Electrical Recovery After Laser-Assisted Discharge for Highly Repetitive Plasma EUV Source

Extreme ultraviolet (EUV) light source is still the key issue for the industrial application of EUV lithography. For the discharge-produced-plasma EUV source, highly repetitive discharge up to tens of kilohertz is required to achieve the sufficient average radiation power at intermediate focus. Electrical recovery of electrode gaps prior to each discharge influences the EUV emission and determines the repetition rate of the discharge. In this paper, electrical recovery after the main discharge is studied for the laser-assisted-discharge (LAD) tin plasma EUV source through the measurement of the electrical breakdown. The electrical recovery process lasts for a few tens to several hundreds of microseconds, and recovery time is proportional to the discharge shot energy. The measured time-varying hold-off and breakdown voltages can provide a basis for the choice of discharge voltage and corresponding maximum repetition rate. Electrical recovery was explained on the basis of dynamic variation of tin discharge products. It suggests that electrical recovery mainly depends on the diffusion of the tin vapor and its density decaying. Tin droplets do not affect the electrical recovery significantly. Larger discharge energy generates denser tin vapor and causes slower electrical recovery process, which limits the maximum repetition rate of the EUV source. In the LAD EUV source, the relative small shot energy is proper for the high-repetition-rate operation.