Hi Rob,

Whenever I have done air-bridge processing, which is distinct from
liftoff, you do want conformal coating of the seed metal in order to
have electrical continuity for the electro-plating.  [Electroless may
be different, but I have no experience with air-bridges and
electroless plating.]  My comment was that you do want rounded PR
profiles to get the electrical continuity, and ideally should use a
sputter technique for the seed metal.  The process flow for a typical
air-bridge (at least those used in GaAs MMICs) is:

Coat and pattern resist 1
Flow resist 1
Conformal metal sputter
Coat and pattern resist 2
Electroplate
Remove resist 2
Remove excess seed metal
Remove resist 1

Liftoff is a distinct process where the metal on top of the resist
should not be continuous with the metal on the wafer.  The best way to
achieve this is with a retrograde profile of the resist, which is a
normal condition created when using an image reversal process.  If you
try to electroplate on a structure like this, it is very likely that
the second resist will partially dissolve the first resist and deform
the seed metal.

Brad

_________________________________

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

LinkedIn: http://www.linkedin.com/in/bradcantos

On Mar 30, 2009, at 11:31 AM, Robert MacDonald wrote:

> Crayon,
>
> With regard the to the bubbles, I didn't notice if you are doing
> your bakes on hot plates rather than in ovens. The hot plates,
> because they are heating from the bottom up, will often be superior
> in driving off the solvents, rather than trapping them in the
> resist. From your email it isn't clear what the thickness of your
> first PR layer is. The thicker this layer is, the more difficult to
> drive off those solvents.
>
> The other point to keep in mind is alluded to by Brad. In order to
> attack the release layer, you want to make sure that it is not con
> formally coated by your seed layer. This is a classic problem with
> lift off processes. Generally it is solved by the generation of
> "undercut". There are many references on how to do this. It is most
> easily done with negative resists, but can also be done with
> positive resists. Without knowing about the geometry of your release
> layer it is hard to know what to recommend. But the key is to
> examine your structure after the deposition of your seed layer (in a
> SEM if possible )and verify that the edges of the release layer will
> be exposed to the AZ solvent.
>
> Finally, I don't want to recommend a complete process change, but I
> am a big fan of dry release for mems. It should be possible with
> your device geometry using isotropic plasma. I have used polymide
> release layer and CF4/O2 plasma with great success.
>
> Good luck,
>
> Rob MacDonald