Inves&ga&on of atomic processes in laser produced plasmas for the short wavelength light sources

Transcription

1 Inves&ga&on of atomic processes in laser produced plasmas for the short wavelength light sources Akira Sasaki Quantum Beam Science Directorate Japan Atomic Energy Agency

2 Introduc&on EUV source at λ=6.5nm is interested for future lithographic applica&ons. Mul&ple charged ions of atomic elements with 50 Z 80 show strong emission in EUV wavelength region. Emission spectrum and CE from Gd and Tb LPP sources are es&mated by applying the theore&cal model used to inves&gate Sn plasmas. Subjects for theore&cal inves&ga&on of short wavelength light sources are discussed.

3 Proper&es of XUV emission. Gd and Tb ions have emission through 4d 4f transi&ons around λ=6.5nm. At higher temperature, narrower emission spectrum is observed from 4d open shell ( Rh to Rb isoelectronic sequence) ions. S. S. Churilov, Phys. Scr. 80, (2009).

4 Proper&es of 4d 4f + 4p 4d transi&on arrays Similar spectral profile from Sn to W. Wavelength decreases as atomic number increases. Weak dependence on ion charge.

5 Tb plasma T e =105eV, n i =10 19 /cm 3 4d 4f + 4p 4d transi&on array dominates the emission and absorp&on in photon energy 190eV band.

6 Calcula&on of CR (collisional radia&ve) model We calculate the level popula&on using the CR model, and coefficients of radia&ve transfer by applying detailed spectral profile of emission lines. 4d ions of Gd is obtained in the T e range of eV at the typical density of n i =10 19 /cm 3. 4d ions

7 Spectral efficiency of Sn, Xe, Nd and Gd T e [ev] Sn Nd Xe Gd n i [/cm 3 ]

8 Calculated spectrum of Tb assuming exponen&al density profile reproduces the experiment, except for side peaks originates from lower charged ions. T e =80eV n 0 =10 19 /cm 3 scale length = 50µm spectral shih= 0.15nm S. S. Churilov, Phys. Scr. 80, (2009).

9 Es&ma&on of CE using power balance model Self similar profile of plasma is sustained by a balance of incoming laser power and outgoing thermal, kine&c and radia&ve power at the cri&cal density point. target surface cri&cal density point n 0 plasma I laser log(n i ) T e I kin + I thermal + I rad I laser = I kin + I thermal + I rad I rad = CE = ( ) j ν n i x ( ),T e [ ] dxdν exp κ ν ( n ( i x ),T )d x e n i x ( x) = n 0 exp x, c s = zkt e c s t I EUV I kin + I thermal + I rad M i K. Nishihara, et al., Phys. Plasmas 15, (2008)

10 Op&miza&on of pumping condi&ons Radia&ve power loss is calculated using emissivity and opacity, taking absorp&on of radia&on in the plasma into account. Laser intensity is determined from the radia&ve power loss, which is calculated from ini&al density n 0, temperature T e, and pulse dura&on τ. At each n 0 and T e, op&mum pulse dura&on can be determined.

11 Op&mum point shihs towards higher Te and ni Sn 1019 ni [/cm3] Nd Te [ev] Te [ev] Te [ev] Te [ev] ni [/cm3] laser intensity [W/cm2] 8 CE(%) 4 40 Xe 1019 ni [/cm3] Gd ni [/cm3] 1020

12 CE of Tb sources Max. SE is obtained at zbar slightly less than 20. CE comparable to Sn sources can be obtained. laser intensity [W/cm 2 ] zbar spectral efficiency (%) CE (%)

13 Discussion on the result of Gd and Tb sources Calcula&on shows EUV sources based on 4d 4f + 4p 4d transi&on array can be scalable to λ=65å, using the similar atomic structure of 4d open shell ions. Half the emission wavelength results in 10 &mes increase of pumping power from Sn sources, as expected from Planck s law. Much higher pumping power ( W/cm 2 ) and short demands innova&on in the laser technology. Theore&cal methods should be improve for the op&miza&on of the source with high accuracy.

14 Subjects of atomic codes Calculated wavelength differs from experiment due to the effect of CI. Calcula&on including CI is impossible for 4f open shell ions with any exis&ng atomic codes. Development of new atomic code is necessary. Tb T e =80eV n 0 =10 19 /cm 3 Iden&fica&on of emission lines is very difficult for complex spectrum. Machine learning algorithms help spectroscopy.

15 Agreement between calc. and exp. some&me occurs at more than one condi&ons, making determina&on of T e and n i difficult. Result from each code is different depending on the atomic model and rate coefficients (a) 26eV exp. calc wavelength [nm] 1.0 (b) exp eV calc wavelength [nm] Code benchmark result Sn n i =8x10 20 /cm 3 A. Sasaki, et al. J. Appl. Phys.107, (2010)

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17 Subjects of radia&on hydrodynamics simula&on (1) As the op&cal thickness of the plasma increases, difference between calcula&on and experiment. Coupled hydrodynamics, atomic process, and radia&on transport is difficult to calculate.

18 Subjects of radia&on hydrodynamics simula&on (2) Ini&al laser and target interac&on some&mes creates nonuniform structure in the plasma, which is difficult to calculate. Mechanism of non uniform abla&on is also important for debris forma&on and development of the method of mi&ga&on. D. Nakamura, et al., J. Phys. D: Appl. Phys. 41 (2008)

19 Summary For the development of short wavelength light sources, not only laser technology improved theore&cal methods will also be useful. To overcome difficulty to produce high temperature plasmas. To find new atomic transi&ons which have strong emission in the λ=6.5nm region. Subjects: Development of a new atomic code. Development of new methods to calculate non uniform structure and radia&on transport in plasmas.