Rarefaction windows in a high-power impulse magnetron sputtering plasmaMaria Palmucci, Nikolay Britun, Stephanos Konstantinidis, and Rony Snyders

Citation: Journal of Applied Physics 114, 113302 (2013); doi: 10.1063/1.4821514 View online: http://dx.doi.org/10.1063/1.4821514 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Time-resolved temperature study in a high-power impulse magnetron sputtering discharge J. Appl. Phys. 114, 013301 (2013); 10.1063/1.4812579 Plasma diagnostics of low pressure high power impulse magnetron sputtering assisted by electron cyclotronwave resonance plasma J. Appl. Phys. 112, 093305 (2012); 10.1063/1.4764102 Fast relaxation of the velocity distribution function of neutral and ionized species in high-power impulsemagnetron sputtering Appl. Phys. Lett. 99, 131504 (2011); 10.1063/1.3644989 In situ plasma diagnostics study of a commercial high-power hollow cathode magnetron deposition tool J. Vac. Sci. Technol. A 28, 112 (2010); 10.1116/1.3271132 High power impulse magnetron sputtering: Current-voltage-time characteristics indicate the onset of sustainedself-sputtering J. Appl. Phys. 102, 113303 (2007); 10.1063/1.2817812

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Rarefaction windows in a high-power impulse magnetron sputtering plasma

Maria Palmucci,1 Nikolay Britun,1,a) Stephanos Konstantinidis,1 and Rony Snyders1,2

1Chimie des Interactions Plasma-Surface (ChIPS), CIRMAP, Universit�e de Mons, 23 Place du Parc,B-7000 Mons, Belgium2Materia Nova Research Center, Parc Initialis, B-7000 Mons, Belgium

(Received 30 July 2013; accepted 2 September 2013; published online 19 September 2013)

The velocity distribution function of the sputtered particles in the direction parallel to the planar

magnetron cathode is studied by spatially- and time-resolved laser-induced fluorescence

spectroscopy in a short-duration (20 ls) high-power impulse magnetron sputtering discharge. The

experimental evidence for the neutral and ionized sputtered particles to have a constant (saturated)

velocity at the end of the plasma on-time is demonstrated. The velocity component parallel to the

target surface reaches the values of about 5 km/s for Ti atoms and ions, which is higher that the

values typically measured in the direct current sputtering discharges before. The results point out

on the presence of a strong gas rarefaction significantly reducing the sputtered particles energy

dissipation during a certain time interval at the end of the plasma pulse, referred to as “rarefaction

window” in this work. The obtained results agree with and essentially clarify the dynamics of

HiPIMS discharge studied during the plasma off-time previously in the work: N. Britun, Appl.

Phys. Lett. 99, 131504 (2011). VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4821514]

I. INTRODUCTION

The gas-rarefaction in sputtering is a well-known phe-

nomenon occurring in both direct current magnetron sputter-

ing (DCMS) and low duty cycle high-power impulse

magnetron sputtering (HiPIMS) discharges.1–4 The effect of

gas rarefaction implies a significant reduction of the gas

pressure induced by the wave of sputtered particles in these

discharges, i.e., the so-called sputtering wind, as introduced

by Hoffman5 and Rossnagel.6 If the DCMS discharges and

the phenomena related to them are rather well known, the

concept of the HiPIMS sputtering systems has emerged only

in late 1990s.7–9 Since that time the HiPIMS systems attract

attention due to the improved properties of the deposited

coatings with, e.g., controllable composition, phase constitu-

tion, roughness, microstructure, etc.10–13 In the HiPIMS dis-

charges, the rarefaction phenomenon is time-dependent and

especially pronounced due to the fast changing (ls scale) of

the high power impulses applied to the discharge.3 The main

reason for this is the high peak current level (typically about

100 A, i.e., much higher than in DCMS), which is used to

produce a HiPIMS discharge, as well as the high voltage

(typically about 1 kV) during the pulse.4 The fact that the

high voltage is normally applied during rather short time

(5–500 ls) results in an intensive sputtering of the target ma-

terial during the pulse, as well as in high ionization

degree.14,15 The plasma processes are even more “squeezed”

