What material(s) are used in IC's as insulating layers between metal layers?
From The Photon's observation and a little more research, the dielectric can come from a whole host of possible options. 
From here, it appears spin-on dielectrics can vary from organic based insulators/dielectrics to silicon based dielectrics. Most of the knowledge in this area appears locked away in published papers and as company specific IP.

From the link, some examples of polymer based insulators are polyimide, polynorbornenes, benzocyclobutene, and PTFE while silicon based spin-on dielectrics can be hydrogen silsesquioxane (HSQ) and methylsilsesquioxane (MSQ).