Energy efficiency analysis of ArF excimer laser system

Abstract

The reliable functioning and continual optimizing of ArF excimer laser system is of importance when it comes to productization into the market from a laboratory test machine. The analysis of dynamic characteristics of the system is vital to understanding its operating mechanism and optimizing the design theoretically. In this article, one-dimensional fluid model is used to analyze the excimer laser discharge mechanism, and the content ratio of fluorine gas, argon gas, and neon gas, which constitute a gas mixture, is studied in a simulated ArF excimer laser system. Particles are treated as a fluid, which significantly reduces the computing cost in fluid model, and therefore is suitable for high-pressure situation. Four equations are included in one-dimensional fluid model, i.e., Boltzmann equation that describes electron energy distribution, ion continue equation that illustrates ion number density, Poisson's equation that shows the distribution of electric field, and photon rate equation that demonstrates laser outputting process. By combining these four equations, high pressure plasma discharge process and particles stimulated radiation process are studied, and calculation continues from one time step to another until the end of discharging process. The result of the calculation presents energy transfer process from three aspects:energy deposition efficiency, ArF* formation, and laser outputting. In the energy deposition process, the energy deposition efficiency is sensitive to the change of fluorine gas ratio while the variation of the content ratio of other two gases has a less influence on this process. In addition, there exists an optimal fluorine gas ratio that causes the highest energy deposition efficiency. In the ArF* formation process, the reaction between excited argon ions and fluorine gas is the main channel that generates ArF*. The proper increasing of fluorine gas ratio helps form ArF*. In the laser outputting process, photon loss is mainly because of the reaction between fluorine negative ions and photons. Therefore superfluous fluorine gas in the mixture leads to less photons, which eventually results in low energy efficiency of laser. By summarizing the three aspects of energy transfer process, the fluorine gas ratio in the gas mixture plays a significant role in determining the energy efficiency of ArF excimer laser system. This theory is verified by experiments, showing that the deviation of the optimized fluorine gas ratio severely reduces energy efficiency. This conclusion can guide us in optimizing the design and steady reliable function of ArF excimer laser system.