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

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1 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 FA C Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government. DEPS

2 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 thermooptic 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) DEPS

3 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 systems Yb:YAG Absorption Spectrum DEPS K Pump Array Laser Wavelength 300 K Wavelength (nm) Spectroscopic data from: Sumida and Fan, OSA Proceedings ASSL 20, 100 (1994) Gain Cross Section (in cm 2 ) σ 21 (10-19 cm 2 ) Energy Levels in Yb:YAG Pump: 940 nm Laser: 1030 nm 3k B 300K, 9k B 100K Yb:YAG Laser Properties Temperature (K) 10 Temperature (K) Saturation Intensity (kw/cm 2 ) I sat (kw/cm 2 )

4 Thermo-Optic Properties of YAG Thermal Conductivity (W/m K) UNDOPED YAG Temperature (K) CTE (ppm/k), dn/dt (ppm/k) Data from: Aggarwal et al., J. Appl. Phys, 98, (2005) 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) DEPS

5 Comparison of Yb and Nd Doped YAG Laser Gain Media for High-Power Applications Laser Gain Medium Parameter Nd:YAG 300 K (4-level laser) Yb:YAG 300 K (~3-level laser) Yb:YAG 100 K (4-level laser) Thermal conductivity (W/cm-K) Thermal expansion (ppm/k) dn/dt (ppm/k) Quantum-limited defect thermal load per unit output Nominal absorption bandwidth (nm) ~4 ~18 ~18 Pump intensity needed for transparency (kw/cm 2 ) < <0.01 Saturation flux at laser wavelength (kw/cm 2 ) Storage time (ms) Better Worse DEPS

6 Cryo-Yb:YAG Power Oscillator LN 2 Dewar Yb:YAG Crystal Thin-Film Polarizers Output Output Coupler Pump Diodes Features Yb:YAG cryogenically cooled with LN 2 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 DEPS *Ripin et al., Opt. Lett., 29, 2154 (2004)

7 300-W Power Oscillator Near-Field Profile at 275 W Output Coupler LN 2 Dewar Yb:YAG Crystals Polarizers Output Power (W) DEPS Unpolarized Linearly Polarized Incident Pump Power (W) Polarization Multiplexing Pump Diodes 308-W average power polarized 64% optical-optical efficiency M 2 ~ 1.2 (wavefront sensor) > 99% linearly polarized OC reflectivity = 25%, L = 1 m, Near-flatflat resonator 455-W achieved with fiber-coupled pumps Fan et al., IEEE Sel. Top. Quan. Elec., 13 (3), pg. 448 (2007)

8 Thermal Sources for Yb:YAG Lasers Pump Photons Absorbed Pump Cooled Yb:YAG Quantum Defect Unabsorbed Pump Laser Output Untrapped Fluorescence Trapped 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 N 2 DEPS

9 Liquid N 2 Costs A 10-kW cryo-yb:yag laser would consume ~1500 liters of L N 2 per day $290/day using a liquid-nitrogen generator that consumes 120 kw electrical power LN 2 tank at MIT/LL micro-electronics laboratory LN 2 does not drive the operating cost of cryo-yb:yag lasers DEPS

10 Impact of Beam Quality A significant attribute of cryo-yb:yag is its inherent ability to generate excellent beam quality with no efficiency penalty. e.g. multiplexing 10 beams at a working distance of 50 cm 10x 1-kW input beams f=50 cm 100-μm spot M 2 Input beam diameter Required lens diameter* cm 6 cm cm 30 cm cm 90 cm *assumes 100% fill factor The excellent beam quality of cryo-yb:yag lasers can simplify spatial multiplexing DEPS

11 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. DEPS

12 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 DEPS

13 Structured Laser Beam Particle flow Detected Time- Domain Waveform Laser Beamlets Particle traversing the SLB y DEPS x 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 *Herzog et al., Appl. Opt., 46(16), (2007)

14 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 DEPS