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\begin{document}

\title{Formation of pure anatase TiO\textsubscript{2} by reactive pulsed dc
magnetron sputtering: method for controlling target poisoning state}



\author[1]{Nikolai Desch}%
\author[1]{Markus Lake}%
\affil[1]{Hochschule Niederrhein - Campus Krefeld Sud}%


\vspace{-1em}



  
  \date{May 25, 2024}


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\selectlanguage{english}
\begin{abstract}
The use of pulsed dc-sputtering sources for reactive magnetron
sputtering with oxygen offers a possibility to suppress the negative
effects of target poisoning (such as arcing). This results in a wide
process range for the selection of a desired operating point. The
control of target poisoning plays a major role in maintaining constant
coating properties and affects the stoichiometry of the reactive
coating, as well as the coating rate and the economic impact of the
coating process. In a hysteresis, the target poisoning during the
reactive sputtering of titanium under oxygen addition proceeds
nonlinearly. Without the use of a suitable target poisoning control
technique, the sputtering process can abruptly change to an unstable
state. As a result, variations of stoichiometry can occur during the
deposition process. A proven method for maintaining a stable reactive
sputtering process is the control of oxygen flow with the input variable
target voltage. By determining the typical oxygen hysteresis at constant
target power and constant argon flow, an operating point for the control
loop is derived. The desired target voltage then serves as the input
variable for the control loop of the target poisoning. The controlling
technique for target poisoning is a basic requirement for the production
of the photolytic active anatase phase of titanium dioxide
(TiO\textsubscript{2}) using reactive magnetron sputtering. The
photocatalytic equipment of surfaces with a titanium dioxide coating in
the anatase phase can be realized with the reactive pulsed dc magnetron
sputter ion plating process (DC-MSIP). The pulsed DC-MSIP process
facilitates coating a variety of surfaces at temperatures below 200 \selectlanguage{ngerman}°C
in an environmentally friendly manner.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy




Nikolai Desch ORCID iD: 0000-0002-0384-8248

Markus Lake ORCIC iD:~0000-0002-3535-8454

Formation of pure anatase TiO\textsubscript{2} by reactive pulsed dc
magnetron sputtering

Method for controlling target poisoning state

Nikolai Desch\textsuperscript{1*}, Markus Lake\textsuperscript{1}

\textsuperscript{1} Hochschule Niederrhein, Reinarzstra\selectlanguage{ngerman}ße 49, 47805
Krefeld, Germany

\textsuperscript{*}corresponding author: + 49 2151 822-5086 \textbar{}
\href{mailto:nikolai.desch@hs-niederrhein.de}{\nolinkurl{nikolai.desch@hs-niederrhein.de}}

\textbf{Keywords:} reactive sputtering, dc magnetron sputtering
(DC-MSIP), pulsed magnetron sputtering (PMS), oxygen, target poisoning,
magnetron sputter ion plating, TiO\textsubscript{2}, anatase

\section*{Abstract}

{\label{abstract}}

The use of pulsed dc-sputtering sources for reactive magnetron
sputtering with oxygen offers a possibility to suppress negative effects
of target poisoning (such as arcing). This results in a wide process
range for the selection of a desired operating point. The control of
target poisoning plays a major role in maintaining constant coating
properties and affects the stoichiometry of the reactive coating, as
well as the coating rate and the economic impact of the coating process.
The target poisoning during the reactive sputtering of titanium under
oxygen addition proceeds nonlinear in a hysteresis. Without the use of a
suited target poisoning control technique the sputtering process can
abruptly change to an unstable state. As a result, variations of the
stoichiometry can occur during the deposition process. A proven method
for maintaining a stable reactive sputtering process is the control of
oxygen flow with the input variable target voltage. By determining the
typical oxygen hysteresis at constant target power and constant argon
flow, an operating point for the control loop is determined. The desired
target voltage then serves as the input variable for the control loop of
the target poisoning. The controlling technique for target poisoning is
a basic requirement for the production of the photolytic active anatase
phase of titanium dioxide by means of reactive magnetron sputtering. The
photocatalytic equipment of surfaces with a titanium dioxide coating in
the anatase phase can be realized with the reactive pulsed dc magnetron
sputter ion plating process (MSIP). The pulsed dc MSIP process provides
the ability to coat a variety of surfaces at temperatures below 200 °C
in an environmentally friendly manner.

