ON HIGH POWER IMPULSE MAGNETRON SPUTTERING

University of South Bohemia
Faculty of Science
Institute of Physics and Biophysics
ON HIGH POWER IMPULSE
MAGNETRON SPUTTERING
Habilitation Thesis
by
Vítězslav Straňák
December 2011, Greifswald

The work described in this thesis has been performed at:
University of South Bohemia
Faculty of Science
Institute of Physics and Biophysics
Branišovská 31
370 05 České Budéjovice
Czech Republic
E.M.A. University of Greifswald
Faculty of Mathematics and Natural Sciences
Institute of Physics
Felix-Huasdorff Strasse 6
174 89 Greifswald
Germany
Academy of Science of the Czech Republic
Division of optics
Institute of Physics, v.v.i.
Na Slovance 2
180 00 Praha 8
Czech Republic

Acknowledgements
I am happy that I can do what I really like in my professional carrier: to discover new
phenomena of plasma physics. It would have not been possible without help, support and
company of a great number of people. I would like to thank all of them at this point.
In particular I have to express my gratitude to closest collaborators Prof. Rainer Hippler
(University of Greifswald), Prof. Milan Tichý (Charles University in Prague), Dr. Zdeněk
Hubička and Dr. Martin Čada (both Academy of Sciences of the Czech Republic). I am
much obliged to them, not only for their professional help but also for personal support
which turns our relations into friendship.
Last but not least, I am deeply grateful to my wife Šárka for her personal support and
encouragement. I can not forget to acknowledge unflagging optimism coming from our
little daughters Viktorie and Terezie.
The work has been financially supported through grants and projects of German
(SFB, BMBF, DFG) and the Czech Republic (MSMT, GACR, TACR, GAAV) governments.

Contents
Preface 2
1 Introduction to HiPIMS discharges phenomena 5
1.1 Definition of HiPIMS discharges . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Plasma target interaction - sputtering . . . . . . . . . . . . . . . . . . . . . 6
1.3 Transportation processes in HiPIMS - deposition . . . . . . . . . . . . . . 7
2 HiPIMS power supplies and discharge generation 10
2.1 HiPIMS in reactive atmosphere . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Advanced HiPIMS systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Diagnostics of HiPIMS discharges 17
3.1 Methods of plasma diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 Properties of non-reactive HiPIMS discharges . . . . . . . . . . . . . . . . 19
3.2.1 Time-resolved Langmuir probe measurements . . . . . . . . . . . . 19
3.2.2 Time-resolved optical emission spectroscopy . . . . . . . . . . . . . 22
3.2.3 Ion and power fluxes towards substrate . . . . . . . . . . . . . . . . 26
3.2.4 Ion energy in HiPIMS discharges . . . . . . . . . . . . . . . . . . . 28
3.3 Properties of reactive HiPIMS discharges . . . . . . . . . . . . . . . . . . . 30
3.3.1 Estimation of negative (oxygen) ion density . . . . . . . . . . . . . 32
4 HiPIMS for thin films deposition 37
4.1 Deposition of photocatalytic TiO x films . . . . . . . . . . . . . . . . . . . . 37
4.2 Effect of nitrogen doping on formation of TiO x N y . . . . . . . . . . . . . . 41
4.3 Deposition of functional metallic Ti-Cu films . . . . . . . . . . . . . . . . . 44
4.3.1 Formation of Ti-Cu films . . . . . . . . . . . . . . . . . . . . . . . 45
4.3.2 Copper release from Ti-Cu films . . . . . . . . . . . . . . . . . . . . 47
4.3.3 Effect of copper on cell and bacteria growth . . . . . . . . . . . . . 48
4.4 Size-controlled formation of Cu nanoclusters . . . . . . . . . . . . . . . . . 50
4.4.1 Effect of pulses on cluster formation . . . . . . . . . . . . . . . . . . 51
4.4.2 Cluster mass and cluster particle flux . . . . . . . . . . . . . . . . . 53
5 Conclusion 55
References 56
List of own publications related to the topic 61
1

Preface
Plasma physics and plasma based technology applications have made large progress in recent
years [1,2]. Energetic particles, which occur in the plasma 1 volume, initiate unique
physical and/or chemical reactions that can be realized only hardly at ordinary conditions.
Among others applications plasma discharges have become an effective tool for plasmaassisted
deposition of thin films with typical range of thickness from nanometers to units
of micrometers (∼10 nm-10µm) [2]. These thin films are highly required in different
areas of micro- and nanotechnology and can be utilized in micro-electronic, bio-medicine
and optical coating industry. Further, the thin films can serve as protective layers (with
mechanical or electrical features) as films with unique properties (ferrolectrics, ferromagnetics)
or decorative coating. Because of various possible applications a wide variety of
plasma-based methods for elemental and/or compound thin films have been invented [5].
electron trap region
B
E
E x B drift path
target
erosion rill
S
N
S
N
S
N
photo of Cu discharge (diameter 50 mm)
outer
magnet
water
cooling
inner
magnet
-V C
cathode
insulator
sputtered Cu target (diameter 50 mm)
Fig. 1: Planar magnetron sputtering. Planar circular magnetron sputtering is based on crossed magnetic
and electric fields E×B. Buffer gas ions, produced in the "electron trap region", cause sputtering of
target made from the material to be deposited.
Physical vapor deposition (PVD), using the ion sputtering effect, is one of the most
often used plasma assisted deposition methods [6]. The principle of ion sputtering is
based on positive ions bombardment of a negatively biased target (cathode) made from
the material to be deposited. Impinging of energetic ions results in ejection of target atoms
which condense on a substrate forming a thin film [7]. This most simple process is called
1 The term plasma was first introduced by Tonks and Langmuir in 1929 [3]. Chen defined plasma
as electrically quasi-neutral gas of charged and neutral particles which exhibits collective behaviour [4].
Plasma quasi-neutrality is meant in sense that one can assume for single charged particles n e ≈n i , where
n e andn i denote electron and positive ions density, respectively. The particles can be in different quantum
states - can be excited or de-excited to and from various energy levels. During these processes photons
and subsequently light are produced. Collective behaviour means that not only local properties but also
plasma conditions in remote regions influence plasma particles.
2

diode sputtering. To increase the production of ions in plasma volume, i.e.to enhance the
number of ions impinging on the target, the diode sputtering was modified by additional
external magnets placed behind the cathode/target. The external magnetic field B is
parallel (at least locally) to the cathode (i.e. perpendicular to electric field E) and forms
an electron trap (E×B field drift), see Fig.1. This improved version, so-called magnetron
sputtering 2 , was introduced first by Waits and Thornton in 1978 [8–11]. Despite theE×B
electron confinement, the conventional dc-magnetron discharge is sustained mainly close
the target and the ionization degree and ion flux rapidly decreases outwards from the
cathode/target [12].
The flux of ions and energetic particles towards the substrate plays an important role
during deposition of thin films [13,14]. The microstructure and morphology of films is
significantly influenced by energetic conditions on the top of layer surface. Since the
ionization of sputtered metal atoms in dc-magnetron discharges is low (typically < 1%)
the majority of ions hitting the substrate is represented by buffer gas 3 ions Ar + [15,16].
This fact may cause subplantation of Ar atoms into the film [17] which deteriorates its
quality (lattice defects [18], residual stresses [19], low adhesion [20]). Hence, it is generally
desired to increase the ionization level of sputtered particles in the discharge volume. In
case that ionization of metal atoms is > 50% we usually talk about ionized physical vapor
deposition (IPVD) [21]. For that reason conventional magnetron sputtering systems have
been modified by addition of the RF electric field component [22], ECRionization [23],
plasma confinement [24] or they were operated in pulse mode [25, 26] to increase the
fraction of metal ions.
The operation of magnetron discharges in pulsed regime with low repetition frequency
and short pulse width leads to significant increase of ionization [25,26]. In the pioneering
work by Kouznetsov et al. in 1999 [27], high power pulse magnetron sputtering was used
for the first time for IPVD of Cu film. In this early work, 50µs pulses of rather high
power (nominal peak power density p p >3kW/cm 2 ) provided ion flux higher by about
two orders of magnitude (compared with dc magnetron) and ionization of metal species
about 70%. Such type of pulsed discharges is called High Power Impulse Magnetron
Sputtering (HiPIMS) 4 and it represents the last trend in magnetron deposition.
It is shown in the paper by Sarakinos et al. [17] that roughly 3-4 publications per year
related to HiPIMS were published in the period 1999-2004. After that time the number of
publications rapidly increases (up to now it is expected that there are about 200 original
papers). In 2010, the first annual International Conference on Fundamentals and Applications
of HiPIMS was organized in Sheffield (UK). EU COST Action MP0804 [28], focused
on highly ionized pulse plasma processes where the HiPIMS plays a key role, was estab-
2 Magnetron sputtering is almost exclusively used for practical applications. The ion bombardment is
insufficient (usually because of low plasma density) during the diode sputtering, which results in significant
reduction of ejected atoms, i.e. lower deposition rate.
3 Magnetron discharges are typically operated with Ar buffer gas. Argon is relatively cheap and
available inert gas with higher mass which is important for sufficient sputtering.
4 The original name was High Power Pulsed Magnetron Sputtering (HPPMS) which is still used in
English speaking countries.
3

lished in the same year. Such expansion of activities in HiPIMS area well demonstrates
high interest of academia as well as industrial communities.
This habilitation thesis is focused on fundamental aspects of HiPIMS discharges and
their application for deposition of different thin films. The author of the thesis works in
the field of HiPIMS from the very beginning (his first related work was published in 2006)
and his home laboratory is well established in magnetron sputtering deposition processes.
The basic aspects of HiPIMS and HiPIMS deposition processes are presented in the thesis
in form of annotated summary of essential results obtained by the author of the thesis
and his coworkers. These results have been already published in international journals
after peer review and the offprints are attached to the thesis as its integral part; in the
text they are cited using Roman numerical system, e.g. [IV]. However, these author’s
outcomes are not presented separately but they are embedded into a deeper context of
already published works (cited using Arabic numbers, e.g. [4]).
The thesis is practically divided in four main chapters. The first one brings an introduction
to the phenomena of HiPIMS discharges. The second chapter deals with facilities
and systems used for generation of HiPIMS discharges. The third chapter is focused on
time resolved diagnostic and the attention of the fourth chapter is paid to deposition of
various thin films and their properties.
4

1. Introduction to HiPIMS discharges
phenomena
The first conventional dc-magnetron sputtering deposition system, in a correct technical
configuration, was presented in the seventies of the last century. Since then, numerous
review papers, e.g. [29], and books, e.g. [10,11,30], have been published, where details
can be found. However, here we point at least to the typical and most important features
of dc-magnetron sputtering (dc-MS). The dc-MS discharges are usually operated at a
nominal discharge current density of the order of i dc ∼10-100mA/cm 2 , which somewhat
corresponds with power densities p dc ≤15W/cm 2 . At such conditions the ionization level
is low with typical electron density level n e ∼10 14 - 10 15 m −3 at pressures about 1Pa [30].
In plasma discharge volume, mostly atoms of buffer gas (Ar) are ionized, resulting in ion
flux of about ζ∼0.1-1.0mA/cm 2 towards the substrate. Densities of above mentioned
parameters represents (technical) limits which are hard to exceed in dc-MS. Further increase
of discharge current leads to overheating of the magnets, melting of the target or
to occurrence of arc especially during reactive sputtering when the target is covered by
electrically insulating layer [31].
1.1 Definition of HiPIMS discharges
The HiPIMS discharges are operated in pulse regime with low repetition frequency
(f ∼100Hz) and short pulse width (duty cycle is usually ≤1%) [16, 27,32–37]. Experimentally
HiPIMS discharges are operated with the same facilities as dc-MS just only
require pulse power supplies. Pulse operation with small duty cycles allows keeping mean
discharge current I m and mean discharge power P m (i.e. the current and power averaged
over the pulse period T) at similar or even lower values as in dc-MS to prevent target
overheating. However, values obtained in the pulse are much higher according to the
formula
∫ T
I m = 1 I d (t)dt, (1.1)
T 0
where I m is the mean discharge current and I d is the discharge current varying in time. In
this way the pulse current density i p can reach several A/cm 2 and the target is loaded with
pulse power density p p in the range of a few kW/cm 2 . The electron density measured in
HiPIMS systems usually exceeds n e ∼10 18 m −3 [33,37], which is by about two-three orders
of magnitude higher than in conventional dc-MS [30].
5

So far, no official definition of HiPIMS has been given. In general, pulse magnetron discharges
which exceed pulse power density p p ≥1.0kW/cm 2 are considered as HiPIMS 1 [17].
Further, the HiPIMS can be differentiated according to the pulse duration to groups with
regular (50-200µs), short (1-20µs), large (200-400µs) and X-large (400-4000µs) pulse
widths. The discharge breakdown through the ionizing collision cascade needs some time
to propagate, which causes delay of discharge current after the cathode voltage onset typically
observed in discharges with regular pulse width [27,38]. The delay between voltage
and current is influenced by pressure, frequency, pulse current, etc [38].
Short HiPIMS pulses used in combination with secondary plasma source decrease the
voltage-current delay [39–41]. The secondary discharge provides pre-ionization of electrons
that respond to fast increase of cathode voltage. Low dc-voltage or low power
RF discharges used to be employed to achieve pre-ionization effect. Low power "background"
discharges also sustain certain level of ionization which allows steady operation
without arc [42,43]. Large pulse-width discharges, operated at very low frequencies, attain
high pulse power densities; p p =10kW/cm 2 is reported in [44]. Long pulse widths
allow the discharge to evolve into the metal discharge phase. In this phase the discharge is
sustained by self-sputtering [44], i.e. the sputtering is mostly provided by metal ions and
the discharge can sustain, in principle, even without Ar + sputtering (see next section).
1.2 Plasma target interaction - sputtering
Plasma target interaction and particle transportation are the key parameters during deposition
process. Hence, this area has considerable importance and has been studied in
several papers [17,44,45]. In next paragraphs only a brief summary of sputtering and particle
transportation in non-reactive discharges is presented. The steady state and regular
magnetron operation producing plasma is assumed at the beginning. The positive ions
presented in plasma volume are accelerated towards negatively biased cathode/target.
The ions hitting the target surface cause two processes: (i) production of secondary electrons
and (ii) sputtering of target atoms. The secondary electrons are repelled from the
cathode, gain energy from the cathode dark space, and enter the plasma, where they
ionize the buffer gas to produce ion-electron pairs. The most intensive production of
ions occurs in the region of "electron trap" where E and B are perpendicular, see Fig.1.
Closed E×B drift path leads to Hall current which may be even higher than the discharge
current itself [8,10]. Hence, secondary electrons are crucially important since they
initiate the buffer gas ionization.
Ion-induced potential electron emission (PEE) yield from the target surface can be
expressed by semi-empirical equation [44–46]
γ SE = 0.032(0.78E i −2φ), (1.2)
1 Other HiPIMS definition says: As HiPIMS are considered discharges, the pulse power density of
which is increased about two orders of magnitude due to operation in pulse mode if the averaged values
are the same like in dc-MS.
6

where E i is the ionization potential of arriving ions and φ denotes the work function of
sputtered material; in Eq.(1.2) the kinetic emission is neglected [45] since the ion kinetic
energy (even at HiPIMS conditions) is relatively low ≪ 300eV. Looking closer to the
first ionization potentials and work functions (summarized in [44] for HiPIMS related
processes and generally listed in [47]), it is obvious that secondary electron emission can
not be caused by any singly charged metal ions. In other words, the secondary electrons,
necessary for discharge operation, are provided only by Ar + ions or by multiply charged
metal ions [45]. Only those ions have ionization energy high enough to fulfill the criterion
resulting from Eq.(1.2) 0.78E i >2φ, providing potential emission of secondary electrons
from the target. Hence, the discharge is sustained mainly due to bombardment of buffer
gas ions Ar + at early stage of the pulse. Self-sustained self-sputtering in HiPIMS, i.e.
operation of discharge without necessary support of gas ions, may occur later when doubly,
and perhaps even higher, charged metal ions are present [44].
The condition for self-sustained self-sputtering can be expressed by equation [45]
α·β ·γ SS ≥ 1, (1.3)
where α is the probability for the sputtered metal atom to be ionized, β is the probability
for the metal ion to return to the target and γ SS denotes the yield of self-sputtering 2 .
Usually the self-sputtering of the metals is about γ SS ≈1 for HiPIMS voltages about
−1000 V [44, 47–49]. For this reason the probability coefficients α and β have to be
high. Therefore, it can be assumed that in HiPIMS discharges there is larger fraction of
multiply charged metal atoms which was also presented in several works [33,50,51].
1.3 Transportation processes in HiPIMS - deposition
Presence of charged metal particles in HiPIMS discharge affects the transportation processes
and subsequently the deposition rate, too. These phenomena are studied in [52]
and only main outcomes are summarized here. In the ideal case the deposition rate a d
should be equal to target erosion rate Φ. However, due to losses in the discharge volume
(diffusion towards the wall, ionization etc.) the deposition rate is usually lower: a d ≪Φ.
The target erosion rate is proportional to discharge current j i , i.e. the number of ions
impinging the target surface, and regarding the sputtering yield γ can be expressed by
the equation
Φ = j i ·γ(E). (1.4)
As expressed by Eq. (1.4), the sputtering yield is a function of ion bombarding energy
E =q·e·V C where q is the charge state of ion, e is the elementary charge and V C denotes
2 The sputtering yield represents the number of target atoms sputtered (ejected from the target) per
energetic particle striking the target with a kinetic energy. The sputtering yield is a function of ion kinetic
energy and mass, incident angle and the material to be sputtered. Self-sputtering denotes the process at
which target bombarding ion is formed from already sputtered (target) atom.
7

