Effect of Negative Substrate Bias Voltage and Pressure on the Structure and Properties of Tungsten Films Deposited by Magnetron Sputtering Technique

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is devoted to studies of thin tungsten films obtained by radio frequency magnetron sputtering at various pressures in the chamber and the substrate potentials. The paper does not contain fundamentally new results. The results obtained should be analyzed in more detail so that the article can be published. During the review, the following comments and suggestions have arose.

1. The authors should justify the use of RF magnetron sputtering for deposition of tungsten films. DC magnetron sputtering is simpler, but also makes it possible to obtain tungsten films, since it is a highly conductive metal.
2. The authors practically do not use the measured plasma characteristics in the article to explain the properties of the resulting films. Neither the plasma density nor the electron temperature appear in the discussion and conclusions. The question arises about the need for section 1 in this article.
3. The concept of "mean free path" is used to explain the properties of films produced at different pressures. The authors should estimate this mean free path at different pressures and compare it with the target-substrate distance for a more reliable justification of their reasoning. Perhaps an article “Mitin, D.M., Serdobintsev, A.A. Effect of scattering of sputtered atoms on the growth rate of films fabricated by magnetron sputtering.  Phys. Lett. 43, 814–816 (2017). https://doi.org/10.1134/S1063785017090073” will be useful for this.
4. The quality of the article can be improved by estimating the mean size or size distribution of columns and pores in tungsten films obtained under various conditions.
5. Page 3, lines 6-9 of section 2.5. The name of the AFM setup is repeated The sentence on lines 6-7 should probably be deleted.
6. Page 3, lines 14 of section 2.5. The superfluous word "Their".

Author Response

Comment 1: The authors should justify the use of RF magnetron sputtering for deposition of tungsten films. DC magnetron sputtering is simpler, but also makes it possible to obtain tungsten films, since it is a highly conductive metal.

Response: We understand the referee's comment, it is understandable and certainly correct.

Without delving into the complexity and costs (DC: easier to implement, less expensive... RF: more complex, requiring impedance matching and more expensive generators), it's worth noting that in RF, ionization efficiency is higher, allowing for operation at lower pressures. Working at lower pressures during the deposition process helps produce purer, denser, more adherent, and directional films, ultimately enhancing overall quality and performance, particularly for possible high-tech applications. Of course, the specific requirements for the film are crucial. Generally, RF results in better thin films than DC. RF-sputtered films tend to be smoother and have a higher packing density. For these reasons, we believe it’s valuable to investigate metal deposition using RF. Notably, only a few studies have explored RF magnetron sputtering of pure tungsten, in particular focusing on the correlation between plasma parameters and the resulting coating characteristics. The following sentences have been added to the text.

REVISED TEXT: “Since most of the studies have investigated the influence of deposition parameters on the structure, morphology and optical properties of metallic W films produced using DC magnetron sputtering [7, 12-19] this experimental work explored this topic in the context of RF magnetron sputtering. It's worth noting that the ionization efficiency is higher in RF, allowing for operation at lower pressures. Working at lower pressures during the deposition process helps produce purer, denser, more adherent, and directional films, ultimately enhancing overall quality and performance, particularly for possible high-tech applications”

 

12. S. Chen, L.C. Yang, H.S. Tian, C.S. Hsu, Evaluating substrate bias on the phase-forming behavior of tungsten thin films deposited by diode and ionized magnetron sputtering, Thin Solid Films, 484, 1–2, 2005, 83-89,
13. G Vijaya et al 2016 IOP Conf. Ser.: Mater. Sci. Eng. 149 012075
14. Chargui A, Beainou RE, Mosset A, Euphrasie S, Potin V, Vairac P, Martin N. Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films. Nanomaterials (Basel). 2020 Jan 1;10(1):81.
15. -na. Zhu, G.-l. Li, H.-d. Wang, B.-s. Xu, D.-m. Zhuang, and J.-j. Liu, Current Applied Physics 9(2), 510 (2009).
16. L. H. Maier, M. Balden, J. Linke, F. Koch, and H. Bolt, Surf. Coat. Technol. 142-144, 5 (2001).
17. T. Grant, M. J. O’Keefe, and J. S. Solomon, Journal of Electronic Materials 24, 7 (1995).
18. Zheng, B. D. Ozsdolay, and D. Gall, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 33(6), 061505 (2015)
19. K. Rane, S. Menzel, T. Gemming, and J. Eckert, Thin Solid Films 571, 1 (2014).

