Conversion efficiency of a laser-produced Sn plasma at 13.5 nm, simulated with a one-dimensional hydrodynamic model and treated as a multi-component blackbody

Abstract

Efficiency optimization of a stable and debris free plasma source at 13.5 nm, is at the forefront of current extreme ultraviolet lithographic (EUVL) research efforts. To date, 1–2.5% soft x-ray conversion efficiencies (CEs) within a 2% bandwidth (BW) around 13.5 nm and into 2π steradians have been attained experimentally for laser-produced plasmas containing Sn at power densities of 0.5–5 × 1011 W cm−2. In order to complement these experimental endeavours, we have undertaken to study the CE, for the given wavelength regime, in the optically thick limit. We have achieved this by coupling time-dependent and steady-state collisional-radiative (CR) equations to the output of the one-dimensional hydrodynamic code MED103 (MEDUSA), where a solid sphere of radius 50 µm was uniformly irradiated by a high intensity laser pulse with a Gaussian temporal profile. The ion populations obtained from these CR results were then used in an integrated spatio-temporal figure of merit (FOM) together with in-band weighted dipole oscillator strengths and transition energies. The maximum FOM, when divided by the laser energy, was found to occur in the range of peak power densities of 2–3 × 1011 W cm−2 for the steady-state and time-dependent models, respectively. The hydrodynamic variables of these peak power densities were then used in a radiative transfer calculation in which the many-celled spherical plasma was treated as a multi-component blackbody. It is found that CEs of 3.5–6% within the 2% BW per 2π steradians may be achieved. These results are of particular relevance to EUVL technologies where a minimum CE of 3% is required by industry.

The effect of viewing angle on the spectral behavior of a Gd plasma source near 6.7 nm

“…For the plasma viewed at the orientations of Fig. 2(c) and 2(d), because of opacity effects, the bulk of the observed emission is known to originate in a layer close to the plasma boundary and the ion distribution there largely controls the emission profile [17]. For an expanding plasma, the highest stages are generally found close to the centre of the plume while lower stages dominate close to the edge.…”

Simulation of Ni-like and Co-like X-rays emitted from laser produced tin plasmas

“…For laserproduced tin plasmas, (1 ÷ 2.5) % soft X-ray CEs at power densities of (0.5 ÷ 5) × 10 11 W cm −2 within a 2 % bandwidth around 13.5 nm and into 2π sr have been obtained experimentally [7]. In this paper, the simulation of the Ni-like and Co-like X-ray resonance lines together with free-free and free-bound emission between 1Å and 30Å emitted from tin plasmas produced by a Nd:YAG laser is described.…”

“…To use X-rays for extreme ultraviolet (EUV) lithography [2], X-ray microscopy [3], and analytic techniques [4,5] have drawn much attention because of the need for the new lithography and microscopy techniques [6]. Extreme ultraviolet lithography (EUVL) will require bright, efficient plasma sources for use at soft X-ray wavelengths of 13.5 nm [7]. A suitable scheme for lithography requires at least 3 % conversion efficiency (CE) of incident laser energy to soft X-rays in a 2 % bandwidth centred at the 13.5 nm peak [8].…”

Laser-produced plasma EUV source based on tin-rich, thin-layer targets

“…The debris-free double-stream Xe/He gas-puff target reaches 0.43% CE (R. Rakowski, et al, to be published). The literature reports the possibility of increasing the conversion efficiency up to 6% for tin target [22,23]. Presumably, tin-based plasma might be utilized for 13.5-nm lithography, because it exceeds the demanded 3% CE [11,24].…”