PHOTOINDUCED TRANSFORMATIONS IN AMORPHOUS SE75GE25 THIN-FILM BY XECL EXCIMER-LASER EXPOSURE

The scalar photoinduced changes of the optical absorption edge and the optical constants in amorphous Se75Ge25 chalcogenide thin film have b een studied using XeCl excimer laser Light with a wavelength of 306 nm . The glass transition temperature (T-g) of this material is evaluated to be approximately 220 degrees C, at which the as-deposited film is initially annealed. The optical energy gaps E-op of as-deposited and i nitially annealed Se75Ge25 thin films are 1.90 and 1.84 eV, respective ly. When initially annealed films are exposed to a 4.05 eV excimer las er (much greater than E-op) with energy densities of 100, 120 and 180 mJ/cm(2), a photodarkening effect that exhibits a decrease in the opti cal transmittance (T-op) and E-op and an increase of the refractive in dex n and the extinction coefficient k in the vicinity of the absorpti on edge (1.4-3.0 eV) is observed. In particular, in the case of a thin film exposed by 120 mJ/cm(2), E-op and n + ik measured at 673.7 nm(1. 84 eV), i.e., an absorption edge of initially annealed thin him, are 1 60 eV and 2.680 + i0.118 and the changes of Delta E-op and Delta n + i Delta k are 0.24 eV and 0.215 + i0.100, respectively. Furthermore, th ese photodarkened thin films recovered (bleached) to the initial annea led state by re-annealing at T-g. 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 chang es in Se75Ge25 thin film lead to its structural changes, we could expe ct high etch selectivity in using this film as a deep ultraviolet lith ography resist. (C) 1998 American Institute of Physics.