Islands (divots or dimples) is a common problem which has been
discussed before many times with not much of a conclusive outcome!
I personally think there are three main causes:

1). Wafer contamination or poor prep. which sounds most likely in your
case. Wafers should be soaked in clean Piranha or conc. H2SO4 for at
least an hour and then thoroughly dehydrated (I do 220deg C on a
hotplate for 30-60mins).

2). Bubbles or particles in the SU-8 itself. Bubbles can be alleviated
by placing the (closed) bottle in an oven at 60deg C for 1 hour -
allow to cool before using. The particles can be due to build up of
SU-8 around the neck of the bottle when pouring directly from it.

3). High humidity. I am not 100% sure about this but would like to
hear opinions from others. Recently I have been having poor results
using identical protocols I have had success with before but in a
cleanroom in which the humidity is often around 55%. SU-25 is just
about tolerable but we have virtually given up on SU-8 2100. SU-2 is
also quite poor.

Has anyone else noticed a link between high humidity and divots (and
generally poor wafer coverage)?


2008/7/2 郑瑞麟(Ruilin Zheng) :
> Hello, everyone,
>
> I've got a problem with SU-8 soft-bake as I shift from 2inch silicon wafer to
> 4inch: some islands form on the surface during soft bake ( start to appear
> at 65 Celsius degree, and remain after 95 Celsius baking ). Does this
> problem have something to do with contamination on wafer?
> At first, I thought it was caused by uneven solvent content of SU-8 stored
> for two or three weeks in covered beaker. But I also tried the fresh
> photoresist, and got the same result.
>
> Does anyone have some ideas with this?  Please give me some advice.
>