Controlling ion kinetic energy distributions in laser produced plasma sources by means of a picosecond pulse pair

Abstract

The next generation of lithography machines uses extreme ultraviolet (EUV) light originating from laser-produced plasma (LPP) sources, where a small tin droplet is ionized by an intense laser pulse to emit the requested light at 13.5 nm. Numerous irradiation schemes have been explored to increase conversion efficiency (CE), out of which a double-pulse approach comprising a weak picosecond Nd:YAG pre-pulse followed by a powerful pulse is considered to be very promising [1]. Nevertheless, even for such CE-optimized schemes, ion debris ejected from the plasma with kinetic energies up to several keV remain a factor that hampers the maximum performance of LPP sources. In this letter we propose a novel pre-pulse scheme consisting of a picosecond pulse pair at 1064 nm, which decreases the amount of undesirable fast ions, avoids back-reflections to the lasers and enables one to tailor the target shape.In the past two decades a large number of theoretical and experimental studies have been conducted on possible light sources for EUV lithography, including synchrotron radiation [2, 3], free-electron lasers [4,5], plasma sources [6-10] and high-harmonic generation [11]. From the aforementioned solutions, a tin-based laser-produced plasma source received the most attention due to its high conversion efficiency, robustness and scalability [12,13], resulting in a first commercial machine launched in 2010. In such an LPP source, narrowband radiation around 13.5 nm comes from multiple ionic states, Sn 8+ to Sn 14+ [14,15], collisionally excited by plasma electrons heated through interaction with a powerful CO2 laser. An effective coupling between laser light and plasma occurs near the critical density, which for CO2-laser-driven plasma is around 10 19 cm -3 , meaning that a mass-limited tin target should be expanded to reduce its density. At the same time the size of the EUV source cannot be too large to match the requirements for the maximum etendue [16]. The precise control of the target shape is thus crucial for the production of EUV light in an industrial setting.

New Insight into the Gas Phase Reaction Dynamics in Pulsed Laser Deposition of Multi-Elemental Oxides

“…Another observation for all the measured kinetic energy distributions was the appearance of a structure at lower kinetic energies with increasing background pressure ( Figure 3 , insets), in particular for the ablation in Ar. Analysing these distributions more closely by fitting energy-shifted Maxwell–Boltzmann (MB) distributions, we noted that these kinetic energy distributions could be described by two or three MB distributions, indicating that the stopping of these species fell into several kinetic energy windows [ 23 , 24 ]. Examples are shown in the Supplementary Materials Figure S2 .…”

High-energy ions from Nd:YAG laser ablation of tin microdroplets: comparison between experiment and a single-fluid hydrodynamic model

“…Plasmas driven by shorter, ps-duration pulses [23,24] exhibit ion energy distributions whose shapes are better described by the planar isothermal expansion model of Mora [25]. In this case the ion energy distribution reads…”

Physics of laser-driven tin plasma sources of EUV radiation for nanolithography

“…This understanding enables the optimization of both η rad and SP, finding a global optimum of the conversion efficiency CE=η rad SP. A record-high 3.2% CE was obtained and further paths were identified for improving this efficiency, capitalizing on the ongoing development of advanced YAG laser systems [34,151]. YAG lasers may also find application in two-color pumping schemes.…”

“…The impact of such energetic particles would affect the performance of light collecting multilayer optics. As such, plasma expansion and the generation of energetic particles is a subject of particular interest [9,[31][32][33][34][35].…”