Preparation of low density poly(methylsilsesquioxane)s for LSI interlayer dielectrics with low dielectric constant.: Fabrication of Ångstrom size pores prepared by baking trifluoropropylsilyl copolymers

We report a method to fabricate Angstrom size pores in poly (methylsilsesquioxane) films in order to decrease the density and the dielectric constant of the film. Copolymers bearing methyl(trisiloxysily1)units and alkyl (trisiloxysilyl) units are spin-coated and baked at 250 degrees C to provide rigid siloxane matrixes. Films are baked up to 450 or 500 degrees C to remove thermally labile alkyl groups and holes are left corresponding to the sizes of the substituents. Trifluoropropyl, cyanoethyl, phenethyl and propyl groups are investigated as thermally labile substituents. Only the trifluoropropyl group works well under the limitation of baking temperatures up to 450 or 500 degrees C, the pores collapsing for films bearing other substituents. A film P5 with a dielectric constant of 2.3 was obtained for trifluoropropyl. The total surface area of this film was 1.34 times that of poly (methylsilsesquioxane) film (P1) prepared by a conventional sol-gel method. The peak of pore distributions is in the region < 10 Angstrom. The peculiarity of the trifluoropropyl group is discussed.