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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