Simulation Research Of Pulse CO₂ Laser Ablated Tin Target Plasma

Extreme-ultraviolet?EUV?source at 13.5 nm from pulse CO2 laser-produced tin plasma is the most promising candidate of the next generation EUV lithography?EUVL?.Increasing the conversion efficiency?CE?of laser energy to EUV emission and decreasing the plasma debris are the key technologies of the development of EUV source.Electron temperature and Electron density determine the radiation characteristics,while ion temperature and density determine the debris characteristics.Thus,the study on the properties of tin plasma is important to obtain stronger EUV intensity and higher CE.With the help of 1-D radiation hydrodynamic code MULTI,we simulate the distribution of electron temperature,electron density,ion temperature,ion density and expansion velocity during the process of plasma expansion.Then,we discuss the influence of target material,initial target density,laser wavelength,pulse duration,peak power intensity,laser tailing,etc.on the emission from laser-produced plasma?LPP?and the characteristics of plasma debris.First of all,the MULTI code is introduced detailedly and then the specific expansion processes of tin plasmas produced by pulse CO2 and Nd:YAG laser are compared,revealing that laser is absorbed mainly by inverse bremsstrahlung?IB?absorption,and most laser energy is deposited below critical electron density area.The critical electron density of pulse CO₂ and Nd:YAG laser are about 10¹⁹ cm^-3,10²¹ cm^-3 respectively.Thus Nd:YAG laser produces a hotter and denser plasma,which results in more EUV reabsorption and electron-ion recombination.Therefore,the CE and spectral purity of plasma produced by CO2 laser are higher than that by Nd:YAG laser,and CO2 laser is more conductive to the development of EUV source.Due to the advantages of pulse CO2 laser in the application of laser produced EUV light sources,this thesis focus on the research of CO2 laser produced plasma.Firstly,the process of pulse CO2 laser interacting with tin and xenon target is simulated.It's found that electron temperature of xenon plasma is higher and the central wavelength of EUV emission is shorter.Then,we simulate the process of pulse CO2 laser irradiation on tin target with different density.We find that decreasing the target density does not change the absorption mechanism of laser and the main plasma parameters.As a result,the characteristics of the EUV emission and the plasma debris remain unchanged.In the end,we study the influence of pulse duration,peak power intensity and laser tailing on tin LPP.High-power CO2 laser increases the electron temperature,and thus enhances EUV emission without impairing spectral purity.Long CO2 pulse prolongs EUV radiation duration,which enhances EUV emission intensity without decreasing the CE.The higher power and longer pulse of CO2 laser result in the increasing debris.However,debris can be mitigated effectively using the electromagnetic field and ambient gas.Therefore,CO2 laser with the higher power and longer pulse is more suitable for EUV radiation.The peak power density and the pulse duration of the CO2 laser we use are 10¹00 W/cm²,300 ns respectively.