Industrial Size High Power Impulse Magnetron Sputtering

Key Words:
Industrial Size High Power Impulse Magnetron Sputtering
A.P. Ehiasarian, Materials Research Institute, Sheffield Hallam University, Sheffield, United
Kingdom; and R. Bugyi, AC Sp. z o.o., Warsaw, Poland
Ion assisted deposition Sputter deposition
Plasma source Power supplies
ABSTRACT
High power impulse magnetron sputtering (HIPIMS) is a
novel physical vapor deposition technology producing large
area uniform discharges with high concentration of metal
ions. Until now it has been available on cathodes with area up
to 180 cm2 (30 in2 ). The present work reports on the utilization
of magnetron cathodes with area of 400 cm2 (62 in2 ) to
generate HIPIMS discharges in the presence of Ar. The
discharge was driven at a frequency of 100 Hz (10 ms) and
pulse duration of 200 µs. The peak currents were in excess of
2.5 kA when operated with a titanium target. The discharge
was stable at Ar pressures down to 7x10-4 mbar (0.5 mTorr).
The HIPIMS plasma composition was studied as a function of
discharge current using time resolved optical emission spectroscopy.
Strong emission from Ti(2+) and Ti(1+) ions was
observed. The discharge was observed to transit from Ar
plasma dominated to metal ion dominated as reported previously
for small cathodes. The optical emission intensity of
individual species, I( Ti(1+) ), varied with discharge current,
Id, according to a power law I( (Ti1+) ) ~ Idn . The exponent
n was found to be 2 for Ti(2+); 1.5 for Ti(1+); and 0.5-0.8 for
Ti(0). This signifies a steep dependence of highly charged
metal ions on the discharge current. It also points to a strong
increase in metal ion to metal neutral ratio as a function of
discharge current.
INTRODUCTION
Industrial uses of hard physical vapor deposited (PVD) coatings
are associated with ever increasing requirements for
microstructure density, defect-free processing and smooth
surface. Scientific research and industrial practice in plasmaassisted
PVD processes have provided substantial evidence
that meeting these demands requires high degree of ionization
and high ion-to-neutral ratio in the deposition flux. Although
a number of plasma deposition sources have been developed
on a laboratory scale, they show difficulties when applied to
industrial size machines. One of the most promising new
plasma deposition sources is the high power impulse magnetron
sputtering (HIPIMS) introduced recently [1] and demonstrated
up to now on laboratory scale equipment.
The HIPIMS discharge is operated on magnetron cathodes
and is characterized with extremely high power densities—up
to 3 kWcm -2 generated at discharge voltages of up to 2 kV.
Overheating is avoided by pulsing the power at low duty cycle
of ~1%. The plasma densities achieved in HIPIMS are of the
order of 10 13 cm -3 , [1, 2]. The sputtered metal flux is ionized
efficiently in the HIPIMS plasma with metal ion charge states
reaching 2+ for Cr [2]. HIPIMS has been successfully
implemented to enhance adhesion by substrate pretreatments
[3] and to deposit wear and corrosion-resistant CrN coatings
[4, 5, 6].
The current paper reports on the utilization of HIPIMS on
industrial size rectangular cathodes with area of more than
400 cm 2 . The plasma composition and its time evolution have
been investigated with optical emission spectroscopy (OES),
and the influence of power on the chemistry of the deposition
flux is discussed.
EXPERIMENT DETAILS
HIPIMS was operated on rectangular cathodes with area >400
cm2 (62 in2 ). The discharge was driven with a power generator
HMP 6/16 (AC Sp z o.o., Warsaw, Poland) capable of supplying
power pulses with duration in the range 0-200 µs at a
frequency of 0-100 Hz (10 ms) equivalent to a duty cycle of
2%. The power supply was capable of delivering peak
currents of up to 3000 A and at a voltage of 2000 V. Arcing
energy was minimized by arc suppression design that allowed
switch off of the power supply even at the maximum current.
