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Inverted fireball deposition of carbon films with extremely low surface roughness

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The surface of carbon films deposited with inverted plasma fireballs is analysed in this paper. Measurements were conducted with Raman spectroscopy, atomic force microscopy and nanoindentation. The latter was used to obtain Young's modulus as well as Martens and Vickers hardness. The roughness of the film was measured by atomic force microscopy and its thickness was measured. It was shown with Raman spectroscopy that the films are homogeneous in terms of atomic composition and layer thickness over an area of about 125 × 125 mm. Furthermore, it was demonstrated that inverted plasma fireballs are a viable tool for obtaining homogeneous, large-area carbon films with rapid growth and very little energy consumption. The obtained films show very low roughness.
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Carbon Letters
Inverted fireball deposition ofcarbon films withextremely low surface
J.Gruenwald1 · G.Eichenhofer2· G.Filipič3· Ž.Federl3· W.Feuchtenberger5· K.Panos5· G.HernándezRodríguez4·
Received: 1 August 2022 / Revised: 26 September 2022 / Accepted: 10 October 2022
© The Author(s), under exclusive licence to Korean Carbon Society 2022
The surface of carbon films deposited with inverted plasma fireballs is analysed in this paper. Measurements were conducted
with Raman spectroscopy, atomic force microscopy and nanoindentation. The latter was used to obtain Young’s modulus as
well as Martens and Vickers hardness. The roughness of the film was measured by atomic force microscopy and its thickness
was measured. It was shown with Raman spectroscopy that the films are homogeneous in terms of atomic composition and
layer thickness over an area of about 125 × 125mm. Furthermore, it was demonstrated that inverted plasma fireballs are a
viable tool for obtaining homogeneous, large area carbon films with rapid growth and very little energy consumption. The
obtained films show very low roughness.
Keywords PECVD· Carbon· Inverted fireballs· Deposition
1 Introduction
Various carbon containing films, ranging from graphene over
amorphous carbon and nanostructured surfaces to diamond
like carbon layers have gained more and more momentum in
recent years. This is mostly due to their vast number of prop-
erties that can be tailored by fine tuning the kind of carbon
allotrope deposited onto different substrates. Amongst many
possible technologies for carbon film deposition, plasma
enhanced chemical vapour deposition (PECVD) is a stand-
ard procedure. It has successfully been used to obtain carbon
nano structures [1, 2], graphene [3, 4], diamond like carbon
(DLC) [5, 6], diamond [7, 8] or amorphous carbon films [9,
10]. In recent years, a novel PECVD process, which is based
on so-called inverted fireballs (IFBs) has gained increased
attention, since it allows the creation of very homogeneous
plasmas with enhanced plasma densities [1115]. There have
also been attempts to incorporate an IFB setup into a sput-
ter chamber but so far with mixed results [16]. The favour-
able combination of high plasma density and homogeneity
is obtained by a highly transparent grid that is put onto a
suitable positive potential with respect to ground. If the bias
becomes larger than the ionisation potential of the working
gas, electrons are attracted from the surrounding background
plasma and accelerated towards the gridded anode. Most of
these highly energetic electrons will pass through the grid
and induce additional ionisation processes in the closed wire
cage. At the same time, the Debye sheath around the mesh
wires overlaps with the sheath of the neighbouring wires
and, thus, forms an equipotential surface around the entire
gridded anode. This leads to a formation of a Faraday cage,
in which the denser plasma is trapped and forms the IFB.
