Review Reports - Characterization of Boron Coatings Produced by RF Planar Magnetron Sputtering

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear colleagues, your work on the deposition of a pure boron film using RF magnetron sputtering may be useful for industrial applications. The manuscript describes the research results in detail. However, to improve the quality of the article, I recommend making changes and additions in accordance with the comments below.

General note

I advise against using the term "plasma." The magnetron operates on a gas discharge, which may contain a region remote from the cathode that exhibits plasma properties. This region could also be located 80 mm from the cathode, at the point where you installed the Langmuir probe. However, your paper lacks information on the ion concentration at this point, so referring to plasma is physically incorrect. I recommend replacing the term "plasma" with "gas discharge" in the manuscript.

Abstract:

It's important that after reading the Abstract, the reader understands:

the purpose of the work. It can be assumed that the purpose of your work was to study the effect of argon pressure on the deposition of pure boron films;
the main results;
the novelty of the results.

This structure is necessary so that the reader has a good understanding of the article after reading the Abstract. The Abstract is the most important part of the manuscript, published separately from the article. It should pique the reader's interest in your work, so it must be written very thoughtfully.

Introduction

Typically, this section contains an analysis of the authors' stated problem, which allows them to justify and formulate the research objective.

You have clearly described the reasons for your interest in pure boron films and indicated the methods used to deposit them. To improve the quality of your article, I recommend that you describe the specific results obtained by different authors (composition, structure, and physical properties of the films). Please indicate what lack of information about these films or their technology prompted you to begin your own research.
This section lacks information on the crystal structure of pure boron films. Is this characteristic of the films irrelevant?
Please include the objective of the work, as stated in the Abstract, in the last paragraph of this section.

Materials and Methods

2.1 Experimental Apparatus

The subsection is written in detail and clearly. No comments.

2.2 Plasma Diagnostics

See general note.
Please indicate how these measurements relate to the overall goal of this work, which is the deposition of pure boron films.

2.3 Mass Density (ρ) Measurements

The subsection is written in detail and clearly. No comments.

2.4 Materials

I recommend moving the text of this subsection to paragraph 2.1

2.5 Characterization of Coatings

The subsection is written in detail and clearly. No comments.

Results

3.1 Plasma Characterization.

An increase in pressure by almost a factor of 10 should proportionally reduce the electron mean free path (e.g., doi:10.1063/5.0022308 or doi:10.1134/S0018151X15020121). Moreover, based on Fig. 1, one would expect a significant effect of pressure on the discharge properties shown in Figs. 3 and 4. However, this was not observed. Therefore, it is advisable to:

describe the method for calculating the electron temperature and concentration in more detail;
comment on the obtained results, discussing their physical causes.

3.2 Coating deposition

This section describes in detail the problems that arise during the deposition of pure boron films. It would be more appropriate to move this information to the introduction, where you could point out the methods you used in your work to overcome the known problems.
The statement "This result can … (see nₑ as a function of sputtering pressure in Fig. 4)" (lines 229-231) is questionable. Figure 4 lacks information on the ion concentration.
The statement "In sputtering systems, the … from internal surfaces." (lines 253-254) requires confirmation in the form of a source citation.
The proportional increase in oxygen concentration shown in Fig. 8, in my opinion, indicates the presence of oxygen in the argon. If you have information on increased oxygen desorption from the chamber walls with increasing sputtering gas pressure, please cite it.

The study of the film composition left a good impression. There are no comments on this part of the manuscript.

Conclusions

Please indicate whether the objective of the work, as stated in the introduction, was achieved.
Please indicate the novelty of the results obtained.

References

No comments.

Author Response

Comment 1:

General note

Response: We understand the referee’s comment; the term ‘plasma’ appears 31 times in the manuscript, including 13 occurrences in the reference list. In the main text, the term is primarily used in relation to the Langmuir probe characterization phase. To address the referee’s concern, we have revised the terminology where appropriate. Specifically, in the Introduction, ‘low-pressure plasma discharges’ has been changed to ‘low-pressure gas discharges,’ and in Section 2.1 Experimental Apparatus, ‘The plasma system’ has been replaced with ‘The magnetron system’.

Regarding the ion concentration, in accordance with the referee's comment, the ion flux has been included in Section 3.1 Plasma Characterization, together with the corresponding description.

Comment 2:

Abstract

It's important that after reading the Abstract, the reader understands:

the purpose of the work. It can be assumed that the purpose of your work was to study the effect of argon pressure on the deposition of pure boron films;
the main results;
the novelty of the results.

