Comparison of plasma chemistries and structure-property relationships of fluorocarbon films deposited from octafluorocyclobutane and pentafluoroethane monomers

Abstract

Fluorocarbon films are deposited from mixtures of pentafluoroethane (PFE)/argon and octafluorocyclobutane/argon in a parallel plate reactor at pressures of 0.75 and 1 Torr and substrate temperatures between 120 and 240 °C. These monomers are compared in terms of plasma dissociation chemistries as well as chemical structure and properties (electrical, thermal, chemical, and optical) of the resulting films. Deposition rates with C4F8 are significantly higher than CF3CHF2. With both monomers, the deposition rate decreases with an increase in substrate temperature, indicative of an adsorption limited process. Films deposited from both monomers are analyzed using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and capacitance measurements. XPS analyses indicate a higher flourine to carbon ratio for films deposited from C4F8. Likewise, these films have lower refractive indices (n∼1.38–1.39) and lower dielectric constants (k∼2.18–2.33) than PFE films (n=1.41, k=2.23–2.55). Films deposited from both monomers exhibit high dielectric losses (∼10−2–10−3) at deposition temperatures >200 °C. TGA analyses of films deposited from both monomers indicate comparable thermal stabilities up to a temperature of 300 °C. However, the decomposition rate is much higher for C4F8 films in the temperature range between 300 and 425 °C. Thermal stability of films deposited from both monomers is enhanced at higher deposition temperatures. However, the sensitivity of thermal stability to deposition temperature is lower for films deposited from C4F8. A tradeoff exists between the electrical properties and thermal stability of films deposited from both monomers.