Time-resolved analysis of electron and ion currents in reactive HiPIMS
deposition of titanium nitride
1
1
1
Rajesh Ganesan , Marcela M. Bilek , David R. McKenzie and Liuhe Li
1.
2.
1,2
Applied and Plasma Physics, School of Physics, Building A.28, University of Sydney, NSW 2006,
Australia.
School of Mechanical Engineering & Automation, Beijing University of Aeronautics & Astronautics,
Beijing 100191，China.
Abstract:
The temporally resolved behaviour of ion and electron currents arriving at the substrate during reactive highpower impulse magnetron sputtering (HiPIMS) of Ti in an atmosphere of pure Ar and a mixture of Ar and N is
investigated. The effect of Ar partial pressure, bias voltage and the inclusion of reactive N gas on the substrate
ion currents are determined. The inclusion of N in the gas mix substantially reduces the ion current arriving at
the substrate. Increasing Ar partial pressure in the Ar/N gas mix, results in increased ion current to the substrate
as well as an increase in the deposition rate of TiN. Experiments measuring the ion current as described here are
well suited to elucidate the optimum process parameters for effective deposition of TiN in HiPIMS mode.
Keywords: HiPIMS, ion current, electron current, substrate bias, Titanium nitride
1.
Introduction: Titanium Nitride (TiN) is an
extremely hard ceramic material, which possesses
excellent mechanical, electrical and interesting
optical properties [1]. To enhance the performance
and longevity of industrial tools and equipment,
TiN is frequently coated on their surfaces. Due to
its relatively high electrical conductivity and
thermal stability, thin film electrodes made of TiN
have
been
employed
in
microelectronic
applications [2]. TiN has been developed as a
plasmonic material for visible and near-infrared
wavelengths [3]. Furthermore, TiN coating on glass
has been also popular for the production of
decorative panels in architecture as solar control
coating in the automotive industry. The colours of
the reflected light are silver and blue, while the
brown and grey colours are transmitted [4]. TiN
coatings on photovoltaic panels reduce the solar
heat inside the photovoltaic panels, as it possesses
enhanced near-infrared reflectivity [5].
To produce efficient functional coatings and thin
films in industry, DC magnetron sputtering
processes are preferred. Coating of TiN by this
process is advantageous, as it can homogeneously
coat large areas. Also it has excellent repeatability,
once the favourable process parameters are
identified. However, the production of high density
and lattice defect free films by a DC magnetron
process is still a challenge [6].
In order to overcome this limitation, high power
was applied in short pulses to drive the magnetron
system, rather than applying a continuous DC.
Magnetron sputtering with this mode of powering
the target is termed High Power Impulse
Magnetron
Sputtering
(HiPIMS).
HIPIMS
technology shows that it is possible to combine the
advantages of high ionization as in arc evaporation
with the advantages of magnetron sputtering. Due
to the high peak power of up to 8 MW, a proportion
of the atoms sputtered in HIPIMS become ionised
allowing them to be accelerated towards the
substrate. Energetic deposition, such as ion assisted
deposition, is known to give dense and defect free
conformal coatings with very good adhesion to the
substrate.
HiPIMS can be employed in industrial coating of
TiN. However, a limitation is that it has a low
deposition rate in comparison with conventional
DC and RF deposition techniques. In order to
overcome this limitation, several techniques have
been tried. Unbalanced magnetron using external
magnetic field has achieved deposition rates in the
range of 30 – 100 % (depending on sample position
and field strength) by directing more ions to the
substrate [7].
As an alternative, we have applied a negative
voltage bias to the substrate in order to attract more
metal ions. Our intention is to increase the
deposition rate and simultaneously improve coating
quality by energetic deposition at relatively low
temperatures and thus make HiPIMS more
attractive in industrial applications. This may also
open up the possibility of achieving a tuneable
residual stress on the films while maintaining a
high hardness [8].
Studying the ion and electron currents reaching the
substrate is crucial to elucidate the effective target
Methodology:
The experiments were performed in a high vacuum
stainless steel chamber with a base pressure below
1.1 x 10-7 torr. The magnetron sputtering system
was supplied by AJA International, Inc and the
HiPIMS RUP 7m model power supply by GBS
Elektronik GmbH, Germany. The power supply can
deliver up to the maximum value of 1 kV and 300
A.
