Post-discharge study of laser-triggered vacuum discharge for highly repetitive powerful EUV source

Extreme ultraviolet (EUV) source remains a key issue for industrial application of EUV lithography. Currently, laser-triggered discharge (LTD) tin plasma is the most promising discharge produced plasma EUV source. High frequency discharge of up to tens of kHz is required to achieve sufficient average radiation power. However, the choice of discharge frequency is restricted by the recovery time of the tin fuel plasma. Although much experimental and theoretical work has been performed to investigate discharge plasma dynamics and its corresponding EUV radiation process during main discharge, study regarding post-discharge stage after main discharge is scant.This paper presents a study of plasma decay and tin fuel residual recovery following a laser triggered discharge for high repetition rate EUV source. First, the insulation strength recovery of tin fuel residual between electrodes after main discharge was measured by time lapse electrical breakdown voltage test. A breakdown voltage measurement system was designed composed mainly of a second high voltage pulse circuit and its isolation and protection circuits with the main discharge. The electrical recovery curve was measured. The tin vapor decay process was also characterized by means of time-resolved Mach-Zender interferometer. Additionally, the influence of tin fuel plasma recovery process on highly repetitive EUV source performance is discussed. In the nonfull electrical recovery region, secondary self-discharge was observed and its affect on plasma dynamics and EUV emission was studied. After full electrical recovery, the following laser-triggered discharge was investigated using the second pulsed laser to confirm the reproducibility of plasma dynamics and EUV pulse stability relative to the proceeding main discharge for different discharge time intervals.