high-power cryogenic yb:yag lasers and optical particle...

DEPS 2007- 1JDH 9/14/2008

MIT Lincoln Laboratory

High-power Cryogenic Yb:YAG Lasers and Optical Particle Targeting for EUV

Sources *J.D. Hybl**, T.Y. Fan, W.D. Herzog, T.H. Jeys, D.J.Ripin, and A. Sanchez

2008 International Workshop on EUV Lithography

11 June 2008

* This work is sponsored by the Department of the Air Force under Air Force Contract FA8721- 05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government.

MIT Lincoln LaboratoryDEPS 2007- 2JDH 9/14/2008

Motivation for Cryo-cooling Yb:YAG Lasers

• EUV LPP sources require laser sources with– High power– High efficiency– Short-pulse waveforms (5-15 ns at multi-kHz PRF)– Good beam quality?

• Average power and beam quality of solid-state lasers are limited by thermo- optic effects

– Thermo-optic distortion– Thermally-induced birefringence

• Cost, size, and weight of solid-state laser systems are generally limited by low efficiency

• Cooled (~100 K) Yb:YAG offers potential for improvements in both of these areas:

– Reduced thermo-optic effects for power scalability with good beam quality– Higher electrical-to-optical efficiency (~2x Nd:YAG in pulsed waveform)


Low-Temperature Spectroscopic Properties

• Yb:YAG is a four-level system at low temperature

• Small quantum defect, 9%• Saturation intensity decreases by ~ 5x• Broad absorption band maintained at low

temperature– Pump wavelength control requirements are

less stringent than for Nd:YAG systemsYb:YAG Laser Properties

Spectroscopic data from:Sumida and Fan, OSA Proceedings ASSL 20, 100 (1994)

Energy Levels in Yb:YAG

Laser: 1030 nm

Pump: 940 nm

3kB T @ 300K, 9kB T @ 100K

Yb:YAG Absorption Spectrum

900Wavelength (nm)

0

2

4

6

8

10

920 940 960 980 1000 1020 1040

Laser Wavelength

77 K

300 K

PumpArray

100 150 200 250 300

0.5

1.0

1.5

2

4

6

8

10

Temperature (K)

Gai

n C

ross

Sec

tio

n (

in 1

0-19

cm

2 )

Sat

ura

tio

n In

ten

sity

(kW

/cm

2 )

0.5

1.0

1.5

100 150 200 250 300

2

8

6

4

10

σ 21

(10-

19cm

2 ) Isat (kW/cm

2)

Temperature (K)


Thermo-Optic Properties of YAG

• Key material properties (κ, α, dn/dT) scale favorably at lower temperature in bulk single crystals

• Thermo-optic effects expected to be > 30x smaller in 100 K Yb:YAG compared with 300 K Nd:YAG

– ~ >12x smaller than 300 K Yb:YAG (assuming equal optical efficiencies)

10

15

20

25

30

35

40

45

50

0

1

2

3

4

5

6

7

8

100 150 200 250 300

UNDOPED YAGTh

erm

al C

ondu

ctiv

ity (W

/m K

) CTE (ppm

/K), dn/dT (ppm

/K)

Temperature (K)

Data from:Aggarwal et al., J. Appl.

Phys, 98, 103514-1 (2005)


Comparison of Yb and Nd Doped YAG Laser Gain Media for High-Power Applications

Laser Gain Medium ParameterNd:YAG

300 K(4-level laser)

Yb:YAG300 K

(~3-level laser)

Yb:YAG100 K

(4-level laser)

Thermal conductivity (W/cm-K) 0.11 0.11 0.4

Thermal expansion (ppm/K) 6.2 6.2 2

dn/dT (ppm/K) 7.9 7.9 0.9

Quantum defect thermal load 0.32 0.111 0.111

Nominal absorption bandwidth (nm) ~4 ~18 ~18

Pump intensity needed for transparency (kW/cm2)


Cryo-Yb:YAG Power Oscillator

Pump Diodes

Output Coupler

LN2 Dewar

Yb:YAGCrystal

Thin-Film Polarizers

• Features– Yb:YAG cryogenically cooled with LN2 cryostat– Efficient end-pumping with high-brightness diode pump lasers – Yb:YAG crystal indium soldered to copper mount for heat-sinking– Large beam radius to avoid optical damage

Output

*Ripin et al., Opt. Lett., 29, 2154 (2004)


