Abstract
The emission spectra from an industrial, droplet-based, laser-produced plasma, extreme ultraviolet light source for nanolithography are here presented and analyzed. The dependence of spectral features on the CO2-drive-laser intensity is studied by changing the beam spot size at constant pulse energy and duration. We characterize the spectrum by fitting the results of atomic structure calculations to the short-wavelength region (7–11 nm), where the contributions from various charge states can be resolved, and obtain the relative contributions of charge states Sn9+ –Sn15+ . These relative contributions are compared to charge state populations as calculated with the non-equilibrium plasma kinetics code flychk. The calculations are shown to be in good qualitative agreement with the results, showing that the effective plasma temperature, and with it, the shape of the unresolved, main emission feature at 13.5 nm, is a remarkably weak function of laser intensity under this source normal operating conditions.
Highlights
Extreme ultraviolet (EUV) lithography [1, 2] is the main candidate for replacement of current 193 nm immersion lithography [3,4,5,6,7], with further miniaturization, fewer processing steps enabled by the shorter wavelength of the radiation
We present spectra obtained from an industrial laser-produced plasma (LPP) EUV light source
This emission feature is composed of thousands of atomic transitions in the ions Sn8+ –Sn14+ [12, 13, 15, 20, 24], which are clustered together in what are commonly known as unresolved trans ition arrays (UTAs) [38]
Summary
Extreme ultraviolet (EUV) lithography [1, 2] is the main candidate for replacement of current 193 nm immersion lithography [3,4,5,6,7], with further miniaturization, fewer processing steps enabled by the shorter wavelength of the radiation. EUV radiation is generated using laser-produced plasma (LPP) light sources. 53 (2020) 055204 employed as projection optics in EUV lithography scanners [1]. The projection optics [1], EUV pellicle [25] in the scanner are sensitively dependent on the spectral emission characteristics as, e.g. the out-of-band EUV spectrum, i.e. photons outside the 2% reflection bandwidth, may influence coating lifetime, further introduce undesirable thermal effects [26, 27]. The result of atomic structure calculations given in [28] are used to characterize the charge state contribution to the spectrum by employing a fitting procedure. Using the n on-equilibrium plasma kinetics code flychk [29], the fit results are compared to the calculated charge state populations, obtaining an effective plasma temperature, scaling thereof with laser intensity
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