Effects of ultraviolet (UV) irradiation in air and under vacuum on low-k dielectrics

This work addresses the effect of ultraviolet radiation of wavelengths longer than 250 nm on Si-CH₃ bonds in porous low-k dielectrics. Porous low-k films (k = 2.3) were exposed to 4.9 eV (254 nm) ultraviolet (UV) radiation in both air and vacuum for one hour. Using Fourier Transform Infrared (FTIR) spectroscopy, the chemical structures of the dielectric films were analyzed before and after the UV exposure. UV irradiation in air led to Si-CH₃ bond depletion in the low-k material and made the films hydrophilic. However, no change in Si-CH₃ bond concentration was observed when the same samples were exposed to UV under vacuum with a similar fluence. These results indicate that UV exposures in vacuum with wavelengths longer than ∼250 nm do not result in Si-CH₃ depletion in low-k films. However, if the irradiation takes place in air, the UV irradiation removes Si-CH₃ although direct photolysis of air species does not occur above ∼242 nm. We propose that photons along with molecular oxygen and, water, synergistically demethylate the low-k films.