Hi
I was wondering if anyone had experience with exposing SU8 from the backside of a transparent substrate and could give me an estimate for the relative exposure dose compared to a front side exposure.
The backside exposure is to prevent the overhanging profile if I exposed from the front, as positively sloped features are required.
Im trying to create 50um features which correlates to an exposure dose of 150-250mJ/cm^2. When I used the same protocol as I would for a topside exposure on silicon (produces good results) the features werent even close to being the correct height; as if they hadnt even been exposed really.
Would it be a fair estimate to try an exposure dose around 600-900mJ/cm^2, or could there be any other issue
Thanks
Alex
Hello Alexander Hamilton ,
Unfortunately I cannot provide any information on backside exposure.
However, to prevent overhang or "T-topping" you could use a long-pass filter to remove wavelengths below 350 nm. In the past I used one from Omega Optical (pn: ZJB360, or now PL-360LP).
Jules Lloyd Hammond Thanks for your answer,
The reason Im trying to do a backside exposure approach is to create a positive draft angle as I plan on using the SU-8 on a TOPAS substrate as an injection moulding inlay
Hi Alexander,
When you do back-side exposure, one of the critical problem that you are going to face is the irregularity in the dimensions of the feature on SU-8. After passing through the mask, the light has to pass through the thick glass slide, which will cause significant diffraction and other effects.
What is the wavelength of the aligner that you are using? The transmission properties of different types of glass varies significantly in the UV region. Therefore, you need to thoroughly analyze all these effects before choosing a dosage for your experiments.
Jose
The substrate I'm using is TOPAS which has 91% UV transmission. The UV lamp is 365nm.
After doing various exposure doses it seems that a significantly lower exposure dose (one third of what I'd use for silicon) lead togood profiles which was surprising!
I met the problem of dose for SU8 for backside too，I used more does than SI
and got excellent result， but just one time！
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