Dear Mr. Farrington,

We encountered the same problem working with a commercial series
of SU-8 2025, which was made by MicroChem (Newton, MA).  MicroChem
informed us that the SU-8 was sensitive to light waves below 350nm
and suggested implementing a filter to rectify the problem.  We inserted
a Hoya UV-34 filter, 2.5mm thick, into the Quintel Q7000 filter slot and
the T-Topping problem was no longer an issue.

The reason the filter worked is because the SU-8 polymerizes faster,
relative to the polymerization rate when exposed to light with wavelengths
greater than 350nm, when exposed to lower wavelength light (< 350nm) .
This being the case, the upper layer of SU-8 becomes overexposed and
prevents
the remaining SU-8 material from fully polymerizing.  The result is the
T-Topping effect.

I included some pictures and the transmittance curve of the Hoya UV-34
filter.  I hope you can view these.

With Filter: FILT1S1B.TIF
Without Filter: NOFL2S1B.TIF

Good Luck,

Steven Hobson
Project Engineer
FraunhoferUSA Center for Manufacturing and Innovation
15 Saint Mary's Street
Brookline, MA 02446
Ph: 617-358-1989
Fax: 617-353-1896
email: [email protected]







Message: 2
From: "N. Farrington" 
Organization: University of Leeds
To: [email protected]
Date: Tue, 4 Dec 2001 13:51:18 -0000
Subject: [mems-talk] SU-8 'T-topping' problem and solution using i-line
filter

Hi there,

I have been using various formulations of SU-8 which has been
made in house (although commercial mixes have also been used).
Unfortunately the structures have been suffering from an
exaggerated negative sloping sidewall problem commonly referred
to as 'T-topping'.  The latest base processing guidelines on the
MCC website blame this problem on the lower wavelength (313nm
and 334nm) output lines of mercury vapour lamps and suggest that
the problem can be eliminated by optimising the spectral
bandwidth (ie using an i-line filter).

I was just wondering if anyone had experienced this problem
and taken this step to solved the problem.

Thanks in advance,

Novak Farrington
_____________________________________

Novak Farrington
Institute of Microwaves and Photonics
School of Electronic and Electrical Engineering
University of Leeds
Leeds LS2 9JT
UK

Tel. +44 (0)113 233 2092
Fax. +44 (0)113 233 2032
email: [email protected]

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