Characterization of a Multi-wavelength Laser-sustained Plasma

Laser-Sustained Plasma (LSP) is a type of dense plasma generated through the interaction of lasers with ionized gases, with temperature reaching tens of thousands Kelvin. ^[1] Due to its high radiation intensity, broadband radiation spectrum, and notable stability, LSP has been adopted as a mainstream light source for wafer optical detection in the semiconductor manufacturing. Since various noble gases (e.g., Ar and Xe) in LSP configurations absorb photons of different wavelengths, there could be a wide range of parameter variations to determine an optimal wavelength of the incident laser for LSPs at different temperatures. For example, it has been recognized that an Ar LSP at 30 atmand 15,000 K exhibits a higher absorptivity for infrared (IR) lasers (700-1,100 nm) compared to visible (VIS) lasers (400-700 nm). However, at a temperature of 25,000 K, the absorptivity for VIS lasers significantly surpasses that ofIR lasers in Ar LSP. Based on the above principle, a multi-wavelength LSP approach is proposed to create a core of reduced size but higher temperature. Here, the IR laser is used to heat the plasma up to 10,000 K; meanwhile, the VIS laser can go through the plasma periphery and is absorbed by the plasma center, resulting in LSP core(s) with lengths of $\sim 100 \mu \mathrm{m}$ and temperatures of $\sim 30,000 \mathrm{~K}$. In this work, a two-dimensional time-dependent fluid model is established to reveal the multi-physical processes of multi-wavelength LSP. ^[2–3] The simulation results show that, an extremely high temperature LSP core can form adjacent to the focal point of laser beam, with a large LSP background at a relative lower temperature. The effects of the laser wavelength, intensity and F-number on the LSP properties are examined and discussed. The results from this study could offer a valuable guidanc for the parameter optimization in LSP experiments.