While the ions are trapped in the potential well that forms
inside the mesh cage, electrons can escape through the grid
or passing between two opposite sides of the cage. If this
happens, the escaping electrons are influenced by the electric
field of the mesh as soon as they leave the IFB. This field
accelerates them back into the IFB, artificially prolonging
Online ISSN 2233-4998
Print ISSN 1976-4251
* J. Gruenwald
1 Gruenwald Laboratories GmbH, Taxberg 50,
5660Taxenbach, Austria
2 4A-PLASMA, Aichtalstraße 66, 71088Holzgerlingen,
3 Jozef Stefan Institute, Jamova Cesta 39, 1000Ljubljana,
4 Institut Für Festkörperphysik, Graz University
ofTechnology, NAWI Graz, Petersgasse 16, 8010Graz,
5 Institut Für Elektronik Und Messtechnik, Helmut Fischer
GmbH, Industriestraße 21, 71069Sindelfingen, Germany
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their collision mean free path and enhancing their kinetic
energy. Therefore, those electrons trigger additional ionising
collisions within the IFB, enhancing the plasma density by
up to an order of magnitude. Although the plasma potential
is essentially flat throughout an IFB, it was experimentally
demonstrated in reference [15] that the potential profile can
be tailored to a certain extent, if one or more sides of the
cage are electrically insulated against the others and biassed
slightly different than the main cage anode [15]. This may
be used in setups where a gradient in the plasma potential
and, hence, in the difference of ion and electron densities
is desirable. IFBs are a novel technology, which offers the
possibility of homogeneous depositions over large areas.
Furthermore, the growth rates are quite high even with low
additional power consumption by the IFB electrode. Nev-
ertheless, the focus of this paper lies on a standard IFB in
a mixture of argon and n-hexane and the films that were
deposited with this method. After the successful deposition
of the carbon films, the physical properties of the layers were
studied with different methods, which will be discussed in
detail in the next section.
2 Materials andmethods
The carbon films were deposited in a vacuum chamber,
type PA 100 from Plasma Technology GmbH at a working
pressure of 3Pa. The chamber was evacuated by a turbo
pump. The background plasma was maintained at 40kHz,
50W. For applying the DC Voltage to the grid, a power
supply from EDF electronics was used. The IFB anode was
manufactured from a stainless steel grid with (wire diameter
120µm, grid constant 250µm). It had a height of 8.5cm and
an edge length of 18.5cm and consumed 20W of electrical
power at a voltage of 174V. The substrate was not addition-
ally heated and remained at room temperature due to the lit-
tle overall power. The IFB setup and the plasma parameters
are shown in detail in reference [17]. A schematic diagram
of the electrical setup of the IFB cage is shown in Fig.1.
The deposition was conducted on standard labora-
tory soda-lime glass slides (as used for microscopy) with
25 × 75 mm2 area. 7 of these glass plates were covering
the substrate holder. An additional slide was put on top of
the others to cover the glass substrate and have a reference
of uncovered glass samples after the deposition. Before
the coating process an IFB was ignited in a mixture of
100sccm H2 and 7.5sccm Ar to clean the substrates. This
cleaning process was upheld for 5min. After the initial
cleaning, a second IFB plasma was ignited in a mixture
of 50sccm Ar and 50sccm tetramethylsilane (TMS) for
1min to obtain an adhesive mediation layer between the
glass substrate and the carbon film. After this, the coating
process was conducted in an Ar/n-hexane IFB (with 100
sccm each) for 15min. The result of this coating experi-
ment is depicted in Fig.2.
It is evident that a smooth carbon containing film was
obtained within the IFB grid. After the coating procedure,
Fig. 1 Schematic diagram of the IFB setup with electrical connections (left) and a photograph of the anode grid on top of the substrate holder
(right, taken from reference [17].)
Fig. 2 Samples after 15 min of IFB deposition. The substrates 1–8
were glass slides with 25 × 75 mm2 area. The slight dislocation of
slide one was due to mechanical vibrations that occurred when open-
ing the chamber
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the samples were analysed with different methods that are
outlined in the next section.
3 Results
3.1 Raman measurements
Raman spectroscopy was performed on the two different
areas, shown in Fig.3. On the uncoated glass sample (red
dot) and the carbon film itself (pink dot). The transition
region (green dot) was also investigated with AFM. These
results will be presented below. The samples were ana-
lysed with a Witec Raman spectroscope using a green laser
with 532nm and 4mW output power on the glass substrate
to get Raman peaks with reasonable intensity. However,
the power of the laser was reduced to 1.6mW on the car-
bon films to prevent possible damages. This is still enough
due to the increased Raman sensitivity of carbon films.
