I've never heard of SU-8 that thin but it might be possible. Just keep diluting the SU-8 with PGMEA (SU-8 developer is the same stuff) to make it thinner and thinner. You will need to cool down the SU-8 very slowly after the post bake to stop it cracking. Maybe over hours.

Following David John Garrett's suggestion, you may get SU-8 polymer film with thin thickness around 10 nm. According our experience, it is possible, but difficult.

In my experience I diluted SU-8 2000.5 in Cyclopentanone to achieve thin layers of SU-8 but I never tried to reach 10 nm. In this reference the authors claim to have reached 20/30 nm thicknes with a roughness of 0.5 nm rms. 

DOI:10.1002/adfm.201102789

I'm also interested in this topic. I'm trying to get 400 nm thickness of SU-8 film. I mixed SU-8 3050:Cyclopentanone = 1:4.2 in volume using magnetic agitator in 15 min. But I alway get problem after spin coating: the SU-8 layer was not uniform,  transparent and there are many droplets on the surface when observing with microscope. The wafer was cleaned with acetone, IPA, de-ionized water and heated at 150oC in 15 min before spin coating.  Do you have any suggestion for me?

Thank you all for nice information, just i wonder about it could possible to make rectangle Grating with smaller thickness(around 30 nm) and we could not do thin film with similar thickness