Study On Soft X-ray Radiation Characteristics Of Laser Produced Plasmas Of Bi-Pb-Sn Alloy,Low-density Targets And Colliding Plasma

With the development of laser produced plasma（LPP）13.5 nm extreme ultraviolet（EUV）lithography sources,researchers have focused on the soft X-ray（SXR）field with shorter wavelength to further improve lithography resolution.On the other hand,new technical routes of low-density targets and colliding plasma are also proposed to replace the current complex scheme of dual-pulse laser interacting with droplet targets.The two dimensional（2D）optical emission spectra（OES）of Bi-Pb-Sn alloy with low melting point plasma were recorded by a Dove prism and visible spectrograph.According to the OES,the spatial and temporal distribution and evolution of electron temperature and density were analyzed by Boltzmann plotting method and Stark broadening method,respectively.It is found that the emission lines of Bi-Pb-Sn plasma are relatively denser in the short wavelength range of 330～380 nm,and most of them came from Bi and Pb atoms with high-atomic numbers in the alloy target.Under different laser energies,the electron temperature at the position 1 mm away from the target kept a constant value within the errors with time,while the electron density decreased rapidly with the distance away from the target surface.A plume splitting phenomenon was found in the plumes of Pb II and Bi II.The 13.5 nm EUV conversion efficiency（CE）of Sn plasma was higher than that of alloy plasma.The CE of the two targets increases rapidly first and then tends to saturation with the increase of laser intensity.The SXR radiation intensity of Bi-Pb-Sn alloy plasma was much higher than that of BN and C targets with low atomic number elements.It is expected to obtain high brightness LPP SXR sources in the future by using this alloy target.Low-density foam Ti and Ni targets with different structures were used to investigate the difference of plasma characteristics between foam and solid targets.It is found that the time-of-flight（TOF）peak voltage and average kinetic energy of ions decreased with the decrease of target initial densities.With the increase of the laser intensity,the difference of TOF peak voltage between the solid and foam targets became more obvious.The ion emission of solid target plasmas showed a stronger angular dependence than that of low-density foam targets,which was consistent with the shape of the plume.The variation of ablation mass rate of solid and foam Ni targets with laser intensity demonstrates that the target initial density is an important factor for the plasma shielding effects.The SXR study of low-density Ti plasma showed that properly reducing the target initial density can significantly decrease the ion kinetic energy without affecting the SXR radiation,which is very important for increasing the lifetime of optical elements.The laser beam was divided into two parts using a polarization cube and focused on the planar Sn target surface at a distance of 2.5 mm to generate the colliding plasma.The formation and evolution of the stagnation layer generated by the Sn colliding plasma was recorded and investigated.It is found that the optical radiation intensity of the stagnation layer is much higher than that of single pulse plasma under the same laser energy.Furthermore,the stagnation layer could exist for a longer time with a higher optical radiation intensity,which means that the formation of the stagnation layer effectively converted the ion kinetic energy into optical radiation energy.At the same laser energy,the ion kinetic energy of the colliding plasma was lower than that of the single pulse plasma.The TOF peak voltage and total charge showed a stronger angular dependence for the colliding plasma.When the laser energy was high,the colliding plasma could reduce the ion kinetic energy of ions without changing the EUV CE.The simulation results of colliding plasma by FLASH code showed that the electron temperature and density in the stagnation layer were significantly increased due to the collision.The stagnation layer reheated by a CO₂ laser can obtain better heating effect than that of single pulse plasma.The ultimate goal of the LPP SXR source development is to increase the radiation power of the required source as much as possible and reduce the damage of ion debris to the optical elements in the system.Compared with three LPP routes,Bi（Z=83）and Pb（Z=82）are the non-radioactive elements with the largest atomic number.Bi LPP and Pb LPP have strong radiation in the SXR and Bi-Pb-Sn alloy has a low melting point which is suitable for the preparation of droplet targets.However,the droplet targets route requires pre-pulse laser and high precision of spatial-temporal synchronization,which limits the application of Bi-Pb-Sn alloy plasma as SXR source.The low-density foam targets have a low-density state before laser illumination and lower ion kinetic energy,which are expected to replace the current complex scheme of dual-pulse laser interacting with droplet targets.Furthermore,the stagnation layer generated by the colliding plasma can not only convert part of the kinetic energy of the plasma into optical radiation,but also can be used as the pre-plasma for CO₂ laser to obtain a better heating effect.Therefore,low-density Bi-Pb-Sn alloy targets can be prepared as the candidates for LPP SXR sources in future.Meanwhile,combined with the reheating technology of stagnation layer,it is expected to obtain high CE and clean LPP SXR sources.