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

This work addresses the issue as to whether ultraviolet radiation of wavelengths longer than 200 nm can break Si-CH3 bonds in porous low-k dielectrics. To resolve this issue, porous SiCOH films (k = 2.3) were exposed to 4.9 eV (254 nm) ultraviolet (UV) radiation in both air and vacuum for 1 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 photons themselves with wavelengths longer than ~250 nm do not modify the chemical structure of the low-k film. However, if the irradiation takes place in air, the photons can indirectly cause damage to the samples. This is due to the fact that UV radiation can break down the air components and create radicals and active species that affect the low-k films.