The spectrum of a 1- μ m-wavelength-driven tin microdroplet laser-produced plasma source in the 5.5–265.5 nm wavelength range

Abstract

We present a calibrated spectrum in the 5.5–265.5 nm range from a microdroplet-tin Nd:YAG-laser-produced plasma under conditions relevant for the production of extreme ultraviolet (EUV) light at 13.5 nm for nanolithography. The plasma emission spectrum obtained using a custom-built transmission grating spectrometer results from a careful calibration of a series of filters enabling measurements free of any higher diffraction orders. Specifically, Zr, Si, and Al thin-foil filters and bulk LiF, MgF2, and UV fused silica filters are employed. A further filter using four SiC mirrors is used to record the otherwise inaccessible 40–100 nm range. The resulting corrected and concatenated spectra are shown to accurately match in their respective overlap regions. The possibility to measure spectra over this broad range enables the optimization of current and future sources of EUV light for nanolithography by providing the diagnostics required for minimizing the emission of unwanted wavelength bands.

Microdroplet-tin plasma sources of EUV radiation driven by solid-state-lasers (Topical Review)

“…Using the method developed by Scheers et al [94] for charge-state-resolved spectroscopy, Bouza et al [95] compared laser-driven plasma emission with electron beam ion trap spectroscopies at longer EUV wavelengths. This plasma spectroscopy work was later extended to include DUV and UV wavelengths, where an intensity-calibrated spectrum from 5 all the way up to 265 nm wavelength [96] was recorded using a novel transmission grating spectrometer (TGS) developed by the MESA+ XUV Optics group at Twente University, in tandem with a smart choice of filters. The TGS will soon be upgraded with one-dimensional imaging capabilities, enabling space-resolved characterization of plasma ionicity (see Byers et al [97]).…”

Development of an ultra-compact inline transmission grating spectrograph for extreme ultraviolet wavelengths

“…Depending on the target application, the design of the spectrograph can be either in reflection or transmission mode. Although reflective spectrographs generally offer a higher resolution due to the possibility of applying aberration corrections and focusing options within the grating 8,9 , the main benefits of a transmission grating spectrograph are its compactness and possibility of inline positioning while still offering sub-Ångström spectral resolution if designed carefully 10,11 . Here, we report about the realization of such a compact EUV-TGS, designed for in-situ spectral monitoring in an EUV laboratory exposure tool.…”

Extreme ultraviolet broadband imaging spectrometer using dispersion-matched zone plates

“…The reliability, throughput, and efficiency of extreme ultraviolet (EUV) lithography machines depend on the characteristics of the EUV light source, which is currently a laser-produced plasma (LPP) generated from microdroplets of tin [1][2][3][4]. This tin LPP emits light from the soft x ray to the visible region, with its emission peaking in the 13.5 nm ± 1% "in-band" region where multilayer mirrors (MLMs) can be employed to guide the light [5]. Maximizing in-band emission compared with the complementary out-of-band emission improves the quality of the EUV source [6,7].…”

“…Out-of-band EUV light contributes to collector mirror heating and ionization of ambient hydrogen gas which can damage nearby optics. Additionally, out-of-band light cannot be reflected by the narrow-bandwidth multilayer mirror (MLM) optical systems employed in EUV lithography and contributes to power loss [5,8]. Longer-wavelength, deep ultraviolet emissions may still be reflected by the MLMs and expose the wafer, potentially reducing the pattern contrast [9,10].…”

“…Two fundamental techniques for characterizing plasma sources are spectroscopy and imaging. The spectrum of a microdroplet tin LPP in a 5.5-265.5-nm wavelength range was previously studied [5]. However, this emission spectrum represents the spatial average across the plasma and does not provide information on the spatial distribution of the emission.…”