Response: In according to the referee's comment, the abstract has been revised.

Comment 3:

Introduction

Typically, this section contains an analysis of the authors' stated problem, which allows them to justify and formulate the research objective.

You have clearly described the reasons for your interest in pure boron films and indicated the methods used to deposit them. To improve the quality of your article, I recommend that you describe the specific results obtained by different authors (composition, structure, and physical properties of the films). Please indicate what lack of information about these films or their technology prompted you to begin your own research.

Response: We thank the referee for this observation, we revised the introduction in several parts (lines 71-91).

This section lacks information on the crystal structure of pure boron films. Is this characteristic of the films irrelevant?

Response: We understand the referee’s comment. Unfortunately, we did not have sufficient time to perform a detailed XRD analysis. Only one film was analyzed, and it exhibited an amorphous structure. A comprehensive XRD study will be included in our future work.

Please include the objective of the work, as stated in the Abstract, in the last paragraph of this section.

Response: We thank the referee for this observation, the aim of this study is to clarify how sputtering pressure affects plasma conditions and the resulting microstructure, density, and oxygen uptake of boron thin films. We revised the introduction with reference to these points.

Comment 4

Materials and Methods

2.1 Experimental Apparatus

The subsection is written in detail and clearly. No comments.

2.2 Plasma Diagnostics

See general note.
Please indicate how these measurements relate to the overall goal of this work, which is the deposition of pure boron films.

Response: We understand the referee’s comment. We reiterate that the aim of this study is to clarify how sputtering pressure affects plasma conditions and the resulting microstructure, density, and oxygen uptake of boron thin films. Plasma parameters were calculated in order to explore possible correlations between these parameters and the type of coating subsequently grown. However, our experimental results show that, within the analyzed pressure range, the plasma parameters do not vary substantially (please also see Comment 4). Nevertheless, as emphasized in the manuscript, there are very few (if any) studies on RF sputtering of boron in this pressure range. Therefore, the calculated plasma parameters still provide valuable reference data for databases of these processes.

2.3 Mass Density (ρ) Measurements

The subsection is written in detail and clearly. No comments.

2.4 Materials

I recommend moving the text of this subsection to paragraph 2.1

Response: In accordance with the referee’s comment, Section 2.4 (Materials) has been moved to Section 2.1.

2.5 Characterization of Coatings

The subsection is written in detail and clearly. No comments.

Comment 4:

Results

3.1 Plasma Characterization.

describe the method for calculating the electron temperature and concentration in more detail;
comment on the obtained results, discussing their physical causes.

Response: As we stated in section 2.3 Plasma Characterisation, the method used to determine electron density and electron temperature is the Druyvesteyn method [references 24–26], which is the correct method non-equilibrium (non-Maxwellian) plasmas. Anyway, for a more comprehensive characterization, we have added the discussion regarding the ion flux in lines 208–216.

Regarding the effect of pressure on the discharge properties, it is useful to analyze Figure 2 of the paper (Electron Energy Distribution Functions, EEDFs). The shape of the EEDFs does not change substantially, and examining the high-energy tail (>10 eV) shows that the most energetic electrons are not significantly affected by the increase in pressure. This may indicate that most of the electrons reaching the probe do not lose significant energy in collisions, and therefore the electron temperature and density remain similar, as the more energetic electrons dominate the probe measurement. These energetic electrons are not significantly thermalized by collisions with neutrals.

Based on the above considerations, we have added the following text in Section 3.1:

“This is supported by the analysis of the EEDFs (Fig. 2), the shape of the EEDFs does not change substantially, and examining the high-energy tail (>10 eV) shows that the most energetic electrons are not significantly affected by the increase in pressure. This may indicate that most of the electrons reaching the probe do not lose significant energy in collisions, and therefore, the electron temperature and density remain similar, as the more energetic electrons dominate the probe measurement. These energetic electrons are not significantly thermalized by collisions with neutrals.”

3.2 Coating deposition

This section describes in detail the problems that arise during the deposition of pure boron films. It would be more appropriate to move this information to the introduction, where you could point out the methods you used in your work to overcome the known problems.

Response: We understand the referee’s comment. The Introduction (lines 71–84) has been partially revised in accordance with this observation. However, Section 3.2 (Coating deposition) has been kept largely unchanged (with minor additions in lines 247–251), as we believe this content is important for the discussion.

2. The statement "This result can … (see nₑ as a function of sputtering pressure in Fig. 4)" (lines 229-231) is questionable. Figure 4 lacks information on the ion concentration.

