I would avoid using Piranha for cleaning anything other than bare Si
or glass wafers since Piranha actually can etch metals and it is
possible that the spots you see are etched spots on the Cr.  The
clean I usually do for masks is a simple acetone wash to get off the
large amounts of photoresist and then a 5 minute ash (oxygen plasma
at 25-50W).  This requires an asher/RIE that can hold your mask which
I know can be hard to find (we only have one that will fit our 5 inch
masks and anything larger needs to go to another fab), but this is
the safest way to clean your mask.

If you cannot find an RIE large enough for your masks you might want
to try just using heated acetone, posistrip, nanostrip, etc (some
sort of organic solvent).

Nicolas "Nik" Duarte
Ph.D. candidate at the Pennsylvania State University
Department of Electrical Engineering


At 08:15 AM 9/6/2007, you wrote:
>Hello all,
>I am currently using a 7"x7" photomask for my work.I had procurred the
>Photomask blank  (Cr blank) from a company called as Telic in USA.
>To clean the photomask I use 70% Sulphuric Acid +30% Hydrogen Peroxide
>(Pirana solution). I dip my photomask in this solution for 7 minutes and
>then clean in thoroughly in DI water.
>Can anyone suggest whether the procedure is correct and do I have to
>something else to clean the mask. After Cleaning the photomask, the Cr side
>looks pretty much clean, but the glass side of the photomask still has
>stains/spots. I suspect that the stains/spots are not on the glass side, but
>inside the glass and hence I would like to ask whether anyone has experience
>in using photomask blanks from Telic.
>Regards,
>Sudesh