 

 

Comment 2: The authors practically do not use the measured plasma characteristics in the article to explain the properties of the resulting films. Neither the plasma density nor the electron temperature appear in the discussion and conclusions. The question arises about the need for section 1 in this article.

Response: We thank the referee for this observation, we revised the article in several parts by inserting the plasma parameters in the discussion and conclusions.

NEW TEXT (Section 3.2):

“This increase in DR can be largely attributed to the rise in G_i, a key parameter governing the sputtering process. G_i increases with pressure primarily because of the increased ne arising from enhanced collisional ionization. Although the Te decreases with pressure due to collisional cooling, its overall impact on G_i is less pronounced. While Te appears in the Bohm velocity equation (v_b = √(eTe /M_i), where e is the elementary charge and M_i is the ion mass), the square root relationship moderates its influence. Therefore, the dominant effect of the increasing ne leads to a net increase in G_i, which in turn enhances the sputtering yield and, consequently, the DR.”

….

….

“Furthermore, while the increase in ne promotes ionization and sputtering, the concurrent decrease in Te can influence the energy of ions reaching the substrate indirectly via its effect on the plasma potential. This complex interplay between pressure, ne, and Te plays a critical role in determining the final coating density and structure. Thus, the observed trends in DR and ? are consistent with the variations in plasma parameters, demonstrating how fundamental plasma characteristics significantly influence both the DR and the resulting coating properties.”

 

REVISED TEXT (Section 3.2.1, page 12):

The observed trends suggest that the energy of the ions impinging on the growth surface plays a critical role in determining the crystallinity of the films. Specifically, the variation in peak width with pressure and bias indicates that higher ion energies promote the formation of larger crystalline domains and improved crystallinity, while lower ion energies result in smaller domains and reduced crystallinity. The observed trends lead to infer that as the pressure increases i.e., the mean free path of the species inside the deposition chamber reduces, the atoms contributing to the growing film are more thermalized. On the contrary, the application of a polarization enhances the energy of the ionized species arriving on the growing film, allowing a greater mobility of the adatoms and therefore a higher crystallinity of the final product.

 

 

NEW TEXT (Conclusions):

….“These improvements in the film properties with V_neg can be attributed to the increased energy of ions impinging on the substrate, which enhances adatom mobility and promotes densification.”

……

“Specifically, increasing the pressure leads to a higher ne and a lower Te, which have a significant impact on the ion flux and the energy of ions reaching the substrate.”

…………

“This degradation of the film properties with increasing pressure can be directly linked to the changes in ne and Te, which result in a modified ion flux and a reduced ion energy at the substrate.”

 

Comment 3: The concept of "mean free path" is used to explain the properties of films produced at different pressures. The authors should estimate this mean free path at different pressures and compare it with the target-substrate distance for a more reliable justification of their reasoning. Perhaps an article “Mitin, D.M., Serdobintsev, A.A. Effect of scattering of sputtered atoms on the growth rate of films fabricated by magnetron sputtering. Phys. Lett. 43, 814–816 (2017). https://doi.org/10.1134/S1063785017090073” will be useful for this.

Response: We thank the referee for this observation, we added figure 5 to support and improve understanding in our discussions. We have also added the text.

