Plasma EUV source has been studied to achieve 180W of power at λ=13.5nm, which is required for the next generation microlithography

Transcription

1 Acknowledgement K. Nishihara, H. Nishimura, S. Fujioka Institute for Laser Engineering, Osaka University A. Sunahara, H. Furukawa Institute for Laser Technology T. Nishikawa, Okayama University F. Koike, Kitazato University H. Tanuma, Tokyo Metropolitan University C. Suzuki, T. Kato National Institute for Fusion Science

2 Contents Introduction of EUV source using Sn plasmas Radiation-hydro simulation of Sn plasmas Improvement of atomic model of Sn Atomic structure and radiative property of Sn Development of CR-model Validation of the model

3 Plasma EUV source has been studied to achieve 180W of power at λ=13.5nm, which is required for the next generation microlithography illumination optics reflection type mask laser driver target projection optics collector optics laser produced plasma source (LPP) wafer stage

4 Present status of the EUV source development Commercial source (>100W) using Sn plasmas will be developed by U.S. and Japanese companies by , based on the results of fundamental studies. Significant progress has been achieved also in resist, optics and other technologies required to realize EUV lithography. Laser produced plasma (LPP) and discharge pumped plasma (DPP) sources are considered, but both still have scaling problems.

5 Background of plasma x-ray sources Highly charged ions such as Al have been known to emit intense x-ray radiation through K-shell emission. Application was limited to scientific purposes, due to difficulty of producing hot dense plasmas. Coherent radiation is also obtained over variety of wavelength.

6 Background of plasma x-ray sources Highly charged ions such as Al have been known to emit intense x-ray radiation through K-shell emission. Application was limited to scientific purposes, due to difficulty of producing hot dense plasmas. Coherent radiation is also Efficient emission is obtained through 4-4 transitions obtained of highz ions. over variety of wavelength.

7 Background of plasma x-ray sources High-Z (z 50) ions have strong emission in different wavelength in EUV. Wavelength of Sn ions matches the Mo/Si multilayer optics.

8 Sn plasma emits broad spectrum, which originates from a large number of transition from multiply- and inner-shell excited states.

9 Sn plasma emits broad spectrum, which originates from a large number of transition from multiply- and inner-shell excited states. Sn ions are produced in relatively low temperature ( 20 ev) plasmas, which can be produced using industrially feasible moderate laser or discharge pumping.

10 Contents Introduction Radiation-hydro simulation of Sn plasmas Improvement of atomic model of Sn Atomic structure and radiative property of Sn Development of CR-model Validation of the model

11 Modeling of laser produced plasma Sn EUV sources based on computational atomic data Atomic structure codes HULLAC GRASP Sn 12+ Sn 11+ Sn 10+ Sn 9+ energy level benchmark 4d - 4f transition Transmission Atomic kinetic codes JATOM (CRE) opacity 10 Sn T e ~ 30 ev Wavelength (nm) T e ~ 0 ev Intensity (arb. unit) Radiation hydrodynamic code STAR (1d, 2d) (photo excitation) simulation Simulation Wavelength (nm) 20 benchmark Intensity (arb. unit) Experiment atomic data EUV spectra CE experiment Experiment Wavelength (nm) 20 Analytical model Power balance (CE) 10 9 W/cm W/cm W/cm 2 EUV CE (%) Sn Spherical 1µm, 1.2ns Optimization of laser & target conditions 12% 10%8%6%4% 2% Laser intensity (W/cm 2 )

12 Radiation-hydro simulation of LPP EUV sources Hydro simulation is carried out using emissivity and opacity calculated by atomic process code. Output EUV spectrum and conversion efficiency are calculated for various conditions. *Approximation method is used to take photo-ionization into account. EUV Conversion efficiency (%) Experiment SimulaCon with photo ionizacon SimulaCon with CRE Laser intensity (W/cm 2 ) CE = output EUV energy(13.5nm, 2%BW ) laser energy

13 Simulation reproduces Sn emission spectrum Sn, planar target 1 µm, 2 ns Intensity (arb. unit) Intensity (arb. unit) laser intensity laser intensity laser intensity 9 x W/cm 2 3 x W/cm 2 9 x W/cm 2 experiment Wavelength (nm) simulation 15 Wavelength (nm) Intensity (arb. unit) Intensity (arb. unit) Wavelength (nm) 15 Wavelength (nm) 20 Intensity (arb. unit) Intensity (arb. unit) Wavelength (nm) 15 Wavelength (nm) 20 20

14 Radiative property of Sn suggests CO 2 laser pumped plasmas to achieve high output power. Efficiency increases at low density because of less satellite contribution and narrow emission spectrum. spectral efficiency CE 8% may be obtained by optimizing the plasma profile

15 Motivation toward the improvement of atomic model Accurate radiation-hydro model is required to design industrial EUV source. Ad hoc model looks fine, however, for validation of data, and for other applications, reproducible modeling method is more useful.