Response: We thank the referee for this observation. This information has been supplied in Figure 4 with discussion in section 3.1 (lines 208-216).

3. The statement "In sputtering systems, the … from internal surfaces." (lines 253-254) requires confirmation in the form of a source citation.

Response: In accordance with the referee’s comment, the reference has been added.

4. The proportional increase in oxygen concentration shown in Fig. 8, in my opinion, indicates the presence of oxygen in the argon. If you have information on increased oxygen desorption from the chamber walls with increasing sputtering gas pressure, please cite it.

Response: We understand the referee’s comment. Although we do not have a direct measurement of the increase in oxygen content with increasing pressure, in the revised version of the Introduction (lines 71–84) we have addressed the issue of oxygen impurities in the argon gas cylinder. This issue is also discussed in Section 3.2 (lines 247–251).

The study of the film composition left a good impression. There are no comments on this part of the manuscript.

Comment 5:

Conclusions

Please indicate whether the objective of the work, as stated in the introduction, was achieved.
Please indicate the novelty of the results obtained.

Response: We thank the referee for this observation. The Conclusion section has been updated accordingly and now includes the following information:

“Overall, the results show that the influence of sputtering pressure on plasma parameters and coating properties has been successfully assessed, providing new insights into the pressure-dependent trends in boron coating density, surface morphology, and oxygen incorporation, for which no experimental studies under conditions comparable to ours have been reported in the literature.”

References

No comments.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

There are some open points which must be answered/explained in the text prior to publication.

Section 2.1: deposition time must be provided.

Sec. 2.2: A few words regarding the RF compensation of the probe are required. The plasma and floating potential should be presented as well. The difference between these two potentials is a measure of the electron temperature, see Eq. 8 of Merlino, Understanding Langmuir probe current-voltage characteristics, American Journal of Physics 75, 1078 (2007); doi: 10.1119/1.2772282

Page 6, Lines 199/200: I do not understand this. What do you mean with “conventional thin –film deposition”?

Fig. 5: add “scale” to each figure. Right now it is not clear what the scale is.

Fig. 6: In order to “guide the eye” some lines could be added.

Page 6, line 234 and figure 7: how was the curve calculated?

Page 9, line 295: with heights “in the range of one micro-metre”?

Page 11, line 314: the W signal is rather large, see figure 11(c), so this should not provide a “high uncertainty”. Anyhow, what is the proposed mechanism for W incorporation into the B film?

Page 11, line 316: the “adventitious contamination” gives rise to a C-1s peak close to 284.8 eV, see www.xpsfitting.com/search/label/Adventitious , while in your sample most C-1s peaks have considerably smaller binding energy. Must be explained.

Author Response

There are some open points which must be answered/explained in the text prior to publication.

Section 2.1: deposition time must be provided.

Response: The section has been revised as follows: The deposition time has now been added to Section 2.1.

Response: The section has been revised as follows: “The probe has a compensation circuit in order to minimize the RF interference with the measured probe signal”, and a reference has been added.

Regarding the difference between the plasma potential and the floating potential, it is expected to be proportional to the average electron temperature in the case of a Maxwellian distribution (Chen F.F., 2001, Physics of Plasmas, 8, 3029). However, as demonstrated in this experimental work, the electron energy distribution function (EEDF) is not Maxwellian (figure 2).

Page 6, Lines 199/200: I do not understand this. What do you mean with “conventional thin –film deposition”?

Response: Conventional thin film growth refers to a deposition process in which the film develops through surface nucleation followed by island coalescence and continuous growth, according to the classical growth mechanisms of Frank–van der Merwe, Volmer–Weber, or Stranski–Krastanov. As clarified in the subsequent sentence, boron coatings deposited by sputtering may exhibit cluster formation.

Fig. 5: add “scale” to each figure. Right now it is not clear what the scale is.

Response: In accordance with the referee’s comment, the figures have been revised as suggested.

Fig. 6: In order to “guide the eye” some lines could be added.

Response: In accordance with the referee’s comment, the figure has been revised as suggested.

Page 6, line 234 and figure 7: how was the curve calculated?

Response: The mean free path of sputtered atoms can be estimated with different approaches. We used the formula in International Journal of Materials Science and Applications 2014; 3(2): 29-36 (10.11648/j.ijmsa.20140302.14)

Page 9, line 295: with heights “in the range of one micrometre”?

Response: In accordance with the referee’s comment, the text has been revised.

