Hi John,

By blurry line, do you mean that the rim of the mould, at the depth of the
top of the first (50um) layer, appears cloudy? Or do you mean there is some
sort of residual overhang that forms at the interface between 1st and 2nd
layers, follwoing development of the top layer?

I presume the two layers are patterned to form different patterns, creating
a stepped/terraced mould.

Michael

> -----Original Message-----
> From: John Hilton [mailto:spudcrazy@gmail.com]
> Sent: Thursday, November 13, 2008 6:33 PM
> To: General MEMS discussion
> Subject: [mems-talk] Blurring of SU-8 Layers upon development
>
> Hi, I'm making multilayer SU-8 molds and I have a recurring problem
> where the bottom layer appears to be blurry or reflowed upon
> development.  This bottom layer is a 50um layer of SU-8 2050 (spun at
> 3000 RPM, soft bake 1min 65C 6min 95C, expose for 45s at 4.2 W/cm2
> thru a long pass UV filter, PEB 1min 65C 6min 95C).  The top layer is
> a 225um layer of SU-8 2100 (spun at 1350rpm, soft baked for a few
> hours, exposed for 82s at 4.2 W/cm2 in roughly 20 second increments to
> eliminate resist heating, followed by PEB and then development.
> Basically around all of my structures on the bottom 50um layer there
> is a blurry line when viewed in the microscope, between 10 and 100 um
> wide.  Ashing w/ 02 plasma can remove some of this effect, but I don't
> know how that will effect the overall height of the mold and it makes
> it harder to de-mold PDMS later.
>
> Has anyone else had this problem?  Testing has yet to indicate one
> single parameter that is the cause of the effect, but exposure time
> and bake times definitely come into play somehow.  Vastly changing
> exposure times results in delamination of the bottom layer, however,
> and renders the final mold unusable.
>
> JP Hilton
> Graduate Student
> Columbia University BioMEMS Laboratory