MESA+ University of Twente
Industrial focus group XUV Optics


Approved Industrial Partnership Programme FOM 

Number: I10  
Title: Extreme UV Multilayer Optics (XMO)
Executive organisational unit: Rijnhuizen
Programme management: Prof. Dr. F. Bijkerk (
Duration: 2005-2010
Cost estimate: M€ 7.9
Carl Zeiss SMT AG


To develop and apply the physics and associated process technology of compounded periodic multilayer structures, which have atomically sharp, flat interfaces, and are chemically stable, radiation damage resistant, and dimension controlled down to the sub-nanometer range.

Background, relevance and implementation

Multilayer mirrors, i.e. periodical stacks of alternating high-Z and low-Z materials, allow Bragg reflection of short-wavelength radiation. When engineered with sub-nanometer precision, Mo/Si mirrors reflect up to 69.5% at near-normal incidence of 13.5 nm radiation. As such, they are enabling the class of optics needed for Extreme UV Lithography, the high-resolution integrated-circuit manufacturing method now developed by ASML and Carl Zeiss SMT. The commercial feasibility however, necessitates major improvements of mirror lifetime and reflectivity, and a revised and considerably more detailed study on the physics of the short-wavelength reflection process, the multilayer design, and the various layer growth processes. To realize lifetime improvements, for instance, ultra thin, yet fully stable capping layers can be applied to cope with surface oxidation. This involves studies on surface photo-chemistry, compound formation and oxidation, and materials migration. The combination of high photon energy and high flux leads to localized fields in the multilayer structure, the generation of secondary electrons, layer interdiffusion, void formation, and microcrystallites with pre-set orientations. Interface instabilities resulting from these effects, will be investigated on specially prepared diffusion barrier layers with sub-nm thickness. These may reduce the intermixing, and preserve the optical bi-layer contrast of both periodic and aperiodic structures. Applied to complex optics elements, these multilayer systems are targeted to meet the requirements of Extreme UV Lithography concerning EUV reflectivity and mirror lifetime. The main part of this research programme is closely connected to the industrial Zeiss-ASML EUVL development road map.