Dear All,

I have been reading the various comments on bubbles in SU-8 coatings and I
wanted to offer some suggestions, as well as raise some additional questions....

As the manufacturer of SU-8 we are focused on evaluating and solving the various
processing issues associated with this unique material...In the past we have had
reports of bubbling in the SU-8 films and have investigated the root causes of
the problem and by far the dispensing method was found to be the largest
contributing factor, followed closely by the fact that the sample had bubbles in
it before actually dispensing the material onto the substrate surface...

We have found that the best way to generate high quality coatings with SU-8 is
to use an Eppendorf Repeating Pipettor which incorporates a syringe type plunger
and the ability to repeatably dispense the same volume to the surface... These
Eppendorf Pipettor's are available from most laboratory supply companies and are
priced at around
$300.00.....On filling of the syringe adapter some air may become entrapped and
if so these can be removed very easily by placing the syringe into an
oven/heated environment set at around 60C for several minutes....This same
method should be used if bubbles are seen in the bottle prior to
dispensing....This heating step reduces the viscosity of the SU-8 and allows the
bubbles to rise to the surface and extinguish....Following this heating cycle
the SU-8 should be allowed to attain room temperature before processing in order
to attain the targeted film thicknesses....

Recently, I have found that some of these bubbles were in fact particulates
caused by 'dried' SU-8 being dispensed onto the wafer surface when pouring
directly from the bottle, therefore I would recommend that the opening of the
bottle be wiped down prior to dispensing with a clean room wipe containing a
small amount of the  SU-8 Developer....Just a word of advice.....

I can concur with some of the comments about the effect of the 65C bake on the
removal of bubbles, this happens due to the fact that the glass transition
temperature of the material (prior to exposure and PEB) is around 55C, therefore
the material will soften and planarise slightly due to the reduction in the
viscosity and hence the bubbles will extinguish...However, if the bubbles are
being produced during the bake step another issue is involved and is most
commonly associated with substrate cleanliness...

The questions I have revolve around what specific formulations are more prone to
'bubbling', is it the original SU-8 formulation or the improved SU-8 2000 family
? Is there a particular film thickness that is more prone to this than others ?

I appreciate any feedback and the opportunity to assist any of the SU-8 users
who are experiencing problems in solving the issues...

Best regards,

Mark Shaw
Technical Sales & Applications Support
MicroChem Corp.

email: mshaw@microchem.com
url: www.microchem.com