Combined effects of pre-pulsing and target geometry on efficient EUV production from laser produced plasma experiments and modeling

Abstract

Laser produced plasmas (LPP) is currently a promising source of an efficient extreme ultraviolet (EUV) photon source production for advanced lithography. Optimum laser pulse parameters with adjusted wavelength, energy, and duration for simple planar or spherical tin target can provide 2-3% conversion efficiency (CE) in laboratory experiments. These values are also in good agreement with modeling results. Additional effects such as targets with complex geometry and tin-doped targets using pre-pulsing of laser beams can significantly increase CE. Recent studies showed that such improvements in LPP system are due to reduction in laser energy losses by decreasing photons transmission (higher harmonic of Nd:YAG laser) or photons reflection (for CO 2 laser). Optimization of target heating using pre-pulses or ablating low-density and nanoporous tin oxide can further improve LLP sources by creating more efficient plasma plumes and as a result increasing CE, the most important parameter for EUV sources. The second important challenge in developing LPP devices is to decrease fast ions and target debris to protect the optical collection system and increase its lifetime.We investigated the combined effects of pre-pulsing with various parameters and different target geometries on EUV conversion efficiency and on energetic ions production. The much higher reflectivity of CO 2 laser from a tin target leads to two possible ways for system improvement using pre-pulses with shorter laser wavelengths or using more complex targets geometries with special grooves as developed previously by the authors.

Microdroplet-tin plasma sources of EUV radiation driven by solid-state-lasers (Topical Review)

“…Subsequent pulses of even lower energy may be used to rarefy a tin target in order to, e.g. make a preheated plasma [29] that is later to be laser-reheated [13,30,31]. Vaporization of the several 10 nm thick targets [27] can also be employed to obtain further information about the thickness profile itself.…”

Modeling and optimization of mass-limited targets for EUV lithography

“…In this regard we can consider two modes for utilizing prepared target matter. The first one is based on the heating by the main laser of plasma plume created at the pre-pulse stage and expanded during the short time, up to 100 ns [3] [4]. The second one is related to utilizing the evaporated and fragmented parts of the droplet [1].…”