Configuration Interaction Effects in Unresolved 5p65dN+1−5p55dN+2+5p65dN5f1 Transition Arrays in Ions Z = 79–92

Luning Liu,Gerry O’Sullivan,Padraig Dunne,Xinbing Wang,Deirdre Kilbane

doi:10.3390/atoms5020020

Abstract

Configuration interaction (CI) effects can greatly influence the way in which extreme ultraviolet (EUV) and soft X-ray (SXR) spectra of heavier ions are dominated by emission from unresolved transition arrays (UTAs), the most intense of which originate from Δn = 0, 4p64dN+1−4p54dN+2+4p64dN4f1 transitions. Changing the principle quantum number n, from 4 to 5, changes the origin of the UTA from Δn = 0, 4p64dN+1−4p54dN+2+4p64dN4f1 to Δn = 0, 5p65dN+1−5p55dN+2+5p65dN5f1 transitions. This causes unexpected and significant changes in the impact of configuration interaction from that observed in the heavily studied n = 4 – n = 4 arrays. In this study, the properties of n = 5 – n = 5 arrays have been investigated theoretically with the aid of Hartree-Fock with configuration interaction (HFCI) calculations. In addition to predicting the wavelengths and spectral details of the anticipated features, the calculations show that the effects of configuration interaction are quite different for the two different families of Δn = 0 transitions, a conclusion which is reinforced by comparison with experimental results.

Highlights

Laser produced plasmas (LPPs) from tin droplet targets have been adopted as the optimum extreme ultraviolet (EUV) light sources for generation lithography for high-volume manufacturing (HVM) of semiconductor circuits with feature sizes of 10 nm or less [1,2]
Which contains thousands of individual lines and emits strongly in such a plasma at an electron temperature of ~30 eV in a narrow wavelength range around 13.5 nm [4,5]. This value coincides with the wavelength of peak reflectance of ~70% of the Mo/Si multilayer mirrors (MLMs) that are used in the scanning tools [6] and tin plasmas are the brightest sources at this wavelength
Other recent research has concentrated on investigating future-generation lithographic sources at shorter wavelengths, in particular at 6.75 nm where an intense Unresolved transition arrays (UTAs) is emitted by gadolinium and terbium plasmas with an electron temperature of close to 100 eV [7,8,9], and where LaB4 C and LaNB4 C MLMs have a peak theoretical reflectivity of close to ~80% [10]

Summary

Introduction

Laser produced plasmas (LPPs) from tin droplet targets have been adopted as the optimum extreme ultraviolet (EUV) light sources for generation lithography for high-volume manufacturing (HVM) of semiconductor circuits with feature sizes of 10 nm or less [1,2]. 4p 1/2 4p4 3/2 4dN 3/2 −4p2 1/2 4p3 3/2 4dN+1 3/2 +4p1 1/2 4p4 3/2 4dN+1 3/2 +4p2 1/2 4p4 3/2 4dN-1 3/2 4f with the 4p1/2 –4d3/2 contribution appearing on the short wavelength side of the UTA, or, if the 4p spin orbit splitting is sufficiently large, forming a second UTA at a shorter wavelength Both sets of transitions are responsible for absorption by ions in the plasma periphery which is the major problem that must be overcome to attain the maximum conversion efficiency of laser to spectral emission energy in EUV source development. In performing calculations for low ion stages of the lighter lanthanides and the elements preceding them, it is necessary to expand the excited state basis to include higher nf orbitals or reduce the effective exchange interaction This is achieved by scaling the G1 (4d, 4f) parameter, as is done in calculations with the Cowan code, in order to obtain good agreement between calculated and observed results.

Results

Findings

Conclusions