Multi-pass Slab CO 2 Amplifiers for Application in EUV Lithography

Transcription

1 Multi-pass Slab CO 2 Amplifiers for Application in EUV Lithography V. Sherstobitov*, A. Rodionov**, D. Goryachkin*, N. Romanov*, L. Kovalchuk*, A. Endo***, K. Nowak*** *JSC Laser Physics, St. Petersburg, Russia **Vavilov Optical Institute, St. Petersburg, Russia ***Gigaphoton Inc., 1200 Manda Hiratsuka, Kanagawa, Japan 2008 International Workshop on EUV Lithography Wailea Beach Mariott, Maui, Hawaii, USA June 10 12, 2008

2 Outline Introduction Multi-pass amplifier arrangements based on CO 2 slab lasers with RF-pumping Computer simulation of the optimum arrangement for a small-scale CO 2 amplifier Experimental verification of simulation results Possibility of power scaling to multi-kw level Summary

3 EUV-source and the problem of laser driver in LPP concept Required power of EUV ~115 W in the intermediate focus Required driver laser average power ~ kw for CE ~ 2 % (Sn) Pulse duration ~10 20 ns, repetition rate ~ 100 khz MOPA system based on CO 2 Lasers a promising approach to design a laser driver ( EUVA ) Increase of efficiency and compactness of the existing MOPA system is needed Use of RF pumped Slab CO 2 Amplifiers a possible solution

4 CW slab CO 2 lasers as possible candidates for development of amplifiers of short pulses Merits: Mature technology Compactness (RF pump power density up to 70 W/cm 3 ) No gas flow (diffusion cooling in the gap of mm) High small-signal gain ( ~ 0.6 m -1 ) High specific power extraction ( W/cm 2 ) Scalability of electrode area to thousands of cm 2 Multiple pass geometry possible

5 Requirements imposed on the multi-pass amplifier optical arrangement Maximum number of beam transits High filling of the gain medium volume by radiation Minimum number of mirrors Low sensitivity to mirror misalignment High stability against self-excitation High optical quality of the amplifier output beam

6 Three-mirror multi-pass telescopic amplifier 3 x z Input beam Output beam 4 5 1, 2 - a concave and a convex mirror of the telescopic system; 3 - concave mirror; 4, 5 - input and output windows; 6-electrode. Merits: A small number of optical elements to be aligned A comparatively good filling of the gain medium The filling could be controlled by varying magnification M of the telescopic system.

7 Two-mirror multi-pass amplifier with plano-concave geometry x z Input beam Output beam 3 4 1, 2-a concave and a flat mirror; 3, 4-entrance and exit windows; 5-electrode Merits: The simplicity of the arrangement Amplifier can be realized by simple modification of a commercially produced slab laser with unstable resonator

8 Numerical model of the multi-pass amplifier Diagram of short-pulse amplifier operation in time I(t) t p Light amplification T p RF pumping t Simulation of light pulse propagation through the amplifier x Сoncave Mirror 0 L i-1 i i+1 m-1 m Flat Mirror Input beam Output beam A G i inp (x,y,z = 0, t) (x,y,i Δz,t) Δz A out (x,y,z = L, t) n i (x,y,i Δz,t)

9 General Description of the Numerical Model Computation of RF discharge RF pump density Distribution of the pump among vibrational levels Computation of vibration kinetics of gain medium Computation of rotational kinetics of gain medium Computation of light propagation over the slab laser Translational and vibrational temperatures Refraction index Medium gain Spatial distribution of radiation intensity in the amplifier

10 Results of small-scale model simulation (Electrode area ~ cm 2 ) The optimum arrangement is the two-mirror plano-concave system The arrangement permits realization of different numbers of pulse transits The pass amplifier arrangement is believed to be feasible A 5-W input beam (10 ns, 100 khz) is expected to be boosted to W at pump power density of ~20-25 W/cm 3 in the gain medium

11 CW slab CO 2 laser with unstable resonator used for experimental verification of the amplifier Electrode area mm 2 Gap between electrodes 2 mm RF frequency 81 MHz Output power in CW mode ~ 175 W RF frequency 81 MHz Gain medium CO 2 : N 2 : He + Xe = 1 : 1 : 6 + 4%; Gas mixture pressure 55 Torr; Pump power 1500 W Pump power density 20 W/cm 3

12 Goals of verification experiments Transformation of CO 2 slab laser into a multi-pass amplifier Investigation of self-excitation at different number of pulse transits Measurement of radiation spectral composition and amplification Recording spatial parameters of output radiation Comparison of input and output radiation temporal profiles

13 General view of laser head of the amplifier with RF power supply 1 Laser head (1) Length 800 mm Tube diameter 160 mm 2 RF power supply (2) ( mm 3 )

14 Multi-pass CO 2 slab amplifier geometry in the experiments Concave mirror R 2 =9000 mm 600 mm Flat mirror R 1 = Input beam 10 mm Electrodes 60 mm 10 mm Output beam Input KCl (ZnSe) window Output KCl (ZnSe) window The number of passes can be changed from 3 to 13 by adjusting the concave and/or flat mirrors

