Wei,

You will probably have to experiment a bit to get the optimal
parameters for the reflow of the first PR.  SEM the sidewall to make
sure you have the profile you need.  What I have done in the past is
flood expose the first PR for 2-3x standard exposure after develop to
destroy the PAC.  This will help to prevent some crosslinking when the
second PR is baked, and will also make it easier to remove when you
are stripping the stack.  I would also recommend sputtering the metals
rather than evaporation, but if you only have an evaporator (hopefully
with a planetary dome), and assuming that your sidewall is completely
flowed and has no sharp edges, you may get away with 250ÅTi/1500ÅAu.
I wouldn't go much thinner.  The exposure conditions for the second PR
may change slightly, but only due to the back reflection of the light
from the metals.  If you are using contact or proximity lithography,
you may have some difficulties getting good image fidelity.  To remove
the Au seed layer, a simple way is to use a wet etch (potassium-iodide/
iodine solution).  I believe Transene offers a commercial version of
this etch in 2 strengths.

Brad
_________________________________

Brad Cantos
brad.cantos@holage.com
http://holage.com


On Sep 24, 2009, at 3:46 PM, 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