Stress compensation of a Mo/Si/C highly reflective multilayer by means of an optimised buffer layer and heat treatment

Abstract The dependency of the stress present in Mo/Si/C multilayers on the constituent individual layers has been investigated. The heat treatment of a multilayer stack, consisting of a highly reflective multilayer and an optimised buffer layer, has resulted in a large reduction of the internal stress in the stack without significantly reducing the extreme ultraviolet (EUV) reflectivity of the highly reflective multilayer. The stress in the Mo layer was found to increase with an increasing thickness in the amorphous region, up to approximately 2.45 nm, and decrease after the Mo layer was mainly crystalline, after approximately 2.6 nm. The reflectivity of the highly reflective multilayer was found to decrease by 1.6% as the thickness of the Si layer used in the buffer layer increased from 1.25 to 2.35 nm. Using the best results from all of these investigations in terms of the stress and roughness produced, an optimised buffer layer was made to compensate the stress of a highly reflective multilayer whilst ensuring a high reflectivity was maintained. This sample achieved a stress of just −13 MPa and a reflectivity reduction of under 0.1%, from the unannealed highly reflective multilayer without buffer layer (with an internal stress of −300 MPa), down to R EUV =69.6%.

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