Growth of Magnetron-Sputtered Ultrathin Chromium Films: In Situ Monitoring and Ex Situ Film Properties
, Anna Sytchkova 2,*, Kazimieras Baltrusaitis 1, Viktoras Vaicikauskas 1, Vitalija Jasulaitiene 1 and Tatjana Gric 1,3Round 1
Reviewer 1 Report
This article is devoted to a topical issue - the development of new semitransparent electrically conductive coatings based on ultrathin metal layers. In this paper, the features of applying chromium layers by the method of reactive deposition on direct current are systematically studied. The relevance of this topic is beyond doubt, however, there are a number of questions regarding the presentation and interpretation of data.
Namely:
1. The question of why the authors study the plasma composition during chromium deposition requires clarification. That it consist from chromium and argon ions seems obvious.
2. The authors give the average thickness of the obtained films with high accuracy, in some cases up to a tenth of a nanometer. The diameter of the obtained samples was quite large - up to 25.4 mm (line 85), however, the authors studied the uniformity of applying samples using the AFM method only in a small area - 1x1 µm (line 119, it should also be noted that Fig. 3. shows an area of 10x10 µm , probably a typo in line. 119). Therefore, the question arises as to how uniformly the film was deposited over the entire area of the substrate and how correctly the average film thickness was indicated.
3. The authors indicate that the final films contain up to 8% oxygen (line 145) and assume that this oxygen arose by desorption from the walls of the vacuum chamber. However, it is a well-known fact that it is possible to remove sorbed oxygen from the chamber walls by heating it in a high vacuum followed by a long-term purging with argon. It is not clear whether the authors performed these procedures. It is also worth noting that in the study of the composition of the plasma in the process of deposition, the authors did not observe oxygen ions, which also raises questions.
4. Due to the difference in electrical resistance for different samples by several orders of magnitude, it would be better to present Fig. 9 in semilogarithmic coordinates
Author Response
Firstly, we would like to thank Reviewers for careful reading and valuable comments.
Reviewer 1. This article is devoted to a topical issue - the development of new semitransparent electrically conductive coatings based on ultrathin metal layers. In this paper, the features of applying chromium layers by the method of reactive deposition on direct current are systematically studied. The relevance of this topic is beyond doubt, however, there are a number of questions regarding the presentation and interpretation of data.
Namely:
- The question of why the authors study the plasma composition during chromium deposition requires clarification. That it consist from chromium and argon ions seems obvious.
We agree that in our case it was quite clear result. It was found only chromium and argon ions. Nevertheless, this investigation is important. We have got confirmation that in the process there were neither significant amount of oxygen (system leakage) nor other elements (sputtering target contamination).
- The authors give the average thickness of the obtained films with high accuracy, in some cases up to a tenth of a nanometer. The diameter of the obtained samples was quite large - up to 25.4 mm (line 85), however, the authors studied the uniformity of applying samples using the AFM method only in a small area - 1x1 µm (line 119, it should also be noted that Fig. 3. shows an area of 10x10 µm , probably a typo in line. 119). Therefore, the question arises as to how uniformly the film was deposited over the entire area of the substrate and how correctly the average film thickness was indicated.
Yes, there is a typo, the area of 10x10 µm was investigated by AFM. Thank you for pointing that out. We have a magnetron-equipped target of 4 inch in diameter while the substrate diameter is only 1 inch. Moreover, we have done optimization of the substrate holder to target distance and optimization of the sample placement for the best uniformity. Measured values confirm that we have +/- 0.1 % uniformity for the 25.4 mm (1 inch) area. For all the samples for this investigation, an area mostly positioned in the middle of the sample was selected, for both AFM and ellipsometric characterization. For ellipsometric characterization, the light spot size varied from 1 to 2 mm2, approximately, increasing with the incidence angle. One-digit accuracy of the film thickness was provided only for the thinnest sample, because in this case, a difference of for example 0.25 nm represents variation of 10% of it’s thickness of 2.5 nm. In fact, such a difference significantly influences the retrieval of the film optical constants as it was discussed in our previous work doi:10.1016/j.optmat.2021.111530.
The following clarifications has been added in Experimental section, 1st paragraph:
“The target-to-substrate distance was 230 mm. Optimization of the substrate holder to target distance and of the sample placement for the best uniformity was performed. Measured values confirm +/- 0.1 % uniformity over the 25.4 mm area where the substrates were positioned for the coating depositions. For all the samples, an area positioned in the middle of the sample was selected.”
- The authors indicate that the final films contain up to 8% oxygen (line 145) and assume that this oxygen arose by desorption from the walls of the vacuum chamber. However, it is a well-known fact that it is possible to remove sorbed oxygen from the chamber walls by heating it in a high vacuum followed by a long-term purging with argon. It is not clear whether the authors performed these procedures. It is also worth noting that in the study of the composition of the plasma in the process of deposition, the authors did not observe oxygen ions, which also raises questions.
We thank for the recommendation. Unfortunately, our chamber’s walls are not equipped neither heater nor water cooling systems.