NEW TEXT:

“For example, the l_mfp at 1 Pa is approximately 1.8 mm, which is less than one-third of the target-to-substrate distance in our system. Therefore, when sputtering in the pressure range of 1–5 Pa, the atoms lose much of their original energy through collisions in the gas phase and are not sufficiently energetic when they arrive at the growing surface…”

 

Comment 4: The quality of the article can be improved by estimating the mean size or size distribution of columns and pores in tungsten films obtained under various conditions

 

Response: We fully agree that estimating the mean size or size distribution of columns and pores would further enhance the quality of the article. However, the available images do not allow for an easy and precise quantitative measurement of these features. Nevertheless, as you rightly suggested, to improve the quality of the manuscript, we have revised Section 3.2.1 to include a qualitative comparison of column and pore sizes in the different coatings. Additionally, we have correlated these observations with the calculated density values to provide a more comprehensive analysis.

 

REVISED TEXT:

“W coatings exhibit a typical columnar structure, which varies as a function of pressure (a-c). As discussed previously, increasing the pressure leads to an increase in electron density and a decrease in electron temperature. This increase in nₑ enhances the ion flux towards the substrate, while the decrease in Tₑ reduces their energy. The lower ion energy favours the growth of thinner and more porous columns, as adatoms have less energy to diffuse and fill voids. This effect is evident in the coating morphology: at 1 Pa, the columns are wide and compact, with a density close to that of bulk W. At 3 Pa, the column dimensions decrease, and some pores appear, leading to a reduction in density to approximately 13 g/cm³. At 5 Pa, the columns become even smaller, with a further increase in porosity and a density drop to around 10 g/cm³.

Applying a bias voltage (d-e) significantly modifies these trends by directly increasing the energy of ions bombarding the surface. Unlike pressure, which influences ion energy indirectly through changes in Tₑ and plasma potential, the application of Vₙₑg directly accelerates ions towards the substrate, providing a more directional and controllable energy source. Furthermore, while reducing the pressure from 5 Pa to 1 Pa increases the plasma potential by approximately 4 V—leading to a minor increase in ion energy—the application of Vₙₑg has a much greater effect. This has a pronounced densification effect: at 3 Pa, applying -100 V increases the column width, making some structures resemble those at 1 Pa, while eliminating pores and raising the density to approximately 16 g/cm³. For the more porous 5 Pa coating, applying -150 V results in a columnar structure similar to the unbiased 3 Pa sample, with only a few residual pores and a comparable density of around 13 g/cm³.”

 

Comment 5: Page 3, lines 6-9 of section 2.5. The name of the AFM setup is repeated The sentence on lines 6-7 should probably be deleted.

Response: Thank you, we have removed the misprint [line 121-123]

 

REVISED TEXT: “The surface morphology of the samples was characterized by Atomic Force Microscopy (AFM) (Core AFM, Nanosurf GmbH, Langen, Germany) in dynamic mode and High-Resolution SEM (Tescan mod. MIRA III, Brno, Czech Republic).”

 

Comment 6: Page 3, lines 14 of section 2.5. The superfluous word "Their".

Response: Deleted the unnecessary word [line 127]

 

 

Note: we point out that other parts of the text have been revised/added to respond to other comments from other referees.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a well written and presented manuscript, scientifically sound, within conclusions well supported by the experimental data. In fact there is a very good combination of electron microscopy, AFM, X-ray diffraction and electrical measurements that make the work very comprehensive and given the reader new information. This is despite the relatively explored area of deposition of thin coatings with well established method as magnetron sputtering. 

 

If there is one improvement that can be done that will be in the EDX analysis of the layers and the Oxygen quantification. For many reasons Oxygen is difficult to quantify therefor I suggest presenting the Oxygen quant data as (yes/no) based on the signal seen above/or not above the background in the corresponding spectra. 

 

Author Response

Comment 1: This is a well written and presented manuscript, scientifically sound, within conclusions well supported by the experimental data. In fact there is a very good combination of electron microscopy, AFM, X-ray diffraction and electrical measurements that make the work very comprehensive and given the reader new information. This is despite the relatively explored area of deposition of thin coatings with well established method as magnetron sputtering.

Response: We thank the referee for the positive evaluation of the article.

