Less Energetic Routes for the Production of SiOx Films from Tris(dimethylamino)silane by Plasma Enhanced Atomic Layer Deposition

Round 1

Reviewer 1 Report

Coatings-2612886

The paper describes the production of SiO_x films by plasma enhanced atomic layer deposition and the analytical investigation of these films using three methods, IRRAS, XPS, and EDS. According to the authors, 17 different preparation conditions were used and the results of a part of them are the subject of this paper. In this work, the effect of the plasma oxidation time and the chemical composition of the oxidizing plasma are evaluated.

The manuscript has several deficits and requires thorough revision. Efforts are needed to improve quality.

Keywords:
The keywords are misleading. Mechanisms of plasma oxidation are not described. Information on the analytical methods used is completely missing, although the analytical results are the main content of this paper.

Introduction
The introduction is very short and gives too little information on the current state of research. Results and references by authors who have investigated the use of tris(dimethylaminosilane) are missing, as is work dealing with the subsequent plasma oxidation.

Materials and Methods:
Details on the production of the SiO_x films are missing. For a better understanding, the film production conditions of the individual samples examined in this work should be clearly presented. Add the full set of experimental data for the (four or six?) different sample types described here.

Line 64: “TDMAS [(CH₃)₂N]₃SiH activated(?) at 150°C and mixed with 30 sccm Ar”  
This does not tell anything about the concentration of TDMAS in the argon stream. Is the TDMAS concentration the same in all experiments? How can you realize equal conditions (composition of the TDMAS/Ar mixture? What do you mean the TDMAS is activated? The fact that the silane is gaseous at that temperature?

Line 65: “The deposition step was followed by purging with Ar for 4 or 8 s.”
When was which time used? Has this time any influence on the quality of the film?

In addition, the chapter lacks information on the analytical work.

Line 69: Did you only use IRRAS? What about the samples in Fig. 3a?  Which IR method was used here? How were the two samples prepared?

Add details about the analytical investigations (method, instrumentation, measuring conditions, sample preparation).

Results and Discussion:

The results of the different methods should be carefully examined and compared. Do the results match or are they contradictory? Critical remarks on the accuracy of the determination of C, N and O with the methods used (EDS, XPS) are missing.

Line 76: “The peak at 3379 cm^-1 is assigned to the stretching of N-H bonds”
The assignment of the broad peak at 3379 cm^-1 is uncertain. The intensity of the 12 s and 18 s samples differs greatly, but they have almost the same N atomic percentage. What about associated O-H groups? Can you exclude a contamination (adsorption) of water?

Line 77: “the appearance of the contribution at 1646 cm^-1, also related to N-H groups”
The assignment of an additional absorption band is also not a reliable indication of NH.

Line 106: The Si/O, O/C, and Si/C atomic ratios derived from EDS are quite similar for samples exposed to oxidation periods of 12 s and 18 s, respectively. However, the atomic ratios derived from XPS are very different for the same type of sample.

It seems highly doubtful whether reliable atomic percentage data can be obtained from the XPS measurements.

Line 133: “The organic groups, originated by atmospheric contamination, are present only in the surface of the film.”
How do you come to this conclusion? Did you successively remove the film in a sputtering experiment? And how thick are the films produced anyway? Elsewhere you speak of monolayers. 

Line 136: C 1s peak deconvolution shows significant O-C=O and C-O-C contributions? This should also be visible in the IR spectra. Is there any evidence of C-O vibrational bands? Why isn’t this discussed?

 

The experimental data for the second series of tests are incomplete. How long was the plasma oxidation time? 6 s? It is not written anywhere except in the caption.

Figure 3(a): Replace com and sem with with and without.  

Figure 3(b): The atomic ratios of the sample treated in an O₂ oxidation plasma for 6 s deviate greatly from the data in Figure 1. Did you possibly swap the captions?

Compare the XPS data of this sample (66% C, 20% O, 7% Si, 7% N) with that of the other sample with 6s oxidation time (23% C, 51% O, 26% Si, 0% N)! The differences are very big. How reliable are these data and then how reliable are the results of the other samples (12 s and 18 s plasma oxidation)? How many samples of each type have you made and how many XPS measurements have you taken? Are there any statistically reliable results?