The HiPIMS power (~ 600 V and ~ 80 A) to the
cathode target was applied in the form of unipolar
pulses with the pulse width of 80 µs driven at
different frequencies. Agilent technologies made
100 MHz digital oscilloscope (DSO-X2014A) is
used for pulse generation at different frequencies
and pulse length. The waveforms of sputtering
voltage, discharge voltage & current and substrate
ion and electron current were also recorded.
A Titanium sputtering target (Kurt J. Lesker,
Clairton, PA, USA) 99.995 % pure, with a diameter
of 7.62 cm and a thickness of 0.64 cm, was used in
these experiments. Argon gas of purity 99.997 %
and Nitrogen gas of ultra-high purity (99.999 %)
were used as carrier gas and reactive gas,
respectively. MKS made mass flow controller
controls the flow rate of both the gases. For
venting, nitrogen gas of high purity 99.997 % was
used. All the gases were procured from BOC
Australia. The gas inlets were placed at either sides
of the target. The partial pressures of both gases are
consistently maintained. The DC bias supply, a
custom made equipment, was connected to the
substrate, to maintain a negative bias voltage
during the HiPIMS pulse up to a maximum of
300V.
Additionally, the bias voltage induced variation in
the Ti2+ ion composition in the near sheath region
was determined by in-situ monitoring its
characteristic spectral emission line at 598.58 nm
by a spectrometer (model: Spectropro 2750). The
light coming from the plasma through the quartz
window was focussed into a fibre through a convex
lens. The spectrometer’s field of view was from 1.5
to 2 cm below the substrate to 7 – 8 cm below the
substrate and towards the target, which was at the
distance 15 cm below the substrate. The light from
the target race tract was totally out of view. The
Gaussian fitting method provided by Origin
software was employed to fit the ion and electron
current data. FWHM, height and the central point
3.
Results:
3.1.
Effect of argon pressure
The substrate ion and electron current waveforms
for reactive HiPIMS of Ti in a pure argon gas
environment at various pressures are shown in Fig.
1. Below chamber pressures of 2 mtorr, no ion or
electron current was observed at the substrate, as
the plasma couldn’t be established at such low
pressures in our system. However, for the pressures
≥ 3 mtorr, significant current was observed at the
substrate and it increased proportionally with the
chamber pressure up to 7 mtorr, weakening beyond
that pressure. The electron current traces were
observed earlier than ions, as electrons reach the
substrate faster than the ions. The electron current
reaches its maximum during the pulse (at 63 µs
after pulse triggering), while the ion current reaches
the maximum at ~ 8 µs after the pulse is complete
(i.e. at 88 µs after pulse triggering). The ion
currents of titanium and argon couldn’t be
distinguished at the pressure of 3mtorr (Fig.1a),
while two distinct ion current pulses could be
observed at the pressure ≥ 5 mtorr (Fig.1b).
9
(a)
6
Current (amp)
2.
of the curve were set as variable parameters, by
keeping the base constant.
3
0
-3
Data
Ion
Electron
Pulse
-6
-9
-200
-100
0
100
200
300
400
Time ( s)
9
(b)
6
Current (amp)
utilization and process optimization. In this work,
we have analysed the time evolution of electron
and ion current reaching the substrate and its
dependence on different gas compositions, gas
pressures and bias voltages.
3
0
Data
Ion 1
Ion 2
Electron
Pulse
-3
-6
-9
-200
-100
0
100
200
300
400
Time ( s)
Fig.1 The substrate ion and electron current as a
function of time for a HiPIMS pulse operating in a
pure argon atmosphere (floating voltage ~ -15 V) at
(a) 3 mbar and (b) 5 mbar. The electron current
precedes the ion current and delivers more total
charge.
3.2.
Effect of bias voltage
With increased negative bias at the substrate,
electrons are repelled from it, whereas the ions are
attracted, as is evident by the higher ion current
recorded in Fig. 2.
The ion current measured at a bias -50 V is
relatively higher to the one measured at the floating
bias condition (c.f Fig. 1b), which might be due to
the increased sheath width at the substrate and the
evacuation of more ions from the plasma by the
substrate sheath.
9
Current (amp)
6
3
may be indicative of the sheath moving further into
the field of view of the spectrometer.
