Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

Extreme ultraviolet (EUV) lithography is currently entering high-volume manufacturing to enable the continued miniaturization of semiconductor devices. The required EUV light, at 13.5 nm wavelength, is produced in a hot and dense laser-driven tin plasma. The atomic origins of this light are demonstrably poorly understood. Here we calculate detailed tin opacity spectra using the Los Alamos atomic physics suite ATOMIC and validate these calculations with experimental comparisons. Our key finding is that EUV light largely originates from transitions between multiply-excited states, and not from the singly-excited states decaying to the ground state as is the current paradigm. Moreover, we find that transitions between these multiply-excited states also contribute in the same narrow window around 13.5 nm as those originating from singly-excited states, and this striking property holds over a wide range of charge states. We thus reveal the doubly magic behavior of tin and the origins of the EUV light

Investigation of plume dynamics during picosecond laser ablation of H13 steel using high-speed digital holography

Ablation of H13 tool steel using pulse packets with repetition rates of 400 and 1000 kHz and pulse energies of 75 and 44μJ, respectively, is investigated. A drop in ablation efficiency (defined here as the depth per pulse or μm/μJ) is shown to occur when using pulse energies of E$_{pulse}$>44μJ, accompanied by a marked difference in crater morphology. A pulsed digital holographic system is applied to image the resulting plumes, showing a persistent plume in both cases. Holographic data are used to calculate the plume absorption and subsequently the fraction of pulse energy arriving at the surface after traversing the plume for different pulse arrival times. A significant proportion of the pulse energy is shown to be absorbed in the plume for E$_{pulse}$>44μJ for pulse arrival times corresponding to > 1 MHz pulse repetition rate, shifting the interaction to a vapour-dominated ablation regime, an energetically costlier ablation mechanism.This work was collaboratively carried out under EPSRC Grant Number EP/K030884/1, as part of the EPSRC Centre for Innovative Manufacturing in Laser-based Production Processes. One of the authors acknowledges his PhD studentship by the Federal Government of Nigeria (TETFUND) in conjunction with the Federal University of Petroleum Resources Effurun (FUPRE)