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In reactive atmosphere, the dominating species
were N1 single atom species comprising 40% of the total
ion flux. Some 30% can be attributed to Ti1 and Ti2. The
IEDFs for all the ions were shifted to higher energy as
shown in Figure 2(b). The dominant ions N1 have a
maximum energy of 40 eV for 99.9% of particles. Ti1
and Ti2 reach maximum energies of 35 and 50 eV,
respectively, and Ar ions reach 20 eV. It is interesting to
note that the plasma potential, as derived from the max-
imum in the IEDFs, remains low at approximately 5 V.
DC and Pulsed DC Sputtering
IEDFs for DC and pulsed DC sputtering in the same Ar N2mixture
are shown in Figure 3(a) and 3(b). The ion count
rate and respectively, the ionisation in pulsed DC is higher
than for DC. However, both the ion chemistry and the
IEDFs are quite similar. Here the maximum energies for99.9% of
the ions are less than 10 eV and correspond well
with a Maxwellian distribution. In contrast to the HIPIMS
case, the content of Ti1 is 3% and Ti2 is negligible. The
content of N1 is also strongly reduced, indicating a sign-
ificantly lower degree of activation of both the pulsed DC
and DC plasmas.
Table 1 shows a comparison of average plasma com-
position for HIPIMS, pulsed DC and DC sputtering. It can be
seen that the nitrogen atomic ion and metallic ions have a
significantly higher contribution in HIPIMS than DC and
pulsed DC.
The general trend is that the ion flux in HIPIMS containsfactor
of 5 more Ti1 and Ti2metal ions than pulse DCor DC
technologies. The ratio holds for both inert and reactive
gas
atmospheres. The activation of the reactive gas is
significant
with HIPIMS generating factor of four times more N1
dissociated ions than DC and DC technologies.
Figure 2(b) and Table 1 show that the ion flux in HIPIMS
comprises mainly N1 and Ti1. These ions represent
film-forming species and as such determine directly the
growth conditions with their energy. In contrast, in DC
sputtering, the majority of ions are Ar1 who only
contributes
part of their energy indirectly via bombardment collisions
with the Ti and N atoms that have already nucleated at the
surface or are in the process of surface diffusion.
Figure 4(a) shows IEDF for Ti1 measured in inert
atmosphere for HIPIMS, pulsed DC and DC. It is immedi-
ately obvious that the count rate is two orders of
magnitude higher for HIPIMS indicating a higher ion fluxduring
the pulse. The average energy is a factor of 2 greater
for HIPIMS. A similar trend is observed for the N1 ions as
shown in Figure 4(b).
Both the average and maximum energies of ions in
HIPIMS are a factor of 3 higher in comparison to conv-
entional DC and pulsed DC sputtering. Since these energies
are significantly higher than what is expected from
sputtering of solid targets at the impingement energies
discussed, alternative mechanisms should be considered. A
certain increase in energy can be expected due to the
increase in sputter voltage from 300 to 500 V, however this
alone cannot account for the three-fold increase observed.
The time-averaged measurements were performed for the
duration of the HIPIMS power pulse only, i.e. excluding
the times of switching on and off of the voltage. During
these periods, abrupt changes in the sheath may cause
surges in plasma potential and the generation of high
energy ions as has been measured for pulsed DC dis-
charges.[9] Since the high energy metal ions are generated
during the pulse, this mechanism is excluded as the
plasma is close to equilibrium and the sheaths change
relatively slowly.
A possible explanation can be sought in the transport of
metal ions. As metal ions are created in the dense plasma
region near the cathode, they may be accelerated back tothe
target to take part in a self-sputtering process. A
fraction of these metal ions may be reflected from the
surface with a high energy thus creating fast neutrals. A
similar process is known to occur in conventional
sputtering where approximately 20% of Ar ions can be
reflected as fast neutrals with energy of the order of up to
200 eV.[10] The fast Ti neutrals however have to traverse
through the HIPIMS plasma again where the density is so
high[2] that they may be ionised a second time by electron
collisions. This ionisation path will not change the energy
of the ion and as a result the process will generate high
Time-Resolved Ionisation Studies of the High Power Impulse
Magnetron Discharge . . .
Table 1.Ion content in per cent of the total ion flux.
Technology Gas composition Ar1R Ar2R Ti1R Ti2R N1R2 N1R
HIPIMS Ar 34 5 46 15
Pulse DC Ar 78 9 13 0.2
DC Ar 82 8 11 0.2
HIPIMS ArRN2 19 5 19 10 7 40
Pulse DC ArRN2 52 4 4 0.07 31 9
DC ArRN2 65 3 3 0.03 23 6
Plasma Process. Polym. 2007,4,S309S313 2007 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim www.plasma-polymers.org S311
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energy ions as observed in the Ti1 IEDF. A similar
mechanism may be responsible for the high energy of N1
and Ar2 species.
Time Evolution During the Pulse
The time evolution of the HIPIMS plasma was measured
for inert and reactive gas atmospheres.
Inert Gas Atmosphere
The time evolution in inert gas atmosphere is given in
Figure 5(a). The data are normalised to 1. The evolution can
be separated into two parts. Initially the discharge
consists
of singly and doubly ionised Ar. After 70 ms, the Ti1
and Ti2 ions comprise the majority of ion fluxsome 70%
of the total. This phase coincides with the peak in
discharge current and extends over 100 ms. At this time
the Ar1 and Ar2 content diminish quickly due to the
intense production of metal ions and the rarefaction of the
gas by the high power dissipated in the pulse. At the end of
the pulse at 150 ms the power is switched off and the metal
signal decays as the voltage is insufficient to sustain the
sputtering process. The plasma comprises mainly Ar ions
generated by remnant electrons that ionise the gas present
in the chamber and electron waves generated during the
power pulse and reflected by the chamber walls. [11] These
observations fit well with published OES results measured
for Cr, showing the transition from a gas discharge to a
metal discharge phase.[2] However, the present results
illustrate the point in a quantitative manner and
demonstrate the relatively high content of metal ions
generated by HIPIMS.
Reactive Gas Atmosphere
The time evolution in reactive gas atmosphere is presented
in Figure 5(b). Here again the discharge is initiated as a
gas
plasma containing mainly N1 ions (55%) but also N12and Ar1 ions
comprising 10% each. The N1 signals are
observed to drop and at the same time the Ti1 and Ti2
metal ion count rate is observed to peak together with the
power pulse. This indicates the transition from a gas
plasma to a metal plasma thus diminishing the relative
amount of gas ions at the moment of maximum metal ion
content. Once power is switched off at the end of pulse, Ti
A. P. Ehiasarian, Y. A. Gonzalvo, T. D. Whitmore
Figure 3.IEDFs of ions generated in (a) DC and (b) MF pulsed
DCsputtering of Ti discharges in reactive Ar/N2atmosphere.
Figure 4.IEDFs of the film-forming species: (a) Ti1 and (b) N1
forHIPIMS (triangles), MF pulsed DC (squares) and conventional
DCsputtering (circles) for Ti.
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Plasma Process. Polym. 2007, 4 , S309S313 2007 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim DOI: 10.1002/ppap.200730806
