[mems-talk] How to improve SU-8 adhesion?

[Gareth Jenkins]

Sorry to here you still have problems. How much better is the adhesion 
on bare silicon?
Also, are you making microchannels actually in the SU-8 or is this for a 
mould. I.e. are you exposing a large area of the wafer? If so, then it 
may be worthwhile to try and reduce this area as much as possible and/or 
use a lower temperature PEB. For large area exposures, there will be a 
lot of stress in the crosslinked film which easily leads to cracking and 
delamination. Also remember to ramp up and down with the hotplate.
Another thing I forgot to ask before was how you are pretreating your 
wafer prior to the dehydration bake? I have no experience of PECVD SiO2 
but for silicon/glass you need to soak in conc. H2SO4 or piranha prior 
to the dehydration bake. However, poor pretreatment usually becomes 
apparent during spinning or softbake - are you getting a nice even film 
at this stage?
As for HMDS, I have heard it said that this can actually make adhesion 
worse (never tried myself). There are products I have heard mentioned on 
this list which may be more appropriate such as AP300 and Omnicoat but 
again I have no experience of them.


Steven Yang wrote:
> Hi, all
>
> Asked some questions on SU-8 2050 several days ago, still stuck on it now.
>
> The problem is the peeling off at some SU-8 2050 microchannel
> edge.(SU-8 2050 on PECVD SiO2). Without SiO2 layer, the situation will
> get better with seldom cracking and peeling off. However, on PECVD
> SiO2 it always turns out this problem more or less.
>
> I have increased the temperature and time for dehrydration at 180C for
> 30mins. and exposured for 40s at 7mW/cm2, PEB at 65C, 95C for 5 and 15
> mins respectively (Same as softbake). No much different.
>
> If there is no any further solution based on recipe modification, will
> the HMDS get the thing better? If so, could anyone give some
> suggestion on the HMDS spin procedure? below is what I plan to do.
> 1) dehydrate at 150C for 15mins
> 2) spin HMDS at 7500rpm for 35s
> 3) dehydrate at 150C for another 15mins
> 4) spin SU-8 2050 at 4000rpm for 45s
>
> Any suggestion? Thanks a lot!
>
> Steven
>