The thickening on the edges is referred to as "edge beads." On larger
samples it can be minimized by pouring solvent on the edge of the
sample during spinning.

On smaller samples that is not an option, however, for positive
resist, it can be dealt with by exposing the edges, developing, and
then exposing again.  I use this method all the time, since my samples
are 5x8mm.  For S18xx series resists its very reliable, the edges need
to be exposed with dose that's about 20 times higher than the dose
used for high-resolution patterns, and developing time is the same for
both EBR (edge bead removal) and the high-resolution pattern.
Immersion in the developer seems to have no measurable effect on
unexposed resist, so the process is very well reproducible.

-mikas

On Wed, Feb 10, 2010 at 5:01 PM, leiwangsdu  wrote:
> Hello, everyone. I use the positive photoresist to do some lithography.
> I have a big problem. My samples are rectangular which have dimensions
10mm*15mm
> After the resist spinning, I found that the resist film around the edge of the
sapmle is thicker
> than the resist in the center. This produces a gap when the contact exposure
is applied.
> And the resist profile is not good enough after the development.
> I  shall be highly obliged if anyone can help me out in this and provide some
advices.
> Thanks in advance
>
> Yours sincerely,
>
> Lei Wang