# Ion Energy Tuning for Enhanced sp3 Carbon Fraction in Electron Cyclotron
Resonance Ion Beam Deposited Diamond-Like Carbon Coatings: a Computational and
Experimental Approach

Callum Wiseman a,1 111These authors contributed equally to this work, Marwa
Ben Yaala a,1,2 222These authors contributed equally to this work, Chalisa
Gier a, Laurent Marot b, Christopher McCormick a, Sheila Rowan c and Stuart
Reid a a SUPA, Department of Biomedical Engineering, University of
Strathclyde, Glasgow, United Kingdom;
b Department of Physics, University of Basel, Basel, Switzerland;
c SUPA, Institute for Gravitational Research, University of Glasgow, Glasgow,
United Kingdom;

###### Abstract

A novel high-energy electron cyclotron resonance (ECR) ion beam deposition
(IBD) technique was used to fabricate DLC films at different ion beam
energies. The ratios of $sp^{2}$/$sp^{3}$ bonding in the DLC coatings were
determined by Raman spectroscopy and XPS, with the confirmation of being
hydrogen-free due to the lack of photoluminescence (PL) background in the
Raman spectra. The results indicate that the $sp^{3}$ percentage ranges from
45% - 85% for the ECR-IBD fabricated DLC films in this study. Monte-Carlo
based SRIM simulation was used to extract the energy and angular distribution
of the sputtered particles from the carbon target and correlate it to the
highest $sp^{3}$ fraction in the manufactured ECR-IBD DLCs. This study
demonstrates a method of depositing DLC thin films under ambient conditions
(room temperature with no post-annealing or additional bias voltage applied)
which produces high-$sp^{3}$ coatings (higher than those traditionally
reported for other sputtering methods) suitable for applications where high
quality DLC coatings are required.

###### keywords:

DLC, ECR-IBD, $sp^{3}$, SRIM, Raman, XPS

††journal: Diamond and Related Materials myfootnotemyfootnotefootnotetext:
<EMAIL_ADDRESS>
##  1 Introduction

Diamond-like Carbon (DLC) is a classification of amorphous carbon thin films
containing some fraction of trigonally and tetrahedrally ($sp^{2}$ and
$sp^{3}$, respectively) bonded carbon atoms and may or may not be
hydrogenated. In pure $sp^{2}$ carbon (graphite), each C atom has a double
bond with three other carbons, whereas in pure $sp^{3}$ carbon (diamond), each
C atom has a single bond with four others . Figure 1 depicts a phase diagram
of $sp^{2}$/$sp^{3}$/H-content and the types of DLC coatings permissible by
different combinations of these three properties.

Figure 1: Ternary phase diagram of DLC reproduced from [1]

DLC coatings are desirable for their varied and interesting properties and
their ability to be tuned to meet the unique needs of various applications.
They can exhibit many of the properties of crystalline diamond, including high
mechanical hardness, chemical inertness, high electrical resistivity, and
optical transparency, as well as being a wide band gap semiconductor.
Amorphous carbon approaching a 100% $sp^{2}$ network will have graphitic
character while higher fractions of $sp^{3}$ bonding will have more ‘diamond-
like‘ characteristics. DLC coatings may be deposited using a variety of
techniques such as ion beam deposition IBD [2]; filtered cathodic vacuum arc
(FCVA) [3]; RF magnetron sputtering [4] and RF plasma enhanced chemical vapour
deposition (PECVD)[5]. As shown in [6], each method has its own benefits,
drawbacks, and range of achievable film properties.

Some techniques employed to produce DLC with high $sp^{3}$ content
(particularly chemical vapour deposition - CVD) introduce a high amount of
hydrogen into the coating due to the use of hydrocarbon precursor gasses. The
increased hydrogen content in the coatings may be undesirable for certain
applications as it may lead to a decrease in many of the favourable properties
that are achieved by the high $sp^{3}$ content, mainly friction and wear [7].