It is not possible to establish a clear correlation between the analytical data discussed and the production conditions, since latter are not clearly described for each sample.

Experimental results of the different methods should be compared and checked for compatibility.

Lines 180-182: “…the carbon detected results from post-plasma reactions when the sample is exposed to atmosphere…”
Add the experimental data to support this conclusion.

Both chapters, 2. Materials and Methods and 3. Results and Discussion, are superficially described and need to be thoroughly revised.

Conclusions

The conclusions drawn are not sufficiently supported by the experimental material. IR absorption spectra, XPS and EDS results still show the presence of C groups in/on the SiO_x layer. Proof of the complete removal of methylsilyl groups and that the so-called C traces arose through atmospheric contamination in a post-plasma reaction is still lacking.

Author Response

Reviewer #1

Comment 1:

Keywords: The keywords are misleading. Mechanisms of plasma oxidation are not described. Information on the analytical methods used is completely missing, although the analytical results are the main content of this paper.

Reply: Considering these suggestions, the keywords were changed to include the analytical methodologies whereas the word “methods and mechanisms of plasma oxidation” was removed.

Comment 2: Introduction: The introduction is very short and gives too little information on the current state of research. Results and references by authors who have investigated the use of tris(dimethylaminosilane) are missing, as is work dealing with the subsequent plasma oxidation.

Reply: To overcome this limitation more information about the current state of research was included in the “Introduction” section including those related to the tris(dimethylanmino)silane and the plasma oxidation step.

Comment 3: Materials and Methods: Details on the production of the SiO films are missing. For a better understanding, the film production conditions of the individual samples examined in this work should be clearly presented. Add the full set of experimental data for the (four or six?) different sample types described here. Line 64: “TDMAS [(CH3)2N]3SiH activated(?) at 150°C and mixed with 30 sccm Ar” This does not tell anything about the concentration of TDMAS in the argon stream. Is the TDMAS concentration the same in all experiments? How can you realize equal conditions (composition of the TDMAS/Ar mixture? What do you mean the TDMAS is activated? The fact that the silane is gaseous at that temperature? Line 65: “The deposition step was followed by purging with Ar for 4 or 8 s.” When was which time used? Has this time any influence on the quality of the film? In addition, the chapter lacks information on the analytical work. Line 69: Did you only use IRRAS? What about the samples in Fig. 3a? Which IR method was used here? How were the two samples prepared? Add details about the analytical investigations (method, instrumentation, measuring conditions, sample preparation)

Reply: A complete description about the deposition and characterization parameters was included in the body of this section. Aside to this, it was included a table summarizing the deposition condition of each sample. Concerning the concentration of TDMAS (row # 64) it should be considered that the precursor is pulsed for 20 ms, directly from the bottle, whereas the Ar flow is maintained to carry the volatilized precursor molecules into the chamber. This information was included in the doc. Since the system conditions (table and bottle temperature, pressure, humidity, etc.) are kept constant in all the experiments, the same precursor proportions are introduced in all conditions. The word “activated”, in the 64^th row, was removed to provide the correct idea of the process. The deposition step is a self-limiting ALD process, with the reaction of the precursor molecules occurring with species on the surface on the substrate. Thus, the precursor molecule does not need to be activated, as indicated by this revisor. Now concerning the purge times, they were correctly provided in the text and in the table: 8 s was used in the first set of experiments (investigation of the oxidation time) and 4 s in the second one (investigation on the plasma composition). As different purge time were used in the different batches, the results of the films oxidized for 6 s in both can not be straightly compared. A comment showing the influence of the purge time on the content of C in the films was include. Furthermore, the analytical methods and procedure were included in the last paragraph of the “Materials and Methods” section. The methodology used for acquisition of the infrared spectra of Fig. 3 was indeed IRRAS. This information was also included together with the procedure that were employed in the experiments. In this case, substrates were mirror-like polished aluminum.

