# Influence of Pre-Ionized Plasma on the Dynamics of a Tin Laser-Triggered Discharge-Plasma
## 1 Introduction  
In order to keep pace with Moore's law, photolithography has been widely used in the semiconductor industry during the last few decades. At present, the 13.5 nm extreme ultraviolet lithography (EUVL) is one of the photolithography methods with the most potential [1]. The wavelength is 13.5 nm emission in a 2% bandwidth, which is mainly for the molybdenum-silicon (Mo/Si) multilayer mirrors [2] and the photo resistance [3]. Due to the high convention efficiency of 13.5 nm emission, Sn has been widely used as an extreme ultraviolet (EUV) source [4]. Two types of EUV sources are used for EUVL, which are laser-produced plasma (LPP) [5, 6] and laser-triggered discharge plasma (LTD) [7]. The LTD source has the features of being a low-cost system, high repetition rate, selective spectral wavelength according to target, potential high repetition-rate operation, and precise timing controllability of a light pulse. In this way, it can be used not only for EUVL, but also for EUV-induced plasma [8], nano-patterning [9], and X-ray microscopy [10, 11]. Moreover, it can be used to inspect the key components of the EUVL system, such as the mask and so on [12].  
In recent years, a number of experiments have been conducted with LTD sources. The dynamics of the Z-pinch plasma are investigated by different methods [13, 14], which show that the electron density is about 10\({}^{18}\)-10\({}^{19}\) cm\({}^{-3}\), and the electron temperature is about 20-40 eV. With a special three electrodes system, [PERSON] et al. investigated the post-discharge stage of the LTD EUV source [15], and it shows that that electrical recovery lasts for a few tens to several hundreds of microseconds, and the repetition of the source can exceed 70 kHz, which is useful to achieve high power of the EUV source. Parameters and types of lasers are also investigated by [PERSON] et al. and [PERSON] et al. The former reports that the maximum conversion efficiency is almost the same for CO\({}_{2}\), Nd:YAG, XeF, and KrF laser, and the dependence of the EUV source efficiency on the laser power density isobtained as well [16]. The latter studies the EUV spectra generated by picosecond and nanosecond laser triggering under different laser energy. It reports that ps-triggering is better in order to get a higher EUV conversion efficiency and narrower spectral profiles [7]. Moreover, the effect of current rise time, electrical energy, and inter-electrode distance are also reported [16; 17].  
The plasma characteristics during the Z-pinch and recovery stage have been studied extensively by using optical techniques or spectroscopy. Until now, the research on the expanding of the pre-ionized plasma for Sn LTD sources has been reported very little. For the LTD source, the trigger laser generates an expanding pre-ionized plasma, which is the discharge medium for the latter process. The condition of the pre-ionized plasma varies with the expanding, and it will dictate the subsequent compression dynamics and the spectral properties of the Z-pinch. Moreover, the pre-ionized plasma results in a much lower threshold for the breakdown voltage as well [18]. In this way, study of the delay between the triggering laser and the current, may be useful for the optimizing of the EUV emission. In this paper, we focus on the dynamics under different laser-current delays by time-resolved EUV spectroscopy.