Objective  UV excimer lasers have the characteristics of short wavelength, high power and narrow linewidth, and are widely used in semiconductor lithography, new display manufacturing, corneal refractive correction, etc. Whether in the fields of medical treatment, industry, flat panel processing, or scientific research, the pulse energy stability of excimer lasers is a very important parameter, which directly determines the accuracy of surgery, the critical dimensions of processing, and the uniformity of material processing. Limited by the composition and proportion of working gas, gas circulation system, stability of excitation source, laser resonance amplification mechanism, etc., the energy stability of excimer laser pulses output by current excimer lasers generally fluctuates around 1%-2%, and it is difficult to further improve. However, the pulse energy stability directly affects the development of industrial production, medical treatment, and scientific research. For example, in the field of integrated circuit manufacturing, the lithography process determines the key dimensions of the device, while the exposure light source directly affects the quality of lithography. The instability of the laser light source may cause pattern deformation and poor registration effect. Therefore, it is of great significance to further improve the output energy stability of excimer lasers based on the existing technology. In practical use, it has been found that when two excimer lasers are used in beam combination, the pulse energy stability is also improved to some extent while the output power is increased.

  Methods  In order to obtain more stable and powerful excimer laser output pulses, theoretical derivation, simulation experiments, and beam combination experiments were conducted to investigate the feasibility of improving the stability of output pulse energy through beam combination. Theoretical derivation showed that when the laser output pulse energy follows a normal distribution, beam combination of multiple lasers with similar parameters can reduce the relative standard deviation of output energy. Subsequently, beam combination simulation experiments and actual beam combination experiments were conducted.

  Results and Discussions   The simulation experiment of beam combination for three excimer lasers with output pulse energy distribution characteristics consistent with normal distribution was conducted. The output pulse energies were 152.2 mJ, 152.9 mJ, and 153.3 mJ, respectively, with relative standard deviations of 1.06%, 0.88%, and 0.95%. Among them, two excimer lasers achieved an average output pulse energy of 305 mJ and a relative standard deviation of 0.7% when combined. Three excimer lasers achieved an average output energy of 458 mJ and a relative standard deviation of 0.6% when combined. Three actual beam combination experiments were conducted using one PLD20 excimer laser and one PLD30 excimer laser (Fig.4). The average output energies of PLD20 were 356.6 mJ, 368.4 mJ, and 328.4 mJ, with relative standard deviations of 1.39%, 1.14%, and 1.49%, respectively. The average output energies of PLD30 were 354.5 mJ, 354.5 mJ, and 331.4 mJ, with relative standard deviations of 1.24%, 1.24%, and 1.35%, respectively. After three beam combinations, the average output energies were 687.2 mJ, 694.5 mJ, and 646.8 mJ, with relative standard deviations of 0.86%, 0.79%, and 0.83%, respectively. The relative standard deviation values of the laser pulses after actual beam combination were all lower than those of the single laser (Tab.4). The decrease in the relative standard deviation value indicates that the output pulse laser energy is more stable.

  Conclusions   From theoretical derivation, simulated beam combination data, and actual beam combination data, it can be concluded that multiple lasers with similar output laser pulse parameters can improve the output laser pulse energy of the lasers while reducing the relative standard deviation of the output laser pulses, thereby improving the stability of the laser pulses. When two lasers with similar parameters are combined, the energy can reach 1.9 times that of a single laser, and the relative standard deviation can be reduced to 0.6-0.7 times that of a single laser.