I'd guess pinholes, a dirty Cr-film or cracks in the photoresist. Do you
do anything with it after sputtering the chromium? I've had problems in
the past until I realized that we've incorporated a plasma cleaning step
of the chromium before depositing the photoresist. It worked great for a
while but then started to act like your film does. The reason in  my case
was that someone had started using the plasma cleaner for bonding PDMS and
that contaminated the film enough to make the photoresist peel and then
the chromium was attacked and dissolved pretty fast.

Also check that the photoresist isn't cracked or under severe stress from
baking.

/mikael



-----Original Message-----
From: mems-talk-bounces+evander=stanford.edu@memsnet.org
[mailto:mems-talk-bounces+evander=stanford.edu@memsnet.org] On Behalf Of
Andrew Sarangan
Sent: Monday, December 13, 2010 8:04 AM
To: General MEMS discussion
Subject: [mems-talk] Adhesion problem during 7740 glass wafer wet etching
process

Most likely Cr has pin holes in the film and the HF is penetrating
through it. Sputtered Cr is particularly prone to developing pin
holes. I've had better luck with evaporated Cr.


On Mon, Dec 13, 2010 at 7:49 AM, Xin Yan  wrote:
> Hi everyone,
>
> i want to etch the 7740 about 4 to 20 um deep.  Using Cr with
Photoresist
> 5214E  as the mask,  Cr layer was produced by DC sputtering about 200nm
> thick. The etchant is 1 HF : 2 HNO3 : 2 H2O to get high etch rate about
> 2um/min  to the 7740.
>
> we met a problem that  Cr can not stand in this solution even 2minutes.
>  After about 2 min, Cr begin to  peel off.
>
>  In the 49%HF without HNO3, Cr cannot stand even 1minutes.
>
> what cause the Cr so unstable or it is supposed to be like this?  or  i
need
> to change the etchant ?
>
> Thank you
>
> Yan Xin