As Extreme Ultra Violet lithography (EUVL) is becoming adopted into manufacturing, there is an ongoing need to identify and improve the EUV mask multilayer properties that impact reflectivity. Key properties include the roughness and inter-diffusion depth at the Mo-Si interfaces. During mask usage, on exposure to EUV, the interfaces are impacted during thermal cycling, so interfacial stability is key. We report on the use of X-ray reflectivity (XRR) to probe the interfacial depth and roughness of Mo/Si multilayers deposited via secondary ion beam deposition (IBD). We confirm top-surface roughness by AFM. We measure minimal impact of the underlying substrate on top-surface roughness of Mo-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 60 degrees. We use this angular dependence to systematically vary the interfacial roughness and monitor the impact on the XRR measurement. We demonstrate that XRR, with attention to the Fourier Transform, may also be used to quantify the inter-diffusion depth at the Mo-Si interfaces. We measure inter-diffusion depths of 0.5 - 1.8nm. A simulated model is developed, 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; gas species and pressure; interfacial treatments; thermal treatment; or mask usage.
For future nodes, TaN-based absorber layers on EUV mask-blanks, may need to be replaced with thinner layers of new material systems. Ni and Co based materials are promising material candidates owing to their high EUV absorption. Ion Beam Etching (IBE) is being explored as an option for patterning these metallic systems that are hard to etch by Reactive Ion Etch. In this work we expand our initial work on the IBE of Ni absorber films to include the role of etch beam energy and alternative etch-masks for both Ni and Co based films. We present experimental film level data such as etch uniformity, angular-dependent etch rates, and surface roughness. We extend the modeling of IBE of line-space patterns, to narrower line widths and various etch-mask materials vis-a-vis side wall angle and CD fidelity, both as a function of beam energy and angle of etch.
Development progress and roadmap, for high-reflective Mo/Si multilayers for EUV mask-blanks, are reviewed. We outline the state-of-the-art in low-defect-density secondary ion beam deposition (IBD), and ongoing hardware development for performance improvement and high-volume manufacturing. We further discuss extension of ion beam technology to later steps in the EUV mask manufacturing: deposition of highly-uniform 2.5 – 3nm Ru capping layers; and patterning of novel Ni absorber structures. IBD-deposited Ru films are demonstrated with uniformity of 0.7% 3σ over a 188mm diameter area. By x-ray reflection with Cu Kα radiation, we measure a film density of 12.4 g/cm3, and a roughness of less than 1.0nm. Deposition rates of ~ 1 – 7 nm/min are demonstrated, implying a capping layer deposition time of 20 seconds – 3 minutes. . For advanced absorber patterning, we discuss Argon ion beam etch (IBE) of Ni films. Ni and Ru IBE etch rates of ~ 8 – 80 nm/min are demonstrated, implying absorber etch times of ~ 30 seconds – 5 minutes. IBE Ni:Ru etch selectivity is 1:1 to 1.3:1, so Ru is not a ‘stopping layer’, etch depth must be controlled by time, and Ni uniformity is a requirement. IBE Ni:Photoresist etch selectivity is 0.8:1 to 1.6:1. We simulate the IBE absorber pattern definition for mask features of half-pitch 96nm (24nm at wafer level). Ion beam incidence angle can be optimized to maintain critical dimension within 6% of the pre-etch value.
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