Kinetics of Thickness Growth of Silicon Films During Pulsed Magnetron Sputtering Using the Caroline D12C System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a relevant study on the growth of silicon films during pulsed magnetron sputtering using the Caroline D12C system, which is important for silicon anodes in lithium-ion batteries. The manuscript offers insights into optimizing the sputtering process, specifically addressing key parameters like specific power, working pressure, and voltage frequency, and their effects on film growth, morphology, and silicon entrainment. The study identifies optimal conditions that minimize silicon loss (0.5-1.5%) and maximize film growth rates, which improves process efficiency and cost-effectiveness.

However, several areas need improvement:

1.- The film morphology is non-uniform up to 100-150 nm, which may affect performance. Further analysis of this issue is needed.

2.- The study only investigates frequencies between 20-500 kHz. A broader range of frequencies should be explored for potential optimization.

3.- Increasing frequency reduces the induction period but increases silicon loss (7-15%). The trade-off should be better addressed, along with optimization strategies.

4.- The manuscript does not consider other factors such as substrate material, temperature, or environmental conditions, which may impact film quality and silicon loss.

5.- The study lacks data on the long-term stability of films, which is essential for real-world applications like lithium-ion batteries.

The abstract should be rewritten to provide a general overview of the study’s key findings and implications, without excessive technical details.

In conclusion, while the study offers valuable insights, significant revisions are needed to address the above concerns and meet the journal’s standards. The authors should focus on improving film characterization and including long-term stability data.

 

 

Comments on the Quality of English Language

It could be improved

Author Response

Comments 1: The film morphology is non-uniform up to 100-150 nm, which may affect performance. Further analysis of this issue is needed.

Response 1: Thank you for pointing this out. We agree with this comment. Indeed, although in our work we achieved uniform films of about 100-150 nm, this is probably too thick. If we could achieve uniform coverage with a smaller film thickness, this would be a much better achievement. The thinner the anode film in the battery, the less time it will take to charge the battery, since with a decrease in the anode thickness, the diffusion drag of lithium ions during charging will be reduced. We had data on the basis of which we estimated the uniformity of the film thickness under different conditions. We included this data in the manuscript on page 8, lines 280-298 and on pages 8-9, lines 304-313, and also added Table 2 (page 9, lines 315-316).

 

Comments 2: The study only investigates frequencies between 20-500 kHz. A broader range of frequencies should be explored for potential optimization.

Response 2: Thank you for pointing this out. We agree with this comment. However, unfortunately, we are limited by the capabilities of the magnetron sputtering equipment in terms of varying the voltage frequency. We have made a corresponding remark in this regard in the updated version of the manuscript on page 14, lines 442-450. We will continue research in this direction as far as possible, since increasing the frequency has a positive effect on the productivity of the process. However, at the same time, the losses of rather expensive silicon with the gas leaving the working chamber increase.

 

Comments 3: Increasing frequency reduces the induction period but increases silicon loss (7-15%). The trade-off should be better addressed, along with optimization strategies.

Response 3: Thank you for pointing this out. We agree with this comment. The corresponding additions to the manuscript regarding the trade-off between magnetron sputtering intensification and silicon losses are made on page 14-15, lines 451-476.

Comments 4: The manuscript does not consider other factors such as substrate material, temperature, or environmental conditions, which may impact film quality and silicon loss.

Response 4: Thank you for pointing this out. We agree with this comment. The substrate material can significantly affect the magnetron sputtering process. The material can affect the ability of ionized silicon to deposit from the plasma flow on the substrate and the characteristics of the growth of condensed silicon colonies in the horizontal and vertical directions. However, we are significantly limited in the choice of substrate material if we have in mind the LIB application. The substrate must have good electrical conductivity, and copper or aluminum are most suitable here. Aluminum has lower electrical conductivity, so we primarily wanted to study the deposition of silicon on a copper substrate. Temperature and environmental conditions are also important, but for the initial study we decided to limit ourselves to standard operating conditions of magnetron sputtering equipment, since in this case the research results will be easier to implement in the real production process. The corresponding remarks are made in the corrected version of the manuscript on page 2, lines 72-75 and page 3, lines 98-104.

 

Comments 5: The study lacks data on the long-term stability of films, which is essential for real-world applications like lithium-ion batteries. The abstract should be rewritten to provide a general overview of the study’s key findings and implications, without excessive technical details.

