Damage resistant and low stress Si-based multilayer mirrors

Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and minimal residual stress. A serious problem of Mo-Si multilayers is the structural instability in the case of localized absorption of in- and outband radiation from the EUV source followed by the degradation of the multilayer. A number of solutions have been envisaged in the past, including the use of compound materials (MoSi2/Si) as well as the use of C barrier layers. We focused our interest on two Si-based systems: Mo/Si and Mo2C/Si multilayer mirrors. The mirrors were designed for normal incidence reflectivity at about 13 run wavelength and were deposited by dc magnetron sputtering. Maximum normal incidence reflectivities of 68.4 % @ 12.8 run for Mo/Si multilayer mirrors and 66.8 % @ 12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 300 degreesC to 500 degreesC we found that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 degreesC, whereas the Mo2C/Si multilayers show a superior thermal stability up to 400degreesC... 500degreesC. An other problem of EUV multilayer mirrors is the large residual compressive stress (-400 to -500 MPa), which causes undesirable distortion of the substrate figure. The reduction of residual stress of Mo/Si and Mo2C/Si multilayers with annealing has been investigated. Using a slow thermal annealing (1degrees C/min), it is possible to reduce the stress from -520 MPa to zero by heating the Mo/Si samples up to 310degrees C. However, this results in a reflectivity drop of about 3...4 %. On the other hand one can reduce the stress of a Mo2C/Si multilayer from -490 MPa to zero by annealing without a considerable reflectivity drop.