Effects of pre-pulses on extreme ultraviolet conversion efficiency in laser-produced tin plasmas

Summary form only given. Extreme ultraviolet (EUV) lithography is being considered for manufacturing the next generation of computer chips. However, a suitable source for EUV emission at 13.5 nm must be identified. The source must be able to provide reliable, clean, and powerful EUV emission at 13.5 nm with 2% bandwidth and be capable of meeting the demands of high volume manufacturing. Laser produced plasmas (LPP) have emerged as a promising source for EUV emission, but higher in-band conversion efficiency (CE) and debris control must first be realized. Tin is considered the material of choice for producing this plasma, as its plasma emits strongly in the EUV in-band region, contributed by various ionic stages (Sn⁸⁺ - Sn¹⁴⁺). However, the net emission of 13.5 nm photons is controlled by plasma opacity, which depends on level populations of different ionic states, ionization balance, and electron density. For obtaining the highest CE, ideal plasma temperatures and densities should be created for the longest possible period of time with the maximum collectable size. Reheating of a pre-formed plasma is one of the methods for controlling density and hence optimizing plasma opacity for maximum EUV emission. This study investigates the effect of pre-pulses on EUV conversion efficiency in laser-produced tin plasmas. An Nd:YAG laser (1.06 μm, 8 ns FWHM) was used as a pre pulse to generate the initial plasma from planar tin target. A second, delayed pulse from a CO₂ laser (10.6 μm, 35 ns FWHM) was then used to reheat the existing plasma to enhance EUV emission. The energy of the pre-pulse was varied to study its effect on the CE when later reheated by the second CO₂ laser pulse. Different delay times between laser pulses were used to identify the optimum plasma conditions from the pre-pulse before reheating with the second laser pulse. A significant increase in CE was found by optimizing the pre-pulse energy and t- e delay time between pulses.