HiPIMS Ion Energy Distribution Measurements in Reactive AlN Deposition

Three papers about
HiPIMS and Mass
Spectrometry
Javier García Molleja
In collaboration with:
OAxel Ferrec
OPierre-Yves Jouan
OJulien Keraudy

Generation processes of super-high-
energy atoms and ions
in magnetron sputtering plasma
O Y. Takagi, Y. Sakashita, H. Toyoda, H.
Sugai
O Vacuum 80 (2006) 581–587

1. Introduction
O It is essential to have a better
understanding of a surface process of the
sputtering target, growing films, and gas-
phase processes in the magnetron
plasma.
O The surface qualities of sputtered films
are influenced by incidence of particles
having kinetic energies much higher than
bond energies.

2.Experiment
O A planar unbalanced magnetron discharge is
made in a cubic metal vessel, 0.2 m in
dimension, which is evacuated with a
turbomolecular pump of 200 l/s.
O Rare gas (Ar, Kr or Xe) is fed into the vessel
through a variable leak valve at a pressure of
0.3–0.7 Pa.
O The discharge was maintained by a DC power
(VD~600 V, ID<0.2 A) applied to a 5-cm-
diameter cathode with a grounded coaxial
cylinder anode.

2. Experiment
O A sputter target of tungsten or permalloy (80%
Ni/20% Fe) was set on the cathode.
O Surface magnetic flux densities were ~600 and
~800 G for target materials of permalloy and
tungsten, respectively.
O Ion EDF was measured at a substrate position, z =
0.1 m away from the target surface.
O QMA with an energy filter of resolution ~0.5 eV.
O By a movable spherical Langmuir probe 1 mm in
diameter made from platinum 2D profile plasma
density and electron temperature were obtained.

2. Experiment
O Although the observed peak density was
~1016 m-3 at z~0.01 m, the maximum
density may be much higher near the
target (Te~10 eV).
O Plasma density of 3·1014 m-3 at the
position of the QMA orifice (z = 0.1 m).
Electron temperature ~2 eV.
O Maxima configure a ring pattern.

2. Experiment
O p=0.4 Pa, I=0.1 A.
O Corrected taking account of
ion acceleration by a sheath at
the orifice and the difference
in work functions.

2. Experiment
O Ar (M = 39.9),
Kr (M = 83.8),
and Xe (M =
131.3).

2. Experiment
O High kinetic energies often observed in
EDF measurements are considered to
originate from backscattering processes
on the target.
O The maximum kinetic energies Ebmax given
by a two-body collision model in the first
approximation (head-on collision):

3. Simulation
O Kinetic energies of fast Ar atoms backscattered
from the target surface were simulated by the
TRIM code.
O 105 argon ions with energy 200–600 eV were
perpendicularly injected onto the flat target
(permalloy, Al, W).
O Ion positive charge was assumed to be neutralized
on the target metal surface.
O Eventually a part of injected ions were ejected, as
backscattered atoms, from the target surface
whose kinetic energies and directions were
calculated by the TRIM code and tabulated in
advance.

3. Simulation
O Tungsten (M =
183.9), permalloy
(M = 58.13), and
aluminum (M =
26.98).
O Ebmax coincided
with the shoulder
of the EDF curve.

3. Simulation
O The Ar+ energy distribution measured at
the position of the QMA was simulated in
Monte Carlo method using the EDF of
backscattered Ar atom in the TRIM code.
O Superhigh-energy ions were found at a
position 0.1 m away from the target, which
was attributed to backscattered fast argon
atoms (Ar(fast)).
O There are three ionization processes of
backscattered Ar(fast).

3. Simulation
O Electron–atom ionization, Ar(fast)+e ->
Ar(fast)++2e.
O Charge exchange, Ar(fast)+Ar(slow)+ ->
Ar(fast)++Ar(slow).
O Atom–atom ionization, Ar(fast)+Ar(slow) ->
Ar(fast)++Ar(slow)+e (a slow atom is also
ionized with the same probability as the fast
atom).
O Ar+ ions are lost in gas phase through the
charge exchange process: Ar(fast)++Ar(slow)
-> Ar(fast)+Ar(slow)+.