the cathode voltage [48, 49]. The sputtering yield dependence on the projectile (ion)
energy is described by [53]
γ ∝ E m with m < 1. (1.5)
Taking the processes in HiPIMS into account, the erosion rate is equal to [52]
Φ = j Ar+ ·γ Ar+ (E)+j M+ ·γ M+ (E)−j M+ . (1.6)
The first term, j Ar+ γ Ar+ (E), of Eq. (1.6) represents the target erosion caused by gas ion
Ar + (typical and practically the only one sputtering initiator in dc-MS). The second part,
j M+ γ M+ (E), denotes the self-sputtering provided by metal ion and the current −j M+
represents re-directed metal ions, due to negative cathode potential, which are repeatedly
"deposited" on the target (and cause the self-sputtering).
It is a well known fact that deposition rate is lower in HiPIMS than in dc-MS [33,54].
The first reason is that HiPIMS operate with higher voltages V C to get the same mean
discharge power than dc-MS discharges [55]. SinceP m =I m·V C , the mean HiPIMS current
has to be lower to get the same mean power. Because the discharge current is proportional
to erosion rate, Eq.(1.6), the deposition rate decreases. The second reason of lower
deposition rate is the fact that sputtering yield invoked by gas ion γ Ar+ is for most
metals higher than the self-sputtering yield γ M+ [48,49]. Low self-sputtering yield plays
important role because the ionization degree of metal particles in HiPIMS discharges is
high > 90% [56,57]. Sarakinosetal. [52] expressed the HiPIMS-to-dc-MS ratio as:
a H
a ∝ (E) (E)−1
· γAr+ · γM+ dc fAr+
γ Ar+ (E 0 ) +fM+ γ Ar+ (E 0 ) , (1.7)
wheref Ar+ andf M+ are relative fractions of Ar + and target M + ions, γ Ar+ (E) andγ M+ (E)
are the sputtering and self-sputtering yields as a function of impinging ion energy, while
γ Ar+ (E 0 ) is the sputtering yield at the reference dc-MS conditions, respectively.
Transport of ionized metal particles in HiPIMS discharges strongly affects the deposition
rate. It was observed experimentally that the ionization level and decrease of
deposition rate are observed simultaneously [54]. In principle, the transport of (neutral)
sputtered particles is determined by the velocity distribution and collisions with buffer
gas atoms [10]. However, charged particles, it means also metal ions, are influenced by
electric and magnetic field. It is known that the magnetic field is changed during the
HiPIMS pulses [17, 58]. This phenomenon has not been fully understood yet and it is
proposed that magnetic field is affected by (i) electron drift caused by applied magnetic
field [58] and (ii) by electron drift caused by plasma pressure gradient and the gradient of
magnetic field [59,60]. An important consequence is that the electron and ion transport
affects each other mutually. As a result, a significant fraction of ions is transported radially
away from the cathode. Hence, the ions are lost on the plasma chamber walls and
the deposition rate decreases [59,61]. The side-wall loss of metal ions represents another
important factor decreasing the deposition rate.
8

Ratio of HiPIMS to dc-MS deposition rate as a function of metal atom ionization
degree was derived first by Christie [62] and later extended by Vlcek et al. [57] to the
form
Π = (1−β)(1−χ+χξ i/ξ n )+β(1−σ)ξ i /ξ n
, (1.8)
1+βσ(γ Ar+ −γ M+ )
where β is the ionization degree of sputtered atoms and σ is the fraction of ionized
sputtered atoms re-directed back to the target. The main differences from the Eq. (1.7)
are new parameters which represent the enhanced ionization in the discharge volume due
to magnetic confinement of electrons χ and to transport parameters for the metal ions ξ i
and neutrals ξ n employed as a relative ion-to-atom transport factor ξ i /ξ n [57].
9

2. HiPIMS power supplies and
discharge generation
As already mentioned, HiPIMS systems differ from conventional dc-MS only by pulse
controlled power supply equipment. A sputtering source itself is usually the same as
in dc-MS processes. Typical planar magnetron sputtering source consists of a planar
cathode usually parallel to anode 1 surface, see Fig.1. The cathode assembly consists of
the source material, so-called target (made from the material to be deposited), directly
connected with the backing power electrode. Magnets are placed below the backing
electrode. Typical size of the cathode/target operated at laboratory condition 2 is usually
between 25-150mm. The sputtering sources are mounted into the vacuum chamber, see
Fig.2.1. The vacuum chamber can be equipped with several sputtering sources, e.g. for
deposition of multi-structural film, composites, alloys etc.
current density [A/cm 2 ]
voltage [V]
1.2
0.8
0.4
0.0
0
-200
-400
-600
ON
OFF
O 2
/Ar = 0.015
O 2
/Ar = 0.1
O 2
/Ar = 10
p = 10 Pa
f = 250 Hz
V cathode
V cathode+resistor
p = 10 Pa
f = 250 Hz
O 2
/Ar = 10
0 50 100 150 200 250 300 350
time [ s]
Fig. 2.1: Schema of sputtering sources (red and
green colored parts) mounted into the vacuum
chamber.
Fig. 2.2: Time evolution of peak discharge currents
(upper panel) and voltages V C+R and V c .
For our experiments we used to employ typical arrangement which can be met in
many other laboratories. In our laboratory, the experiments have been carried out using
commercial planar magnetron(s) situated in a stainless-steel vacuum chamber. The magnetron(s)
is(are) equipped by the target(s) of 50mm or 75mm in diameter. The vacuum
1 The anode is not strictly defined in MS discharges. It is usually the wall of the vacuum chamber
or substrate holder which serves as an anode. In most cases the vacuum chamber is grounded while the
substrate holder can be biased or can be floated.
2 Rectangular or rotatable planar magnetron sputtering sources with larger sputtering area are used
for industrial applications. See the information of producers, e.g. Gencoa Ltd.
10

chamber is pumped out by a turbomolecular pump down to ultimate pressure of 10 −6 Pa.
The working pressure in the chamber is adjusted by a throttle valve installed between the
chamber and the turbomolecular pump. The flow rates of working gases employed during
the process are controlled by mass flow controllers. Most of the experiments were done
with typical flow rate of argon (20sccm) in a pressure range of 0.3-20Pa.
A number of research groups have developed high-power pulsed supplies for generation
of HiPIMS. These systems are more or less similar and they are based on a combination
of dc-power supply loading a large capacitor bank of a pulsing unit connected to the
magnetron cathode 3 . In our case, e.g. works [I-III], the capacitors are charged from dcsource
AE Pinnacle 3000 during the idle part of the pulses. Stored energy is released
in pulses activated by hand-made power switch based on IGBT transistors. The high
power switch transistors are controlled by an external multi-channel pulse delay generator,
Quantum 9518. The pulse delay generator serves also as trigger source for diagnostics
tools. A ballast resistor serving as dummy load without magnetron discharge is inserted
in series with the magnetron cathode path. The voltage waveforms measured over the
ballast resistor by the oscilloscope can be used for calculation of discharge current using
Ohm’s law, see Fig.2.2 lower panel. The other option is to use the current oscilloscopic
probe and to measure the discharge current directly.
current density [mA/cm 2 ]
1000
100
10
I V 7 dc-MS
I V 6
I V 2
HiPIMS
dc-MS
HiPIMS
mean discharge current [mA]
750
700
650
600
550
500
450
mean discharge current [mA]
deposition rate [nm/min]
p = 10 Pa, U = -580 V
poissoned target
1.4
1.2
1.0
0.8
0.6
0.4
0.2
deposition rate [nm/min]
400 600 800 1000
cathode voltage [V]
0.1 1 10
ratio O 2
/Ar
Fig. 2.3: Comparison of current - voltage curves
for dc-MS and HiPIMS discharges driven at similar
conditions.
Fig. 2.4: Dependence of deposition rate (right
scale) and mean discharge current (left scale) on
O 2 /Ar ratio.
The voltage waveforms are practically rectangular and drop to zero when the active
part of the modulation cycle finished, Fig.2.2. Slight delay of discharge current after
the cathode voltage onset represents the time needed for propagation of ionizing collision
cascade. In non-reactive mode, when the gas mixture ratio is very small, target voltage
and the discharge current are linked by Thornton’s law [9]
I ∝ V n . (2.1)
3 From last two-three years the HiPIMS power supplies are also commercially available from
Melec GmbH, Huttinger Electronic GmbH and Solvix SA.
11

For dc-MS discharges, n takes on higher values (up to 15) while values close to one (usually
from 1 to 3), i.e. I ∝V , are typical for HiPIMS [17]. Current-voltage characteristic of
dc-MS and HiPIMS copper discharges at p=3Pa are compared in Fig.2.3. Slightly higher
n in our HiPIMS is caused by lower range of applied target voltage. At certain target
voltage -600V, n abruptly decreased due to loss of electron confinement. The similar
behaviour was observed in [63].
2.1 HiPIMS in reactive atmosphere
Sputtering of elemental target in the presence of chemically reactive gases used to be
called reactive sputtering [64]. Gases O 2 and N 2 are most frequently employed during
reactive deposition of -oxide and -nitride films 4 , respectively. This phenomenon causes
that surface of electrically conductive target is covered by insulating layer which results in
hysteresis behaviour of cathode voltage [65,66]. The oxidized/nitrided target state is often
called "poisoned" and is generally considered as unwanted state because it: (i) increases
occurrence of arcs, (ii) decreases film quality due to ejection of micro-sized droplets [67]
and (iii) reduces the deposition rate [I,III].
The sputtering of poisoned target is more complex and other phenomena as sputtering
yield of poisoning layer and its state have to be taken into account [I]. The effect of Ti target
poisoned by O 2 (deposition of TiO 2 films) is clearly demonstrated in Fig.2.4. Presence
of O 2 in the chamber increases the discharge current; the current density i p ∼1A/cm 2
was measured in oxygen mixture O 2 /Ar=10. In this case the discharge current has almost
linear dependence on time and reaches the highest value at the end of the active
pulse. In O 2 /Ar=0.1 mixture the behaviour is different - i p reaches maximum faster but
the maximum peak value of current density is lower despite the constant applied cathode
voltage in both cases. These different waveforms are affected by the amount of O 2 and
the explanation is as follows.
Mean discharge current I m decreases when O 2 is delivered into the discharge, i.e. O 2
decreases the discharge conductivity, see Fig.2.4. After reaching I m minima, near target
poisoning O 2 /Ar∼0.1, I m increases almost linearly and attains original value. Such
behaviour can be explained by the emission of secondary electrons induced by ion bombardment
of the cathode/target (ISEE). Usually ISEE coefficient of Ti target decreases
with oxidation. Abnormal behaviour observed in our case can be explained by a hypothesis
given in [68], which is based on former works [69, 70]: the ISEE coefficient of the
oxidized targets depends on the state of the formed oxide, i.e. a suboxide or fully oxide
state. When a suboxide is formed, the ISEE coefficient is small. In contrast, if the target
is fully oxidized, the ISEE ox coefficient is significantly larger than the ISEE m of the
metal. Hence, after target oxidation the amount of electrons in the discharge increases
4 As examples of oxide films we can list TiO 2 , ZnO, Al 2 O 3 , AZO (Aluminium-Zinc-Oxide), ITO
(Indium-Tin-Oxide), BSTO (Barium Strontium Titanium Oxide) and many others. Nitrogen is added to
the films to increase their hardness, typically TiN, or to get required semiconductor transition, eg. SiN x .
12

due to ISEE but number of sputtered Ti particles/deposition rate is small because of low
sputtering yield of oxides.
In the same Fig.2.4 the dependence of deposition rate on O 2 /Ar ratio is shown, too.
With negligible amount of oxygen the deposition rate reached about a d ∼15-20nm/min
(not presented in the graph). Since the sputtering yield of titanium oxide is lower compared
with pure titanium (Y TiOx =0.015 and Y Ti =0.3 for Ar + energy of 300eV [71,72]) at
higher O 2 /Ar ratio (0.005

100 s
V Ti
HiPIMS
V
MF
pulsed
C p
R p
100 s
V Cu
HiPIMS
R H
- -
C R
V H
DC
H
R s R s
Ti Cu
R H
R H
C H
V DC
L1
L2
+ +
Fig. 2.5: The electric scheme of hybrid-dual-HiPIMS sputtering system with closed magnetic field.
The electrical circuit is based on parallel combination of two identical pulsed power circuits L1 and L2.
Each circuit consists of dc power supply V DC , power switches with large capacitors C H , and sputtering
sources with opposite magnet polarity. Resistors R H serve as ballasting resistors. The mid-frequency
circuit is connected to cathode via a step-down transformer. Configuration without MF-circuit is called
dual-HiPIMS and was developed in our laboratory before, see [V].
The potential of the electrode which acts as an anode is typically close to ground potential
≈ 0 V, i.e. close to potential of the chamber which serves as a reference electrode. In
that way the sputtering process of dual-HiPIMS is electrically and magnetically confined
since the polarity of the magnets is reversed. The HiPIMS process parameters can be
controlled separately for each loop because they are fully independent. This option, to
apply different cathode voltages, is a main feature of the developed dual-pulse system for
deposition of multi-component films with defined chemical composition 6 .
Potentials measured on the Ti (ϕ Ti ) and Cu (ϕ Cu ) electrodes with respect to the
ground are shown in Fig.2.6. The potential drop on the electrode which serves as cathode
is fast with leading edge delay ∼ 5 ns. Sputtering voltages (V s = |ϕ Ti - ϕ Cu |) were set
to keep the mean discharge currents constant I H m−Ti = 400 mA and I H m−Cu = 300 mA.
Because of short time HiPIMS pulses, high peak current density i H p−Ti ≈2.75A/cm2 for Ti
HiPIMS discharge andI H p−Cu≈2.25A/cm 2 for Cu HiPIMS discharge were measured. Such
6 In our laboratory Ti-Cu films are often deposited. However, Cu and Ti has quite different sputtering
yields (Y Ti = 0.4 and Y Cu = 2 for Ar + energies of 500 eV) [48]. For that reason the HiPIMS loops are
controlled separately with different current/power densities to eliminate the different sputtering yields.
14

values correspond to pulse power densities p H p ∼1kW/cm2 and pulse currents I H p ∼55A,
respectively.
Mid-frequency voltage source of V MF is connected in parallel to dual-HiPIMS driven
electrodes separated by a transformer. The winding ratio of employed transformer is stepdown
which prevents primary part of MF-circuit against penetration of HiPIMS pulses,
i.e.protects mid-frequency source. The capacitor C P placed in the primary MF-circuit
serves as a filter for dc-component of the mid-frequency source. Ballasting resistors R S
have a minimum inductance. In our experiments, the mid-frequency source AC-PMP1
Hüttinger Elektronik was operated with repetition frequency f M =94kHz and pulse width
T M a =3µs while the dual-HiPIMS uses f H =100Hz and T H a =100µs.
0
0
potential [V]
peak current [A]
-200
-400
-600
-800 60
MF
45
30
15
0
(9950)
Ti HiPIMS
Ti HiPIMS
dual-HiPIMS
Cu HiPIMS
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
Cu HiPIMS
p = 3.0 Pa
0 50 100 150 200 250
time [ s]
HiPIMS pulses
potential [V]
peak current [A]
-200
-400
0.6
0.4
0.2
0.0
Ti electrode
Cu electrode
MF discharge:
(driven together with dual-HiPIMS)
f = 94 kHz
T a
= 3 s
T d
= 7 s
p = 3.0 Pa
260.3 262.3 264.3 266.3 268.3 2610.3 27
time [ s]
MF pulses
Fig. 2.6: The time evolution of cathode voltage (upper panel) and discharge current density (lower panel)
in hybrid-dual-HiPIMS during sputtering of Ti and Cu targets. Left: time evolutions during the dual-
HiPIMS pulses (Im−Ti H = 400 mA, IH m−Cu = 300 mA. Right: evolution of MF discharge ITi,Cu
m−M
≈ 250 mA
driven in the idle part of HiPIMS frequency.
The MF - potentials applied on electrodes and MF - current densities are shown in
Fig.2.6, too. The potential pulses on electrodes are not rectangular but look more
like triangular outputs. The deformation is partially caused by the power supply itself,
which does not give ideal rectangular output. Pulse discharge current reaches the value
Ip−Ti,Cu M ≈600mA, which corresponds to peak power density pM p ∼13W/cm2 . It can be
seen in Fig. 2.6 that applied MF potential does not appear symmetric immediately after
the end of (Cu) HiPIMS pulses; the Cu MF potential amplitudes increase gradually while
the Ti MF potential operates regularly after the end of Cu HiPIMS pulse. We believe
that this asymmetry is caused by the (i) transient effect in sheath capacitance created at
the electrode region and by (ii) plasma resistivity. The amount of charged particles in the
cathode sheath region is always higher in comparison with anode region in this transient
time. Hence, the potential drop of MF voltage is higher at the anode (Ti) than at the
15

cathode (Cu) relative to the ground potential. The plasma potential at the cathode, at
the end of HiPIMS pulse, is usually several volts, i.e. very close to ground potential [III].
Combination of MF and HiPIMS discharges increases the deposition rate (in our experiments
the deposition rate was increased several times). Nevertheless, the most important
effect of MF discharge in a hybrid system is pre-ionization. MF-discharge produces certain
amount of electrons that respond by fast increase of HiPIMS voltage. This not only allows
faster development of HiPIMS pulses but also affords opportunity of significant pressure
reduction in the vacuum chamber. Under our experimental conditions the hybrid discharge
is stable at p=0.3Pa. If "pure" dual-HiPIMS is operated without MF support,
the pressure has to be higher than p≥1.1Pa. However, the pressure reduction demands
lower cathode voltage (in our case the cathode voltage has to be decreased roughly about
-150V) to keep the same discharge current. A similar effect of reduced pressure during
the HiPIMS process due to assisted MF-driven discharge was reported in [84,85]. This
effect is associated with the presence of charged particles. The assistance of electrons left
from the previous pulse in the plasma re-ignition was studied for MF discharges by Welzel
et al. [86,87]. The pressure reduction is very important because it allows: (i) increase the
purity grade of deposited films and (ii) control of the energy of incoming particles, which
influences the film quality (the energy of particles is affected by collisions in discharge
volume and is proportional to the mean free path length).
16