1.0
Normalized Intensity
The two distinct peaks may represent the arrival of
Ti and Ar ions. One interpretation is that the ion
peak with sharp edge is Ti, while the broad one is
attributed to Ar ions based on the fact that the
creation of Ar ions has been observed to precede
that of Ti ions [9]. Another interpretation is that the
broadening of the peaks reflects differences in the
energy distribution of the populations. Previous
work has observed a broader energy distribution in
the ions of the target (in this case Ti) compared to
that of the background gas (in this case Ar) [10].
This interpretation would assign the narrow peak to
Ar and the broad one to Ti ions. The last part of the
electron current and the initial part of the ion
current can’t be clearly distinguished, due to the
fact that the charges cancel each other during the
period of overlap.
0.8
0.6
0.4
0.2
0.0
0
40
80
120
160
200
Negative Bias (Volts)
Fig. 3 Variation in the Intensity (normalized) of
Ti2+ spectral line emission (598.58 nm) as a
function of applied bias. The light is gathered by
laterally focussing the region in-between of target
and substrate well above the race track.
3.3.
Inclusion of nitrogen gas
A completely different ion current behaviour was
observed when nitrogen gas was introduced into
the plasma at the flow rate of 40 sccm, while the
flow rate of argon was maintained at 15 sccm with
a system pressure of 3mtorr. The magnitude of ion
current reaching the surface is drastically reduced
to 10 % with the inclusion of nitrogen gas in the
plasma. The substantial reduction of ions is
attributed to a chemical reaction between the
titanium and nitrogen ions.
3.3.a. The effect of argon flow rate and bias
voltage for a mixed argon and nitrogen
plasma
0
-3
Data
Ion 1
Ion 2
Pulse
-6
-9
-200
-100
0
100
200
300
400
Time ( s)
Fig. 2 Ion currents measured at the substrate as a
function of time for an argon pressure of 5 mbar
and a substrate bias of -50 V. The ion current is
resolved into a components tentatively attributed to
Ti ions and Ar ions.
No significant variation in the ion current was
observed with further increase in the bias voltages
(up to -200V), indicating that the ion saturation
limit is achieved and the ion current is limited by
the ion flux entering the sheath. The spectroscopic
observation shows that increasing bias voltages
lead to a gradual decline in the intensity of the
spectral line (at 598.58 nm) of Ti2+ ions, in the
region closer to sheath, as shown in Fig. 3. This
The area under the graph of the ion current as a
function of time (ie the total charge delivered by
the ion current) was calculated for various argon
flow rates and bias voltages and is shown in Fig. 4.
The ion current is the sum of the ion currents of
argon, nitrogen and titanium.
The following observations have been made: (i)
when the substrate was floating, there was no
significant variation in the total charge delivered by
ions for the various flow rates of argon gas. (ii) at
– 50 V bias, the total charge delivered to the
substrate is increased by almost a factor of 3 for the
higher argon flow rate (30 sccm) compared to that
of the lower flow rate (15 sccm). (iii) further
increase of negative bias voltage results in a
marginal increase of ion current
for both argon
flow rates, however the values of
seem to be
saturated.
Charge delivered x 10-6
(Amp.sec)
35
30
25
Ar-15 and N-40 sccm
Ar-30and N-40 sccm
20
15
10
5
0
-50
-100
-150
-200
Applied bias (Volts)
Fig. 4 The positive charge delivered, calculated by
integration of the substrate ion current at various
applied bias voltages for two gas compositions.
High partial pressure of argon favours not only
higher ionisation, but also carries more target metal
ions to the substrate. While the substrate is floating
and for the deposition time of 60 minutes, TiN
films of thickness of 39nm and 43nm were
obtained for lower and higher argon flow rate,
respectively. By applying the substrate bias of
-100V, TiN films were obtained with the thickness
of 46nm and 55nm for lower and higher argon flow
rate, respectively. This indicates that substrate bias,
as well as increasing the energy of the ions
impinging on the substrate, is effective in bringing
more of the depositing ions from the plasma to the
substrate.
4.
Conclusion and further work:
The results show that studying the ion currents at
the substrate in HiPIMS provides information that
will be helpful in developing methods of achieving
higher deposition rate and effective target material
utilisation without increasing the applied power to
the cathode. Higher argon content in and Ar/N gas
mix increases the ion current at the substrate. The
substrate bias extracts the ions into the sheath and
thus, contributes to a larger flux of ions reaching
the substrate, which is observed from the
substantial increase in the thickness of deposited
TiN films.
5.
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