3. Simulation
O In the experimental conditions (na = 1.0·1020 m-3, np
= 4·1014 m-3, Te = 2 eV) and plotted as a function of
the impact energy, assuming the slow particle to be
at rest. For reference, the mean free path ael of fast
Ar atom for momentum transfer collision with slow Ar
atoms was indicated.
O The dominant ionization mechanism corresponds to
the shortest ionization free path.
O Above the ionization threshold energy (15.75 eV), the
fast Ar atom is dominantly ionized by atom–atom
collisions. Below the threshold, however, the
ionization occurs mainly by charge exchange
collisions. Electron impact ionization is always
negligible.

3. Simulation
O The high-energy tail caused by backscattered fast Ar atoms
were simulated in the Monte Carlo method.
O The box was filled with slow Ar atoms and a plasma
composed of slow Ar+ ions and electrons.
O When particles collided with the walls, they were absorbed.
O We injected fast Ar atoms (backscattered Ar atoms) from
the origin.
O The ions and atoms arriving at a substrate (0.03 m X 0.03
m) on the wall at z = 0.1 m were counted memorizing the
kinetic energies.
O In the case of the momentum transfer collisions, the
scattering angle was determined in the center of mass
coordinate taking account of the angular dependence of
differential cross section.
O Velocities after collisions were calculated from energy and
momentum conservation and were transformed to the
laboratory frame.

3. Simulation
O If the high-energy particle bombardment
should be relevant to sputter film damage,
it would be caused by injection of fast
atoms rather than fast ions.

4. Comparison between
experiment and simulation
O Fast Ar+ ion flux passing through the QMA orifice can
be expressed.
O The low-energy majority ion flux is approximately
given by the Bohm flux.
O Enables a quantitative comparison of the simulation
results with the measured results on the high-energy
tail of EDF.
O Good agreement between the simulation results and
the measured values was obtained not only in the
relative shape of EDF but also in the quantitative
values.
O The population of fast Ar+ ion for tungsten target was
about ten times higher than for the permalloy case.

5. Summary
O High-energy particle incidence on a substrate
significantly affects qualities of deposited films.
O When the mass number of target material was
larger than that of rare gas, a fast ion tail in the
EDF was observed in a range of super-high
energies (>100 eV), with a majority of slow ions.
O According to the simulation, the backscattered rare
gas atoms of super-high energies are converted to
fast ions, 1% at most, by atom–atom collisions in
the gas phase; however, the rest of them impinge
on the substrate as fast atoms.
O Guideline for reducing the high-energy particle
bombardment on the substrate is proposed.

HiPIMS Ion Energy Distribution
Measurements in Reactive Mode
O Pierre-Yves Jouan, Laurent Le Brizoual,
Mihaï Ganciu, Christophe Cardinaud,
Sylvain Tricot, and Mohamed-Abdou
Djouadi.
O IEEE TRANSACTIONS ON PLASMA
SCIENCE, VOL. 38, NO. 11, NOVEMBER
2010

1. Introduction
O Aluminum nitride is of great interest due to its unique
properties.
O Its piezoelectric behavior and its high acoustic wave
velocity permit excellent acoustic wave applications
(SAW and BAW).
O Parameters like the magnetron configuration
(balanced or unbalanced), the magnetic field
distribution, or even the substrate bias have not been
so widely investigated.
O Setting the substrate to its self-biased potential can
change the energy of ions bombarding the substrate.
O Due to the low ionization rate of the conventional
magnetron systems, only few species (ions) are
controlled, and therefore, the detrimental role of
energetic neutrals cannot be avoided.

2. Experimental
O A 50-mm-diameter and 6-mm-thick aluminum
target was mounted on this cathode.
O The deposition of the AlN thin films is done in
the nitrided mode of the target (N2/(Ar + N2) =
25% to 35%).
O The target is first cleaned in a 100% argon
atmosphere at 0.27 Pa for 15 min to remove
any previous nitride or oxide layer on the Al
target.
O Pressure is adjusted down to 0.27 Pa. The
target is nitrided for 15 min. The total gas
mass flow rate is 40 sccm.

2. Experimental
O The power applied to the target in the dc reactive
magnetron sputtering is 150 W.
O The applied voltage in HiPIMS is −1 kV, the pulse
width is 28 μs, and the frequency (repetition rate) is
1.6 kHz.
O Mass spectrometer from Hiden (EQP 1000). The
spectrometer is differentially pumped by a
turbomolecular pump allowing a lower pressure limit
of 3.7 × 10−6 Pa. The mass spectrometer aperture is
located in front of the target at a distance of 30–100
mm.
O The spectrometer is referenced to the ground (0 V).
Nevertheless, in order to optimize the species
collection, the extractor potential was varied.

