The source output power and lifetime, including the collector optics lifetime, are among the key issues for EUV lithography systems. In order to meet the requirements for the EUV collector mirror, both the reflectivity and the long-term thermal stability of its multilayer coating have been enhanced considerably during recent development efforts. Sub-aperture ellipsoidal mirrors of different substrate materials with outer diameters of about 320 mm were coated with
laterally graded high-temperature multilayers. The interface-engineered Mo/Si multilayer mirror (MLM) coatings were optimized in terms of high peak reflectivity at 13.5 nm and working temperatures above 400°C. Thin barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon and to provide long-term optical stability of the coating at elevated temperatures. A normal-incidence reflectance of R ~ 60 %
at 13.5 nm was measured on Si wafer samples after heating up to 600°C. No degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests and multiple annealing cycles. On highly polished collector substrates with improved surface roughness a reflectance for s-polarized light exceeding peak values of R = 57 % was obtained. With optimized layer gradient the degree of wavelength matching was improved, as well,
resulting in peak reflectivity values above 56 % throughout the clear aperture for a series of measurement points across the mirror. The corresponding area-weighted 2% in-band average reflectance for this collector mirror coating exceeds 52 % for unpolarized light.
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
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°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°C for 100 hours. The total degradation of optical properties above 650°C can be explained by a recrystallization process of MoSi2 layers.
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.
Most applications of Mo/Si multilayer optics in EUVL require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ λ = 13.5 nm. Different interface-engineered Mo/X/Si/X multilayers with maximum reflectivity of 69.6 % at 13.5 nm were developed. These new multilayer mirrors consist of molybdenum and sili-con layers separated by different interdiffusion barriers (X = C and SiC). Microstructure and optical properties of the multilayers have been investigated by small and large angle Cu-Kα scattering, AFM and characterized by EUV reflectometry. A concept for material selection, thickness optimization of interdiffusion barriers and perspectives for their wide application in imaging EUVL optics will be discussed. Some applications of multilayer mirrors in EUVL require not only the highest possible normal incidence reflectivity but also a long-term and thermal stability at the operating temperatures. The Mo/C/Si/C interface-engineered were optimized in terms of high peak reflectivity at a wavelength near 13.5 nm (Rp ⩾ 60.0 %) and broad operating temperature range (T = 20 - 500°C). The best results were obtained with 0.8 nm thickness of carbon interlayers on both interfaces. Annealing in vacuum was carried out at elevated temperatures up to 650 °C for up to 100 hours. 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 EUVL optics.
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 °C and 500 °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 °C, the MoSi2/Si multilayers showed a superior thermal stability up to 500 °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 °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.