Generation of the small tin-droplet streams with a manipulable droplet spacing via the forced velocity perturbation

Abstract

In extreme ultraviolet (EUV) sources, small tin droplets are scattered into the tin mist/disk under the irradiation of the pulsed laser to produce the EUV light. Small droplet size and large droplet spacing are required to suppress debris production to protect expensive collector mirrors. To this end, a tin-droplet generator with a capillary glass nozzle was designed and built to produce uniform tin-droplet streams with the droplet diameter less than 50 μm. Meanwhile, a forced perturbation, generated by a sandwich piezoelectric transducer, was loaded into a liquid tin jet to manipulate the droplet spacing through a stepped rod. The mono-sized tin-droplet streams with an average diameter of 42 μm were successfully produced in both the Rayleigh and the forced jet breakup regimes. A two-dimensional (2D) axisymmetric model was proposed to reveal the influence of the velocity perturbation amplitude on jet breakup patterns at different wavelengths. An F*–λ* (The dimensionless perturbation velocity amplitude F*–The dimensionless wavenumber l*) map was built, and five different droplet breakup patterns were identified based on simulations. Numerical simulations indicated that the droplet spacing could be increased by providing extra momentum to droplets from the forced velocity perturbation. Finally, by increasing the velocity perturbation amplitude, the droplet spacing was increased from ∼9Dd (droplet diameter) to ∼19Dd without significantly increasing the droplet size. This work provides a novel approach to obtaining small mono-sized tin-droplet streams with manipulable droplet spacing.