Gareth,

I recognize a few problems with your process. 200um features in 100um of SU8
resist can be done very reliably on a contact printer such as the SUSS.

You can expect a profile in thick SU8 that will have a slight hourglass
shape but one can easily match bottom to top CD with optimization of
exposure and bakes (soft and PEB).

Using a SUSS tool you will need to filter out the wavelengths below 350nm.
SU-8 is highly absorbing below 350nm and to expose thick SU8 you need to get
light to the bottom without overexposing at the surface.  I-line band pass
filter will do this or a UV400 cut-off filter (if you want to have broadband
available for other resists used on same tool).

Uniform gap is especially important to prevent proximity effect which is
what I believe you are seeing.  First try a profilometer of your wafer
surface.  I know unless you've done multiple coats with edge strips in
between you will have height variation from edge to center.  The 'bead' can
be millimeters wide and microns high even though it appears relaxed.
Typically there will also be a high spot or 'button' in the center.

If doing contact WEC on a non-uniform coating the surface leveling will not
be accurate and therefore your proximity or contact gap will not be
repeatable or reliable.  It would be best to use proximity flags for WEC as
long as they can swing out beyond the edge bead.  Minimizing contact to
resist surface as ProxWEC does will also reduce resist contamination and
build up between mask and wafer which can very dramatically affect gap
accuracy at exposure.

You should also be using a chrome mask or at the very least place your
acetate film between two glass plates to hold flat. On the SUSS tools the
WEC leveling, whether done in proximity or contact, will reference the mask
and the film is too flexible to endure the WEC pressure and you will not get
an accurate gap.

Lastly, with 200um features you could expose SU8 at proximity exposure gaps
of 50-200um which minimizes contamination induced proximity effects as long
as you can keep the WEC head finely tuned.

I'd be happy to discuss any further questions you may have.

Regards,
Suzanne


-----Original Message-----
From: Bill Moffat
Sent: Friday, August 08, 2008 8:05 AM
To: 'General MEMS discussion'
Subject: RE: [mems-talk] SU-8 using Suss contact mask aligner

Gareth,
       I know a real expert in this.  Sue Ericson, used to work for Suss
and is an experienced resist engineer with lots of experience using SU8.
I will copy her this email and hope she gets to you. No I wont I do not
have her email.  I will phone her and give her yours.

Bill Moffat