\par\null

\section*{1. Introduction}

{\label{introduction}}

Surface technology describes the summary of a large variety of processes
that are used when conventional materials no longer meet increasing
demands for wear, corrosion or special tribological or optical
requirements. The overall function of a component is therefore often
divided into subfunctions. In this case, the surface coating takes a
specific function and thus enables the selection of a less expensive
base material while at the same time fulfilling the range of
requirements for the entire component. Today's application areas of
surface technology extend far beyond the function of producing
decorative, tribological or corrosion-protective surface coatings.
Surface engineering processes are nowadays already an integral part of
production and manufacturing, and are often the driving force behind the
implementation of product requirements. Furthermore, economic aspects
play an important role for the use of coating processes. {[}1{]}

All surfaces that we come into contact in our everyday activities have a
special role in spreading pathogens. Especially high traffic touched
surfaces in public transport or health care can be affected unnoticed.
Titanium dioxide is photocatalytically active in its anatase phase and
can therefore be used as a self-cleaning, antimicrobial surface coating
system. Particularly with regard to the formation and spread of bacteria
(e.g. coli bacteria), germs (e.g. MRSA), fungi and algae, anatase can
assist in minimizing their formation and spread. In healthcare,
follow-up diseases caused by contact with germs, bacteria or viruses are
one of the most frequent causes of death worldwide {[}2{]}. The
so-called multi-resistant germs (MRSA) represent a further problem. In
the European economic area, more than 670,000 infections and 33,110
deaths could be attributed to antibiotic-resistant germs during 2015.
The total number of infections has increased in the period from 2007 to
2015. Approximately 64~\% of all cases in this period occurred due to
infections in relation to health care facilities. At least half of the
infections are considered to be preventable {[}3{]}. All surfaces that
patients come into contact with during their stays in a healthcare
facility have a major impact on the spread of infections and diseases. A
common door handle used by patients and visitors can thus become a
critical transmission surface for bacteria and germs. In the analysis of
statistical data examining infections for their cause, it was determined
that in 21.1~\% of the cases the reason was the contact with
contaminated medical equipment and in 19.8~\% of the cases contamination
of the environment was responsible for an infection {[}4{]}. The
photocatalytic activity of titanium dioxide is based on the release of
radicals on the surface which can decompose organic substances in an
advanced oxidation process (AOP). The photocatalytically active surface
coating offers a physical effect that is able to decompose organic
deposits upon irradiation with ultraviolet light. The UV light exposure
initiates an advanced oxidation process of the organic substances
{[}5{]}. A potential formation of a biofilm and a possible risk of
infection is thereby reduced and in the best case completely prevented
{[}6{]}. Further applications of titanium dioxide can be found in the
treatment of wastewater with pharmaceutically contaminated residues.
{[}5, 7{]}

\section*{2 Magnetron Sputter Ion Plating
(MSIP)}

{\label{magnetron-sputter-ion-plating-msip}}

Since the first development of an apparatus for sputtering solids by
bombarding particles in a vacuum by Penning in 1936, numerous
technological developments and research efforts have driven magnetron
sputtering as a coating technology. Approximately 50 years ago, the
development of sputtering technology progressed greatly and was
characterized by successes in improving deposition rates, process
stability, and cost efficiency in the following years. At present,
magnetron sputtering is widely known as a reliable and cost-effective
coating process that has already become established in numerous
industries {[}8{]}. The MSIP-PVD process is also known as cathode
sputtering and can be used for the deposition of metallic coatings
(e.g., Ti, Al, Cr), nitride coatings (e.g., CrAlN, TiAlN, TiN, CrN), or
oxide ceramic coatings (e.g., Al\textsubscript{2}O\textsubscript{3},
Cr\textsubscript{2}O\textsubscript{3}). The coating can also be produced
in several layers, in so-called multilayer coatings. The base material
for the deposition of a coating is called the target and is fixed onto a
target carrier in the deposition source, the magnetron. Here, the target
can be a pure solid material or it can be bonded to a copper backing
plate. Bonded targets are often preferred when the target material is
expensive and, depending on the target material, it can be necessary to
use a backing plate, for instance, if the material is brittle. In the
magnetron, magnets, cooling, and a current feed-through to the target
are present behind the target. The target is also called the cathode
because it is connected to the negative pole of the power supply,
whereby it is electrically isolated from the rest of the magnetron. The
anode of the magnetron is frequently connected to the coating chamber as
a reference potential and the combination of a magnetron source with a
power supply defines the particular sputtering process. The selection of
different power sources for cathode sputtering allows for a large number
of different process variations. Proven processes in practice are direct
current magnetron sputtering (DCMS), pulsed magnetron sputtering (PMS),
and pulsed high current magnetron sputtering (HIPIMS). The DCMS process
is well-suited for sputtering metallic target materials. However, by
choosing the PMS method with a power source that can be operated with a
pulse mode in the range of medium frequencies (e.g., 100 kHz to 350
kHz), even nonconductive target materials (e.g.,
Al\textsubscript{2}O\textsubscript{3} or SiO\textsubscript{2}) can be
sputtered {[}9{]}. The unipolar or bipolar pulsed operation of magnetron
sputtering power supplies offers a wide range of variations in
parameters by changing the pulse frequency and pulse duty cycle. With
regard to the development of new coating processes or the optimization
of existing coating systems, this results in a wide range of potential.