Each sample was measured at least at two points in each
of the relevant regions—glass and carbon. The obtained
data are displayed in Fig.4. Since the most interesting
region is the carbon film, Fig.4b depicts the collected data
from random samples (2, 3, 4 and 8 in this case) to illus-
trate that the Raman peaks are indeed identical, regardless
of the substrate’s position within the IFB.
The peaks in Fig.4a can be identified as the symme-
try stretching mode of Si–O bonds from the glass slide
(570 cm−1) [18], a high frequency Si–O-Si binding mode
(794 cm−1) [19] and the so-called Q3 Si–O stretching
vibration mode (1098 cm−1) [20]. With increasing thick-
ness, the Raman peaks from the glass substrate are van-
ishing and only one broad peak remains. This peak was
deconvoluted into two separate peaks at 1383 cm−1 and
1546 cm−1, respectively. The former is as assumed to be
the stretching mode of the CH3 alkyl group while the lat-
ter is attributed to the aromatic Si–C vibration mode [21].
Since the peak position and intensities are identical on
all the measured samples, it can be concluded that the
chemical composition of the films is very homogeneous
throughout the entire substrate and, thus, throughout the
entire plasma inside the IFB. This assumption is also cor-
roborated by the measurements in reference [17].
Fig. 3 The points of measurements for the Raman and AFM data
Fig. 4 Raman spectra of the glass substrate a and the carbon film
(b). Image c shows a magnified view of the deconvoluted peak at
1546 cm−1
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3.2 AFM measurements
To study the surface morphology, AFM measurements
were performed on the glass and on the carbon coating.
The results are shown in Fig.5.
The uncoated glass as well as the transition region was
analysed on a 3 × 3µm2 area while the carbon coating was
investigated on a 7 × 7µm2 area. It can be seen that the high-
est elevation on the surface is about 22nm with rms rough-
ness of 2.349nm, while the rms roughness on the transi-
tion region decreases down to 0.553nm with the highest
elevations at around 11nm. However, on the carbon coating
the roughness decreases further below the resolution of the
AFM (< 0.25nm). Approaching the resolution limit of the
AFM also causes the artefact at the end of the measure-
ment in Fig.5c. The same results were obtained on all the
samples, indicating again that the morphology of the carbon
layer produced with IFB shows a very smooth and homo-
geneous surface throughout the whole IFB plasma region.
3.3 Nanoindentation measurements
Nanoindention or instrumented indentation testing was per-
formed according to DIN EN ISO 14577, which provides
Fig. 5 AFM images and surface map of the glass substrate (a), the transition region (b) and the carbon film (c)
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access to information about the mechanical properties of the
obtained films. The used instrument in these experiments
was a PICODENTOR HM 500 from the Helmut Fischer
GmbH with a Vickers indenter. A maximum force of 100µN
was used as parameter for the standard measurement proce-
dure. The loading and unloading of the force occurred within
3s. In addition, an enhanced stiffness procedure (ESP) with
a maximum force of 100mN applied during 912 s was
utilised to gain further depth-dependent information. The
measured indentation depth h as a function of the applied
force F is depicted in Fig.6.
The Martens hardness (HM) was calculated from the load
curve and is shown as a function of depth in Fig.7.
While the HIT value can be converted into the Vickers
hardness (HV), the elastic properties have been calculated
from the unloading of the lever in accordance with the
method proposed by Oliver and Pharr [22]. This method
yields the so-called indentation modulus EIT.
The force-penetration depth curves (Fig.6) point out
that the deposited layer exhibits a considerable difference in
mechanical properties compared to the glass substrate. The
increase in the curve fraction of the loading demonstrates
a clear difference. Furthermore, the maximum penetration
depth in the coating is higher than in the glass substrate. It
is therefore evident that the coating must have a lower hard-
ness. This was confirmed by the hardness parameter HM
in Fig.7. The graph shows that the hardness of the coating
is not only lower, but also shows a stable or homogeneous
behaviour in depth without any hardness gradient.