Comment 4: Results and Discussion

• The results of the different methods should be carefully examined and compared. Do the results match or are they contradictory? Critical remarks on the accuracy of the determination of C, N and O with the methods used (EDS, XPS) are missing.
• Line 106: The Si/O, O/C, and Si/C atomic ratios derived from EDS are quite similar for samples exposed to oxidation periods of 12 s and 18 s, respectively. However, the atomic ratios derived from XPS are very different for the same type of sample. It seems highly doubtful whether reliable atomic percentage data can be obtained from the XPS measurements.

Reply: In order to amend this question, atomic proportion determined from RBS results were included. It was not possible to include the atomic proportions derived from EDS since there was interference of the substrate composition in the film results. But a very detailed discussion was elaborated concerning the different depth probed by the different approaches and on the influence of this on the results. The following information was included (row #: 215 to 233).

Comment 5: “It is also presented in Table 2 the atomic proportions of C, O and Si derived from the RBS measurements. In these cases, N was not detected due the low cross-section of this element to the alpha beam. But a trend of elevation in the C proportion with increasing t_ox is promptly detected, then strengthening the previous XPS, IRRAS and EDS findings. The C proportions, derived from RBS inspections are always lower than the detected by XPS, indicating that C is more abundant on the layer surface. This compositional gradient will then influence on the atomic proportions obtained by the different methodologies since they are probing different depth. “

”All these results show that SiO_x-like structures, with less amount of organic groups, are obtained with the shortest oxidation time (6 s) in O₂ plasma, which represents the treatment with the lowest energy cost. It is interesting to mention that the grows in the C content with increasing t_ox is due to recontamination, that is, in all cases the third dimethylamino group is released. Thus, giving extra time to the oxidation process is not beneficial, but instead, detrimental to the SiO_x formation. In the resulting structure, IRRAS showed that hydrogen atoms share the oxygen of the siloxane network, forming hydroxyls (O-H), normally observed in silicon oxides. It can be finally inferred, by comparing the results obtained from different methodologies, that organic groups (C-H, C-C and C-O) are mainly concentrated on the film surface, suggesting they are originated by atmospheric contamination.”

Line 76: “The peak at 3379 cm^-1 is assigned to the stretching of N-H bonds” The assignment of the broad peak at 3379 cm is uncertain. The intensity of the 12 s and 18 s samples differs greatly, but they have almost the same N atomic percentage. What about associated O-H groups? Can you exclude a contamination (adsorption) of water?

Line 77: “the appearance of the contribution at 1646 cm^-1 , also related to N-H groups” The assignment of an additional absorption band is also not a reliable indication of NH.

Reply: It was included, in the discussion of both set of experiments, the possibility of O-H contribution to the band at 3379 cm^-1, as re-commended.  The following comments were added (row #: 151 to 159).

“In the literature, the peak at 3379 cm^-1 is assigned to the stretching of N-H bonds [10], which is in good agreement with the appearance of the contribution at 1646 cm^-1, also related to N-H groups [10]. However, O-H stretching vibrations may also contribute to this band since they are reported in the literature around 3400 cm^-1[11]. This attribution is also consistent with the appearance of a band related to O-H stretching vibrations around 3644 cm^-1. Free water molecules also produce a contribution around 3700 cm^-1 [12] where low intensity peaks are detected. In the spectrum of the film produced with 18 s of oxidation time, the band at 3379 cm^-1 is overlapped with the band ascribed to ν-O-H (3644 cm^-1)[10].”

The lack of N signal in the XPS spectrum of the sample prepared with 6 s of oxidation time, together with the reduction in the intensity of the bands at 3379 and 1646 reinforce the idea that N-H vibrations contribute to these bands in the spectra of the films prepared with 12 and 18 s oxidation time.

Comment 6: Line 133: “The organic groups, originated by atmospheric contamination, are present only in the surface of the film.” How do you come to this conclusion? Did you successively remove the film in a sputtering experiment? And how thick are the films produced anyway? Elsewhere you speak of monolayers.

