Structural characterization of ultrathin Cr and Sc films for soft X-ray mirrors

The structure of multilayers of ultrathin scandium (Sc) and chromium (Cr) films has been characterized by means of transmission electron microscopy (TEM). Face centered cubic Sc was found both in magnetron sputtered thin Sc layers on Si(0 0 1) and in Cr/Sc multilayers for soft X-ray mirrors. The single Sc and Cr layers are polycrystalline with randomly oriented grains, while Sc and Cr within the Cr/Sc multilayer show a strong [0 0 1] texture in the deposition direction. From high-resolution images the orientation-relationship at the Cr/Sc interfaces could be deduced as: Sc_[110]//Cr_[100] and Sc_[010]//Cr_[110], which was confirmed by image simulations.     

Damage to extreme-ultraviolet Sc/Si multilayer mirrors exposed to intense 46.9-nm laser pulses

The damage threshold and damage mechanism of extreme-ultraviolet Sc/Si multilayer mirror coatings are investigated with focused nanosecond pulses at 46.9-nm radiation from a compact capillary-discharge laser. Damage threshold fluences of approximately 0.08 J/cm2 are measured for coatings deposited on both borosilicate glass and Si substrates. The use of scanning and transmission electron microscopy and small-angle x-ray diffraction techniques reveals the thermal nature of the damage mechanism. The results are relevant to the use of newly developed high-flux extreme-ultraviolet sources in applications.     

Reactive diffusion in Sc/Si multilayer X-ray mirrors with CrB2 barrier layers

Processes undergoing in Sc/Si multilayer X-ray mirrors (MXMs) with periods of ∼27 nm and barrier layers of CrB₂0.3- and 0.7-nm thick within the temperature range of 420–780 K were studied by methods of small-angle X-ray reflectivity (λ=0.154 nm) and cross-sectional transmission electron microscopy. All layers with the exception of Sc ones are amorphous. Barrier layers are stable at least up to a temperature of 625 K and double the activation energy of diffusional intermixing at moderate temperatures. Introduction of barriers improves the thermal stability of Sc/Si MXMs at least by 80 degrees. Diffusion of Si atoms through barrier layers into Sc layers with formation of silicides was shown to be the main degradation mechanism of MXMs. A comparison of the stability for Sc/Si MXMs with different barriers published in the literature is conducted. The ways of further improvement of barrier properties are discussed.     

Evolution of the roughness of amorphous quartz surfaces and Cr/Sc multilayer structures upon exposure to ion-beam etching

The effect of ion-beam etching on surfaces of Cr/Sc multilayer structures and amorphous quartz roughness is studied. The dependence of roughness on the depth of etching, incidence angle, and ion energy is considered. Optimal parameters of etching which provide minimal evolution of surface roughness are presented.     

EUV damage threshold measurements of Mo/Si multilayer mirrors

We present 1-on-1 and 10-on-1 damage threshold investigations on Mo/Si multilayers with EUV radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. The experiments were performed on different types of Mo/Si mirror, showing no significant difference in single pulse damage thresholds. However, the damage threshold for ten pulses is ??60?% lower than the single pulse threshold, implying a defect dominated damage process. Using Nomarski (DIC) and atomic force microscopy (AFM) we analysed the damage morphologies, indicating a primarily thermally induced damage mechanism. Additionally, we studied the radiation-induced change of reflectivity upon damage of a multilayer mirror.     

Optimization of platinum-carbon multilayer mirrors for hard X-ray optics

The design of hard X-ray optics for astrophysical technology is one of the key technologies for investigating active galaxies and clusters of galaxies. In the present paper, we have optimized multilayer mirrors of platinum-carbon layer pairs for the hard X-ray region at different grazing angles. The Luus-Jaakola optimization procedure has been implemented for the global optimization of multialyer mirrors. With this algorithm it is not necessary to specify initially the number of layers present in a design.     

