As Extreme Ultra Violet lithography (EUVL) is becoming adopted into manufacturing, there is an ongoing need toidentify and improve the EUV mask multilayer properties that impact reflectivity. Key properties include the roughnessand inter-diffusion depth at the Mo-Si interfaces. During mask usage, on exposure to EUV, the interfaces are impactedduring thermal cycling, so interfacial stability is key. We report on the use of X-ray reflectivity (XRR) to probe theinterfacial depth and roughness of Mo/Si multilayers deposited via secondary ion beam deposition (IBD). We confirmtop-surface roughness by AFM. We measure minimal impact of the underlying substrate on top-surface roughness ofMo-Si multilayer stacks. Mo and Si single-layer roughness are shown to be primarily dependent on deposition angle;with minimal roughness at intermediate angles and significant deterioration beyond a deposition angle of about 60degrees. We use this angular dependence to systematically vary the interfacial roughness and monitor the impact on theXRR measurement. We demonstrate that XRR, with attention to the Fourier Transform, may also be used to quantify theinter-diffusion depth at the Mo-Si interfaces. We measure inter-diffusion depths of 0.5 - 1.8nm. A simulated model isdeveloped, incorporating both interfacial depth and roughness, and the experimental data are compared with this model.The model could be applied to quantify the impact on the interfaces of: beam energy and flux; incidence angles; gasspecies and pressure; interfacial treatments; thermal treatment; or mask usage.
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