\par\null

\section*{2.1 Reactive pulsed DC-MSIP}

{\label{reactive-pulsed-dc-msip}}\par\null

Sputtering of pure metallic coatings usually takes place using argon as
an inert process gas. In this process, the inert argon atoms pass
through the process of ionization and recombination with electrons
several times and are thus frequently reused before they are captured by
the vacuum pumping unit. If a reactive gas, such as oxygen or nitrogen,
is also introduced, it chemically combines with the target plasma and
forms an oxide in the case of oxygen or a nitride in the case of
nitrogen. For homogeneous reactive layer formation, a uniform and
controllable supply of the reactive gas is essential. The reactive gas
is introduced in close range to the target. Usually, tubes with small
holes are placed along the target for a homogeneous supply of the
process and reactive gases. Similarly to inert gas, the reactive gas
becomes ionized and is accelerated onto the target. Therefore, adding to
the reaction with the target plasma also helps to contribute to the
sputtering process. Since the reactive gas can also be ionized, it is
possible to sputter without inert process gas in a pure reactive gas
atmosphere. However, this method is rarely used in practice, and
generally, a constant flow is maintained with the process gas while the
reactive gas is added in a pressure-controlled manner. The control of
the reactive gas via the partial pressure can be used as a manipulated
variable for the stoichiometry of the layer formation {[}10, 11{]}.

Three main processes are involved in a reactive deposition, whereby the
main process of deposition is sputtering on the target surface.
Simultaneously, when reactive gas is introduced into the deposition
chamber, getter processes occur on the target surface, the substrate
surface, and the chamber walls of the deposition equipment. Getter
processes occur when layers are formed by the interaction of reactive
gas atoms and they take place in various forms on all surfaces in a
coating system.~ Getter processes that take place on the target surface
are also known as ``target poisoning.'' As the supply increases, the
reaction with reactant gas causes the sputtering yield to decrease
because of the increasing occupancy of the target. The secondary
electron yield thus increases and the plasma impedance decreases,
resulting in a reduced cathode voltage at the magnetron. The sum of the
events leads to a hysteresis behavior in the process of reactive
magnetron sputtering. While the stoichiometry in the deposition of
nitrides can generally be relatively easily controlled via the partial
pressure of the reactive gas, the process of target poisoning in the
reactive sputtering of oxides behaves much more dynamically and is
therefore more difficult to control with conventional DC-MSIP. As the
target surface is also covered by an oxide layer during the deposition
process, the resulting lower electrical conductivity of the oxide layer
leads to increased arcing, which in turn results in an extremely
unstable sputtering process, creating undesired droplets of molten
target material, and can even destroy the sputtering target. The use of
a pulsed DC source for magnetron sputtering can overcome these
disadvantages as the pulsed mode of the magnetron with frequencies
ranging from 100 kHz to 350 kHz avoids arcing on the target surface,
since it is covered with an oxide layer.



\section*{3. Experimental Design and
Results}

{\label{experimental-design-and-results}}

The Raman analysis of the coatings was performed with a Renishaw InVia
Raman spectroscope, and a green laser with a wavelength of 532 nm and a
laser power of 0.75 mW was used. All titanium dioxide coatings were
produced on the Metaplas Domino.Mini industrial PVD coating system. In
this work, pulsed DC-MSIP is the applied coating technology. The
titanium target used has a size of 456 mm x 81 mm x 10 mm and is made of
99.999 \% pure titanium. A sputtering power of 7.0 kW was applied for
all experiments, which corresponds to a power density of approximately
19 W/cm². Argon with a purity of 99.999 \% was used as the process gas.
The reactive gas oxygen has a purity of 99.998 \%. Conventional slides
were used as substrate material (soda-lime float glass slides 76 mm x 26
mm x 1.5 mm) and the maximum substrate temperature was recorded using a
drag gauge thermometer. The magnetic field strength perpendicular to the
target was 24.9 mT. For the coating experiment, the glass slides were
ultrasonically cleaned in technical ethanol (96 \%) for 10 minutes and
the cleaned slides were subsequently charged on the substrate table.
After loading the coating chamber, the chamber was evacuated to a
starting vacuum of 4.0 x 10\textsuperscript{-3} Pa. Afterwards, without
additional heating, an argon-ion sputter etching was performed for 15
minutes. Following the etching process, the coating process was carried
out using the reactive pulsed magnetron sputtering method, also without
additional heating. The turbomolecular pump (Turbovac Mag W 1600 iP) was
kept constant at 25,200 rpm during the coating experiment. All coating
experiments were operated in power-constant mode with 7.0 kW dc pulsed
sputtering power. For all experiments, the applied pulse parameters were
kept constant. Figure 1 shows the voltage (a) and current (b) curves
during the PMS process, recorded with a Tektronix TDS 2001C
oscilloscope, measured at the Advanced Energy Pinnacle Plus+ power
source on the Metaplas Domino.Mini coating system. The applied pulse
frequency was 350 kHz with a switch-on time
(\emph{t}\textsubscript{\emph{on}}) of 1.7 \selectlanguage{greek}μ\selectlanguage{english}s and the switch-off
(?\textsubscript{\emph{off}}) in this case was 1.1~\selectlanguage{greek}μ\selectlanguage{english}s. The pulse
duration T was 2.8 \selectlanguage{greek}μ\selectlanguage{english}s. The switch-on time corresponds to a 60 \% duty
cycle
(\emph{t}\textsubscript{\emph{on~}}\emph{/t}\textsubscript{\emph{on}}\emph{+t}\textsubscript{\emph{off}}\emph{).}
Further detailed explanations of the pulsed dc mode can be found in
{[}12--14{]}.{}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/Figure-1}