2. Experimental
O The substrate is replaced by a conducting
plate of 30 mm in diameter located in front
of the spectrometer aperture and isolated
from the ground potential. A 380-μm slit is
located at the middle of this plate to allow
plasma species to get through and reach
the spectrometer aperture. When the
plasma is ON, the plate reaches the
floating potential (Vf).

2. Experimental
O Vext=0 V, Vaxis=-40 V, Vtr=3 V, Vdyn=-4000 V.
O These values lead to a transit time of 89 μs
for Ar+, 74 μs for N2+, 72 μs for Al+, and 52 μs
for N+. The difference in the transit time for an
ion at 1 and 100 eV ranges from 2.5 to 3.6 μs
for the investigated ions. Therefore, the gate
width for the acquisition was set to 4 μs.

3. Results
O The IED of a dc
magnetron sputtering
discharge was
performed: power of
150 W and a pressure
of 0.27 Pa in a mixture
of 75% Ar + 25% N2
gas.
O The spectrometer head
was positioned in front
of the target at a
distance of 30 mm and
was grounded.

3. Results
O Two ion populations are visible on the IED of Al+ and N+.
The low-energy peak corresponds to low-energy ions
coming from the gas plasma (Ar and N2). The second peak
and the broad energy tail result from energetic sputtered
neutrals ionized by the electron impact during their
transport between the target and the mass spectrometer
head.
O However, no extraction potential is applied to attract ions
inside the spectrometer (grounded configuration). When the
extractor is referenced to a negative potential, more low-
energy ions are attracted inside the spectrometer.
O The low-energy part of the curve is enhanced, but the
energetic tail remains unchanged.
O It is worth to notice that, as only argon is used, the Al+
distribution is wider than that obtained with a mixture of
Ar/N2.

3. Results
O The added plate with the slit was therefore grounded
or let at a floating potential.
O Curves are normalized to their area. The IED is
broader, and the energetic tail slightly extends at
higher energies in the floating case. In fact, when the
substrate becomes floating from grounded, the
difference between the plasma potential and the
substrate potential increases from 2 to 30 V which
leads to a 1.5 times thicker sheath.
O A floating potential or a slight bias voltage (−10 to
−20) improves significantly the crystalline quality of
AlN films.
O A conventional magnetron sputtering plasma implies
that most of the incoming species at the substrate
surface are neutrals. Energetic backscattered
neutrals could not be controlled.

3. Results

O The HiPIMS produces such a high amount of ions in the
plasma due to an ionization rate of 10% to 70% for power
densities around 1–3 kW/cm2 avoiding energetic neutrals.
O For the HiPIMS, the mean plasma potential value is
between 3 and 4 eV for 35% nitrogen content but does not
vary significantly when the percentage of nitrogen is
changed.

3. Results
O Experimental conditions (pulsewidth =
28 μs, repetition rate = 1.6 kHz, and
applied voltage = −1 kV).
O There is a preionization voltage (−300
V) in between the pulses.
O The first peak minimum in the absolute
value is associated with the primary
electrons generated at the beginning of
the discharge.
O The shoulder that follows this first peak
is enhanced in the reactive Ar+ − N2
case.
O Secondary electrons are emitted by the
target are responsible for the observed
shoulders.
O The secondary electron emission yield
is higher for AlN (0.3 electron/ion) than
for a pure Al target (0.08 electron/ion)
and they sustaing the high negative
floating potential for a longer time.

3. Results
O For a floating substrate located at 30 mm from the
target, ionized Al+, N+, N2+ , and Ar+ were detected,
including low signal from Ar2+ and Al2+.
O The overall signal of each ion investigated is higher
than that observed in the dc mode, particularly at a
high energy.
O Target species (Al+ and N+) have a larger energy tail
of up to 70 eV for aluminum ions and up to 50 eV for
N+ ions.
O The ratio of the Al+/Ar+ signal reaches 0.71.
O Low-energy peak correspond to thermalized ions in
the plasma and of a higher energy tail.
O The high-energy tail originates from sputtered
neutrals (by self-sputtering and Ar+ bombardement)
which are ionized on their way to the substrate.

