Dependence of current rise time on laser-triggered discharge plasma

A powerful, stable extreme ultraviolet (EUV) source is the most important component for EUV lithography and EUV mask inspection. Here, we investigate the characteristics of laser-triggered discharge plasma at three different current rise times, fast, middle and slow. A height-adjustable coaxial birdcage was used to change circuit inductance. The rise time was varied between 30 ns–55 ns with peak current of 10 kA. The time-integrated EUV (at 13.5 nm in 2% bandwidth) intensity for the fast rise time was found to be 55% stronger than that of the slow rise time despite its lower energy. A high-speed Mach–Zehnder interferogram and visible imaging of the pinch plasma were employed to discuss plasma compression processes qualitatively and quantitatively. Also discharge produced debris was investigated using a silicon-crystal witness plate. The fast rise current was found to have advantages such as lower debris, higher EUV intensity, and possibility of suppressing instability in comparison with the slow rise time. As expected, total debris amounts lessened proportionally to the primary charged energy, as found from a comparison of fast and slow rise currents.