Extreme ultraviolet mask multilayer material variation impact on horizontal to vertical pattern bias

Multilayer unit thickness variations in the multilayer extreme ultraviolet (EUV) reflector stack pose a difficult problem for generation of lithographic models for use in Optical Proximity Correction (OPC). The multilayer stack is ideally comprised of alternating layers of molybdenum and silicon. However, there are diffused interface regions between these layers that might have slight variations in thickness, reflectance, and absorption. These interface regions can differ within specified parameters based on variations in the deposition tool, resulting in EUV masks with minor differences in the multilayer. This is a difficult problem for OPC models, because slight variations in the multilayer can result in large variations in the feature printed on the wafer. Also, these stack variations are not precisely known for every reticle, rather a sample stack is used to gather data from a cross section of a representative reticle. This study explores the relationship between EUV mask stack reflectivity and horizontal to vertical pattern bias. In this computational study, the MoSi2 thickness is varied at systematic locations in the mask stack, then data on horizontal to vertical bias (H to V bias) for multiple features are gathered. The data will be used to understand the relationship between mask substrate reflectance, mask material thickness, and H to V bias. The study will also investigate the impact of high numerical aperture (0.55 NA anamorphic) imaging on the final H to V bias. Initial work indicates that a 1% variation in substrate reflectance results in approximately a 4% variation in CD.