in time in the case of so-called “short-pulsed” HiPIMS dis-

charges where pulse duration is <50 ls.16–18 As a result of a

short pulse duration, some stages of the HiPIMS discharge

ignition may be simply absent under these conditions. This is

first of all related to the so-called run away regime which

normally happens at the end of the HiPIMS pulse having

duration longer than about 200 ls, and absent in the short

pulse case, when the plasma pulse is finished before the dis-

charge current gets saturated.4

Talking about the energy distribution of the sputtered

particles in a sputtering discharge, it should be noted that the

knowledge accumulated so far in the DCMS and pulsed-

DCMS domains18–22 is barely applicable to the HiPIMS

case, first of all due to the low duty cycle used in the last

case, as well as due to much higher energy delivered to

plasma per a single pulse (i.e., higher applied energy den-

sity). In particular, the velocity distribution function (vdf) of

the sputtered species, being by definition static in DCMS,21

is essentially time-dependent in the HiPIMS case.

Furthermore, since in a DCMS discharge the rarefaction

region is also static and surrounded by the bulk gas, it might

be hardly accessible for the measurements of initial (high)

velocities of the sputtered species just after their ejection

from the cathode surface, as a result of thermalization in the

target vicinity. On the other hand, one may expect that in the

HiPIMS case, due to its pulsed nature, as well as due to

much higher applied currents, the time evolution of the gas

rarefaction might be directly visualized by the time-resolved

changes of the velocity distribution of the sputtered species

in the region adjacent to the magnetron cathode. The latter

phenomenon is the subject of the present study.

By applying laser-induced fluorescence (LIF) diagnos-

tics, recently it was shown that during the HiPIMS off-time,

the full width at half maximum (FWHM) of the vdf of the

sputtered species decreases super-exponentially, as a result

of the fast thermalization of the sputtered species likely

enhanced by the gas refilling effect after the plasma pulse.23

These measurements, however, are highly sensitive to the

strong plasma emission, so the LIF signal could not be

detected during the plasma on-time, which is primarily due

to utilization of a wide band interference band-pass filter for

signal detection. The method utilized in the present work

a)Author to whom correspondence should be addressed. E-mail:

nikolay.britun@umons.ac.be

0021-8979/2013/114(11)/113302/9/$30.00 VC 2013 AIP Publishing LLC114, 113302-1

JOURNAL OF APPLIED PHYSICS 114, 113302 (2013)

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uses the spectrally resolved LIF approach where the men-

tioned drawback is successfully eliminated. The details on

the applied diagnostics method are given in the Sec. II, fol-

lowing by the results of the measurements and their discus-

sion in the Sec. III, and summary in the Sec. IV.

II. EXPERIMENTAL DETAILS

A. HiPIMS system and the LIF setup

A stainless steel cylindrical HiPIMS reactor (40 cm long,

with inner diameter of 27 cm) having a circular Ti target

(1 cm thick, 10 cm in diameter), and using Ar as a working

gas was utilized as source of sputtered titanium. Two Ar pres-

sures (pgas), 5 and 20 mTorr, were used in the experiments.

The integral energy EP supplied during the plasma pulse was

equal to about 0.3 J.24 Typical current-voltage waveforms of

the Ar-Ti HiPIMS plasma pulse are given in Fig. 1.

A YAG:Nd INDI pumping laser working at the wave-

length of 532 nm coupled with a Sirah Cobra Stretch dye

laser with the second harmonic generation unit were utilized

for the plasma excitation during the LIF measurements. The

generated wavelength range (DCM dye) was about

305–330 nm (Table I). The laser energy was real-time con-

trolled by a laser energy measurement head (Ophir PE-9).

The detailed information on the Ti and Tiþ spectral transi-

tions for LIF used in this study can be found elsewhere.23

In this work, the fluorescence signal was acquired through

the optical fibre and filtered by a monochromator allowing for

the detected light (containing both the laser-induced fluores-

cence emission and spontaneous emission) to be spectrally

resolved for better separation of the spectral peak(s) under in-

terest. For this purpose, an Andor SR750 spectrometer-

monochromator with an iStar 740 Intensified Charged Couple

Device (ICCD) camera synchronized with the pulsed laser

system (with the total jitter <1 ns) was utilized. The optical

measurements were carried out at 15, 45, and 75 mm above

the magnetron target surface. The volumes of the discharge

actually probed by the optical fiber in this study are indicated

in Fig. 2 in magenta. The key parameters of the described ex-

perimental setup are summarized in Table I.