This paper presents a room-temperature high-energy sputtering technique for
producing hydrogen-free DLC coatings with tuneable $sp^{3}$ content along with
a computational study to correlate the sputtering plume energy distribution to
the measured coatings composition. The deposition technique here is based on
electron cyclotron resonance (ECR)-IBD, a thin-film deposition process
previously demonstrated to produce high density hafnium dioxide coatings [8]
and the lowest IR absorption in amorphous silicon thin films [9]. To the best
of the authors’ knowledge, no comprehensive work has previously been
undertaken to assess the viability of the ECR-IBD process, or any comparable
high-energy sputtering technique, for producing high performance DLC coatings;
for example, with a high proportion of $sp^{3}$ bonding relative to other
sputtering methods, as presented here.

##  2 Experiment

###  2.1 Thin film deposition

The DLC thin films in this study were produced using a custom-built ECR-IBD
system, as shown in Figure 2.

Figure 2: A detailed schematic of the ECR-IBD coating system used in this
study.

The ECR source (manufactured by Polygon Physics) ionises and accelerates the
Ar gas to sputter a target of the desired material, with the sputtered
particles contributing to thin film formation. Assuming single ionisation in
the argon plasma, we attribute the nominal beam energy to the accelerating
voltage applied in the range of 4 - 16 kV for these experiments. Argon is
injected into a 1/4$\lambda$ resonant cavity, where it is ionised in the
presence of a superimposed AC electric and static magnetic field. At 2.45 GHz,
a magnetic field strength of 87.5 mT is required. A beam is then extracted
through a series of electrostatic optic-labelled extractors, focus, and
ground, as shown in Figure 2. In this study, the potential at the extractor
was analogous to the nominal beam energy. The focus was set to 80-90% of this
value, while the ground plate was held at 0 V. The incidence angle, $\alpha$
(normal to the target relative to the ion beam propagation direction), was set
to 45∘. The source-target and target-sample distances were kept constant. The
substrates coated in this work were either fused SiO2 glass or stainless steel
316L. All the depositions were performed at room temperature, and no
additional heating was applied to the substrates during the coating process.
Detailed deposition parameters are shown in Table 1

Parameter | Value
---|---
Target material | HOPG
Target purity | 99.99%
Argon pressure |  $3.7\times 10^{-5}$ to $1.2\times 10^{-4}$ mbar
Ion beam voltage | 4-16 kV
Ion beam current | 100-500 $\mu$A
Table 1: Detailed deposition parameters for this study.

###  2.2 Characterisation

SRIM (Stopping and Range of Ions in Matter) simulations were carried out using
ion energies across the range of accelerating voltages that the ECR-IBD system
was capable of achieving. SRIM is a stochastic code that uses the Monte Carlo
approach based on binary collision approximation [10]. In this study, SRIM
[11] simulation (version 2013) was used to determine the spatial and energy
distributions of sputtered atoms from the carbon target. A detailed full-
damage cascade TRIM (TRanport of Ions in Matter) calculation was used to
record the positions and associated energies of sputtered species from the
target. One million Ar ions impinged onto the target material at 45∘ incident
angle and with energies of 4 keV, 10 keV and 16 keV. The sputtered target
parameters, including the surface binding energy, displacement energy, lattice
binding energy, and mass density of the material, were set to the default TRIM
values for the carbon target.

Raman spectroscopy/microscopy was used to measure the peak that occurs in
carbon materials at around 1580 cm-1. Visible Raman spectra were acquired
(Renishaw inVia Raman Microscope, Renishaw, Wooton-under-Edge, UK) using an
argon laser at an excitation wavelength of 532 nm. Measurements were performed
in triplicate, and the spectra were fitted using OriginPro 2021.

X-ray photoelectron spectroscopy (XPS) was used to measure the chemical
composition and extract the $sp^{3}$ content. The electron spectrometer was
equipped with a hemispherical analyser (Leybold EA10/100 MCD) and a non
monochromatised Mg K$\alpha$ X-ray source ($h\nu$ = 1253.6 eV) was used for
core level spectroscopy. The binding energy scale was calibrated using the Au
4f7/2 line of a cleaned gold sample at 84.0 eV. The fitting was performed
using Unifit 2015 with Voigt peak shapes. The fitting procedure for the core
level lines is described in [12].

