[mems-talk] SU-8 2050 Align Problem

[Brubaker Chad]

Steven,

As Gareth noted, you are probably insufficiently baking.  The baking parameters as mentioned in the MicroChem data sheets are just guidelines, and do not take into account local conditions (variations in humidity, air pressure, etc.) Additionally, their recommended process flow includes the use of edge-bead removal, which is not an easy possibility for your substrates.

You are most likely sticking at the edge bead (examine the edge bead closely after removal from the mask; you will probably see "flattened" areas where the mask pressed in, or even areas that looked like pulled taffy, where the material adhered and then "pulled" when the mask lifted).

This means, in all likelihood, that the edge bead is the area with the majority of retained solvent.  If you cannot remove your edge bead, try to use bake parameters on your film as if it were a thickness equal to the height of your edge bead (which, at the thickness you are spinning, may be as high as 80 µm from the surface of your substrate). It is still best to use a hotplate in this case, as use of an oven has been seen to produce a non-diffusion layer ("skin" or "crust") on the surface of your substrate, trapping solvent within the film.  Use of vacuum in conjunction with a hotplate can work well, however, as the vacuum will effectively increase the removal of cyclopentanone (CP) from the film.

As Gareth also mentioned, the edge bead can be managed to some extent using a "relaxation" step by letting the wafer sit post spin on a level surface - you can also do this on a hotplate, as unexposed SU-8 has a glass transition temperature (Tg) of 55ºC - if you let it sit for an extended period of time (perhaps an hour, or even the several hours he mentioned) at temperatures of 65 - 95'C, this may actually solve both problems simultaneously, although you will have to be cautious of in-film stress which may lead to cracking.

Best Regards,
Chad Brubaker

EV Group       invent * innovate * implement
Senior Process Engineer - Technology - Tel:  480.727.9635, Fax:  480.727.9700  e-mail: c.brubaker at EVGroup.com, www.EVGroup.com



-----Original Message-----
From: mems-talk-bounces at memsnet.org [mailto:mems-talk-bounces at memsnet.org] On Behalf Of Gareth Jenkins
Sent: Wednesday, January 02, 2008 10:06 AM
To: General MEMS discussion
Subject: Re: [mems-talk] SU-8 2050 Align Problem

Either your softbake is too short or you are heating the SU-8 during 
exposure. Try having gaps in your exposure to allow the SU-8 to cool 
(e.g. 10s exposure followed by 10s gap and repeat until you have the 
required time). You should also have a long pass filter to reduce 
excessive exposure to deep UV.
Leaving your sample to rest on a leveled surface for several hours 
after spinning (before softbake) may help reduce the edge bead. You 
could also use a cleanroom wipe soaked in acetone to remove the edge 
bead (though may be difficult with an irregular sample).