Silicon / silicon oxide and silicon / silicon nitride multilayers for X-UV optical applications

Si/SiO ₂ and Si/Si ₃N ₄ multilayers have been fabricated using a locally made reactive diode rf-sputtering system. The layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target and argon as sputtering gas. O ₂ and N ₂ partial pressure conditions were optimized to deposit stoechiometric SiO ₂ and Si ₃N ₄ films without significant reaction with the silicon target. In-situ kinetic ellipsometry was used to monitore both thick film and multilayer deposition. The different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by grazing X-ray reflection ( Cu - K α line ), and the reflectivity was measured in the soft X-ray range (120 - 350 angstrom) by synchrotron radiation. Both Si/SiO ₂ and Si/Si ₃N ₄ multilayers exhibit well defined Bragg peaks with very narrow bandpasses (two to three times lower than the conventional Mo/Si multilayer), and high absolute reflectivities (up to ≈ 22% at 130 angstrom). The soft X-ray performances of these mirrors are explained using the physical characteristics deduced from kinetic ellipsometry, grazing X-ray reflection, infrared absorption and transmission electron microscopy measurements.