Optimizing vacuum ultraviolet attenuated phase shift masking materials

Abstract

Extensive vacuum ultraviolet (VUV) optical data have been collected for sputter-deposited thin films of masking material compounds for use at 157 nm. The use of VUV optical data and effective media modeling has lead to 27 possible material candidates for further study. These composite material films consist of a silicon dioxide host combined with various oxides and nitrides, generally of refractory metals. By adjusting material stoichiometry through control of deposition parameters (target composition ratios, pressure, power, flow rates, etc.), films have been tailored for specific optical properties. By using extinction coefficient/refractive index relationships for 180° phase shift, 4%–25% transmission, and <15% reflectivity, evaluation of the materials is possible. Additional detailed analysis of three specific composite materials is shown for use as attenuated phase shift masking at 157 nm. In each case, silicon dioxide is the host material within the film and the various metal oxides provide the appropriate absorption. The use of fluoride film etch stop materials is also described as a method to improve etch selectivity.