\end{center}
\end{figure}

\textbf{Figure 1:~} Voltage (a) and current (b) charts of the reactive
pulsed dc magnetron sputtering process, 350 kHz pulse frequency, 7 kW
constant power mode, 60 \% duty cycle~ ~ \textbf{}

\par\null\par\null

\section*{3.1 Hysteresis tests for the determination of target poisoning
state}\label{hysteresis-tests-for-the-determination-of-target-poisoning-state}

In the common DCMS, a reactive sputtering process of an oxide layer
results in an electrical charging of the magnetron and thus leads to
arcing. The PMS method pulses the sputtering power and in the off times,
a charge of the magnetron can discharge again. The pulsed operation of
the magnetron enables a consistent stable deposition process for oxide
films. Figure 2 shows the magnetron voltage recorded during the pulsed
magnetron sputtering process at a constant argon flow of 120.0 sccm. The
addition of oxygen was linearly varied in each case over a five-minute
period. In this case, the sputter power was 7.0 kW with a dc pulsed mode
at 350~kHz and a switch-on time of 1.7 \selectlanguage{greek}μ\selectlanguage{english}s. The hysteresis behavior of
the reactive sputtering process of titanium dioxide can be used to
explain the effect of target poisoning. Figure 2 shows six states of
target poisoning from a) to f) on a schematic cross-section of a target
with the typical sputtering erosion.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/Figure-2}

\end{center}
\end{figure}

\textbf{Figure 2:} (1) Magnetron voltage during the reactive pulsed dc
magnetron sputtering process with target poisoning regions (R1-R3) and
(2) schematic illustration of target poisoning state

The states of the target poisoning were assigned to the respective areas
in the upper graph in Figure 2 which represents the progression of
magnetron voltage over the linear increase in oxygen flow from 0 sccm to
96 sccm which was followed by a direct linear decrease in oxygen flow to
0 sccm. Recording a hysteresis plot during magnetron sputtering is an
important step in detecting sputtering regions and the target poisoning
state. In a hysteresis experiment, all settings are kept constant except
for the variation of the reactive gas flow. At the beginning of the
hysteresis experiment, the metallic sputtering region was present
(approximately up to 30 sccm), since no target poisoning had occurred (a
in Figure 2). As the oxygen flow increased, the target surface poisoning
commenced outside the sputtering erosion (b in Figure 2). The target
poisoning increased from 30 sccm to approximately 60~sccm oxygen flow to
the point where the target surface was completely poisoned except for
the sputtering erosion (c in Figure 2). The high ion density in this
region provides enhanced sputtering in these areas of the target and is
also responsible for keeping the sputtering erosion clear until shortly
before the transition region (Region 2 in Figure 2). The magnetron
voltage is very constant in the metallic region and only shows a small
increase of approximately 20 V. Just past a slight increase in oxygen
flow, in the metallic region at about 60 sccm oxygen flow, the process
shifted to the unstable transition region (Region 2 in Figure 2). The
transition region was characterized by an increase in target poisoning
until partial saturation of the sputtering erosion (d in Figure 2). A
relatively small increase of 15 sccm from 65 sccm oxygen flux to 80 sccm
caused a rapid decrease in the magnetron voltage from 590 V to 450 V. In
the transition area, the magnetron voltage displayed a rapid change over
to the completely poisoned area. A small increase in oxygen flow of 10
sccm caused a sharp decline in magnetron voltage of 140 V and positioned
the sputtering process to shift into the fully poisoned sputtering area.
A further increase of the oxygen flow above 90 sccm only caused a slight
decrease in the magnetron voltage but led to the complete poisoning of
the target surface (e in Figure 2).~ Once the state of full poisoning
was reached at 96 sccm, the flow was decreased to the starting value for
the same duration. Based on the magnetron voltage plot, as the oxygen
flow decreased, it became evident that the target poisoning was not
behaving similarly to the increase of oxygen flow. The steep decrease in
the transition region was not reversed to the same extent during the
decrease in the oxygen flow. The total target poisoning only weakly
decreased over a relatively large decrease from 70 sccm to approximately
25 sccm (f in Figure 2). If complete target poisoning occurs, the target
is only atomized slowly due to the complete occupancy of titanium
dioxide with the same decrease in oxygen flow, whereby the relatively
slow atomization compared to the initially metallic region is the main
influence. Since the target was completely covered with oxide and oxygen
was constantly supplied, the sputtering process only reached the
metallic sputtering region directly after a large decrease in oxygen
flow. Since the magnetron voltage is a value that can directly be
measured and the flux control is a manipulated value, both values can be
used for a control loop to control the sputtering region of the reactive
deposition. Usually, a magnetron voltage from the transition range, e.g.
500 V, is selected as desired value, and in a regulating concept of this
kind, the oxygen flow is then adjusted to the desired magnetron
voltage.{}Such a feedback system is used on the PVD coating system
Metaplas Domino.Mini, at the Niederrhein University of Applied Sciences,
for the sputtering of oxides. Since hysteresis in reactive magnetron
sputtering is always system dependent and influenced by the respective
systems and plant conditions, the measurement of the hysteresis behavior
is important for the investigation of an efficient and stoichiometric
operating set point for the deposition of oxides.

