Short-term damage and its mechanism of a CaF₂ window for a DUV lithography machine

The CaF₂ window is the laser exit of the DUV lithography machine as well as its sealing component. Bearing the irradiation, high pressure, and discharging pollutants, the window is easy to damage and directly deteriorate the performance and reliability of the laser. In this paper, considering the effects of the above factors, two typical short-term damages to the CaF₂ window - high energy induced damage and pollutant-induced damage are studied quantitatively. Using an experimental design, theoretical calculation, characterization analysis, and numerical simulation, we found that the damage induced by high-energy irradiation is dominated by defect propagation at the initial stage. At the later stage, it is dominated by heat and thermal stress with thermal melting and evenly distributed microcracks of 1∼10 µm in size. Low-energy irradiation only causes expansion and deformation of the window, but the highly absorbent electrode discharging particles with a diameter of 0.1∼1 µm strongly absorb the laser. Strong local heat is caused during the melting and gasification of the particles, which easily leads to ablation and cracks. The damaged area is proportional to the particle size, and the damaged rate is proportional to the average power density at a high repetition frequency. High energy density and the defects with high absorptivity, the electrode discharging particles, and the heat accumulation effect, are the main factors for the two short-term damage, respectively.