B. Details of the LIF measurements

Being spectrally resolved, LIF signal can be detected as

a function of the laser wavelength forming an absorption line

profile of the considered species in a gaseous discharge

depending on their Doppler-shift. The LIF signal for Ti

ground state atoms mixed with the optical emission lines

measured as a function of the displacement Dklas of the laser

excitation wavelength klas from its resonant (central) value

FIG. 1. Typical current- and voltage-waveforms of the high-power magne-

tron plasma pulse used in this work. Pulse duration is 20 ls. The power (P)

waveform is given for the sake of illustration.

TABLE I. Summary of the experimental parameters used in the study.

Parameter Value

Magnetron sputtering system (HiPIMS)

Base pressure <10�6 Torr

Ar pressure used 5, 20 mTorr

HiPIMS pulse duration 20 ls

Magnetron target Ti, 10 cm diameter, 1 cm thick

Magnetron type balanced

Repetition rate 1 kHz

Laser diagnostics measurements

Pumping laser Spectra Physics INDI YAG (532 nm)

Dye laser Sirah Cobra-Stretch

Dye useda DCM

Laser linewidth � 0.8 pm (at 320 nm)b

Ti spectral transition (excitation)c 3F2 – 3D01 (klas¼ 320.585 nm)

Ti spectral transition (fluorescence) 3F2 – 3D01 (kfluor¼ 508.706 nm)

Tiþ spectral transition (excitation) 4F3/2 – 4G05/2 (klas¼ 338.376 nm)

Tiþ spectral transition (fluorescence) 2G7/2 – 4G05/2 (kfluor¼ 486.561 nm)

aThe additional information is available at www.exciton.com.bAccording to the Sirah Cobra-Stretch dye laser User Manual: http://

www.sirah.com/laser/pulsed-dye-lasers/cobra-stretch.cThe information on the spectral transitions is taken from the NIST Atomic

Spectra Database Lines Form: http://physics.nist.gov/PhysRefData/ASD/

lines_form.html.

FIG. 2. Schematics of the LIF spectroscopy measurements in the HiPIMS

discharge, as it was applied in this study.

113302-2 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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kres (Dklas¼ klas - kres) is shown in Fig. 3. The intensity of

the fluorescence signal recorded at kfluor in this way for a def-

inite set of Dklas forms a spectral profile of the plasma

absorption line, in this particular case corresponding to

atomic ground state Ti. After a deconvolution with the essen-

tial broadening mechanisms in plasma (as discussed below),

the resulting profile represents in general case a vdf of the

species under interest. The mentioned vdf corresponds, how-

ever, only to a certain velocity component of the discharge

species, defined by the laser beam direction. Due to the laser

beam geometry used in this work, the distribution of the ve-

locity component parallel to the target surface vk is consid-

ered (see Fig. 2).

The total signal coming to the detector at the fluores-

cence wavelength kfluor can be represented in the following

form:

Iðt;Dt; klasÞ ¼ kwðtÞðIemðDtÞ þ ILIFðDt; klasÞÞ; (1)

where I is the detected light intensity, t is the time (compara-

ble to the time of measurements), Dt is the time delay rela-

tively to the beginning of the plasma pulse (ls scale), klas is

the laser wavelength, kw is the eventual signal attenuation

induced by a gradual viewport contamination caused by dep-

osition, Iem is the spontaneous emission intensity, and ILIF is

the LIF signal intensity. Taking into account well-known

relations for Iem25,26 and ILIF,27 we obtain

Iðt;Dt; klasÞ

¼ kwðtÞ C1Aij�hxijniðDtÞ þ C2elasAijB0i

ðAi0 þ Aij þ QiÞn0ðDt; klasÞ

� �

¼ kwðtÞðC3niðDtÞ þ C4elasn0ðDt; klasÞÞ; (2)

where i, j, 0, stand for the exited (upper), intermediate, and

ground (lower) atomic states correspondingly; C1�4 are the

constants, Aij, Ai0 are the Einstein’s coefficients of spontane-

ous emission; �hxij is the energy difference between i and jenergy levels; niðDtÞ is the delay time-dependent density of

the excited atoms; elas is the laser energy per a single laser

pulse; B0i is the Einstein’s coefficient for absorption between

the levels 0 and i; Qi is the collisional quenching coefficient

for the level i; and n0ðDt; klasÞ is a part of the ground state

species density population corresponding to the resonant ex-

citation, i.e., satisfying the condition28

�hxi0 ¼ hc=klas: (3)