Response 5: Thank you for pointing this out. We agree with this comment. We have conducted initial studies on the long-term stability of the nanofilms we have obtained. In particular, we have selectively experimentally assessed the adhesion of the nanofilms to the substrate. It is essential that the film adheres well enough. In this case, the cathode-anode system of the LIB will not delaminate. The adhesion variability was from 9 to 12 MPa. This is a good enough value that will ensure the mechanical stability of thin silicon films on the substrate. The obtained films were stored in a desiccator for 12 months, and then repeated adhesion studies were carried out. The results showed that the adhesion value of the films did not change, which indicates the long-term stability of the films. Further studies are needed that will indicate good thermal and electrochemical stability, as well as adhesion retention. It is much more important to study not the stability of the films themselves, but the system of multiple alternating cathodes and anodes separated by layers of electrolyte in the LIB. It is necessary to understand how these parameters will change as the number of charge-discharge cycles increases. In this work, we have not yet obtained such a system, so the study of long-term stability was postponed until the time when the production of a working LIB based on silicon nanofilms is completed. The corresponding additions were made to the text of the manuscript on page 6, lines 194-210. Also, at your recommendation, the abstract was rewritten, which now provides a general overview of the main conclusions without unnecessary technical details, page 1, lines 9-23.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

I recommend that this paper be accepted after major revision.

After carefully reviewing this work, I found that the manuscript has good strength and has shown interesting work with highly appreciated protocol. Still, it requires some major revisions, and the questions below are clarified:

1- Figures and Data Visualization that Are Not Included

Important metrics (ie, SEM images in Figures 2–5, AFM images in Figures 6–7) are cited but not present in the manuscript. These are essential for testing morphology claims. You should provide these high-resolution images with scale bars and annotations.

AFM data will typically correlate to roughness parameters (e.g., RMS values) which can help discussions about the homogeneity of the deposited film.

2- Mechanistic Explanations

Plasma dynamics and atom mobility are phong unless it's poolside. For example, why does a higher voltage frequency produce a bigger silicon loss? Make observations (e.g., ion bombardment efficiency, gas-phase collisions) relevant to plasma physics.

Explain the compromise between reduced induction periods and high-frequency silicon loss Is the higher loss due to target overheating or gas-phase scattering?

3- Practical feasibility and scalability

12,800 nm/min at 500 kHz is like sweeping high. Evaluate if the Caroline D12C system can perform under these conditions without target degradation or damage to the substrate. Discuss scale-up of the LIB production at the industrial level

4- Reversal of Pressure Guidelines

It is also mentioned that low pressures (0.5–1.0 Pa) give denser films (page 7), but in the conclusion, it points to low pressures being 2.0 Pa for uniform pores. Bookend this inconsistency by aligning results and recommendations.

5- Comparison to Prior Work

Compare the benchmark growth rates, induction periods, and silicon losses with other studies using magnetron sputtering. For instance, what differences do these results have to those from Catchpole et al. (Ref. 2) or Chan et al. (Ref. 5)?

6- Silicon Loss Implications

Although losses are quantified (0.5–15%), their economic effect on process efficiency is not elaborated. Evaluation of industrial adoption of new technologies

7- Consistent terminologies for silicon loss (e.g., entrainment vs removal). Use full terminology (e.g. “induction period”) in the Methods section.

Comments for author File: Comments.pdf

Author Response

Comments 1: Figures and Data Visualization that Are Not Included

Important metrics (ie, SEM images in Figures 2–5, AFM images in Figures 6–7) are cited but not present in the manuscript. These are essential for testing morphology claims. You should provide these high-resolution images with scale bars and annotations.

AFM data will typically correlate to roughness parameters (e.g., RMS values) which can help discussions about the homogeneity of the deposited film.

Response 1: Thank you for pointing this out. We agree with this comment. In the updated version of the manuscript, we attach the necessary appendices. Also regarding the statistical processing of the data concerning the assessment of the surface roughness of the resulting film. We had data on the basis of which we assessed the uniformity of the film thickness under various conditions. We have included this data in the manuscript on page 8, lines 280-298 and on pages 8-9, lines 304-313, and also added Table 2 (page 9, lines 315-316).

 

Comments 2: Mechanistic Explanations

Plasma dynamics and atom mobility are phong unless it's poolside. For example, why does a higher voltage frequency produce a bigger silicon loss? Make observations (e.g., ion bombardment efficiency, gas-phase collisions) relevant to plasma physics.

Explain the compromise between reduced induction periods and high-frequency silicon loss Is the higher loss due to target overheating or gas-phase scattering?

Response 2: Thank you for pointing this out. We agree with this comment. Indeed, increasing the frequency is traditionally associated with decreasing the ion energy distribution width. In turn, a narrow IED width promotes more uniform deposition of material on the substrate and a decrease in the deposition rate. In this sense, we have obtained a result that is not quite traditionally expected. Probably, this is due to the fact that we accompanied the increase in the voltage frequency with a fairly high power density on the target. The result was an inversion of traditional trends, when increasing the frequency at high powers, although insignificantly, reduces the film quality and an ultra-high deposition rate is achieved. Unfortunately, we cannot use this effect in practice, since the rate increase is achieved at too large a film thickness beyond the induction period time limit. The paradoxical nature of the obtained result could be explained if we studied the experimental ion bombardment spectra. However, unfortunately, our magnetron equipment was not equipped with a differentially pumped delay grating ion flux energy analyzer. We hope that colleagues who have such an opportunity will repeat our study and will be able to explain the effect we have obtained. We had to reflect these features in the "Results and Discussion" section, so we added the corresponding comments on your kind recommendation on page 15, lines 481-498.

