Reactive Ion Etching of X-Cut LiNbO₃ in an ICP/TCP System for the Fabrication of an Optical Ridge Waveguide

Round 1

Reviewer 1 Report

Andrei Kozlov and et. al "Reactive ion etching of X-cut LiNbO3 in ICP/TCP system for fabrication of optical ridge waveguide" demonstrated the technology for producing of ridge waveguides with minimal roughness of sidewalls and material surface in near-waveguide region. The fabrication parameters were studied and analysed to achieve high etching quality. This work may draw much attention for readers in the field of LNOI nanophotonic reasearch. However, I suggest a major revision should be taken before accepting.

 

1.      For the part of “Introduction”, in Table 1, the profile quality (surface roughness) must be quantized. The description of high, very high, relative could not be satisfactory for readers.

 

2.      In the part of “Materials and Methods”, the authors shown some simulation results. In my opipion, it is unnecessary because in the work the etching process were developed on LN crystal and there are no waveguide test results.

 

3.      In the part of “Results”, when the authors discuss the surface roughness, there is no compelling data at all. I think the roughness data is necessary.

 

4.      For the paper structure and writing, the authors should prepared more carefully. The article is loosely organized and is not easy to read.

Author Response

Dear Reviewer, authors of article "Reactive ion etching of X-cut LiNbO3 in ICP/TCP system for fabrication of optical ridge waveguide" are very appreciate to you for kind and very useful revision of our article. We believe, that changes, which we did after your comments made text more clearly for readers. Below we are present our comments to your remarks with explanations.

Remark 1:

For the part of “Introduction”, in Table 1, the profile quality (surface roughness) must be quantized. The description of high, very high, relative could not be satisfactory for readers. 

Our comment:

Thank you for this valuable remark. We have corrected a part of table, which related with articles where authors provide quantitative info from AFM or other analysis tools. However, the rest of articles have no quantitative information. In these articles, authors provide only qualitative description of etching results like "very rough surface" etc.

 

Remark 2:

In the part of “Materials and Methods”, the authors shown some simulation results. In my opipion, it is unnecessary because in the work the etching process were developed on LN crystal and there are no waveguide test results.

Our comment:

We provide our simulation results in order to explain the value of etching depth. Recently we have explained this need in the text (lines 109-123). Current work was performed with bulk X-cut LN, but etching depth is calculated and etched in accordance with thin film structure of LNOI and fundamental optical mode distribution in LNOI waveguide with relevant geometry. 

 

Remark 3:

In the part of “Results”, when the authors discuss the surface roughness, there is no compelling data at all. I think the roughness data is necessary.

Our comment:

Thank you for this comment. We added some information about this remark in the Conclusion section (lines 424-428).

At the stage of technology development, qualitative description from SEM analysis is enough for us. In the future we are going to do some experiments with the aim of reveal the influence of surface roughness on optical losses in waveguides.

Remark 4:

For the paper structure and writing, the authors should prepared more carefully. The article is loosely organized and is not easy to read.

Our comment:

We have tried to keep typical structure for articles about developing of technology.  However, many articles on technology development provide only results but do not explain the effects on which the technology is based. Therefore, reproduction of this technology for other scientists is very complicated. Our key intention is to provide not only technology results, but also brief model explanation of RIE process of LN.

Author Response File: Author Response.docx

Reviewer 2 Report

The effect of technological parameters of RIE on waveguide performances is not given in the manuscript. There are no data that confirms the optical propagation characterization of this ridge waveguide, such as near-filed light intensity distribution and propagation loss. Comparing with the previous publications, there are no enough improvements in either the method or the results in the manuscript. I do not recommend publication of the manuscript in Applied Sciences due to the lack of novelty.

Author Response

Dear Reviewer, authors of article "Reactive ion etching of X-cut LiNbO3 in ICP/TCP system for fabrication of optical ridge waveguide" are very appreciate to you for kind and very useful revision of our article.

Thank you for your opinion. This article was focused on technologies aspect of LN reactive ion etching. This method is one of most suitable for waveguide fabrication on LNOI. However, there is no good model, which explain the kinetics of reactive ion etching of LN especially for thin film configuration (LNOI). We hope that this article will be good start point in the formation of understanding of this important process. Our first goal is to make technology more manageable. The characterization of waveguides is our secondary goal.

Author Response File: Author Response.docx

Reviewer 3 Report

it can be accepted in current state.

Author Response

Dear Reviewer, authors of article "Reactive ion etching of X-cut LiNbO3 in ICP/TCP system for fabrication of optical ridge waveguide" are very appreciate to you for kind and very useful revision of our article.

Author Response File: Author Response.docx

Reviewer 4 Report

The authors have carried out extensive work studying different regimes of RIE to produce ridge waveguides with smooth side walls in thin film LN. 

All parts of their research were described in great detail and with clarity: design simulation, fabrication steps, different etching recipes, modelling of  the etching process and discussion of the results.

The work presents timely and interesting results which will contribute towards the realisation of smooth and near-vertical profile of thin film LN waveguides which are so desired in integrated optics.

I have a concern  that annealing of Cr mask at 300 C could introduce some stress into LN film- did the authors consider this aspect?

Fig.2 has no description of its parts. It is stated that the details are in the text, however, short description of each part of the Fig.2 directly under the figure should be added.

 

 

Author Response

Dear Reviewer, authors of article "Reactive ion etching of X-cut LiNbO3 in ICP/TCP system for fabrication of optical ridge waveguide" are very appreciate to you for kind and very useful revision of our article. We believe, that changes, which we did after your comments made text more clearly for readers. Below we are present our comments to your remarks with explanations. 

Remark 1:

I have a concern  that annealing of Cr mask at 300 C could introduce some stress into LN film- did the authors consider this aspect?

Our comment:

Of course, we should pay attention at this aspect because of different thermal expansion coefficients of materials especially in presence of chromium films.

 

Current works have done with bulk LN wafers, but when we will transfer etching technology on LNOI, this aspect will be crucial for the technology.

 

Going beyond this article, we try to transfer the etching technology on LNOI wafers at this month. Some comments about it.

First, we followed instructions for thermal treatment of LNOI wafers, which provided from NANO LN vendor. Heating/cooling rate ~ 3 °/min.

In addition, our colleagues evaluated the residual stresses in thin chromium films (hardmask) deposited in this deposition mode using a special flexible structure based on silicon dioxide. These films characterized with slight compressive stress. Thin films of LN on oxidized silicon wafer has tensile stress. We suppose that in this configuration (films thickneses, material of hardmask, deposition and annealing modes) there is no threats to the appearance of defects on LNOI. 

Our current practical experience is quite good. We do not have any cracks or any different defects.

Thanks to your timely valuable remark, we added some information in our article (lines 182-187).

 

Remark 2:

Fig.2 has no description of its parts. It is stated that the details are in the text, however, short description of each part of the Fig.2 directly under the figure should be added.

Our comment:

We have corrected the description for Fig.2. (lines 165-167)

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I recommend publication in the current version.

Reviewer 2 Report

I’m pleased to see that most of the concerns were addressed in a satisfactory manner. Therefore, I recommend this article for publication.