XCT and DLW: Synergies of Two Techniques at Sub-Micrometer Resolution

Round 1

Reviewer 1 Report

The paper by Sven Fritzsche et al. describes 3D Direct Laser Writing and characterization of the fabricated structures by XCT. Also the both techniques are well established and known, authors claim that the combination of both at their resolution limit is novel. In my opinion, the combination of these two techniques is just a very particular example, where the XCT is relevant for 3D DLW structures optimization. However, I found the presented results may have practical interest for people working in the field of 3-D lithography. Therefore, this work is well addressing the scope of the journal and I would recommend it for publication in MDPI Applied Sciences after addressing my remarks.

1. Abstract. Lines 3-6. Authors should comment on the difference between "XCT" and "XRM in 3D". They sound the same to me.

I would also encourage the authors to comment in the introductory section about the resolution limits of 3D XCT/XRM, compared to X-ray microscopy in 2D and distinguish cases of the synchrotron and lab-source based instruments, as well as highest resolution reported and routinely available in commercial instruments.

2. Generally, I would expect to see the introduction, as an overview in the field with defining a problem and solution to it proposed in the presented work. I am missing that and I expect it to be clearly defined before accepting the manuscript for publication!

Authors talk about capabilities of 2PP-DWL, DWL and immersion lithography, which are quite different techniques for me. However, authors do not even mention what method they used in the introductory part. perhaps, the introduction should be more focused to the immersion lithography they used.

The content of two paragraphs on lines 66-85 does not reads as Introduction, does not contain a single reference and should rather belong to a discussion section, in my opinion.

3. Chapter 2.1: What is the thickness of the photoresist after the baking step(s)? Did authors always measure it? Perhaps, a naive question, but natural question this missing information causes: can it be that the missing pyramid tips are sometimes caused by insufficient photoresist thickness?.. 

4. The title suggest to see the demonstration of both the XCT and the DLW at the resolution limit. However, only low resolution overview images are presented. It would be essential to present some high resolution images illustrating the highest resolution achieved in this work and give the actual resolution number.

Line 325-328 says that further studies are needed to demonstrate absolute resolution limits. This does not reflect the ambitious title, which either needs to be reformulated or commenting this point in details what are the resolution limiting factors.   

5. I assume authors used XCT in absorption mode. I am wondering if phase contrast methods, such as edge illumination or grating interferometry would be more suitable, especially for such low x-ray absorbing materials as polymers?

Minor remarks:

- The meaning of the sentence on lines 317-318 is not clear.

- Authors mostly (but, not everywhere) use "m" annotation for micrometers throughout the paper. For example, on lines: 7, 114, 115, Table 1, Table 2, lines 133, 134, 140, 157, 160, 170, 179, 254, 267, 297, 308, Figure 3. I might have missed some. Please, check throughout the paper!

- Replace "x" with "×" on lines 159, 190, 205, in Table2 and wherever relevant.

- Line 190: replace "mm₃" with "mm³"

- Abstract, line 11: I would replace "high-absorbance" with "high X-ray absorbance" for clarity.

- Table 2: number of projections should have no units.

 

 

 

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors describe the combination of two state-of-the-art techniques, both offering extraordinary possibilities in terms of manufacturing accuracy for one and imaging precision for another. Direct laser writing (DLW) by two-photon polymerization can produce polymer structure with a resolution down to hundred nm. It is a very complex technique and flaws can be generated at different steps of the process. The novelty of this paper is to combine X-Ray computed tomography (XCT) analysis on different structures produced using the DLW technique. The authors were able to highlight the appearance of defects previously not detectable by other analysis methods.

The paper is supported by a relevant bibliography. It is very well organized, and the various techniques and methods are described in sufficient details. The results are presented accurately with many details and explanations and lead to relevant conclusions.

In conclusion and taking in account  the very minor corrections listed below, I think this document can be published in Applied Sciences.

However, authors should reread the document carefully as there are many unit errors everywhere in the text including in the abstract and in some tables and figure caption (lines 7, 87, etc...). I think probably that the Greek letter µ disappear completely from the pdf I got.

Line 228: Figure 7a is written twice

Ref 26:  has to be completed

However, authors use a lot of acronyms, some of them are not defined before they are used (STL etc…). I think it is necessary that the authors check and correct this point before being published.

Author Response

Please see the attachment

Author Response File: Author Response.pdf