Equation (2) is given here expanded to illustrate the sig-

nal corrections, which are necessary to apply in order to

extract the measurable quantity properly (n0 in our case). In

this study, the optical signal corrections include: (i) viewport

contamination (kw) correction using the time-decay of a

strong emission line available in the spectra, and non-

overlapped with the fluorescence lines, (ii) the spontaneous

emission Iem from plasma subtraction afterwards, (iii) the

normalization of the LIF signal obtained at each klas to the

laser energy elas, monitored real-time.29 As a result of the

described corrections, the quantity n0ðDt; klasÞ proportional

to a corresponding part of the Ti velocity distribution is

obtained. After its collection at different klas, and deconvolu-

tion with the most essential line broadening mechanisms (see

the Sec. III A), the resulting vdfs of the sputtered atoms and

their broadenings can be finally obtained.

III. LIF RESULTS AND DISCUSSION

A. Line broadening mechanisms in a HiPIMS plasma

When dealing with the emission or absorption lines in

plasma, the question about the broadening mechanisms

which can possibly contribute to the total measured line

shape should be analysed. Based on the previously per-

formed estimations of the main broadening sources which

may contribute to the spectral line shapes, it was concluded

that mainly the Doppler and the instrumental broadening (as

a result of finite laser linewidth) contribute to a line shape in

the HiPIMS case. These contributions are still smaller than

the typical line profiles measured by LIF in HiPIMS. The

summary of the possible broadening contributions is given in

Table II. The analytical expressions for the line broadening

contribution Dki of the ith broadening mechanism in a

HiPIMS discharge are summarized elsewhere (see Ref. 23

and therein).

Working in the target vicinity, the role of Zeeman spec-

tral lines splitting DkZ should be estimated as well. To esti-

mate its contribution, the term structure for the used Ti/Tiþ

spectral transitions should be taken into account. According

FIG. 3. Emission spectra of the HiPIMS plasma acquired during the plasma

on-time at different Dklas (Dklas¼ klas - kres) showing the change of the fluo-

rescence emission peak (at 508.7 nm) depending on the laser excitation

wavelength. Time delay Dt¼ 12 ls and pgas¼ 20 mTorr.

113302-3 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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to the theory of Zeeman effect,30 the considered spectral

transitions (see Table I) have either nine (Ti) or twelve (Tiþ)

Zeeman components with a total splitting nearly equal to the

Larmor frequency XL (verified additionally). Thus, the

resulting Zeeman splitting can be upper-estimated as

DkZ � 2XL ¼eB

mec; (4)

where e, me are the electron charge and mass, respectively, Bis the strength of magnetic field in the studied volume, and cis the speed of light. The final estimation of Zeeman effect is

performed in this work based on the following suggestions:

(i) The distance to the target is z¼ 15 mm (closest used), and

(ii) The magnetic field strength is B� 350 Gauss (measured

at z¼ 15 mm by a Hall probe). The estimation for Ti gives

DkZ � 0.3 pm, the estimation for Tiþ leads to a comparable

value. The effective line broadening induced by such a split-

ting, however, might be even smaller taking into account the

intensity drop for the side Zeeman components, resulting in

Zeeman splitting being also negligible comparing to the

dominating broadening mechanisms found before.

It should be noted that the strength of the magnetic field

in a HiPIMS discharge may be additionally perturbed due to

the current-induced magnetic field changes, thus altering the

stationary magnetic field distribution during the plasma on-

time, as reported recently.31 This effect is also neglected

here first of all due to the moderate values of the discharge

current used in this study, as compared to the peak current

value (I�500 A) used in the Ref. 31.

B. vdf broadening results

As a result of the time-resolved LIF measurements, it

was found that the shape of the spectral lines (so the vdfs)

for Ti and Tiþ depends dramatically on the delay time Dt in

the HiPIMS discharge. At this point, the essential clarifica-

tion of the time-resolved line shape behaviour during the

HiPIMS on-time is obtained as compared to the previous

results23 where mainly the plasma off-time is considered.

