Laser Production of Extreme Ultraviolet Light Source for the Next Generation Lithography Application

Extreme ultraviolet lithography (EUVL) is a technology to be used in mass production of the next-generation semiconductor devices. Critical issues in the development of a Sn-based EUV source are in achieving a high conversion efficiency (CE) of incident laser energy into EUV light, reducing debris emanating from light source plasmas, and suppressing out-of-band radiation beside the EUV light. The minimum-mass target, which contains the minimum number of Sn atoms required for sufficient EUV radiation, is a solution to these critical issues. One practical-minimum mass target is a pure Sn microdroplet. Laser-driven expansion of a pure Sn microdroplet is proposed to solve the considerable mismatch between the optimal laser spot diameter (300 µm) and the diameter (20 µm) of microdroplets containing the minimum-mass Sn fuel for generating the required EUV radiant energy (10 mJ/pulse). An expanded microdroplet was irradiated with a CO2 laser pulse to generate EUV light. A combination of low density and long scale length of the expanded microdroplet leads to a higher EUV energy CE (4%) than that (2.5%) obtained from planar Sn bulk targets irradiated by a single CO2 laser pulse. This scheme can be used to produce a practical EUV light source system with an EUV CE of 3.9%.