300-W Power Oscillator

Near-Field Profile at 275 W

Pump Diodes

Output Coupler

Polarizers

PolarizationMultiplexing

LN2 Dewar

Yb:YAGCrystals

0 100 200 300 400 5000

50

100

150

200

250

300

350

Out

put P

ower

(W)

Incident Pump Power (W)

Unpolarized Linearly Polarized

• 308-W average power polarized • 64% optical-optical efficiency• M2 ~ 1.2 (wavefront sensor)• > 99% linearly polarized• OC reflectivity = 25%, L = 1 m, Near-flat-

flat resonator• 455-W achieved with fiber-coupled

pumps– Fan et al., IEEE Sel. Top. Quan. Elec., 13

(3), pg. 448 (2007)


Thermal Sources for Yb:YAG Lasers

• Typical measured heat load is 0.3 W dissipated per W output– 9% of absorbed pump power dissipated in Yb:YAG by quantum defect– Additional contribution to cold-tip thermal load from trapped fluorescence

• Modest amounts of liquid nitrogen are required– A 10-kW laser (3 kW of heat) will consume 1 LPM of L N2

Fluorescence

LaserOutput

Quantum Defect

UnabsorbedPump

Untrapped

Trapped

PumpPhotons

Cooled Yb:YAG

AbsorbedPump


Liquid N2 Costs

• A 10-kW cryo-Yb:YAG laser would consume ~1500 liters of L N2 per day– $290/day using a liquid-nitrogen generator that consumes 120 kW electrical

power

LN2 tank at MIT/LL micro-electronics laboratory

LN2 does not drive the operating cost of cryo-Yb:YAG lasers


Impact of Beam Quality

• A significant attribute of cryo-Yb:YAG is its inherent ability to generate excellent beam quality with no efficiency penalty.

f=50 cm10x 1-kW input beams 100-μm spot

e.g. multiplexing 10 beams at a working distance of 50 cm

M2 Input beam diameter Required lens diameter*2 1.3 cm 6 cm10 6.4 cm 30 cm30 20 cm 90 cm

The excellent beam quality of cryo-Yb:YAG lasers can simplify spatial multiplexing

*assumes 100% fill factor


Current Work

• MIT-LL leads development in the Advanced Track Illuminator Laser (ATILL) program

– Single beam line– Multi-kW average power in 15-ns pulses at 5 kHz PRF– Near diffraction-limited beam quality (1.5x)

• Achieving near-diffraction-limited beam quality at this power is a significant technical challenge for the ATILL program

– Relaxed beam quality requirement (~2x D.L.) for EUV LPP sources simplifies laser design

– Allows spatial multiplexing as a path to achieve the desired power and pulse repetition rate.


Optical Particle Targeting

• The Structured Laser Beam (SLB) provides individual particle trajectories with in a flow stream

– Particle position within sample volume– Particle velocity

• In EUV sources, the SLB could be used to: – Improve shot-to-shot energy stability of EUV light

Improve laser-particle targeting performance– Provide real-time diagnostics for droplet generation systems

Particle velocity fields Spatial map of particle flow


Structured Laser Beam

Laser Beamlets

Particle flow Detected Time- Domain Waveform

1. A diode laser beam is split into four beams with different orientations

2. A particle’s scattering signal is decoded into its position and velocity

• Position accuracy of 8-μm rms* • Velocity accuracy to 1%• Can be scaled to parameters of interest

for LPP

Particle traversing the SLB

x

y

*Herzog et al., Appl. Opt., 46(16), 3150-3155 (2007)


Summary

• Cryogenic Yb:YAG offers a path to high-performance lasers for EUV LPP sources

– Good beam quality for simplified multiplexing– High-average power handling for PRF scaling

• Current effort at MIT/LL for power-scaling cryo-Yb:YAG matches well to the requirements of EUV LPP sources

– Multi-kW power level with a pulsed waveform (15-ns pulses)

• The structured laser beam provides a relatively simple technology for measuring trajectories of the target particles

– Potential as an online diagnostic of particle flow

High-power Cryogenic Yb:YAG Lasers and Optical Particle Targeting for EUV Sources * Motivation for Cryo-cooling Yb:YAG LasersLow-Temperature Spectroscopic PropertiesThermo-Optic Properties of YAGComparison of Yb and Nd Doped YAG Laser Gain Media for High-Power ApplicationsCryo-Yb:YAG Power Oscillator300-W Power OscillatorThermal Sources for Yb:YAG LasersLiquid N2 CostsImpact of Beam QualityCurrent WorkOptical Particle TargetingStructured Laser BeamSummary

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