Multi-pass Slab CO2

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 LithographyWailea Beach Mariott, Maui, Hawaii, USA

June 10 – 12, 2008

Outline

• Introduction• Multi-pass amplifier arrangements based

on CO2 slab lasers with RF-pumping • Computer simulation of the optimum arrangement

for a small-scale CO2 amplifier• Experimental verification of simulation results • Possibility of power scaling to multi-kW level• Summary

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 ~ 10 – 30 kW for CE ~ 2 % (Sn)

• Pulse duration ~10 – 20 ns, repetition rate ~ 100 kHz

• MOPA system based on CO2 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 CO2 Amplifiers – a possible solution

Merits: • Mature technology• Compactness (RF pump power density up to 70 W/cm3)• No gas flow (diffusion cooling in the gap of 1.5–2 mm)• High small-signal gain ( ~ 0.6 m-1) • High specific power extraction (0.5-1.2 W/cm2

)• Scalability of electrode area to thousands of cm2

• Multiple pass geometry possible

CW slab CO2

lasers as possible candidates for development of amplifiers of short pulses

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

Three-mirror multi-pass telescopic amplifier

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.

6

Input beam

Output beam

xz

4

2

5

1

3

Two-mirror multi-pass amplifier with plano-concave geometry

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

3

Input beam

Output beam

xz

4

251

Numerical model of the multi-pass amplifier

Diagram of short-pulse amplifier

operation in time

I(t)

RF pumpingLight amplification

Tp

t

tp

Input beam Output beam

Сoncave

Mirror

Gi(x,y,i⋅Δz,t)ni(x,y,i⋅Δz,t)

Ainp

(x,y,z = 0, t) Aout

(x,y,z =

L, t)Δz

Flat

Mirror

1 2 3 i-1 i i+1 m-1 m

0 Lx

Simulation of light pulse propagation through the amplifier

General Description of the Numerical Model

Computation of RF discharge

Computation of vibration kinetics of gain medium

Computation of light propagation over the slab laser

Computation of rotational kinetics of gain medium

• RF pump density

• Distribution of the pump among vibrational

levels

• Translational and vibrational

temperatures

• Refraction index

• Medium gain

• Spatial distribution of radiation intensity in the amplifier

Results of small-scale model simulation (Electrode area ~60×600 cm2)

• The optimum arrangement is the two-mirror plano-concave system

• The arrangement permits realization of different numbers of pulse transits

• The 9-11-

pass amplifier arrangement is believed to be feasible

• A 5-W input beam (10 ns, 100 kHz) is expected to be boosted to 40 -

50 W at pump power density

of ~20 -

25 W/cm3 in the gain medium

CW slab CO2

laser with unstable resonator used for experimental

verification of the amplifier

• Electrode area 60×600 mm2

• Gap between electrodes 2 mm• RF frequency 81 MHz• Output power in CW mode ~ 175 W• RF

frequency 81 MHz• Gain medium CO2

: N2

: He + Xe

= 1 : 1 : 6 + 4%;• Gas mixture pressure 55 Torr;• Pump power

1500 W• Pump power density 20 W/cm3

Goals of verification experiments

• Transformation of CO2 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

General view of laser head of the amplifier with RF power supply

Laser head (1)

• Length – 800 mm

• Tube diameter –

160 mm

RF power supply

(2)

(630×380×170 mm3)

11

22

Multi-pass CO2

slab amplifier geometry in the experiments

The number of passes can be changed from 3 to 13 by adjusting the concave and/or flat mirrors

Flat mirrorR1

=∞

Concave mirrorR2

=9000 mm 600 mm

60 m

m

10 m

m

10 m

m

Input beam

Output beam

Electrodes

Input KCl

(ZnSe) window

Output KCl

(ZnSe) window

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 ~ 3.4×3.4 mm

• Waist location about 50 cm from the polarizer of MO

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

The spatial profile of the beam incident

at the waveguide inlet

Spatial parameters of input beamat different planes along the optical path

The intensity distribution of the MO output beam in

the plane of its waist

3.2 mm

1.6

mm

5.6 mm

The output beam far-field pattern

Far-field distribution of output radiation

Profiles of output beam far-field

distribution along х

(а) and y (b) coordinate

48 mrad 2λ/d

=10

mra

d

0 10 20 30 40 50αx, mrad

0

1

Inte

nsity

, a.u

.αy, mrad

a

b

0 10 20 30 40 500

1

Inte

nsity

, a.u

.