##  3 Results and discussion

###  3.1 Raman Spectra of the produced DLC coatings

Raman spectra for all samples along with their respective fits are shown in
figure 3. From the Raman spectra, it can be seen that there is no
photoluminescent (PL) background, indicating that the DLCs produced are
hydrogen free coatings as PL is mainly due to the hydrogen saturating the non-
radiative recombination of the electron–hole pairs in the $sp^{2}$ bonded
clusters [13]. The C peak in the Raman spectra was deconvoluted into two
Gaussian peaks representing the D and G components. Clear differences can be
seen in the size and shape of the D and G peaks, as shown in red and green,
respectively. The relationships between various parameters of these peaks can
be used to deduce structural information about the DLC coatings. The position,
width, and intensity of the D and G peaks in the Raman spectra are closely
related to the density, size, and structure of the $sp^{2}$ clusters in DLC
[14]. Both D and G peaks are due to bond stretching modes in the graphitic
structures. The G peak is due to the stretching of carbon bonds either in
chains or rings, whereas the D peak is due to the breathing mode of the six-
fold aromatic rings. The diamond signature in the DLC referred to in the
literature as the T peak (presenting the C-C bond vibration in $sp^{3}$) can
only be seen for UV-excited Raman. For visible excitation, including the
current study at 532 nm, only D and G peaks are measurable. In theory, one
visible Raman spectrum could correspond to different $sp^{3}$ contents as the
$sp^{2}$ clustering varies independently of the $sp^{3}$ bonds so only a
combination of a UV Raman and Vis Raman can fully characterise the structure.
However, a large number of studies show that the properties of $sp^{2}$
clusters are in turn closely related to the $sp^{3}$ content of DLC. In this
paper, the the Visible Raman measurements are therefore used as a qualitative
estimation of $sp^{3}$ content of the produced DLC thin films. Various Raman
parameters are often used to infer structural information about DLCs,
including the POS(G), I(D)/I(G), and FWHM(G). Specifically, FWHM(G) was used
to estimate the $sp^{3}$ content in the coatings based on the following
relationship [15]:

$sp^{3}\;\%=-2.05+1.90\times 10^{-2}=W-3.01\times 10^{-5}W^{2}\pm 0.08,$ (1)

where $W$ is the FWHM(G) measured at $\lambda$ = 514 nm and $\pm 0.08$ is the
standard deviation of the fit of Cui’s data to the quadratic model. The
following relationship was used to account for the Raman measurements taken at
different excitation wavelengths:

$\mathrm{FWHM(G)_{\textrm{514}\;nm}}=\mathrm{FWHM(G)_{\lambda}}+0.21\times(514-\lambda),$
(2)

where $\lambda$ = 532 nm and FWHM(G) are given in wavenumbers. This equation
is based on the dispersion data presented by Ferrari et al. [16] with the
assumption that FWHM(G) disperses linearly and at approximately the same rate
for all types of DLC samples.

Figure 3: All Raman raw spectra (left) and an example fit (right) to the Raman
carbon peak measured on the sample deposited at 10 kV.

Table 2 presents the $sp^{3}$ percentage calculated from the procedure
described in equation 1. According to this analysis, the majority of samples
have $sp^{3}$ amounts between 40-50%, with a very slight parabolic dependence
going from 48.1% at 4 kV to 49.9% at 9 kV and 45.4% at 16 kV. However, the
highest value was 10 kV, with 57.7% of the carbon fraction being $sp^{3}$.

Ion energy (kV) |  $sp^{3}$ estimate from Raman (%)
---|---
4 | 48.1
6 | 49.3
8 | 42.1
9 | 49.9
10 | 57.7
12 | 48.4
14 | 47.1
16 | 45.4
Table 2: Results of the Raman fitting procedure and $sp^{3}$ calculation from
equation 1.

