22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Measurement and modelling of ion and neutral Zn energy distribution
functions during RF magnetron sputtering of ZnO target
V. Roy Garofano1, R.K. Gangwar1, L. Maaloul1, T. Carvalho1 and L. Stafford1
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Département de physique, Université de Montréal, Montréal, Québec H3C 3J7, Canada
Abstract: Measurements of the energy distribution functions of Ar and Zn ions during
magnetron sputtering of ZnO targets in RF Ar plasmas are reported. Based on the
predictions of a recently-developed Boltzmann code, the measured high-energy tail of Zn
ions is ascribed to a partial thermalization of sputtered Zn atoms.
Keywords: Magnetron sputtering, mass spectrometry, IEDF modelling.
1. Introduction
ZnO-based thin films have a wide band gap (~ 3.4 eV),
a high charge carrier concentration and a high optical
transmittance (≥90%) in the visible and near infrared,
making these materials attractive and promising for many
device applications, including thin film transistors,
flexible displays, solar cells, gas and biomedical sensors.
Magnetron sputtering is widely used to elaborate ZnObased thin films. The quality of the coatings being
strongly linked to the plasma characteristics, a complete
understanding on the role of both ions and sputtered
particles on the plasma deposition dynamics is essential.
This work focuses on measurements and modeling of the
energy distribution functions of Zn and Ar ion species
impinging onto the substrate during thin film deposition
in magnetron RF plasmas.
3. Measurements
Ion energy distribution functions (IEDFs) of Ar+ and
Zn+ species obtained from PSMS at 5 mTorr are shown in
Fig. 2 for various absorbed RF powers (related to the selfbias voltage and thus to the energy of the ions impinging
onto the ZnO target).
2. Experimental details
Figure 1 shows a schematic of the experimental setup
used in this study (see ref. [1] for more details). A
stainless steel plate was installed at 16 cm from the
magnetron surface to limit plasma expansion in the axial
direction and to accommodate a plasma sampling mass
spectrometer (PSMS). The discharge was sustained in
nominally pure Ar at pressures ranging from 5 to 50
mTorr using ZnO as the sputtering target. The self-bias
voltage resulting from the 13.56 MHz electric field
applied to the magnetron source was varied between -100
and -250 V by adjusting the absorbed (i.e. incident minus
reflected) RF power between 25 and 65 W.
Fig. 1: Schematic of the experimental setup
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Fig. 2: Ar+ (top) and Zn+ (bottom) IEDFs as a function of
absorbed RF power at 5mTorr.
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In Fig. 2, Ar+ peaks show fairly symmetric energy
profiles, as expected from RF plasmas with low
modulation of the IEDFs (over the range of experimental
conditions investigated, the ion transit time is much
higher than the RF oscillation period). On the other hand,
Zn+ species follow more complex energy distribution
functions, with the presence of a high-energy tail. In
addition, while the energy maximum of both Ar+ and Zn+
peaks increases with injected power (a feature ascribed to
the higher plasma potentials at higher RF powers), the
relative contribution of the high-energy tail in the Zn+
IEDFs noticeably increases. As shown in Fig. 3, the
opposite trend was observed by varying the gas pressure.
While the position of the maximum from both Ar+ and
Zn+ decreases with increasing pressure, the high-energy
tail vanishes between 5 and 50 mTorr.
Fig. 3: Ar+ (top) and Zn+ (bottom) IEDFs as a function of
pressure for a selb-bias voltage of -105 V.
The pressure dependence of the high-energy tail in the
IEDFs of Zn+ species indicates a “thermalization” of Zn
neutrals ejected with high energies from the ZnO target
following their interaction with the cold background gas.
These aspects were examined in more details by solving
the Boltzmann equation for sputtered Zn neutrals as a
function of the distance z from the target, using an
energy-dependant cross section for Zn atoms-Ar atoms
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elastic collisions [2]. For low-energy ions impinging onto
the ZnO target, the energy distribution of Zn atoms close
to the target (z=0) can be described by a two-temperatures
Maxwellian energy distribution function:
Eq. 1
where ε is the energy of sputtered Zn atoms, and T dir and
T ind are the characteristic temperatures for direct and
indirect sputtering collisions. Here, direct sputtering refers
to topmost surface collisions between impinging ions and
target atoms, while indirect sputtering can be ascribed to a
more fully developed collision cascade in the near-surface
region involving primary, secondary, and possibly higher
order knock-on atoms [3].