The spectral line shapes measured at the different Dt are

presented in Fig. 4 for Ti and Tiþ. A definite increase in

FWHM for the obtained spectral profiles representing the

vdfs of the sputtered species (undeconvoluted) can be

observed at the end of the plasma on-time. At the same time,

right after the pulse, the FWHMs shrink due to the increased

collision rate and gas thermalization. Both Ti and Tiþ dem-

onstrate roughly similar time evolution in terms of their

FWHMs.

The obtained time-resolved results in terms of FWHM

are summarized in Fig. 5 where the deconvoluted FWHMs

of the measured vdf of Ti ground state atoms and ions are

presented for typical discharge conditions. Several conclu-

sions on the obtained vdf dynamics can be made from this

figure:

(i) The measured FWHMs increase abruptly during the

plasma pulse showing the saturation in less than about

10 ls after the pulse beginning. This effect is espe-

cially pronounced at low-pressure.

(ii) The saturation time is somewhat shorter at closer dis-

tance (15 mm), and at lower gas pressure.

(iii) After the saturation, the FWHM values remain nearly

constant until the end of the plasma pulse.

(iv) The saturated FWHM value clearly depends on the

gas pressure, as well as on the distance z from the

target.

(v) Right after the on-time, the FWHM starts to decay,

with the decay time much longer that its growth time.

(vi) The found FWHM values are in a good agreement

with the data measured during the HiPIMS off-time

previously (triangles in Fig. 5).

In spite of the fact that the Ti data are mainly represented

in Fig. 5, the dynamics for Tiþ ions is supposed to be similar,

taking into account the appearance of the Tiþ vdfs shown in

Fig. 4, as well as the similarities in Ti and Tiþ behaviour found

during the HiPIMS off-time previously.23 Based on these

observations, it is reasonable to suggest that there is no essen-

tial differences for Ti and Tiþ in a HiPIMS discharge in terms

of their kinetics (see Figs. 5(a) and 5(b), 5 mTorr case), i.e.,

the ions seem to keep the vdf dynamics of neutrals, rather than

being additionally accelerated in the discharge volume.

It should be noted additionally that the vdf of Ar neu-

trals measured at the end of the plasma on-time is

TABLE II. The estimated spectral line broadening mechanisms in a HiPIMS plasma, based on Ref. 23.

Broadening type Nature of the broadening effect Estimated value, (pm)

Doppler (thermal) Thermal motion of the gas particles. In this work this broadening type is included into the determined

spectral profile, since the thermal motion is considered as a part of the total motion of the gas particles.

�0.8

Instrumental A finite laser linewidth, which is equal to about 0.8 pm in our case. �0.8 a

Stark Interaction of the light emitters with the electrons in plasma. �0.1a

Stark shift Asymmetry of the Stark profile. �0.01a

Zeeman Splitting of a spectral peak in the magnetic field (Zeeman effect). �0.3a

Self-absorption Self absorption effect as a result of high density of plasma which is normally negligible

in low-pressure plasmas.

Negligible

Resonance Interactions between the atoms of the same kind when the upper level of the emitter is linked to the

ground state (GS) by a dipole transition.

Totally negligible

Van der Waals Interactions of the different kinds of the ground state atoms or the interaction between the

same atoms in which radiative levels are not linked to GS.

Totally negligible

aIndicates an upper estimation.

113302-4 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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FIG. 4. The evolution of the fluores-

cence emission intensity as a function

of Dklas measured at several time delays

Dt for Ti and Tiþ in HiPIMS.

Lorentzian fit is shown by the red

curves. The corresponding broadening

values (FWHM) are shown in the

upper-right corners. pgas¼ 5 mTorr and

z¼ 15 mm.

FIG. 5. The FWHMs of the measured Ti fluorescence line determined at several Dt and z at two pgas values after deconvolution with the laser profile. The

FWHM data obtained during the HiPIMS off-time are taken from Ref. 23 (triangles). The velocity component v|| parallel to the laser beam (right scale) corre-

sponding to 1/2 FWHM is given for comparison.