Reply:  XPS spectra were obtained with and without the cleaning process by bombardment with Ar ions. The proportions of C obtained were reduced (from 23 to 6%) after surface cleaning, indicating that this element is, in fact, retained in the first monolayers of the film. In good agreement with this, RBS results show a smaller proportion of C.

The following comments were included in the body of the manuscript,

(row # 205 to 214) “The atomic proportions of the elements derived from the XPS spectra are presented in Table 2. As it can be noticed, the shorter the plasma oxidation time, the smaller the proportions of C (23%) and N (0%) on the film surface. Consistently with the previous results, the proportion of C increases and that of silicon decreases for t_ox larger than 6 s. For the latter, the structure is composed of 26% Si, 51% O and 23% C, with no detection of a significant amount of N. As C-containing groups were not necessary for the fitting of the Si high resolution peak of this sample, the contribution of C to the elemental composition of this film is assumed to be due to the adsorption of atmospheric C. Indeed, when the sample was plasma-sputtered with Ar ions prior to the acquisition of the XPS spectrum, only 6% of C was detected.”

(row # 302 to 309): ” It is worthy to verify that C content detected in the sample produced under the best oxidation condition evaluated here was only 15%, but when the sample was plasma sputtered with Ar ions this value decrease to less than 10%. This phenomenon is the same observed in the previous experiments, that were conducted in different conditions. This coincidence of results suggests that, in fact, the carbon detected in both cases results from post-plasma reactions when the sample is exposed to atmosphere and not from the oxidation process. Finally, the third methylamine connection of the precursor was not identified in the chemical structure of the film when this oxidation route was used.”

Comment 7: Line 136: C 1s peak deconvolution shows significant O-C=O and C-O-C contributions? This should also be visible in the IR spectra. Is there any evidence of C-O vibrational bands? Why isn’t this discussed?

Reply: The following comment was included to aboard such aspect: (row # 192 to 204): For fitting the Si 2p peaks of these two last samples, a component related to Si₃N₄ was used instead of Si-CH, since the detection range of this last bond would appear at a higher energy edge. This indicates the absence of Si-CH  groups in the films. However, Si-O and Si-O-C groups were included. Consistently, in the adjustments of the C 1s peaks, groups O-C=O and O-C-O were used. The latter are in good agreement with the contributions of reduced intensities in the infrared spectra of these samples (Figure 2) between 1500-1400 cm^-1, attributed to the stretching of C=O and C-O bonds [20]. It is worth mentioning that the ratio between the intensities of the contributions of oxidized groups (O-C=O + O-C-O) in relation to that of non-oxidized groups (C-C) is greater in the XPS spectrum of the film prepared with 6 s of oxidation time, consistently with the greater intensity of the band at 1511 cm^-1 (C=O and C-O) in the IRRAS spectrum of the same sample. Thus, these results corroborate the infrared results that suggested the deposition of SiO_x-type films with reduced organic contamination.

Comment 8: Figure 3(b): The atomic ratios of the sample treated in an O oxidation plasma for 6 s deviate greatly from the data in Figure 1. Did you possibly swap the captions? Compare the XPS data of this sample (66% C, 20% O, 7% Si, 7% N) with that of the other sample with 6s oxidation time (23% C, 51% O, 26% Si, 0% N)! The differences are very big. How reliable are these data and then how reliable are the results of the other samples (12 s and 18 s plasma oxidation)? How many samples of each type have you made and how many XPS measurements have you taken? Are there any statistically reliable results? It is not possible to establish a clear correlation between the analytical data discussed and the production conditions, since latter are not clearly described for each sample. Experimental results of the different methods should be compared and checked for compatibility.

Reply: To clarify this point, the experimental section was better elaborated, following the revisor recommendation. In the same section was stated that it is not possible to compare both results due to their different experimental conditions.

(row # 93 to 99 )“Despite the same oxidation time of 6 s has been used in samples prepared in the first and second batch of experiments, they cannot be directly compared once the purge time (8 and 4 s) and O₂ flow (100 and 90 sccm) used differ in both. Consistently with the proposal of testing processes with lower temperature, all the steps of this set of experiments were conducted at 150°C. The complete deposition, purging and oxidation parameters used in the different experiments are summarized on table 1.”

