Interface characteristics of Mo/Si and B4C/Mo/Si multilayers using non-destructive X-ray techniques

A methodology combining non-destructive X-ray techniques is proposed to study the interfacial zones of periodic multilayers. The used X-ray techniques are X-ray emission spectroscopy induced by electrons and X-ray reflectivity in the hard and soft X-ray ranges. The first technique evidences the presence of compounds at the interfaces and gives an estimation of the thickness of the interfacial zone. These informations are used to constrain the fit of the X-ray reflectivity curves that enables to determine the thickness and roughness of the various layers of the stacks. The results are validated in the soft X-ray range where the reflectivity curves are very sensitive to the chemical state of the elements present in the stack. The methodology is applied to characterize Mo/Si (1-4 nm/2 nm) and B₄C/Mo/Si (1 nm/2 nm/2 nm) multilayers. It is shown that the two interfacial zones of the Mo/Si multilayers are composed of the silicides MoSi₂ and Mo₅Si₃. It is found that the interface thickness is about to be 0.4-0.8 nm depending on the samples. The molybdenum silicides are also evidenced at the interfaces of the B₄C/Mo/Si multilayers. However, their interface thickness is 0.2 nm thinner than that of the same stack without the B₄C layers, these layers being at the Mo-on-Si side or at the Si-on-Mo side. Thus, the B₄C layers do not stop but only reduce the interdiffusion between the Mo and Si layers.     

Mo/B4C软X射线多层膜结构特性研究

在6.7<λ<10.0nm波段选择Mo/B₄C作为多层膜材料,并采用磁控溅射法制备出多层膜样品.这些多层膜样品的周期结构为3.35nm～5.52nm.采用X射线衍射仪和透射电镜(TEM)对样品的微观结构进行研究.结构表明,这些多层膜样品的结构质量很高,并有很好的热稳定性.     

Interface characterization of B4C‐based multilayers by X‐ray grazing‐incidence reflectivity and diffuse scattering

B₄C‐based multilayers have important applications for soft to hard X‐rays. In this paper, X‐ray grazing‐incidence reflectivity and diffuse scattering, combining various analysis methods, were used to characterize the structure of B₄C‐based multilayers including layer thickness, density, interfacial roughness, interdiffusion, correlation length, etc. Quantitative results for W/B₄C, Mo/B₄C and La/B₄C multilayers were compared. W/B₄C multilayers show the sharpest interfaces and most stable structures. The roughness replications of La/B₄C and Mo/B₄C multilayers are not strong, and oxidations and structure expansions are found in the aging process. This work provides guidance for future fabrication and characterization of B₄C‐based multilayers.     

NiFe/Mo多层膜界面的电子显微学研究

用高分辨电子显微学方法研究了Ni₈₀Fe₂₀/Mo磁性多层膜，结果表明：(1)多层膜的结晶状态，随Mo非磁性层厚度而变化.当Mo层厚度为0.7nm时，多层膜基本为非晶；当Mo层厚度大于1.6nm时，Mo层和NiFe层内分别结晶为体心立方和面心立方多晶，层内晶粒尺寸为2—6nm.(2)在Mo层厚度为1.6和2.1nm的多层膜中，NiFe层和Mo层之间存在两种取向关系：(110)_Mo∥(111)_NiFe,［111］_关键词：     

Characterization of Pd/Y multilayers with B4C barrier layers using GIXR and X‐ray standing wave enhanced HAXPES

Pd/Y multilayers are high‐reflectance mirrors designed to work in the 7.5–11 nm wavelength range. Samples, prepared by magnetron sputtering, are deposited with or without B₄C barrier layers located at the interfaces of the Pd and Y layers to reduce interdiffusion, which is expected from calculating the mixing enthalpy of Pd and Y. Grazing‐incident X‐ray reflectometry is used to characterize these multilayers. B₄C barrier layers are found to be effective in reducing Pd–Y interdiffusion. Details of the composition of the multilayers are revealed by hard X‐ray photoemission spectroscopy with X‐ray standing wave effects. This consists of measuring the photoemission intensity from the samples by performing an angular scan in the region corresponding to the multilayer period and an incident photon energy according to Bragg's law. The experimental results indicate that Pd does not chemically react with B nor C at the Pd–B₄C interface while Y does react at the Y–B₄C interface. The formation of Y–B or Y–C chemical compounds could be the reason why the interfaces are stabilized. By comparing the experimentally obtained angular variation of the characteristic photoemission with theoretical calculations, the depth distribution of each component element can be interpreted.     

Effect of B4C diffusion barriers on the thermal stability of Sc/Si periodic multilayers

The optical properties of Sc/Si periodic multilayers are analyzed at three wavelengths in the X-ray range: 0.154, 0.712 and 12.7 nm. Fitting the reflectivity curves obtained at these three wavelengths enable us to constrain the parameters, thickness, density and roughness of the various layers, of the studied multilayers. Scattering curves were also measured at 12.7 nm on some samples to obtain an estimate of the correlation length of the roughness. Two sets of multilayers are used, with and without B₄C diffusion barrier at the interfaces. To see the efficiency of the B₄C layers the measures are performed after annealing up to 400 °C. A dramatic change of the structure of the Sc/Si multilayer is observed between 100 and 200 °C leading to a strong loss of reflectivity. For the Sc/B₄C/Si/B₄C multilayer the structure is stable up to 200 °C after which a progressive evolution of the stack occurs.     