Page 11, line 314: the W signal is rather large, see figure 11(c), so this should not provide a “high uncertainty”. Anyhow, what is the proposed mechanism for W incorporation into the B film?

Response: W is present as a background impurity originating from the chamber components (in particular from the cathode shield). Therefore, we identify W impurity co-deposition during film growth as the primary mechanism of incorporation into the B film.

Response: As correctly noted by the reviewer, the C 1s spectrum exhibits fitting components at binding energies lower than those associated with hydrocarbon (CHx) contamination (~284.8 eV) and graphitic C=C carbon (284.3 eV). These lower binding energy components were assigned to boron and tungsten carbide species in both stoichiometric and non-stoichiometric forms. Upon sputtering, the relative contributions of the CHx and C=C components decrease, while the B_xC_y and C_xW_y contributions increases, consistent with the increasing boron and tungsten concentrations. At a higher number of sputtering cycles, the components attributed to B_xC_y and C_xW_y become dominant in the C 1s spectrum. Nevertheless, for all sputtering cycles, spectral alignment was performed using the positions of both the CH_x and C=C fitting components as reference. The manuscript has been modified as follows:

=> Section 2.5 Characterization of Coatings Page 4 Lines 173-176.

“Control on the correct spectra alignment on the BE scale was done ensuring that the fitting components assigned to hydrocarbon contamination CH_x and the graphitic C=C carbon were located respectively at 284.8 eV and 284.3 eV.”

=> Section 3.2 Coating deposition

“Most of the observed carbon is due to adventitious contamination, resulting from air exposure and the adsorption of organic molecules.”

Changed in (lines 360-365):

“Most of the detected carbon originates from adventitious contamination caused by air exposure, leading to the adsorption of organic species. The components observed at 284.8 eV and 284.3 eV are attributed to CHx species and graphitic C=C carbon, respectively. The lower binding energy contributions, in the range of 282–283.5 eV, are assigned to carbon – boron and carbon–tungsten (C–W) bonds.” [NIST X-ray Photoelectron Spectroscopy Database - Version 5.0 (2023) on line version, https://dx.doi.org/10.18434/T4T88K]

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Review

Characterization of boron coatings produced by RF planar magnetron sputtering

Line 16 and 81: the paper refers to a study with a variation of the Ar pressure in the chamber. In line 81 a constant flow rate of 20 sccm is mentioned and later a TP with pumping speed of 150 l/s. It is not clear how the pressure is changed. This aspect is important, if the pressure was increased by partially closing a diaphragm (or ~valve) the base pressure was increased proportionally causing additional contamination of the growing thin film.
Comment: MS coating deposition is usually done in a pressure range from 0.1….1 Pa. The lower pressure is especially important for the high target substrate distance of 12 cm. The authors might comment on the high pressure used and the neglection of the low pressure range.
Line 42: “Each technology has its peculiarities, advantages, and disadvantages” sentence without any content.
Line 125 and 137: it is not clear which SEM was used, or if 2 different SEMs were used which was used when.
Line202-225: Essentially describes why it is not common to apply pressure > 2 Pa during the sputtering of any material
Line 214: There is no increased deposition rate at high pressures, on the contrary, atoms are deflected towards the target and the walls, and the deposition rate is reduced.
Line257-276 and fig 8: As described by the authors, the incorporation of oxygen is typical for the deposition of reactive materials like Ti, Cr, B and others. To avoid a notable contribution of the oxygen content in the thin film, either the deposition rate is increased or the base pressure reduced. The latter might not be very efficient in “dirty” chambers. For decorative and abrasion resistant TiN any oxygen content > 10 % is causing a colour change and/or reduction of mechanical properties and the coating is considered being scrap. The 1^st step to avoid the contamination is thorough cleaning (sand blasting of all surfaces) of the chamber.
Line 305: If the result of an experimental study indicates that the optimum conditions are at the extreme of the analyzed experimental value (in this case lowest total pressure), than the question will always be, why was the experimental window not extended.
Table 1: It is not clear which sample was analyzed by XPS (sample produced at 0.6 Pa???)

General Remarks:

The article is well written and describes the deposition of B-coatings by RF magnetron sputtering. The coating characterization is quite complete for the envisaged application (protection of surfaces in fusion reactors). The experimental conditions (choice of pressure range) are very unfortunate and reduce the value for any reader.

Author Response

Comment 1: Line 16 and 81: the paper refers to a study with a variation of the Ar pressure in the chamber. In line 81 a constant flow rate of 20 sccm is mentioned and later a TP with pumping speed of 150 l/s. It is not clear how the pressure is changed. This aspect is important, if the pressure was increased by partially closing a diaphragm (or ~valve) the base pressure was increased proportionally causing additional contamination of the growing thin film.

