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I have a microfluidics problem where hydrogen is forming tiny (nanometer-scale) bubbles which cause problems. The hydrogen is a reaction byproduct that comes from n etch between silicon and TMAH. The bubbles essentially cause etch masking.

We are trying physical/mechanical means of removing the bubbles (lowering pressure, vibration, etc.) but I wonder if we could add something to the etch solution which would react with the hydrogen and form something soluble, thus prevent formation of the gas-phase hydrogen bubbles.

So far I found suggestions for forming various metal hydrides, but metals are considered contaminants. Also I guess sodium will react with it, but sodium isn't allowed either. We could add oxygen to the solution to create H2O but free oxygen in the solution would probably cause oxidation and impact the process. A lot of things will react with hydrogen though...I wonder about citric acid or IPA? Any chemical which would react away the hydrogen.

BetterSense
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    Hydrogen isn’t quite as reactive as you might think - mixing it with oxygen for example doesn’t instantly create water. – NotEvans. Jun 22 '21 at 20:23
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    Adding to what @NotEvans. said, no go for hydrides, sodium, citric acid or either IPA (India Pale Ale or isopropyl alcohol). Maybe an ultra thin palladium membrane to let the hydrogen diffuse though and be safely removed. – Ed V Jun 22 '21 at 20:30
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    I don't see how a membrane would be compatible with etching. All in all, @BetterSense, this is a genuinely tough problem, so don't expect an easy shortcut. Very few things react with hydrogen at room temperature (in particular, none of those you've mentioned). – Ivan Neretin Jun 22 '21 at 20:34
  • How about increasing pressure to dissolve the hydrogen? Perhaps there is a solvent compatible with the etchant that could help dissolve the bubbles... see https://www.bre.com/PDF/Solubility-of-Hydrocarbons-in-Physical-Solvents.pdf for some solvents. – DrMoishe Pippik Jun 23 '21 at 23:39

1 Answers1

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Various methods for improving the alkaline etching of silicon are given in this open source by Pal et al. [1]. In the section on chemical additives they describe the effects of hydroxylamine, $\ce{NH2OH}$, which produces an oxidizing radical that reacts quickly. In the case of a TMAH solution they report from Swarnalatha [2] that 5% (not higher) TMAH plus 10% hydroxylamine gives a maximal etching rate.

The authors also quote results from Schriffrin et al. [3,4] demonstrating that suppression of hydrogen bubbles by adding oxygen or isopropanol (the latter acts as a surfactant causing the bubbles to detach) is effective in producing a smoother etched surface.

References

1. Pal, P., Swarnalatha, V., Rao, A.V.N. et al. High speed silicon wet anisotropic etching for applications in bulk micromachining: a review. Micro and Nano Syst Lett 9, 4 (2021). https://doi.org/10.1186/s40486-021-00129-0

2. Swarnalatha, V (2019). "TMAH based high speed silicon bulk micromachining", PhD thesis, Indian Institute of Technology Hyderabad, India.

3. Campbell SA, Cooper K, Dixon L, Earwaker R, Port SN, Schiffrin DJ (1995). "Inhibition of pyramid formation in the etching of Si P(100) in aqueous potassium hydroxide isopropanol." J Micromech Microeng 5:209–218.

4. Baum T, Schiffrin DJ (1997) AFM study of surface finish improvement by ultrasound in the anisotropic etching of Si <100> in KOH for micromachining applications. J Micromech Microeng 7:338–342.

Oscar Lanzi
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