The peak current of the discharge was limited by the available
power. The parameters of the HIPIMS discharge were measured
with a high voltage probe (Tektronix P6015A) with
bandwidth of 50 MHz and a high current transformer (Tektronix
CT-4) in combination with a current probe (P6021) with
bandwidth of 500 Hz. The traces were recorded by an ultrafast
digital phosphor oscilloscope Tektronix 3032B. The cathode
and power supply were installed in an industrial size batch
coater Hauzer HTC 1000/4, replacing one of the four original
cathodes of the machine as illustrated in Figure 1. The
chamber had a volume of 1 m 3 and was evacuated to a base
pressure of 3x10 -6 mbar (2x10 -6 Torr) with turbomolecular
pumping.
© 2004 Society of Vacuum Coaters 505/856-7188 ISSN 0737-5921 7
47th Annual Technical Conference Proceedings (April 24–29, 2004) Dallas, TX USA

Figure 1: Schematic cross section of Hauzer HTC 1000/4
coater with HIPIMS cathode and optical probe. Not to scale.
HIPIMS of Ti was performed in Ar atmosphere at a pressure
of 1x10 -3 mbar (0.75 mTorr). The optical emission from the
dense plasma region of the cathode was collected in situ by a
quartz fibre bundle with collimator positioned in the center of
the chamber (Figure 1). This optical probe was arranged to
view the racetrack of magnetron target surface along its
normal. The light was analyzed with a Jobin Yvon Triax 320
spectrometer (Czerny-Turner geometry) with resolution of
0.12 nm. A photomultiplier tube (Hamamatsu R955) was
used as detector. The entire system was sensitive to the
spectral range 200-950 nm.
The chemistry of the HIPIMS plasma was obtained from OES
measurements averaging over 50 pulses. Time resolved
measurements of the plasma composition were obtained by
monitoring the light signal from a single emission line by
measuring the voltage across the 100 kOhm terminated PMT
output. 16 averages were taken in order to improve the signalto-noise
ratio.
The electron temperature was estimated qualitatively by taking
the ratio of two optical emission lines of Ti (0), namely
363.39 nm with excitation energy of 3.41 eV and the line at
521.04 nm with excitation energy of 2.43 eV. Assuming a
constant species density ratio, a Corona discharge and a
Maxwellian electron energy distribution, the ratio of emission
lines is a direct measure of the electron temperature (Te) in the
plasma [7]. The sensitivity of this method, however, is
confined to a particular range of Te, which in turn is determined
by the difference in excitation energies in the two
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species. For the particular lines quoted above, the sensitivity
range is for Te < 5 eV. For higher Te > 5 eV, the line ratio is
constant within +/-10 %. These particular lines were chosen
because of the high intensity and because they belong to the
same species and therefore have a constant ratio of density of
species. Plots of discharge current vs. optical emission
intensity were obtained from the peak values obtained in timeresolved
measurements of the current and OES signal respectively.
RESULTS AND DISCUSSION
Plasma Composition
Optical emission spectra of the HIPIMS discharge operated at
a pressure of 1x10-3 mbar on an industrial size rectangular
cathode of area 400 cm2 are displayed in Figure 2.
Figure 2: Optical emission spectrum from HIPIMS on an
industrial size cathode.
Overall, the spectrum contains high intensities of Ti metal
ions. Strong emission from Ti(2+) ions is detected in the
wavelength region 241-256 nm. Dominating the spectrum is
the Ti(1+) emission at 368 and 375 nm. Bands of Ti neutral
lines are visible centered around 363 nm and 374 nm. The
intensities of the ions 2+ and 1+ are extremely high relative to
the intensity of neutral lines. In conventional unbalanced
magnetron (UBM) sputtering, for example, 2+ emission is
almost never observed while 1+ emission at 368.4 nm and
375.8 nm is significantly lower than the neutral lines in the
same region. These findings confirm previous investigations
[2, 8] on plasma composition of HIPIMS on laboratory size
cathodes with area up to 180 cm 2 .

It is well known that the sputtering process generates mainly
a neutral flux with approximately 0.1% metal ion content [9].
Therefore, any metal ionization is due to collisions of the
sputtered atoms with electrons in the dense plasma region.
The probability of ionization depends on the temperature and
density of the plasma electrons.
It is well documented that the energy of electrons is sufficient
to ionize that Ar with ionization potential IP = 15.6 eV even
in conventional UBM plasmas. Thus electrons in HIPIMS
would have sufficient energy to also doubly-ionize Ti with
IP = 13.637 eV. Furthermore, HIPIMS plasma electrons may
have a greater electron temperature because the potentialcarrying
sheath and presheath regions may be extended by the
high discharge voltage conditions.