3. Results
O The floating potential, which is referenced to the ground,
varies during the pulse.
O It is worth to notice that ions are detected after at least 20
μs, and therefore, the floating potential seen by ions is
around −20 V or even less.
O Moreover, the integrated measurements are done during a
larger time than the pulse duration: the floating potential is
mainly around zero.
O The time-resolved measurements were performed to
investigate the plasma composition during the impulse
sequence. The HiPIMS discharge is dominated by Al+ ions.
O A high power density at the target causes the gas to
undergo very strong rarefaction and could explain the lower
detected signal of Ar+.

3. Results
O For each ion, the signal increases quickly, shortly after the
end of the pulse (28 μs), and is the maximum for a delay of
50 μs.
O The collected ion current at the substrate is delayed from
the beginning of the pulse. This delay can be explained by
geometrical consideration, magnetic field configuration,
target–mass-spectrometer first electrode distance etc.
O The diffusion rate of the plasma can be estimated to ≈ 0.6
cm·μs−1. Diffusion is dominated by elastic scattering.
O Most of the ion flux reaches the substrate at the end of the
pulse and during the OFF time of the impulse sequence.
O Despite these large measured energies, AlN films have
better crystallinity in the HiPIMS than in the dc mode.
O The rocking curve measurements of the (0002) diffraction
peak indicate an FWHM of 1.6º in the dc reactive
magnetron sputtering and an FWHM of 1.1° in the HiPIMS.
O The momentum transfer seems to be lower than that of the
energetic backscattered neutrals.

4. Conclusion
O The floating substrate configuration leads to an
energetic tail in the dc configuration.
O This one is sustained to a high negative value for a
longer time when the target is nitrided due to the high
secondary electron emission yield of AlN.
O The main ionic species detected is Al+ whose
intensity is 150 times higher than that in the dc mode.
O The HiPIMS plasma show that most of the ionic flux
reaches the substrate during the pulse OFF time.
O Large fraction of energetic ions coming from the
target have been used for momentum transfer, and
better crystallized AlN films were obtained.

Time and energy resolved ion mass spectroscopy
studies of the ion flux during
high power pulsed magnetron sputtering of Cr in Ar
and Ar/N2 atmospheres
O G. Greczynski, L. Hultman
O Vacuum 84 (2010) 1159–1170

1. Introduction
O In HiPIMS the high temporal electron density in the
plasma results in an effective ionization of sputtered
material.
O Since the influence of ion bombardment during film
growth on properties of resulting coatings is well
proven it is of primary importance to understand what
factors affect the ion energy distribution function
(IEDF) in the case of HIPIMS processing.
O HiPIMS produces a high intensity peak of low energy
ions and more energetic ions produce a high-energy
tail, significantly broader than for the DC discharge.
O Metal and gas ions could be detected in the after
glow plasma on the ms time scales, thus long after
the current pulse was turned off.

2. Experimental
O Industrial CC800/9 coating system
manufactured by CemeCon AG in Germany
upgraded with the HIPIMS technology.
O The base pressure in the vacuum chamber
after the overnight bake-out was 4·10-5 Pa.
O Chromium target of dimensions 88x500 mm2
was sputtered in Ar or Ar/N2 atmosphere at
the total pressure of 0.4 Pa.
O The cathode was operated in the frequency
range between 100 Hz and 300 Hz. The
average power was between 500 W and 4500
W, and the pulse duration was 200 s.

2. Experimental
O The spectrometer was facing the center of the target from
the distance of 21 cm.
O Sputtered atoms with an average energy of 10–30 eV are
expected to make several collisions (get thermalized).
O IEDFs were measured both in time-averaged and time-
resolved mode with commercially available mass
spectrometer PSM003 (from Hiden Analytical, UK), mass
discrimination up to 300 amu with a 0.01 amu resolution.
O The orifice of the spectrometer was grounded and aligned
along the target surface normal.
O The IEDF were often recorded for more than one isotope.
Data presented in this paper are scaled by the abundance
values.
O Due to the fact that quadrupole mass analyzers transmit
more at low mass than at high mass, the 1/mass
transmission function is often assumed.
O Data quantification in the absolute terms is not attempted
due to the fact that the energy-dependent transmission
function of our mass spectrometer is not known.

