Behavior of laser assisted tin discharge EUV emitting plasma

Extreme Ultraviolet (EUV) lithography is considered as the most promising candidate of the next generation of lithography for manufacturing ever smaller and faster chips. In our laboratory, a laser assisted tin target discharge produced plasma EUV source has been studied. The system comprises an Nd:YAG laser, focusing on a tin (Sn) rod embedded in one of electrodes to create plasma; and a power supply system to generate a sinusoidal discharge current of 22 kA amplitude, 250 ns half cycle that flows through the plasma between the electrodes to pinch the plasma and produce EUV radiation. Since the EUV emission directly relates to the plasma behavior, the dynamics of Z-pinch plasma during discharge was studied by a visible region high speed camera, with 10 ns exposure time. The Zipper-effect of the pinch plasma has been observed: the pinch spot is moving from anode to the tin cathode. From the EUV intensity and the spot size obtained by a EUV region pinhole camera, we found the optimum electrodes distance and laser energy for EUV generation is 4mm and 51mJ, respectively. Besides, since the YAG laser energy strongly affects the EUV emission, the expansion characteristics of the laser created plasma, especially the evolution of excited Sn neutral and Sn⁺¹ species were studied by the optical diagnostics of special plasma emission spectrum lines (452.5 nm for Sn I and 579.8 nm for Sn II), when there is no external electric field between the electrodes. From the experimental results, we found that the MAX- intensity-position of Sn I moves slower than that of Sn II, however, the velocity of the edge position along the axis of Sn I is larger than that of Sn II. And the velocities of the MAX-intensity-position and edge position of Sn II is 0.45-1.3times10⁴ m/s and 1.8times10⁴ m/s, respectively.