The energy distribution of sputtered Zn atoms following
their interaction with the cold background gas at z=16 cm,
i.e. close to the PSMS, was obtained from the resolution
of the Boltzmann equation assuming the energy
distribution of Zn atoms at z=0 given by Eq. (1). These Zn
neutral particles were then “transformed” into Zn ions by
gas-phase ionization collisions. In this work, creation of
Zn ions from Zn neutrals is assumed to result from
(i) electron-impact ionization on ground state Zn atoms,
(ii) Penning ionization reactions with metastable Ar
atoms, and (iii) charge transfer reactions with Ar ions.
The resulting Zn ion population is then accelerated in the
sheath voltage close to the grounded PSMS. Since the
sheath voltage depends on the operating conditions (see,
for example, the variations in the IEDFs from Ar ions), it
was treated as an adjustable parameter in our calculations.
Fig. 4: Gas-phase Ionization rate of Zn atoms as a
function of their energy
The reaction rate of the 3 possible ionization
mechanisms mentioned above as a function of the energy
of Zn atoms is presented in Fig. 4. The electron-impact
ionization rate constant is shown for an electron
temperature of 3 eV, which corresponds to the 48W, 5
mTorr condition where the high-energy tail in the IEDFs
of Zn+ is significant (see Fig. 3). Over the range of
experimental conditions investigated in the present study,
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the electron-impact ionization rate on ground state Zn
atoms dominates at very low energy of Zn atoms (below
0.1 eV), whereas charge transfer reactions with Ar ions
become more important at higher energies.
The predictions of the Boltzmann model for Zn atoms
coupled with the creation rate constants of Zn ions
displayed in Fig. 4 are shown in Fig. 5 for a typical
condition, RF power 48 W, pressure of 5 mTorr.
Excellent agreement is achieved between the model and
the measured IEDF of Zn+. Three populations can be
observed: (i) a low-energy population of Zn ions
accelerated in the sheath voltage and formed by ionization
of Zn atoms fully thermalized with the background gas,
(ii) a mid-energy population of Zn ions accelerated in the
sheath voltage and formed by ionization of partially
thermalized Zn atoms arising from indirect sputtering
processes, and (iii) a high-energy population of Zn ions
accelerated in the sheath voltage and formed by ionization
of the partially thermalized Zn atoms arising from direct
sputtering processes. For the experimental conditions
shown in Fig. 5, the characteristic temperatures of direct
and indirect sputtering processes (deduced from the
comparison between measured and calculated IEDFs) are
T dir = 1.74 ± 0.07 eV and T ind = 0.39 ± 0.02 eV. The higher
temperature for direct sputtering is consistent with the
more fully coordinated collision cascade for indirect
sputtering reactions.
Fig. 5. Example of fit of the measured Zn+ IEDFs for a
pressure of 5 mTorr and a power of 48 W. The results
were obtained at z=16 cm.
The pressure dependence of the IEDFs obtained from
the model using all other input parameters as constants is
presented in Fig. 6. As expected, the high-energy tails
from both direct and indirect sputtering processes vanish
with increasing pressure due to more fully developed
thermalization of sputtered Zn atoms.
the RF power) on the relative populations of fully and
partially thermalized species as well as on their
characteristic temperatures. The contribution from the
high-energy tails from both sputtering processes increases
with the RF power due to the higher number of highenergy species with increasing ion impinging energy. On
the other hand, the temperatures from the high-energy
species were found to decrease with increasing power; a
feature ascribed to the higher ion penetration length and
thus to the more fully developed collision cascade as the
energy of the ions impinging onto the target increases.
Fig. 6. Influence of pressure on the IEDFs of Zn+
predicted by the model at 65 W. The results were
obtained at z=16 cm.
4. Conclusion
Plasma sampling mass spectrometry measurements
during magnetron of ZnO in low-pressure Ar plasmas
revealed a high-energy tail in the IEDF of Zn ions. This
high-energy group of ions was found to result from
sputtered neutrals not being completely thermalized by
collision with cold Ar atoms, and then ionized in the gas
phase mostly by electron-impact ionization on ground
state Zn atoms and charge transfer collisions. A model
accounting for both thermalization and ionization
processes showed an excellent agreement with our
experimental data. This was then used to probe the
evolution of the sputtering temperatures from both direct
and indirect reactions over a wide range of RF powers and
gas pressures.
5. References
[1] L. Maaloul and L. Stafford, J. Vac. Sci. Technol. A
Vacuum, Surfaces, Film. 31, 061306 (2013).
[2] a V Phelps, C.H. Greene, J.P. Burke, and a V Phelps,
J. Phys. B At. Mol. Opt. Phys. 33, 2965 (2000).
[3] P. Sigmund, Phys. Rev. 184, (1969).
The comparison between the model predictions and the
experiments can also be used to deduce the influence of
the ion energy (related to the self-bias voltage, and thus to
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