Evidence of production of keV Sn⁺ ions in the H₂ buffer gas surrounding an Sn-plasma EUV source

Abstract

Charge-state-resolved kinetic energy spectra of Sn ions ejected from a laser-produced plasma (LPP) of Sn have been measured at different densities of the H2 buffer gas surrounding a micro-droplet LPP. In the absence of H2, energetic keV Sn ions with charge states ranging from 4+ to 8+ are measured. For the H2 densities used in the experiments no appreciable stopping or energy loss of the ions is observed. However, electron capture by Sn ions from H2 results in a rapid shift towards lower charge states. At the highest H2 pressure of 6×10−4 mbar, only Sn2+ and Sn+ ions are measured. The occurrence of Sn+ ions is remarkable due to the endothermic nature of electron capture by Sn2+ ions from H2. To explain the production of keV Sn+ ions, it is proposed that their generation is due to electron capture by metastable Sn2+∗ ions. The gateway role of metastable Sn2+∗ is underpinned by model simulations using atomic collision cross sections to track the charge states of Sn ions while traversing the H2 buffer gas.

Electron Capture from Molecular Hydrogen by Metastable Sn2+* Ions

“…is endothermic with an energy difference of 1.3 eV for the Franck-Condon ionization of H 2 at its equilibrium distance, it has been believed that the consecutive charge state lowering basically stops at 2+. However, in a recent study [11], we have shown that the successive capture reactions do lead to the formation of Sn + ions. The work of Rai et al [11] explains the high production of Sn + as a consequence of the preceding single-capture reaction between Sn 3+ and H 2 , which was shown [15] to mainly produce long-lived metastable Sn 2+ (5s5p…”

“…However, in a recent study [11], we have shown that the successive capture reactions do lead to the formation of Sn + ions. The work of Rai et al [11] explains the high production of Sn + as a consequence of the preceding single-capture reaction between Sn 3+ and H 2 , which was shown [15] to mainly produce long-lived metastable Sn 2+ (5s5p…”

“…In particular, a dense Sn plasma of about 40 eV in temperature is created in which Sn ions with charge states between 9+ and 15+ are the atomic sources of EUV light at 13.5 nm [4][5][6][7][8]. Energetic Sn ions are emitted from the plasma in lower charge states, typically from 4+ to 8+ [9][10][11], which is attributed to recombination in the peripheral part of the plasma [12]. To protect the plasma-facing optics from these energetic and highly charged ions, an H 2 buffer gas is introduced [2,13].…”

“…Collisions of the Sn q+ ions with the H 2 molecules give rise to one-and two-electron capture reactions stepping down the average charge state of the ions. The initial steps down till Sn 3+ are expected to happen rapidly due to the large cross-sections for the higher charge states [11]. In the whole series of consecutive scattering events while traveling through the buffer gas, the ions' kinetic energy is reduced to values that allow for the efficient mitigation of the Sn ions.…”

Elastic scattering cross sections and transport of tin ions in extreme ultraviolet lithography sources

“…2 where the difference in ionization levels is only 0.44 eV. However, work by Rai et al [16,33] has shown evidence of single ionized tin coming from a laser-produced-plasma. These singly ionized tin particles only appear after hydrogen gas is introduced and higher charged tin ions are charge exchanged down.…”