Indeed, no oxygen ions were observed in the plasma. It means that the oxygen amount was negligible or at least low enough to not appear in the spectra. Moreover, the XPS analysis was done in a separate chamber, after exposure of the sample to air. Therefore, the oxygen detected by XPS most probably wasn’t sorbet during the deposition process.
- Due to the difference in electrical resistance for different samples by several orders of magnitude, it would be better to present Fig. 9 in semilogarithmic coordinates
Thank you for the recommendation. In present research we have selected the same scale that was selected by all the other research groups cited in the work
Reviewer 2 Report
Review of „Growth of Magnetron Sputtered Ultrathin Chromium Films: in-situ Monitoring at Nanoscale and ex-situ Film Properties”
In their presented submission, Belosludtsev et al. present characterization data regarding ultrathin sputtered Chromium films with respect to film purity, morphology, air stability and conductivity. The submitted draft constitutes a solid piece of material analysis and can in principle be published once the issues listed below will have been addressed.
Before we go into detail: Is the title of the paper really complete in the current way? The second line of it looks like there is something missing for completion or was meant to be arranged differently, in the current shape it sounds strange in my opinion.
Small details + language aspects:
line 14 (a) when declaring an abbreviation, please do it in full and write optical for the O (b) a mechanism of film formation is announced here, but the only other time the word “mechanism” appears is in the title of one of the references. I wouldn’t call the AFM experiments a mechanistic study yet, so this claim should be removed from the abstract.
l.30 plasmonicS l.58 “Before [the publication of] our research” instead of “until”
l.131 the OES lines of Ar can be looked up as it was done for Cr, e.g. in Siepa et al. J. Phys. D 47, 445201, DOI 10.1088/0022-3727/47/44/445201 , so no need to formulate this in a speculative manner.
l. 136 Is the XPS part of the in-situ characterization or were the samples transferred via air/argon/vacuum box? The absence of hydroxide indicates it wasn’t air, or was it?
l. 138 substrate WHICH complicated
l. 139-140 Does this mean the range 12-57 doesn’t exhibit lots of differences or is it even the same for this given range AND also the ultrathin films?
l. 142 not Cr atoms (that’s reserved for the gas phase), but metallic Cr
l. 145 (a) which cross sections and other parameters were used for the calculation of the stoichiometry? It doesn’t require a lot of comments, I just think they should be cited. (b) Would it be possible that parts of Cr2O3 were also generated by Cr reducing small fractions of SiO2 at the interface?
l.150 Would it be possible to provide an exact thickness here?
l.154 Rq unformatted
l.156 if there is a feature exactly in the middle of an AFM image, that may be an indicator that a small piece of the tip broke off during the approach. This may not make a huge difference in this case, but if sharper features are observed, this carries the danger of artifacts shaped like the breaking area of the tip, so in the future better take a new tip after you see something like this.
l. 178/fig. 4 I would recommend not to tilt the numbers on the thickness axis, if your software allows for that
l.183/fig. 5 Recommendation: use “ in-situ (this work)” instead of “our”. There is no official standard for that, but I think that’s the most common way for such a label.
l.188 Aside from the mere fact of transparency, what else can we learn from this? Does this somehow complement the other methods? This paragraph is a bit too basic, I think. l.189 which instead of what
l.209/fig.8 another recommendation to use “this work” instead of “our” l.217/fig.217 with the given scale, it is barely possible to read out any trend of the given work beyond 10 nm. There is plenty of space, so why not make a second panel for the low sheet resistances, e.g. <50Ω/sq?
l. 220 Well, there is agreement with Lozanova and Udachan, but Lin et al. had it much higher. Any indicators about reasons? Maybe they had a higher oxygen contaminant? [If you can’t find any reason, it’s OK, but if it’s possible to find one, it should be mentioned.]
l.221+230 It doesn’t stabilize, as the article also explaines below, there is still a functional behaviour which for a clean material would be Rsh~1/d. In my own sputtered metal films, I can quite precisely monitor the film thickness by dividing the specific resistivity by the sheet resistance, the agreement with XRR is quite good as soon as I cross 20-25 nm. At this value, the mentioned scattering of electrons becomes basically negligible with respect to bulk behaviour.
l.227 ~ means “proportional to”, “approximately equal” would be ≈ according to ISO 80000-1 (\approx in LaTeX, if you’re using that).
l.247 Was the conclusion meant to end here or was there more to come? It ends without a point.
Author Response
Firstly, we would like to thank Reviewers for careful reading and valuable comments.
Reviewer 2. In their presented submission, Belosludtsev et al. present characterization data regarding ultrathin sputtered Chromium films with respect to film purity, morphology, air stability and conductivity. The submitted draft constitutes a solid piece of material analysis and can in principle be published once the issues listed below will have been addressed.
Before we go into detail: Is the title of the paper really complete in the current way? The second line of it looks like there is something missing for completion or was meant to be arranged differently, in the current shape it sounds strange in my opinion.
We left “at Nanoscale” to appear more coherent with other journals, for previous submissions. I believe we should simply remove it. So that the title becomes: “Growth of Magnetron Sputtered Ultrathin Chromium Films: in-situ Monitoring and ex-situ Film Properties”
We simplified the title like: “Growth of Magnetron Sputtered Ultrathin Chromium Films: in-situ Monitoring and ex-situ Film Properties.” We hope this version sounds clearer now.