113302-5 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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represented at the same time by two characteristic parts,

namely by rather narrow thermalized and wide non-

thermalized ones, as demonstrated recently.24 The broaden-

ing of the vdf part corresponding to the thermalized Ar

roughly corresponds to the gas temperature measured at the

end of the HiPIMS plasma on-time in the same work,

whereas the broadening of the non-thermalized part is in a

good agreement with the typical velocity values found in the

present study for Ti and Tiþ.

C. Discussion of the obtained results

The observed dynamic behaviour of both Ti and Tiþ

vdfs and their FWHMs should be directly related to the proc-

esses of avalanche sputtering and gas rarefaction during the

HiPIMS pulse, as well as to the gas thermalization and refill-

ing processes coming afterwards. In order to analyse the

obtained results properly, however, the position of the

plasma volume excited by LIF along with the direction of

the laser beam should be taken into account.

The found abrupt increase in Ti FWHMs during the

plasma on-time is well consistent with the time-resolved op-

tical emission spectroscopy (OES) observations of the Ti

emission lines made in the HiPIMS discharge under the simi-

lar conditions (not shown), as well as in the HiPIMS dis-

charges elsewhere,32 showing a nearly exponential increase

of the excited states density of species sputtered during the

HiPIMS plasma pulse. These results are also supported by

the Ti/Tiþ ground state density data obtained previously by

optical absorption spectroscopy diagnostics.33 Mentioned

studies point out on the fact that a certain time interval nec-

essary for the FWHM to get saturated should exist due to the

finite speed of the sputtered species, as well as due to the fi-

nite time necessary for rarefaction in the discharge to take

place. These arguments are also supported by our recent

LIF-imaging results obtained in HiPIMS discharge for Ti,

Tiþ, and Ar metastables (ArM) at the same conditions.34

The fact that the velocity component vk stops to grow at

a certain moment of time, as found in this work, clearly indi-

cates the formation of a “rarefaction window,” which persists

until the end of the plasma pulse and during which the mo-

mentum exchange between the sputtered atoms and the back-

ground gas atoms is minimal. As a consequence, the velocity

of the sputtered species remains rather high. The time-

duration as well as the spatial extension of this rarefaction

zone above the magnetron cathode should obviously depend

on the gas pressure and the discharge current, as well as on

the other discharge parameters, such as the size of the cath-

ode, etc.

As we can see from Figs. 5(a) and 5(c), the described

saturation effect for Ti happens somewhat earlier at lower Ar

pressure, which can be explained by faster arrival of the

energetic sputtered species to the probed plasma volume in

this case. Having the space separation between the laser-

excited volumes equal to about 30 mm in this work and the

arrival time difference of about 3 ls (estimated from Fig. 5),

we can deduce that the particle velocity in the direction per-

pendicular to the target is v?� 10 km/s. This value correlates

well with the recent results obtained by time-resolved mass

spectrometry in HiPIMS under the close conditions.35,36 At

the same time, it is about twice as large as the maximum

value of the velocity vk determined in this work (see Fig. 6).

This ratio, however, agrees well with the angular velocity

distribution of sputtered species measured in DCMS plasmas

earlier.21 It should be pointed out additionally that, in the

case of the mass spectrometry measurements, the v? velocity

component was still detectable at about 80 mm away from

the target surface,35 whereas there is no essential rarefaction

effect at already 75 mm away from the target surface based

on the findings of this work, as can be seen in Figs. 5(b) and

5(d). The last observation is supported by the vertical direc-

tivity of the energetic particles in HiPIMS, as suggested by

Anders et al.37 The abovementioned results on the particles

directivity are, at the same time, in a partial contradiction

with the hypothesis of the azimuthal acceleration of ions in

HiPIMS discharges, which should favour the mass transport

in the direction parallel to the target surface, as suggested by

Lundin et al.38

Finally, the presence of the high-energy peak in the

energy spectra of the sputtered species which has been

detected both in perpendicular35,39 and parallel38 directions

to the target surface is generally consistent with our findings.

The additional vdf measurements of the vertical velocitycomponent which were not possible to realize in the current

reactor geometry and which are scheduled for the future may

essentially clarify these contradictions.

