Industrial Size High Power Impulse Magnetron Sputtering
Industrial Size High Power Impulse Magnetron Sputtering
Industrial Size High Power Impulse Magnetron Sputtering
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Key Words:<br />
<strong>Industrial</strong> <strong>Size</strong> <strong>High</strong> <strong>Power</strong> <strong>Impulse</strong> <strong>Magnetron</strong> <strong>Sputtering</strong><br />
A.P. Ehiasarian, Materials Research Institute, Sheffield Hallam University, Sheffield, United<br />
Kingdom; and R. Bugyi, AC Sp. z o.o., Warsaw, Poland<br />
Ion assisted deposition Sputter deposition<br />
Plasma source <strong>Power</strong> supplies<br />
ABSTRACT<br />
<strong>High</strong> power impulse magnetron sputtering (HIPIMS) is a<br />
novel physical vapor deposition technology producing large<br />
area uniform discharges with high concentration of metal<br />
ions. Until now it has been available on cathodes with area up<br />
to 180 cm2 (30 in2 ). The present work reports on the utilization<br />
of magnetron cathodes with area of 400 cm2 (62 in2 ) to<br />
generate HIPIMS discharges in the presence of Ar. The<br />
discharge was driven at a frequency of 100 Hz (10 ms) and<br />
pulse duration of 200 µs. The peak currents were in excess of<br />
2.5 kA when operated with a titanium target. The discharge<br />
was stable at Ar pressures down to 7x10-4 mbar (0.5 mTorr).<br />
The HIPIMS plasma composition was studied as a function of<br />
discharge current using time resolved optical emission spectroscopy.<br />
Strong emission from Ti(2+) and Ti(1+) ions was<br />
observed. The discharge was observed to transit from Ar<br />
plasma dominated to metal ion dominated as reported previously<br />
for small cathodes. The optical emission intensity of<br />
individual species, I( Ti(1+) ), varied with discharge current,<br />
Id, according to a power law I( (Ti1+) ) ~ Idn . The exponent<br />
n was found to be 2 for Ti(2+); 1.5 for Ti(1+); and 0.5-0.8 for<br />
Ti(0). This signifies a steep dependence of highly charged<br />
metal ions on the discharge current. It also points to a strong<br />
increase in metal ion to metal neutral ratio as a function of<br />
discharge current.<br />
INTRODUCTION<br />
<strong>Industrial</strong> uses of hard physical vapor deposited (PVD) coatings<br />
are associated with ever increasing requirements for<br />
microstructure density, defect-free processing and smooth<br />
surface. Scientific research and industrial practice in plasmaassisted<br />
PVD processes have provided substantial evidence<br />
that meeting these demands requires high degree of ionization<br />
and high ion-to-neutral ratio in the deposition flux. Although<br />
a number of plasma deposition sources have been developed<br />
on a laboratory scale, they show difficulties when applied to<br />
industrial size machines. One of the most promising new<br />
plasma deposition sources is the high power impulse magnetron<br />
sputtering (HIPIMS) introduced recently [1] and demonstrated<br />
up to now on laboratory scale equipment.<br />
The HIPIMS discharge is operated on magnetron cathodes<br />
and is characterized with extremely high power densities—up<br />
to 3 kWcm -2 generated at discharge voltages of up to 2 kV.<br />
Overheating is avoided by pulsing the power at low duty cycle<br />
of ~1%. The plasma densities achieved in HIPIMS are of the<br />
order of 10 13 cm -3 , [1, 2]. The sputtered metal flux is ionized<br />
efficiently in the HIPIMS plasma with metal ion charge states<br />
reaching 2+ for Cr [2]. HIPIMS has been successfully<br />
implemented to enhance adhesion by substrate pretreatments<br />
[3] and to deposit wear and corrosion-resistant CrN coatings<br />
[4, 5, 6].<br />
The current paper reports on the utilization of HIPIMS on<br />
industrial size rectangular cathodes with area of more than<br />
400 cm 2 . The plasma composition and its time evolution have<br />
been investigated with optical emission spectroscopy (OES),<br />
and the influence of power on the chemistry of the deposition<br />
flux is discussed.<br />
EXPERIMENT DETAILS<br />
HIPIMS was operated on rectangular cathodes with area >400<br />
cm2 (62 in2 ). The discharge was driven with a power generator<br />
HMP 6/16 (AC Sp z o.o., Warsaw, Poland) capable of supplying<br />
power pulses with duration in the range 0-200 µs at a<br />
frequency of 0-100 Hz (10 ms) equivalent to a duty cycle of<br />
2%. The power supply was capable of delivering peak<br />
currents of up to 3000 A and at a voltage of 2000 V. Arcing<br />