CEMS-study of the interface in Fe/Cr multilayer systems

The magnetic hyperfine (hf.) fields at the Fe/Cr interface were analyzed in epitaxial Fe/Cr thin film structures of (100)- and (110)-orientation with monolayer resolution by means of in-situ⁵⁷Fe Conversion Electron Mössbauer Spectroscopy (CEMS). The hf. field (300 K) in the 1st Fe-monolayer (ML) at the interface has been found to be strongly reduced to 22.0/20.9 T for (110)-/(100)-orientation, whereas the 2nd and 3rd ML reveal a slightly increased hf. field of 33.7 T as compared with the Fe-bulk value of 33.4 T. The temperature dependence of the hf. field at the interface shows aT ^3/2 spin wave law. The spin wave parameters are enlarged with respect to the bulk value indicating a reduced exchange interaction. A discontinuity in theT ^3/2-dependence is interpreted by the onset of magnetic order (Néel-temperature) of the Cr layers adjoining the⁵⁷Fe probe layer.     

Computer simulation of blistering in multilayer mirrors for EUV lithography

Progress in extreme UV lithography depends on the quality of multilayer radiation collecting mirrors (made of Mo/Si), in which radiation damage can cause porosity. Accumulation of bulk defects (blisters) is represented by the model of the first order phase transition at a fluctuation stage. Computer simulation of defect distribution functions depending on the sizes and arrangement in layers allows the determination of possible ways for the properties of the layer material properties to deteriorate.     

Graphs for the design of laser mirrors at normal incidence

The results of computer calculations concerning the reflectivity in the stop-band at normal incidence are given for periodic multilayer systems made out of layers of ZnS and MgF₂ of equal optical thickness. General formulae are derived to distinguish between the maxima and minima of the reflectivity at the centre of the stop-bands and for the calculation of the width of the stop-band of higher order mirrors as a function of the wavelength.The bandwidth of higher order mirrors and the reflectivity at the centre of the stop-band are presented in a number of graphs for systems with up to 18 layers on substrates with varying refractive index. Quarter-wave layer anti-reflection coatings are treated as special cases. The process of designing laser mirrors can be simplified using the graphs. They may also be useful during the evaporation process when thickness monitoring is done by optical methods. An example of a partially reflecting mirror is given. This uses a flat minimum of reflectivity at the centre of the stop-band to obtain an almost constant reflectivity over the total width of the stop-band.     

Molybdenum-strontium multilayer mirrors for the 8-12-nm extreme-ultraviolet wavelength region

Mo-Sr multilayer mirrors were successfully deposited by dc-magnetron sputtering and characterized in situ with synchrotron radiation. Normal-incidence (3.6 degrees ) reflectance of 23.0% at 8.8 nm, 40.8% at 9.4 nm, and 48.3% at 10.5 nm were measured before the samples were exposed to air. After exposure, as a result of the reactivity of Sr with oxygen and water vapor, the reflectance of these multilayers decreased rapidly. Attempts to use thin layers of C to passivate the surface of these Mo-Sr multilayers were unsuccessful.     

Reduction of interlayer thickness by low-temperature deposition of Mo/Si multilayer mirrors for X-ray reflection

Thin interlayers are essential for high-quality multilayer optics. We present the first investigation of reducing the interlayer thickness of Mo/Si multilayer structures by cooling the substrate with liquid nitrogen during the deposition. The structures were deposited by means of electron beam evaporation. Even after warming up to room temperature prior to analysis, the interlayers that formed upon cryogenic deposition were found to be approximately 60% thinner compared to room temperature deposition. The interlayer thickness reduction at low temperature and its preservation upon warming up are attributed to a lower mobility of adatoms, reduced surface segregation of Si during Mo-on-Si growth, and/or crystallization of Mo.     

Thermal stability of Mo/Si multilayer soft-X-ray mirrors fabricated by electron-beam evaporation

Mo/Si multilayers are fabricated by electron-beam evaporation in UHV at different temperatures (30° C, 150° C, 200° C) during deposition. After completion their thermal stability is tested by baking them at temperatures (T _bak) between 200° C and 800° C in steps of 50° C or 100° C. After each baking step the multilayers are characterized by small angle Cu_K-X-ray diffraction. Additionally, the normal incidence soft-X-ray reflectivity for wavelengths between 11 nm and 19 nm is determined after baking at 500° C. Furthermore, the layer structure of the multilayers is investigated by means of Rutherford Backscattering Spectroscopy (RBS) and sputter/Auger Electron Spectroscopy (AES) technique. While the reflectivity turns out to be highest for a deposition temperature of 150° C, the thermal stability of the multilayer increases with deposition temperature. The multilayer deposited at 200° C stands even a 20 min 500° C baking without considerable changes in the reflectivity behaviour.     

