Dear Masoud Jedari Ghourichaei , thanks for sharing this interesting question. Etching away some unwanted part of our structures is indeed a problem and with free standing structures it can be even worst. Most of these fabrication techniques are implemented for substractive or top-down processes and often the best idea when you make a mistake is to start from scratch. In your case, this would be the best option if that etched SiO2 volume under the cantilever is important from a structural, optical or other aspect.

About the chances to rebuild the SiO2 layer under the cantilever by PECVD or other methods I would say it will depend on the cantilever and gap dimensions. A very thin gap and a wide and large cantilever can leave areas below the cantilever without filling while a relatively large gap and a thin and short cantilever would facilitate an homogeneous filling, giving that the cantilever shadow will be smaller. This shadow exists because there is a directional deposition component with a PECVD. This could be used in your favour if the gap is placed facing the plama orientation, but this would be difficult in general because of the aspect ratio of wafers or substrates where you are doing the deposition.

I couldn´t find any paper about a similar case, but the following one describe a method to obtain thick films of SiO2 by PECVD, that could be useful for you:

https://www.sciencedirect.com/science/article/abs/pii/S0040609098010219

Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures M.I. Alayo, I. Pereyra*, M.N.P. Carreño

Thin Solid Films 332 (1998) 40,45

Sadly it is still not available at RG, but may be you can access through your university library or contacting the authors.

An important point to have into account is the quality of the PECVD SiO2, its properties compared to the etched SiO2. Alayo et al studied this effect and compared the PECVD SiO2 etching rate compared to the etching rate of thermally deposited SiO2 and they observed that for thin SiO2 films deposited at 0.2 um/h the etching rates were comparable, but for thick SiO2 films deposited at 3 um /h the etching rate was 2 times the observed for thermal SiO2. Thicker SiO2 films dpeosited faster (6 um/h) showed 7 times faster etching rates. The refractive index also changes with deposition rate, but less. So, if SiO2 structure and properties matter in your MEMS, you should take this into account. If this SiO2 layer should be removed at some point of your fabrication process it could be useful to have a material easier to etch.

Another thing to take into account: if your cantilever is very thin, its width large and its length large too, it could easily bend when you deposited the SiO2, specially if you need a relatively thick SiO2 layer. In this case you could end up with the volume under your cantilever filled with SiO2, but the cantilever itself bended, changing the original design and may be its functionality.

Hope this helps. Good luck with your research work and my best wishes.

I agree with Manuel Gómez.

It depends on dimensions of Your device, but I wouldn't expect PECVD to fully fill area under cantilever. In the best case You'll propably end up with some gas bubbles trapped between SiO2 walls under cantliver.

Manuel Gómez thank you very much for the thorough and extensive answer. It seems PECVD cannot be an option for me. As Krzysztof Chmielewski said in his nice provided picture; because of the directional nature of the PECVD I will end up with such configuration with trapped gas under the cantilever. however, here is my suggestion: what if we use LPCVD? does it work for my case?

Dear Masoud Jedari Ghourichaei , what Krzysztof Chmielewski drawing shows is a possibility, and as we both pointed it would depend on the cantilever geometry.

Remember that CVD techniques are Deposition methods, and that they involve both the gas reactants and the substrate surface a part of the particular ways to do promote the reaction be it LPCVD or PECVD.

In the case of PECVD, I talked about certain directionality because there are an anode and a cathode that affects the plasma. However the plasma function is to enhance the CVD process as it is the temperature in LPCVD.

In general these methods are depositions and therefore there are just one substrate side exposed. In your case the cantilever and the baser have all their surfaces exposed, so the CVD process can happen in all of them, so SiO2 would be growing from the base of your silicon substrate, the wall between the silicon substrate and the cantilever and also from the bottom, top and sides of the cantilver. Certainly the plasma can be shadowed partially below the cantilever and may be the deposition rate under the cantilever can be lower than in other areas but while precursors can go into the space between the SiO2 growing below the bottom of the cantilever and the one growing from the silicon substrate they will close the gap unlees the SiO2 film coat and close the space as depicted on the Krzysztof´s picture. If precursors cannot flow inside that buble the SiO2 growth will stop (only) there.

This could also happen with LPCVD I think.

Thank you all for the valuable answers

Manuel Gómez

Krzysztof Chmielewski

Saad Zahraw Sekhi

you welcome Masoud Jedari Ghourichaei.

Dear Masoud Jedari Ghourichaei

You can use a solution which is composed of suspended silicon dioxide particles in a flying solvent. After covering the areas that to remain free from the oxide with adhesive film you can simply dip the structure in the solution after each dipping you need to dry it. You need to repeat the process sometimes to fill the gap as you want. There is such chemical solution that is normally used to deposit silicon dioxide on the silicon wafers to getter metals from them.

best wishes

Abdelhalim abdelnaby Zekry

Thank you so much for your reply. But, could you please recommend which solution is used for this process. and how silicon dioxide is dissolved in the solution?

thanks