Vacuum ultraviolet spectral emission properties of Ga, In and Sn droplet-based laser produced plasmas

• Emission Spectra of gallium, indium and tin droplet-based laser produced plasmas
• Plasma Emission from atomic transitions in the Vacuum Ultraviolet Region
• Spectra calibrated in units of absolute spectral radiance
• Emission spectroscopy measurements in an almost unexplored wavelength region
• Results relevant for Extreme Ultraviolet Lithography and Inspection applications

The Emission Spectra of gallium, indium and tin droplet-based laser produced plasmas are presented in the Vacuum Ultraviolet (VUV) emission range from 30 nm to 160 nm. The Ga ion transitions are investigated in detail as a function of background pressure level and laser irradiance. Different wavelength emission regions were detected according to the level of background gas. At short wavelengths (i.e. 30–50 nm) the line emission from the higher charge states is reduced with increasing pressure, while at longer wavelengths (i.e. 100–160 nm) the trend is inverted, as the plasma emission intensity of the lower charge states increases with higher background gas pressure level. The emitted lines are fitted with Voigt profiles to determine the electron density. The electron temperature is obtained from a fit based on the Planck distribution. These estimations are then used to identify the relevant processes that lead to the different charge state emissions as a function of background gas. Langmuir Probe measurements are also reported for evaluating the ion kinetic energy as a function of background gas. The gallium spectra are calibrated in units of spectral radiance, together with spectra from indium and tin. This calibration allows absolute power estimations from the light source in the VUV region. The presented experimental results are relevant as fundamental plasma emission spectroscopic measurements in an almost unexplored wavelength region as well as for applications such as Extreme Ultraviolet Lithography to determine the so-called Out-of-Band (OoB) radiation emission and for metrology applications for future inspection tools.