VUV/UV radiation interaction with silicon dioxide: Towards the next generation of 157 nm optical lithography materials.

摘要

The effects of VUV/UV radiation on high purity fused silica (HPFS) glasses were examined. In particular, the study focused on VUV/UV-induced absorption in an attempt to determine the viability of silica as an optical material for 157 nm optical lithography systems.; “Wet” silica (∼800 ppm OH), “dry” silica (1 ppm OH), and a fluorine-doped silica were irradiated at 248 nm, 193 nm, and 157 nm. A custom VUV point-discharge lamp was constructed in order to achieve the latter wavelengths. A KrF excimer laser was used for 248 nm exposures. After irradiations, a dual-beam spectrophotometer was utilized to identify the presence of any induced absorption bands. Samples exhibiting photoluminescence were characterized in situ with fluorescence spectroscopy. Electron spin resonance (ESR) was used as a tool for determining the presence of paramagnetic defects. Certain samples were also examined with Fourier transform infrared (FTIR) spectroscopy. Initial experiments were carried out at room temperature. However, it was established that the induced defects were undergoing a rapid recovery. Irradiations were then attempted at 77 K to minimize thermal bleaching. A time-resolved absorption study was also implemented to qualitatively study recovery rates.; Results showed the E′ centers and non-bridging oxygen hole centers (NBOHC) were the primary defects generated by the VUV/UV radiation. Their concentration and precursors however were specific to each sample and irradiation wavelength. It was also established that the 157 nm light was capable of inducing structural rearrangement via a single-photon process while 193 nm and 248 nm require a two-photon process. Therefore, defect concentrations were found to be an order of magnitude higher with an equivalent dose of radiation at 157 nm. The high energy (7.9 eV) photons are believed to dissociate strained Si-O bonds that exist in three- and four-membered rings. Conversely, the lower energy 6.4 eV and 5.0 eV photons (193 nm and 248 nm respectively) relied on a two-photon absorbed exciton for band-to-band excitation. Lastly, it was postulated that while “wet” silica remains an excellent material for ArF and KrF excimer lasers, fluorine-doped silica currently represents the only viable candidate for 157 nm lithography due to its high transmission and resistance to 7.9 eV radiation.