{}~

The coating process, consisting of the generation of a bonding layer and
the functional layer, was carried out in four consecutive phases. First,
the magnetron was ramped up for three minutes at a constant argon flow
of 120 sccm in front of a shading plate. During start-up, the sputter
power was ramped up in two minutes from 0.5 kW to 7.0 kW followed by a
ramp-down from 7.0 kW to 2.0~kW in one minute. During this process, the
target surface was cleaned of any oxide layers. In the second phase, the
shading plate was moved away from the magnetron and a pure titanium
layer was deposited on the substrate for three minutes. The shading
plate was subsequently moved back in front of the magnetron. In the
third phase, the sputtering power was again increased to 7.0 kW. As soon
as the sputtering power of 7.0 kW was reached, the oxygen control
circuit was activated with the desired magnetron voltage. For a period
of three minutes, the shading plate continued to be coated. During the
fourth phase, the shading plate was moved away from the magnetron and
the reactive coating process with titanium dioxide was carried out for
five hours. The coating phase was followed by a one-hour cooling phase.
All coating experiments were conducted with the same etching process,
and with no additional heating and no BIAS during the coating process.
The process gas flow was changed in three stages starting at 120 sccm,
subsequently moving to 160~sccm, and finally up to 200 sccm. For each
process gas flow, three operating points out of the transit region of
target poisoning were chosen (Region 2 in Figure 2). The hysteresis
behavior was generally divided into two stable sputtering areas, namely
metallic (Region 1 in Figure 2) and reactive (Region 3 in Figure 2). An
unstable transition area (Region 2 in Figure 2) was observed between the
two stable areas. Although the metallic mode of operation has the
advantage of high deposition rates, reactively deposited films from this
region are substoichiometric. In the reactive sputtering region, the
deposited layers are ideally stoichiometric with the disadvantage that
the deposition rates are a factor of 5 to 10 times smaller than in the
metallic sputtering region. The hysteresis behavior is prominent in the
sputtering of oxide layers, e.g. Al\textsubscript{2}O\textsubscript{3}
or TiO\textsubscript{2}, and the process window for an economical and
stoichiometric deposition is located in the transition area between the
reactive and metallic sputtering areas. The sputtering behavior in the
transition region poses unique challenges for process control since a
relatively small change in the gas regulation of oxygen can lead to
strong change target poisoning {[}15{]}.



\section*{3.2 Experimental design}

{\label{experimental-design}}\par\null

For the synthesis of the anatase titanium dioxide modification, the
relevant literature was reviewed. Based on the findings, the absolute
process pressure proved to be an important process parameter whereas it
was concluded that additional heating and application of BIAS voltage
were not important to improve the formation of the anatase phase. Table
1 presents an overview of successfully prepared anatase phases from
various publications. {}