16 Calculation of coefficients of radiative transfer for the hydrodynamics simulation of Sn plasmas (1) Calculate atomic data using the Hullac code. (2) Construct a CR-model based on nl averaged level, and calculate population at CRE. (3) Calculate emissivity and opacity taking the detailed structure of strong lines into account. in-band emissivity [W/cm 3 ] absorption coefficient [/cm]

17 Improvement of atomic model for EUV sources Use of accurate wavelength of emission lines. Determination of dominant emission channels.

18 Sn ions emit EUV radiation in the 13.5nm band through 4d-4f transition. H. Tanuma, J. Phys. Conf. Ser. 58, 231 (2007).

19 Sn ions emit EUV radiation in the 13.5nm band through 4d-4f transition. Charge exchange Spectroscopy analyzer ECR ion source ion selector EUV spectrometer

20 Accurate wavelength is required for the optimization of EUV source using hydro-simulation As optical depth increases, absorption feature appears. Absorption peaks coincide with those observed in the charge exchange spectroscopy. EUV CXS 20

21 Accurate wavelength is required for the optimization of EUV source using hydro-simulation Effect of CI decides the wavelength and spectral profile of 4d-4f + 4p-4d transition. Calculated wavelength should be verified by experiment.

22 Wavelength of 4d-4f + 4p-4d transitions are corrected according to the measurement. We determine shift using least square fitting between calculated and experimental spectrum. calculation experiment (Hullac) (CXS) [A] wavelength width wavelength width shift

23 Wavelength of spectator satellite lines Wavelength correction analogous to resonance lines. Wavelength and profile are calculated including dominant CI, such as 4di-1nl, 4di-24fnl, 4p54dinl. Spectrum converges to the resonance line at the limit of spectator electron with n. 4d-4f + 4p-4d resonance lines 23

24 Improvement of atomic model for EUV sources Use of accurate wavelength of emission lines. Determination of dominant emission channels.

25 Satellite line emission from multiple- and innershell excited configurations should be included.

26 Methods to determine the atomic model Levels are defined by nl. We include levels with low excited energy, which are likely to populate. 4s4d i nl 4p 5 4d i nl 4d i-2 5snl 4d i-2 4fnl 5pnl 5dnl 5fnl 4d i-1 nl

27 We define a group of levels which have same core configuration and one excited electron (4d i-1 nl). We decide number of groups of levels to be included in the model, according to the excitation energy of core state. 4s4d i nl 4p 5 4d i nl 4d i-2 5snl 4d i-2 4fnl 5pnl 5dnl 5fnl 4d i-1 nl

28 Set of groups is chosen for each ion, because level structure changes depending on ion charge. Sn 4+ Sn 5+ Sn 6+ Sn 7+ Sn 8+ Sn 9 Sn 10+ Sn 11+ 4d 9 4d 8 4d 7 4d 6 4d 5 4d 4 4d 3 4d 2 4d 8 5s 4d 7 5s 4d 6 5s 4d 5 5s 4d 4 5s 4d 3 5s 4d 2 5s 4d5s 4d 8 5p 4d 7 5p 4d 6 5p 4d 5 5p 4d 4 5p 4p 5 4d 5 4p 5 4d 4 4p 5 4d 3 4d 8 5d 4d 7 5d 4d 6 4f 4d 5 4f 4d 4 4f 4d 3 5p 4d 2 4f 4d4f 4d 8 6s 4d 7 4f 4d 6 5d 4p 5 4d 7 4p 5 4d 6 4d 3 4f 4d 2 5p 4d5p 4d 7 5s 2 4d 7 6s 4p 5 4d 8 4d 5 5d 4d 4 5d 4d 3 5d 4d 2 5d 4d5d 4d 8 4f 4d 6 5s 2 4d 6 6s 4d 5 6s 4d 4 6s 4d 3 6s 4s4d 4 4s4d 3 4d 8 6p 4d 7 6p 4d 5 5s 2 4d 4 5s 2 4d 3 5s 2 4d 2 5s 2 4d 2 6s 4d6s 4d 7 5s5p4p 5 4d 9 4d 6 6p 4d 5 6p 4d 4 6p 4d 3 5f 4d 2 5f 4p 5 4d 2 5s 4d 8 6d 4d 7 5f 4d 6 5f 4d 5 5f 4d 4 5f 4d 3 6p 4d5s 2 4d5f

29 Problems in developing atomic model A large number of atomic levels must be generated, and tested in terms of level energy and transition probability. Validation of the atomic model relies on theoretical methods, because comparison with experiment is limited to very few conditions.

