Microchem has their SU-8 2000.5 that I've used as an adhesion layer for thicker SU-8 before, but I'm sure would work for your purposes. If you're looking for exactly 1 micron, then I'd suggest spinning the SU-8 twice at 2000 rpm. I've recently been successful with spinning two layers if I do a soft bake at 55 C for 5 minutes in between spins.
It might not answer your question but choosing SU8 for 1um thick might not be a good option. It'll make more sense to use positive resist that mostly thinner & you can do image reversal to get similar profile with negative resist. One good option for that is AZ-5214 and check its spin curve
You can probably thin the 3000 series to spin that low or more easily spin the 2000.5 slower to get 1 micron. A slight modification to Isaac's answer is that I would not expect to get "exactly" 1 micron by spinning 2000RPM twice because the second layer tends to spin thicker (better SU8 on SU8 adhesion). It depends on your tolerances; check your results with a profilometer/optical microscope because the spin curves are the most variable step in the whole SU8 process.
Dennis, I'm curious why you don't suggest SU8 for such thin films?
Philip, as far as i know possitive photoresist in general is less viscous and easier to apply with less potential problem like bubble and adhesion. Especially SU8 also has very high CTE (even higher than other negative resist like BPR or AZ15) will have a high internal stress that might not be good for a very thin layer. In my opinion, the selection of SU8 is normally because of it's capability to build high aspect ratio feature and it's nearly vertical sidewall or as a structural material. Unless your critical dimension is around 50-100 nm, then i don't see the benefit of using SU8 for the 1 um thick layer
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