15 Master oscillator (MO) of EUVA Pulse duration 20 ns; Repetition rate from 10 to 100 khz; Output power (at f = 100 khz) 5W; Gaussian output beam Waist size ~ mm Waist location about 50 cm from the polarizer of MO

16 General experimental arrangement Measured in the experiments Spatial parameters of master oscillator output radiation Amplification of radiation in the multi-pass amplifier Saturation in the gain medium Spectral composition of the MO beam Pulse profiles of the input and output radiation

17 Spatial parameters of input beam at different planes along the optical path 1.6 mm 3.2 mm 5.6 mm The intensity distribution of the MO output beam in the plane of its waist The spatial profile of the beam incident at the waveguide inlet

18 The output beam far-field pattern 48 mrad Far-field distribution of output radiation 2λ/d =10 mrad Intensity, a.u. Intensity, a.u a α x, mrad b α y, mrad Profiles of output beam far-field distribution along х (а) and y (b) coordinate

19 Input, output signal power, W Amplification of radiation in the 9-pass and 11-pass amplifier 9 pass P out G* Time, sec G P inp 2 Gain 6 4 Input, output signal power, W pass P out G* Time, sec G P inp Gain Input power P inp (green curve); Output power P out (red); Gain G=P out /P inp (blue); Gain G*=P* out /P* inp (violet). P inp P* inp Amplifier P* out KCl windows P out

20 Amplification of radiation in the 13-pass amplifier (AR coated ZnSe windows) 40 Output signal power, W Input power ~ 4.2 W Time, sec Relative power efficiency η= (P out P inp )/P osc = 21.6% is demonstrated

21 Estimation of saturation in the gain medium (9-pass-amplifier) 30 P out *, W P out * Gain Gain P* inp, W 2

22 Dynamics of input and output power and spectral composition of the MO radiation Input, output signal power, W Time, sec P out G* G P inp Gain P30 t=25 sec 60 sec 70 sec P28 P26 P24 P22 P20 80 sec 90 sec 100 sec Relative power, W Time, sec P20 P22 P24 P26 P sec 120 sec 130 sec

23 The pulse profiles of the input and output radiation Power, a.u input pulses output pulses input and output pulses simultaneously Power, a.u Time, ns Time, ns Time, ns Simultaneous recording of input and output signals at the same detector Optical path difference in output and input channels l d ~ 13.5 m Oscilloscope band width 500 MHz No pulse elongation (~ 14 ns FWHM for both pulses) No pedestal Time delay between pulses ~ 45 ns ( equal to 2 l d /c)

24 Results of experimental verification 9, 11 and 13-pass amplifiers realized in a CO 2 slab laser head ( mm 2, 20 W/cm 3,15 ns, 100 khz ) The output of 42 W achieved in 13-pass amplifier at input power of 4.2 W ( power gain G ~ 10) Saturation of amplification observed Extraction efficiency of 21.6% as compared to CW laser High beam quality of output radiation demonstrated No pedestal in the output pulse profile observed Experimental results agree with theoretical predictions Numerical model can be used for optimization of power-scaled multi-pass amplifiers

25 Simulation of a large-scale CO 2 slab laser amplifier based on 8-kW CW industrial slab laser with unstable resonator Gain medium area mm 2 RF pump density ~ 75 W/cm 3 Parameters to be optimized: 2w 1, R, ρ 1,β 1, α 1, α 2

26 Simulation of a large-scale CO 2 slab laser amplifier based on 8-kW CW industrial laser with unstable resonator Gain distribution after a pulse transit over 9-pass amplifier for R=40 m (a) and 21 m (b) ( Effect of mirror curvature) a) g, 1/cm y, cm b) y, cm z, cm z, cm y, cm y, cm x, mm x, mm

27 Large-scale CO 2 slab laser amplifier (range of parameters and output pulse energy) 2.4 β 1, degree mj R = 40 m 35.6 mj R = 50 m 34.2 mj 2.0 R = 33 m P inp = 2 mj mj 32.2 mj 35.4 mj 1.4 Relative extraction efficiency ~49 % is achievable! Δα, degree Specific combination of β and Δα is required for each R Filling and output power increase at small Δα Decreasing Δα is limited by self-oscillation Maximum output predicted ~ 39 mj ( 3.9 kw at 100 khz)

28 Summary Two-mirror multi-pass slab CO 2 amplifier proposed and tested experimentally (42 W, 100 khz, 15 ns) Numerical model for simulation of multi-pass amplifiers elaborated and verified 3.9- kw output predicted for short-pulse amplifier based on commercially available 8-kW CW slab CO 2 laser ~ 49% relative extraction efficiency achievable Additive summation of parallel amplifier channels is expedient for further power scaling Use of multi-pass slab CO 2 amplifiers is a promising approach to development of a compact EUV source for lithography