Wide-view-angle λ/4 plates for diagnosing 193-nm lithography tools

We developed new λ/4 plates, which are insensitive to angle of incidence at 193 nm. Thanks to the development of the wide-view-angle λ/4 plates, a polarimeter reticle is able to measure the polarization of illumination of immersion lithography tools. The wide-view-angle λ/4 plates are composed of four crystal plates; two are made of crystalline quartz categorized as positive uniaxial crystals, and two are made of sapphire categorized as negative uniaxial crystals. Since the birefringent crystals differ in regard to which is the higher of the two refractive indices for the ordinary ray and for the extraordinary ray, their combination mitigates the retardation change at oblique incidence. We measure the retardation change of the new λ/4 plates by the Senarmont method. Results show that the change of retardation is mitigated within an angular range of incidence of ±20°.