0 40 80 120Time, sec

Inpu

t, ou

tput

sig

nal p

ower

, W

Gai

n

Pout

G*G

Pinp2

4

6

8

10

12

0

4

8

12

16

20

24

28

Amplification of radiation in the 9-pass and 11-pass amplifier

• Input power Pinp (green curve); • Output power Pout (red); • Gain G=Pout /Pinp (blue); • Gain G*=P*out /P*inp (violet) .

Pout

PinpP*inp

P*out

KCl windows

Amplifier

9 pass

0 40 80 120

2

4

6

8

10

12

Gai

n

Time, sec

Pout

G*

G

PinpInpu

t, ou

tput

sig

nal p

ower

, W

0

8

4

12

16

20

24

28

11 pass

Amplification of radiation in the 13-pass amplifier

(AR coated ZnSe

windows)

Relative power efficiency η= (Pout

– Pinp

)/Posc

= 21.6% is demonstrated

Input power ~ 4.2 W

Time, sec

Out

put s

igna

l pow

er, W

0 40 80 120 160 2000

8

16

24

32

40

Estimation of saturation in the gain medium (9-pass-amplifier)

0 1 2 3 4

2

6

10

14

0

10

20

30

Pout

, W

Pinp

, W

*

*

Gain

Pout*

Gain

Dynamics of input and output power and spectral composition of the MO radiation

P20

P22P24

P26P28

P30 60 sec 70 sec

80 sec 90 sec 100 sec

110 sec 120 sec 130 sec

t=25 sec

-2

0

4

8

12

16

20

0 50 100 150 200 250-2

12

8

4

Time, sec

Inpu

t, ou

tput

sig

nal p

ower

, W

Gai

n

Pout

G*

GPinp

1000

2000

3000

0 50 100 150 200 2500

P20P22P24P26P28

Time, sec

Rel

ativ

e po

wer

, W

The pulse profiles of the input and output radiation

• No pulse elongation (~ 14 ns FWHM for both pulses)• No pedestal• Time delay between pulses ~ 45 ns ( equal to 2 ld /c)

input pulses output pulsesinput and output pulses

simultaneously

• 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

-0.040

0.040.080.120.160.2

Powe

r ,a.

u .

-0.040

0.040.080.120.160.2

Powe

r,a.

u.

-80 -40 0 40 80 120Time, ns

-80 -40 0 40 80 120

Time, ns-80 -40 0 40 80 120

Time, ns

Results of experimental verification• 9, 11 and 13-pass amplifiers realized in a CO2 slab

laser head (60×600 mm2, 20 W/cm3 ,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

Simulation of a large-scale CO2

slab laser amplifier based on 8-kW CW industrial slab laser

with unstable resonator

• Gain medium area 300×1800 mm2

• RF pump density ~ 75 W/cm3

• Parameters to be optimized:

2w1

, R, ρ1

,β1

, α1

, α2

Simulation of a large-scale CO2

slab laser amplifier based on 8-kW CW industrial laser

with unstable resonatorGain distribution after a pulse transit over 9-pass amplifier

for R=40 m (a) and 21 m (b) ( Effect of mirror curvature)

0 30 60 90 120 150 180

z, cm

0

10

20

30

y, c

m

0 30 60 90 120 150 180

z, cm

0

10

20

30

y, c

m

x, mm

0

10

20

30

y, c

m

x, mm

0

10

20

30

y, c

m0.00120.00140.00160.00180.00200.00220.00240.00260.00280.00300.00320.00340.00360.00380.00400.00420.00440.00460.0048

g, 1/cma)

b)

-0.75 0 0.75

-0.75 0 0.75

Large-scale CO2 slab laser amplifier (range of parameters and output pulse energy)

• 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)

Δα, degree0.481.4

29.9 mJ

Pinp = 2 mJ

0.440.40

β1

, degree

1.6

1.8

2.0

2.2

2.4

32.2 mJ35.4 mJ

35.6 mJ

R = 33 m

34.2 mJ

38.6 mJR = 40 m

R = 50 m

Relative extraction efficiency ~49 % is achievable !

Summary

• Two-mirror multi-pass slab CO2 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 CO2 laser

• ~ 49% relative extraction efficiency achievable• Additive summation of parallel amplifier channels is

expedient for further power scaling• Use of multi-pass slab CO2 amplifiers is a promising

approach to development of a compact EUV source for lithography