3rd International EUVL Symposium NOVEMBER 1-4, 2004 Miyazaki, Japan Visualization of Xe and Sn Atoms Generated from Laser-Produced Plasma for EUV Light Source H. Tanaka, A. Matsumoto, K. Akinaga, A. Takahashi * and T. Okada Department of Electrical and Electronic Systems Engineering, ISEE, Kyushu University * Department of Health Science, School of Medicine, Kyushu University
EUV light source for lithography 1 Generation methods for EUV Light source LPP (Laser Produced Plasma) We employed LPP in our study. DPP (Discharge Produced Plasma) Problems of LPP The conversion efficiency from laser light to EUV light is very low. Debris generation that limits the lifetime of the optics in the lithographic system Mo-Si multilayer mirror with a reflectivity of 67 % at 13.5nm EUV light source having an average power of more than 100 W at 13.5 nm is required. Debris Nd:YAG lasers at a wavelength of 1.06 µm have been exclusively employed as a driver laser for LPP. Xe and Sn are used as target materials for laser irradiation.
Fast ion Background of measurement of neutral debris behavior Electromagnetic shield 2 Debris Droplet Neutral atom Fast atom Xe Sn Gas curtain & Debris shield Mirror damage Self absorption Mirror contamination It is confirmed that these shields are effective in eliminating debris. They are bound to interfere with transmission of EUV light. In order to estimate debris generation and hold shields to the minimum necessary, we visualize debris behavior by Rayleigh scattering and laser-induced fluorescence (LIF). In this time, we measured the behavior of Xe gas jet target on a trial basis
Dye laser Experimental setup for generation and measurement of EUV light Xe gas jet target Plasma Photon drag detector Exit window (BaF 2 ) Vacuum chamber (Top View) Aperture 1 st order diffracted beam X-ray CCD camera Data acquisition system ICCD camera Band pass filter Scattered light, Fluorescence 3 Condenser lens (BaF 2 ) Photon drag detector EUV light Entrance window (BaF 2 ) CO laser Beam stop Data acquisition system Transmission grating (φ = 50 µm, d = 1/1000 mm) Zr filter (φ = 2 mm, t = 100 nm)
Experimental arrangement for imaging LIF and Rayleigh scattering 4 Side View Xe gas Nozzle Sheet beam Scattered light and ( Fluorescence ) Beam expander Dye laser Top View CO 2 laser Nozzle Condenser lens Sheet beam Beam expander Dye laser Scattered light and ( Fluorescence ) ( Band pass filter ) ICCD camera SPEC Wavelength : 224 ~ 226 nm Output : < 5 mj/pulse Pulse width : < 10 ns
Visualization of Xe gas jet target by Rayleigh scattering 5 Nozzle Laser beam 10mm Delay 100 µs 200 µs 300 µs 400 µs 500 µs 600 µs 700 µs
Estimation of target gas density and transmission 6 Xe density [1/cc] Xe density [1/cc] 2 10 17 Transmission of 13.5 nm light 1 1.5 10 17 0.8 0.6 1 10 17 0.4 5 10 16 0.2 0 0-30 -20-10 0 10 20 30 Distance [mm] CO 2 laser spot position Transmission of 13.5 nm light Signal Intensity [arb. units] 800 700 600 500 400 300 200 100 0 EUV spectrum with Xe gas target CO 2 laser 2.5 W/cm 2 Xe gas 12 atm. 10 12 14 16 18 20 Wavelength [nm] Transmission curve of Xe gas pressure of Xe gas : 10-3 Torr light path length : 70 cm Measured conversion efficiency (C.E.) from the laser light to 13.5nm light 0.15 % ( Jpn. J. Appl. Phys. 43 (2004) L 585-L587) 200 µs Density distribution Estimated C.E. at the emission point 0.3 % We can raise C.E. by improving a structure of the nozzle
Energy level diagram of Xe for LIF 7 Fluorescence 480.83 nm 828.23 nm 7p [0 1/2 ] 7p [1 1/2 ] 7p [2 1/2 ] 7p [0 1/2 ] 6p [1/2] 0 6p [3/2] 2 6p [3/2] 1 6p [5/2] 3 6p [5/2] 2 6p [1/2] 1 In LIF, the species resonantly excited by a probe laser beam decay into the lower-energy states by radiative and/or nonradiative transitions. Therefore by observing the fluorescence, we can detect species with high selectivity. 6s [1/2] 1 6s [1/2] 0 6s [1 1/2 ] 1 6s [1 1/2 ] 0 129.56 nm 146.96 nm 225.1 nm 2 249.6 nm 2 5p [0] 0 In this time, Xe atoms were excited respectively by two 225.1 nm photons with the dye-laser and the fluorescence at 480.83 nm was observed.
Wavelength calibration of the dye laser with the Xe gas cell 8 Dye laser beam (wavelength scanning) Gas cell ( Xe : ~10 Torr) Singal intensity [arb. units] 8 7 6 5 4 3 2 1 Excitation spectrum LIF 7p[0 1/2 ] 0 224 224.5 225 225.5 226 Dye laser wavelength [nm] Box car Photo multiplier tube Optical fiber 7p[1 1/2 ] Singal intensity [arb. units] 10 1 Two-photon excitation LIF signal intensity (Probe laser intensity) 2 Slope 2 Excitation spectrum 0.1 0.1 1 10 Laser pulse energy [mj]
One-dimensional imaging by LIF 9 Nozzle (Xe gas jet) Rayleigh scattering & LIF LIF Dye laser beam Without filter Scattered light and Fluorescence ICCD camera UV cut filter LIF LIF Two-dimensional imaging by by Rayleigh scattering (for (for reference) With filter
Estimation of Doppler broadening 10 Spectral bandwidth of probe laser λ L = 0.05 nm Doppler broadening of Xe at room temperature ν = D Therefore ν D = 2.88 GHz λ D = 0.12 pm 2kT Mc 2ν 0 2 ln 2 Signal intensity of LIF is improvable by optimizing the spectral bandwidth of probe Doppler broadening in LPP Emission angular distribution of laser-ablated particles : cos n θ (n>4) Velocity of the particle emission : 10~30 ev Assuming that the angular distribution is cos 5 12 θ Doppler broadening [pm] 10 8 6 4 2 Xe Sn 0 0 20 40 60 80 100 Velocity of the particle emission [ev]
Emission from CO 2 -LPP with Xe gas jet target 11 Emission of neutral Xe (480 nm) Gate Width of ICCD camera : 5ns, 5ns, 50ns I.I. Gain : 0 10mm 70ns 100ns 2µs Backing pressure : 12atm 20,000 counts 40,000 counts 7,000 counts Singal intensity [arb. units] 0.8 0.7 0.6 EUV pulse 0.5 0.4 0.3 0.2 0.1 0 0 50 100 150 200 250 300 Time [ns] Gas jet target Diffusion CO 2 -laser Long-pulse Spatially extended and long-time emission It is difficult to detect LIF signal near the spot immediately. Measurement with solid target by LIF
Summary 12 We developed the measurement system to visualize debris. Rayleigh scattering Two dimensional images of Xe gas jet target were measured. Self-absorption of Xe gas jet target was estimated. LIF One dimensional images of Xe gas jet target were measured. In the future Measurement of Xe particles behavior Optimizing the measurement system for a improvement of LIF signal intensity. Imaging of debris from a cryogenic Xe target by LIF and Rayleigh scattering. Measurement of Sn particles behavior Estimation of emission characteristics of droplet by Rayleigh and Mie scattering. Visualization of neutral Sn behavior by LIF (one-photon excitation).