Nitrogen Passivation in La/B4C Layered Structures

In theory La/B4C multilayer mirrors are a most promising optical elements for the future generation of 6 nm photolithography tools. One of the most serious phenomena limiting the reflectivity of La/B4C mirrors is interface intermixing of La and B4C. Recent research on La/B4C multilayer mirrors with 3.4 nm bi-layer thickness has shown an intermixed interface layer with a thickness of more than 2 nm, i.e. more then half the multilayer period. Application of barriers that prevent intermixing for such a sort period multilayer will significantly reduce the optical contrast. One of the ways of suppressing interface intermixing is passivation of the La layer, for instance by nitridation. In this work we will present the results of X-ray photoelectron spectroscopy analysis of La nitride formation using N-ion and N2 treatment of the La layer. All samples have been deposited on super polished Si substrates and transferred from the deposition facility to the XPS spectrometer in vacuum to avoid contamination. N-ion post treatment produces two types of La nitride as can be deduced from the N1s spectrum (figure 1). The peak width of the lower binding energy (indicated as N-) represents a compound formed due to surface interaction of La with N2, the peak with higher binding energy represents a compound formed by interaction of La with N-ions. Tsarfati et al. [1] has connected the formation of two types of nitride with the formation of LaN inside the layer and La(N2) at the surface.