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17 April 2014 Study of alternative capping and absorber layers for extreme ultraviolet (EUV) masks for sub-16nm half-pitch nodes
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Abstract
Multiple challenges, including the availability of a reliable high power source, defect free mask, and proper resist material, have forced extreme ultraviolet (EUV) lithography to be considered for sub-10 nm half-pitch nodes. Therefore, techniques such as phase shift masks (PSMs) or high numerical aperture (NA) lithography might be considered. Such techniques require thin EUV absorber materials to be optimized to reduce EUV mask shadowing effects. Despite the challenges in dry etching of Ni and finding proper chemistries with a high etch selectivity to suitable capping materials, we decided to examine the chemical stability of Ni for existing mask cleaning chemistries. Ni, after Ag, has the highest absorption in EUV light at λ = 13.5 nm, which makes it a proper candidate—in pure form or in mixing with other elements—for thin absorber film. Depending on the composition of the final material, proper integration schemes will be developed. We studied Ni stability in commonly used mask cleaning processes based on ammonium hydroxide/ hydrogen peroxide (APM) and water mixtures. Ni films deposited with an ion beam deposition technique with a thickness of 35 nm are sufficient to totally absorb EUV light at λ = 13.5 nm. Multiple cleanings of these Ni films resulted in Ni oxidation— confirmed by time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis as NiO with thickness about 1.5 nm. Furthermore, Ni oxidation processes are self-limiting and oxide layer thickness did not increase with a further cleaning. A three minute exposure to sulfuric acid/hydrogen peroxide mixture (SPM) can remove NiO and Ni totally. To protect Ni film from etching by SPM chemistry a 3 nm Si capping was used on top of Ni film. However, Si capping was removed by APM chemistry and could not protect Ni film against SPM chemistry. TiO2 may be a very good capping layer for EUV optics but it is not suitable for EUV mask blanks and will be removed by APM chemistries.
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Abbas Rastegar, Matthew House, Ruahi Tian, Thomas Laursen, Alin Antohe, and Patrick Kearney "Study of alternative capping and absorber layers for extreme ultraviolet (EUV) masks for sub-16nm half-pitch nodes", Proc. SPIE 9048, Extreme Ultraviolet (EUV) Lithography V, 90480L (17 April 2014); doi: 10.1117/12.2048074; https://doi.org/10.1117/12.2048074
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