Greg,
One thing to note is that the SU-8 2000 series was designed with a new
solvent system. The old solvent system was based on PGMEA (Propylene Glycol
Methyl Ethyl Acetate), which is a relatively low vapor pressure solvent.
Resists using this solvent typically have cure temperatures in the 110C
range. However, due to the low glass transition temperature of SU-8 (55C),
the cure temperature was limited to 95 C.
SU-8 2000 uses a higher vapor pressure solvent. This means that the nominal
cure times are much reduced, since the solvent will be driven off much
faster. This could be the reason for the stress issues you have been
seeing. It is possible that, with the old SU-8, the bake cycle you were
using allowed a small amount of solvent to be retained in the film.
Since SU-8 has a pretty large TCE mismatch from silicon, if all of the
solvent is removed from the film, cooling from 95 C to room temperature
fairly rapidly (by taking the wafer off of the hot plate or out of the oven
into room temperature air, for instance) can create a lot of stress. If
there is a small amount of solvent left, it can allow the SU-8 to "relax" a
little bit, and accumulate less stress during cool down.
In regards to the SU-8 2000, a couple things can be tried. One would be to
reduce the bake time of the material. According to the data sheets on the
MCC website, for a 50 um thick layer of SU-8 50, the recommended bake time
is 26 minutes on a hot plate (6 at 65, 20 at 90). For the SU-8 2000, for a
layer of the same thickness, the recommended bake time is a total of 9
minutes (3 at 65, 6 at 95).
A second thing to try would be to "anneal" the SU-8 layer. Basically, a big
part of the problem is that the SU-8 reaches a relaxed state at 95 C, due to
the fact that, even without solvent, it will flow somewhat at elevated
temperatures. If the layer is given a chance to reach a relaxed state at a
lower temperature (65 - 70 C, for instance), it will reduce the overall
stress in the film once the film has reached room temperature.
Be sure to perform a two step post exposure bake, as well, although you were
probably doing that already with the old SU-8.
Best Regards,
Chad Brubaker
EV Group-Technology, Tel: (602) 437 9492 x 119, Fax: (602) 437 9435
E-mail: [email protected], Web: www.EVGroup.com, 06/25/02
-----Original Message-----
From: Greg Miller [mailto:[email protected]]
Sent: Wednesday, June 19, 2002 2:44 PM
To: [email protected]
Subject: [mems-talk] SU-8 adhesion
I've been working with SU-8 50 over the last year and things have been
working well -
However, recently I order a bottle of SU-8 they sent me the new 2000
series-
after I re-optimize the spin speed and followed there standard cure
process on silicon wafer - We still have adhesion problems or film
stress issue that do not occur with the old SU-8
I was wondering if anyone has run into this problem and could offer a
solution.
Thanks,
Greg Miller
KVH Industries
Email: [email protected]
_______________________________________________
[email protected] mailing list: to unsubscribe or change your list
options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk
Hosted by the MEMS Exchange, providers of MEMS processing services.
Visit us at http://www.memsnet.org/