Abstract
The results of a systematic study performed on Pb-Sn alloys of concentration 65–35% and 94–6% by weight along with spectra from pure Pb and Sn in the wavelength range of 9.8–18 nm are presented. The dynamics of the Nd:YAG laser produced plasma were changed by varying the focused spot size and input energy of the laser pulse; the laser irradiance at the target varied from 7.3 × 109 W cm−2 to 1.2 × 1012 W cm−2. The contributing ion stages and line emission are identified using the steady state collisional radiative model of Colombant and Tonon, and the Cowan suite of atomic structure codes. The Sn spectrum was dominated in each case by the well-known unresolved transition array (UTA) near 13.5 nm. However, a surprising result was the lack of any enhancement or narrowing of this feature at low concentrations of Sn in the alloy spectra whose emission was essentially dominated by Pb ions.
Highlights
IntroductionThe emission from Sn in a 2% bandwidth at 13.5 nm arising from ∆n = 0, n = 4–4 transitions, has become the definitive source of choice for extreme ultraviolet (EUV) lithography systems
The emission from Sn in a 2% bandwidth at 13.5 nm arising from ∆n = 0, n = 4–4 transitions, has become the definitive source of choice for extreme ultraviolet (EUV) lithography systems.This emission results from an intense unresolved transition array (UTA) whose width and intensity are strongly influenced by plasma opacity [1]
For plasmas produced with CO2 lasers (λ = 10.6 μm) the reduction in critical electron density results in a greater UTA intensity and laser to EUV emission conversion efficiency (CE); again due to the reduction in plasma opacity resulting from a lower ion density [3]
Summary
The emission from Sn in a 2% bandwidth at 13.5 nm arising from ∆n = 0, n = 4–4 transitions, has become the definitive source of choice for extreme ultraviolet (EUV) lithography systems This emission results from an intense unresolved transition array (UTA) whose width and intensity are strongly influenced by plasma opacity [1]. Fahy et al proposed a liquid collector optic to mitigate the debris associated with laser produced plasmas (LPPs) and multilayer optics [5] Following this Kambali et al proposed using LPPs from Au or an Au-Sn alloy as a metrology source in the 10 to 18 nm region where Au has a relatively flat spectrum with the added attraction that the alloy had a melting temperature of 280 ◦ C and could be used as an renewable plasma fuel [6].
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