Emission characteristics of debris from Nd:YAG LPP and CO 2 LPP A Takahashi 1, K Tamaru 2, T Akiyama 2, D Nakamura 2 and T Okada 2 1 Department of Health Sciences, Kyushu University, 3-1-1, Maidashi, Fukuoka 812-8582, Japan 2 Graduate School of Information Science and Electrical Engineering, Kyushu University, 6-1-1, Hakozaki, Fukuoka 812-8581, Japan
CO 2 LPP vs. Nd:YAG LPP - Conversion Efficiency 1.8.6.4.2 Nd:YAG LPP CO2 LPP In the previous work, we presented the CO 2 LPP for the EUV light source, and showed that the CE of CO 2 laser-produced tin plasma is comparable with that of Nd: YAG laser *. *Tanaka H, Matsumoto A, Akinaga K, Takahashi A and Okada T 25 Appl. Phys. Lett. 87 4153 1 9 1 1 1 11 Laser Intensity [W/cm 2 ]
Current problem for LPP-EUV lithography is debris Debris (droplet, ion, neutral atom) harm the optics and limit the lifetime. Mo/Si multilayer mirror (Reflectivity of 67 % at 13.5 nm) Lifetime > 3, hour EUV output at intermediate focus (I. F.) > 115 W (18 W) (@ 13.5 nm, 2% BW) 7~1 khz Debris Driver laser Nd:YAG laser (1.6 µm) CO 2 laser (1.6 µm) Plasma Target : Sn, Li, Xe
Measures against debris are Kyushu University Electromagnetic shield Fast Ion Gas curtain & Debris shield Mass limited target Droplet & Neutral Atom Anyway, it is necessary to understand the characteristics of debris emission for the effective measures. The purpose of our study is to investigate the emission characteristics of debris from laser-produced tin plasma to develop efficient shield.
In this work, we investigated Kyushu University the emission characteristics of debris from laser-produced tin plasma for both of CO 2 and Nd: YAG laser. Sn atoms & droplet were detected by QCM (Quartz Crystal Micro-balance) Si Plate Fast Ions were detected by Faraday Cup Mirror Sn Target
Schematic of the experimental arrangement
Plasma Shutter Kyushu University CO 2 laser Laser-produced plasma cuts off the tail.4.3.2.1 without P.S. with P.S. In order to avoid the complexity of analysis and investigate the effect of laser pulse shape, we arranged a plasma shutter (PS) to cut off the tail of the CO 2 laser pulse. 1 2 3 4 5 6 7 8 Time [nsec]
Ion Signals measured by Faraday Cup 6 Nd: YAG LPP @ 6 x 1 1 W/cm 2 CO 2 LPP @ 9 x 1 9 W/cm 2 6 5 4 3 2 (a) 29 41 67 5 4 3 2 (b) 29 41 67 1 1 1 2 3 4 5 6 7 8 Time [μsec] 1 2 3 4 5 6 7 8 Time [μsec] The ion signal of CO 2 LPP without the plasma shutter has complex structure, and the time axis does not correspond to the time-of-flight of ions because of the long pulse width and the tail..
Ion Signals for Short-pulse CO 2 LPP Without Plasma Shutter With Plasma Shutter 6 5 5 4 3 2 (b) 29 41 67 4 3 2 29 41 67 1 1 1 2 3 4 5 6 7 8 Time [μsec] 1 2 3 4 5 6 7 8 Time [μsec] The initial signal around 1 μsec and the late signal after 3 μsec disappeared. This result suggests that the tail part of CO 2 laser pulse generates not only lowenergy ions but also high-energy ions.
Ion Energy Distribution of Nd:YAG and CO 2 LPP Normarized Spectrum [/ev].2.15.1.5 29 CO2 LPP Nd:YAG LPP 2 4 6 Ion Energy [ev] Although Nd: YAG laser intensity is larger than that of CO 2 laser by several factors, the ion kinetic energy of CO 2 LPP is higher than that of Nd: YAG LPP.
Debris Emission measured by QCM Nd: YAG LPP CO 2 LPP (Short pulse) 25 25 2 15 21 41 2 15 21 41 67 1 1 5 5 5 1 15 2 Shot Number 5 1 15 2 Shot Number The debris emission of short-pulse (2 nsec FWHM) CO 2 LPP is much less than that of Nd: YAG LPP for the same laser energy (~4 mj/pulse).
Debris Emission for Short/Long-Pulse CO 2 LPP 25 CO 2 LPP (short pulse) 25 CO 2 -LPP (LONG PULSE) 2 15 21 41 67 2 15 21 41 67 1 1 5 5 5 1 15 2 Shot Number 5 1 15 2 Shot Number The debris emission of short-pulse CO 2 LPP is much less than that of long-pulse (without the plasma shutter ) for the same laser energy (~4 mj/pulse).
SEM Images of Focal Point on Sn Target suggest the results of debris emission Short-Pulse CO 2 LPP (2 nsec FWHM) Nd: YAG LPP (8 nsec FWHM) ~4 mj X 2 shots Long-Pulse CO 2 LPP
SEM Images of Sn Droplets on Si Plates for CO 2 LPP 29 41 1μm 67
SEM Images of Sn Droplets on Si Plates for Nd: YAG LPP 29 41 1μm The droplet sizes of Nd: YAG LPP are much larger than those of CO 2 LPP. 67
CO 2 laser is absorbed in superficial low-density region CO 2 LPP Nd: YAG LPP of the plasma due to the long wavelength, in contrast, Nd: YAG laser penetrates into the high-density region on the target surface. Therefore, the superficial temperature of CO 2 LPP will be much larger than that of Nd: YAG laser for the same laser energy. The ions emitted from Nd: YAG LPP will loss the energy due to the collisions with other particles in the plasma. On the other hand, the ions in CO 2 LPP will not loss so much energy because the almost all of ions generate the superficial low-density region. This also explains the effect of pulse shape on the ion signals. The tail of CO 2 laser pulse is efficiently absorbed in the expanded lowdensity plume, and generates high-energy ions.
The characteristic of droplets emission is. Nd: YAG LPP also attributed to the absorption mechanism of laser energy. In the case of Nd: YAG LPP, the laser penetrates to the target surface, and the target surface is superheated into liquid phase, and then droplets are formed. CO 2 LPP In contrast, in the case of CO 2 LPP, once plasma is produced, the energy is absorbed in the plasma surface; therefore the target surface is not heated so much as the Nd: YAG LPP.
Summary Kyushu University The emission characteristics of debris from laser-produced tin plasma were investigated for an EUV light source. The ions and droplets emitted from tin plasma produced by a CO 2 laser or an Nd: YAG laser were detected with Faraday cups and QCM detectors, respectively. A higher ion kinetic energy and a lower droplet emission were observed in the case of CO 2 laser in compared with Nd: YAG laser for the same laser energy. The reason comes from the difference between the interactions of the laser pulse with the plasma.
Aknowlegement Kyushu University This work was performed under the auspices of MEXT (Ministry of Education, Culture, Science and Technology, Japan) under contract subject Leading Project for EUV lithography source development.