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


On Wed, Feb 25, 2009 at 6:54 PM, Edward Sebesta  wrote:

> I would like to clarify something on Hard Bake, reflow and UV cure.
>
> 1. Reflow and hard bake are not the same thing. Reflow is where the hard
> bake is hot enough to cause the resist to flow and the resist features
> to adopt a minimum surface. It is a reshaping driven by surface tension
> and the resist being softened to the point where the resist will flow.
> However, you can have hard bakes where the resist doesn't reflow. Reflow
> is a subset of hard bake.
>
> 2. Hard bake does have some definition. It is a bake higher than would
> be acceptable for a softbake due to loss of photospeed and Photo Active
> compound. It is always a post develop bake.
>
> 3. The Deep UV curing machines, like the Fusions, would ramp a bake
> while doing the UV cure. There isn't a need for bake afterwords. I
> suppose there might be some esoteric application where there is, but I
> never ran into it.
>
> 4. Finally, for wet etching, hard baking only goes so far to help. Some
> hard bake will always be better than no hard bake. However, beyond 140
> degrees you will rarely see much improvement, and at higher temperatures
> you might introduce stress in the resist film to make it try to pull way
> from the substrate.
>
> Deep UV curing is to harden resists to withstand plasma etch. Usually
> metal plasma etch. I don't see it helping resist adhension.
>
> Higher temperature hardbakes can also make the resist very difficult to
> remove. You have to use plasma ashes to take the "skin" off and then
> sulfuric peroxide strips.
>
> If you can't get good adhesion at 130 deg. Or 140 deg. C, I would look
> at surface treatments. Try HMDS, even if it isn't silicon, silicon
> dioxide, or silicon nitride. HMDS removes anything with an OH group, and
> often it can solve an adhesion problem even if it isn't a "textbook"
> case.
>
> Before working on adhesion enhancements, make sure your substrate is
> actually clean, and has no residues from vapors, cleaning solvents,
> vacuum ovens, etc. A fab or lab, can have a lot of little devils to do
> in your adhesion. For example, spilled machine oil has a small vapor
> pressure and you only need a monomolecular layer of junk to have bad
> adhesion.
>
> Also, HMDS isn't the only material available for vapor priming. There
> are multiple chemicals. I used to use some Aluminum analog to HMDS for
> aluminum metal layers. Probably was a waste of time and a superstition
> of the previous engineer, but I didn't dare remove it.
>
> Edward H. Sebesta
> Independent Semiconductor & MEMS engineer

--
EVELYN BENABE
Graduate Research Assistant
RF Microsystems Research Group
University of South Florida
4202 East Fowler Avenue
Tampa, FL 33620
Office: ENB 412
Office Phone: (813)-974-4851
Email: benabe@mail.usf.edu