Photoinduced transformations in amorphous Se75Ge25 thin film by XeCl excimer-laser exposure

Abstract

The scalar photoinduced changes of the optical absorption edge and the optical constants in amorphous Se75Ge25 chalcogenide thin film have been studied using XeCl excimer laser light with a wavelength of 306 nm. The glass transition temperature (Tg) of this material is evaluated to be approximately 220 °C, at which the as-deposited film is initially annealed. The optical energy gaps Eop of as-deposited and initially annealed Se75Ge25 thin films are 1.90 and 1.84 eV, respectively. When initially annealed films are exposed to a 4.05 eV excimer laser (≫Eop) with energy densities of 100, 120 and 180 mJ/cm2, a photodarkening effect that exhibits a decrease in the optical transmittance (Top) and Eop and an increase of the refractive index n and the extinction coefficient k in the vicinity of the absorption edge (1.4–3.0 eV) is observed. In particular, in the case of a thin film exposed by 120 mJ/cm2, Eop and n+ik measured at 673.7 nm(1.84 eV), i.e., an absorption edge of initially annealed thin film, are 160 eV and 2.680+i0.118 and the changes of ΔEop and Δn+iΔk are 0.24 eV and 0.215+i0.100, respectively. Furthermore, these photodarkened thin films recovered (bleached) to the initial annealed state by re-annealing at Tg. As a result of the x-ray diffraction analysis, we can conclude that these changes are due to an amorphous to amorphous transition. Therefore, considering that these large changes in Se75Ge25 thin film lead to its structural changes, we could expect high etch selectivity in using this film as a deep ultraviolet lithography resist.