Spectroscopy of highly charged ions and its relevance to EUV and soft x-ray source development

Abstract

The primary requirement for the development of tools for extreme ultraviolet lithography (EUVL) has been the identification and optimization of suitable sources. These sources must be capable of producing hundreds of watts of extreme ultraviolet (EUV) radiation within a wavelength bandwidth of 2% centred on 13.5 nm, based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ∼70% at this wavelength. Since, with the exception of large scale facilities, such as free electron lasers, such radiation is only emitted from plasmas containing moderately to highly charged ions, the source development prompted a large volume of studies of laser produced and discharge plasmas in order to identify which ions were the strongest emitters at this wavelength and the plasma conditions under which their emission was optimized. It quickly emerged that transitions of the type 4p64dn − 4p54dn+1 + 4dn−14f in the spectra of Sn IX to SnXIV were the best candidates and work is still ongoing to establish the plasma conditions under which their emission at 13.5 nm is maximized. In addition, development of other sources at 6.X nm, where X ∼ 0.7, has been identified as the wavelength of choice for so-called Beyond EUVL (BEUVL), based on the availability of La/B based MLMs, with theoretical reflectance approaching 80% at this wavelength. Laser produced plasmas of Gd and Tb have been identified as potential source elements, as n = 4 − n = 4 transitions in their ions emit strongly near this wavelength. However to date, the highest conversion efficiency (CE) obtained, for laser to BEUV energy emitted within the 0.6% wavelength bandwidth of the available mirrors is only 0.8%, compared with values of 5% for the 2% bandwidth relevant for the Mo/Si mirrors at 13.5 nm. This suggests a need to identify other potential sources or the selection of other wavelengths for BEUVL. This review deals with the atomic physics of the highly-charged ions relevant to EUV emission at these wavelengths. It considers the developments that have contributed to the realization of the 5% CE at 13.5 nm which underpins the production of high-volume lithography tools, and those that will be required to realize BEUV lithography.

Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target

“…In the experiments, we average over five shots (shots No. [10][11][12][13][14] per target position as well as over 30 separate target positions, i.e., 150 shots in total. Shots later than shot No.…”

“…There they could be used for generating a fine-dispersed liquid-metal target 10 before the arrival of a high-energy main-pulse responsible for the EUV emission, enhancing laser-plasma coupling. 11 The utilization of a fs-ps laser system could strongly reduce fast ionic and neutral debris from EUV sources compared with nanosecondpulses, 12 enabling a better machine lifetime. 13 Since the 1990s, many experiments have been performed and models developed 2,3 for laser-matter interaction at this particular time scale.…”

High-space resolution imaging plate analysis of extreme ultraviolet (EUV) light from tin laser-produced plasmas

“…These light intensities were chosen based on the previous optimization for 13.5 nm generation. 22,23 The target vacuum chamber pressure was typically in the order of 1 × 10 5 Torr. The phosphor imaging plates (Fujifilm, Japan) consisted of a BaFBr:Eu 2+ photosensitive film, and the BAS-TR type was used exclusively at Tokyo Tech.…”

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

“…Laser produced plasmas (LPPs) from tin droplet targets have been adopted as the optimum extreme ultraviolet (EUV) light sources for next generation lithography for high-volume manufacturing (HVM) of semiconductor circuits with feature sizes of 10 nm or less [1,2]. Transitions of the type 4p 6 4d N+1 −4p 5 4d N+2 +4p 6 4d N 4f 1 in Sn 8+ -Sn 13+ merge to form an unresolved transition array (UTA) [3] 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].…”

“…So one would expect the p-d contribution to overtake that for d-f with increasing ionization around n = 3 [17]. Moreover, if one allows for spin orbit splitting of the 4p and 4d subshells, for N > 4 the lowest configuration will be 4p 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.…”