It is worth mentioning here that Cui [15] model is an empirical model
established to fit a specific set of experimental data. This can present a
discrepancy for the DLC coatings presented here, and is used for estimating
the $sp^{3}$ content in the coatings. In addition, recent studies have not
recommended using Raman spectroscopy to measure the $sp^{3}$ content of DLCs
[17, 18]. Therefore, a direct measurement of the composition is performed
using XPS to precisely quantify the percentage fractions of carbon bonds in
the $sp^{2}$ and $sp^{3}$ configurations. Results are presented in the
following section.

###  3.2 $sp^{3}$ ratio measured by XPS

XPS analysis/fitting of the coatings are shown in figure 4. The C1s peak has
two components for C=C ($sp^{2}$) and C-C ($sp^{3}$) bonded carbons and a
’shoulder’ towards higher binding energy for C-O and C=O bonds. Surface oxygen
contamination is unavoidable if the samples are exposed to atmosphere after
deposition.

Figure 4: The C1s peak from XPS spectra acquired for samples deposited at
different energies. The black curves are the raw measured data; blue, green,
cyan, and magenta fitted curves are the individual chemical states; and red
curves are the sum curves. The $sp^{3}$ content is derived from the ratio of
the areas of the C-C and C=C peaks fitted in green and blue lines
respectively.

Samples at different ion energies from 4 kV to 16 kV were measured. Shown in
figure 4 are the C1s core level spectra measured for all samples. The C1s core
level peak was deconvoluted into four singlets. The peaks at 286.8($\pm 0.2$)
eV and 288.4($\pm 0.2$) eV are attributed to C-O and C=O [19] while the peaks
at 284.7($\pm 0.2$) eV and 285.6 ($\pm 0.2$) eV are assigned to the $sp^{2}$
and $sp^{3}$ components of the DLC [20]. Although not shown here, some XPS
spectra shifted to higher binding energies during the measurement due to
surface charging on the insulating glass substrate. The shift was corrected
with respect to the main $sp^{2}$ and $sp^{3}$ peaks. The measurements also
revealed the presence of carbon and oxygen. The presence of oxygen on the
surface was due to the air exposure of the samples after deposition.

The output of the C1s peak fitting are presented in a scatter plot in figure
5. All samples had $sp^{3}$ values higher than 45%, reaching a maximum of 85%
at 10 kV. This value is notably high compared with $sp^{3}$ fraction in DLCs
manufactured by other sputtering techniques which generally produce DLC
coatings with $sp^{3}$ contents lower than 50% [6]. It is important to note
that the ECR IBD process requires no bias voltage on the substrate,which
provides more flexibility in the choice of substrate (insulating, temperature
sensitive, etc.) to produce high $sp^{3}$ DLCs for different applications.

Figure 5: $sp^{3}$ content estimated from XPS fitting results.

The large disparity between the $sp^{3}$ fractions estimated from the Raman
and measured by XPS is found where raman gives lower estimates overall than
XPS. Raman is only a qualitative method, it does not measure the exact
$sp^{3}$ content however the position of the G peak indicates a ta-C phase
(see Robertson [14] for POS(G) at 513 nm). Both XPS and Raman show that
$sp^{3}$ depends on the energy of the ions sputtering the target and,
consequently, on the energy of the sputtered atoms.

The growth mechanism of DLC films is best understood by consideration of
subplantation of incident particles [21]. Up to a certain value, incoming
film-forming particles will not have enough energy to cause the densification
required to produce a high proportion of $sp^{3}$ bonds, whereas at higher
energies, too much thermal dissipation will occur, resulting in a lower
fraction of $sp^{3}$ bonds being formed. Robertson [14] suggests that carbon
ions with energies of around 100 eV will produce high $sp^{3}$ content DLCs,
although Zia [22] notes that value may vary with deposition technique: some
producing high $sp^{3}$ DLCs at 140 eV [23]; others producing adequate
$sp^{3}$ content at as low as 30 eV [24]. Therefore, it is important to
estimate the energies of the sputtered particles that contribute to film
formation. When using a process such as FCVA, a common technique for producing
DLC with a high proportion of $sp^{3}$ bonding, the energy of the film-forming
particles is the same as the bias voltage applied to the substrate, so is
directly known. However, in the ECR IBD process, the energy of the Ar+ ions
are known, unlike the energy of the film-forming particles sputtered from the
target. Hence, SRIM simulation was used to calculate the energy distribution
of the film-forming particles.

