Hi Lorenzo,
This kind of 'grass' is a very common problem in strongly anisotropic dry
etches.
It's caused by a thin layer of etch-resistant material, which breaks
through unevenly, leaving thin pillars with a 'capping stone' on top.
Type 'black silicon' into Google for lots of discussion of this in silicon
etching.
In your case, this is most likely due to an oxide layer (Al2O3?), which may
be present before you start etching, or may be due to an air leak.
The other candidate to produce the same effect is AlF3, but there should be
no F in your system.

I suggest starting with a short 'breakthrough' step with heavy ion
bombardment, to clear away any oxide from the surface.
Use high Ar content (for heavy ions) and high RIE power (for high ion
energy).
Not for too long, though, or your mask will disappear!

Alternatively, you can deliberately compromise the anisotropy of the
process (eg with higher pressure or temperature) to undercut the capping
stones.

Good luck.
Nick

Dr Nick Appleyard
Research Physicist
Etch Product Group, Trikon Technologies Ltd
Ringland Way, Newport, South Wales, NP18 2TA
Tel    (44) 01633 414187
Fax   (44) 01633 414180
Mob  (44) 0788 178 0727


Hello,
I'm trying to etch by RIE very deep pillars (16 um) in a GaAs/AlGaAs
superlattice (30% Al content) with SiCl4/Cl2/Ar sources.
The result is often not very good since sometimes I have filaments with
a resulting rough surface. I've been told that one problem could be an
excess of Cl in the mixture but its reduction did not lead always to
better results.
Do you have some ideas?
Is it maybe a kind of contamination or leak in the chamber?
Thank you

Lorenzo



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