Han Park wrote:
>
> I am trying to obtain a good undercut profile of the photoresist to get
> a good liftoff.
>
> The thickness of the photoresist needs to be about 6~7microns.
> The deposited material is about 3microns thick.
>
> Soaking in chlorobenzene for 30 minutes was not enough to give a good
> enough undercut profile for a good liftoff.
>
> Is there anyone who can help me with this?  Is there an alternative to
> this process?
>
We haven't used AZ for lift-off, but the Shipley series. You don't get
that thick of a resist layer, of course.

However, my understanding is that you need only a relatively short soak
in clorobenzene to get a thin and hard surface layer. (like a minute or
so, with Shipley the layer starts cracking otherwise). The undercut of
this hard surface layer is then realized by overdeveloping. That means,
after breaking through the surface layer, which can be seen as reddish
clouds appearing, you develop like 5 min or so, much longer than for a
standard lithography. That creates the undercut under the hardened
surface layer, the lid. This can be actually seen with a microscope and
experience. Then, you also shouldn't need that thick of resist for a 3um
metal layer. (I know, people say you need twice the thickness of the
metal for the resist, but I have seen it working great with much less).
So like 4um should do it, as long as your undercut/overhanging lid is
good.

An alternative to lift-off is always etching. The definition won't be as
good, especially with the thicker metal layers, and with several metal
layers you need the right selective etchers. The underetch of the metal
is often more than expected, because of grain boundary etching. You can
use your lift-off mask with negative resist, or make a inverted mask for
positive resist for the etching step.

Good luck.

Alexander Hoelke
Center for Microelectronic Sensors and MEMS (CMSM)
University of Cincinnati
Cincinnati, Ohio, USA