\par\null

\textbf{Table 1:} Comparison of PVD process parameters for the
deposition of anatase titanium dioxide\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\textbf{Sputter Process} & \textbf{Pressure {[}Pa{]}} & \textbf{Oxygen
ratio} \textbf{in Argon/Oxygen-Mix {[}\%{]}} &
\textbf{Reference}\tabularnewline
\midrule
\endhead
DC & 1.0 & 20~~~~~~~~~~~~ & {[}10{]}\tabularnewline
& 1.0 & 30 & {[}26{]}\tabularnewline
& 2.7 x 10\textsuperscript{-1} & 100 & {[}27{]}\tabularnewline
& 2.7 x 10\textsuperscript{-1} & 16.6 & {[}28{]}\tabularnewline
& 7.0 x 10\textsuperscript{-1} & 30 & {[}29{]}\tabularnewline
& 7.0 x 10\textsuperscript{-1} & 14.6 & {[}27{]}\tabularnewline
pulsed DC & 4.0 & 34.4 & {[}16{]}\tabularnewline
& 4.0 x 10\textsuperscript{-1} & 30 & {[}30{]}\tabularnewline
& 1.5 & 30 & {[}17{]}\tabularnewline
& 6.0 x 10\textsuperscript{-1} & 20 & {[}31{]}\tabularnewline
\bottomrule
\end{longtable}

To ensure that this range can be achieved during reactive pulsed
magnetron sputtering on the present coating system, the process pressure
is initially determined at different process gas settings and constant
reactive gas flow (Table 2). The supply of argon and oxygen combined
with the power control of the vacuum pumping system affects the final
chamber pressure. For a first rough selection, an extra oxygen addition
of 70 ml/min has been chosen. This acted as an initial guess for the
choice of argon flow. In the real sputtering process, deviations are to
be expected due to getter processes of the titanium atomization and the
subsequently selected operating point. Table 2 shows the results of this
experiment at constant addition of 70 ml/min oxygen and under variation
of the argon flow. Here, the pure process pressure was recorded without
the sputtering process being in progress. The turbomolecular pump was
kept at a constant 60~\% power. The argon flow was varied from 100
ml/min in 20 ml/min steps up to a maximum of 240 ml/min. Possible
deviations in the real process were accepted at this point and the
determination at this point was only used to generate suitable changes
in the process pressure via the control of the argon flow. As an
approach, the real experiments will proceed from lower range of process
pressure to higher range process pressure.

\par\null

\textbf{Table 2:} Determination of the absolute process pressure\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
\textbf{Argon flow {[}ml/min{]}} & \textbf{Process pressure {[}Pa{]}} &
\textbf{Process pressuse with addition of 70 sccm Oxygen flow
{[}Pa{]}}\tabularnewline
\midrule
\endhead
100 & 3.7 x 10\textsuperscript{-1} & 0.65 x
10\textsuperscript{-1}\tabularnewline
1200 & 4.4 x 10\textsuperscript{-1} & 0.75 x
10\textsuperscript{-1}\tabularnewline
140 & 5.3 x 10\textsuperscript{-1} & 9.3 x
10\textsuperscript{-1}\tabularnewline
160 & 6.3 x 10\textsuperscript{-1} & 1.1\tabularnewline
180 & 7.2 x 10\textsuperscript{-1} & 1.5\tabularnewline
200 & 8.4 x 10\textsuperscript{-1} & 2.1\tabularnewline
220 & 1.0 & 2.8\tabularnewline
240 & 1.4 & 3.2\tabularnewline
\bottomrule
\end{longtable}

Possible deviations in the real process were accepted at this point and
the determination was only used to generate suitable changes in the
process pressure via the control of the argon flow, whereas the actual
experiments proceeded from a lower range of process pressure to a higher
range process pressure. In a real sputtering process, deviations are to
be expected due to target poisoning, getter processes, and the chosen
operating point, which means that the chamber pressure will be slightly
lower or higher than predicted, depending on the choice of the operating
point. The values of 120 sccm, 160 sccm, and 200 sccm were selected as
suitable process gas settings and the investigation of the hysteresis
behavior at the three predefined process gas settings commenced. As
described in Chapter 3.1 in Figure 2, three hysteresis courses of the
target poisoning were recorded using this procedure of hysteresis
determination. Figures~3, Figure~4, and Figure~5 portray the
determination of working points from three hysteresis experiments,
whereby the choice of the working points in each case was made from the
transition area of the target poisoning. All three experiments were
determined at a constant target power of 7.0 kW. The hysteresis of
Figure 3 was determined at a constant argon flow of 120 sccm, while that
in Figure 4 was measured at 160 sccm and Figure 5 at 200 sccm. Three
operating points were selected from each of the three hysteresis
patterns, resulting in a total of nine experimental settings.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/Figure-3}

