We experimentally study the morphology of a radially expanding sheet of liquid tin, formed by nanosecond-pulse Nd:YAG laser impact on a spherical microdroplet. Specifically, the sheet thickness profile and its time evolution are captured in detail over a range of laser-pulse energies and for two droplet sizes. Two complementary methods to determine this thickness are employed and shown to be in excellent agreement. All obtained thickness profiles collapse onto a single self-similar curve. Spatial integration of the thickness profiles allows us to determine the volume of the sheet. Remarkably, less than half of the initial amount of tin remains in the sheet under conditions relevant for industrial sources of extreme ultraviolet light, where these thin tin sheets serve as target material. Further analysis shows that the dominant fraction of the mass lost from the sheet during its expansion ends up as fine fragments. We propose that such mass loss can be minimized by producing the sheet targets on the shortest possible time scale. These findings may be particularly valuable for ongoing developments in state-of-the-art nanolithography.

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.

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