Response: The sputtering process pressure was controlled by partially closing a throttle valve between the vacuum chamber and the pump. This information has been inserted in section 2.2 Experimental Apparatus.

Comment 2: Comment: MS coating deposition is usually done in a pressure range from 0.1….1 Pa. The lower pressure is especially important for the high target substrate distance of 12 cm. The authors might comment on the high pressure used and the neglection of the low pressure range.

Response: We understand the referee’s comment, this experimental work has the character of a general study aimed at exploring the production of boron coatings by non-reactive sputtering and at characterizing the plasma in order to assess whether correlations can be established between plasma parameters and the properties of the deposited coatings. The results obtained may contribute to a better understanding of this process and its potential industrial applications.

Among the possible applications, we also consider fusion-related ones. In fusion devices, boronization of the walls produces thin boron-based films (typically on the order of a few hundred nanometers). Therefore, the coatings produced in this work may serve as a proxy for the boron films formed in tokamaks, helping to better understand their characteristics.

Remaining within the fusion context, boronization processes can occur at pressures in the range of approximately 0.1–2 Pa; therefore, the pressure range investigated in this work may be relevant for this field.

Comment 3: Line 42: “Each technology has its peculiarities, advantages, and disadvantages” sentence without any content.

Response: We understand the referee’s comment, we have removed the sentence “Each technology has its peculiarities, advantages, and disadvantages.”

Comment 4: Line 125 and 137: it is not clear which SEM was used, or if 2 different SEMs were used which was used when.

Response: We thank the referee for this observation. Indeed, a second SEM instrument was indicated by mistake. We have therefore removed the following sentence:

“Cross-sectional observations of the sputtered B coatings were performed using a CrossBeam 1540 XB system (Carl Zeiss AG, Oberkochen, Germany), equipped with a focused ion beam (FIB; resolution: 7 nm) and a field-emission SEM (FESEM; resolution: 1.1 nm). The FIB was used for sample milling, while the FESEM was employed for imaging both the surface and cross sections of the samples.”

Comment 5: Line 202-225: Essentially describes why it is not common to apply pressure > 2 Pa during the sputtering of any material.

Response: We agree with this comment. In the response to Comment 2, we justify the pressure range used in this study.

Comment 6: Line 214: There is no increased deposition rate at high pressures, on the contrary, atoms are deflected towards the target and the walls, and the deposition rate is reduced.

Response: We understand this comment. In this part of the text we refer to pressures in the range of 4–5 Pa, where diffusive processes of boron atoms indeed dominate. Since we were not able to obtain a measurable deposition rate at these pressures (as explained in the manuscript), we have erased the following sentence at pag. 7:

“Another concomitant factor promoting cluster growth is the higher deposition rate at these pressures, which leads to rapid accumulation of boron atoms on the substrate surface, leaving insufficient time for diffusion and the formation of a continuous coating“

Comment 7: Line257-276 and fig 8: As described by the authors, the incorporation of oxygen is typical for the deposition of reactive materials like Ti, Cr, B and others. To avoid a notable contribution of the oxygen content in the thin film, either the deposition rate is increased or the base pressure reduced. The latter might not be very efficient in “dirty” chambers. For decorative and abrasion resistant TiN any oxygen content > 10 % is causing a colour change and/or reduction of mechanical properties and the coating is considered being scrap. The 1^st step to avoid the contamination is thorough cleaning (sand blasting of all surfaces) of the chamber.

Response: We thank the referee for this suggestion.

Comment 8: Line 305: If the result of an experimental study indicates that the optimum conditions are at the extreme of the analyzed experimental value (in this case lowest total pressure), than the question will always be, why was the experimental window not extended.

Response: We understand the referee’s comment, in the response to Comment 2, we justify the pressure range used in this study.

Comment 9: Table 1: It is not clear which sample was analyzed by XPS (sample produced at 0.6 Pa???)

Response: The analyzed sample is specified in lines 303–306. However, for greater clarity, we have also added an indication of the sample type in the caption of the table.

General Remarks:

Response: We would like to thank the referee for the positive assessment of our paper. Regarding the range of pressure, the comment 2 explains the choice.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have considered all points of this reviewer. The manuscript is improved and now ready for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Boronization in fusion reactors will not be done by RF sputtering. By other techniques, the effect of the working pressure is very different