3. Diagnostics of HiPIMS discharges
HiPIMS pulses produce plasma characterized by the level of ionization, energy distribution
of electrons, ions and neutrals, drift of particles and many other parameters. These
properties are the key input quantities which influence complex processes in the plasma
volume and subsequently growth of the thin films. Hence, the diagnostics of plasmas
plays an important role in all experiments and becomes an inseparable part of plasmaassisted
deposition. Knowledge of internal plasma parameters either serves for the study of
elementary processes or helps to find optimal technological conditions. This section is an
overview of HiPIMS plasma diagnostics during reactive as well as non-reactive sputtering.
The comprehensive plasma characterization is reached because of combination of timeresolved
Langmuir probe measurements, optical emission spectroscopy, ion flux and ion
energy resolved investigations.
3.1 Methods of plasma diagnostics
The probe diagnostics of low temperature plasma, developed by Langmuir and Mott-Smith
in twenties of last century [88], belong to the oldest as well as most often used diagnostic
methods; comprehensively described in [89]. Distribution of electron energy is one of
the most important parameters which are usually determined from IV characterictics
measured by Langmuir probe. The second derivative of the measured probe current i
with respect to probe potential V in transition region yields the so-called Druyvesteyn
formula [90]
f (ε) = 4 √ me
e 3 A p 2
√ d 2 i e ε
dV 2, (ε = −eV). (3.1)
Here ε =m e v 2 /2 is the electron energy, A p is the probe surface, i e denotes the electron
current and V =V p -V pl , where V p and V pl represents probe and plasma potentials, respectively.
Because of further processing, it is necessary to know whether EEDF is Maxwellian
or not. The Maxwellian distribution function is described
f maxwell (ε) = 2 √ π
n e (kT e ) −3/2√ εexp
( −ε
kT e
)
, (3.2)
and it is characterized by electron temperature T e and electron density n e . Low-pressure
discharges has often the electron energy distribution that departs from a Maxwellian one.
It can be easily distinguished by the second derivative of IV characteristic in transition
area; the (d 2 i e /dV 2 ) of Maxwellian distribution is linear to ε (voltage) in semi-logarithmic
17

scale. The Electron Energy Probability Function (EEPF) F(ε)=ε −1/2 f(ε) is sometimes
introduced instead of EEDF [91]:
F (ε) = ε −1/2 f(ε) = 4 √ me
e 3 A p 2
d 2 i e
dV 2.
(3.3)
Assuming Maxwellian distribution, the electron temperature can be estimated from
the inverse slope of the logarithmic electron probe current with respect to probe voltage
(in volts)
[ ] −1
dlogie (V)
T e = . (3.4)
dV
The non-Maxwellian plasmas can not be characterized by electron temperature and, instead,
the mean electron energy E m has to be determined using EEDF
E m = 〈ε〉 = 1 ∫∞
n e
0
εf (ε)dε. (3.5)
The effective temperature defined as T eff =2/3〈ε〉 is a suitable characteristic for non-
Maxwellian plasma. For Maxwellian plasma T eff =T e .
It is possible to estimate the electron density n e directly from EEDF, Eq.(3.1), too.
The method is based on integral evaluation
n e =
∫∞
0
f (ε)dε. (3.6)
However, the results are burdened by larger computational error caused by inaccurate
estimation of (i) f(ε)∼d 2 i/dV 2 especially at low energies and (ii) definition of proper
integral limits.
There exists also other method how to determine the electron density directly employing
only electron current [89] to reduce the computational error. According to the orbit
motion limit regime [92], the electron current in the electron acceleration region should
be square-root dependent on the V p , referenced to the V pl . Hence, the slope of i 2 e vs.V
plot is proportional to n 2 e and the electron density can be determined from
∆i 2 e
∆V = 2 e
π 2A2 p n 2
m
e. (3.7)
e
Using Eq.(3.7) the n e is computed directly from the probe data, no other quantities as
T e being required. Thus the computational error is reduced to a minimum.
Ion Velocity Distribution Function (IVDF) can be estimated from characteristics of
ion current measured with respect to barrier potential in the so-called Retarding Field
Analyzer (RFA) measurements [93, 94]. Let us consider a velocity distribution f(v,r)
of ions where v =(v x ,v y ,v z ,) and r =(r x ,r y ,r z ,) represent the velocity and space vectors,
respectively. In case of a homogeneous velocity distribution in the plane x-y, the partial
18

integration over all v x and v y reduces f(v,r) to a one dimensional distribution f(v z ). In
case of almost perfectly anisotropic velocity distribution with respect to the z-axis the
IVDF is described by the formula [95,96]:
f(IVDF) = − m e 2 ·
1
· dI c(ϕ r )
, (3.8)
T g A 0 dϕ r
where m represents the mass of ions, T g is the total geometrical transparency of grids, A 0
is the total open area of the entrance orifice, e denotes elementary charge andI c introduces
detector current versus the applied retarding grid potential ϕ r .
However, the RFA device is not usually equipped with mass selector, i.e. the measured
I c is the total sum of all ion contributions. This makes data interpretation more difficult
since magnetron sputtering discharge is a complex system of gas and metal ions which can
not be distinguished by RFA. Because we want to avoid errors and confusing elements,
the first derivative dI c (ϕ r )/dϕ r is called IVDF, presented as a function of energy E in
arbitrary units, since f(IVDF)∝dI c (ϕ r )/dϕ r . Energies E in graphs 1 are calculated using
E =eϕ r . Only single, elementary charged, ions are assumed using Eq.(3.8). To calculate
Ion Energy Distribution Function (IEDF) from RFA measurements is more difficult and
can significantly increase an error. However, the difference between IVDF and IEDF is
carefully presented in [97,98].
3.2 Properties of non-reactive HiPIMS discharges
3.2.1 Time-resolved Langmuir probe measurements
The mechanism of HiPIMS plasma ignition is similar to breakdown of an insulating material.
The process starts with free electrons 2 accelerated outwards from the negatively
biased cathode. The accelerated electrons collide with neutral gas which can cause electron
impact ionization
(e)+Ar −→ Ar + +e+(e), (3.9)
producing gas ions and free electrons. The ions are attracted by the cathode and cause
sputtering while electrons collide with other neutral atoms. These effects result in gas
breakdown and plasma launching. Because of high power density there is an enormous
increase of charged particles in the HiPIMS pulse. Hence, the electron density, in close
vicinity (a few centimeters) of the target, is high, n e ≈5·10 18 m −3 . Its time-evolution,
obtained from Langmuir probe measurements, is shown in Fig. 3.1. High plasma density
results from large amount of metal sputtered particles since it is proportional to ionizing
1 One could claim that f(IVDF) should be plotted against v rather than against E. However, rescaling
the abscissa using v =(2E/m) 1/2 is not possible because of different masses of different measured ions.
Because the energy is usually expressed in terms of energy units eV, the discrepancy in reading f(IVDF)
vs. E can be accepted [95].
2 Free electrons are essential for discharge breakdown. They occur naturally due to background radiation
or thermal energy.
19

collision frequency. For such a high electron density the ionization mean free path is
short and significant fraction of sputtered atoms is ionized; e.g. the fraction of ionized Ti
atoms was estimated about 90-99% [99]. The increase of n e usually corresponds with the
evolution of peak discharge current reaching maximum typically at the end of the pulse.
Exponential decrease of electron density follows after the pulse end.
electron density [10 18 m -3 ]
5 electron density
mean electron energy
4
3
2
1
p = 3 Pa
f = 100 Hz, duty 1%
copper target
0.1 0
pulse width off
0
0 50 100 150 200
A [us]
6
5
4
3
2
1
mean electron energy [eV]
EEPF [eV -3/2 ]
1
0.1
0.01
dual HiPIMS - delay t d
= 500 s
f = 100 Hz, t a
= 100 s
Ti discharge ignated at t a
= 0 s
60 s
100 s
120 s
250 s
500 s
0 2 4 6 8 10
energy [eV]
Fig. 3.1: The time evolution of electron density
and mean electron energy in HiPIMS discharge,
f = 100 Hz, p = 3 Pa.
Fig. 3.2: Time evolution of Electron Energy probability
Functions of Ti-HiPIMS pulse, f = 100 Hz,
p = 3 Pa.
Typical time evolution of mean electron energy E m is also shown in Fig. 3.1. The
most energetic electrons are usually observed at early stage of HiPIMS pulse onset. After
reaching maxima, E m steeply decreases which is caused by energy losses due to preferential
ionization of sputtered metallic particles; ionization potential of metals (Cu + =7.73eV,
Ti + =6.82eV) is significantly lower than the potential of buffer gas (Ar + =15.75eV). Metal
sputtered particles produced at the onset of the pulse collide with electrons resulting
in ionization. The density of metal atoms in the plasma gradually increases, which corresponds
with decrease of E m .
Time-evolution of Electron Energy Probability Function (EEPF) for Ti HiPIMS pulse
is shown in Fig.3.2. The measurements were performed during a period of 60-500µs
(let us to remind that the pulse width was 100µs). The Druyvesteyn EEPFs with broad
energy distribution during the pulse to a double-Maxwellian distribution towards the end
of the pulse and finally a Maxwellian-like distribution after the pulse are usually observed.
Rapid cooling after the end of the pulse is typical. Cooling time is about 100µs during
which the electrons lose half of their mean kinetic energy.
The repetition frequency and duty cycle are key input parameters which influence
plasma properties according to Eq.(1.1). Their effect on the electron density and electron
energy was investigated in dual-systems during deposition of Ti-Cu film [V]. In this work
dual-HiPIMS (f H =100Hz, duty 1%) and dual-MS (f D =4.65kHz, duty 45%) discharges 3
3 Experimental arrangement of both systems can be found in chapter 2.2
20

are compared. The highest electron density n H e−Ti ≈8·1018 m −3 was observed at the end
of Ti pulse in dual-HiPIMS mode, see Fig.3.3. When the Cu discharge ignites, i.e. 15µs
after the end of the Ti-pulse, the electron density shortly increases for a few microseconds
but finally decreases down to the value n H e−Cu≈1·10 18 m −3 (n H e−Cu

high energy electrons dominate in bi-Maxwellian distribution. However, during a few
microsecond, energy of fast electrons is reduced due to collisions in discharge volume and
"cold" electron group become significant, having electron densities larger by about half
an order of magnitude. Different electron groups were also detected in [100,101].
3.2.2 Time-resolved optical emission spectroscopy
The optical emission spectroscopy can qualify discharge properties and ionization level
of sputtered particles. Time-averaged emission spectra for dual-MS and dual-HiPIMS
discharges in UV-VIS wavelength range are compared in Fig.3.5. The atomic Ti and
Cu emission lines dominate in the dual-MS working with higher repetition frequency.
The dual-HiPIMS discharge shows similar spectral composition with additional emission
of Ti + and Cu + . Presence of metallic (Ti + , Cu + ) ions in dual-HiPIMS is caused by
higher plasma density which increases the probability of ionizing collisions and excitation
of sputtered materials [56]. Intense emission of Ar lines was observed at wavelengths
λ>650nm in both sputtering configurations. Because of much higher intensity emitted
by Ar, the spectrum above 650nm is not included in Fig.3.5 [V].
time [ s]
normalized relative intensity [a.u.]
2.0
1.5
1.0
0.5
0.0
Cu
Ti
Cu
Ti + Ti + Ti
Cu Ti + Ti +
Ti +
Ti
Ti
Ti Ti
dual HIPIMS
f = 100 Hz, duty 1 %
delay 15 s Ar
Cu +
300 350 400 450 500 550 600 650
wavelength [nm]
Ar +
Ar +
Ti
Ti
dual MS
f = 4.65 kHz, duty 50 %
delay 15 s
Cu +
Cu +
normalized intensity [a.u.]
0 50 100 150 200 250
1.2
0.9
0.6
0.3
0.0
0.9
0.6
0.3
0.0
dual HiPIMS
dual MS
Ti - ON
delay
Cu - ON
OFF
Ti
Cu
Ti +
Cu +
0 50 100 150 200 250
time [ s]
Fig. 3.5: Comparison of time-averaged spectra
overviews emitted by dual-MS and dual-HiPIMS.
Upper spectrum - dual-MS, lower spectrum - dual-
HiPIMS.
Fig. 3.6: Time evolution of particular line intens.
for excited metals λ Ti = 453.3 nm, λ Cu = 324.7 nm
and for λ Ti+ = 334.9 nm, λ Cu+ = 615.4 nm ionized
metals.
Selected spectral lines of neutral Ti (λ Ti =453.3nm), Cu (λ Cu =324.7nm) and of ionized
Ti + (λ Ti+ =334.9nm), Cu + (λ Cu+ =615.4nm) metal elements are compared for dual-
MS and dual-HiPIMS in Fig.3.6. The intensity evolutions of Ti line, observed for different
sputtering modes, exhibit different behaviour. In dual-HiPIMS, the Ti intensity increases
in two (nearly) linear steps. The intensity increases slightly during the first half of
the Ti-pulse while a steep rise is observed for Ta−Ti H >50µs. Two-step evolution of Ti
line intensity in dual-MS is observed, too. However, the first linear part is much shorter:
22

oughlyTa−Ti D

elative intensity of Ar [a.u.]
9
6
3
0
Ti - HiPIMS
Cu - HiPIMS
dual-HiPIMS
hybrid-dual-HiPIMS
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
p = 3.0 Pa
= 811.5 nm (Ar)
0 100 200 300
time [ s]
relative intensity Ti [a.u]
7 Ti - HiPIMS Cu - HiPIMS
6
5
4
3
2
1
0
Ti, hybrid-DH
Ti, DH
Cu, hybrid-DH
Cu, DH
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
p = 3.0 Pa
= 327.4 nm (Cu)
= 453.3 nm (Ti)
0 100 200 300
time [ s]
22
19
15
11
7
4
0
relative intensity Cu [a.u.]
Fig. 3.7: Time resolved optical emission measurements
of Ar line λ Ar = 811.5 nm.
Fig. 3.8: Time resolved optical emission measurements
of λ Cu = 327.4 nm and λ Ti = 453.3 nm.
the electrons. In this way the deposition rate on the substrate is reduced and the anode
can be contaminated (covered) by the cathode material [VII]. Hence, some fraction of Ti
vapour is deposited on the inactive Cu anode during the Ti-HiPIMS pulse and it has to
be sputtered first at the beginning of the (second) Cu-HiPIMS pulse (and vice-versa for
reversed polarity of Ti-Cu electrodes). The second source of the emission observed might
be represented by ions still persisting in the cathode vicinity from previous pulse when it
served as anode (see the explanation given above).
A more probable explanation is the first one, based on target contamination. The
reasons are as follows: A well-pronounced emission of different materials at the beginning
of HiPIMS pulses is observable only for dual-HiPIMS mode despite the fact that the
intensity of Cu emission at the beginning of the Ti pulse is much lower, see Fig.3.8
(this effect corresponds with the low sensitivity of the iCCD chip which is needed for
measurement of massive sputtering during the Cu-HiPIMS pulse). When the hybrid dual-
HiPIMS discharge is operated, the emission of only Ti is observed during Cu-HiPIMS.
The absence of Cu emission at the beginning of the Ti-HiPIMS pulse is caused by the
operation of an MF-discharge in the idle time of HiPIMS pulses which provides a "cleaning
effect".
To study the discharge expansion dynamics and target contamination, the fast optical
emission imaging was employed [IV,VIII]. Images for both Ti-driven (right side of images)
and Cu-driven (left side of images) discharges in hybrid-dual-HiPIMS configuration are
shown in Fig.3.9. The total light emission 5 is presented in Fig.3.9. The Ti-pulse is ignited
first (T a =0µs) and the emission in the vicinity of Ti target is immediately detected. This
5 Measurements with optical filters for particular (Ar, Ar + , Ti, Ti + , Cu, Cu + ) wavelengths were also
done to verify the origin of imaged light emission. The function of automatic iCCD chip sensitivity adjustment
was activated during all recording of time-resolved image sequences since the emission intensity
dynamically varies in time. In other words, the iCCD chip sensitivity was adjusted automatically by the
gain (G=1-255) of the amplifier to obtain optimal images.
24