Other commonly used hardness properties determined
for the coating, that cannot be represented as a function
of depth in a standard measuring procedure, are HIT of
8.8GPa ± 0.07GPa and the Vickers hardness value of 827
HV ± 7 HV.
In addition to the hardness properties, the elastic proper-
ties were also determined using the characteristic value EIT/
(1-νs2). The mentioned parameter approximately character-
ises the elastic modulus of the coating, without considering
the Poisson’s ratio (νs). Based on these measurements, an
EIT/(1-νs2) of 70.7GPa ± 2.6GPa was determined for the
glass and 47.9GPa ± 0.3GPa for the coating.
When comparing the measurements from the standard
measurement procedures with those from the ESP in Fig.8,
it is clearly evident that this allows an estimation of the layer
thickness. The previously determined characteristic values
from the standard measurement procedure can be seen again
within the measurement uncertainty. Only the characteristic
values EIT/(1-νs2) and HM were shown for clarity.
For both considered characteristic values, a comparable
measured value from the standard procedure can be deter-
mined from the diagram in Fig.8. The special feature here
is that a significantly greater penetration depth was run. This
means that, according to the 10% rule, the properties of the
coating are represented by only one point at approx. 28nm.
Fig. 6 Averaged indentation depth h as a function of the applied force
Fig. 7 Depth dependent Martens hardness (HM) as a function of the
penetration depth
Fig. 8 Averaged depth-dependent characteristic value of HM and
EIT/(1-νs2), measured with ESP
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The hardness value is influenced by the substrate material
at greater depths. This can also be seen in the elastic behav-
iour, so that two curve components with different slopes
can be distinguished. At the turning point of the slopes, a
layer thickness can be estimated, which can be assumed to
be approx. 200–300nm.
For the analysis of the layer homogeneity, an array of
measuring points was taken on a sample with the nanoin-
dentation instrument. A matrix with a measurement area of
12.65mm by 14.2mm was applied to the sample to obtain
an impression of the homogeneity.
The 341 measurement curves obtained in this area were
used to generate a colour-coded plot of various properties of
the measurement matrix, which can be seen in Fig.9.
The thin layer exhibits a satisfactory homogeneity over
the measured sample area in terms of hardness and Young's
modulus, which can be seen from HM and EIT/(1- νs2). No
gradients of the considered properties over a large area of
the sample can be seen. The results are statistically normal-
distributed. This allows the conclusion that a homogeneous
deposition result was obtained in the process.
3.4 Measurements ofthefilm thickness
The evaluation of the coating thickness was also performed
by Helmut Fischer GmbH. These measurements were con-
ducted with a BETASCOPE using a radioactive source C14
at a temperature of 22°C with a measuring time of 100s.
We measured a mean value of 263nm and a standard devia-
tion of 50nm for the coating thickness by assuming 1.395g/
cm3 for the density of the coating.
4 Conclusions
In this paper, the deposition of large area amorphous carbon
coatings with excellent homogeneity by applying IFBs is
reported. The measurements of the mechanical and surface
properties showed that films with low surface roughness
and homogeneous chemical composition were obtained
with growth rates of about 15–20nm/min at very low over-
all input power (approx. 70W). The coating experiments
demonstrate that IFBs are a versatile tool, especially for the
fabrication of large area carbon films, which demand a small
surface roughness and high level of homogeneity. An over-
view of the results from the mechanical analysis and the
relevant statistical parameters from these measurements are
listed in Table1.