###  3.3 Sputtered particles energy: simulation results

The angular energy distribution of the sputtered particles from the carbon
target was simulated using the Monte Carlo software SRIM [10] for different
ion beam energies. Figure 6 shows the energy and angular distribution of the
film forming particle and their relative frequency. High flux of low energy
particles are found in the middle of the sputtering plume, while a lower flux
but higher energy particles are sputtered towards higher emission angles.

Figure 6: Heatmaps (left) from SRIM simulation of the energy/angular
distribution of film forming C atoms sputtered using 4, 10, and 16 kV (top-
bottom) ion beam and the flux (right) or energy distribution of sputtered
atoms against their relative frequency. Figure 7: Sections of the SRIM
simulations with 4 kV (top), 10 kV (middle), and 16 kV (bottom) ion beams. The
energies seen by the substrates positioned at various positions within the
plume are annotated in white in the figure.

Figure 7 presents a plane section of the sputtering plume energy distribution
representing the energies seen by the substrates positioned at various
positions within the plume. The C atoms were sputtered from the target with
energies of approximately 30-60 eV at 4 kV, 40-100 eV at 10 kV, and even as
high as 125 eV at 16 kV. High $sp^{3}$ content is correlated with high density
in DLC films, with pure diamond having a density of 3.52 g/cm3. As the density
of the deposited films approaches this value, the $sp^{3}$ fraction also
increases. High density is achieved when incoming atoms have an optimum
energy, which is high enough to be sub-planted in the deposited film but low
enough not to cause thermal dissipation in the film (Robertson 2011). If the
incoming atom energy is too low, it will stop at the surface, resulting in low
density; if the energy is too high, some of that energy will be dissipated
thermally in the film, resulting in the dispersion of already implanted atoms,
in addition to thermally driven structural changes, for example, localised
phase transitions and potential modifications to internal stresses. According
to the XPS results presented in the previous section, the optimum ion energy
for the highest $sp^{3}$ fraction is at 10 kV which corresponds to 41 eV
sputtered atom energy in the middle of the plume, where the flux is the
highest.

##  4 Conclusion

Hydrogen-free DLC coatings were deposited at room temperature using the ECR-
IBD process. Raman spectra of the coatings were analysed and fitted and the
fitting parameters were used to estimate the $sp^{3}$ ratio in the deposited
films. Direct quantitative measurements were performed using XPS. The impact
of the energy of both the ion beam and the sputtered particle on the structure
of the manufactured coatings was investigated. The results show that the ECR-
IBD technique produces DLC thin films with $sp^{3}$ content up to 85%,
surpassing the values previously reported for other sputtering techniques.

##  5 Acknowledgments

We are grateful for financial support from STFC (ST/V005642/1, ST/W005778/1,
and ST/S001832/1) and the University of Strathclyde. S.R. is supported by a
Royal Society Industry Fellowship (INF$\backslash$R1$\backslash$201072). We
are grateful to the National Manufacturing Institute for Scotland (NMIS) for
support, and we thank our colleagues within SUPA for their interest in this
work.

