Preparation and characterization of multilayers for EUV applications
The dependency of EUV reflectance and of the stress present in Mo/Si/C multilayers on the constituent individual layers has been investigated. The heat treatment of highly EUV reflective Mo/Si/C multilayers has been examined in terms of annealing time and temperature. Irreversible stress change was found at annealing temperatures above 130 °C, where the change of period thickness and the change of stress increase more rapidly. It was found, that there is a saturation of stress change depending on the annealing time. Annealing at 100 °C reveals a saturation of stress change after 10 h, whereas annealing at 150 °C still shows stress reduction after 50 h. A second annealing of two samples shows a reversible stress-temperature behavior of the multilayers. Stress compensation layers for the coating of stress-mitigated Mo/Si/C multilayers were developed. The best results of composition for reflective multilayers and stress-compensation multilayers were joint together in order to fabricate stress-mitigated Mo/Si/C multilayers. Taking the condition of achieving an overall stress below ±100-150 MPa into account, two types of stress-mitigated multilayers were coated. The first type includes a buffer layer in order to compensate the compressive stress of the reflective multilayers without annealing. The stress was reduced to -76 MPa by keeping a high EUV near-normal incidence reflectance of ~70.0 %. The second type contains a thinner buffer layer and the stress reduction of the complete multilayer system was assisted by a post-annealing at 100 °C for 10 h. The stress was measured to be -108 MPa and an EUV reflectance of 69.5 % was obtained.