Small details + language aspects
Namely:
line 14 (a) when declaring an abbreviation, please do it in full and write optical for the O (b) a mechanism of film formation is announced here, but the only other time the word “mechanism” appears is in the title of one of the references. I wouldn’t call the AFM experiments a mechanistic study yet, so this claim should be removed from the abstract.
Thank you for the note, this sentence was removed.
l.30 plasmonicS l.58 “Before [the publication of] our research” instead of “until”
Corrections were done.
l.131 the OES lines of Ar can be looked up as it was done for Cr, e.g. in Siepa et al. J. Phys. D 47, 445201, DOI 10.1088/0022-3727/47/44/445201 , so no need to formulate this in a speculative manner.
New reference was added. Reference numbers were updated.
- 136 Is the XPS part of the in-situ characterization or were the samples transferred via air/argon/vacuum box? The absence of hydroxide indicates it wasn’t air, or was it?
The XPS analysis were done in a separate chamber. The samples were prepared in one chamber, than packed in clean room, and transferred in sealed box in other clean room where XPS was located.
- 138 substrate WHICH complicated
Correction was done.
- 139-140 Does this mean the range 12-57 doesn’t exhibit lots of differences or is it even the same for this given range AND also the ultrathin films?
The results on the elemental composition for 12-57 nm films were similar and presented in Figure 2. For the ultrathin film it was complicated to evaluate XPS date as it was influenced by the signal that came from the substrate.
- 142 not Cr atoms (that’s reserved for the gas phase), but metallic Cr
Correction was done.
- 145 (a) which cross sections and other parameters were used for the calculation of the stoichiometry? It doesn’t require a lot of comments, I just think they should be cited. (b) Would it be possible that parts of Cr2O3 were also generated by Cr reducing small fractions of SiO2 at the interface?
(a) On the question of stoichiometry and at.% calculation: XPS data processing was performed using the Avantage 3.962 software. It was used for further data processing, like e.g. fitting of high resolution peaks by components having mixed Gaussian-Lorentzian character corresponding to different chemical states of elements. The background for all spectra was subtracted using a Shirley baseline. In our opinion, it is unlikely that chromium oxide would form in the interlayer between the SiO2 and the Cr film, especially since fused silica substrate was double-side polished at λ/10.
l.150 Would it be possible to provide an exact thickness here?
As we have mentioned that these results might be associated with the films with thicknesses 12-57 nm. As we considering group of films that is why in capture not mentioned one value.
l.154 Rq unformatted
Correction was done.
l.156 if there is a feature exactly in the middle of an AFM image, that may be an indicator that a small piece of the tip broke off during the approach. This may not make a huge difference in this case, but if sharper features are observed, this carries the danger of artifacts shaped like the breaking area of the tip, so in the future better take a new tip after you see something like this.
Thank you for a valuable recommendation. We will consider for the future investigations.
- 178/fig. 4 I would recommend not to tilt the numbers on the thickness axis, if your software allows for that
Unfortunately, our software does tilt these numbers. We believe that the labels of thickness axis are readable, what it important for the Readers.
l.183/fig. 5 Recommendation: use “ in-situ (this work)” instead of “our”. There is no official standard for that, but I think that’s the most common way for such a label.
Correction was done.
l.188 Aside from the mere fact of transparency, what else can we learn from this? Does this somehow complement the other methods? This paragraph is a bit too basic, I think. l.189 which instead of what
Information that “Slight difference in the results might be caused by difference in the deposition conditions” was added.
l.209/fig.8 another recommendation to use “this work” instead of “our” l.217/fig.217 with the given scale, it is barely possible to read out any trend of the given work beyond 10 nm. There is plenty of space, so why not make a second panel for the low sheet resistances, e.g. <50Ω/sq?
Information that “Slight difference in the results might be caused by difference in the deposition conditions” was added.
- 220 Well, there is agreement with Lozanova and Udachan, but Lin et al. had it much higher. Any indicators about reasons? Maybe they had a higher oxygen contaminant? [If you can’t find any reason, it’s OK, but if it’s possible to find one, it should be mentioned.]
Unfortunately, Lin et al. have not done investigation the purity of their films, that is why we could not assume “higher oxygen contaminant” or other reason for higher Sheet resistance.
l.221+230 It doesn’t stabilize, as the article also explaines below, there is still a functional behaviour which for a clean material would be Rsh~1/d. In my own sputtered metal films, I can quite precisely monitor the film thickness by dividing the specific resistivity by the sheet resistance, the agreement with XRR is quite good as soon as I cross 20-25 nm. At this value, the mentioned scattering of electrons becomes basically negligible with respect to bulk behaviour.
We have changed the term “stabilize” for “the change is less significant”.
l.227 ~ means “proportional to”, “approximately equal” would be ≈ according to ISO 80000-1 (\approx in LaTeX, if you’re using that).
Correction was done.
l.247 Was the conclusion meant to end here or was there more to come? It ends without a point.
The point was added.
