Evelyn,

       The reason you need a more powerful plasma for ashing, is that
the resist usually is more difficult to remove after it goes through
additional processing. It is more than just the thickness.

If you do a descum just after develop, or even after a mild hard bake,
it is usually a thin residue, and the resist isn't cross-linked.
However, after various processing, the resist might be cross-linked, and
more resistant for other reasons. For example, if you did a lift off
process, it means you did a metal deposition in a vacuum chamber. The
resist will be much tougher to remove, and as you point out, you
probably have larger amounts to remove. A vacuum tends to drive
cross-linking at lower temperatures.

I am not sure what the acrynom "PE" means. However, for the resist
removal, you just need a strong O2 plasma. A RIE plasma would be
necessary for a really difficult resist removal, if the resist is
"fried." "Fried" means the resist after a really high temperature bake
or ion implant or something where the resist is really cross-linked,
even carbonized (charred). However, if you are removing most of the
resist with solvent, it can't be that cross-linked or difficult and you
probably don't have thick layers.

Regardless of all the issues I mention above, it comes down to putting
it in the plasma you have available or you are constrained to by your
process requirements, and seeing if it works. It might be that an RIE
plasma would work in a certain period of time, and with the plasma you
have it will take longer. I don't think a barrel asher would work if you
have any heavy residues that are very resistant.

Solvents also vary considerably in their ability to strip resist and
that might reduce your resist residue to levels manageable with the
plasma you have available.

Since you don't want to find out that you can't strip resist on your
device when you get the step, I would create test vehicles to work out
the process and then you know whether you really need RIE.

Best of luck.

Edward (Ed) H. Sebesta
Independent Semiconductor & MEMS Process Engineer



-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Evelyn B
Sent: Saturday, February 28, 2009 7:28 AM
To: General MEMS discussion; Evelyn Benabe
Subject: [mems-talk] Hard baking. Adhesion. RE: definition of hard bake
andhard bake recipes


Edward,

I have one question regarding your discussion below.  I have heard that
descum is done after development and ashing is done after lift-off to
remove resist residue not removed by the resist solvent.  I also
understand, that ashing is supposed to be done at a higher power level
than descum to increase the rate of removal.  But, I have also being
told that ashing is done in RIE mode, at high power levels to remove
very thick layers of resist.  Could you clarify?  I am trying to
determine if I need to do an ashing step after 3000PY  lift-off in RR4
and whether this ashing needs to be done in either PE or RIE mode, the
latter having different pressure requirements.

Thanks,

Evelyn