Dear Wei,

-For the reflow of photoresist, you can use a hotplate at 110 for a
couple of minutes (4-5 mins). This works for our applications and
shipley's photoresists.

-I recommend sputtering instead of evaporation for a better step
coverage of the side walls. I have never tried 500 A for a seed layer
and my feeling is that you should prefer at least 1500 A of gold to have
a uniformly distrubuted current across the wafer.

-You may need to increase the exposure (slightly) and development
durations to be able to open the via parts.

-If the electroplated thickness is a few microns, you can directly use
Transene's gold etchants to remove the seed layer (considering the big
difference between the thickness of the electroplated gold and seed
gold). Certainly, gold etchant will give a damage  to the electroplated
gold (you may observe an increase in the roughness of the top of the
electroplated gold) but you can minimize this damage if you work very
precisely (by taking the wafer out of the etchant just after the removal
of seed layer) .   Ti etching will not give any damage to the
electroplated gold.
Best regards,

Kagan TOPALLI, Ph. D.
Senior Research Engineer
METU-MEMS Center
Middle East Technical University
TR-06531 Ankara Turkey
Phone: +90 312 210 45 46
Fax: +90 312 210 23 04
http://www.mems.eee.metu.edu.tr/~topalli/
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Wei Cheng wrote:
> Dear all,
>
> I have some question regarding air-bridge fabrication. Thanks for
> sharing and helping.
>
> 1. I plan to use AZ4210 as the 1st layer PR (2.9um thickness) to open
> the VIA contact and reflow the PR to round the sidewall. Is 120C/30min
> in oven appropriate for re-flow?
>
> 2. I will use e-beam evaporator to deposit seed metal layer (Ti/Au) for
> carrying the electroplating current. What is the proper metal thickness
> for this purpose? (500A vs. 1500A) What if I use sputter instead of
> e-beam evaporate to deposit? My consideration is its efficiency of
> electric conducting continuity and easiness to remove the 1st layer PR
> later.
>
> 3. I will use AZ5218 as the 2nd layer PR (2.7um on planar surface). Will
> the exposure, bake and develop condition remain the usual? My concern is
> AZ5218 is now on top of another PR layer and locally it is much thicker
> (greater than 5um) at Via contact.
>
> 4. After Au electroplating, what is the better way to remove the excess
> seed metal layer? Do I have to use an extra etch step (dry or wet) to
> remove it?
>
> Wei Cheng
>