As for gas-phase collisions, they probably could hardly have had a significant effect on the deposition rate and the removal of silicon from the target. An increase in the intensity of gas-phase collisions is usually associated with a change in the pressure in the working chamber. With an increase in pressure, the intensity of gas-phase collisions increases. This reduces the average energy of ions and the deposition rate decreases. In our work, the pressure was varied within very small limits. As can be seen from the data presented in our manuscript, the change in pressure did not significantly affect the deposition kinetics, but only the density of the resulting films.

In connection with all of the above, we propose to accept for now as a working hypothesis the position that you defined as a mechanistic explanation. Later, when colleagues join in the study of these effects, we may have more substantiated explanations.

We have added these working formulations on page 15, lines 481-498.

Increased power supplied to the target leads to an increase in the ion energy, which contributes to a more intense sputtering of the material. This may explain the increased entrainment of silicon into the gas phase.

With an increase in the voltage frequency, the ions acquire a higher energy, which increases the efficiency of sputtering the material from the target. This leads to a larger number of sputtered silicon atoms that pass into the gas phase.

As for the trade-off between the reduction of the induction period and the loss of silicon, this trade-off is purely economic in nature. Taking into account your recommendations and the recommendations of other reviewers, we have added the corresponding comments to the updated manuscript on pages 14-15, lines 451-476.

We associate silicon losses with dispersion in the gas phase, as stated above, and not with target overheating. No target overheating was observed during our experiments, and the magnetron equipment operated in normal mode.

 

Comments 3: Practical feasibility and scalability

12,800 nm/min at 500 kHz is like sweeping high. Evaluate if the Caroline D12C system can perform under these conditions without target degradation or damage to the substrate. Discuss scale-up of the LIB production at the industrial level

Response 3: Thank you for pointing this out. We agree with this comment. Indeed, the combination of increased power density and increased frequency gave an unusual effect of increasing the deposition rate, which we reported to you in the previous reply. However, unfortunately, such rates are achieved after the induction period, when the silicon film thickness is already several micrometers. Such a large film thickness is unacceptable for us. Therefore, the actual rates during process scaling will be much lower than 500-1000 nm/min, in order to complete the process by the time the film thickness is 100-150 nm. To exclude excessive optimism in this regard, we added the corresponding comments to the updated version of the manuscript on page 16, lines 528-532.

 

Comments 4: Reversal of Pressure Guidelines

It is also mentioned that low pressures (0.5–1.0 Pa) give denser films (page 7), but in the conclusion, it points to low pressures being 2.0 Pa for uniform pores. Bookend this inconsistency by aligning results and recommendations.

Response 4: Thank you for pointing this out. We agree with this comment. There was a lively discussion among our authors on the effect of pressure in the working chamber and interpretation of scanning electron microscopy data. The opinions were very varied. We discussed this issue again and came to the general opinion that the porosity will increase as the pressure increases and the most optimal option will be films obtained at a pressure of 2 Pa. The fact is that there is no need for films in LIB to have a very dense film structure, since excessive film density will contribute to its destruction during the diffusion of lithium ions during the charging and discharging of LIB. Free expansion of the film as a result of the absorption of lithium ions will be ensured by the presence of pores in the film structure. However, an excessively porous structure is also undesirable, since such a structure will worsen the mechanical strength of LIB. In any case, the choice of the final film version will be determined by its stability during long-term tests of the LIB (charge-discharge) that will be made from these films. The relevant additional comments have been added to the updated manuscript pages 7-8, lines 261-271, and page 16, lines 511-519.

 

Comments 5: Comparison to Prior Work

Compare the benchmark growth rates, induction periods, and silicon losses with other studies using magnetron sputtering. For instance, what differences do these results have to those from Catchpole et al. (Ref. 2) or Chan et al. (Ref. 5)?

