Try puddling acetone on the wafer before dispensing the resist, then dispense
the resist and slowly ramp up the spin rate.  The acetone dilutes the resist in
the trenches, dropping the viscosity of the resist significantly, but is
displaced by undiluted resist as you slowly spin the resist on.  Be sure to use
a resist towards the thicker end of the viscosity spectrum in order to fully
planarize your trenches.  I've used this approach for refilling long trenches
(25um deep, 5um wide), but not for vias so your mileage may vary...

                Good luck!
                      Matt Last

    I'm trying to fill some deep vias (25um wide, 70um deep) with
resist. I can't seem to get the resist to go any deeper than about
40um. The best results were with using a PGMA pre-wet.  I've tried some
55cp resist that I coated three times to fill (based on top down zygo)
and some 120 cp resist. X-sections show both resists to only be 40um
down the vias.
    Any suggestions?

Robert