I am ICP etching AlGaAs Bragg structures using Ar, Cl2 and BCl3.
After etching (typically a few microns), there is material deposited on the surface and sidewalls of my photonic structures. I assume it is something like AlCl3... but I'm not sure.
I want to remove this deposited material as it increases the roughness.
I have tried H2O2, up to ~20% concentration, but this does not seem to help.
Does anyone know what the deposited material is, and how could I dissolve it away?
It is likely the result of your sample heating during the etch. I've run into this same problem, and there is no good way to remove the residue. You can prevent it by controlling the sample heating. I am assuming you are using He backside cooling, but I would also recommend using a small amount of vacuum oil between your sample and the carrier wafer (this is what solved it for me). Additionally, you can do piece-wise etching, i.e. etch for 30s, let sample cool with He for 60s, etch for 30s, and so on.
Good luck!
Hi Robert,
I've seen a recipe which uses a flow of SF6 just after the etch to reduce the roughness. The corresponding pressure is some 50-100 mT, the flow value is the maximum allowed by the MFC, the gas is flowing for 2 minutes.
Never tried it in our research, also no idea how this really works (instead looks like a magic). But maybe worth trying, it's a simple&easy experiment.
In case you try it, please inform about the results.
Also the temperature control during the process is truly important, as mentioned by Tyler Growden , especially when a photoresist is used as the etch mask. Deep etches generally mean long time and/or high discharge powers involved, then the overheating might become a problem. You should use a reliable bonding of your small pieces to the carrier (cooled from backside).
Thanks for the replies. Geoff Pain we also see this when etching with only Al/Cl2, so I was going with it being chlorine-based. However, we could look into Aluminium Borides, do you have any ideas of how to dissolve this?
Tyler Growden I use vacuum grease on the sample holder, which has He backside cooling. That said, the sample holder gets to ~50C during the etch process (~5 minutes). At the start I didn't use vacuum grease and the resist really burned onto the sample and would not come off. With the vacuum grease, I think temperature is probably okay.
Victor V Solovyev I would try this, but at the moment we don't have SF6 connected to our ICP. We do have CF4 - do you think the result would be similar?
I have attached an image of the etched structure to show you what we are dealing with. It's not the greatest picture, but the sidewall has a rough deposited layer.
Robert J Chapman , I don't think CF4 will replace SF6 in this particular case. (Maybe worth trying only in case all other attempts failed.)
In the image you attached it is the top surface that looks really terrible - though I'd suppose it was covered by photoresist. Was it?
Regarding the sidewall: it's not clearly seen that some deposited residue is present. It's definetely rough when compared to the cleaved edge, however it might have resulted from the rough edge of the mask. It would be instructive to see the image where the PR mask is not removed yet.
You could also try some hard mask (e.g. thin Ni layer) instead of photoresist. The etch result will be then less sensitive to the sample temperature during the process, also it will eliminate PR as a possible source of contamination.
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