Influence of a laser intensity on EUV brightness and ion speed from a laser-assisted discharge-produced plasma

Laser-assisted discharge-produced plasma (LDP) is one of the ways to generate extreme ultraviolet (EUV) light used in the semiconductor manufacturing processes. This light source uses a pulsed laser and a high-current pulsed electrical discharge to make a high-temperature and high-density tin plasma. One of two rotating disk electrodes, of which surfaces are coated by liquid tin (Sn), is irradiated by the laser to produce tin plasma. The plasma propagates from one electrode (cathode) to the other (anode) and ignites an electrical breakdown between the electrodes. The low-inductance circuit connected to the electrodes provides a current of approximately 15 kA and 150 ns to the tin plasma. The plasma implodes due to its own magnetic pressure, and EUV radiation is emitted from the resultant hot and dense plasma. High-speed ions are also emitted from the plasma and limit the lifetime of the mirror used to collect the EUV light. We need to maximize the light emission and minimize the ion emission. The role of the laser is essential in the LDP EUV source not only to ignite the discharge but also to condition the initial plasma. It influences EUV energy, EUV brightness, and emitted ion speed distribution of the plasma. The experiment suggested that laser intensity higher than 25 GW/cm2 produced high EUV brightness and low emitted ion speed.