The extreme-ultraviolet (EUV)-induced oxidation of Mo/Si multilayer mirrors was characterized by several methods: EUV reflectivity, x-ray photoelectron spectroscopy, small-angle x-ray reflectometry, atomic force microscopy, and EUV scattering measurements. Based on the results of the different investigation techniques, an oxidation model was developed to explain the degradation of the mirrors under EUV radiation.

Scattering resulting from interface imperfections crucially affects the throughput and image contrast of EUV optics. Since both the substrate finish and the intrinsic thin film roughness influence the scattering, thorough investigations are needed to separate the different effects and to identify the most promising starting points for further optimizations. Mo/Si multilayer coatings deposited onto different substrates are investigated by utilizing an instrument for EUV reflectance and scattering measurements at 13.5 nm recently developed at the Fraunhofer IOF. The influences of the substrate finish and the deposition process onto the scattering are separated. Furthermore, the instrument allowed the EUV-induced degradation of Mo/Si mirrors to be investigated at the wavelength of application. In particular the impact of top-layer oxidation and roughening on the scattering properties is discussed.

The lifetime of Mo/Si multilayer-coated projection optics is one of the outstanding issues on the road of commercialization of extreme-ultraviolet lithography (EUVL). The application of Mo/Si multilayer optics in EUVL requires both sufficient radiation stability and also the highest possible normal-incidence reflectivity. A serious problem of conventional high-reflective Mo/Si multilayers capped by silicon is the considerable degradation of reflective properties due to carbonization and oxidation of the silicon surface layer under exposure by EUV radiation. In this study, we focus on titanium dioxide (TiO2) and ruthenium dioxide (RuO2) as promising capping layer materials for EUVL multilayer coatings. The multilayer designs as well as the deposition parameters of the Mo/Si systems with different capping layers were optimized in terms of maximum peak reflectivity at the wavelength of 13.5 nm and longterm stability under high-intensive irradiation. Optimized TiO2-capped Mo/Si multilayer mirrors with an initial reflectivity of 67.0% presented a reflectivity drop of 0.6% after an irradiation dose of 760 J/mm2. The reflectivity drop was explained by the partial oxidation of the silicon sub-layer. No reflectivity loss after similar irradiation dose was found for RuO2-capped Mo/Si multilayer mirrors having initial peak reflectivity of 66%. In this paper we present data on improved reflectivity of interface-engineered TiO2- and RuO2-capped Mo/Si multilayer mirrors due to the minimization of both interdiffusion processes inside the multilayer stack and absorption loss in the oxide layer. Reflectivities of 68.5% at the wavelength of 13.4 nm were achieved for both TiO2- and RuO2-capped Mo/Si multilayer mirrors.

Most applications of Mo/Si multilayer optics in EUV lithographic systems require a high normal-incidence reflectivity. Using dc-magnetron sputtering we achieved R=68.8% at the wavelength of 13.5nm. Different interface-engineered Mo/X/Si/X multilayers with a maximum reflectivity of 69.6% were developed. These new multilayer mirrors consist of molybdenum and silicon layers separated by different interdiffusion barriers (X=C and SiC). The Mo/C/Si/C interface-engineered mirrors were optimized in terms of high peak reflectivity at a wavelength near 13.5nm (Rp⩾60.0%) and broad operating temperature range (T=20–500°C). The best results were obtained with 0.8nm thicknesses of carbon interlayers on both interfaces. The combination of good optical properties and high thermal stability of interface-engineered Mo/C/Si/C multilayer mirrors underlines their potential for their use in EUV optics.

According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUV lithography development today. To meet these requirements, a new dc magnetron sputtering system NESSY and technologies to coat laterally graded EUV multilayers on curved optics were developed. The major characteristics of the deposition tool and results of sputtered multilayer optics are presented in this paper.

The EUV source output power and the collector optics lifetime have been identified as critical key issues for EUV lithography. In order to meet these requirements a heated collector concept was realized for the first time. An ellipsoidal collector substrate with an outer diameter of 320 mm was coated with a laterally graded high-temperature multilayer. The interface-engineered Mo/Si multilayer coating was optimized in terms of high peak reflectivity at 13.5 nm and a working temperature of 400 °C. Barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon to provide long-term optical stability of the multilayer at elevated temperatures. A normal-incidence reflectance of more than 40 % at 13.55 nm was measured after heating. After initial annealing at 400 °C for one hour, no degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests for up to 100 hours and multiple annealing cycles. The successful realization of this high-temperature sub-aperture collector mirror represents a major step towards the implementation of the heated collector concept and illustrates the great potential of high-temperature EUV multilayer coatings.