30 String library to manipulate information of electron configuration is developed 4d 8 4d 7 4f 4d 6 excitation ionization 4d 8 4d 7 4f 4d-4f transition We usually use string expression of configuration to discuss atomic process. If string expression is manipulated using computer programs, it will be useful to analyze atomic model with a large number of levels. String expressions are already used in atomic physics and kinetics codes (Hullac).

31 String library with simple atomic physics functions is developed Input configuration string 4s 2 4p 5 4d 10 Output configuration string 4s 2 4p 5 4d 10 4f 1 sort by nl 4s 4p 4d check range of nl and n e add one electron + f 4s 4p 4d f *Information of configuration is stored using associative array.

32 input configuration 1s 2 2s 2 2p 6 3s 1 3s-3p -3s +3s excitation 1s 2 2s 2 2p 6 3p 1 ionization 1s 2 2s 2 2p 6 recombination 1s 2 2s 2 2p 6 3s 2 Low excitated states of He-like ion Configurations of Li-like ion 1s 2 1s 3 1s 2 2s 1 1s 2 2s 1 1s 1 2s 2 1s 2 2p 1 1s 1 2s 1 2p 1 1s 1 2s 1 1s 2 2p 1 1s 1 2s 1 2p 1 1s 1 2p 2 1s 2 3s 1 1s 1 2s 1 3s 1 1s 1 2p 1 3s 1 1s 1 2p 1 1s 1s 2 3p 1 1s 1 2s 1 3p 1 1s 1 2p 1 3p 1 core configurations 2s 1s 2 3d 1 1s 1 2s 1 3d 1 1s 1 2p 1 3d 1 2p 3s 3p 3d TreeSet of electron orbits to be added to excited the core electrons configuration

33 Use of string library to find likely strong emission lines and include the effect of CI To calculate 4f state, 4p state should be included to take CI into account 4s 2 4p 5 4d 2 4s 2 4p 6 4f 4s4p 5 4d 3 4s4p 6 4d4f 4s 2 4p 6 4d ground state Resonant 4d-4f transition should be strong 4s4p 6 4d 2 4s -1 state may also be populated

34 New methods of developing atomic model A large number of atomic levels must be generated, and tested in terms of level energy and transition probability. Validation of the atomic model relies on theoretical methods, because comparison with experiment is limited to very few conditions.

35 Atomic model is decided by increasing number of groups of levels until zbar and opacity converges groups of configurations 1 4d i-1 nl 2 4p 5 4d i nl 3 4d i-2 5snl 4 4d i-2 5pnl 5 4d i-2 4fnl increase

36 Model dependence of spectral opacity (CRE) n i =10 21 /cm 3, T e =30eV Emissivity and opacity converges by including more than 5 groups of configurations.

37 Model dependence of emission spectrum (LTE) n i =10 21 /cm 3, T e =30eV r=0.5µm As optical depth increases, spectrum becomes broad due to increase of intensity of satellite lines.

38 Opacity of tamped Sn sample heated by radiation (T R =50eV) is measured. n i =8x10 20 /cm 3, l=1.24x10-4 cm -measurement of ρr Te 30eV -code calculation

39 Calculated opacity agree with measurement within experimental uncertainty. 24eV transmission experiment calculation Wavelength [nm]

40 Calculated opacity agree with measurement within experimental uncertainty. transmission Dependence of square error on temperature is weak. Position of peak agrees at 40eV, whereas profile is similar at 24eV, which suggests plasma non-uniformity. 40eV Wavelength [nm]

41 Calculated spectrum using new opacity data still disagree with measurement with respect to the detailed structure in the 13.5nm band.

42 Summary We show atomic model of Sn, which is useful for optimizing the laser plasma EUV source. Methods to generate atomic model automatically are proposed. For validation of the model, mean charge and spectral emissivity are converged in terms of atomic model. Detailed comparison of emission spectrum is in progress.