Electrical recovery after a vacuum tin discharge for highly repetitive plasma EUV source

The laser assisted vacuum tin discharges produce high energy-density micro-plasmas, which can be used for high power extreme ultraviolet (EUV) sources for the next generation semiconductor nano-lithography. Highly repetitive operation of the discharge more than 20 kHz are required to obtain the EUV yield for high volume manufacturing (HVM). Here we describe the electrical recovery of the electrode gap and the behavior of the discharge products in the post-discharge phase to consider the physical limitation of the repetitive operation. The second voltage pulse was applied to the electrode at a certain time after the main discharge to evaluate the electrical recovery. The experiment shows that the hold-off voltage was gradually increased with the time and the recovery was delayed with increasing the discharge energy. The measured voltage recovery suggests the maximum allowable repetition rate and the operation voltage as a function of the energy dissipated in the main discharge. For example, when the shot energy is set to 20 J the pulse repetition rate is limited below 5 kHz, whereas the repetition rate can be increased up to 50 kHz for the small discharge energy of 5 J. Laser scattering and laser-induced fluorescence (LIF) were used to observe the discharge products including tin droplets and vapor. Tin vapor density decreased with the time, and the decrease was delayed with increasing the discharge energy. The electrical recovery of the gap seems to be in accordance with the history of the vapor density. Tin droplets, which stagnate for 10 ms around the electrodes, are unlikely to influence the electrical recovery significantly.