Ti will fly of within seconds in HF. Cr/Au is the best combination. However,
usually with this thin coating, there are pinholes in the surface. If you
can add some gold using electroplating, most of the pinholes will be sealed
off

shay

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Albert Henning
Sent: Monday, March 01, 2010 7:39 PM
To: General MEMS discussion
Subject: [mems-talk] Survivability of Cr/Au metallization in 49% HF for 30
min?

This problem was observed on bulk micromachined devices at Redwood
Microsystems.  Ultimately, our process was compatible with nitride, so
we ended up using nitride instead of CrAu.  Our pinholes, however, were
lithographic in nature:  the CrAu was on bare Si.

Speculating, then:  the poly has a different grain structure than bare
Si.  The grain structure of your CrAu depends not only on deposition
conditions (temperature, base pressure, Ar sputter clean [if any]), but
also on the substrate structure.  I suspect the underlying structure is
contributing to the nanoscale pinholes, which the HF then attacks.
Since you cannot work on the poly deposition (it is part of the Summit
process), your choice it seems to me is limited to the metallization.
Perhaps Ti would work better than Cr as an adhesion layer?  Cr has a lot
of stress, and I believe (but am not certain) Ti has lower stress, which
may work to your advantage in terms of grain growth in the Au.

Al