Laser-induced vaporization of a stretching sheet of liquid tin

Abstract

We experimentally study the mass distribution of a sheet of liquid tin formed by the impact of a ns-laser pulse on a spherical microdroplet. The mass distribution is obtained using a low-intensity, second ns-laser pulse, which induces vaporization of the stretching thin tin sheet. This careful vaporization enables the investigation of the thickness profile of the sheet, and its mass, at early times after laser pulse impact on a droplet, which have remained inaccessible by the methods used in recent work [B. Liu et al., Phys. Rev. Appl. 13, 024035 (2020)]. The vaporization method, moreover, allows the visualization of the thick rim that bounds the thin sheet. Our results unambiguously demonstrate that increasing the energy of the ns-laser pulse incident on the droplet, which enables reaching a predetermined target radius more quickly, results in a larger mass fraction remaining in the sheet. Specifically, our studies show a doubling of the sheet mass fraction by reducing the required expansion time. As a corollary, less tin will end up in other channels of the mass distribution, such as fragments surrounding the sheet. Accordingly, more mass would be available in the target sheet for interaction with the more energetic, main laser pulse that is used in the industry to produce a hot and dense plasma from tin sheet targets in order to create extreme ultraviolet light for nanolithography.

Nanosecond laser ablation threshold of liquid tin microdroplets

“…The current work could be extended by measuring the transition between bulk and thin-film ablation regimes, utilizing the sheet thinning over time resulting from pre-pulse impact [47]. Plasma shielding is likely undesired in an advanced EUV-source vapor-target preparation scheme [8] as it prevents vaporization of part of the liquid mass and possibly disturbs the formation of a uniform "cloud" target. By measuring the plasma onset thresholds for the case of thin sheets, we can therefore identify a parameter range possibly beneficial to target preparation for dropletbased EUV sources.…”

“…Experimental investigations into the ablation threshold and ablation mechanics may therefore be of value to advance our understanding of laser-droplet interaction, and be of significant benefit to EUV source development in the context of droplet deformation, debris mitigation, and metrology. Furthermore, accurate knowledge of ablation thresholds and an understanding of the possibility of phase explosion is valuable in the context of advanced target preparation, where laser vaporization of the liquid tin below the plasma formation threshold can possibly be used to generate low-density vapor targets [8].…”

Energy- and charge-state-resolved spectrometry of tin laser-produced plasma using a retarding field energy analyzer

“…The shadowgraphy imaging setup is described in earlier studies (see, e.g. [17,18]) and enables obtaining the velocity of the plasma-propelled liquid mass. This propulsion velocity is a convenient metric for laser-to-droplet alignment [19].…”

Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

“…In the experiments, micrometer-sized liquid tin droplets are first irradiated with a relatively low intensity (∼10 9 W cm −2 ), 1 µm wavelength PP (see figure 1(a)) from an Nd:YAG laser. The PP propels the droplets and deforms them into extended, disk-like targets of diameter d T [50][51][52][53] with typical radial expansion speeds of ∼90 m s −1 . The target diameter is precisely controlled via the time delay between pre-and main pulse, which ranges 0-3 µs.…”