Elaine Long

Sources for beyond extreme ultraviolet lithography and water window imaging

Lithography tools are being built and shipped to semiconductor manufacturers for high volume manufacturing using extreme ultraviolet lithography (EUVL) at a wavelength of 13.5 nm. This wavelength is based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ~70% at this wavelength. Moreover, the primary lithography tool manufacturer, ASML, has identified 6.x nm, where x~7, as the wavelength of choice for so-called Beyond EUVL, based on the availability of La/B4C MLMs, with theoretical reflectance approaching 80% at this wavelength. The optimum sources have been identified as laser produced plasmas of Gd and Tb, as n = 4–n = 4 transitions in their ions emit strongly near this wavelength. However, to date, the highest conversion efficiency obtained, for laser to EUV energy emitted within the 0.6% wavelength bandwidth of the mirror is only 0.8%, pointing to the need to identify other potential sources or consider the selection of other wavelengths. At the same time, sources for other applications are being developed. Conventional sources for soft x-ray microscopy use H-like line emission from liquid nitrogen or carbon containing liquid jets which can be focused using zone plates. Recently the possibility of using MLMs with n = 4−n = 4 emission from a highly charged Bi plasma was proposed and subsequently the possibility of using Δn = 1 transitions in 3rd row transition elements was identified. All of these studies seek to identify spectral features that coincide with the reflectance characteristics of available MLMs, determine the conditions under which they are optimized and establish the maximum conversion efficiencies obtainable. Thus, there is a need for systematic studies of laser produced plasmas of a wide range of elements as some of the challenges are similar for all of these sources and some recent results will be presented.

XUV spectra of 2nd transition row elements: identification of 3d–4p and 3d–4f transition arrays

The use of laser produced plasmas (LPPs) in extreme ultraviolet/soft x-ray lithography and metrology at 13.5 nm has been widely reported and recent research efforts have focused on developing next generation sources for lithography, surface morphology, patterning and microscopy at shorter wavelengths. In this paper, the spectra emitted from LPPs of the 2nd transition row elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of zirconium (Z = 40) and technetium (Z = 43), produced by two Nd:YAG lasers which delivered up to 600 mJ in 7 ns and 230 mJ in 170 ps, respectively, are reported. Intense emission was observed in the 2–8 nm spectral region resulting from unresolved transition arrays (UTAs) due to 3d–4p, 3d–4f and 3p–3d transitions. These transitions in a number of ion stages of yttrium, niobium, ruthenium and rhodium were identified by comparison with results from Cowan code calculations and previous studies. The theoretical data were parameterized using the UTA formalism and the mean wavelength and widths were calculated and compared with experimental results.

EUV spectral analysis of ns-laser produced bismuth plasmas at 8-17 nm

Extreme ultraviolet (EUV) spectra from laser produced bismuth plasmas were recorded in the 8-17 nm spectral region using a Nd:YAG laser with a pulse length of 8 ns operating at a range of laser power densities. Due to the broad-band emission at 8-17 nm, bismuth plasmas show promise as sources of quasicontinuous radiation in the extreme ultraviolet. When varying the incident laser power density, ionic populations of Bi ions at different power densities were estimated by the collisional-radiative (CR) model for explanation of changes in the spectral profile. Comparison of experimental spectra with atomic structure calculations using the Hartree-Fock with configuration interaction (HFCI) code of Cowan was performed in order to identify most of the features in the spectra.

Source development for extreme ultraviolet lithography and water window imaging

Sources based on laser produced plasmas of tin (Sn) are currently being developed for extreme ultraviolet lithography for semiconductor fabrication. Since they operate at short wavelength (13.5 nm) they are capable of producing features with critical dimensions in the 10 nm range. Already next generation lithography sources operating at an even lower wavelength of around 6.7-6.8 nm have been proposed and research is ongoing on their feasibility for both large scale manufacturing and ‘at wavelength’ metrology. The high resolution afforded by such short wavelengths is also of use for applications such as surface patterning and microscopy and the results of recent experiments to identify sources for operation in the ‘water window’ (2.34-4.2 nm), where carbon absorbs strongly but water does not are summarized.

Spectra of plasmas of Ru, Rh, Pd and Mo produced with nanosecond and picosecond laser pulses

This paper describes the extreme ultraviolet and soft x-ray emission recorded in the 2-12 nm region from Mo, Ru, Rh and Pd ions present in the laser produced plasmas. The spectra were found to be dominated by 3p-3d transitions in the 5-8 nm region, which shift slowly to shorter wavelengths with the increasing atomic number, and by 3d-4p and 3d-4f transitions at shorter wavelengths. These transitions, in a number of neighbouring ion stages, were distinguished by comparison with Cowan code calculations and previously reported data. The experimental results show that strong emission can be observed at the 6.X nm region for Ru, Rh and Pd plasmas.

EUV spectra from highly charged terbium ions in optically thin and thick plasmas

We have observed extreme ultraviolet (EUV) spectra from terbium (Tb) ions in optically thin and thick plasmas for a comparative study. The experimental spectra are recorded in optically thin, magnetically confined torus plasmas and dense laser-produced plasmas (LPPs). The main feature of the spectra is quasicontinuum emission with a peak around 6.5-6.6 nm, the bandwidth of which is narrower in the torus plasmas than in the LPPs. A comparison between the two types of spectra also suggests strong opacity effects in the LPPs. A comparison with the calculated line strength distributions gives a qualitative interpretation of the observed spectra.

Coincidence photoelectron measurements following 2p photoionization in Mg

Coincidence photoelectron spectroscopy has been used to test the two-step model of Auger decay following 2p photoionization in magnesium. Fitting of the data has yielded information on the dipole photoionization matrix elements as a function of photon energy.

Properties of the extreme ultraviolet emission from germanium and gallium plasmas

We report on the usefulness of germanium (Ge) and gallium (Ga) plasmas as potential extreme ultraviolet (EUV) sources at both 6.x and 13.5 nm, wavelengths of interest for lithography and metrology applications. Spectra from plasmas produced using neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers with pulse durations of 170 ps and 7 ns [full width at half-maximum] and a compact electron beam ion trap have been used to determine the charge states responsible while theoretical calculations using atomic structure Cowan and HULLAC codes enabled the identification of structure in the Ge spectrum arising from strong transitions of the type 3d – 4p and 3d – 4f. Transitions of the type 3d – 4f in Ge10+ and Ga11+ were found to lie in the 6.x nm region, while 3p−3d transitions from a range of stages in both elements contribute in the 13.5 nm region. We also studied the emission spectra of galinstan plasmas, and the results imply that galinstan might provide a promising candidate for EUV applications that explo...