured distribution functions ͑Fig. 1,
solid line͒, it can be concluded that the theoretical distribu-
tion does not describe the measured distribution well and that
the notion of sputtered O− ions is not consistent with the
measured data. Since the energy of the high energy O− ions
͑Fig. 1, solid line͒ corresponds to the target potential of the
negative half cycle ͑Fig. 2͒, it is reasonable to assume that
the O− ions are accelerated by the target potential. The en-
ergy of these ions appears to be exclusively determined by
the target potential distribution function,
tϽT
f͑E ͒ = f͑qeU ͒ ϳ
x͑t͒, Ui ෈ ͗Umin,Umax͘,
͑3͒
͚
i
i
t=0
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051502-3
S. Mráz, and J. M. Schneider
Appl. Phys. Lett. 89, 051502 ͑2006͒
alumina ͑17.17 eV͒ per Al atom at temperature of
298.15 K͔.9
tered alumina films can be understood. These findings may
be of importance for understanding plasma energetics and
growth involving electronegative species.
Low energy ions may be formed via dissociative elec-
tron attachment: e+O2→O+O−,7 and subsequent accelera-
tion of the O− ions by a fraction of the target potential. It is
reasonable to assume that this mechanism is primarily re-
sponsible for the low energy ͑40–120 eV͒ population.
In summary, we have measured the energy distribution
function of O− ions for sputtered Al in an Ar/O2 atmosphere,
while the target was covered with an oxide layer. Three ion
populations were identified: low, medium, and high energy
O− ions. Based on comparing the measured and the calcu-
lated energy distribution functions for pulsed dc plasma we
have shown that the high energy O− ions are not sputtered
and suggest that they may be generated at the cathode sur-
face through desorption of O− or O followed by electron
attachment and subsequent acceleration over the cathode fall.
Furthermore, we suggest that the medium energy O− ions
may be formed through sputtering of AlO− and AlO2− clus-
ters, their acceleration over the cathode fall and subsequent
dissociation. Based on the presented data here the origin of
the high and the medium energy O− ions in magnetron sput-
The authors gratefully acknowledge the financial support
granted by Deutsche Forschungsgemeinschaft ͑DFG͒ within
the project Schn 735/9-1.
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