Hi Chad and Peng,

Thanks for Your comments.

Syringes with the black o-ring are not used in the cleanroom, because of
those things You mentioned.
The way of dispensing is not the cause of those bubbles of 1 um ; it
causes bigger bubbles, different in size.
I cover the whole wafer with SU-8 by "painting" with the syringe before
spinning. If I leave the edge of the wafer free from resist, after
spinning, these areas have the bigger - irregular in size - bubbles.
The homogeneous spread 1 um bubbles have an other cause. Boiling effects
during bake? Heating of the resist during lithography by the chromium
mask? (I use hard contact mode near UV).

Maybe it is related with the thickness I am using (10-20 um).  They are
not easy to observe, so maybe they exist in thicker layers. With a
microscope -light on top and some filters make them visible.
Peng, with SEM , You can not always see them on the surface (pivots).
Ones I had badly prepared a wafer with a too thin gold layer for SEM-ing
and could see the "craters". A "too thick" layer gold covers them
completely.

Now I degassed (by heating) the SU-8, already present in syringes, and
will look how it turns out.

Thanks again for discussing this topic!

Regards,

Herman.

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Brubaker Chad
Sent: dinsdag 18 december 2007 22:23
To: General MEMS discussion
Cc: Brubaker Chad
Subject: RE: [mems-talk] SU-8 (micro)bubbles

Also regarding the syringe - does this syringe have a black o-ring?  If
so, it is very possible that it also contains a silicone lubricant.
This lubricant does not typically interact (in my experience), but it
can create non-homogenous regions within the resist that can lead to
defects.  Additionally, the o-ring itself may be vulnerable to attack
from GBL (the solvent in the original formulation of SU-8).

The previous poster may also have a point regarding inclusion of bubbles
from syringe loading.  I have quite a bit of experience loading SU-8
(original, 2000, and 3000 formulations, plus KMPR) into syringes
(non-o-ring types - Air Tite offers a syringe from HSW called Norm-Ject)
with no introduction of bubbles in the manner you mentioned - by pouring
into the back of the syringe with the plunger removed.

The trick is to pour it in using a manner similar to that used to pour
beer with a minimum of foam - by pouring down the side.  This allows the
material to maintain a laminar profile and not experience any turbulence
that can introduce cavitation.