Fluorine incorporation in plasma-polymerized octofluorocyclobutane, hexafluoropropylene and trifluoroethylene

The need for increased signal transmission speed and device density in the next generation of multilevel integrated circuits (ICs) places stringent de mands on materials performance. There is a requirement for interlayer diele ctrics with permittivities under 3 (low kappa dielectrics) that have compat ibility with copper and copper processing. Plasma polymerization is a solve nt-free, room temperature process that can be used to rapidly deposit thin polymer films on a wide variety of substrates. This paper describes the dep osition of plasma polymers from several fluorinated monomers (octofluorocyc lobutane (OFCB), hexafluoropropylene (HFP) and trifluoroethylene (TrFE)), a nd evaluates their molecular structures. Films with relatively high F/C rat ios were investigated in detail. The refractive index, n, of plasma-polymer ized OFCB (PPOFCB), 1.37 at a wavelength of 900 nm, indicates that it has a high frequency permittivity (n(2)) of about 2.0. The plasma fluoropolymers were transparent, yellow films that adhered strongly to the substrates and were deposited at constant deposition rates that ranged from 0.03 mum/min for PPOFCB to 0.34 mum/min for PPHFP. The AFM-determined roughness of PPOFC B on copper is 0.46 nm, half the 0.97 nm roughness of the substrate. The si gnificantly rougher PPTrFE and PPHFP consist of spherical particles from pr edominantly gas phase polymerizations. The incorporation of fluorine in the polymer is greater and more efficient for PPOFCB and PPHFP than far PPTrFE . For PPOFCB, F/C increases with decreasing W/F-m (where W is the plasma po wer and F-m is the mass flow rate) and, in a less sensitive manner, with in creasing pressure. A typical PPOFCB has an F/C of approximately 1.5 and app roximately 1.5% oxygen resulting from the reaction of long lived radicals i n the plasma polymer with atmospheric oxygen. PPOFCB and PPHFP have similar molecular structures, consisting of random assemblies of fluorinated carbo n groups. CF2 groups are more prevalent in PPOFCB, reflecting the monomer s tructure and the low W/F-m,. CF groups and unsaturation are more prevalent in PPHFP, reflecting the monomer structure and the high W/F-m. (C) 2001 Els evier Science Ltd. All rights reserved.