## References

  * [1] J. Robertson, Plasma Deposition of Diamond-Like Carbon 50 01AF01.  doi:10.1143/JJAP.50.01AF01.
URL https://iopscience.iop.org/article/10.1143/JJAP.50.01AF01

  * [2] S. Aisenberg, R. Chabot, Ion‐Beam Deposition of Thin Films of Diamondlike Carbon 42 (7) 2953–2958.  doi:10.1063/1.1660654.
URL https://aip.scitation.org/doi/10.1063/1.1660654

  * [3] M. Chhowalla, J. Robertson, C. W. Chen, S. R. P. Silva, C. A. Davis, G. A. J. Amaratunga, W. I. Milne, Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon ( _ta_ -C) films 81 (1) 139–145.  doi:10.1063/1.364000.
URL http://aip.scitation.org/doi/10.1063/1.364000

  * [4] G. Zambrano, H. Riascos, P. Prieto, E. Restrepo, A. Devia, C. Rincón, Optical emission spectroscopy study of r.f. magnetron sputtering discharge used for multilayers thin film deposition 172 (2-3) 144–149.  doi:10.1016/S0257-8972(03)00339-6.
URL https://linkinghub.elsevier.com/retrieve/pii/S0257897203003396

  * [5] A. Varade, A. Krishna, K. N. Reddy, M. Chellamalai, P. Shashikumar, Diamond-like carbon coating made by rf plasma enhanced chemical vapour deposition for protective antireflective coatings on germanium, Procedia Materials Science 5 (2014) 1015–1019.  doi:10.1016/j.mspro.2014.07.390.
URL https://doi.org/10.1016/j.mspro.2014.07.390

  * [6] J. Vetter, 60 years of DLC coatings: Historical highlights and technical review of cathodic arc processes to synthesize various DLC types, and their evolution for industrial applications 257 213–240.  doi:10.1016/j.surfcoat.2014.08.017.
URL https://linkinghub.elsevier.com/retrieve/pii/S0257897214007257

  * [7] T. Higuchi, Y. Mabuchi, H. Ichihara, T. Murata, M. Moronuki, Development of hydrogen-free diamond-like carbon coating for piston rings, Tribology Online 12 (3) (2017) 117–122.  doi:10.2474/trol.12.117.
URL https://doi.org/10.2474/trol.12.117

  * [8] C. Gier, M. B. Yaala, C. Wiseman, S. MacFoy, M. Chicoine, F. Schiettekatte, J. Hough, S. Rowan, I. Martin, P. MacKay, et al., Controlling the optical properties of hafnium dioxide thin films deposited with electron cyclotron resonance ion beam deposition, Thin Solid Films 771 (2023) 139781.  doi:10.1016/j.tsf.2023.139781.
URL https://doi.org/10.1016/j.tsf.2023.139781

  * [9] R. Birney, J. Steinlechner, Z. Tornasi, S. MacFoy, D. Vine, A. Bell, D. Gibson, J. Hough, S. Rowan, P. Sortais, et al., Amorphous silicon with extremely low absorption: Beating thermal noise in gravitational astronomy, Physical review letters 121 (19) (2018) 191101.  doi:10.1103/PhysRevLett.121.191101.
URL https://doi.org/10.1103/PhysRevLett.121.191101

  * [10] J. F. Ziegler, M. D. Ziegler, J. P. Biersack, SRIM - The stopping and range of ions in matter (2010) 268 1818–1823.  doi:10.1016/j.nimb.2010.02.091.
URL https://ui.adsabs.harvard.edu/abs/2010NIMPB.268.1818Z

  * [11] J. F. Ziegler, J. Biersack, U. Littmark, The stopping and ranges of ions in solids, vol. 1, The Stopping and Ranges of Ions in Matter.
  * [12] B. Eren, L. Marot, M. Langer, R. Steiner, M. Wisse, D. Mathys, E. Meyer, The effect of low temperature deuterium plasma on molybdenum reflectivity 51 (10) 103025.  doi:10.1088/0029-5515/51/10/103025.
URL https://iopscience.iop.org/article/10.1088/0029-5515/51/10/103025

  * [13] C. Casiraghi, A. C. Ferrari, J. Robertson, Raman spectroscopy of hydrogenated amorphous carbons 72 (8) 085401.  doi:10.1103/PhysRevB.72.085401.
URL https://link.aps.org/doi/10.1103/PhysRevB.72.085401