2. Experimental
O Special care has been taken to account for a potential
signal drop caused by sputter-deposited material on the
spectrometer orifice.
O The target was sputtered for 60 s in pure Ar prior to such
control scans. Following that, Cr+ and Ar+ spectra were
recorded. The total signal drop after performing all
measurements did not exceed a factor of 2.
O The scanned energy range for singly charged ions was
between 0 eV and 100 eV in 0.5 eV steps.
O The detector gate window was in this case set to 10 s and
a delay time with respect to the onset of the voltage pulse
to the cathode was varied from 20 s up to 240 s in 10 s
intervals. The total acquisition time per data point was 1ms.
For singly charged ions, the ion energy was scanned
between 0 eV and 50 eV in 1 eV steps.
O For ions in the energy range between 0 and 100 eV, the
TOF does not vary more than 10%.

3. Results and discussion
O The exact shape of current and voltage waveforms
is to a great extent determined by the size of the
capacitor bank.
O The voltage is not constant throughout the pulse,
but rather drops from the peak value. The resulting
target current is thus proportional to the rate at
which the voltage drops.
O Between 0 s and 100 s discharge operates at
typical HIPIMS conditions with large dynamic
changes in both current and voltage, that later
stabilize in the second phase (100–200 s) to
resemble that of DC sputtering.

O Current levels achieved
during the second phase
constitute only a fraction
of typical DC currents for
a corresponding target
voltage. This observation
clearly indicates that this,
DC-like discharge,
operates under Ar-
depleted conditions
owing to the severe gas
rarefaction.
O Not even a 750 s long
pulse-off time (after high
current pulse) was
enough to completely
restore the initial
conditions.

O Film growth will be determined by ion flux
generated during the HIPIMS phase.
O Ar2+ to the total ion signal is below 2%, owing to
the very high second ionization potential of 27.76
eV (for comparison: 2nd ionization potential for Cr
is 16.57 eV).
O IEDFs of singly and doubly charged Cr ions
comprises low-energy peaks and very pronounced
high-energy tails that completely dominate the ion
energy spectrum for energies larger than 5 eV.
O ~90% reduction of the original energy flux under
the present conditions.

O The high energy tail that
develops with increasing Ep
represents primarily the
original distribution function of
sputter-ejected metal atoms
convoluted with (i) the
probability function for
electron-impact ionization, (ii)
the probability function for
collisions of metal ions with Ar
neutrals, and finally (iii) the
energy transmission function of
the spectrometer.
O In parallel, a contribution from
the backreflected metal ions to
the high-energy tail is also
expected.
O Cr ions that arrive at the target
with sufficiently high energy
can kick out another Cr atom
(self-sputtering) or get
neutralized and reflected.

O The position of this low-energy peak reflects the value of plasma
potential in the region where ions were created, averaged over the
entire time period of the pulse.
O Due to the fact that the plasma potential varies quite substantially
during the relatively short voltage pulse and tends to saturate in the
long post-discharge phase on the millisecond scale, the position of
the low-energy peak in time averaged measurements is determined
by the plasma potential in the post-discharge phase.
O Typical value of Vpd decreases with increasing pulse energy.
O Higher energy Ar+ ions are thought to originate either from
momentum transfer in collisions with sputtered species or from Ar
ions that after being accelerated towards the cathode got reflected
as energetic neutrals to undergo post-ionization in the next stage.
O The intensity of Ar+ signal is independent of the pulse energy (ions
contributing to this spectral feature are created in the post-
discharge phase).
O Cr+ and Cr2+ ions in the high-energy portion of the IEDFs increases
with increasing Ep. This effect may be ascribed to a higher
probability of electron-impact ionization.

O There is an increased amount of back-reflected
metal ions.
O A severe drop in the deposition rate is observed for
increasing pulse energy.
O The increase of temporal plasma density with
pulse energy has only a small effect on the
intensity of Ar+.
O Low-energy peak of Cr+ IEDF although noticeable,
is still small.
O The uniform increase in the intensity of Cr2+ IEDF
suggests that most of these ions are created
during the most energetic phase of the discharge.

O Metallic mode
O In the Cr+ IEDF the first ions are detected 35 s after the beginning
of the voltage pulse the intensity in the low-energy part of the
energy spectrum (below 10 eV) is several times lower than in the
high energy part.
O In the next time interval (55–75 s) the Cr+ ions increase
tremendously in intensity to a large extent preserving the original
shape of IEDF.
O Times longer than 150 s after the onset of the pulse, the IEDF
consists of a single peak.The position of this low-energy peak does
not coincide with a thermalized peak observed in time-averaged
measurements (time-averaged data include also information about
the plasma state after 200 s).
O The difference in detected energy of thermalized ions is caused by
the drop in plasma potential after the pulse is turned off.
O The intensity of the low energy peak at 2 eV is significantly lower in
the case of Cr2+, which is a consequence of a significantly lower
probability for double-ionization events in the post-discharge
plasma, but its behavior is the same than singly ionized Cr.
O Ion spectrum is significantly faster for Cr2+ ions.

