Hi Jie,

I have made Cr photomasks using 150 nm thickness PMMA to define the
pattern, as you are trying to do. I have found that HMDS vapour priming
the mask prior to applying the PMMA improves its adhesion sufficiently
that it does not part from the plate during wet etching. To etch the Cr
I use an etchant composed of 41 g ceric ammonium nitrate, 10.5 ml of
perchloric acid and 250 ml of water; I don't know if this is the same as
"CR-12S", which you have been using. It will etch through 100 nm of Cr
in around a minute.

The PMMA undercut is very small, and is not an issue for my ~1 micron
features. I don't see it being an issue for your 0.8 micron features
either. However, if the tolerances required are very tight then spending
a little time characterising the Cr etching will be worthwhile.

Regards,

Arthur Blackburn

Microelectronics Research Centre
Cavendish Laboratory
Cambridge
United Kingdom