Laser-induced droplet deformation: curvature inversion explained from instantaneous pressure impulse

TL;DR

This paper explains the counterintuitive curvature inversion of laser-formed tin sheets by combining simulations and experiments, revealing that an instantaneous pressure impulse with low kurtosis causes the observed shape changes relevant to EUV nanolithography.

Contribution

It introduces a new explanation for curvature inversion using pressure impulse modeling, resolving previous theoretical discrepancies with experimental data.

Findings

Curvature inversion explained by low-kurtosis pressure impulse.

Successful reproduction of experimental curvature across various ratios.

Application of simulation methods to EUV nanolithography processes.

Abstract

We investigate the shape of a tin sheet formed from a droplet struck by a nanosecond laser pulse. Specifically, we examine the dynamics of the process as a function of laser beam properties, focusing on the outstanding riddle of curvature inversion: tin sheets produced in experiments and state-of-the-art extreme ultraviolet (EUV) nanolithography light sources curve in a direction opposite to previous theoretical predictions. We resolve this discrepancy by combining direct numerical simulations with experimental data, demonstrating that curvature inversion can be explained by an instantaneous pressure impulse with low kurtosis. Specifically, we parametrize a dimensionless pressure width, $W$, using a raised cosine function and successfully reproduce the experimentally observed curvature over a wide range of laser-to-droplet diameter ratios, $0.3 < d/D_0 < 0.8$. The simulation process described in this work has applications in the EUV nanolithography industry, where a laser pulse deforms a droplet into a sheet, which is subsequently ionized by a second pulse to produce EUV-emitting plasma.