A
G = 233
T = 13 s
Ti-HiPIMS pulse
B
G = 31
T = 64 s
Ti-HiPIMS pulse
C
G = 255
T = 109 s
HiPIMS pulse delay
D
G = 240
T = 123 s
Cu-HiPIMS pulse
D
G = 180
T = 171 s
Cu-HiPIMS pulse
E
G = 255
T = 276 s
Ti-MF pulse
F
G = 255
T = 280 s
Cu-MF pulse
Fig. 3.9: Time resolved images of Ti-Cu hybrid-dual-
HiPIMS discharge. Images: A and B - Ti HiPIMS pulse
(0 - 100µs), C - delay between pulses (100 - 115µs), D and
E-Cu HiPIMS pulse (115 - 215µs), F-MF pulse of Ti, G -
MF pulse of Cu (MF driven in the idle time of HiPIMS). Ti
magnetron is shown on the right and Cu on the left-hand
side of images, respectively. The gain of iCCD amplifier G
was automatically varied in range 0 - 255.
light is mostly produced by Ar ∗
(verified by measurements with
optical filters and measurements
with OES). However, after a few
microseconds, T a ∼1-3µs, visible
emission in neighborhood of Cu
electrode is observed as well. It
becomes more pronounced in the
vicinity of Cu-target and creates
well-defined bond between Cu (anode)
and Ti (cathode), Fig.3.9 A.
This effect is due to magnetic
(reversed polarity of magnets)
and electric (anode-cathode) confinements.
Electrons, repelled
from the cathode ϕ Ti ≃-800V
(Fig.2.6), move along the magnetic
field line towards the anode, due
to |ϕ Ti - ϕ Cu | ∼ 800 V, and cause
electron impact ionization of Ar
gas. Apparent bonding between
electrodes is somewhat similar to
the edge of a virtual tubular-like
anode of dual magnetron system.
However, bounding between
sputtering sources, represented by
emission of Ar ∗ and Ar + , between
electrodes disappears after
full propagation of the Ti-HiPIMS
pulse, see Fig.3.9 B. This is not
true because the emission of sputtered
Ti gradually exceeds the intensity
of Ar ∗ and Ar + lines and
the sensitivity of iCCD chip is automatically
decreased. After 25-
30µs, massive sputtering of Ti is
dominant and a larger cloud is
formed nearby Ti target. The
cloud, formed mainly by neutral
Ti and Ar (with some fraction of
Ar + and Ti + ), propagates downwards
with a speed roughly about
2-3mm/µs. Measured speed some-
25

what corresponds with the diffusion velocity; the diffusion coefficient of Ti in Ar atmosphere
gives the velocity at discussed experimental condition about 1.5mm/µs. The
situation is more complex since there is a strong production of (energetic) ions during
HiPIMS pulses which are dragged due to ambipolar diffusion. Similar velocities of metal
propagation in HiPIMS discharges were published elsewhere [104]. The movement could
be also caused by drift (forcing the particle to move in a certain direction). Nevertheless,
owing to the formation of a virtual tubular-like anode, we believe that fundamental drift
currents flow between the electrodes due to electric and magnetic confinement. Hence,
the drift in substrate direction is limited and we suppose that it is ambipolar diffusion
which provides for the presence of ions in the substrate region.
Fig.3.9C depicts the discharge between the HiPIMS pulses, i.e. time period 100-
115µs. The discharge is not fully stable, which corresponds with voltage measured between
the electrodes, see Fig.2.6. Despite instabilities, the emission appears in the Ti
target vicinity and, weaker but still detectable, is observable near the Cu target, see
Fig. 3.9C. Despite weak intensity of MF discharge in the delay between the HiPIMS
pulses, the ions persist in the volume and support fast re-ignition of Cu-HiPIMS pulse.
The sputtering yield of Cu is higher in comparison with Ti [48]. For that reason we can
suppose intense sputtering of copper and its subsequent ionization. The effect of magnetic
confinement is not as pronounced as in the case of Ti-HiPIMS pulse because of lower sensitivity
of the iCCD chip. The last two images show MF-pulses: Fig. 3.9E sputtering of
Ti and Fig. 3.9 F sputtering of Cu, respectively. The most important is that the emission
is due to excited atoms Ar ∗ , Cu ∗ and Ti ∗ .
3.2.3 Ion and power fluxes towards substrate
High ionization degree of metal particles and their flux towards the substrate crucially
influence film properties [105–108]. However, the degree of ionization n i /(n i +n n ) is not
equal to ionized flux fraction ζ i /(ζ i +ζ n ) because the flux of ions is governed by electron
temperature T e while the neutral flux by the gas temperature T g (T e ≫T g ). This means
that the ion flux fraction is larger than the degree of ionization and therefore it is not
uncommon to find that the ionized flux fraction at the substrate can be more than 90%
although the degree of ionization reaches only about 50% [105,109]. Positive ion flux ζ i
can be determined e.g. from measurements of ion currents on a planar probe biased by
pulsed voltage 6 [110].
Because of high ionization degree the ion flux is usually higher in HiPIMS than in
mid-frequency or dc-MS discharges. The difference is shown in Fig.3.10 where ion fluxes
of dual-HiPIMS and dual-MS are compared [V]. The highest ion flux ζ i ≈80mA/cm 2 was
measured during Ti-HiPIMS pulse driven in dual arrangement, see Fig.3.10. High ion
flux is a sum of all ion contributions (ζi
Ar+ , ζi
Ti+ or ζi
Cu+ ) in the presented case. During
the Cu-HiPIMS pulse the ion flux is slightly lower, which is caused by lower applied
6 The angular frequency of the applied pulsed bias ω has to be significantly lower than the ion plasma
frequency ω i . Due to the inequality ω < ω i , the ions instantly respond to the changes of substrate/probe
voltage and the time evolution ζ i (t) can be determined precisely.
26

power density and higher ionization potential of Cu, but still it is by about two orders of
magnitude larger than the values obtained in dual-MS or fluxes reported elsewhere [110].
The dependence of the mean ion flux, averaged over the pulse width, on substrate bias
is shown in Fig.3.11. Ion flux depends nearly linearly on the bias voltage, but the flux
from dual-HiPIMS is always significantly bigger ζ H i ≫ζ D i . Hence, biasing of the substrate
during dc-MS or dual-MS somewhat increases ion flux but hardly can reach impact of
HiPIMS.
ion flux - dual MS [mA/cm 2 ]
3.0
Ti - ON Cu - ON pause dual HiPIMS
dual MS
dual HiPIMS
2.5
2.0
1.5
Ti - ON Cu - ON Ti - ON Cu - ON
dual MS
1.0
0 100 200 300 400
time [ s]
100
80
60
40
20
0
ion flux - dual HiPIMS [mA/cm 2 ]
ion flux [mA.cm -2 ]
140
120
100
80
2
1
dual HiPIMS
dual MS
0
20 40 60 80 100 120 140 160
probe bias [V]
Fig. 3.10: Comparison of ion fluxes for dual-MS
(two periods) and dual-HiPIMS (one period) to
substrate/probe biased at U b = -30 V.
Fig. 3.11: Averaged ion flux to the negatively biased
substrate for different voltages. The graph
clearly shows differences of two orders of magnitude.
Ions are the most important contributors to the total power density flux. The total
power flux somehow represents the total energy, coming from discharge, which affects
the growing film. High power flux usually causes substrate heating which can damage
thermally sensitive materials, e.g. plastics etc. Hence, it is generally wanted to obtain
low power flux mostly represented by metal ions. In our case [V] the total power density
flux, estimated from calorimetric probe measurements [111,112], is roughly twice larger in
dual-HiPIMS (P H Γ =38mW/cm 2 ) than in dual-MS mode (P D Γ =17mW/cm 2 ). The energy
fluxes of particular species (ions, electrons and neutral particles) can be estimated from the
model derived in [78]. It was calculated, within computational error, that the major part
of the power density flux to floating substrate is represented by charged particles; power
density from ions ∼75% for dual-HiPIMS and ∼60% for dual-MS and power density
from electrons ∼25% for both sputtering configurations of the total power density flux.
The power density flux from neutral particles in dual-MS reaches about 10% while in
dual-HiPIMS is nearly negligible (≤1%) because of high ionization.
According to Gras-Marti’s and Valles-Abarca’s analytical model [113], more than 90 %
of high-energy neutral particles (sputtered Ti, Cu atoms and Auger neutralized reflected
Ar ions) will be thermalized before they reach the substrate surface and their energetic
contribution to the substrate can be neglected. The validity of power density flux values
27

estimated from the model can be demonstrated by their relative comparison with ion flux
measurements. The ratio of ion fluxes ζi H /ζi D ≈32 and ratio of power density flux from
ions is Pi H /Pi D ≈30.5. We can conclude that: (i) ion contribution dominates in power
density flux, (ii) ion flux to substrate is nearly about two orders of magnitude higher
in dual-HiPIMS than in dual-MS [V]. However, the deposition rate (i.e. the number of
incoming particles onto substrate per time unit) is significantly lower in HiPIMS processes.
Taking the lower deposition rate into account, a higher energy per incoming particle in
HiPIMS has to be expected [114].
3.2.4 Ion energy in HiPIMS discharges
It was already mentioned in the previous chapter that ions are major carriers of energy
towards the substrate in HiPIMS discharges. Time-resolved ion distribution functions,
measured in hybrid-dual-HiPIMS discharge by retarding field analyzer (RFA) are presented
in Fig.3.12 [IV]. Generally, we can say that typical energy of ions hitting the electrically
isolated substrate is about ∼10 eV during HiPIMS pulses; ions gain the energy in the substrate/RFA
detector sheath owing to difference between plasma and floating potentials.
The maxima of IVDFs measured for the first Ti-HiPIMS pulse are slightly higher than
for the second Cu-HiPIMS pulse. This can be explained by higher pulse power densities
reached during Ti-HiPIMS pulse, since mean discharge currents were different.
The IVDFs, measured during Ti-pulse, starts to develop roughly at T a ≈30µs. Most
probably this behaviour corresponds with production and ambipolar diffusion of ions
generated in the cathode region 7 . The diffusion rate of emission cloud, represented by
excited atoms and ions, was estimated to about 2-3mm/µs from OEI measurements. This
speed most probably pertain to ions (accelerated by ambipolar diffusion) while the neutral
atoms are slower 0.5mm/µs at a pressure of 3Pa [102]. Since the distance between the
RFA probe and magnetron cathode is 50mm in vertical direction, we can conclude that
weak IVDFs in first microseconds are caused by absence of ions travelling from cathode
towards the RFA sensor.
The situation is more complex, since the detected ions are not created only in the
vicinity of the target moving downwards due to the process of diffusion. Some ions can
be created in the plasma bulk and can then reach the substrate area at the same time
as fast ions travelling from the target. Further, ions of sputtered particles, together with
argon ions, might move towards the substrate due to an ion-acoustic solitary wave, as
reported in [115, 116]. However, because both targets are electrically and magnetically
confined, a tubular-like virtual anode is formed between the magnetrons. Hence, ions
are electrostatically forced to stay in the vicinity of the electrons and the effect of an
ion-acoustic solitary wave moving towards the substrate is most probably limited in our
case.
7 There exist also some other (less probable) mechanisms which may be responsible for observed phenomena,
e.g. production of secondary ions in discharge volume, effect of E×B drift, creation of ionizing
avalanches etc. However, these effects have not been studied in detail as yet.
28

IVDF
3
2
1
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
energy [eV]
20
10
0
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
0
100
200
time [ s]
300
400
-10
0
10
20
energy [eV]
Ti-HiPIMS Cu-HiPIMS MF
ON OFF
-10
0 50 100 150 200 250 300 350 400
time [ s]
3D projection
2D projection
Fig. 3.12: Time resolved measurements of IVDFs in hybrid-dual-HiPIMS discharge. Left panel: time
resolved measurements presented in 3D graph. Right panel: the same data figured in 2D projection.
Mean HiPIMS discharge currents Im−Ti H = 400 mA, IH m−Cu = 300 mA and mean MF-discharge currents
≈ 250 mA were kept constant, p = 3.0 Pa.
I M m
Pre-ionization also increases the ion energy [IV], as illustrated in Fig.3.13. The figure
shows a situation when only the first, Ti-HiPIMS, pulse is driven (zero potential is applied
on Cu cathode) in hybrid configuration. The IVDFs during active Ti-HiPIMS pulse is
nearly identical with measurements presented in Fig.3.12. Nevertheless, ions incoming
with high energies (∼12eV) were detected also after the end of the HiPIMS pulse, which
is probably caused by ambipolar diffusion motion, see above. Ions with lower energies
(5eV) were observed at the same time (T ∼120µs), too, see Fig.3.13. This low energy
peak originates from MF-discharge operation activated after Ti-HiPIMS pulse. Ion distributions
of MF discharge occur regularly with period of ∼10µs (corresponding with MF
frequency 94kHz) at T a ≥250µs. Afterwards, the MF-discharge is stabilized and produces
ions with incoming energies of about 1-3eV, see Fig.3.13.
Since the time of flight is given by the speed of the particle, we can easily deduce
that the speed of 12eV ions will be 1.6 times higher then of 5eV ion. Furthermore, we
can assume that Ti ions will comprise a substantial part of the 12eV ions and that their
speed will be somewhat lower. On the other hand, 5eV ions comprise mainly Ar ions.
We can conclude that the time of flight will be similar and differences will be negligible
(i.e. speeds are approx. 6mm/µs versus 4.5mm/µs). Then we believe that ions created in
plasma bulk or in the vicinity of the target during the Ti-HiPIMS pulse reach the analyzer
at a distance of 50mm from the target face 10-20µs after the end of the Ti-HiPIMS pulse
and are detected as a second peak in measured IVDF at the time T a ∼120µs.
29