\end{center}
\end{figure}

\textbf{Figure 3:} Hysteresis characteristics at constant argon flow 120
ml/min

\par\null\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/Figure-4}

\end{center}
\end{figure}

\textbf{Figure 4:} Hysteresis characteristics at constant argon flow 160
ml/min

\par\null\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/Figure-5}

\end{center}
\end{figure}

\textbf{Figure 5:} Hysteresis characteristics at constant argon flow 200
ml/min

\par\null

\section*{3.3 Results}

{\label{results}}\par\null

The most important process parameters of the nine coating experiments
are summarized in Table 3, whereby the oxygen control range presents the
limits of regulation to maintain the specified magnetron voltage. During
the coating process, over the duration of five hours, the real magnetron
voltage was controlled to the specified magnetron voltage with the
oxygen control loop, which was able to maintain the operating points
very closely, and thus the absolute process pressure remained constant
over the entire coating period. The maximum substrate temperature was
measured with a drag indicator thermometer, whereby the lowest substrate
temperature measured is 110 \selectlanguage{ngerman}°C and the highest substrate temperature is
220 °C. Furthermore, the lowest settled oxygen ratio is 13 \% and the
highest is 33 \%. The scanning electron microscopy images of the
fracture surfaces of coating experiments No.~1 to No. 9 were captured
using a Zeiss DSM 982 scanning electron microscope. The coated slides
were broken in such a way as to crack the coating under tensile load so
that the layer structure of the coating thus becomes visible.

\par\null

\textbf{Table 3:} Overview of main process variables in sputtering
process\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\textbf{Exp. {[}No.{]}} & \textbf{Argon {[}sccm{]}} & \textbf{Operating
Point {[}V{]}} & \textbf{Operating Point Range {[}V{]}} & \textbf{Oxygen
Control Range} \textbf{{[}sccm{]}} & \textbf{Resulting Oxygen Ratio
{[}\%{]}} & \textbf{Absolute Chamber Pressure {[}Pa{]}} &
\selectlanguage{ngerman}\textbf{Surface Temperature {[}°C{]}}\tabularnewline
\midrule
\endhead
1 & 120 & 480 & 470 - 485 & 52 - 60 & 30 - 33 & 7.3 x
10\textsuperscript{-1} & 160\tabularnewline
2 & & 500 & 496 - 502 & 48 - 53 & 29 - 31 & 6.8 x 10\textsuperscript{-1}
& 130\tabularnewline
3 & & 520 & 509 - 523 & 35 - 45 & 23 - 27 & 6.0 x 10\textsuperscript{-1}
& 135\tabularnewline
4 & 160 & 410 & 405 - 416 & 58 - 76 & 27 - 32 & 1.5 & 150\tabularnewline
5 & & 440 & 436 - 445 & 48 - 57 & 23 - 26 & 1.1 & 220\tabularnewline
6 & & 470 & 465 - 473 & 39 - 46 & 20 - 22 & 9.3 x 10\textsuperscript{-1}
& 135\tabularnewline
7 & 200 & 380 & 373 - 388 & 38 - 55 & 16 - 22 & 2.0 & 170\tabularnewline
8 & & 400 & 390 - 407 & 35 - 51 & 15 - 20 & 1.7 & 130\tabularnewline
9 & & 420 & 416 - 425 & 30 - 44 & 13 - 18 & 1.5 & 110\tabularnewline
\bottomrule
\end{longtable}

Figure 6 shows the SEM image of coating No. 6 in which a columnar layer
structure is observed for all coatings. The thin titanium adhesion
layer, with an average layer thickness of 0.2 \selectlanguage{greek}μ\selectlanguage{english}m, has a defect-free
interface to the substrate material in all coating experiments. The
layer structures of the titanium dioxide coatings are consistently
homogeneous, free of defects, and display feature defect-free bonding to
the adhesive layer while there is no significant change in the layer
structure between coating experiments No. 1 to No. 9.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-61/Figure-6}
\caption{{~
{\label{836444}}%
}}
\end{center}
\end{figure}

\textbf{Figure 6:} Scanning electron image of a surface break of
titanium dioxide 1.0 \selectlanguage{greek}µ\selectlanguage{english}m on slide glass (coating experiment No. 9)

Figure 7 shows an image of the coated slide specimens. Figure 8 shows
the Raman spectra of the coating experiments No.~1 to No.~3, Figure 9
presents the findings of experiments No.~4 to No.~6, and Figure 10 shows
the results of experiments No.~7 to No.~9. The wavenumbers found in all
coatings correspond to the anatase phase.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 7:} Image of TiO\textsubscript{2} anatase coatings on
glass slide from left Experiment No. 1 to right Experiment No. 9\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/Figure-8}

\end{center}
\end{figure}

\textbf{Figure 8:} Raman spectrum, coating experiments No. 1 (bottom),
No. 2 (middle) and No. 3 (top)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/Figure-9}

\end{center}
\end{figure}

\textbf{Figure 9:} Raman spectrum, coating experiments No. 4 (bottom),
No. 5 (middle) and No. 6 (top)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/Figure-10}

\end{center}
\end{figure}

\textbf{Figure 10:} Raman spectrum, coating experiments No. 7 (bottom),
No. 8 (middle) and No. 9 (top)

\section*{4. Conclusion}

{\label{conclusion}}\par\null

In this work, pulsed DC magnetron sputtering was used for the reactive
deposition of titanium dioxide in the anatase phase. The pure anatase
phase was detected by Raman spectroscopy on all prepared coatings,
whereby the maximum substrate temperature was 220 \selectlanguage{ngerman}°C and the lowest
substrate temperature was measured in coating experiment No. 9 and
amounts to 110 °C. Over a wide process window covering nine coating
experiments, the coating remained in the pure anatase phase.

