SciPost Submission Page
Ether cleavage and chemical removal of SU-8
by José M. Ripalda, Raquel Álvaro, and María Luisa Dotor
|As Contributors:||José M. Ripalda|
|Date submitted:||2022-03-14 15:25|
|Submitted by:||Ripalda, José M.|
|Submitted to:||SciPost Chemistry|
The high chemical stability of SU-8 makes it irreplaceable for a wide range of applications, most notably as a lithography photoresist for micro and nanotechnology. This advantage becomes a problem when there is a need to remove SU-8 from the fabricated devices. Researchers have been struggling for two decades with this problem, and although a number of partial solutions have been found, this difficulty has limited the applications of SU-8. Here we demonstrate a fast, reproducible, and comparatively gentle method to chemically remove SU-8 photoresist. An ether cleavage mechanism for the observed reaction is proposed, and the hypothesis is tested with ab initio quantum chemical calculations. Also described are a polymer-metal adhesion treatment, and a complementary removal method, based on atomic hydrogen inductively coupled plasma.
Submission & Refereeing History
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Reports on this Submission
Anonymous Report 1 on 2022-6-5 (Contributed Report)
1- The topic is of clear interest and relevance to applications.
2- The introduction provides a clear and concise presentation of the knowledge of the field.
1- The actual results presented are relatively scarce, and do not go in depth into any of the areas studied.
2- The claims made are overall too general and overreaching compared to the data obtained. For example, in the conclusion: "Three current technological issues with SU-8 have been addressed in this work" seems a very big claim compared to the results available.
3- The authors present a lot of hypotheses and suppositions, and often lack actual data to confirm or infirm them. As such, the manuscript is mostly speculative/argumentative, rather than scientific findings.
4- The DFT section, done at a reasonable but very routine level of theory, presents only one possible pathway. It does not compare pathways, or explain some of the choices in the pathway studied.
The content of the paper is overall interesting, but it feels a bit like a mixed pot of very different results (surface preparation, UV exposure, removal, computational results), where none of the areas involved have really been pursued in depth: it does not mean that the findings have no merit, but it raises more questions than it answers.
1- The authors present, in the "Polymer-metal adhesion treatment" section, a "an in house developed procedure to chemically modify the metal surface […] described for the first time". However, this procedure is not explicitly compared to the other state-of-the-art methods (in the Methods section), and its impact on the sample obtained is not presented in the manuscript. Is the adhesion obtained better? (one supposes so) If so, how was this improvement characterized, what quantitative and qualitative factors were compared? Etc.
2- In the section about chemical removal, only three experimental conclusions are investigated and reported. It is not entirely clear to me why these three were chosen, and why the different factors were not investigated separately. It seems to be difficult to reach conclusions, as the authors do, on the basis of only these three experiments.
3- In that same section, how is "complete removal" characterized? Is it visual inspection, or are tests actually performed to quantify the removal? Figure 3 is not very useful in this regard, as a very macroscopic view of dissolution, with no real information provided.
4- Figure 4 needs a scale bar, the diameter of the field of vision is not sufficient for accurate measurements.
5- Section 3.2 and Figure 5: the authors present this reaction in the text as "a hypothesis", but the Figure is less clear and presents it as a fact. The author should perform analysis to confirm whether the tertiary amine is indeed observed after treatment with dimethylamine, which would either confirm the presence of this species (and validate the findings), or invalidate the hypothesis. More than half a page is spent presenting this hypothesis, so it would need to be checked experimentally.
6- This is a major point, which I do not understand: the mechanism proposed and studied by DFT implies the pre-existence of a stable carbocation, that exists as a stable species? The authors state in the caption of Figure 1 "The carbocation persists after polymerization", but that seems surprising to me, so it needs to be argue: how has it been observed and confirmed? what is the anion associated? This is a very unexpected hypothesis, so it needs a strong confirmation in my view.
7- In the discussion of the DFT section, the authors discuss a lowering of the "energy barrier", but transition states have not been characterized or discussed at all. The transitions states need to be determined, and the alternative pathways characterized as well, for comparison and confirmation that the chosen mechanism is representative. The current presentation is way too speculative.
7bis- The authors acknowledge so, saying: "the results are consistent with the proposed hypothesis, but this does not confirm the hypothesis as valid, it merely serves to gain confidence in its validity" and "only a few of all the possible reaction pathways can be explored, and only thermodynamic results on stability are easily achievable, results on reaction kinetics being much more challenging to obtain". But this is not true, there are a lot of DFT studies of chemical reactivity and mechanisms that study both intermediates and transitions states, and compare different mechanisms to conclude: this is, in fact, what DFT studies are routinely used for, in order to reach conclusions about reactivity. Just confirming one set of relative stabilities seems well under the typically bar for mechanistic studies, in this respect.