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| Experimental and Computational Study of Laser-Maintained Argon Plasma under High Pressure |
| Wang Dazhi, Yuan Bowen, Lu Qi, Qiao Junjie, Xiong Qing |
| State Key Laboratory of Power Transmission Equipment & System Security and New Technology Chongqing University Chongqing 400044 China |
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Abstract Laser-sustained plasma (LSP), as an advanced light source, has the characteristics of high stability, wide spectrum, and high brightness which has value in applications like highly sensitive defect detection of semiconductor surface based on LSP vacuum ultraviolet radiation. The generation process and optimization of the vacuum ultraviolet radiation produced by the LSP source have always attracted much attention. Based on the experimental spectral measurement and zero-dimensional global simulation model, this paper explored the self-sustaining discharge process of argon LSP (pressure>10 bar), the main generation and disappearance path of Ar2*, and the mechanism of argon gas pressure and laser power on Ar2*. Based on these works, the optimization trend of Ar2* 126 nm vacuum ultraviolet radiation was predicted.
An LSP generator in high-pressure pure argon environment was designed and constructed. The experimental conditions were divided into 11 groups according to argon pressure and laser power. It was observed that the brightness of Ar LSP decreased after the ignition power was turned off, which was caused by the process of electrons loss due to recombination of free electrons with Ar ions. In addition, with the increase of laser power (123~201 W) and Ar pressure (13~20 bar), the relative intensity of the LSP emission spectrum increased significantly.
Combined with the experimental results, a zero-dimensional global model of argon LSP containing 23 kinds of particles and 326 reactions was established, the laser power and Ar pressure were coupled with particle density and temperature parameters. The particle number density generated in the model was calculated in balance. The simulated radiation spectrum of LSP is obtained based on emission spectroscopy theory, and is in good agreement with the experimental spectrum. The simulation time started from the electrode was powered off to the stable Ar LSP phase. The time evolution behavior of each particle under typical working conditions (laser power 201 W, argon pressure 14 bar) was discussed.
Finally, the reaction path of Ar* (taking Ar(1s5) and Ar(2p10) as examples) under different working conditions were analyzed based on the calculated data of model balance. The results show that electron collision is the main reaction path of Ar*, that is, electron density determines particle density in balance. Increasing laser power and Ar gas pressure changes the number of ground-state Ar, resulting in the maximum electron density changes to 2.59×1016 cm-3 and 4.18×1016 cm-3, respectively.. Therefore, increasing Ar gas pressure is a more efficient way to increase the density of Ar*. Then, the reaction path of Ar2* particles shows that the three-body collision involving ground-state Ar and 1s-level excited Ar are the main generation path of Ar2*, and the electron-collision reaction is the main loss path. Gas pressure can significantly increase Ar2* density in balance by increasing the three-body collision rate.
The following conclusions can be drawn from the experimental and simulation analysis: (1) Inverse bremsstrahlung absorption plays a key role in the process of LSP generation and maintenance. It is necessary to provide a medium with a certain electron density level (>1016 cm-3) to realize a stable LSP generation. (2) The intensity of Ar2* at vacuum ultraviolet radiation of 126 nm increases with the increase of gas pressure, but does not continue to increase with the continuous rise of laser power. Increasing the gas pressure is a more economical and effective way to optimize the 126 nm radiation output of argon LSP. (3) To get high brightness (including 126 nm radiation) and stable output, Ar LSP should be run with gas pressure set at 20 bar (even higher), laser power controlled between 120 to 150 W.
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Received: 12 May 2022
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2023, Vol. 38 