~

The layer thickness was determined using the calotte grinding method.
For evaluation, five coating thickness measurements were averaged in
each case and the deviation of the coating thickness measurement was
between 0.01 \selectlanguage{greek}µ\selectlanguage{english}m and 0.10 \selectlanguage{greek}µ\selectlanguage{english}m. The lowest coating thickness was observed
in Experiment No. 4 and measures 0.5 \selectlanguage{greek}µ\selectlanguage{english}m. Experiment No. 9 has the
highest measured film thickness with 1.2 \selectlanguage{greek}µ\selectlanguage{english}m while Experiment No. 1 has a
film thickness of 0.7 \selectlanguage{greek}µ\selectlanguage{english}m, followed by No. 2 at 0.8 \selectlanguage{greek}µ\selectlanguage{english}m, No. 3 at 1.1 \selectlanguage{greek}µ\selectlanguage{english}m,
No. 5 at 0.8 \selectlanguage{greek}µ\selectlanguage{english}m, No. 6 at 1.0 \selectlanguage{greek}µ\selectlanguage{english}m, and No. 7 and No. 8 display a film
thickness of 0.6 \selectlanguage{greek}µ\selectlanguage{english}m.

~

The design of the ideal proportion of oxygen in the oxygen/argon gas
mixture reported in the literature for the deposition of the anatase
phase during the reactive deposition of titanium dioxide, which is
described as ranging from 14.6 \% {[}20{]} to 34.4 \% {[}23{]}, was
indirectly determined in this work by establishing the operating points
in the hysteresis experiment. In the coating tests No.~1 to No. 9, the
ratio was measured at a minimum of 13 \% (coating test No. 9) and a
maximum of 33 \% (coating test No. 1). The main influencing factor of
absolute process pressure, as described in the comparison of coating
parameters in the literature from Table 1, was interpreted by attempting
to determine the process pressures (Table 2). According to the
evaluation of the real process pressure experiment, the pressure ranged
between a minimum of 6.0 x 10\textsuperscript{-1} Pa (coating test
No.~3) and a maximum of 2.0 Pa (coating test No.~7). Thus, the range of
absolute process pressure of this work is within the promising absolute
process pressure of a minimum of 2.7 x 10\textsuperscript{-1} Pa to 4.0
Pa summarized from the literature as shown in Table 1. The promising
absolute process pressure for the deposition of titanium dioxide in the
anatase phase according to Table 1 was confirmed using Raman
spectroscopic investigations.

~

The Raman spectroscopic studies show that the pure anatase phase of
titanium dioxide is present in all coatings. Five wavenumbers specific
to the anatase phase were found in the coating tests No.~1 to No.~9. The
characteristic wavenumbers found correspond -- with minor deviations --
to the six specific wavenumbers of anatase, namely 144
cm\textsuperscript{-1}, 197 cm\textsuperscript{-1}, 399
cm\textsuperscript{-1}, 513 cm\textsuperscript{-1},
519~cm\textsuperscript{-1}, and 639 cm\textsuperscript{-1} according to
the calculations and observations of {[}26{]}, whereby the two
wavenumbers 513~cm\textsuperscript{-1} and 519 cm\textsuperscript{-1}
are so close to each other that they overlap. The specific wavenumbers
were found to be temperature-dependent {[}27{]}. Furthermore, different
wavelengths of the laser used can lead to a slight deviation in the
wavenumber.

~

The scanning electron microscopic observation of the coatings showed a
columnar and homogeneous layer structure in all coating tests, whereby
the columnar layer structure was favored due to the relatively low
temperature of a maximum of 220 \selectlanguage{ngerman}°C, the deposition pressure of minimum
{6.0~x~10\textsuperscript{-1} Pa}, and the non-use of an additional BIAS
voltage.



\section*{Conflict of Interest
Statement}

{\label{conflict-of-interest-statement}}

The authors declare no conflict of interest.

\par\null

\section*{Data availability statement}

{\label{data-availability-statement}}

The data that support the findings of this study are available from the
corresponding author upon reasonable request

\par\null

\section*{References}

{\label{256795}}\par\null

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