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. DEPS 2007-1
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 2007-2
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 10 8 6 4 2 Yb:YAG Absorption Spectrum 0 900 DEPS 2007-3 77 K Pump Array Laser Wavelength 300 K 920 940 960 980 1000 1020 1040 Wavelength (nm) Spectroscopic data from: Sumida and Fan, OSA Proceedings ASSL 20, 100 (1994) Gain Cross Section (in 10-19 cm 2 ) σ 21 (10-19 cm 2 ) Energy Levels in Yb:YAG 1.5 1.5 1.0 1.0 0.5 0.5 Pump: 940 nm Laser: 1030 nm 3k B T @ 300K, 9k B T @ 100K Yb:YAG Laser Properties 100 150 200 250 300 100 150 200 250 300 Temperature (K) 10 Temperature (K) 8 6 4 2 10 8 6 4 2 Saturation Intensity (kw/cm 2 ) I sat (kw/cm 2 )
Thermo-Optic Properties of YAG Thermal Conductivity (W/m K) 50 45 40 35 30 25 20 15 UNDOPED YAG 10 0 100 150 200 250 300 Temperature (K) 8 7 6 5 4 3 2 1 CTE (ppm/k), dn/dt (ppm/k) Data from: Aggarwal et al., J. Appl. Phys, 98, 103514-1 (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 2007-4
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) 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-limited defect thermal load per unit output 0.32 0.11 0.11 Nominal absorption bandwidth (nm) ~4 ~18 ~18 Pump intensity needed for transparency (kw/cm 2 ) <0.01 1.5 <0.01 Saturation flux at laser wavelength (kw/cm 2 ) 2.6 9.8 2.0 Storage time (ms) 0.24 0.95 0.86 Better Worse DEPS 2007-5
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 2007-6 *Ripin et al., Opt. Lett., 29, 2154 (2004)
300-W Power Oscillator Near-Field Profile at 275 W Output Coupler LN 2 Dewar Yb:YAG Crystals Polarizers Output Power (W) 350 300 250 200 150 100 50 0 0 100 200 300 400 500 DEPS 2007-7 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)
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 2007-8
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 2007-9
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* 2 1.3 cm 6 cm 10 6.4 cm 30 cm 30 20 cm 90 cm *assumes 100% fill factor The excellent beam quality of cryo-yb:yag lasers can simplify spatial multiplexing DEPS 2007-10
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 2007-11
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 2007-12
Structured Laser Beam Particle flow Detected Time- Domain Waveform Laser Beamlets Particle traversing the SLB y DEPS 2007-13 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), 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 DEPS 2007-14