The main purpose of this paper is to show that IFB can
produce carbon layers with relevant properties. However,
there will be more work necessary in the future regarding
suitable precursor gases or gas mixtures and the fine tuning
of other physical parameters such substrate temperature, gas
pressure, et cetera which are expected to enhance the number
of varieties for other carbon allotropes on different substrate
Acknowlegdements The authors want to thank J. Eisenlohr from
Plasma Technology GmbH, 71083 Herrenberg-Gültstein, Germany for
the making the PA 100 plasma system available for the experiments
and his technical support. J. Gruenwald: Member of the GET-Plasma
Consortium, G. Eichenhofer: Member of the GET-Plasma Consortium.
The authors declare that they have no conflicts of interest.
Fig. 9 Colour-coded plot of EIT/(1-νs2) (a) and HM (b) for testing
the layer homogeneity
Table 1 Results and statistical parameters of the mechanical proper-
ties of the carbon films and maximum indentation depth hmax
HM EIT/(1-νs2) HIT HV hmax
N/mm2GPa N/mm2nm
X 2966 47.9 8752 827 27
U (X.) 14 0.25 65 7 0.1
S 137 2.4 608 58 0.7
V/% 4.6 4.9 7 7 2,6
Min 2658 42.3 7148 675 25
Max 3451 55.2 11060 1045 29
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... For example, they can be used to convert dc voltage signals directly into rf signals [2]. However, the strongest research focus has been put on their usage in surface treatment technologies, especially in the field of carbon coatings [3,4,5] or sputtering [6]. It has also been demonstrated recently that the plasma parameter inside and IFB can be controlled to a certain extent by segmented biasing of the IFB anode [7]. ...
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This paper describes the derivation of a simple engineering equation to calculate the minimal necessary bias voltage for the onset of an inverted fireball. The calculation uses the electron density in the absence of an inverted fireball as well as the grid constant and the working gas species as input parameters. It will be shown that the interplay between ionsation potential, dissociation potential (for molecular working gases), as well as the electron density in the background plasma, play an important role in the necessary minimum bias on the fireball electrode. Some of the most common working gases and their relevant parameters will also be listed in this paper. This should give experimentalists and engineers a practical equation that can be used to quickly determine the most important electrical properties of the anode that is used to trap the inverted fireball. Thus, the planning and design of experimental setups or technological devices will be made much less time-consuming and, thus, more convenient.
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In this paper we present for the first time the measurement of inverted fireballs in a low-pressure Ar/n-hexane plasma by using a Multipole Resonance Probe. The basic plasma parameters such as electron density, electron temperature and Debye length as well as the particle collision frequency have been obtained for different experimental settings. These measurements show that inverted fireballs that are created in chemically active plasma offer advantageous properties regarding deposition techniques. Furthermore, the influence of the plasma parameters on grid bias and changes in neutral gas pressure has been studied within this work. It is observed that the plasma density within an inverted fireball configuration can be raised by more than an order of magnitude compared to the background plasma or without biasing the grid electrode, respectively. The obtained results are promising for future coating and surface modification applications with inverted fireballs and show that the Multipole Resonance Probe is a viable diagnostic tool for reactive plasmas.
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Oriented carbon nanostructures (OCNs) with dominant graphitic characteristics have attracted research interest for various applications due to the excellent electrical and optical properties owing to their vertical orientation, interconnected structures, electronic properties, and large surface area. Plasma enhanced chemical vapor deposition (PECVD) is considered as a promising method for the large-scale synthesis of OCNs. Alternatively, structural reformation of natural carbon precursor or phenol-based polymers using plasma-assisted surface treatment is also considered for the fabrication of OCNs. In this work, we have demonstrated a fast technique for the synthesis of OCNs by plasma-assisted structure reformation of resorcinol-formaldehyde (RF) polymer gels using radio-frequency inductively coupled plasma (rf-ICP). A thin layer of RF polymer gel cast on a glass substrate was used as the carbon source and treated with rf plasma under different plasma discharge conditions. Argon and hydrogen gases were used in surface treatment, and the growth of carbon nanostructures at different discharge parameters was systematically examined. This study explored the influence of the gas flow rate, the plasma power, and the treatment time on the structural reformation of polymer gel to produce OCNs. Moreover, the gas-sensing properties of as-prepared OCNs towards ethanol at atmospheric conditions were also investigated.