energy was minimized by arc suppression design that allowed<br />
switch off of the power supply even at the maximum current.<br />
The peak current of the discharge was limited by the available<br />
power. The parameters of the HIPIMS discharge were measured<br />
with a high voltage probe (Tektronix P6015A) with<br />
bandwidth of 50 MHz and a high current transformer (Tektronix<br />
CT-4) in combination with a current probe (P6021) with<br />
bandwidth of 500 Hz. The traces were recorded by an ultrafast<br />
digital phosphor oscilloscope Tektronix 3032B. The cathode<br />
and power supply were installed in an industrial size batch<br />
coater Hauzer HTC 1000/4, replacing one of the four original<br />
cathodes of the machine as illustrated in Figure 1. The<br />
chamber had a volume of 1 m 3 and was evacuated to a base<br />
pressure of 3x10 -6 mbar (2x10 -6 Torr) with turbomolecular<br />
pumping.<br />
© 2004 Society of Vacuum Coaters 505/856-7188 ISSN 0737-5921 7<br />
47th Annual Technical Conference Proceedings (April 24–29, 2004) Dallas, TX USA
Figure 1: Schematic cross section of Hauzer HTC 1000/4<br />
coater with HIPIMS cathode and optical probe. Not to scale.<br />
HIPIMS of Ti was performed in Ar atmosphere at a pressure<br />
of 1x10 -3 mbar (0.75 mTorr). The optical emission from the<br />
dense plasma region of the cathode was collected in situ by a<br />
quartz fibre bundle with collimator positioned in the center of<br />
the chamber (Figure 1). This optical probe was arranged to<br />
view the racetrack of magnetron target surface along its<br />
normal. The light was analyzed with a Jobin Yvon Triax 320<br />
spectrometer (Czerny-Turner geometry) with resolution of<br />
0.12 nm. A photomultiplier tube (Hamamatsu R955) was<br />
used as detector. The entire system was sensitive to the<br />
spectral range 200-950 nm.<br />
The chemistry of the HIPIMS plasma was obtained from OES<br />
measurements averaging over 50 pulses. Time resolved<br />
measurements of the plasma composition were obtained by<br />
monitoring the light signal from a single emission line by<br />
measuring the voltage across the 100 kOhm terminated PMT<br />
output. 16 averages were taken in order to improve the signalto-noise<br />
ratio.<br />
The electron temperature was estimated qualitatively by taking<br />
the ratio of two optical emission lines of Ti (0), namely<br />
363.39 nm with excitation energy of 3.41 eV and the line at<br />
521.04 nm with excitation energy of 2.43 eV. Assuming a<br />
constant species density ratio, a Corona discharge and a<br />
Maxwellian electron energy distribution, the ratio of emission<br />
lines is a direct measure of the electron temperature (Te) in the<br />
plasma [7]. The sensitivity of this method, however, is<br />
confined to a particular range of Te, which in turn is determined<br />
by the difference in excitation energies in the two<br />
8<br />
species. For the particular lines quoted above, the sensitivity<br />
range is for Te < 5 eV. For higher Te > 5 eV, the line ratio is<br />
constant within +/-10 %. These particular lines were chosen<br />
because of the high intensity and because they belong to the<br />
same species and therefore have a constant ratio of density of<br />
species. Plots of discharge current vs. optical emission<br />
intensity were obtained from the peak values obtained in timeresolved<br />
measurements of the current and OES signal respectively.<br />
RESULTS AND DISCUSSION<br />
Plasma Composition<br />
Optical emission spectra of the HIPIMS discharge operated at<br />
a pressure of 1x10-3 mbar on an industrial size rectangular<br />
cathode of area 400 cm2 are displayed in Figure 2.<br />
Figure 2: Optical emission spectrum from HIPIMS on an<br />
industrial size cathode.<br />
Overall, the spectrum contains high intensities of Ti metal<br />
ions. Strong emission from Ti(2+) ions is detected in the<br />
wavelength region 241-256 nm. Dominating the spectrum is<br />
the Ti(1+) emission at 368 and 375 nm. Bands of Ti neutral<br />
lines are visible centered around 363 nm and 374 nm. The<br />
intensities of the ions 2+ and 1+ are extremely high relative to<br />
the intensity of neutral lines. In conventional unbalanced<br />
magnetron (UBM) sputtering, for example, 2+ emission is<br />
almost never observed while 1+ emission at 368.4 nm and<br />
375.8 nm is significantly lower than the neutral lines in the<br />
same region. These findings confirm previous investigations<br />
[2, 8] on plasma composition of HIPIMS on laboratory size<br />
cathodes with area up to 180 cm 2 .
