Characteristics of extreme ultraviolet emission from mid-infrared laser-produced rare-earth Gd plasmas

Abstract

We characterize extreme ultraviolet (EUV) emission from mid-infrared (mid-IR) laser-produced plasmas (LPPs) of the rare-earth element Gd. The energy conversion efficiency (CE) and the spectral purity in the mid-IR LPPs at λL = 10.6 μm were higher than for solid-state LPPs at λL = 1.06 μm, because the plasma produced is optically thin due to the lower critical density, resulting in a CE of 0.7%. The peak wavelength remained fixed at 6.76 nm for all laser intensities studied. Plasma parameters at a mid-IR laser intensity of 1.3×10(11) W/cm(2) was also evaluated by use of the hydrodynamic simulation code to produce the EUV emission at 6.76 nm.

Laser-Produced Heavy Ion Plasmas as Efficient Soft X-Ray Sources

“…the peak wavelength of 6.76 nm was constant with high spectral efficiency (purity) and energy CE in optically thin CO 2 -LPPs of Gd [14]. In order to infer the laser parameters that maximize 6.x-nm Gd-LPP emission, direct comparison between emissions from a laser-produced Gd plasma and that of Gd ions from welldefined charge states is necessary, as the charge state dependence of the emission at 6.x nm is defined by the electron temperature.…”

“…The EUV emission was observed using a monochromatic EUV pinhole camera. We present benchmark data for the electron density profile with the dominant EUV emission at 6.7 nm occurring in a region with an electron density close to 10 19 cm −3 [14], which was corresponded to the critical density of the CO 2 (carbon dioxide) laser LPP, as shown in Figure 3 [22]. Interferograms were produced by a Mach-Zehnder interferometer by the use of a Nd:YAG laser at a wavelength of 532 nm with a pulse duration of 6 ns (FWHM) [22].…”

“…Since at high plasma electron densities, the opacity effects reduce the intensity of the resonance lines thereby limiting the output power, methods of reducing the effects of reabsorption (opacity) were evaluated to achieve high energy conversion efficiency (CE) from the incident laser energy to the EUV emission energy and the spectral purity. The effect of optical thickness was evaluated by changing the laser wavelength to alter the plasma electron density [7,14]. In order to increase the EUV energy CE and spectral efficiency (purity), the optical thickness in the dominant region of the EUV emission of high-Z highly charged plasmas should be controlled.…”

“…Optically, thin plasmas provide more efficient sources. Therefore, systematic LPP UTA source studies with up-to-date intense picosecond pulse lasers [17] or middle infrared laser, such as the CO 2 laser [14], are needed to determine available light source wavelengths for future applications.…”

Tuning extreme ultraviolet emission for optimum coupling with multilayer mirrors for future lithography through control of ionic charge states

“…With any subsequent increase of the Nd:YAG laser power density, the CE is almost constant at 0.4%, the value obtained at the optimum laser intensity of 4 Â 10 12 W/cm 2 with a pulse duration of 10 ns. 23 Since the transitions responsible are similar to those involved in Sn emission at 13.5 nm and are known to be optically thick, 24 higher CEs should be obtained using CO 2 lasers and this will be the subject of future studies.…”