As mentioned above, the rarefaction windows observed

at the end of the HiPIMS plasma pulse allow the initial ve-

locity of the sputtered species to be measured. To measure

this value, however, a very close target proximity or/and low

pressure is required. Indeed, even at pgas¼ 5 mTorr, the

essential dissipation of the gas velocity is already visible in

the space region z¼ 15–45 mm (see Figs. 5(a) and 5(b)). If

at z¼ 15 mm, vk� 4.8 km/s during the rarefaction, at

z¼ 45 mm, this value is reduced to about 3 km/s (right scale

in Fig. 5). This implies definite collisional energy dissipation

FIG. 6. Velocity components v|| determined based on the Figure 5 at the end

of the plasma pulse (Dt¼ 20 ls) as a function of the distance z from the tar-

get surface. pgas¼ 5 and 20 mTorr. Possible ways for the velocity (energy)

dissipation are sketched by the dashed lines. The thermalization lengths

Ltherm corresponding to e-times velocity drop are shown for illustration.

113302-6 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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during the flight of the sputtered particles in the considered

space interval. At the same time, in the volume located at

z¼ 15 mm, the species should undergo only one collision or

so. The determination of the “real” velocity of species right

after being ejected from the target surface thus might be

accomplished by fitting the obtained velocity (energy) dissi-

pation data, as sketched in Fig. 6. In particular, the corre-

sponding thermalization lengths (Ltherm) can be deduced

when the velocity of particles (or the corresponding FWHM)

drops e times from its maximum value. It is worth to note

that the obtained Ltherm values correspond to �5(7) collisions

necessary for thermalization to take place in the 5(20) mTorr

case, which is well consistent with the previous works.40–42

These numbers of collisions is obtained, however, based on

the classical definition of the mean free path in a gas (see,

e.g., Ref. 43) and may need to be corrected for the HiPIMS

case depending on the considered moment of time.

Another point to stress is the absolute value of the

observed rarefaction, i.e., a drop of Ar density during the rar-

efaction window in the discharge. Since the rarefaction phe-

nomenon is usually applied to the bulk gas atoms in their

ground state, this question is rather complicated. This is first

of all related to the spectroscopic limitations for detection of

Ar ground state atoms by LIF for which the utilisation of

deep UV spectral transitions is necessary. The question

might be partially resolved considering the Ar metastable

atoms (in the 4s state).44 As follows from our recent study on

ArM density mapping,34 an ArM density drop roughly up to 1

order of magnitude is observable under certain conditions af-

ter the plasma pulse relatively to the “background” density

measured in the off-time. This result agrees with the estima-

tions of the rarefaction in HiPIMS discharge based on the

DCMS data.45 Somewhat smaller, but still comparable val-

ues of the bulk gas rarefaction are also obtained in the other

studies.44,46 The observable discrepancies with the present

work arise due likely to much longer plasma pulses (200 ls)

considered in the last two cases, when the rarefaction is

observed yet during the on-time. The time position of the rar-

efaction window may change essentially for different pulse

duration, which has to be studied additionally. All in all, a

fair comparison between the ground state and metastables Ar

is barely possible due to essential excitation of ArM caused

by electrons near the target during the HiPIMS pulse.

At last, the comparability of the obtained vdf results

with the well-know Sigmund-Thompson (ST) distribution

for sputtered species47,48 should be mentioned. The data on

the velocities for different species sputtered in the non-

HiPIMS discharges collected in the literature so far generally

agree well with ST distribution (see Fig. 7, and Table III)

where the value was found to be equal to 1–3 km/s.

