Ablation depth in planar Sn targets during the interaction with a Nd:YAG laser for extreme ultraviolet lithography

The depth of mass ablation in planar Sn targets during the interaction with a pulsed Nd:YAG laser was investigated. The pulse duration and laser spot size were fixed, and the laser energy was varied to achieve laser intensities from approximately 10 to 10 W/cm, which is relevant to the extreme ultraviolet (EUV) lithography application. The ablation depth was measured by irradiating layered targets consisting of a thin Sn coating with variable thickness evaporated onto a Si wafer. The Sn coating thickness at which the signatures of Si ions in the expanding plasma disappeared indicated the ablation depth. Redundant diagnostics were utilized to detect Si ions in the expanding plasma, including a calibrated EUV energy monitor, an EUV spectrometer, and an electrostatic ion energy analyzer. The ablation depth was found to scale with the laser intensity to the (5/9) power, which is consistent with analytical models of steady-state laser ablation developed for the laser fusion application, at which laser intensity is generally higher than present in the EUV lithography application. This work was supported by Cymer Inc. and by the University of California (UC) under the UC Industry-University Cooperative Research Program (ele06-10278).