 

Comment 2: If there is one improvement that can be done that will be in the EDX analysis of the layers and the Oxygen quantification. For many reasons Oxygen is difficult to quantify therefor I suggest presenting the Oxygen quant data as (yes/no) based on the signal seen above/or not above the background in the corresponding spectra.

Response: We understand the referee's comment, but we don't think we'll change it. Below you will find our motivation.

 

The two spectra below describe the results of the analyses on samples in which the oxygen content was determined as 3.9% wt (A) and 0.4% wt (B). The red line is the background signal, as calculated by EDS software, and in both cases, a difference between the background and the actual X-ray signal can be seen, the difference in (A) being way more obvious than in (B). A commonly-agreed-upon detection limit is 0.1 wt%, which is lower than the amount calculated in spectrum (B), so the quantification of the different amounts of oxygen appears reasonable, at least within the reliability of EDS analysis.

 

Fig (A) included in the pdf

 

Fig (B)  included in the pdf

Note: we point out that other parts of the text have been revised/added to respond to other comments from other referees.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors investigate the effects of substrate bias and sputtering pressure on the properties of tungsten films deposited using RF plasma magnetron sputtering. The findings are well-supported by the presented data; however, the following concerns should be addressed before the manuscript is considered for publication:

1. The influence of bias and pressure during sputter deposition has been extensively reported in the literature. While the authors claim that studies specifically focusing on substrate bias for tungsten coatings are limited, there are additional reports beyond reference 22 that examine the effects of bias on tungsten films. The authors should clearly articulate the novelty of their work beyond presenting a series of experimental results and discussions. This reviewer presumes that the key distinction may lie in the effects of bias in RF sputtering, as opposed to the well-documented bias effects in DC sputtering. If so, the authors should discuss the differences in bias effects between DC and RF sputtering.
2. The results should be compared with existing literature to contextualize the properties of tungsten coatings under varying sputtering conditions. For example, the sputtering parameters used in this study may overlap with those reported in prior research, such as the conditions surveyed in the article (https://doi.org/10.1016/j.tsf.2015.01.030). Identifying general trends and distinctions could help highlight the manuscript’s novelty.
3. To ensure the conditions for beta-phase formation, the authors should also present the films deposited under 3Pa/-150V, 1Pa/-100V, and 1Pa/-150V.

Author Response

Comment 1: The influence of bias and pressure during sputter deposition has been extensively reported in the literature. While the authors claim that studies specifically focusing on substrate bias for tungsten coatings are limited, there are additional reports beyond reference 22 that examine the effects of bias on tungsten films. The authors should clearly articulate the novelty of their work beyond presenting a series of experimental results and discussions. This reviewer presumes that the key distinction may lie in the effects of bias in RF sputtering, as opposed to the well-documented bias effects in DC sputtering. If so, the authors should discuss the differences in bias effects between DC and RF sputtering.

Response: We understand the referee's comment, it is understandable and certainly correct. Without delving into the complexity and costs (DC: easier to implement, less expensive... RF: more complex, requiring impedance matching and more expensive generators), it's worth noting that in RF, ionization efficiency is higher, allowing for operation at lower pressures. Working at lower pressures during the deposition process helps produce purer, denser, more adherent, and directional films, ultimately enhancing overall quality and performance, particularly for possible high-tech applications. Of course, the specific requirements for the film are crucial. Generally, RF results in better thin films than DC. RF-sputtered films tend to be smoother and have a higher packing density. For these reasons, we believe it’s valuable to investigate metal deposition using RF. Notably, only a few studies have explored RF magnetron sputtering of pure tungsten, in particular focusing on the correlation between plasma parameters and the resulting W coating characteristics. Regarding the negative substrate bias, we tried to emphasize more in the introduction the importance of our study in the RF case.

 

NEW TEXT (39-46 lines): “Since most of the studies have investigated the influence of deposition parameters on the structure, morphology and optical properties of metallic W films produced using DC magnetron sputtering [7, 12-19], this experimental work explored this topic in the context of RF magnetron sputtering. It's worth noting that the ionization efficiency is higher in RF, allowing for operation at lower pressures. Working at lower pressures during the deposition process helps produce purer, denser, more adherent, and directional films, ultimately enhancing overall quality and performance, particularly for possible high-tech applications.”