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The Raman spectra of alkali silicate glasses containing 5 to 30 mol % M2O (M = Li, Na, K, Rb, and Cs) have been fit successfully with pseudo‐Voigt lineshapes of dominantly Lorentzian character in order to quantify the Qn species distributions. This differs from the more popular Gaussian lineshapes which have been used for the past four decades. There is an increase in asymmetry in the Q3 band, with increasing M2O content which appears to result from the weakened Si‐O force constants of some Q3 bands due to charge transfer via M‐BO bonds. With charge transfer to the tetrahedra, the negative charge accumulates preferentially on Si atoms thus decreasing Si‐O Coulombic interactions, weakening Si‐O force constants, and shifting the Qn A1 symmetric stretch vibrational frequencies to lower values (eg, from ∼1100 cm−1 to ∼1050 cm−1). The fraction of affected Q3 species increases with alkali content, as does the Q3 peak asymmetry. We propose that this extends through to all the Qn species and postulate that there are multiple vibrational modes for each Qn species which are dictated by their proximity to network modifier cations.
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In this letter we present measurements of the influence on inhomogeneous electrode biasing on the basic plasma parameters of inverted fireballs in a hydrogen plasma. The measurements were performed in hydrogen because it is often used in many reactive plasmas, which are very important for technical or industrial applications. The dependence of the plasma parameters on voltages and currents on the electrodes are described in this work. It will be shown that the density profiles and the plasma potentials inside an inverted fireball can be shaped to a certain extend by asymmetric potentials on the anode.
The use of {113} oriented single crystal diamond substrates has been reported for CVD growth of high-quality p-type layers with increased doping efficiency, compared to the {100} orientation, and excellent electrical properties. In this work we expand the range of crystal orientations to include the {118} and {115} planes and study the effect of B doping level in the gas phase with B/C ratios from 250 up to 2000 ppm. We demonstrate the growth of high-quality B doped SCD layers, with low RMS roughness (<2 nm) and high surface chemical purity (99% C with >1% B) for all orientations. Boron incorporation is shown to be dependent not only on the B/C in the gas phase, but also on the crystallographic orientation, with orientations with a higher density of {100}/{111} steps and risers exhibiting higher B incorporation into the solid from the gas phase (4 × 10¹⁹ cm⁻³ up to 1 × 10²¹ cm⁻³). Finally, using electro-chemical techniques, we confirm, for the first-time, high-quality electrodes on such orientations. Novelty statement In this work we report, for the first time, MW PECVD of high-quality boron doped diamond layers on vicinal {118} and {115} single crystal substrates, which are confirmed, via electro- chemical characterisation, to be excellent electrodes.
In this paper the results of a first attempt to ignite an inverted fireball with a magnetron sputter source are presented. The conducted measurements show that there is no inverted fireball visible to the naked eye present during the experiments. However, some features are observed that indicate at least an initial onset of such an inverted fireball with an interruption before it is fully developed. In the course of the experiments magnetron sputtered molybdenum films have been deposited inside a highly transparent gridded anode and the obtained films have been analysed. It turns out that even without a visible inverted fireball being present, the film morphology and mechanical properties such as elastic modulus, hardness and surface roughness have changed significantly compared to a setup without inverted fireball anode. Additionally, the deposition rate is elevated by up to 20 % inside the inverted fireball anode compared to the reference setup, although with a nearly threefold increase in total input power (cathode power + grid anode power). This increase in deposition rate was achieved despite the fact that a mesh electrode with only about 37% transparency was used. On the other hand, it was demonstrated that the use of additional gridded electrodes in such a device triggers instabilities and non-linearities in the plasma, which might be of interest for further fundamental research.
