Hi Z.Z.,

What you are describing here is, I think, the formation of a galvanic-
enhanced etch due to the presence of the metals, and that is why you
see a difference between the n-type and p-type materials.  You can try
to keep photoresist on top of the metal to minimize the contact to the
etchant to reduce the galvanic effect.  I have also had good
experience with dilute phosphoric acid-based etchants (try
100:10:5::DIW:H3PO4:H2O2 as a starting point).  You may still
experience some difference in etch rates regardless.  How much
difference can you tolerate?

Brad Cantos


On Nov 11, 2008, at 2:42 PM, Zhaoyu Zhang wrote:

> Sorry if this is of no interest of you.
>
> Hello,
>
> I need to selective etch partially(undercut) AlGaAs v.s. GaAs of p-
> GaAs/p-AlGaAs/i-GaAs/n-AlGaAs epi from the side of the mesa. And we
> need to have same undercut of p-AlGaAs and n-AlGaAs with gold on top
> of p-GaAs.  Now the problem is: Without gold or platinum on top of p-
> GaAs(only tried this two metals), I can get P-AlGaAs and n-AlGaAs
> undercut the same. But with gold or platinum deposited first on p-
> GaAs, there's huge difference: p-AlGaAs being etched(undercut) while
> n-AlGaAs staying intact.  I will really appreciate if anyone has
> some solutions to etch evenly with gold in present?  We do need
> metal deposition first.
>
> Note:
> .. p-AlGaAs denotes p type AlGaAs
> .. n-AlGaAs denotes n type AlGaAs
> .. the wet etch chemistry I use: buffered HF
>
> Many thanks!
> Z.Z.

Brad Cantos
[email protected]
http://holage.com