Response 5: Thank you for pointing this out. We agree with this comment. Indeed, very few authors devote their publications to the kinetics of silicon film growth during magnetron sputtering. There are practically no works with which direct comparisons could be made. Therefore, only trends have to be compared. The works that you mentioned in your comment are indirectly related to the influence of process parameters on the process indicators. They talk about the importance of work on improving the production technology and quality of thin silicon films for LIA applications. In this regard, we conducted an additional search for sources of information on this issue and added them to the updated version of the manuscript (pos. 10 and 11 in the list of sources). Thus, in the work of R. Edrei et al. [10], with a silicon film thickness of less than 150 nm (60-80 nm), significant unevenness of the substrate coverage with condensed silicon is observed. Just like us, the authors used the method of sections of a three-dimensional AFM image model to estimate the film unevenness. However, unlike our work, this study did not find a film thickness above which the film is leveled and the roughness is reduced. Unlike this work, we found that with an increase in the film thickness of more than 100-150 nm, the film thickness is leveled over the entire surface of the substrate. As for the effect of the power supplied to the target on the kinetics of thin film growth. We found a work similar to ours. In this work, sputtering was carried out not with silicon, but with copper on the surface of a glass substrate. In addition, the work provides absolute power values, and not those reduced to the area of ​​the working surface of the target. Also, the geometric parameters of the target are not specified. All this does not allow us to quantitatively compare their results with the results of our experiments. However, a similar trend can be noted. In the study [11], it was found that the growth rate increases exponentially with an increase in the power supplied to the target. In this case, the speed was increased by two orders of magnitude (up to 42,000 nm/min or 700 nm/s) compared to the usual values ​​under similar conditions at lower power (up to 300 nm/min or 5 nm/s). In our case, we also observe a multiple increase in the sputtering speed, but the trend is not exponentially growing, but power-law with an exit to a plateau.

Unfortunately, we did not find similar works on the effect of voltage frequency on the kinetics of magnetron sputtering.

The corresponding comments are added to the text of the updated manuscript on pages 9-10, lines 321-333, page 12, lines 392-403.

 

Comments 6: Silicon Loss Implications

Although losses are quantified (0.5–15%), their economic effect on process efficiency is not elaborated. Evaluation of industrial adoption of new technologies

Response 6: Thank you for pointing this out. We agree with this comment. This issue was of interest not only to you, but to other reviewers as well. As noted above, the trade-off between reducing the induction period and silicon losses is purely economic in nature. In the process of industrial optimization of the process, we observe how the specific consumption of electrical energy and the costs of the installation operator's wages will change with an increase in the voltage frequency. Since productivity increases with an increase in the frequency, the time for manufacturing LIB will decrease. The power of the sputtering installation is constant, regardless of the frequency. Therefore, reducing the time for manufacturing LIB will reduce the energy costs for manufacturing a unit of LIB power. The same applies to the costs of the magnetron sputtering installation operator's wages. Since silicon carryover increases with an increase in the voltage frequency, the specific consumption of silicon for manufacturing LIB increases accordingly. We transform the mass specific consumption of silicon into financial costs. We sum up the costs of electricity, wages, and silicon. For this, we will need data on the cost of the above-mentioned resources at the enterprise that will commercialize the technology. We ensure that in each case the technical characteristics of the LIB remain at the same level and that the magnetron sputtering equipment operates stably in a safe mode. We plot a graph of the dependence of total costs on the voltage frequency. Perhaps this graph will have a minimum, since at some point the positive effect of the increase in productivity will be leveled by the negative effect of the increase in the specific consumption of silicon. It is possible that the minimum will not be achieved due to technical limitations of the equipment in terms of voltage frequency or due to deterioration in the quality of the LIB when a certain voltage frequency level is exceeded. In this work, we do not carry out such optimization, but we plan to as our project develops. The corresponding additions to the manuscript regarding the compromise between the intensification of magnetron sputtering and silicon losses are made on pages 14-15, lines 451-476.

Comments 7: Consistent terminologies for silicon loss (e.g., entrainment vs removal). Use full terminology (e.g. “induction period”) in the Methods section.

Response 7: Thank you for pointing this out. We agree with this comment. Indeed, we missed this point in our manuscript. We have brought the terminology to uniformity by replacing 3 mentions of the term "entrainment" in the text of the manuscript with the term "removal", which is more commonly used in the manuscript and more appropriate in meaning.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have effectively addressed all the points I raised with detailed responses and corresponding manuscript revisions.

Film Morphology (Comment 1) – The authors provide supporting data and acknowledge the impact of thickness on performance, making appropriate manuscript additions.

Frequency Range (Comment 2) – They justify the limitation due to equipment constraints and include a corresponding remark. While further discussion on sufficiency would be beneficial, their response is reasonable.

Frequency vs. Silicon Loss Trade-Off (Comment 3) – The response effectively discusses the balance between process intensification and material loss, with manuscript updates.

Other Factors Affecting Film Quality (Comment 4) – The authors clarify their focus on standard conditions and justify substrate selection for LIB applications.

Long-Term Stability & Abstract Revision (Comment 5) – Initial adhesion stability data is provided, and the abstract is revised for clarity. 

Overall, the authors have provided well-reasoned explanations and appropriate manuscript revisions, incorporating additional data and clarifications where necessary. While some limitations remain, they have been sufficiently justified. Therefore, my previous concerns have been adequately addressed in the revised manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

I accept this paper in the present form; the authors addressed all the comments.