It is well known that the sputtering process generates mainly<br />
a neutral flux with approximately 0.1% metal ion content [9].<br />
Therefore, any metal ionization is due to collisions of the<br />
sputtered atoms with electrons in the dense plasma region.<br />
The probability of ionization depends on the temperature and<br />
density of the plasma electrons.<br />
It is well documented that the energy of electrons is sufficient<br />
to ionize that Ar with ionization potential IP = 15.6 eV even<br />
in conventional UBM plasmas. Thus electrons in HIPIMS<br />
would have sufficient energy to also doubly-ionize Ti with<br />
IP = 13.637 eV. Furthermore, HIPIMS plasma electrons may<br />
have a greater electron temperature because the potentialcarrying<br />
sheath and presheath regions may be extended by the<br />
high discharge voltage conditions.<br />
Figure 3: Time evolution of the plasma at high discharge<br />
current density.<br />
The density of electrons in the HIPIMS plasma is extremely<br />
high, reaching 10 13 cm -3 [2, 9]. This value represents a two<br />
orders of magnitude increase in plasma density over conventional<br />
UBM discharge plasmas. This is expected to promote<br />
high frequency of ionizing collisions between sputtered atoms<br />
and electrons in dense plasma region.<br />
Time Evolution of the Discharge<br />
The time evolution of the HIPIMS plasma on an industrial size<br />
rectangular magnetron is shown in Figure 3. The discharge<br />
current trace is compared to the traces obtained from OES<br />
measurements. It can be seen that simultaneously with the<br />
initiation of the discharge current at 40 µs, the Ar(0) neutral<br />
emission is developed. The Ar(0) signal reaches a local<br />
maximum by 50 µs, some 10 µs after initiation, and then<br />
continues to rise slowly to its absolute maximum at 200 µs.<br />
The Ti(0) emission is detected with a small delay of ~10 µs<br />
relative to the Ar(0) emission. The Ti(1+) and Ti(2+) emission<br />
are initiated one after another with similar delays of ~10<br />
µs. The Ti(0) and Ti(1+) emission peaks at ~160 µs, while the<br />
Ti(2+) emission peaks at ~200 µs. It is interesting to note that<br />
the rate of increase in Ti(2+) emission is significantly slower<br />
than the rest of the species. A similar increase (not shown) has<br />
been observed for a number of other Ti(2+) lines (see Table 1<br />
for a list of observed lines).<br />
Figure 4: Ratio of Ti(0) emission lines and temporal evolution<br />
of the Ti(0) 363.39 nm line.<br />
The overall behavior of the plasma can be separated in two<br />
halves. From 20-50 µs the discharge is comprised almost<br />
entirely of Ar. As the discharge current rises further, a<br />
significant influx of metal atoms and ions in the plasma is<br />
observed. It can be speculated that sputtering is initiated by Ar<br />
bombardment but is then superceded by self-sputtering.<br />
9
Figure 5: Optical emission from Ti(0), Ti(1+), and Ti(2+)<br />
species as a function of HIPIMS discharge current.<br />
Electron Temperature<br />
The temporal evolution of two emission lines of Ti (0) at<br />
363.39 nm and 521.04 nm was measured and the ratio between<br />
them was calculated in an attempt to estimate changes in the<br />
electron temperature during the pulse. Figure 4 shows the<br />
temporal evolution of the Ti (0) 363 nm line and the ratio<br />
calculated from the two lines. The initiation of the discharge<br />
is marked by the detection of emission from the Ti (0) 363.39<br />
nm line at 20 µs. The electron temperature is seen to increase<br />
significantly to approximately 70% of the maximum within<br />
the first 20 µs (
Equation 1 and the corresponding slopes show that the production<br />
of metal ions—Ti(1+) and, especially, the highly<br />
charged Ti(2+)—is strongly influenced by the discharge current.<br />
At the same time, the relationship for Ti(0) is less strong.<br />
This discussion shows that increasing the power of the HIPIMS<br />
discharge leads not only to an increase in deposition rate, but,<br />
significantly, to an increase in the metal ion-to-neutral ratio in<br />
the deposition flux.<br />
CONCLUSIONS<br />
<strong>High</strong> power impulse magnetron sputtering (HIPIMS) has<br />
been performed successfully in industrial size coaters on<br />
industrial size cathodes with area > 400 cm2 . A dedicated<br />
power supply with peak current capability of 2.5 kA was built<br />
(AC Sp. z o.o., Warsaw, Poland) in order to drive the discharge.<br />
Electronic arc suppression technology ensured the<br />
stable operation of the discharge in deposition runs over<br />
several hours.<br />
The high power supplied to the discharge enabled the generation<br />
of highly dense plasmas containing doubly charged<br />
Ti(2+) metal ions and extremely high metal ion-to-neutral<br />
ratios. Investigations of the time evolution of the discharge<br />
showed that a transition from gas to metal plasma occurred<br />
within the pulse. The electron temperature during the pulse<br />
was found to increase rapidly in the initiation phase of the<br />
discharge and then continue increasing at a slower rate until<br />
the end of the power pulse. The plasma composition was<br />
strongly influenced by the power supplied to the discharge.<br />
Increasing the discharge current gave rise to a rapid increase<br />
in Ti(2+), Ti(1+) metal ion intensity and a slower increase in<br />
Ti(0) emission, thus pushing the metal ion-to-neutral ratio to<br />
high values.<br />
ACKNOWLEDGMENTS<br />
This work was financially supported by the European Union<br />
GROWTH (CRAFT) program, contract No. G5ST-CT-2002-<br />
50355<br />
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