AZ9260 is not the ideal resist for DRIE. Try AZ6632, it will withstand a
hardbake up to 130° without reflowing too much. For 550 um you will need
to spin it at 500rpm, I use a ramped process for spincoating: 800rpm for
3s, 500rpm for 30s, 1500rpm for 1s.
 From my experience with an ASE-tool, 600W coil power is too much for the
resist, try 300-400 and the resist will withstand the 550um etch.
Alternatively, you can use SiO2 as a mask.

Marco