>Hi Everybody,
>               Can any one suggest a protective coating (polymer etc)for
>protecting the top surface of silicon wafer with aluminium lines while
>etching the back side of the wafer using KOH solution???? We have already
>tried out wax, black wax coatings but it did not work.
>Any help and references in this regard will be highly appreciated....
>Thanks
>Kanishka
>

Hi Kanishka,

SU-8 and polyimide are highly chemically stable in KOH solution. I have
tried the former, but the only problem is adhesion with the substrate. The
SU-8 tends to peel off within minutes of immersion in KOH (my solution was
at 75 Deg C). I do not think the adhesion problem can be resolved, so an
alternative may be needed. I may try polyimide next. Another possibility is
a newly developed polymer pretective coating developed by Brewer Science -
the following paper is available on their website "Polymer protective
coating for wet deep silicon etching". These people seem to be applying the
polymer onto a silicon nitride substrate so I don't know how the adhesion
would be on other substrates. The polymer they use (which they don't
specify) is resistant to KOH.

Is your Al directly on Si or is there an intermediate layer of (e.g. Cr)?
You might experience adhesion problems between Al and the Si substrate once
immersed in KOH as well. You could try protecting your topside by sputtering
a Cr layer over everything. After KOH etching the back of the wafer, you
could remove the Cr with wet etchant and rinse once the Al lines become
visible. The Al should not be etched until all the Cr is removed.

Let me know if you find out anything new on the polymer side.

Regards,

Michael

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