Development of High-peak, High-average-power LD Pumped Solid-State Laser System for EUV Generation

Transcription

1 Development of High-peak, High-average-power LD Pumped Solid-State Laser System for EUV Generation Koji Tsubakimoto, Hidetsugu Yoshida, Hisanori Fujita, Masahiro Nakatsuka, Noriaki Miyanaga, Yasukazu Izawa, Kunioki Mima Institute of Laser Engineering, Osaka University October, 6, World Trade Center, Barcerona, Spain

2 Conceptual Design of 5 kw Laser System Requirements for laser system Output :1-3 J/pulse Pulse width:1-1 ns Repetition :5 - khz Stability :±.3 % Basic concepts of ILE Oscillator :Fiber laser Main Amplifier:CW LD Pumped Nd:YAG amplifier Output:1 J/pulse Pulse width:1-1 ns Repetition:5 khz 5 W (Regen. Amp) Fiber Front-end 5 W W x 6 mm Rod, pass x 4 mm Rod, pass ~5 W /SBS Pulse Compression /Higher Harmonic Gen. /Control of Focus Pattern to Target Chamber 8 x 1 mm Rod, pass SBS PC Mirror

3 Pre-Amplifier Aperture Polarizer Spatial Filter Rod 9 degree Rotator Faraday Rotator Mirror The lens sets (spatial filter) compensate the thermal lens. 9 degree rotator and double pass arrangement also compensate thermal birefringence. 1st stage (4 mm ) nd stage (6 mm ) Output(W) Input vs Output Input(W) Near-Field Pattern Far-Field Pattern Output(W) pass pass Input(W)

4 Main Amplifier Chain Amplifier Head

5 Input-output property of main amplifier chains -1 khz operation from Pre-Amp Aperture NFP FFP Input1 Rod 1 mm,.8 at% LD:.3 kw CW / head Pulse Width 7 ns Rep. Rate 1 khz Input power control Output (W) Spatial filter 1 st stage nd stage Reduce beam size Propagation loss Output Input (W) 34 shutter Alignment Laser (13 nm) Focus Lens f = 5 = 5 1 st Stage after 1 Pass 1 st Stage after Passes 1 st Stage Passes + nd Stage 1 Pass Beam Diameter 7. mm 1 st Stage Passes + nd Stage 1 Pass Beam Diameter 6.4 mm For EUV Ex. Vacuum chamber

6 Problems caused by High power density pumping Gain dropping Spontaneous emission cross section depends on temperature. Gain range shifts with temperature K/nm from A. Kaminski, Laser Crystals Lower level population of laser transition increases. Beam quality is degraded by thermal lens effect. d T dr 1 dt r dr 3 19 o T 1 cm, T in C Q k If the pumping is uniform and thermal conductivity is constant, thermal lens is ideal parabolic form. Q T( r) T( r ) r r 4k Nd:YAG A. Rapaport, et. al. Appl. Opt. V41,N33, 75. Nonuniform pumping Temperature dependence of thermal conductivity.14 W/cmK(Room).1 W/cmK(1 ) High order wavefront distortions occur. We improved the cooling system of the laser rod. Center spot disappears with free propagation.

7 Gain Property for two stages (1 mm,.6 at %) From Front-End: a few W, 1 ns, 5 khz Thermal lens effect is compensated by adjusting distance of spatial filters. Measurement Gain Gain Saturation?? We checked fluorescence spectrum. Center wavelength sifted according to pump power Improvement of cooling system for Nd:YAG rods Pump Power (W)

8 Center wavelength of gain on each amplifier Center wavelength Fiber Osc. Reg-Amp Pre-Amp 4mmRod 6mmRod Main Amp (Variable) (.6 at%) (.8 at%) (1.8 kw pump old) (. kw pump new) 164 Nd:YAG OSC Increasing pumping power Gain property of main amplifier Nd:YAG 1mmφ.8at% New cooling system Old cooling system Gain PumpPower(W)

9 Measured patterns near image-points after amplification Before image-point On image-point After image-point Propagation property is seemed like Bessel beam.

10 Numerical calculations of temperature distribution in laser rod Temperature(K) d T dr dt r dr Q k T:Temperate r : Radius Q :Thermal load Radius of Rod:1 mm Heat transfer coefficient:.6,3. Pumping power:1,15, W Coolant temperature: W/OK(T) Radius(cm) Thermal Conductivity A k( T) B T T( r) Pumping power 1W K const. 1W 15W W a r b r a.6-9.x x x x1-4 N. P. Padture and P. G. Klements : J. Am. Ceram. Soc. 8 (1997) 118. c r b d r c x x x x1-4 e d x x x x1-1

11 Effect of thermal distortion on beam propagation Radius of Rod:1 mm Heat transfer coefficient:.6 Pumping power: W Coolant temperature: First order A k nmesh 1 Is the spot caused by first order distortion? k Third order A k nmesh k Forth order A k nmesh k 56

12 Is the laser system required to keep supplying the high reputation pulse? Laser system will supply the laser pulse when the stepper requires. Rod temperature vs averaged pump power CW pumping Pulse pumping Rod diameter 1mm, Pumping length 7mm Pumping pulse (~Hz) Temperture(K) h=.6 h=3.3 Laser pulse (~ khz) Some advantages by pulse pumping operation PumpPower(W) Thermal load is relieved. The diopter of the thermal lens decreases. The temperature of the laser medium becomes low. The gain increases.

13 System arrangement and Operation Mode of Main-amp Chain from PreAmp Apeture FFP Input power control by Half Wave Plate Input1 Alignment Laser (13 nm) Slide Shutter NFP Macro-pulse Operation Pumping pulse power ROD:1 mmφ,.8 at% LD: 3. kw (Duty 5 %) Output1 5 Hz pump Duty 5 % Pumping pulse 1 ms Focus Lens f = 5 = 5 Laser pulse Pumping power Average1.5 kw Peak 3. kw

14 Input-Output property of Intermittent Operation Output(W) Input vs Output 4unit-pass 4unit-1pass 4unit-pass+4unit-1pass 4unit-1pass(Duty5%) 8unit-1pass(Duty5%) Pulse shape Intensity (a.u.) FWHM 4 ns.8 Input.6 Output Time(ns) Output patterns Input(W) Near-field Far-field 1pulse energy 95 mj(1 khz) 1 mj(5 khz,duty 5 %) Pulse width 7 ns 4 ns Peak power 13.6 MW 5.5 MW

15 Conclusions Pre-Amplifire chains Output power 16 W(pulse width 4 ns) We confirmed compensation of the thermal effects and the imaging relay. Main Amplifiers Output power 11 W(5 % duty, Pulse width 4 ns) with 8 heads, single pass. Peak power of single pulse is 5.5 MW. High power density pumping Gain dropping is mainly caused by thermal shift of gain range. So we improved the cooling system of the main amplifier head. Non parabolic wavefront distortions is caused due to nonuniform pumping and temperature dependence of thermal conductivity. 4 th order distortion increases remarkably, but 1 st order distortion may cause a central peak on the propagated beam. Future plans Up to 3 kw output power with 8 heads, double passes. Wavefront compensation with SBS and deformable mirrors.