Stress control of Mo/Si-based multilayer coatings deposited by ion-beam sputtering

We have developed a new stress control technique in which modified molybdenum (Mo)/silicon (Si) multilayers are deposited by ion-beam sputtering, together with a method of sub-multilayering of each Mo layer into a trilayer of Mo/ruthenium (Ru)/Mo and a method of ion-beam polishing (IBP) after the deposition of each Mo layer. We fabricated conventional and stress-controlled Mo/Si multilayer coatings using an ion-beam sputtering system and measured the internal stress of these coatings. The conventional Mo/Si multilayers had a compressive stress of approximately - 450 MPa, while the stress-controlled multilayers had a tensile stress of +13 MPa. The modified Mo/Si multilayers had the same reflectivity as and a slightly larger bandwidth than the conventional ones. For the purpose of investigating the mechanism of stress control, we have developed an in situ stress monitoring system using an electrostatic capacitance. For single-layer Mo films, tensile stress was observed in the early stage of film growth. In the case of the conventional Mo/Si multilayers, partial stress was tensile during the deposition of Mo layers, but became compressive during the deposition of Si layers. In the case of the stress-controlled multilayers, it was observed that IBP of the surface of each Mo layer suppressed the generation of compressive stress in the ensuing Si layers.