Jiang,

Soft bake is a diffusion process, right?  The first soft bake sufficiently dries
the first layer.  The second layer soft bake only needs to be long enough to dry
the second layer plus any solvent that has diffused back into the first layer.

Basically, I have had good results running the process as a loop with the second
soft bake process equal to the first.  The only exception is at 500 µm of SU-8
(2x 250 µm process).  At that point, a simple loop process made the total soft
bake time for the first layer too long and the resist appeared to begin a
thermal crosslinking process at the wafer interface.  With respect to trying a
4x coating process -- be mindful of this thermal crosslinking.  The first layer
is going to be extremely dry if you simply perform the process 4x.  You might
need to modify the bake recipes so that the first layer is baked only to the
point where a new layer can be cast on top of it.  Each subsequent bake can be
extended slightly. By the time you are at the final bake step the first layer
should be ever so slightly wet and just in need of a slight amount of drying to
be ready for litho.

FYI, I have run double coat processes in manufacturing other than SU-8.  I have
found that the soft bake time can be proportional to thickness again supporting
a first order diffusion argument.

Best Regards,
Garrett Oakes

EV Group
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-----Original Message-----
From: jiang Liudi [mailto:ldjiang@hotmail.com]
Sent: Tuesday, August 12, 2008 9:50 AM
To: General MEMS discussion
Subject: Re: [mems-talk] Su 8 thickness

hi,

Thanks again for the information.

sorry one more question,

If I coat two layers of 50um thick SU8, how long should I bake the 2nd layer
which has total 100um thickness? should it be soft baked as a 50um one or 100um
one?

Also, I am thinking of coating 4 layers of 50um thick SU8 to make up to 200um.
would you also have some experience on such multilayers? How would be the soft
bake time like for the 2nd, 3rd and 4th layer?

much appreciated again.

Best wishes,
Jiang