A comment was also presented during the discussion of the results:

(row # 240 to 259) “The spectrum of the film produced using only oxygen in the oxidation process reveals the presence of peaks characteristic of the Si-O bond at 1219, 1146, 1038, 944 and a doublet at 825-755 cm^-1. In addition, absorptions ascribed to Si-CH bond (1300-1500 cm^-1), N-H and/or O-H bonds (1600 cm^-1) [10,11,13,21] are also observed. Furthermore, a broad band observed in the range of 3713 to 2948 cm^-1, suggests the presence of peaks assigned to vibrations of N-H (3400 cm^-1), O-H and H₂O (3700 cm^-1) and C-H (2957 cm^-1). It is interesting to mention the substantial difference between the infrared spectrum of this sample and that of the sample prepared with the same oxidation time (150°C, 260 – 270 mTorr, 6 s) but with 8 s of purging time (Figure 2(d)). The higher contribution of C-containing groups in the spectrum of the film prepared with 4 s of purging time, shows that the deposition and oxidation kinetics are also sensitive to this parameter.”

Comment 9: Lines 180-182: “…the carbon detected results from post-plasma reactions when the sample is exposed to atmosphere…” Add the experimental data to support this conclusion.

Reply:  RBS results were included to show that the average C proportion throughout the film thickness is lower than the derived from its surface (XPS). Furthermore, after plasma sputtering the surfaces prior acquisition of XPS spectra, the content of C was substantially reduced. All these comments were included in the body of the manuscript, as discussed in the previous questions.

Comment 10: Conclusions: The conclusions drawn are not sufficiently supported by the experimental material. IR absorption spectra, XPS and EDS results still show the presence of C groups in/on the SiO layer. Proof of the complete removal of methylsilyl groups and that the so-called C traces arose through atmospheric contamination in a post-plasma reaction is still lacking.  

Reply: After including the RBS analysis, better explain the results, doing correlations between XPS, RBS and IRRAS, the conclusions could be validated. Finally, English corrections were performed throughout the body of the text and the modifications are highlighted.

Reviewer 2 Report

The aim of this work is to improve the quality of SiOx films obtained by atomic layer deposition. A detailed analysis of chemical bonds in the obtained film was carried out. It is shown that increasing the time of oxidation by plasma increases the concentration of contaminants. On the contrary, the increase of argon concentration promotes the purification of the film from organic contaminants. Therefore, the minimum possible oxidation time of 6 sec in combination with high argon concentration provides the best quality of SiOx film obtained by atomic layer deposition. This article is of purely technical interest for those readers who need to obtain pure SiOx layers by a low-cost method PEALD.

Author Response

No comments 

Reviewer 3 Report

1. What’s the thickness of each film?

2. The author concludes that “oxidation time in pure oxygen plasmas was decisive for attaining the complete removal of carbon”, and among 6s, 12s, and 18 s, only 6s “allowed the removal of organic moieties in the deposited layer preventing the incorporation of residual groups from the oxidative atmosphere”. Has lower plasma oxidation time (< 6s) been attempted? How is the result compared to 6s? 

3. Figure 1(a): “seg” should be changed to “sec” if that is representing second.

4. Figure 1(a), 1(b), 1(c), 3(a): Transmittance should use “%” as unit, with scale specified in y-axis.

5. For XPS plots, y-axis should be “Intensity (a.u.)” if arbitrary unit is used for intensity. 

Author Response

Reviewer #3

Comment 1: What’s the thickness of each film? 2. The author concludes that “oxidation time in pure oxygen plasmas was decisive for attaining the complete removal of carbon”, and among 6s, 12s, and 18 s, only 6s “allowed the removal of organic moieties in the deposited layer preventing the incorporation of residual groups from the oxidative atmosphere”. Has lower plasma oxidation time (< 6s) been attempted? How is the result compared to 6s? 3. Figure 1(a): “seg” should be changed to “sec” if that is representing second. 4. Figure 1(a), 1(b), 1(c), 3(a): Transmittance should use “%” as unit, with scale specified in y-axis. 5. For XPS plots, y-axis should be “Intensity (a.u.)” if arbitrary unit is used for intensity.