energy [eV]
20
10
0
Ti-HiPIMS
ON
Cu-HiPIMS
OFF
dual-HiPIMS:
f = 100 Hz
T a
= 100 s
T d
= 15 s
HF discharge:
f = 94 kHz
T a
= 3 s
T d
= 7 s
MF
-10
0 50 100 150 200 250 300 350 400
time [ s]
IVDF [a.u.]
2.0
1.5
1.0
0.5
p = 0.5 Pa
p = 3.0 Pa
p = 5.0 Pa
time-averaged
hybrid-dual-HiPIMS
f HiPIMS
= 100 Hz
f MF
= 94 kHz
0.0
0 5 10 15 20 25 30 35
energy [eV]
Fig. 3.13: Effect of hybrid mode on IVDFs formation.
Only Ti-HiPIMS pulse was operated supported
by regularly driven MF discharge. Pressure
was kept constant, p = 3.0 Pa.
Fig. 3.14: Time-averaged measurement of IVDF
as a function of pressure in the chamber. The discharge
was operated in hybrid-dual-HiPIMS mode.
Other important effect of hybrid system is the possibility of reducing the working pressure.
Since the MF discharge provides pre-ionization and supplies charged particles before
HiPIMS pulses, the pressure can be reduced by more than one order of magnitude. The
pressure affects the ion energy, shown in Fig.3.14, and it can be confirmed that (i) measured
ion energy increases and (ii) full width at half maximum of IVDFs also increases
with decreasing pressure. This behaviour is well known since the energetic losses due to
collisions are lower at lower pressures. At low pressures we observed slightly pronounced
double-peak distribution, see Fig.3.14. It is expected that the high energy peak is formed
by ions during the HiPIMS-pulse whereas the low energy peak originates from the time
after the end of the pulse [117,118] or might be caused by an effect of the difference in
the time of flight for low and high energy ions generated simultaneously.
3.3 Properties of reactive HiPIMS discharges
Chemically reactive species, generated due to dissociation of reactive gases, lead to reactions:
(i) in plasma volume, (ii) at the target surface and (iii) at coated surfaces. All these
reactions are responsible for different behavior of reactive plasma sputtering. The overview
spectrum of Ar/N 2 +O 2 HiPIMS discharge during sputtering of Ti target is shown in
Fig.3.15. The spectrum mostly consists of the atomic lines of Ar ∗ , Ar + , Ti ∗ , Ti + , O ∗ ,
molecular bands of NO γ (A 2 Σ + −→X 2 Π) system, which was not observed in the dc-mode,
and of the N 2 first and second positive systems (B 3 Π g −→A 3 Σ + u , C 3 Π u −→B 3 Π g ) [VIII].
Strong N 2 bands emission was observed in the spectra while weak nitrogen line
N (λ N =821.1-821.6nm) was hardly detectable. The spectra and intensities of particular
spectral lines are affected by the amount of reactive component O 2 added into the
30

elative intensity [a.u]
10
8
6
4
2
NO
(A 2 + - X 2 )
(C 3 N 2
SPS
u B 3 g )
Ti II
Ti II
Ti I
Ti I
Ar II
p = 0.75 Pa, O2 = 0.4 sccm
Ar I
O I
N 2
FPS
(B 3 g A 3 + u )
intensity [a.u.]
6
5
4
p = 0.75 Pa, Ar/N 2
+ O 2
= 20/10 + X
NO , 227 nm
N 2
SPS, 357 nm
O, 777 nm
10
8
6
4
2
intensity [a.u.]
0
200 300 400 500 600 700 800
vawelenght [nm]
3
0 2 4 6 8 10
amount of O 2
[sccm]
0
Fig. 3.15: Overview of the spectrum measured in
Ar discharge with O 2 (0.4 sccm) and N 2 (10.0 sccm)
reactive gases. The spectrum was recorded at
T a = 50µs.
Fig. 3.16: Plot of intensities of atomic line O and
molecular bands of systems N 2 2 nd positive and
NO γ vs. amount of O 2 added in the discharge.
discharge. As expected, the intensity of atomic oxygen line (λ O =777 nm) depends nearly
linearly on the amount of added O 2 , see Fig.3.16. The intensity of the N 2 2 nd positive
system (λ N2−SPS =357nm) is observed to be almost constant for higher O 2 flow rates.
This means that the rate of N 2 excitation in the discharge volume is not significantly
affected by oxygen in the plasma. An increase of the NO γ system (λ NOγ =227nm) was
observed with increasing of oxygen partial pressure. It was shown in works [119–121] that
NO is preferentially produced as a result of nitrogen and oxygen reaction.
Plasma density is influenced by partial pressure of reactive gases. Electropositive
gas mixtures usually provide similar behaviour and do not influence plasma parameters
drastically. It was presented in work [IX] that the electron density slightly increases when
N 2 is added to the discharge, while the mean electron energy decreases. This effect is
caused by enhanced ionization of nitrogen via Ar metastables.
Different and much more complicated situation occurs in electronegative discharges due
to presence of negative ions. Negative atomic and molecular oxygen ions were detected
in oxygen magnetron discharges many times [III]. Fig.3.17 shows the time evolution of
electron density n e within the modulation cycle in Ar/O 2 HiPIMS discharge. During the
active part of modulation cycle, n e always reached a maximum at the end of pulse. The
same figure shows that with increasing oxygen percentage in the gas mixture n e decreases
although the instant discharge current is the same. The electron density was more than
twice lower in Ar/O 2 mixture than in pure Ar discharge. It is supposed that this decrease
is due to the creation of negative ions by dissociative electron attachment. The dissociative
attachment is a resonant process with maximum probability at electron energies around
6eV [122].
31

On the other hand, the presence of O 2 in the plasma increases the electron temperature
8 . Two phenomena can affect electron energy at the presented experiment: (i) It is
well known that after adding O 2 into the plasma the sputtering rate of Ti sharply decreases
(target oxidization). Instead of fast sputtering of Ti atoms only slow sputtering
of TiO x layer from the target surface predominates. This implies that the loss of electron
energy by inelastic collisions with Ti atoms is less intensive and T e is effectively higher.
(ii) The second phenomenon is connected with creation of negative oxygen ions. T e
can be increased because of the energy released with the magnitude of electron affinity
energy [III].
electron density [10 16 m -3 ]
25
20
15
10
5
0
Ar (pure)
Ar/O 2
= 40
Ar/O 2
= 13
Ar/O 2
= 4
Ar+O 2
= 2
ON OFF
0 100 200 300 400 500
time [ s]
electron temperature [eV]
3.0 ON OFF
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
Ar/O 2
= 2
Ar/O 2
= 4
Ar/O 2
= 13
Ar/O 2
= 40
Ar (pure)
0 100 200 300 400 500
time [ s]
Fig. 3.17: Time evolution of electron density in
different Ar/O 2 HiPIMS discharges.
Fig. 3.18: Time evolution of electron temperature
in different Ar/O 2 admixtures.
Time evolution of the plasma potential V p during modulation HiPIMS cycle is shown
in Fig.3.19. The plasma potential is usually measured as negative at the pulse beginning
and|V p | decreases with time. Negative V p and|V p | increase was also observed in [123,124].
The authors explain it by a thin oxide layer covering the walls of the reaction chamber.
Vetushkaetal. [124] also measured negative V p in the pulse magnetron equipped by C
target and pure Ar gas (the wall oxidation can not occur). It is supposed that negative
values of V p and |V p | decrease during the pulse width are attributed to strong electric
field that appears between cathode and grounded reactor in the first instant of the active
part of the modulation cycle. This field gradually relaxes because it becomes partially
shielded due to the formation of the cathode fall with positive space charge of ions and
|V p | decreases.
3.3.1 Estimation of negative (oxygen) ion density
Langmuir probe investigation of negative-ion properties belongs to the most interesting
subjects in electronegative plasmas. However, presence of negative ions distorts the I-V
8 The electron temperature was calculated using Eq. (3.4).
32

plsma potential [V]
6
3
0
-3
-6
-9
-12
-15
-18
-21
-24
Ar,
Ar/O 2
,
z = 15 mm
z = 15 mm
Ar,
Ar/O 2
,
z = 65 mm
z = 65 mm
ON OFF
0 100 200 300 400 500
time [ s]
plasma potential [V]
5
0
-5
-10
-15
-20
axis
race-track
Ar, z = 65 mm
Ar, z = 15 mm
Ar/O 2
, z = 65 mm
Ar/O 2
, z = 15 mm
0 10 20 30 40 50
radial distance [mm]
axial dependence
radial dependence
Fig. 3.19: Dependences of plasma potential measured in different positions above the target. The
HiPIMS discharge was driven in pure Ar and Ar/O 2 = 13 gas mixture. Left: time evolution of V p .
Right: V p as a function of radial position above the target.
probe characteristics and complicates the interpretation of Langmuir probe data. Methods
determining negative-ion density in electronegative plasma from Langmuir probe measurements
have been already proposed, e.g., in [125–127]. The general approach written in
paper [X] is based on comparison of two probe characteristics, the first one taken in pure
electropositive (Ar) plasma and the second measured in electronegative (Ar/O 2 ) plasma.
The methods and the way of negative ion density estimation is briefly described below
(details are in [X]).
In the following text the subscripts ’e’, ’+’ and ’−’ denote quantities related to electrons,
positive and negative ions, respectively. As electron and positive-ion number densities
are different in electronegative plasma, it is important for probe evaluation to specify
clearly the formulae for electronic and ionic part of the characteristics. Proposed probe
evaluation technique is based on the assumption that a small amount of negative ions
does not change significantly the general form of expressions for electron and positive-ion
parts of the characteristics. Then the procedure is as follows: (i) First the probe measurement
is done in a reference electropositive plasma existing in a state close to the state
of electronegative one. (ii) From this reference characteristics specific formulae (laws) for
electronic and ionic currents can be derived. Both formulae may contain optional parameters,
which can be specified, e.g. by the least squares method. During this procedure
the quasineutrality condition equating electron and positive-ion number density is taken
into account. (iii) The same formulae applied to the probe characteristics measured in
electronegative plasma can be used to determine the electron and positive-ion densities
and other plasma quantities.
33

To be more specific, let suppose that the electronic part of the reference electropositive
IV characteristic is well-described by the Maxwellian electron distribution function. Then
√
( ) kTe eV
i e = eA p n e exp , V ≡ V probe −V pl < 0, (3.10)
2πm e kT e
where A p is probe area, V is voltage bias between the probe and plasma, n e is electron
number density andT e is electron temperature measured in kelvins. By facing this formula
with measured data one obtains plasma potential, electron number density and electron
temperature (of electropositive plasma) in a standard way.
A few theories have been proposed for description of the positive-ion current and the
ion current theories do not provide sufficiently reliable and undoubted results. Initially
we will consider a positive-ion current of quite general form
i + = eA p n +
√
kTe
m +
F (V, T e , λ), V ≪ 0, (3.11)
with a function F representing particular ion-current model and depending on a set of
optional parameters λ, if there are any. After substituting for plasma potential, electron
temperature and plasma density (n e =n + ) determined from the electronic part Eq.(3.10)
into this formula and comparing it with the probe characteristics for highly negative bias,
optional parameter(s) λ can be determined by the least square method.
Now the probe characteristics of electronegative plasma will be discussed. The quantities
concerning the electronegative plasma are distinguished from their ’electropositive’
counterparts by prime symbol. Accepting the Eq.(3.10) and, due to the inequality
T e ′/m′ e ≫T′ − /m′ − neglecting the contribution of negative ions, new values of plasma
potential V pl, ′ electron temperature T e ′ and electron number density n ′ e can be estimated.
Substituting electron temperature and plasma potential into Eq.(3.11) and comparing
it with the measured ionic part of the characteristics, ion density n ′ + can be obtained.
Finally, the negative ion density can be determined since the quasineutrality criterion has
to be fulfilled n ′ − =n′ + -n′ e .
Function F can be proposed from theoretical principles or can merely represent a
numeric approximation of experimentally obtained data. In our numerical computations
we prefer the ionic model of a simple form
F (V) = C
(
1 + e|V|
kT e
) κ
, V ≪ 0, (3.12)
with two optional parameters λ = {C,κ} determined numerically. Particular choice
Eq.(3.12) of function F yields
with coefficients
n ′ − = (γ −ε)n e,
ε ≡ n′ e
n e
, γ ≡ n′ +
n +
≈
n ′ −
n ′ +
= 1− ε γ
( ) T
′ κ−1/2 〈 〉
e i
′
+ (V)
T e i + (V)
V≪−T e,−T e
′
34
(3.13)
(3.14)

Angle brackets in the last expression denote averaging over the region of probe potentials,
where the currents can be considered as purely ionic.
For comparison, the method described in [127] is also included in the presented general
scheme of Eq.(3.13), now with coefficients
ε =
√
Te
T ′ e
i ′ e (0)
i e (0) ,
γ = √
Te
T ′ e
i ′ +
i +
. (3.15)
The first expression follows from the Maxwellian electron characteristic Eq.(3.10) evaluated
at the plasma potential, the second one is a consequence of the simplest but for
cylindrical probe not quite adequate model of constant positive-ion saturation current,
independent not only of any optional parameters but also of the probe potential.
There are two main sources of errors in evaluation of the characteristics in electronegative
plasma. The first one is due to the assumption that parameters λ in Eq.(3.11) (or
C, κ in Eq.(3.12)) are identical for both electropositive and electronegative plasma. To
minimize this error, the negative-ion density should be low and the state of the reference
electropositive plasma close to the state of electronegative plasma (so-called slightly
electronegative plasma). Then one can expect that the Bohm criterion is not significantly
modified. Braithwaite and Allen [128] obtained for low pressures a modified Bohm
criterion
v 2 B = kT e
m +
n +,s T −
n e,s T − + n −,s T e
, (3.16)
where the subscript ’s’ denotes local values at the sheath edge. At low collisionality the
negative ions are confined in the positively charged plasma bulk, thus n −,s ≈0 and the
normal Bohm criterion applies. Here it is supposed, with some uncertainty, that the same
model Eq.(3.11) is valid for both plasmas.
The second error arises due to the positive-ion mass uncertainty. The mixture of gases
may contain a variety of positive-ion species. Further, there is a high fraction of ionized
sputtered metal particles (nearly total in HiPIMS discharges). The effective mass m + of
a mixture of positive ions is defined by the formula
1
√ = ∑ ν(G,M)
√
m+
G,Mp(G,M)
m(G,M)
(3.17)
where p(G) is the fraction of a positive-ion gas species G ( ∑ p(G)=1), ν(G) is its
ionization degree (ν =1,2,...) and m(G) is its mass. Symbol M represents the fraction
of positive metal ions. The kind and fraction of positive ions in Ar/O 2 plasmas have
been already estimated, e.g. by energy-resolved mass spectrometry measurements [129].
At particular conditions existing during deposition of TiO 2 films, the fraction of gas
ions (Ar + ) was estimated as dominant, followed by metal ions (Ti + , Ti ++ ) and multiply
charged gas ions. On the other hand, the fraction of positive oxygen atoms O + and O ++
can be neglected completely. On account of plasma monitor analysis we estimated from
Eq. (3.17) √ m ′ +/m + =1±0.4. Hence, the error of the parameter γ from Eq.(3.13) is due
to the positive-ion mass uncertainty estimated up to 40%.
35

Despite relatively large error the derived method gives reasonable results comparable
with values published elsewhere [130,131]. It is shown in the paper [X] that density of
negative ions is roughly one order of magnitude lower than density of electrons in slightly
electronegative ArO 2 mixtures. It is assumed from complex diagnostic of electron density,
negative ion density and electron energy that negative ion are created by dissociative
electron attachment. The oxygen molecule is first dissociated by electron impact and
then the negative atomic ion is created by electron attachment. Radial distribution of
negative ion densities exhibit maximum above the erosion rill (see Fig.1), i.e. shifted from
the target axis. The shift could be a consequence of azimuthal rotation of ions, caused
by crossed radial electric and axial magnetic fields (E×B drift).
Qualitatively similar behaviour was also observed in HiPIMS discharges. The density
of negative ions reached significantly lower values than the electron density (differences
from one to three orders of magnitude were observed at particular conditions). It was
found from time-resolved measurements that the electron density n e reaches maxima during
the pulse while the negative ion density remains nearly constant (even relatively long
after HiPIMS pulse). This is due to much lower diffusion rate of heavy ions. Nevertheless,
determination of negative ion density by the proposed method is based on comparison of
electropositive and electronegative probe characteristics. Since the characteristics measured
after the pulse are weak and noisy, the error rises dramatically 9 . For that reason
the negative ion densities obtained after the HiPIMS pulse are more or less tentative.
9 Moreover, plasma decays after HiPIMS pulse and quasinetrality n ′ − =n ′ + -n ′ e is disturbed. When
quasinetrality condition is not valid the negative ion density determination is not relevant at all.
36