Design of broadband and broad angular Cr/C multilayer reflector for “near water window” region

Traditional periodic multilayer reflector working in the soft X-ray region should be placed at the designed incidence angle and wavelength, which is extremely difficult and complicate in practice especially for “near water window region” (4.43-6.50 nm). In this region, Ni-like-Ta laser working at the wavelength 4.48 nm provides a possible source which has caught a lot attention. In this paper, to overcome the shortcoming of the periodic structure, top-flat broad band (normal incidence) and broad angular (around quasi-Brewster angle) Cr/C non-periodic multilayer reflector have been designed for the 4.48 nm soft X-ray optical system. In the wavelength region 4.473-4.488 nm, 4.455-4.496 nm and 4.435-4.529 nm, mean theoretical normal-incidence reflectivity can get 35.0%, 24.2% and 12.6%, respectively, by the top-flat broad band reflectors. In the grazing incidence angle region 44.85-45.16°, 44.54-45.46° and 44.02-45.98°, mean s-component theoretical reflectivity at 4.48 nm can get 39.7%, 23.1% and 13.0%, respectively, by the top-flat broad angular reflectors.     

高反射率Mo/B4C多层膜设计及制备

运用遗传算法优化设计了Mo/B4C多层膜结构。入射光入射角度取10°时,设计的理想多层膜膜对数为150,周期为3.59 nm,Gamma值（Mo膜厚与周期的比值）为0.41,峰值反射率为33.29%。采用恒功率模式直流磁控溅射方法制作Mo/B4C多层膜。通过在Mo/B4C多层膜与基底之间增加15 nm厚的Cr粘附层,提高多层膜与基底的粘附力。另外,还采用调整多层膜Gamma值的方法减小其内应力,调整后多层膜结构周期为3.59 nm, Mo膜厚1.97 nm, B4C膜厚1.62 nm,峰值反射率26.34％。制备了膜对数为150的Mo/B4C膜并测量了其反射率,在波长7.03 nm处,Mo/B4C多层膜的近正入射反射率为21.0％。最后对测量结果进行了拟合,拟合得到Mo/B4C多层膜的周期为3.60 nm,Gamma值0.60,界面粗糙度为0.30 nm。     

高反射率Mo/B4C多层膜设计及制备

 运用遗传算法优化设计了Mo/B₄C多层膜结构。入射光入射角度取10°时,设计的理想多层膜膜对数为150,周期为3.59 nm,Gamma值（Mo膜厚与周期的比值）为0.41,峰值反射率为33.29%。采用恒功率模式直流磁控溅射方法制作Mo/B₄C多层膜。通过在Mo/B4C多层膜与基底之间增加15 nm厚的Cr粘附层,提高多层膜与基底的粘附力。另外,还采用调整多层膜Gamma值的方法减小其内应力,调整后多层膜结构周期为3.59 nm, Mo膜厚1.97 nm, B4C膜厚1.62 nm,峰值反射率26.34％。制备了膜对数为150的Mo/B₄C膜并测量了其反射率,在波长7.03 nm处,Mo/B₄C多层膜的近正入射反射率为21.0％。最后对测量结果进行了拟合,拟合得到Mo/B₄C多层膜的周期为3.60 nm,Gamma值0.60,界面粗糙度为0.30 nm。     

On the preparation of high reflectance aspherical mirrors by thin-film deposition

The replacement of thick aluminium layers by dielectric films during the manufacture of high reflectivity aspherical mirrors was investigated. Aluminium oxide was found to be a suitable material for this purpose.     

Electronic structure and magnetism of Cr/Sn multilayer systems

The electronic structure and magnetism of Cr/Sn and Fe/Cr/Sn/Cr multilayer systems with monolayer Sn are studied by means of a first-principles method. The calculated hyperfine field at Sn site is significantly large (∼20 T) in Cr/Sn multilayers, while the value is remarkably diminished (∼4 T) in the case of Fe/Cr/Sn/Cr multilayers. This trend of the hyperfine field is consistent with recently reported experimental results. It is found that the hyperfine field at Sn site is determined by the spin magnetic moment at the interface Cr site. The most important feature in the electronic structure of the multilayer systems is the existence of an interface state at the Cr interface layer near the Fermi energy region.