There are many defects in the bending part of U-type pipe, which will directly affect its safety and service life. DLC (Diamond Like Carbon) film is considered as an effective protective film because of its excellent properties. However, it is very difficult to deposit DLC films on the inner wall of U-type pipes with double stress. The purpose of this paper is to deposit DLC film with high hardness, wear resistance and corrosion resistance on the inner wall of U-type pipes by hollow cathode PECVD. The microstructure, mechanical properties, tribological and corrosive behaviors were characterized by the Raman spectroscopy, ATR-FTIR, SEM, nano-indention, polarization, electrochemical impedance spectroscopy and ball-on-disk tribological tests. Results indicates that the U-type pipe inner DLC films shows the lower stress level. Compared with the bare substrate, it presents the higher hardness (7~14 GPa), elastic factor (> 0.1) and plastic factor (> 0.1) and the above DLC film exhibits the lower corrosion current density, which is about 1-2 orders of magnitude lower than the bare substrate. Moreover, the DLC films also possesses the lower friction coefficient (0.04~0.06) and wear rate (2.67~5.93E-7 mm³N⁻¹m⁻¹). Therefore, the DLC films exhibits the greater mechanical, tribological and anti-corrosion performance, indicating that the above DLC films effectively protects U-shaped pipe and prolongs its service life.
The need for 2D vertical graphene nanosheets (VGNs) is driven by its great potential in diverse energy, electronics, and sensor applications, wherein many cases a low-temperature synthesis is preferred due to requirements of the manufacturing process. Unfortunately, most of today's known methods, including plasma, require either relatively high temperatures or high plasma powers. Herein, we report on a controllable synthesis of VGNs at a pushed down low-temperature boundary for synthesis, the low temperatures (450 °C) and low plasma powers (30 W) using capacitively coupled plasma (CCP) driven by radio-frequency power at 13.56 MHz. The strategies implemented also include unrevealing the role of Nickel (Ni) catalyst thin film on the substrates (Si/Al). It was found that the Ni catalyst on Si/Al initiates the nucleation/growth of VGNs at 450 °C in comparison to the substrates without Ni catalyst. With increasing temperature, the graphene nanosheets become bigger in size, well-structured and well separated. The role of Ni catalysts is hence to boost the growth rate, density, and quality of the growing VGNs. Furthermore, this CCP method can be used to synthesize VGNs at the lowest temperatures possible so far on a variety of substrates and provide new opportunities in the practical application of VGNs.
Long service life of implants is of high importance to patients. This study deposited amorphous carbon (a-C) coatings on biomedical grade Ti6Al4V substrate using plasma-enhanced chemical vapor deposition (PECVD). The effect of substrate bias power on microstructure and corrosion of the coatings is investigated and compared with the bare Ti6Al4V substrate. The experimental results show that the substrate bias power of 30 W leads to amorphous carbon in microstructure. With increased bias nanocrystalline graphite appears. All coated samples have significant improvements in corrosion resistivity as compared to Ti6Al4V alone. The sample deposited at bias power of 30 W with a thickness of 150 nm demonstrated the lowest corrosion current of 6.47 nA/cm² and the lowest corrosion rate of 8.05 nm/year, which can postpone the corrosion of the Ti6Al4V substrate for at least 18 years. From the in-vitro cell culture studies, the best sample also registered the higher cell viability and biocompatibility as compared to bare Ti6Al4V, thus could be helpful in promoting healing.
In this paper, we present systematic measurements of the basic plasma parameters of inverted fireballs (FBs) in a helium plasma for the first time. Inverted FBs in helium with a diameter of 10 cm are generated in a highly transparent hollow electrode via impact ionisation of accelerated electrons from an existing background plasma. The measured quantities are the plasma potential, the electron density and the electron temperature. The dependence of the plasma parameters on the discharge current was studied experimentally inside as well as outside the inverted FB. Furthermore, estimations of the neutral gas temperature inside the inverted FBs are made and possible explanations of the creation of macroscopic gas flows from the inverted FBs are given. It is shown in this paper that due to efficient thermalisation of electrons with neutrals the gas temperature inside the inverted FB can reach several thousand Kelvin, which also leads to a thermal equilibrium of ions and electrons inside this plasma structure.