4. HiPIMS for thin films deposition
High ionization of sputtered metal particles is a main feature of HiPIMS discharges which
is utilized during the deposition of thin films. Large quantity of ionized sputtered atoms
leads to the growth of smooth and dense films [105,107,132] and allows controlling the
crystallography, phase composition and microstructure, as well as mechanical and optical
properties [106, 133]. Improvement of film adhesion [108], possibility of reducing the
substrate thermal load [134] and deposition on complex-shaped surfaces [135] has been
reported, too. This chapter is devoted to the effects of HiPIMS discharge on formation of
oxide (TiO 2 , TiO x N y ) and metallic (Ti-Cu) nanostructured thin films.
4.1 Deposition of photocatalytic TiO x films
In the Ti-O system, different crystalline phases are described: hexagonal TiO x (x/= 0.04 sccm form TiO 2
, rutile
reference
-Ti
TiO
experimental
V hexagonal
TiO 0.55
0.00 0.01 0.02 0.03 0.04
O 2
/Ar
Fig. 4.1: Different crystalline structures of TiO x
as functions of pressure and O 2 /Ar ratio. T -
α-Ti, TiO, a - amorphous phase (TiO 2 ), R - rutile
(TiO 2 ), A-anatase (TiO 2 ).
Fig. 4.2: Lattice volume of α-Ti and TiO vs.
O 2 /Ar ratio. The volume was calculated using a
unit cell transformation from cubic to hexagonal
(α-Ti).
The anatase crystal structure is known to have higher photocatalytic activity compared
to rutile, owing to its larger band-gap (anatase ∼3.2eV, rutile ∼3.0eV). Anatase
37

can be obtained from amorphous TiO 2 at an annealing temperature of∼300 ◦ C, rutile can
be formed from amorphous TiO 2 or anatase at higher temperatures ∼900 ◦ C [139,140].
Titanium dioxides deposited by classical magnetrons are mostly amorphous and postdeposition
thermal annealing is necessary for crystallization which is unsuitable for deposition
on plastic or heat sensitive materials. One way how to deliver sufficient amount
of energy and deposit crystalline structures directly, without thermal annealing, is the
application of HiPIMS discharges [I,II].
X-ray diffractometry investigations of deposited films reveal structures of α-Ti, TiO,
amorphous (or x-ray amorphous) films and TiO 2 as rutile and/or anatase as a function
of oxygen amount and pressure in the chamber, see Fig.4.1. At low amounts of O 2
(O 2 /Ar≤0.015) α-Ti or amorphous structures were found. Higher amount of oxygen at
low pressures forms mainly rutile. A phase change to anatase is observed with increasing
pressure. In the middle range of pressures p∼5-10Pa peaks of both crystalline phases
were found.
Films deposited at the lowest pressure represent different titanium-oxygen phase formations
with increasing O 2 flow, see Fig.4.2. A strongly preferred orientation with closely
packed lattice planes (002) was observed in the films deposited with O 2 /Ar=0.005. For
increasing O 2 flows the preferred orientation gradually disappears and for ratio 0.025 the
α-Ti phase was observed. In the range from 0.005 up to 0.035 the α-Ti reflections shift towards
smaller angles, which indicates an increase of the lattice constants. The increasing
cell volume is caused by incorporation of oxygen into α-Ti.
Higher O 2 amounts prevent the formation of highly oriented (002 texture) titanium
layers and changes the layer stacking sequence. Crystallographic facets and orientations
are preserved on islands and interfaces between initially disoriented (coalesced) particles.
The crystalline positions, separated by grain boundaries, are statistically distributed. At
O 2 /Ar=0.0375 a new phase, the cubic TiO (hongquite phase [138]) was formed. Under
the assumption of Vegard’s rule, the unit cell volume of 36.6 Å 3 corresponds to oxygen
incorporation of about 55% (that means the hexagonal α-Ti structure is preserved up to
TiO 0.55 ). This is in agreement with reference data [136, 137]. Only a small increase of
O 2 flow (up to ratio 0.04) leads to the formation of rutile structure with the cell volume
independent of O 2 amount. In the intermediate range of pressure p∼5-10Pa, rutile and
anatase phases were found only if sufficient amount of O 2 was delivered into Ar discharge.
It is known that crystalline phases are formed on heated substrates where surface temperature
is higher than the crystallization temperature, T surf ≥T cr . Musil et al. estimated
substrate surface temperature, measured by thermostrip, about 100 ◦ C higher than the
substrate temperature measured by a thermocouple [83]. According to this presumption
we can roughly estimate our surface substrate temperature to about T surf ∼150 ◦ C. This
is not sufficient for classical thermal annealing. The process or phenomenon responsible
for crystallization is based on energy E cr delivered to the growing film. A pressure increase
p in the vacuum chamber induces a decrease of E cr due to particle collisions.
The energy delivered into the growing film affects the surface morphology, too. The
images obtained by Atomic Force Microscopy (AFM) measurements show a homogenous
38

p =15Pa
p =0.75Pa
Fig. 4.3: The 3DAFM images of TiO 2 surface structure deposited under different experimental conditions.
Left: anatase/amrphous phase, 15 Pa, mean cluster surface 330 nm 2 . Right: rutile phase, 0.75 Pa,
mean cluster surface 783 nm 2 .
distribution of small grains/clusters, Fig.4.3. The roughness expressed by roots-meansquare
(RMS) value is of the order of nanometers (σ RMS ≤3-4nm). This result is in
good agreement with roughness σ estimated by XR measurements, see Table4.1. The
highest grain/cluster radius 〈r MC 〉, was estimated for pressure p

Fig.4.4 presents band-gap energy E g of the TiO 2 as a function of particle/grain size 1 .
The band-gap of rutile phase reaches Eg R =2.99eV while the gap of anatase with finer
morphology is higher Eg A =3.26eV, see also Table4.1. The dependence of energy-gap
E g on particle size for ultra-fine anatase TiO 2 has been already described in the literature
[142] (E g increased from 3.173 to 3.289eV when particle diameter decreased from 17
down to 3.8nm). This behaviour was attributed to delocalization of molecular orbital or
to the size quantization effect. The Q-size effect of semiconductor clusters usually appears
between 1 and 12nm, as reported in [143,144] for TiO 2 . Hence, it is expected that for
larger grains the band gap is determined by crystallographic phase. This can be illustrated
on the anatase/amorphous film where energy band-gap is low (E g =2.97 eV) despite small
grain, see Fig.4.4.
band-gap E g [eV]
3.30
3.25
3.20
3.15
3.10
3.05
3.00
2.95
anatase
p = 10 Pa
anatase / amrph.
p = 15 Pa
anatase / rutile
p = 5 Pa
rutile
p = 0.75 Pa
10 11 12 13 14 15 16
grain radius [nm]
current density i [ A.cm -2 ]
40
30
20
10
p = 0.75 Pa, rutile
p = 5 Pa, anatase/rutile
p = 10 Pa, anatase
p = 15 Pa, anatase/amrph.
0
0 200 400 600 800 1000
potential [mV] vs. Ag/AgCl
Fig. 4.4: Dependence of band-gap energy E g on
mean grain/cluster radius 〈r MC 〉 for different crystallographic
phases, see Table 4.1.
Fig. 4.5: Polarization curves for different crystallographic
phases recorded during 5 s/5 s intervals
of UV radiation/dark period.
Semiconductor TiO 2 produces electron-hole pairs after UV radiation. Basically, when
the TiO 2 particle absorbs a photon, an electron is transferred from the semiconductor
valence band to the conduction band. The generated holes initiate oxidation reaction
on the surface of the photoanode while the electrons diffuse through the film towards
the conducting back contact of the support. The electrons and holes are termed charge
carriers. The most limiting aspect of this process is relatively high recombination rate
of photoinduced electron-hole pairs. Photo-electrochemical properties of irradiated TiO 2
1 The optical band-gap energy E g can be determined from the absorption coefficient α as a function
of incident photon energy E(hν). In our case the E g was obtained by extrapolating the linear portion to
the photon energy in α vs. E(hν) graph from spectroscopic ellipsometry measurements [141].
40

films were estimated from measurement of polarization curves 2 Fig.4.5. The currents were
not constant but linearly increasing with time and applied potential. After switching the
light on, the initial current spikes, which dropped to some steady state current, were
observed. This behaviour was also described in [145] and formerly in [146]. The authors
of [145] explain this effect by initial concentration of negative charge carriers accumulated
in the film during deposition.
The highest currents were measured for anatase and anatase/rutile phase films prepared
at 10 and 5Pa, respectively. Rutile (p=0.75Pa) and low crystalline anatase
(p=5Pa) films attained much lower values of photocurrents. Lower photoactivity of rutile
is mostly due to the low extent of surface hydroxylation which is, on the other hand,
a characteristic feature of anatase. With respect to the fast recombination processes of
the photoinduced couple (h + , e − ), surface abundance of the OH groups may significantly
affect the final photoactivity. Produced electron vacancies oxidize to hydroxyl radicals,
which are afterwards highly active oxidation species. Electrons diffusing through the film
can be trapped by oxygen vacancies. The electron mobility decreases in densely packed
Ti particles of a cluster-like organization, see Fig.4.3, and consequently the probability
of recombination with holes and/or trapping by oxygen vacancies increases.
In order to compare films with different thicknesses and crystallographic structures,
Incident Photon-current Conversion Efficiency (IPCE) was determined (results are listed
in Table4.1). The IPCEs have similar qualitative features as measured photocurrents.
Low IPCE for anatase/amorphous film (p=15Pa) can be due to three reasons: (i) low
crystallinity of the film with small fraction of anatase phase, (ii) small thickness of the
film and (iii) low value of band-gap energy. Hence, it is supposed that crystalline volume
fraction of the film is the most important factor which influences photo-electrochemical
properties.
4.2 Effect of nitrogen doping on formation of TiO x N y
As mentioned in the previous chapter, titanium dioxide is well known photocatalytic material.
However, photocatalytic efficiency of pure TiO 2 after irradiation by solar spectrum
is very low. Because of high energy band-gap of TiO 2 only the UV part of the solar spectra
(

thermal instability [147–151], and (ii) non-metal anions such as N − [152–155], C − [156],
S − [157], F − [158], which shift the band-gap towards lower spectral frequencies. Among
the doping elements, N was found to be most effective because nitrogen has comparable
ionic radius to oxygen and the p states of N contribute to band-gap narrowing by mixing
with O 2p states [154,159].
0.2 sccm O 2
, 0.75 Pa
content [%]
50
40
30
20
10
0
N
O
Ti
0.2 0.4 1 5 10
amount of O 2
[sccm]
intensity [cps]
2500
2000
1500
1000
500
Ti(III) 2p 1/2
Ti(IV) 2p 1/2 Ti(II) 2p 1/2
Ti(IV) 2p 3/2
Ti(III) 2p 3/2
Ti(II) 2p 3/2
468 466 464 462 460 458 456 454 452
binding energy [eV]
Fig. 4.6: Chemical composition of TiO x N y -
only Ti, O and N elements are shown (impurities
are excluded from evaluation). p = 0.75 Pa,
F N2 = 10 sccm.
Fig. 4.7: The Ti 2p 3/2 peak of TiO x N y film deposited
at low pressure p = 0.75 Pa and low oxygen
flow F O2 = 0.2 sccm measured by high resolution
XPS.
Magnetron sputtering deposition of crystalline TiO x N y by dc magnetron sputtering
method is difficult and the process must be usually followed by thermal annealing [155,
160–163] or high discharge current during deposition has to be applied [162,164]. However,
crystalline TiO x N y films were achieved using HiPIMS technique at room temperature
[VIII]. In this work the film properties were analyzed with respect to varying the O 2
partial pressure (flow rates for Ar and N 2 were kept constant).
The deposited films consist mainly of Ti, N and O elements, see Fig.4.6. Impurity
level reaches about 20% and it is excluded from further evaluation. A decrease of nitrogen
content is observed for increasing O/N ratio. Obviously, there is a preferred incorporation
of oxygen into the growing film. If a sufficient amount of oxygen is present in the discharge
volume, nitrogen is virtually not incorporated. With increasing oxygen flow, not only the
intensity of the N1s peak but also the contributions of TiN to the nitrogen 1s spectrum
decreases while O1s increases.
This observation is confirmed by high resolution measurements of the Ti2p peak.
Fig.4.7 shows the Ti2p peak of the film deposited at low pressure and low oxygen flow.
Three possible bonds to fit the measured profile has to be assumed: 458.5eV, 457.1eV and
455.7eV correspond to Ti(IV)2p3/2, Ti(III)2p3/2 and Ti(II)2p3/2, respectively. The
other three functions in Fig. 4.7 correspond to 2p1/2 peak. Unfortunately, TiO and TiN
cannot be clearly distinguished, because both bondings contribute to the Ti(II) peak. The
formation of TiO 2 can be attributed to surface oxidation of the film during transport of
42

the sample from the vacuum chamber to the XPS device. The amount of Ti(III) and Ti(II)
components is decreasing with increasing oxygen ratio. For oxygen flow F O2 >1sccm the
Ti2p region shows only two peaks which belong to Ti(IV)2p3/2 and 1/2 peaks, see [VIII].
Because of negligible content of N in the film, the peak at Ti(II)/Ti(III) position was not
detected. Hence we can conclude that only TiO 2 is formed for higher oxygen flow rates.
XRD measurements were performed to get information about the phase composition.
As expected, TiO/TiN as well as TiO 2 crystallographic phases are observed (Fig.4.8) as
a function of particular experimental conditions. Thin films deposited at high oxygen
flow rate (F O2 ∼10sccm) consist of TiO 2 , which is found to be x-ray amorphous for high
deposition pressure (p∼10Pa) or forms rutile phases at a low pressure (p∼0.75Pa),
respectively. The deposition of x-ray amorphous films at 10Pa is probably due to lower
energy transfer to the growing layer caused by increased number of particle collisions in
plasma.
intensity
200
180
160
140
120
100
80
60
40
20
0
TiO x N y
O 2
= 0.2 sccm, p = 10 Pa
TiO x N y
O 2
= 10.0 sccm, p = 10 Pa
O 2
= 0.2 sccm, p = 0.75 Pa
O 2
= 10.0 sccm, p = 0.75 Pa
p = 0.75 Pa p = 10 Pa
TiO x N y
10 sccm O 2
TiO 2
0.2 sccm O 2
TiO 2 TiO 2
10 sccm O 2
0.2 sccm O 2
25 30 35 40 45
2 [°]
energy band-gap [eV]
3.3
3.0
2.7
2.4
2.1
3.2
2.8
2.4
2.0
1.6
energy band-gap
refractive index - Moss
refractive index - measured
p = 10 Pa
energy band-gap
refractive index - Moss
refractive index - measured
p= 0.75 Pa
0 2 4 6 8 10
O 2
amount [sccm]
2.6
2.5
2.4
2.3
2.8
2.6
2.4
2.2
refractive index
Fig. 4.8: X-ray peaks of the films prepared at low
flow rate of O 2 were identified as TiO x N y .
Fig. 4.9: Energy band-gap and refractive index
vs. O 2 amount for films prepared atp = 10 Pa (upper
panel) and p = 0.75 Pa (lower panel).
Fcc TiX (where X=O or N) is found at low oxygen flow. However, it is difficult to
distinguish between TiO and TiN phases by x-ray diffractometry. Both structures show
a face centred cubic Ti sublattice, with oxygen or nitrogen incorporated in octahedral
sites. Octahedral sites are filled completely if the ratio X/Ti=1. This ratio can vary
between 0-1 as a function of different deposition conditions [165–168]. However, the
lattice constant depends on the amount of octahedral sites filled by anions and, of course,
also on the incorporated elements. For this reason it is impossible to determine both the
type of elements and the amount of incorporated atoms only from the lattice constant
measurement by XRD technique. Hence, incorporation of both oxygen and nitrogen into
the Ti lattice is assumed. This assumption is confirmed by XPS measurements and their
composition is therefore referred to TiO x N y . The lattice constant is influenced by pressure
in the chamber; determined from peak position are 4.23Å for p=0.75Pa and 4.15Å
for p=10Pa. Theoretical lattice constants values are a TiN =4.242Å and a TiO =4.185Å,
respectively. However, all deposited crystalline phases show comparably small grain sizes.
43

