Physics of laser-driven tin plasma sources of EUV radiation for nanolithography

Abstract

Laser-produced transient tin plasmas are the sources of extreme ultraviolet (EUV) light at 13.5 nm wavelength for next-generation nanolithography, enabling the continued miniaturization of the features on chips. Generating the required EUV light at sufficient power, reliability, and stability presents a formidable multi-faceted task, combining industrial innovations with attractive scientific questions. This topical review presents a contemporary overview of the status of the field, discussing the key processes that govern the dynamics in each step in the process of generating EUV light. Relevant physical processes span over a challenging six orders of magnitude in time scale, ranging from the (sub-)ps and ns time scales of laser-driven atomic plasma processes to the several μs required for the fluid dynamic tin target deformation that is set in motion by them.

Mass Loss from a Stretching Semitransparent Sheet of Liquid Tin

“…Tin microdroplets serve as mass-limited targets for highenergy lasers to produce a hot and dense plasma that emits extreme ultraviolet (EUV) light for state-of-the-art nanolithography [1][2][3][4][5][6][7]. The generation of EUV light is a two-step process, in which a first laser prepulse deforms the tin droplet into a liquid sheet that is suited to serving as a target for a second, more energetic main laser pulse that produces the EUV light.…”

“…A detailed understanding of the prepulse-induced deformation is required to optimally produce EUV light with a minimal production of microparticulate "debris." Such debris could limit the lifetime of nearby light-collection optics [4,[6][7][8][9]. Several recent studies have provided insight into the response of a tin microdroplet to a nanosecond laser pulse in which the propulsion [10,11] accompanies a deformation [10][11][12][13][14] of the droplet into a thin sheet.…”

“…[16,17]. Droplet deformation and fragmentation upon impact, be it onto a solid substrate or by a laser pulse, is a topic of particular current interest in fields ranging from epidemiology, governing the transport of pathogens from sneezes and coughs [17,18], through sprays relevant in, e.g., agriculture [18,19], to state-of-the-art nanolithography [7,[10][11][12][13][14]. Knowledge of the thickness profile of the thin liquid sheet formed upon impact is crucial for understanding the deformation and complex fragmentation processes involved.…”

Time- and space-resolved optical Stark spectroscopy in the afterglow of laser-produced tin-droplet plasma

“…Laser-produced tin plasma is used in state-of-the-art extreme-ultraviolet (EUV) nanolithography machines where highly charged tin ions produce the required EUV light. Hot (10-100 eV) and dense (10 19 -10 21 e − cm −3 ) plasma is produced when molten Sn microdroplets are illuminated by high-energy laser pulses [1]. The responsible ions for emitting EUV photons near 13.5 nm are Sn XI-Sn XV with their resonance transitions 4p 6 4d m -4p 5 4d m+1 and 4d m -4d m−1 4 f (m = 4-0) [1][2][3][4][5].…”

“…Hot (10-100 eV) and dense (10 19 -10 21 e − cm −3 ) plasma is produced when molten Sn microdroplets are illuminated by high-energy laser pulses [1]. The responsible ions for emitting EUV photons near 13.5 nm are Sn XI-Sn XV with their resonance transitions 4p 6 4d m -4p 5 4d m+1 and 4d m -4d m−1 4 f (m = 4-0) [1][2][3][4][5]. Besides the lithographically applicable EUV radiation, the laser-matter interaction produces a violently expanding plasma with a broad range of charge states down to neutral Sn I.…”