  * [14] J. Robertson, Diamond-like amorphous carbon 37 (4-6) 129–281.  doi:10.1016/S0927-796X(02)00005-0.
URL https://linkinghub.elsevier.com/retrieve/pii/S0927796X02000050

  * [15] W. Cui, Q. Lai, L. Zhang, F. Wang, Quantitative measurements of sp3 content in DLC films with Raman spectroscopy 205 (7) 1995–1999.  doi:10.1016/j.surfcoat.2010.08.093.
URL https://linkinghub.elsevier.com/retrieve/pii/S0257897210007425

  * [16] A. C. Ferrari, J. Robertson, Raman spectroscopy of amorphous, nanostructured, diamond–like carbon, and nanodiamond, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences 362 (1824) (2004) 2477–2512.  doi:10.1098/rsta.2004.1452.
URL https://doi.org/10.1098/rsta.2004.1452

  * [17] W.-D. Münz, T. Zufrass, Industrial scale deposition of well adherent superhard and low friction c-dlc coatings grown by hipims and anode assisted unbalanced magnetron sputtering, Surface and Coatings Technology 387 (2020) 125485.  doi:10.1016/j.surfcoat.2020.125485.
URL https://doi.org/10.1016/j.surfcoat.2020.125485

  * [18] E. C. T. Ba, M. R. Dumont, P. S. Martins, B. da Silva Pinheiro, M. P. M. da Cruz, J. W. Barbosa, Deconvolution process approach in raman spectra of dlc coating to determine the sp3 hybridization content using the id/ig ratio in relation to the quantification determined by x-ray photoelectron spectroscopy, Diamond and Related Materials 122 (2022) 108818\.  doi:10.1016/j.diamond.2021.108818.
URL https://doi.org/10.1016/j.diamond.2021.108818

  * [19] J. Lomon, P. Chaiyabin, T. Saisopa, K. Seawsakul, N. Saowiang, K. Promsakha, P. Poolcharuansin, N. Pasaja, A. Chingsungnoen, R. Supruangnet, et al., Xps and xas preliminary studies of diamond-like carbon films prepared by hipims technique, in: Journal of Physics: Conference Series, Vol. 1144, IOP Publishing, 2018, p. 012048.  doi:10.1088/1742-6596/1144/1/012048.
URL https://iopscience.iop.org/article/10.1088/1742-6596/1144/1/012048

  * [20] X. Li, H. Miao, H. Zhu, Deposition and characterization of diamond-like carbon films by electron cyclotron resonance microwave plasma enhanced unbalanced magnetron sputtering, Chemical Engineering Transactions 66 (2018) 19–24.  doi:10.3303/CET1866004.
URL https://doi.org/10.3303/CET1866004

  * [21] Y. Lifshitz, S. R. Kasi, J. W. Rabalais, Subplantation model for film growth from hyperthermal species: Application to diamond 62 (11) 1290–1293.  doi:10.1103/PhysRevLett.62.1290.
URL https://link.aps.org/doi/10.1103/PhysRevLett.62.1290

  * [22] A. W. Zia, S. A. Hussain, M. M. F. A. Baig, Optimizing diamond-like carbon coatings - From experimental era to artificial intelligence 48 (24) 36000–36011.  doi:10.1016/j.ceramint.2022.10.149.
URL https://linkinghub.elsevier.com/retrieve/pii/S0272884222037300

  * [23] P. J. Fallon, V. S. Veerasamy, C. A. Davis, J. Robertson, G. A. J. Amaratunga, W. I. Milne, J. Koskinen, Properties of filtered-ion-beam-deposited diamondlike carbon as a function of ion energy 48 (7) 4777–4782.  doi:10.1103/PhysRevB.48.4777.
URL https://link.aps.org/doi/10.1103/PhysRevB.48.4777

  * [24] D. R. McKenzie, D. Muller, B. A. Pailthorpe, Compressive-stress-induced formation of thin-film tetrahedral amorphous carbon 67 (6) 773–776.  doi:10.1103/PhysRevLett.67.773.
URL https://link.aps.org/doi/10.1103/PhysRevLett.67.773