Pulsed plasma polymerization of an aromatic perfluorocarbon monomer: Formation of low dielectric constant, high thermal stability films

The plasma polymerization of the perfluoroallyl benzene monomer $\mathrm{[(}{\mathrm{C}}_{\mathrm{6}}{\mathrm{F}}_{\mathrm{5}}{\mathrm{)CF}}_{\mathrm{2}}{\mathrm{CF=CF}}_2]$ was carried out under pulsed discharge conditions. Progressive changes in the composition of the plasma generated polymers were observed with sequential changes in the duty cycle of the pulsed discharge, as other reaction variables remaining constant. In particular, an increased retention of the aromatic ring of the starting monomer in the polymers is obtained with decreasing plasma duty cycles during film formation, as shown by x-ray photoelectron spectroscopy and Fourier-transform infrared analyses of these films. Dielectric constants of these perfluorinated films are consistently below a value of 2.0, measured over a frequency range of ${\text{5×10}}^2{\text{–1×10}}^5 \mathrm{Hz}.$ Although thermal gravimetric analysis of the plasma films revealed significant weight loss beginning at temperature below 300 °C, up to 70%, dramatic improvement in the thermal stability of these films was observed following their thermal annealing at temperature of 350–400 °C under ${\mathrm{N}}_2.$ The resultant annealed films exhibit negligible weight loss during subsequent thermogravimetric analysis at temperature as high as 420 °C. Most importantly, this gain in thermal stability of the plasma polymers is achieved with only minor increases in the dielectric constants, along with relatively minor changes in film compositions, following the thermal annealing processes. The implications of this study with respect to low dielectric constant requirements for future smaller feature size integrated circuits are discussed in this work.