Are droplet-impact and laser-ablation splashing commutable?

In nanolithography industry, extreme ultraviolet (EUV) light sources are used for semiconductor
device fabrication. This EUV source is based on laser ablating liquid tin to create a plasma that
emits the required 13.5 nm wavelength light. In current day nanolithography devices, a train
of tin droplets is ablated with high frequency limited by the droplet dispenser. An alternative
concept for an EUV source is investigated, where this limiting droplet dispenser is absent.

In this research, a new EUV source concept derived from nuclear fusion engineering is adopted:
a liquid tin film on top of a tungsten substrate. In nuclear fusion reactors, liquid metal films
are considered as heat shields. This study focuses on the fluid dynamics after laser impact on a
liquid tin film. In the measured laser energy regime, splashing is observed; a crown forms and
droplets detach from the crown rim. This splashing is undesirable in an EUV source, where
contamination, in the form of ejected droplets, has to be minimized. In addition, during local
and temporal heat fluxes in nuclear fusion reactors on liquid metal heat shields (called divertors)
droplets could also be ejected, and disturb reactor operation. Thus, there is a joint interest in
suppressing splashing caused by heat fluxes.

Since there is an abundance of literature on droplet-impact splashing, and virtually no literature
on laser ablation splashing, we compare laser-ablation splashing to droplet-impact splashing. If
the splashing dynamics are found to be comparable, knowledge of droplet-impact splashing can
be applied to laser-ablation splashing. This knowledge could eventually contribute to designing
an EUV source, and liquid metal divertor concept, that suppresses splashing.

The crown evolution of a laser-ablation splash on a 3 mm thick liquid tin film is studied ex-
perimentally using shadowgraphy for laser-impact energies of 2.5-30 mJ, focused to a 66 µm
full width half maximum (FWHM) Gaussian spot using 8 ns pulses. The results are compared
qualitatively, as well as quantitatively to literature describing droplet-impact splashing. Many
similarities are observed, apart from the difference in crown collapse. This difference of crown
height evolution at late-time can be attributed to the relative importance of gravity on the
different length-scale, expressed by the bond number. To make crown measurements and time
dimensionless, an energy-dependent characteristic length scale and velocity are defined. In di-
mensionless units, early-time evolution of crown width (W ∗) of the laser-ablation splash is found
to be independent of impact laser energy and follows W ∗ − 1 ∝ √τ . This relation agrees with
droplet-impact literature. A linear relation between the crown height (H) and width (W ) is
measured during the crown growth phase. The collection of these linear relations for various
laser energies is found to approximately follow the quadratic relation H ≈ 0.01 + 0.33W 2.

In conclusion, the dimensionless crown growth of the laser-ablation splash correlates well with
the dimensionless crown growth of the droplet-impact splash, both theoretically and empirically.
Consequently, within the measured regime, laser ablation and droplet impact are approximately
interchangeable, considering the early-time crown evolution of the splash