However, this is not the case for the values of velocity meas-

ured in the present study. There are two main reasons why

the velocity values obtained during HiPIMS discharges are

essentially higher. First, it might be due to the static nature

of the rarefaction region and faster thermalization outside of

this region in the non-HiPIMS discharges, as mentioned

above. Second, the deviation from the ST distribution is gen-

erally one of the main features of HiPIMS plasmas, which

happens due to the presence of the high-energy peaks in the

energy spectrum of the sputtered species. The presence of

this peak is verified in the numerous studies,4,35,39,49 where it

is explained by several factors, including the abnormal trans-

port in HiPIMS,38,39,49 the contribution of the back-reflected

energetic ions,35 etc. In general, the mentioned works point

out on a coexistence of the high-energy particles (with the

energies of �10–30 eV) and the low-energy particles

(�1.5–2 eV) in HiPIMS plasma. Since the low-energy group

of particles has the typical velocity which correlates well

with the values widely measured in the literature (1.5–3 km/s

for Ti), the velocity value determined in this work should

probably represent the high-energy group of species sput-

tered in HiPIMS. These considerations are particularly sup-

ported by the known angular distribution for the sputtered

species. According to these measurements,21 the v? velocity

component might be 2–3 times larger, resulting in the energy

of the sputtered species to be equal or above 20 eV, which

corresponds well to the high energy particles detected in

FIG. 7. The sputtering velocities (v?) of Ti and Cu averaged based on the

values available in the literature (see Table III), comparing to the value

measured in this work (vk) in the HiPIMS discharge (target vicinity,

z¼ 15 mm and pgas¼ 5 mTorr). The normalized Sigmund-Thompson distri-

butions calculated for Ti and Cu (Eb(Ti)¼ 4.9 eV, Eb(Cu)¼ 3.6 eV)47 are

given for comparison.

TABLE III. The velocities after sputtering for Ti and Cu (given for compari-

son) measured in the literature by different methods representing the non-

HiPIMS sputtering discharges.

Sputtered Ti Sputtered Cu

Found

velocity (km/s)

Method

used Reference

Found

velocity (km/s)

Method

used Reference

3 LIF 50 2,5a TOF, FPIb 52

3 LIF 51 2,3a Simulation 55

4a TOF, FPI 52 3,5 TOF 56

2.5 FPI 21 2 TOF 57

2.5 FPI 22

3.5a LIF 53

3.5a TOF, LIF 54

�2 LIF 18

Average¼ 3.0 6 0.6 km/s Average¼ 2.6 6 0.6 km/s

aCalculated based on the measured energy value.bTOF stands for Time-of-Flight, and FPI for Fabry-Perot Interferometry

measurements correspondingly.

113302-7 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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HiPIMS. As mentioned above, the direct v? velocity studies

based on LIF are scheduled for the future work.

IV. SUMMARIZING REMARKS

The time-resolved measurements of the sputtered par-

ticles velocity distribution function in the direction parallel

to the magnetron cathode surface is performed by laser-

induced fluorescence spectroscopy in a HiPIMS discharge.

The main processes happening during the HiPIMS on- and

off-time in terms of the sputtered particles velocity distribu-

tion determined experimentally in this work are summarized

schematically in Fig. 8. Based on this representation, the

time-evolution of the velocity distribution in HiPIMS can be

described as follow: at the beginning of the plasma pulse,

where the gas rarefaction is not yet strong, the broadening

(FWHM) of the measured vdfs remains rather narrow. As

the discharge current and the amount of the sputtered species

grow-up, the FWHM increases due to increasing of the gas

rarefaction and decreasing the number of collisions in this

region. Finally, when the rarefaction surpasses the threshold

when the mean free path for sputtered species is comparable

or greater than the considered plasma volume, the FWHM of

the vdf saturates showing the initial velocity (or its certain

component) of the species after their ejection from the

cathode surface. Such time interval is referred in this work to

as “rarefaction window.” The duration of this interval as

well as the particle velocity (energy) during this time should

strongly depend on the strength of rarefaction, i.e., on such

discharge parameters as the gas pressure, distance from the

magnetron target, applied power, target size, atomic mass of

the bulk gas particles, etc.

The further relaxation of the velocity (energy) distribu-

tion after the plasma pulse, till the end of the off-time, is

governed by the collisions with the background gas as it

cools down and refills the depleted volume in front of the

sputtering target. The rate of decrease of a given velocity

component is roughly constant till the end of the plasma off-

time.

ACKNOWLEDGMENTS

This work is supported by Belgian Government through

the «Pole d’Attraction Interuniversitaire» (PAI, P7/34,

“Plasma-Surface Interaction”, W). The technical contribution

of M. Michiels and D. Walrave (Materia Nova) is strongly

appreciated. N. Britun is a postdoc researcher, and S.

Konstantinidis is a research associate of the FNRS (Fonds

National de la Recherche Scientifique), Belgium.

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comparison.

113302-8 Palmucci et al. J. Appl. Phys. 114, 113302 (2013)

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