Laser-absorption diagnostics of electron density in laser-ablation brass plasma under vacuum

High-density laser-induced plasmas (LIP) are widely employed in semiconductor manufacturing—such as in EUV lithography and pulsed laser deposition—where processes typically occur under vacuum. Accurate diagnosis and monitoring of these plasmas are essential for process development and optimization. Electron density is a key plasma parameter, commonly measured using techniques such as the Langmuir probe and optical emission spectroscopy. However, these methods have not been validated for electron densities exceeding 10¹⁹ cm^-3, which are typical in LIP. In our study, we apply a laser absorption technique to measure time- and space-resolved electron density profiles. A probe beam is transmitted through the plasma along multiple lines of sight. The resulting absorbance data are converted into absorption coefficients via tomographic reconstruction and subsequently into electron densities. Measurements were performed on laser ablation plasmas generated on a brass target under vacuum conditions (~ 1 Torr). Additionally, for monitoring purposes, the technique was applied in a single-line configuration while varying the laser energies used for plasma generation. Our results demonstrate that this technique can be effectively employed not only for detailed plasma diagnostics but also for real-time monitoring of high-density LIP.