The Diagnostic Study On The Species In Nanosecond Laser Produced Sn Plasma Using Optical Emission Spectroscopy And Time-of-flight Mass Spectroscopy

The extreme ultraviolet(EUV)lithography using a wavelength of 13.5nm has been widely regarded as one of the most advanced lithography technologies of the next generation.Due to the unique properties such as high conversion efficiency,low debris,and small size of the light source when generating the extreme ultraviolet light source,the laser produced plasma(LPP)is considered to be the most promising EUV light source generating method.The EUV radiation is emitted with the electronic transition from the states of high-energy levels in the multi-charged ions.Hence,the study of the multi-charged ions' distribution and their dynamic evolution in the plasma becomes essential for better optimizing EUV conversion efficiency(CE),which is also the crucial aspect for further understanding the basic physical process of LPP.In this work,two methods including optical emission spectroscopy(OES)and time-of-flight mass spectroscopy(TOF-MS)were applied to investigate the species in the nanosecond laser ablated Sn plasma.The ion's charge state distribution in Sn plasma and their temporal evolution were studied systematically.The expansion characteristics of Sn plasma produced by laser ablation were analyzed.The basic physical process of the generation of multi-charged ions was discussed.The specific research results are summarized as follows:1.The resolution of the TOF-MS device system has been improved.One particle's velocity limiter is developed to reduce the transversal diffusion of the laser-produced plasma and species in the plasma is also confined in a small space.The profile of the spectral line signal is consequently reduced and the resolution is greatly improved after the application of this limiter.2.The temporal evolution of ionic and atomic lines emitted from the laser produced Sn plasma produced in vacuum at various laser pulse energy was investigated using time-resolved optical emission spectroscopy.The emission lines of divalent ions,monovalent ions and atoms were observed in the experimental spectral range.Both of the ionic and atomic spectral line intensities were found to be decreasing with the time,and ionic line decreased more sharply.With the laser energy decreases,the number of characteristic lines also decreases,and meanwhile the continuum radiation,ionic and atomic optical emission decrease significantly.The electron density and electron temperature were determined byStark width and Boltzmann plot,respectively.The lower the laser energy,the lower the electron temperature and electron density.3.The time-of-flight mass spectroscopy(TOF-MS)was used to diagnose the distribution and temporal evolution of charged species in laser-induced Sn plasma with different power densities varies from 0.55 GW/cm2 to 7 GW/cm2.The results show that there exited monovalent ion and a number of multi-charged ions in Sn plasma.As the laser power density increased,the number of multi-charged state in the Sn plasma increases.When the laser power density is 7.0 GW/cm2,the charge state is up to 6 in the present investigations.Temporal evolution of the multi-charged ions presents higher charged Sn ions have higher velocities and their velocities also are positively related laser power density.This reveals that the ions with the different charge states are spatially separated during the plasma expansion process.It is estimated that the range of velocity of the different charge states is from 0.82km/s to 6.7 km/s,and the corresponding kinetic energy range is from 0.041 ke V to 2.77 ke V.The spatial separation of ions with different charge states is observed and this is due to the acceleration of the transient plasma sheath formed during laser ablation.