A comparative study of the interfacial roughness correlation and propagation in Mo/Si multilayers deposited using RF-magnetron sputtering on silicon, ule and zerodur substrates

摘要

Mo/Si multilayer (ML) mirrors play a decisive role in an extreme-ultraviolet (EUV) lithography process. In this study, the surface and interfacial roughness, as well as the lateral and vertical correlation lengths, of a series of Mo/Si MLs deposited by RF-magnetron sputtering (RF-MS) have been characterized using diffuse x-ray scattering and atomic force microscopy. We have investigated the influence of the substrate quality and material (silicon, ule and zerodur) on the propagation and the value of ML roughness. We show that, whatever the substrate is, the film deposited by RF-MS presents a reduced roughness compared with that of the substrate. Moreover, rocking-curve analyses show that, for Si and ule substrates, the ML average roughness is very low (< 1.5 Angstrom), associated with high spatial frequency oscillations, while in the case of zerodur substrates, the roughness is significantly increased (> 2 Angstrom) and the high spatial frequency oscillations are reduced. Finally, the combination of specular and non-specular small-angle x-ray results allows us to evaluate another key parameter, namely, the uncorrelated roughness which is an intrinsic characteristic related to the choice of both the deposition technique and the materials. This intrinsic roughness is found to be very low (2 Angstrom) and constitutes a good argument in favour of the use of the RF-MS technique for EUV mirror deposition. [References: 34]