Identifying extreme ultraviolet lithography attenuated phase shifting mask absorber materials using effective media approximation modeling

Pattern transfer in an extreme ultraviolet lithography (EUVL) system requires reflective optical elements illuminated at oblique illumination angles. This, in combination with the three-dimensional effects at the mask, is the source of the so-called mask 3D (M3D) effects that include shadowing, best focus shifts, and contrast fading. Alternative mask absorbers at lower thickness possess the ability to extenuate the M3D effects and improve overall imaging performance. An approach to recognize candidate material combinations as alternative EUVL mask absorbers through dielectric constant modeling by using methods involving Wiener bounds and effective media approximation (EMA) is presented. Using EMA, several stoichiometrically stable binary alloys of refractory metals that may serve as attenuated phase shifting mask absorbers have been studied. The optical properties and thickness of each absorber candidate alloy in a standard EUV mask stack are optimized for imaging performance. The best performing alloys are characterized through rigorous 3D image modeling of near-field intensity and phase at varied illumination angles.