With the continuous decrease in feature size in the semiconductor industry, extreme ultraviolet lithography (EUVL) is becoming increasingly crucial in ultrahigh integrated circuit manufacturing. The emission characteristics of tin (Sn) in terms of its high conversion efficiency (CE) and spectral purity (SP) make it the best choice for current EUVL systems. Laser-produced plasma (LPP) and laser-induced discharge plasma (LDP) are the most important technical methods for generating extreme ultraviolet rays. LDP has the low cost, simple structure, high operation rate, and high energy injection, and it is promising for mask inspection, microscopic imaging, and spectral metrology. Unlike the LPP source, many parameters, such as the electrode structure, discharge gap, laser wavelength, and current rise rate, make it difficult to design an LDP source; therefore, it is necessary to investigate the influence of these parameters on the performance of the LDP EUV source.

An experimental setup is designed to investigate the laser-induced discharge of tin plasma. A pulsed carbon dioxide laser is used to ablate a tin plate cathode and produce an expanding pre-ionized plasma as the discharge medium, decreasing the threshold for the breakdown voltage. A vacuum arc gradually formes between the cathode and the stainless-steel hemisphere anode, and the current bombards the electrodes and generates more plasma. The time-domain waveform of the current is recorded, and the extreme ultraviolet spectra of the LPP and LDP are analyzed. A radiative magneto-hydrodynamic program Z* is used to simulate the laser plasma and discharge plasma.

The experimental LPP-EUV spectrum shows a peak at 13.7 nm when the laser energy is 145 mJ, whereas the peaks of the LDP spectra show a significant red shift compared to those of the LPP spectrum (Fig.3). When the voltage is 7 kV, the in-band (bandwidth of 2% at 13.5 nm wavelength) spectral intensity of the LDP does not significantly increase compared with that under the LPP condition. At a voltage of 15 kV, the in-band spectral intensity increases significantly. The simulation results show that the time-domain signals of the LDP-EUV have multiple peaks, and when the voltage reaches 15 kV, the second peak of the EUV radiation is higher than the first peak (Fig.6). When the current intensity is sufficiently high, the Joule heat generated by the current is sufficient to compensate for the energy transmitted via plasma diffusion and thermal radiation, causing the plasma temperature to increase and the EUV radiation area to extend. Subsequently, the plasma between the electrodes rapidly collapses as the current intensity oscillates and decays. The total and EUV radiation powers reach their maximum values near the current peak. When the discharge voltage increases from 7 kV to 15 kV, the total and EUV radiation powers both increase, and the highest EUV radiation power reaches 0.025 MW. The total radiation energy increases from 842.00 mJ to 3.85 J, and the total EUV radiation energy increases from 3.5 mJ to 65.0 mJ. The CE increases from 0.054% to 0.23%, and the SP increases from 0.42% to 1.69%. At a voltage of 7 kV, the maximum EUV radiation power density is 0.09 MW/cm³, and the EUV radiation is mainly concentrated near the anode (Fig.7). When the voltage is 15 kV, the maximum EUV radiation power density can reach 0.3 MW/cm³, and the EUV radiation is mainly concentrated on both sides of the electrodes (Fig.8). The average ionization degree of the plasma in the main area of EUV radiation is 10‒12.

In this study, the EUV radiation emitted by the LDP and LPP is conducted experimentally and theoretically. Compared to that of the LPP source, the temperature of the LDP source is significantly higher, and more Sn¹⁰⁺, Sn¹¹⁺, and Sn¹²⁺ ions are present. Transitions between multiple excited states gradually replace those between single excited and ground states. However, the plasma size of the LDP source is very large, resulting in a low radiation power density. The light source for mask inspection requires strong brightness; therefore, further research on the Z-pinch mechanism is required to reduce the plasma size and improve brightness. The discharge voltage significantly influences the in-band EUV radiation of the LDP source. This phenomenon demonstrates the major advantage of the LDP light source: the extreme ultraviolet output power can be increased by increasing the injection of electrical energy. However, CE and SP still need to be improved by increasing the current rise rate. Shortening the current rise time and reducing the inductance of the discharge circuit can be good approaches.