Reply:  The results presented here represent a part of a PhD work. We are first creating a better route of deposition of SiOx films from TDMAS. We have already prepared Al2O3 films too. The ideia now is to combine both layers in order to create a montmorillonite-like structure. Until this point the thickness of films wasn’t determined, but the number of cycles of deposition was constant in all the depositions to guarantee that the thickness was the same. The thickness was estimated to be 170 nm. Besides that, with the better conditions of deposition stablished in the present work (lowest oxidation time of 6 s and using Ar in the oxidative plasma), new samples were prepared with lower plasma oxidation time (4 s) and using Ar in the oxidative atmosphere. It was observed a still lower proportion of organics incorporated in the films. The results will be provided as a second manuscript.

All the suggestions of this revisor were all accepted and changed in the manuscript.  

Reviewer 4 Report

The paper is very interesting, especially for applications (technological point of view)

Some queries must be improved:

- The authors mentioned that the obtained SiOx films are "dense with nanometric thicknesses". Please explain the meaning of "dense" (compared to other methods or quantitatively estimated?) and provide the thickness of the prepared SiOx films.

- The route described in the paper, for obtaining SiOx films with less energetic routes, is feasible for obtaining thicker SiOx films (hunders of nm)?

- How about the adherence of the SiOx films on the subtrates used for deposition (aluminum and carbon steel)? Can this method extended to other substrates, for example flexible ones?

- If possible, the authors are kindly asked to ass some images (AFM for example) would be of interest for the morphology of the prepared SiOx films

 

 

The English is fine (minor corrections to be checked)

Author Response

Reviewer #4

Comments:

• The authors mentioned that the obtained SiOx films are "dense with nanometric thicknesses". Please explain the meaning of "dense" (compared to other methods or quantitatively estimated?) and provide the thickness of the prepared SiOx films.
• The route described in the paper, for obtaining SiOx films with less energetic routes, is feasible for obtaining thicker SiOx films (hunders of nm)?
• How about the adherence of the SiOx films on the substrates used for deposition (aluminum and carbon steel)? Can this method extended to other substrates, for example flexible ones?
• If possible, the authors are kindly asked to ass some images (AFM for example) would be of interest for the morphology of the prepared SiOx films.

Reply: To address the point of the film density and of the film morphology, SEM micrographs were included. The structure has no porous or defects that would turn permeation of species through it ease. Considering different substrates, yes. It is possible to prepare the same coatings in different kinds of substrates. Simple pre-deposition treatments may be required in some case, but it is feasible. The obtention of thicker films is also possible, just tailoring the number of cycles in the process.

 

Round 2

Reviewer 1 Report

The authors have largely taken the reviewer's critical comments into account and made the desired additions to the content.

The quality of the manuscript was significantly improved.

The text should be read carefully again as it contains some spelling errors (e.g., in the lines 98, 107, 109, 294).

The abbreviation SEM stands for scanning electron microscopy (see lines 114, 131).

Author Response

Reviewer #1

Comment 1: 

The text should be read carefully again as it contains some spelling errors (e.g., in the lines 98, 107, 109, 294).

The abbreviation SEM stands for scanning electron microscopy (see lines 114, 131).

Reply: The spelling errors mentioned by the revisor were amended. Some others were also corrected and highlighted in red.

The Secondary Electrons Microscopy was changed by Scanning Electron Microscopy.

Two new paragraphs were included in the “Introduction” section to furnish a broader and updated literature review of PEALD of TDMAS.

Reviewer 3 Report

Again, Transmittance should be in the unit of percent. Figure 2a,b,c and 4a need to be revised.

Author Response

Reviewer #3

Comment 1:  Again, Transmittance should be in the unit of percent. Figure

2a,b,c and 4a need to be revised.

Reply:  The figures were changed accordingly.