Fluorine incorporation in plasma-polymerized octofluorocyclobutane, hexafluoropropylene and trifluoroethylene

Abstract

The need for increased signal transmission speed and device density in the next generation of multilevel integrated circuits (ICs) places stringent demands on materials performance. There is a requirement for interlayer dielectrics with permittivities under 3 (low κ dielectrics) that have compatibility with copper and copper processing. Plasma polymerization is a solvent-free, room temperature process that can be used to rapidly deposit thin polymer films on a wide variety of substrates. This paper describes the deposition of plasma polymers from several fluorinated monomers (octofluorocyclobutane (OFCB), hexafluoropropylene (HFP) and trifluoroethylene (TrFE)), and evaluates their molecular structures. Films with relatively high F/C ratios were investigated in detail. The refractive index, n, of plasma-polymerized 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 μm/min for PPOFCB to 0.34 μm/min for PPHFP. The AFM-determined roughness of PPOFCB on copper is 0.46 nm, half the 0.97 nm roughness of the substrate. The significantly rougher PPTrFE and PPHFP consist of spherical particles from predominantly gas phase polymerizations. The incorporation of fluorine in the polymer is greater and more efficient for PPOFCB and PPHFP than for PPTrFE. For PPOFCB, F/C increases with decreasing W/ F m (where W is the plasma power and F m is the mass flow rate) and, in a less sensitive manner, with increasing pressure. A typical PPOFCB has an F/C of approximately 1.5 and approximately 1.5% oxygen resulting from the reaction of long lived radicals in the plasma polymer with atmospheric oxygen. PPOFCB and PPHFP have similar molecular structures, consisting of random assemblies of fluorinated carbon groups. CF 2 groups are more prevalent in PPOFCB, reflecting the monomer structure 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.