Also, I would also look into your exposure source - as Gareth mentions in an
earlier message in this thread, SU-8 is very sensitive to DUV (exposure <350 nm
wavelength) - many broadband exposure sources (i.e., 350 - 450 nm) will have a
small amount of "leakage" below 350 nm (depends on the quality of the cut-off
filter) - SU-8 rapidly absorbs these wavelengths will little to no transparency
or bleaching - ultimately this leads to a T-topped profile (which, when viewed
from above, can have the appearance of dark edges).
On the other hand, if the film is underexposed, but uses the proper wavelength
band, the profile can slope in either direction (positive slope due to longer
developer attack to the upper regions, or negative slope, due to top surface
being exposed, and bottom not - this is less likely, due to the transparency of
Regarding the bake parameters you are currently using, I almost think I could
read it as "95 seconds at 120 C" or "95C for 120 seconds).
If you are PEB at 120C, then underexposure is not your concern - SU-8 uses a
catalyzed photo-acid - the exposure product is not a reactant for the cross-
linking process, it's a catalyst. Longer exposures simply increase the
concentration of the catalyst, which influences reaction rate. Variation in PEB
temperature will also influence the reaction rate (similar to exposure - the
higher the temperature, the higher the reaction rate). In general, SU-8, once
exposed, will crosslink to near-completion; its just a question of "how long".
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From: firstname.lastname@example.org [mailto:mems-talk-
email@example.com] On Behalf Of Gareth Jenkins
Sent: Saturday, January 19, 2013 5:38 PM
To: General MEMS discussion
Subject: Re: [mems-talk] SU-8 2005 overexposed?
Your bake parameters do not sound right. The datasheets recommend only
about 2mins softbake at 95C for this thickness and 3-4mins post exposure
bake at 95C (120C is too high).
Looking at the datasheet, I think Andrew may be correct that it is also
underexposed - 2 or three times the dose may be required. This would lead
to the profile not being straight (if the top of the structure is equal or
less than the size of the mask feature then this would confirm it).
On 19 January 2013 22:17, Jocelyn Ng wrote:
> After post exposure bake, still no darkening, but after developing,
> darkening occurs. After seeing the profile using SEM, the profile is not
> straight. I soft bake it at 65C for about 9mins, 95C at 120C for post
> exposé bake. Or does it require higher soft bake temperature for this
> Thank you.
> Best Regards,
> Siew Mun
> Sent from my iPhone
> On 18 Jan, 2013, at 10:24, Gareth Jenkins wrote:
> > Deep UV or heating of the wafer may also cause the problem. When does the
> > darkening occur? (Is it immediately visible after exposure or at some
> > point in the process)? Do your structures look overexposed after
> > development?
> > On 17 January 2013 11:11, Jocelyn Ng wrote:
> >> Hello,
> >> I'm doing single layer SU8 2005, thickness ~6-7um. I use 60mJ/cm^-2 on
> >> sputtered gold surface. But somehow sometimes I notice that the edges
> of my
> >> pattern darkens, I suspect it's due to overexposed. But 60mJ is quite a
> >> dose compared to the recommended one. I would appreciate any comments
> >> given. Thank you.
> >> Best Regards,
> >> Jocelyn
> >> Sent from my iPhone
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