O Ar+ ions exhibit high intensity already between 15 s and 35
s after the onset of the voltage pulse clearly preceding the
Cr ions.
O Between 35 s and 55 s, the IEDF increases in intensity
and broadens up to 20 eV to collapse drastically in the next
20 s. After 75 s the IEDF gradually narrows down to a
single peak.
O A sudden drop observed after 55 s coincides with an
equally dramatic increase in the number of Cr+ and Cr2+
ions.
O The density of Ar gas is reduced in the vicinity of the
cathode due to effective energy exchange with high
temporal flux of sputter-ejected metal neutrals.
O The observed drop in the intensity of Ar+ signal can also
result from lower ionization probability.
O With increasing pulse energy, all peaks (including the table
current) move to the left on the time axis that is caused by
the fact that the discharge was operated at 100 Hz.

O With increasing frequency time between the pulses becomes short
enough to ensure immediate ignition and no such effects are
observed.
O The first detected species are in all cases Ar+ ions. It is remarkable
that the intensity of Ar+ signal increases up to the point when first
Cr+ ions are detected.
O Increasing intensity of Cr+ IEDF is then accompanied by a sudden
drop in the Ar+ signal.
O On the later stage Cr+ IEDF decays and the Ar+ IEDF slowly picks
up. One reason for this could be rarefaction effects and quenching
of electron density.
O The high-energy tails drop off after the peak maximum is reached.
O The maxima of the Cr2+ signal coincide with the rising portions of
the Cr+ peaks meaning that doubly charged ions are most
effectively produced during the time interval when single charged
ions possess a very broad energy spectrum.
O The rapid decrease of Cr2+ intensity may be indicative of a plasma
cooling effect induced by the still-increasing concentration of metal
atoms.

O Reactive mode
O The total gas pressure was kept constant at 0.4 Pa and the N2-to-Ar
gas ratio, fN2/Ar, was varied between 0 (metallic mode) and 5
(sputtering in heavily poisoned mode).
O The signal detected at m/e=15 is assumed to be entirely dominated
by N+ ions; the contribution from N22+ ions is expected to be orders
of magnitude lower due to its very high ionization potential.
O The IEDF of singly charged Cr ions for increasing fN2/Ar comprises
the low-energy peak and very pronounced high-energy tails. With
increasing N2 content in the plasma the low-energy peak moves
from 1.1 eV to 0.5 eV, indicating that the post-discharge, Vpd,
decreases.
O The IEDF of Ar+ shows a more or less uniform increase in intensity
with increasing nitrogen content in the plasma up to fN2/Ar =0.2.
Thereafter the count rate decreases, preserving the overall shape
of IEDF, to end up at approximately half of the initial value at fN2/Ar
=5.
O The overall shape of N2+ IEDF is similar to that of Ar+, especially at
higher nitrogen flows.

O The IEDF of N+ is qualitatively different
as it possesses the high-energy tail
typical for Cr+.
O This energetic stream of monoatomic
nitrogen ions is created through the
electron-impact ionization of nitrogen
atoms originating from the target:
either sputtered from the nitrided
portion of the target or being the result
of dissociation of back-attracted N2+
ions at the target surface.
O IEDF of Cr+ it can be seen that the
high-energy tail of the most energetic
distributions (between 55 s and 95
s) falls off faster with increasing ion
energy for fN2/Ar = 2 (15 J, 300 Hz)
than in the case of sputtering in pure
Ar.
O Qualitatively similar changes with
respect to the operation in metallic
mode are also observed in the case of
Ar+ despite the fact that Ar ions are by
far less energetic than Cr ions.

