Alignment method for reflective focusing systems in high-power laser-generated plasma light sources

• A 12 kW LGP system employing reflective focusing with a low F-number laser. • Novel method to achieve precise optical alignment for a sealed optical system. • Quantitative relationship between output power and the alignment deviation. • High repeatability and operator-independent alignment process. Laser-generated plasma (LGP) light sources are critical for high-resolution bright field inspection of modern semiconductor wafer defects. This paper presents a 12 kW LGP system to enhance plasma radiance, employing reflective focusing with a low F-number to suppress plasma elongation. All optics are housed in a sealed chamber for operational safety. A novel alignment method is introduced to achieve precise optical alignment within the sealed chamber. This method uses a metal sphere to regularize the spot image and calculates component offset through image processing. The principle is analytically derived and verified via ray-tracing simulations, achieving a theoretical alignment accuracy of 0.01 mm. Experimental results demonstrate the robustness of the method: realignment consistently converged within 30 iterations across multiple disassembly-reassembly cycles. Moreover, a quantitative study reveals a clear decrease in output power as the offset of the optical axis increases. At a pump laser power of 6.0 kW, the system achieved an average output power of 315.8 W, with <0.4% variation over repeated cycles. This work provides a reliable, operator-independent alignment solution to ensure optimal performance of high-power LGP light sources.