A mass-limited Sn target irradiated by dual laser pulses for an EUVL source

We present efforts to mitigate debris from laser-produced Sn plasma by introducing a low energy pre-pulse while keeping high in-band conversion efficiency from laser to 13.5 nm extreme ultraviolet (EUV) light. The basic idea is to separate the processes of plasma production and 13.5 nm EUV light generation. A low energy pre-pulse is introduced to create a pre-plume; the main pulse then heats up the pre-plume to the optimum temperature for efficient 13.5 nm EUV light generation. Much lower ion energy and nearly the same conversion efficiency were simultaneously observed from plasma driven by a dual-pulse as compared with that of a single pulse. Thin Sn coating were investigated as a form of mass-limited target. It was found that the higher ion energy normally accompanying the use of a mass-limited Sn target is effectively maintained under 100 eV by using the dual pulse irradiation technique. A Sn coating as thin as 30 nm could generate almost the same conversion efficiency as that obtained with a single pulse and a massive target. It was noted that less gas is required to mitigate ions with lower energy when dual pulses are used. This research enables an efficient, clean, and high-speed mass-limited target supply based on pure Sn for a high volume manufacturing (HVM) EUVL source.