O Intensity and the maximum ion energy are greatly reduced when
sputtering in the poisoned mode. This is, however, accompanied by
the increase in intensity on the later stage (after 135 s) when only
thermalized species are present.
O The IEDF of N2+ ions is quite narrow with most energetic ions
appearing between 35 s and 75 s, thus about the same time as
for Ar+ ions.
O IEDFs of N+ ions, bring yet another evidence that supports their
origin from the target; energetic ions appear at the same time
interval as most energetic Cr+ ions (55–95 s) and with a certain
delay with respect to N2+ ions.
O Independent of fN2/Ar the first ions detected are always Ar+ and N2+.
O Both ion types gain intensity as time goes by up to the point where
Cr+ and N+ signals start to take off, at around 50 s.
O The increase of the Cr+ and N+ ion flux (that reach maximum at ca.
80 s and 70 s, respectively) is accompanied by a decrease in a
number of Ar+ and N2+ ions detected. The situation is reversed in
the following time interval.
O Ar+ and N2+ features two characteristic peaks: one at early stage of
discharge associated with more energetic ions and one broader
bump composed of thermalized ions.

O The intensity of Cr+ decays with a longer time
constant than that of N+.
O This phenomenon is related to the fact that
after 100 s the character of the discharge
changes from HIPIMS to DC-like.
O Comparing DC discharge varying the nitrogen
percentage N+ content is 4–10 times lower
than in the case of the HIPIMS discharge
operated at the same average power.

O Very high temporal energy density on the target can
enhance the dissociative sputtering of CrN, as well as, lead
to more effective decomposition of back-reflected N2+ ions.
O Similar to Ar ions, the number of N2+ ions is affected by the
energetic flux of Cr atoms from the target.
O The intensity of the Ar+ and Cr+ signals increases with
decreasing Ar flow, at least for the lower values of fN2/Ar.
O Varying gas composition together with related poisoning of
the target surface gives a number of effects that can
account for this unexpected result: (i) an increased volume
density of species to be ionized, (ii) an increased probability
for ionization event per each gas atom, (iii) the plasma
volume probed in experiment may change.
O Certain changes of the gas density in front of the target can
not be excluded, as the primary cause of gas rarefaction.
O The secondary electron emission from nitrided surfaces is
believed to be higher than for metals.

O Ions available during film growth with HIPIMS may
constitute a major part of the particle flux incident on the
substrate.
O With increasing pulse energy, the emission from Cr+ ions
increases faster than that from Cr neutrals.
O For cases where formation of a compound is energetically
favorable even without the presence of activated species,
surface reactions may continue also during the pulse-off
time and it has to be taken into account in any analysis of
film growth processes.
O If the extra energy is required to promote compound
formation, film growth will proceed mainly during the high-
energy pulse.
O At Ep =3 J ion flux contributions are as follows: Cr+ – 32.3%,
Ar+ – 65.3%, Cr2+ – 1.2% and Ar2+ – 1.2%.
O At Ep =30 J, respectively: Cr+ – 52.2%, Ar+ – 41.1%, Cr2+ –
5.4% and Ar2+ – 1.3%.
O For the pulse energy of 30 J, the energy contributions are
as follows: Cr+ – 76%, Cr 2+ – 14%, Ar+ – 9% and Ar2+ – 1%.

O Doubly charged ions upon application of
typical bias voltages in the range 50–100
V is energetic enough to cause high
lattice defect density that may eventually
lead to disruption of epitaxial growth.
O The films would then become denser,
but exhibit high compressive internal
stress and inert gas incorporation.
O In the experimental set up used in our
experiment, the application of a
synchronized biasing pulse between 0
s and 40 s should affect the average
energy of a significant portion of the Ar+
ions, without having a noticeable effect
on the energy spectrum of the Cr+ and
Cr2+ ions.
O One may envision different biasing
scenarios for growth of CrNx films since,
in case of Cr, nitride formation is
believed to take place mainly during the
time when intense plasma is present. In
particular, it would be interesting to
evaluate separately the roles of low
energy N2+ ions and energetic N+ ions in
the nitride formation process.

4. Conclusions
O Increasing the pulse energy during sputtering in the metallic
mode leads to a rapid (linear) increase of the number of
doubly charged Cr ions.
O The composition (and energy) of the ion flux can be
significantly altered by varying the pulse energy.
O Low energy N2+ molecular ions and energetic N+ ions are
present while sputtering in reactive mode in contrast to the
reactive DC sputtering.
O Time-resolved studies reveal a significant variation in the
composition of the ion flux throughout the pulse.
O The properties (composition and energy) of the ion flux
incident on the substrate can be adjusted varying the pulse
energy and with time variations in the composition of ion
flux.
O Time evolution of ion flux composition is proposed to be an
inherent feature of the high power pulsed discharges.

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