Measurement of Electron Temperature and Density Using Stark Broadening of the Coaxial Focused Plasma for Extreme Ultraviolet Lithography

We have generated an Ar plasma in a dense plasma focus device with coaxial electrodes for extreme ultraviolet (EUV) lithography and investigated an emitted visible light for electrooptical plasma diagnostics. We have applied an input voltage of 4.5 kV to the capacitor bank of 1.53 ??F, and the diode chamber has been filled with Ar gas of 8-mtorr pressure. The inner surface of the cylindrical cathode has been attached by an acetal insulator. Also, the anode is made of tin metal. If we assumed that the focused plasma regions satisfy the local thermodynamic equilibrium conditions, the electron temperature and density of the coaxial plasma focus could be obtained by the Stark broadening of optical emission spectroscopy. The Lorentzian profile for the emission lines of Ar I of 426.629 nm and Ar II of 487.99 nm were measured with a visible monochromator. In addition, the electron density has been estimated by the full-width at half-maximum (FWHM) of its profile. To find the exact value of FWHM, we observed the instrumental line broadening of the monochromator with a Hg-Ar reference lamp. The electron temperature has been calculated using the two relative electron-density ratios of the Stark profiles. In case of electron density, it has been observed by the Stark broadening method. This experimental result shows the temporal behavior of the electron temperature and density characteristics for the focused plasma. The EUV emission signal whose wavelength is about 6-16 nm has been detected by using a photodetector (AXUV-100 Zr/C, IRD). The result compared the electron temperature and density with the temporal EUV signal. The electron density and temperature were observed to be 10¹⁶ cm^-3 and 20-30 eV, respectively.