Hey Mikas,

I am a follower of this MEMS forum and I would be interested to know about a
little bit more in regards to to S18XX processing and S1818 processing which
you have mentioned in your post.

Recently I am doing lift off with S1813 and my processing is failing
everytime over the past few weeks.

Here is my process conditions with S1813.

Substrate: silicon nitride on Si
Spin coating: HMDS for proper adhesion
                    for S1813 500 rpm for 10 secs, 1000 rpm for 1000 secs,
1800 rpm for 15 secs
the thickness in the alpha step shows around 1.3 microns
Pre baking time: 110'c for 3 mins
Exposure: Intensity is 10mW/cm2 Time; 15 secs for a total dose of
150mJ/cm2**
development: MF 319 for 30 secs
Smallest feature size: 10 ums

Physical vapor deposition: Sputtering
                                        Ti about 60nm
                                        Cu about 200nm

The sample is then placed under Acetone ultrasonics for 1 hr for lift off.
** the contact of the mask with the sample is extremely good since S1813
hardly gives you any edge bead and the exposure is done in contact.

But unfortunately the process is failing since either most of the patterns
are peeled away after lift off or are poorly lifted off which chunks of
metal still remaining in the sample.

I will be interested to know about any suggestions for the above processing.
Further, it will be nice if you can let us know about the routine procedure
of S1818 that you follow for lift off.

Thanks a lot
Anirban

On Tue, Jul 14, 2009 at 1:57 PM, mikas remeika wrote:

> Hi, Shah,
>
> I have had many people recommend me to do lift-off by "overnight soak
> in acetone" however, in my experience it results in very poor
> lift-off.  I always do liftoff in an ultrasonic bath at moderate
> power, and achieve complete liftoff in a matter of minutes, with no
> damage to the pattern (if film adhesion is good).  I've done 0.5
> micron features without any noticeable damage due to sonication.
>
> Also, about the film thickness/resist thickness ratio - I routinely do
> lift-off with a 0.5 um gold film over a 2 um-thick S1818, without any
> problems (without using PMGI or any other undercut layer) so an long
> as the resist is 2-3 times thicker than the film, you should be OK.
> In this case the film is also sputtered at an angle (with rotation),
> but that does not cause problems.
>
> One factor which I may affect liftoff is the quality of resist edges.
> I find that if the mask makes good contact to the resist - so that the
> shadow edges are sharper - the liftoff is more reliable.  Therefore I
> recommend removing edge beads even when doing low resolution
> lithography.
>
> I have never worked with SPR 220-3, I have done liftoff with PMMA,
> SU-8 2000.5 and S18xx series resists:
>
> For Shipley 18xx series it is best to use resist stripper 1165,
> acetone does not do very well.
>
> For PMMA/MMA acetone works well.
>
> For SU-8 2000.5, even without any underlayer  (e.g. Omnicoat) Remover
> PG works very well, as long as it is heated to above 60C.
>
> Hope this helps,
>
> -Mikas.

ANIRBAN
Electrical Engineering
Louisiana State University
Baton Rouge
70802,USA