Figure 3: Time evolution of the plasma at high discharge
current density.
The density of electrons in the HIPIMS plasma is extremely
high, reaching 10 13 cm -3 [2, 9]. This value represents a two
orders of magnitude increase in plasma density over conventional
UBM discharge plasmas. This is expected to promote
high frequency of ionizing collisions between sputtered atoms
and electrons in dense plasma region.
Time Evolution of the Discharge
The time evolution of the HIPIMS plasma on an industrial size
rectangular magnetron is shown in Figure 3. The discharge
current trace is compared to the traces obtained from OES
measurements. It can be seen that simultaneously with the
initiation of the discharge current at 40 µs, the Ar(0) neutral
emission is developed. The Ar(0) signal reaches a local
maximum by 50 µs, some 10 µs after initiation, and then
continues to rise slowly to its absolute maximum at 200 µs.
The Ti(0) emission is detected with a small delay of ~10 µs
relative to the Ar(0) emission. The Ti(1+) and Ti(2+) emission
are initiated one after another with similar delays of ~10
µs. The Ti(0) and Ti(1+) emission peaks at ~160 µs, while the
Ti(2+) emission peaks at ~200 µs. It is interesting to note that
the rate of increase in Ti(2+) emission is significantly slower
than the rest of the species. A similar increase (not shown) has
been observed for a number of other Ti(2+) lines (see Table 1
for a list of observed lines).
Figure 4: Ratio of Ti(0) emission lines and temporal evolution
of the Ti(0) 363.39 nm line.
The overall behavior of the plasma can be separated in two
halves. From 20-50 µs the discharge is comprised almost
entirely of Ar. As the discharge current rises further, a
significant influx of metal atoms and ions in the plasma is
observed. It can be speculated that sputtering is initiated by Ar
bombardment but is then superceded by self-sputtering.
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Figure 5: Optical emission from Ti(0), Ti(1+), and Ti(2+)
species as a function of HIPIMS discharge current.
Electron Temperature
The temporal evolution of two emission lines of Ti (0) at
363.39 nm and 521.04 nm was measured and the ratio between
them was calculated in an attempt to estimate changes in the
electron temperature during the pulse. Figure 4 shows the
temporal evolution of the Ti (0) 363 nm line and the ratio
calculated from the two lines. The initiation of the discharge
is marked by the detection of emission from the Ti (0) 363.39
nm line at 20 µs. The electron temperature is seen to increase
significantly to approximately 70% of the maximum within
the first 20 µs (

Equation 1 and the corresponding slopes show that the production
of metal ions—Ti(1+) and, especially, the highly
charged Ti(2+)—is strongly influenced by the discharge current.
At the same time, the relationship for Ti(0) is less strong.
This discussion shows that increasing the power of the HIPIMS
discharge leads not only to an increase in deposition rate, but,
significantly, to an increase in the metal ion-to-neutral ratio in
the deposition flux.
CONCLUSIONS
High power impulse magnetron sputtering (HIPIMS) has
been performed successfully in industrial size coaters on
industrial size cathodes with area > 400 cm2 . A dedicated
power supply with peak current capability of 2.5 kA was built
(AC Sp. z o.o., Warsaw, Poland) in order to drive the discharge.
Electronic arc suppression technology ensured the
stable operation of the discharge in deposition runs over
several hours.
The high power supplied to the discharge enabled the generation
of highly dense plasmas containing doubly charged
Ti(2+) metal ions and extremely high metal ion-to-neutral
ratios. Investigations of the time evolution of the discharge
showed that a transition from gas to metal plasma occurred
within the pulse. The electron temperature during the pulse
was found to increase rapidly in the initiation phase of the
discharge and then continue increasing at a slower rate until
the end of the power pulse. The plasma composition was
strongly influenced by the power supplied to the discharge.
Increasing the discharge current gave rise to a rapid increase
in Ti(2+), Ti(1+) metal ion intensity and a slower increase in
Ti(0) emission, thus pushing the metal ion-to-neutral ratio to
high values.
ACKNOWLEDGMENTS
This work was financially supported by the European Union
GROWTH (CRAFT) program, contract No. G5ST-CT-2002-
50355
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