The application of multilayer optics in EUV lithography requires not only the highest possible normal-incidence reflectivity but also a long-term thermal and radiation stability at operating temperatures. This requirement is most important in the case of the collector mirror of the illumination system close to the EUV source where a short-time decrease in reflectivity is most likely. Mo/Si multilayer mirrors, designed for high normal reflectivity at the wavelength of 13.5 nm and deposited by dc magnetron sputtering, were directly exposed to EUV radiation without mitigation system. They presented a loss of reflectivity of more than 18% after only 8 hours of irradiation by a Xe-discharge source. Another problem of Mo/Si multilayers is the instability of reflectivity and peak wavelength under high heat load. It becomes especially critical at temperatures above 200 degrees C, where interdiffusion between the molybdenum and the silicon layers is observed. The development of high-temperature multilayers was focused on two alternative Si-based systems: MOSi2/Si and interface engineered Mo/C/Si/C multilayer mirrors. The multilayer designs as well as the deposition parameters of all systems were optimized in terms of high peak reflectivity (>= 60%) at a wavelength of 13.5 nm and high thermal stability. Small thermally induced changes of the MOSi2/Si multilayer properties were found but they were independent of the annealing time at all temperatures examined. A wavelength shift of -1.7% and a reflectivity drop of 1.0% have been found after annealing at 500 degrees C for 100 hours. The total degradation of optical properties above 650 degrees C can be explained by a recrystallization process of MOSi2 layers.

The effect of elevated temperatures on the optical and structural stability of MOSi2/Si and Mo/C/Si/C multilayer coatings was investigated. The multilayer mirrors were designed for normal-incidence reflectivity at a wavelength of about 13.5 nm. The multilayers were deposited by dc-magnetron sputtering and subsequently annealed at temperatures of 400 degrees C and 500 degrees C for 1, 10 and 100 hours. X-ray scattering, transmission electron microscopy, atomic force microscopy and normal-incidence reflectivity measurements were used for the characterization of the multilayer structures. We achieved maximal normal-incidence reflectivities of 41.2 % and 59.6 % for as-deposited MOSi2/S and Mo/C/Si/C multilayer mirrors. While the optical properties of Mo/C/Si/C multilayers changed monotonically during annealing time at temperatures of more than 400 degrees C, the MOSi2/Si mulitilayers showed a superior thermal stability up to 500 degrees C. New barrier layer materials were also suggested to enhance the thermal stability of Mo/Si multilayers. Interface-engineered Mo/Si multilayer mirrors were designed to combine both a high reflectivity of more than 60 % at 13.5 nm and a superior long-term thermal stability of up to 500 degrees C.

The paper proposes to review briefly steps of classical experimental progress towards resistant VUV-XUV coatings. It intends to address some of the new challenges of the VUV-XUV radiation resistant coatings, including material investigations, manufacturing, characterizations and active optical components.

Most applications of Mo/Si multilayer optics in Extreme ultraviolet lithography (EUVL) require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ λ = 13.45 nm. High-reflective Mo/Si/C and high-temperature stable Mo/C/Si/C multilayer mirrors with reflectivity of 69.6 % and 61.0 % at 13.5 nm were developed. Microstructure and optical properties of the multilayers have been investigated by small and large angle Cu-Kα scattering and characterized by EUV reflectivity. Beside the periodic multilayer design, Mo/Si multilayer mirrors with increased as well as reduced bandwidth in their spectral and angular reflectance have been designed and deposited. A reflectivity of more than 20 % was achieved in the wavelength range from 13 nm to 15 nm. In addition, narrowband multilayer mirrors with a significantly reduced band-width (FWHM = 0.077 nm) basing on high order reflection have been designed and fabricated. Both the increase and the reduction of the reflection bandwidth are unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such specially designed mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology of EUV sources. According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUVL development today. To meet these requirements, a new dc magnetron sputtering system has been developed.

The stability of Mo/Si multilayer mirrors under extreme ultraviolet (EUV) radiations has been investigated. Mirrors were deposited by DC magnetron sputtering and designed for maximal reflectivity at the wavelength of 13.5nm. Investigating samples after irradiation by three different EUV sources (Xe-gas discharge, compact laser and X-ray tube), a layer was found on top of mirrors. These contamination layers mainly consisted of oxygen and carbon but with thicknesses depending on the kind of source which was used. A maximal carbon thickness of 45nm after 210min of irradiation existed with the compact laser source and a minimal one of 1nm after 10h of irradiation was found with the X-ray tube. Furthermore, this deposition process was demonstrated to have a two stages of contamination rates.