Thus it is assumed that x-ray amorphous amounts of TiO/TiN or TiO 2 are also present
within the films.
The band-gap energyE g depends on oxygen amount; if the flow rate of oxygen decreases,
E g is decreased, too (see Fig.4.9). The lowest values of E g ≈1.7-2.1eV are related
to TiO x N y structure. Conversely, the highest values of band-gap energy, which correspond
to the band gap of TiO 2 , were measured for highly oxidized gas mixtures. The
band-gap is related to the refractive index by semi-empirical relation known as the Moss
rule [169,170]), n 4·E g =const., where const.=95eV. Comparison of measured refractive
indices 3 with values estimated from E g employing the Moss rule are displayed in Fig.4.9,
too. The crystalline phase of TiO x N y shows higher values of refractive index n and also
shifts of n(λ) maxima towards lower photon energies (not presented as a figure here).
The explanation could be based on former research of Futsuhara et al. [171] who found
that the band-gap is related to the difference in ionicity of metal-O and metal-N bonds.
The electronegativity of oxygen is larger than that of nitrogen, which indicates that the
Ti-O bonds involve a larger charge transfer than Ti-N bonds. Assuming that both, Ti-O
as well as Ti-N, bonds coexist in the films, the shift of the band-gap to lower energy can
be attributed to a decrease of ionicity due to formation of Ti-N bonds at lower oxygen
flow rate.
4.3 Deposition of functional metallic Ti-Cu films
The problems associated with infection of implants/endoprostheses in orthopaedic surgery
initiate the necessity to develop new strategies of antimicrobial coatings. Such implant
surface coating can be represented by metal-ion based films [172,173] with antimicrobial
and biofilm preventing properties [173]. Several metal ions (Cu 2+ , Ag + , Zn + ) are described
as antibacterial agents which could be used for metal-ion based deposition on implant
surfaces [174–176]. For example, silver is a proven antibacterial coating and used in a few
medical devices [177,178]. However, copper is more promising metal ion for deposition
applications because of its lower toxicity and higher cytocompatibility [179]. Furthermore,
copper is a metabolizable agent, which can be naturally removed from a living body when
it is released from the film.
For this reason, our research is also motivated by preparation of copper rich intermetallic
Ti-Cu thin films. Ti-Cu phase diagram exhibits mutual solubilities of Ti in
fcc-Cu (up to 8mol% [180]) and Cu in hcp-Ti (about 1mol% [181]), which predicts
the possibility of copper release. Released copper serves as an antimicrobial agent while
the role of titanium is to increase the adhesion of the human cells on the coated implant
and to increase the adhesion of the film [XI]. In this study, Ti-Cu films with antibacterial
effect were prepared by different ways of magnetron sputtering: dc-MS, dual-MS and
dual-HiPIMS. Cu discharge current was varied (Im Cu =10-400mA) for particular exper-
3 Refractive index is usually obtained from ellipsometry measurements as a function of photon energy,
i.e. n(λ). Presented refractive indices are refer to λ=500 nm.
44

iments while other parameters were kept constant to see the effects on film formation,
crystallographic structure[XII] and subsequently on bio-properties.
4.3.1 Formation of Ti - Cu films
Normalized composition to 100% of pure Ti-Cu film prepared by dc-MS, dual-MS and
dual-HiPIMS (see chapter3.2.1) technique is presented in Fig.4.10. To clarify the results,
detected impurities were excluded from the evaluation. Concerning the chemical compositions,
estimated from Ti2p and Cu2p peaks as a function of mean copper discharge
current Im Cu , one would naturally expect: higher Cu content in the film at larger Im Cu .
However, the situation is different for dual-HiPIMS films where fraction of copper is high
in the whole range of Im Cu , see Fig.4.10. This effect is caused by intensive sputtering of
copper due to high pulse power density. For low Cu discharge current the power density
was found by about order of magnitude higher than in dc-MS [XII].
These findings correspond well with estimated film densities ρ. Fig.4.11 shows ρ vs.
Im Cu (varied in range from 0 to 400mA)4 . This indicates that the film density increases
with increasing Cu discharge current. However, dual-HiPIMS films are highly dense in
general, even at low Im Cu current, which corresponds with chemical composition revealed
by XPS. In principle, the film density could be increased by ion-bombardment in HiPIMS
discharge, but it is believed that this effect is minor if compared with that of chemical
composition.
normalized content of Ti and Cu elements [%]
100
80
60
40
20
0
Ti (2p)
Cu (2p)
dc-MS dual-MS dual-HiPIMS
10 100 200 400 10 100 200 400 10 100 200 400
Cu discharge current [mA]
density [g/cm 3 ]
9
8
7
6
5
4
A)
dc-MS
dual-MS
dual-HiPIMS
}
I Cu
m
I Ti
m
= 100 - 400 mA, varied
= 400 mA
dc-MS
dual-MS
dual-HiPIMS
}
I Cu = 100 mA
m
I Ti = 0 mA
m
B)
0 100 200 300 400 100 100
I Cu
I
[mA] varied, ITi = 400 mA constant
Cu = 100 mA
m
m m
I Ti = 0 mA
m
density [g/cm 3 ]
Fig. 4.10: Chemical composition of Ti-Cu films
for different Im Cu;
ITi m = 400 mA was kept constant
permanently.
Fig. 4.11: Film densities vs. Cu currents for films
deposited in dc-MS, dual-MS and dual-HiPIMS
modes.
4 The densities of pure Cu films deposited at I Cu
m
= 100 mA (ITi m = 0 mA) are shown in the same figure,
too: right-hand side of graph in Fig. 4.11 labelled as B). Densities of pure Ti and Cu films show values
comparable to the theoretical values ρ Ti = 4.5 g/cm 3 and ρ Cu = 8.9 g/cm 3 , respectively. The lower values
of deposited pure metallic films are due to voids or other defects.
45

Table 4.2: Cu domain sizes [nm] calculated from the Fourier transforms of the pure physical line profile.
∗ denotes cases when no line profile, i.e. no observable Cu reflections in the x-ray pattern, occurs.
method/Cu current 10 mA 50 mA 100 mA 200 mA 400 mA
dc-MS ∗ ∗ 2.2 2.7 5.0
dual-MS ∗ ∗ 2.2 2.8 5.5
dual-HiPIMS 4.4 44 7.1 7.9 7.9
Arrangement of dc-MS, dual-MS and dual-HiPIMS x-ray patterns of layers deposited
at identical Im Cu/ITi
m current conditions exhibit increasing intensities of Cu reflections from
dc to dual-HiPIMS mode. Independent of the deposition method and the Cu current
used, the x-ray patterns for all films do not show any reflections of metallic hcp-Ti or any
other Ti compound. Only reflections of fcc-Cu are observable [XII]. The GIXD-patterns
of thin films prepared at dc-MS and dual-MS conditions do not show any crystalline Cu
for low currents Im Cu ≤100mA, too. The Cu crystallites grow and the reflections appear
in the x-ray patterns only at higher Cu discharge current. However, the dual-HiPIMS
mode produces polycrystalline metallic Cu at any Im Cu . The domain sizes estimated from
GIXD peak line profile are presented in Table 4.2. The dual-HiPIMS film lattice parameters
correspond to the values for pure fcc-Cu [182] while the lattice parameters calculated
from dc-MS and dual-MS films are in accordance with those found by Krull et al. [180]
for Cu 0.92 Ti 0.08 .
The absence of Ti x-ray reflections suggests amorphous state of Ti particles in Ti-Cu
films, which indicates different growth mechanisms of Ti and Cu during film formation.
Ti could be a part of solid solution as mentioned above for dc-MS and dual-MS mode.
Hence, the Ti-Cu films were annealed with intention to form crystalline TiO 2 (anatase or
rutile) besides crystalline CuO (Cu 2 O) to prove the presence of Ti in the films. During
post-deposition heat treatment under atmospheric condition the formation of crystalline
titanium dioxide and copper oxide in the films was observed. Fig.4.13 shows the phase
transformation of crystalline metallic Cu into crystalline CuO and the formation of crystalline
TiO 2 . Annealed films (Fig.4.13) exhibit TiO 2 in both, anatase and rutile, phases.
Different oxidation and crystallization processes were observed between dc-MS, dual-MS
films and dual-HiPIMS produced films. After an annealing time of 7h, titanium dioxide
is detectable only in dc-MS and dual-MS films. The formation of crystalline TiO 2 in
dual-HiPIMS film needs more time (32h), suggesting a longer crystallization process in
which small TiO 2 particles grow to sufficiently large crystalline domains.
XR and GIXD investigations confirm the results of XPS measurements and plasma
diagnostics. With increasing Im Cu current the amount of Cu in the films grows. Titanium
incorporated in the deposited films is x-ray amorphous. On the other hand, Cu particles
form crystals large enough for x-ray diffractometry signals. High mobility of atoms
(ions) impinging on crystal surfaces enables the particles to occupy favourable energetic
positions. This effect corresponds with energy delivered into the growing films which is
supported by the fact that Cu and Cu + are expected as major carriers of energy (especially
in dual-HiPIMS, see chapter2.2). Formation of Cu-Ti solid solutions was not observed
46

lattice parameters [A]
3.655
3.650
3.645
3.640
3.635
3.630
3.625
3.620
3.615
dc-MS
dual-MS
dual-HiPIMS
I Cu
= 400 mA
I Ti
= 400 mA
p = 3 Pa
reference: pure Cu fcc
(ICSD 53247, 627114)
intensity [cps]
350
300
250
200
150
100
50
TiO 2
anatase
TiO 2
rutile
CuO
CuO
CuO
TiO 2
anatase
CuO
dc-MS
dual-MS
dual-HiPIMS
I Cu
= 100 mA
I Cu
= 400 mA
p = 3 Pa
TiO 2
anatase
TiO 2
rutile
TiO 2
anatase
3.610
1 2 3 4
mode of deposition
0
25 30 35 40 45 50 55
2 [°]
Fig. 4.12: Lattice parameters of fcc-Cu in films
deposited at Im Ti = 400 mA and ICu m = 400 mA for
dc-MS, dual-MS and dual-HiPIMS mode.
Fig. 4.13: X-ray patterns of films after annealing
under atmospheric conditions at 730 ◦ C for 7 h (dc-
MS, dual-MS), and 32 h (dual-HiPIMS).
at dual-HiPIMS conditions and is probably due to Cu/Ti element ratio, see Fig.4.10.
The formation of Cu-Ti solid solution is known up to a composition of fcc Cu 0.92 Ti 0.08 ,
see [180]. According to Vegard’s rule the lattice parameter of fcc-Cu increases depending
on the Ti amount. Hence, from the observed fcc-Cu lattice parameters (Fig. 4.12) we
draw the conclusion that Ti is partially dissolved in dc-MS and dual-MS films but not in
dual-HiPIMS films. In dual-HiPIMS, the ratio is higher and copper-related mechanisms
prevail.
4.3.2 Copper release from Ti-Cu films
The antimicrobial effect is caused by copper species released from the metallic Ti-Cu films.
The results of cumulative copper releaseR Cu , measured by atomic absorption spectroscopy
(AAS), from the samples stored in 700µl of Dulbecco’s Modified Eagle Medium (DMEM) 5
are presented in Fig.4.14 as a function of storage time. The highest copper release was
measured for dual-HiPIMS films at about R Cu ≈6 mmol/l, i.e. about 250µg of copper.
The total amount of deposited copper, estimated by the calculation m=V ·ρ (employing
the area of the substrate, chemical composition from XPS, density and thickness of the
film from XR presented in chapter4.3.1), is roughly up to 200µg.
The difference is about 20% and can be considered as an acceptable total error of all
employed techniques. Despite the error, however, this indicates that copper is released
relatively quickly (within 24 hours) and completely from dual-HiPIMS films. The situation
is different for dc-MS and dual-MS films. The total cumulative Cu release, measured by
AAS, amounted to approx. 30µg but the calculation of copper mass from the density
yields roughly 60-70µg. From this we can conclude that the copper release from dc-MS
5 Dulbecco’s Modified Eagle Medium is a cell culture medium that can be used to maintain cells in
tissue culture. It contains amino acids, salts, glucose and vitamins.
47

and dual-MS in not total, but that only half of "stored Cu" is released. The presence of
Cu particles in dc-MS and dual-MS films after 10 days in DMEM was also confirmed by
XPS measurements.
Copper release mechanism is not fully understood yet, but it most probably corresponds
with the crystallographic properties of thin films: (i) there is good relative agreement
between the intensities of XRD patterns and cumulative copper release, and (ii) the grains
of dc-MS and dual-MS are much smaller, see Table4.2. The second important result is that
the dual-HiPIMS technique produces denser films containing more Cu species. Finally, we
can conclude that the dual-HiPIMS technique produces Ti-Cu films with a higher density
(higher amount of Cu), which can be completely released in DMEM within 24 hours.
cumulative Cu-release [mmol/l]
8
7
6
5
4
3
2
1
***
*** ***
#
#
dc
dual-MS
dual-HiPIMS
I Ti
= 400 mA
I Cu
= 100 mA
vitality [%] + SD
100
80
60
40
20
MTS after 24 hours
MTS 24 hours rinsing
+ after 24 hours
0
10 min 60 min 24 h 48 h 4 days 10 days
0
TiAlV dc-MS dual-MS dual-HiPIMS
Fig. 4.14: Dependence of cumulative Cu release
vs. storage time in DMEM. ONEWAY ANOVA
(Post Hoc LSD) test, ∗ ∗ ∗p < 0.001 significance
is based on dual-MS and dc-MS, ♯p < 0.01 significance
is based on dual-MS.
Fig. 4.15: Vitality of MG-63 osteoblasts. (i) blue
colour - 24 hours of cultivation on unrinsed samples,
(ii) red colour - 24 hours rinsing followed by
24 hours of cultivation. n = 3, Mann-Whitney-Utest,
∗∗∗p < 0.001.
4.3.3 Effect of copper on cell and bacteria growth
The biomedical properties of Ti-Cu films, in particular combination of antimicrobial effect
(against planktonic and biofilm Staphylococcus bacteria) with sufficient growth and vitality
of the MG-63 osteoblast cells, have been the main object of interest in our investigation
[XI]. The scanning electron microscope (SEM) images 6 of osteoblastic cells growing on the
reference TiAlV substrate surface and on the different types of Ti-Cu thin films prepared
by various sputtering techniques are shown in Fig.4.16. Two sets of sample images are
shown in Fig. 4.16: A - cells on unrinsed Ti-Cu surfaces after 24 hours of cultivation and B
- samples rinsed for 24h in DMEM before 24 hours of cultivation. On the non-deposited
6 The visible rough surface of the samples is caused by corundum blasting (titanium alloy Ti6Al4V with
surface modified by blasting with corundum particles at roughness R z ≈ 20µm was used as a substrate).
48

TiAlV reference surface, the osteoblasts span the ridges and demonstrate a flattened,
well-spread phenotype. However, the cells are markedly less spread and sparsely attached
on unrinsed Ti-Cu surfaces. This effect is most obvious for dual-HiPIMS film; because
of high amount of cytotoxic copper (Fig.4.14), the cell morphology is drastically impaired
and only very few rounded cells can be found, see Fig.4.16A - unrinsed samples. This
behaviour is also expressed in Fig.4.15 where the cell vitality is presented: the vitality
is significantly reduced after cell cultivation for 24 hours directly on the Ti-Cu deposited
surfaces. Particularly, the cell vitality decreases to a few percent on dual-HiPIMS films.
Interestingly, DMEM rinsing of the samples for 24h before cell cultivation leads to an
increase in cell vitality. The positive effect of sample rinsing on cell growth can be seen in
SEM images (Fig.4.16B - rinsed samples). During the rinsing time a substantial amount
of copper from the Ti-Cu film is released and soon afterwards the cells can grow onto the
substrate surface. This effect is especially obvious for films prepared under dual-HiPIMS
conditions when vitality increases by over 60%, i.e. an increase in cell vitality by about
one order of magnitude.
TiAlV
dc-MS
TiAlV
dc-MS
dual-MS
dual-HiPIMS
dual-MS
dual-HiPIMS
A - unrinsed surfaces
B - rinsed surfaces
Fig. 4.16: SEM images of MG-63 osteoblasts after 24 hours of cultivation immediately on the coppercontaining
surfaces (A) or after rinsing for 24 hours followed by 24 hours of cultivation (B). Cells on the
surfaces are marked with arrows. Bars = 20µm.
Staphylococcus epidermidis strain (ATCC35984) and Staphylococcus aureus strain
(ATCC25923) were employed for the study of the antimicrobial effect. The antimicrobial
effect of thin films on both planktonic bacteria is presented in Fig.4.17. Only Ti-Cu films
prepared using dual-HiPIMS technique demonstrated growth inhibition or killing effects on
both staphylococcal species. Killing occurs during the first 24 to 96 hours, which indicates
49

a high diffusion rate of copper species released into the cultivation medium. In contrast,
in the presence of the films prepared by dc-MS and dual-MS methods, both bacterial
species grew to the same density as in the presence of titanium reference samples. Thus,
these films do not show any antibacterial effect, which is caused by the low amount of
released copper (less than 30µg of copper), see Fig.4.14.
Finally we can conclude that cytotoxic effect is caused by copper species released from
film(s). It is shown that copper is released completely and quickly (within 24 hours) only
from dual-HiPIMS films. This effect probably results from the different crystallographic
structure characterized by large domains and a small lattice parameter. After that period
the copper release is finished and osteoblastic cells start to grow on the surface. It is
clearly shown in SEM images Fig.4.16 that the vitality of osteoblastic cells significantly
increases after 24 hours of rinsing, i.e. after the total release of copper particles. Hence,
the dual-HiPIMS is a promising technique for the deposition of films with a cytotoxic
effect followed by sufficient growth of osteoblastic cells.
CFU/ml
1E8
1E7
1
dc
dual-MS
dual-HiPIMS
reference
Plantonic bacteria - S. epidermidis
I Ti
= 400 mA
I Cu
= 100 mA
0 2 4 6 8 10
time in days
S.epidermidis
CFU/ml
1E8
1E7
dc
dual-MS
dual-HiPIMS
reference
10
Planktonic bacteria - S. aureus
I Ti
= 400 mA
I Cu
= 100 mA
1
0 2 4 6 8 10
time in days
S.aureus
Fig. 4.17: The antibacterial effect of Ti-Cu films tested on planktonic bacteria S.epidermidis (left graph)
and S.aureus (right graph). The tests ran for 10 days. CFU = colony forming units.
4.4 Size-controlled formation of Cu nanoclusters
Clusters are aggregates of atoms or molecules, generally intermediate in size between individual
atoms and particles large enough to be called bulk matter [183], usually composed
from 2 up to 10 6 - 10 7 atoms. Clusters and nanostructured surfaces are important mainly
for two reasons. First, the size regime of 1-100nm corresponds to the size of important
biological molecules. For example, clusters can immobilize proteins [184] or deactivate
bacteria [185], and can be used in cell-based assay, biosensors, microfabricated medical
50