 

 

OLD Text (from line 66-73 lines)

Despite numerous studies on the influence of V_neg on the microstructure and properties of various metal coatings deposited by RF plasma magnetron methods, there are limited reports on the specific use of substrate bias to modify the properties of W coatings [..]. This study investigates the effects…….

 

REVISED TEXT: “Despite numerous studies on the influence of V_neg on the microstructure and properties of various metal coatings deposited by RF plasma magnetron methods, there are limited reports on the specific use of substrate bias to modify the properties of W coatings [..,..], in fact, as already mentioned for pressure, most of the articles refer to DC sources. The effect of substrate bias voltage on DC and RF sputtered coatings can be quite different. For example, E. Eser et al reported [..] on the different microstructure of WC-Co coatings, which was generated during DC and RF sputtering process with the same voltage applied to the substrate. Therefore, this study investigates the effects….”

 

 

Comment 2: The results should be compared with existing literature to contextualize the properties of tungsten coatings under varying sputtering conditions. For example, the sputtering parameters used in this study may overlap with those reported in prior research, such as the conditions surveyed in the article (https://doi.org/10.1016/j.tsf.2015.01.030). Identifying general trends and distinctions could help highlight the manuscript’s novelty.

Response: We thank the referee for the observation. We have inserted in different parts of the paper references that have similar W sputtering conditions with results that confirm our trends. Furthermore, for the electrical resistivity part, we have highlighted a different result concerning the aforementioned articles.

 

REVISED TEXT (section 3.2.3): “Lastly, we note that only the experiment at 1 Pa produced an R_r300K like that of the W bulk with a r_300K range from 5´10⁻⁸ to 5´10⁻⁷ Ω m [..]. Although in literature [.., ..] the increase in resistivity has been associated with the presence of β-W, assuming high-pressure coating had the same density as low-pressure coatings, vice versa, our results indicate that the coating density is the main factor to be controlled in order to have a low resistivity.”

 

Comment 3: To ensure the conditions for beta-phase formation, the authors should also present the films deposited under 3Pa/-150V, 1Pa/-100V, and 1Pa/-150V.

Response: We understand the referee's comment, but as explained in the article (page 5 and section 3.2), the deposition process was optimized to the threshold where stress phenomena emerged. At 1 Pa, even a few volts induced stress phenomena, and also at 3Pa/-150V, thus we could not make the XRD measurements.

 

Note: we point out that other parts of the text have been revised/added to respond to other comments from other referees.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewer should note the responsible attitude of the authors of the article to the comments made. All questions have been clearly answered, and the article has been adequately modified. It is only worth noting that the version of the corrected work available to the reviewer has not been corrected in relation to comments 5 and 6 (paragraph 2.5) from the previous review. This technical error should be corrected in accordance with the responses to the comments. Otherwise, the article can be accepted for publication in unchanged form.

Author Response

We are sorry for the technical error. The corrections are below.

Comment 5: Page 3, lines 6-9 of section 2.5. The name of the AFM setup is repeated The sentence on lines 6-7 should probably be deleted.

Response: Thank you, we have removed the misprint [line 131-132]

 

REVISED TEXT: “The morphological features of the samples were examined by AFM (Core AFM, Nanosurf GmbH, Langen, Germany) in dynamic mode and High-Resolution SEM (Tescan mod. MIRA III, Brno, Czech Republic).”

Comment 6: Page 3, lines 14 of section 2.5. The superfluous word "Their".

Response: Deleted the unnecessary word [line 139]

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript considering the comments of this reviewer and I find the revised version much improved.

Author Response

We thank you again and appreciate the time and consideration you have given to reviewing our manuscript, “Effect of the negative substrate bias voltage and pressure on the structure and properties of tungsten films deposited by magnetron sputtering technique”, for potential publication in Coatings.