Development of Mo/Si multilayers deposited by low-pressure rotary magnet cathode sputtering for EUV lithography

Abstract

Mo (molybdenum)/Si (silicon) multilayers were deposited by low-pressure RMC (rotary magnet cathode) sputtering, which can operate at a gas pressure (about 0.1Pa) lower than that conventional magnetron sputtering. We obtained a high EUV (extreme ultraviolet) reflectivity in the Mo/Si multilayers sputtered with low-pressure Xe gas. The measured maximum EUV reflectivity was about 71%. We confirmed that the multilayers sputtered at a low pressure exhibited high EUV reflectivity. From the TEM (transmission electron microscopy) images of the multilayers we observed thinner interdiffusion layers between Mo and Si layers in the multilayers deposited by RMC sputtering than in those deposited by ion-beam sputtering. The Mo single layer deposited by RMC sputtering has tensile stress and the Si single layer has compressive stress. By changing the Γ ratio (the fractional thickness ratio of a Mo layer to the total thickness of a Mo layer and a Si layer) of multilayer coatings, film stress can be converted from tensile to compressive. However, for larger Γ ratio, interface roughness increases and EUV reflectivity decreases. We have developed a doubly stacked multilayer structure that has low stress and high reflectivity. Using this technique, the stress of multilayer coatings can be reduced to -6 MPa.