Optimization of laser-produced plasmas for nanolithography and materials detection

Justin R Freeman, Purdue University

Abstract

In this work, laser-matter interactions and resultant plasma emission using traditional short pulsed lasers are studied in the context of semiconductor lithography and material sensing applications. Ultrafast laser ablation and plasma emission results are then compared to those using traditional short pulsed lasers. Then fundamental laser-matter interactions and ablation processes of ultrafast lasers are investigated. This work focuses on laser-produced plasma (LPP) light sources at extreme ultraviolet (EUV) wavelengths. The out-of-band (OoB) light emission as well as ionic and atomic debris from the plasma source, which are capable of damaging collection optics, have been studied as a function of incident laser wavelength to characterize the angular distributions of debris and identify the differences in debris from longer and shorter laser excitation wavelengths. By applying a prepulse to create improved laser-target coupling conditions, conversion efficiency (CE) from laser energy to 13.5 nm light emission from the plasma source can be improved by 30% or higher. Energetic ions escaping from the plasma can cause significant damage to light collection optics, greatly reducing their lifetimes, but by implementing a prepulse, it has been shown that most-probable ion energies can be reduced significantly, minimizing damage caused to collection optics. Laser-induced breakdown spectroscopy (LIBS) is a technique used to identify the elemental constituents of unknown samples by studying the optical light spectra emitted from a LPP. Despite advantages such as in situ capabilities and near-instant results, detection limits of LIBS systems are not as competitive as other laboratory-based systems. To overcome such limitations, a double pulse (DP) LIBS system is arranged using a long-wavelength laser for the second pulse and heating of the plume created by the first pulse. Detector gating parameters were optimized and different first-pulse laser energies were investigated to study improvements with increasing mass ablation. The long-wavelength laser does not increase mass ablation in DP-LIBS and through optimization, it is found that maximum enhancements are observed for cases of smallest mass ablation; an important consideration for analysis of delicate samples. For bulk element analysis, enhancements of 14 and 10 times for S/N and S/B, respectively, are seen, and for trace element analysis, enhancements of 7 and 3 times for S/N and S/B, respectively, are seen. Due to extremely short pulse durations, the ablation mechanisms for ultrafast lasers are not fully understood, meaning their implementation in existing and novel laser applications are hindered. The differences in visible emission dynamics from nanosecond (ns) and femtosecond (fs) laser ablation (LA) plumes are reported and the effects that vacuum and ambient pressure environments play on plasma plume expansion dynamics. Lastly, a fundamental study of ultrafast laser ablation is performed to better understand ablation mechanisms and resultant plasma plume properties. Under ns laser ablation, ion time of flight analysis typically shows a single-peak profile, however, under fs laser ablation a double-peak profile is observed and the source of the faster peak is heavily disputed. To better understand the nature of the fast peak, ion time of flight profiles are investigated for several high-purity metals under ns and fs laser irradiation. Ion peak velocities are compared to material thermal properties to confirm the thermal nature of the slower peak observed under fs laser ablation and its correlation to the ns laser ablation results. The faster ion peak from fs laser ablation does not show any relation to thermal properties and in fact shows similar velocity for all elements investigated, despite widely varying atomic mass. The results combine to confirm the non-thermal nature of the fast ion peak observed under ultrafast laser ablation. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Harilal, Purdue University.

Subject Area

Nuclear engineering|Plasma physics

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