devices [186] etc. In addition, nano-sized particles exhibit specific catalytic properties
which are lost in related bulk materials [187]. Secondly, the semiconductor industry is
now entering the nanometre regime [188]. Control of surface features on this length scale
is therefore essential for the fabrication of microelectronic devices (memories, processors
etc.). In particular, small islands of deposited materials can be considered as quantum
objects, too [189–192].
The cluster source that combines magnetron plasma sputtering with gas condensation
was introduced by Haberland [193, 194]. In this system the cluster growth is strongly
related with pressure, which is set to higher values (p c ∼100Pa) [193,195–197]. However,
sputtering sources are usually designed to be operated at low pressure p≤10Pa [192,197].
For that reason a novel way of magnetron sputtering with gas condensation are looked
for. It was shown in work [XIII] that the size of the clusters can be controlled by pulsed
magnetron sputtering. The details of the experimental arrangement are given there.
However, a few sentences about essential experiment facts are mentioned here, too.
The nanocluster source, manufactured by Oxford Applied Research, consists of three
main parts: (i) sputtering and aggregation chamber followed by (ii) quadrupole mass
filter and (iii) deposition chamber. The mass filter was used to analyze negatively charged
clusters 7 . The copper cluster mass/size and cluster efficiency growth were estimated from
the measurement of Mass Distribution Function (MDF) by the quadrupole mass filter
or from images of atomic force microscopy (AFM) see Fig.4.18, which was used as a
complementary technique. The cluster mass is expressed in atomic mass units (amu,
1amu=1.66·10 −27 kg) and attains usually the values 10 5 - 10 6 amu, which corresponds
to cluster size of about 10nm, see Fig.4.18. The polydispersity of the MDF can be
characterized by Full Width at Half Maximum (FWHM).
4.4.1 Effect of pulses on cluster formation
The formation of clusters is mainly determined by the number of atom particles and by
the pressure in gas condensation chamber p c , see Fig.4.19. It is obvious from the figure
that the cluster mass m CM increases nearly linearly with mean discharge current I m since
the amount of sputtered particles is proportional to the discharge current n Cu ∝(I m ,V c ).
However, at lower pressures pT g−cr ) and atoms evaporate from
the cluster surface. The absence of well pronounced critical temperature T g−cr for higher
7 Generally, it is expected that clusters are small separated particles on floating potential in the discharge
volume, i.e. negatively charged. However, there exist some works where larger fraction of positively
charged clusters was detected, too. Hence, this topic is still open and cluster charging mechanism has not
been fully understood yet. Similar phenomena of charging are solved in complex dusty plasmas whereby
the size of particles is expected to be larger, roughly micrometers.
51

2
1.8
1.6
AFM data
A
log−normal: m CM = 1.43 10
6 AMU
1.4
Frequency [%]
1.2
1
0.8
0.6
AFM image
0.4
0.2
0
10 5 10 6 10 7
cluster mass [AMU]
Mass Distribution Function
Fig. 4.18: The AFM image (left) of isolated Cu clusters on Si substrate deposited under pulsed magnetron
sputtering conditions: f = 1 kHz, t a /T = 50 %, p c = 25 Pa. The graph on the right shows the mass
distribution function of Cu cluster mass derived from AFM images.
pressures p>30Pa, see Fig.4.19, is caused by cooling effect of the buffer gas. Because
pressure is proportional to temperature p g ∝T g , free metallic atoms are thermalized and
can aggregate with primary clusters even at higher discharge currents. However, it is
supposed that T g−cr exists for higher pressures, too.
Fig.4.19 shows another important effect: cluster masses m CM formed in dc-pulse modulated
regime are by a few hundreds of percent higher. Effect of pulses on cluster mass is
also presented in Fig.4.20; cluster mass depends on the repetition frequency reaching a
maximum at about ∼1kHz. The explanation of cluster growth efficiency is probably as
follows. Relaxation time, i.e. pulse-off time, at lower discharge frequencies f ≈0.1-1kHz
reaches values of the order of ∼1ms. The time of atom diffusion and heat transport
towards the aggregation chamber walls of our dimensions is much higher; typically estimated
to be of the order of ∼0.1s for conventional dc mode [198]. Hence, during the
relaxation period metallic atoms are mostly thermalized in the buffer gas, propagate along
the aggregation tube due to pressure gradient and convert into clusters. The effect of
pulsed discharge vanishes at higher frequencies when relaxation time is too short and the
buffer gas is heated above the critical temperature.
This indicates that extension of relaxation time, realized by alteration of pulse, can
make cluster production more efficient, Fig.4.21. Depending on the frequency, the cluster
size/mass decreases or increases with increasing duty cycle. The biggest Cu clusters,
m CM ≈4·10 5 amu, are formed at lower frequencies (f =1kHz) and low duty cycles
(t a /T =20%). Effective cluster growth is caused by longer relaxation time, i.e. by thermalization
of sputtered metallic atoms in buffer gas during idle part of pulse period. The
effect of pulse discharge vanishes at high duty cycles and the mass of clusters is nearly
equal to masses produced in dc-regime.
52

Q
mass of clusters m CM
[10 5 amu]
1.8 p c
= 25 Pa, DC
p c
= 40 Pa, DC
1.5
1.2
0.9
0.6
0.3
p c
= 25 Pa, f = 1 kHz
p c
= 40 Pa, f = 1 kHz
0.0
0 200 400 600 800
discharge current I m
[mA]
mass of clusters m CM
[10 5 amu]
Q
1.5
1.2
0.9
0.6
0.3
0.1 1 10
discharge frequency f [kHz]
p c
= 25 Pa
p c
= 40 Pa
p c
= 90 Pa
I m
= 400 mA
exponential fit
Fig. 4.19: Comparison of dependencies m CM vs.
I m for dc and pulsed discharges.
Fig. 4.20: The dependencies of cluster mass m CM
on discharge repetition frequency f.
However, at very low frequencies, m CM is small and even decreases with increasing
duty cycle. Low frequencies and short duty cycles, i.e. HiPIMS discharges, suffer by
the lack of neutral sputtered atoms, which can easily form clusters. The same cause,
insufficient amount of sputtered particles, is responsible for smaller clusters formed at
high frequencies 8 (f =25kHz in Fig.4.21). Another important effect of different duty
cycles is shown in Fig.4.22. The mass distributions are broader, with asymmetry towards
high cluster mass (characterized by points 2 in FWHM) at low duty cycles, and become
narrow and symmetric with increasing duty cycle.
Table 4.3: Summary of AFM measurements for dc and dc-pulsed modulated regime for f = 1 kHz. The
abbreviations denote: m CM mass of bare Cu clusters, γ is cluster particle flux and Φ is cluster mass flux.
abbreviation unit dc t a /T =50 % t a /T = 20 %
m A CM [10 5 amu] 0.61 2.57 8.29
γ [1 cluster/1 µm 2 /1 min] 35 10 4
Φ [10 5 amu/1 µm 2 /1 min] 23.5 25.6 32.8
4.4.2 Cluster mass and cluster particle flux
Cluster masses estimated by quadrupole filter qualitatively corresponds with cluster masses
obtained by AFM measurement, see Fig. 4.22. Cluster surface coverage Γ (in particles
8 Differences measured between peak discharge current (pulse mode) and mean discharge current (dcmode)
are negligible at higher frequencies. From that one can assume that instantaneous amount of
sputtered Cu atoms in the active part of pulse will be comparable with instantaneous number of sputtered
particles in dc-mode. This implies that the total number of sputtered particles, integrated over the time
corresponding to one period, is lower in pulse mode than in the dc-regime because of sputtering absence
in the idle part of the period.
53

mass of clusters m CM
[amu]
Q
4x10 5
1x10 5
3x10 4
f = 1 kHz
f = 0.7 kHz
f = 25 kHz
I m
= 400 mA, p c
= 25 Pa
mass of clusters [amu]
1.0x10 6
8.0x10 5
FWMH-2
6.0x10 5
4.0x10 5
2.0x10 5
0.0
FWMH-1
cluster mass m
A
CM
(CuO)
cluster mass m Q CM
(Cu)
FWHM points 1 and 2
10 20 30 40 50 60 70 80 90 100
duty cycle t a
/T [%]
20 40 60 80 100
duty cycle t a
/T [%]
Fig. 4.21: The cluster mass m CM as a function of
duty cycles t a /T for different repetition discharge
frequencies.
Fig. 4.22: Comparison of cluster masses obtained
from mass filter m Q CM
and AFM measurements
m A CM .
per unit area) can be determined using the AFM, too. The measurements performed at
various deposition times t dep showed that the surface coverage Γ depends almost linearly
on the deposition time. Hence, to compare the surface coverage under different deposition
conditions, we refer to the cluster particle flux γ, which is calculated by dividing the
particle count per unit area by the deposition time, i.e. γ =Γ/t dep . Furthermore, we can
calculate the cluster mass flux Φ, which is defined by multiplying the cluster particle flux
with the most probable cluster mass m CM , i.e. Φ=γ·m CM .
The results for dc and dc-pulse modulated regime at f =1kHz are given in Tab4.3.
The highest cluster particle flux γ is obtained for dc conditions. Interestingly, the particle
flux γ increases with increasing duty cycle, whereas the cluster mass flux Φ is nearly
constant (within experimental error). This means that similar mass is deposited onto
the surface and the increase of cluster mass is due to decrease of number of deposited
particles. Such behaviour could be considered as some kind of mass conservation, which
might be again explained using the critical gas temperature: for low duty cycles the buffer
gas temperature is lower than critical and large clusters can be formed. Increasing the
duty cycle might also increase the temperature of the buffer gas, shifting the aggregationevaporation
equilibrium to smaller clusters.
54

5. Conclusion
High power impulse magnetron sputtering (HiPIMS) discharges, their properties and their
effect on growth of thin films are presented in the thesis. The first work related with
HiPIMS appeared in 1999 [27] and a few years later the topic has been established as
an individual field 1 . Most important aspects and features of HiPIMS discharges are
illustrated on our own results, published in several scientific journals; related papers
compose the second part of this work. Hence, the thesis brings a review on scientific and
engineering state of HiPIMS art.
HiPIMS discharges produce ultra-high dense plasma (plasma density is higher by about
two-three orders of magnitude than in dc magnetron discharges) with large fraction of
metal ionized species. Hence, the ion flux towards the substrate is large and leads to
growth of smooth and dense films with higher adhesion, allows to control the crystallographic
phase and microstructure, and improves mechanical and optical features. It is
important that plasma density during HiPIMS pulse is typically huge, due to high applied
power density, but their mean values, averaged over the whole period, are similar to those
of with dc discharges. Hence, total heat load is low and allows to deposit thermal sensitive
substrates. Further, it is shown that pulse operation provides new and additional parameters
to control the deposition process, tailor the properties and optimize the performance
of elemental composition. For these reasons HiPIMS is an attractive alternative which
should be implemented in industrial coating processes 2 . However, HiPIMS have still some
disadvantages, such as lower deposition rate and higher cost of pulse power supplies.
HiPIMS discharges have been intensively studied since the last six, seven years. This
period is too short to make distant future forecast of further development. However, for
reasons mentioned above we do not think that HiPIMS will become a regular tool in
the coating industry. We expect that HiPIMS will be soon established as a special, but
frequently used, technique for specific, highly precise deposition of smaller and complex
shaped substrates. For example, deposition of complex thin films with required crystallography
is the main HiPIMS outcome in the near future. Further challenges of HiPIMS
improvement can be optimization of reactive deposition (especially -oxide films) and upscaling
related with large discharge volume production (construction and development
of sufficient pulse power supplies has to be solved). These new challenges need better
understanding of the underlying physical phenomena and call for advanced exploration
of all the related processes. We hope that we will successfully continue in our research in
this exciting area.
1 HiPIMS as a separate conference topic was introduced first during the Tenth International Conference
on Plasma Surface Engineering 2006 held in Garmisch-Partenkirchen.
2 A few designers and producers of deposition technology equipment (Hauser TechnoCoating, CemeCon,
IonBond Netherlands, SVS Vacuum Coating Technologies) have already partially implemented
HiPIMS in their programme.
55

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60

List of own publications
In the list are pointed out our publications which are referred in the thesis. Attached
papers compose the second part of the work.
[I] V. Stranak, M. Quaas, H. Wulff, Z. Hubicka, S. Wrehde, M. Tichy, R. Hippler, Fromation of
TiO x films produced by high-power pulsed magnetron sputtering. J. Phys. D: Appl. Phys. 41,
(2008), 055202.
[II] V. Stranak, M. Cada, M. Quaas, S. Block, R. Bogdanowicz, S. Kment, H. Wulff, Z. Hubicka,
Ch. A. Helm, M. Tichy, R. Hippler, Physical properties of homogeneous TiO x films prepared by
high power impulse magnetron sputtering as a function of crystallographic phase and nanostructure
J. Phys. D: Appl. Phys. 42, (2009), 105204.
[III] V. Stranak, Z. Hubicka, P. Adamek, J. Blazek, M. Tichy, P. Spatenka, R. Hippler, S. Wrehde,
Time-resolved probe diagnostics of pulsed dc magnetron discharge during deposition of TiO x layers.
Surf. Coat. Technol. 21, (2006), 2512–2519.
[IV] V. Stranak, S. Drache, R. Bogdanowicz, H. Wulff, A-P. Herrendorf, Z. Hubicka, M. Cada,
M. Tichy, R. Hippler, Effect of mid-frequency discharge assistance on dual-high power impulse
magnetron sputtering. Surf. Coat. Technol., DOI 10.1016/j.surfcoat.2011.11.043, in print, available
on-line.
[V] V. Stranak, M. Cada, Z. Hubicka, M. Tichy, R. Hippler, Time-resolved investigation of dual high
power impulse magnetron sputtering with closed magnetic field during deposition of Ti-Cu thin
films. J. Appl. Phys. 108, (2010), 043305.
[VI] V. Stranak, S. Drache, M. Cada, Z. Hubicka, M. Tichy, R. Hippler, Time-resolved diagnostics of
dual high power impulse magnetron sputtering with pulse delays of 15µs and 500µs. Contrib. Plasma
Phys. 51(2-3), (2011), 237–245.
[VII] V. Stranak, R. Bogdanowicz, S. Drache, M. Cada, Z. Hubicka, R. Hippler, Dynamic Study
of Dual High-Power Impulse Magnetron Sputtering Discharge by Optical Emission Imaging.
IEEE Trans. Plasma Sci. 39(11), (2011), 2454–2455.
[VIII] V. Stranak, M. Quaas, R. Bogdanowicz, H. Steffen, H. Wulff, Z. Hubicka, M. Tichy, R. Hippler,
Effect of nitrogen doping on TiO x N y thin film formation at reactive high-power pulsed magnetron
sputtering. J. Phys D: Appl. Phys. 43, (2010), 28523.
[IX] V. Stranak, M. Tichy, H. Steffen, S. Wrehde, R. Hippler, Investigation of plasma parameters
in the DC planar magnetron in balanced and unbalanced mode. Czech. J. Phys. 54, (2004),
C822–C827.
[X] V. Stranak, J. Blazek, S. Wrehde, P. Adamek, Z. Hubicka, M. Tichy, P. Spatenka, R. Hippler,
Study of Electronegative Ar/O 2 discharge by means of Langmuir probe. Contrib. Plasma
Phys. 48(5-7), (2008), 503–508.
[XI] V. Stranak, H. Wulff, H. Rebl, C. Zietz, K. Arndt, R. Bogdanowicz, B. Nebe, R. Bader, A. Podbielski,
Z. Hubicka, R. Hippler, Deposition of Thin Titanium-Copper Films with Antimicrobial
Effect by Advanced Magnetron Sputtering Methods. Mat. Sci. Eng. C 31, (2011), 01512–1519.
[XII] V. Stranak, H. Wulff, R. Bogdanowicz, S. Drache, Z. Hubicka, M. Cada, M. Tichy, R. Hippler,
Growth and properties of Ti-Cu films with respect to plasma parameters in dual-magnetron
sputtering discharges. Eur. Phys. J. D: Appl. Phys. 64, (2011), 2454–2455.
[XIII] V. Stranak, S. Block, S. Drache, Z. Hubicka, C. A. Helm, L. Jastrabik, M. Tichy, R. Hippler,
Size-controlled formation of Cu nanoclusters in pulsed magnetron sputtering system. Surf. Coat.
Technol. 205, (2011), 2755–2762.
61