Understanding and measuring EUV mask 3D effects

EUV lithography is rapidly being pushed to its resolution limit, where tradeoffs are heightened among resolution, throughput, and stochastics. Mitigation strategies include attenuated phase shift masks (aPSMs) and thinner high-k absorbers. Furthermore, multilayer bandwidth and phase shift may need to be reassessed. All these improvements relate to mask 3D effects (M3Ds), arising from several causes: first, phase shift versus pitch, which sets the aPSM target phase shift around 1.2π instead of π; second, absorber thickness effects, which directly relate to the promise of high-k absorbers; and third, multilayer effects such as bandwidth and phase versus angle. We quantify these effects in simulation for different EUV scanner generations (0.33 and 0.55 NA). Moreover, we demonstrate the measurement of these effects with the EUV Tech ENK (EUV N&K and Phase Measurement Tool) using actinic scatterometry. The complexity of M3Ds suggests that new metrics of multilayer and absorber performance beyond reflectivity will need to be considered. Actinic scatterometry provides a promising route to measuring M3Ds due to its sensitivity to wavelength, angle, and feature size.