Formation of short-period multilayer W/B4C compositions

The quantitative characteristics of the interlayer interaction in multilayer W/B₄C periodic compositions produced by magnetron sputtering are studied by small-angle X-ray diffraction using CuK _?? radiation and by electron microscopy of transverse cuts. It is found that approximately 0.85 nm of the tungsten layer thickness is consumed for the formation of mixed zones at layer boundaries. The mixed layers have a density of 13.4 ± 0.7 g/cm³ and contain tungsten in a bound chemical state. The effect of these mixed zones on the X-ray reflectivity of multilayer W/B₄C compositions is estimated. A method is proposed to determine the layer thickness at a small number of peaks in an X-ray diffraction pattern.     

Investigation in the interface roughness of DC-sputtered Mo/B4C multilayer mirrors with variable layer pairs for 7-nm soft X-ray polarizers

The surface and interface roughness of Mo/B₄C multilayer mirrors for 7-nm soft X-ray polarizer with variable layer pairs (N = 50, 70, 90 and 110), fabricated by DC sputtering technique is investigated by atomic force microscopy and X-ray scattering and reflecting. The experimental results present that the surface and interface roughness of Mo/B₄C multilayer mirrors increase layer by layer from its substrate as its Mo layer thickness greater than 2 nm, and the roughness grown tendency could be characterized by a quadratic function.     

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。     

软X射线Mo/B_4C多层膜应力和热稳定性研究

为了实现7nm波段Mo/B4C多层膜反射镜元件的制备,研究了不同退火方式对Mo/B4C多层膜应力和热稳定性的影响。首先,采用直流磁控溅射方法分别基于石英和硅基板制作Mo/B4C多层膜样品,设计周期为3.58nm、周期数为60,Mo膜层厚度与周期的比值为0.4。其次,采用不同的退火方式对所制作的样品进行退火实验,最高退火温度500℃。最后,分别采用X射线掠入射反射、X射线散射和光学干涉仪的方法对退火前后的Mo/B4C多层膜的周期、界面粗糙度和应力进行测试。测试结果表明采用真空退火方式能够有效降低Mo/B4C多层膜的应力,且退火前后Mo/B4C多层膜的周期和界面粗糙度无明显变化,证明Mo/B4C多层膜在500℃以内具有很好的热稳定性。     

XMCD studies of magnetic polarization at Mo atoms in CoMo alloy and magnetically coupled Co/Mo multilayers

Magnetic polarization of Mo atoms in Co₉₆Mo₄ alloy film and Co/Mo multilayered structures has been studied by X‐ray magnetic circular dichroism. Samples with Mo spacers of two different thicknesses (0.9 nm and 1.8 nm) were investigated. Mo atoms receive a magnetic moment of ?0.21μ_B in the alloy. In the multilayer with the thinner Mo spacer (d_Mo = 0.9 nm) the magnetic moment is much smaller (?0.03μ_B). In both cases the measured induced moment at the Mo site is oriented antiparallel to the moment at the Co atoms. The presence of the induced moment in the Mo spacer coincides with antiferromagnetic coupling between the Co component slabs. In contrast, neither measurable induced moment at the Mo site nor interlayer coupling between the Co layers has been found for the multilayer with the thicker Mo spacer. Possible mechanisms of the coupling associated with the induced moment are discussed in detail.     

Mo/Si软X射线多层膜的界面粗糙度研究

 用磁控溅射法分别制备了以Mo膜层和Si膜层为顶层的Mo/Si多层膜系列, 利用小角X射线衍射确定了各多层膜的周期厚度。以不同周期数的Mo/Si多层膜的新鲜表面近似等同于同一多层膜的内界面,通过原子力显微镜研究了多层膜界面粗糙度随膜层数的变化规律。并在国家同步辐射实验室测量了各多层膜的软X射线反射率。研究表明：随着膜层数的增加,Mo膜层和Si膜层的界面粗糙度先减小后增加然后再减小,多层膜的峰值反射率先增加后减小。     

Analysis of periodic Mo/Si multilayers: Influence of the Mo thickness

A set of Mo/Si periodic multilayers is studied by non-destructive analysis methods. The thickness of the Si layers is 5 nm while the thickness of the Mo layers changes from one multilayer to another, from 2 to 4 nm. This enables us to probe the effect of the transition between the amorphous and crystalline state of the Mo layers near the interfaces with Si on the optical performances of the multilayers. This transition results in the variation of the refractive index (density variation) of the Mo layers, as observed by X-ray reflectivity (XRR) at a wavelength of 0.154 nm. Combining X-ray emission spectroscopy (XES) and XRR, the parameters (composition, thickness and roughness) of the interfacial layers formed by the interaction between the Mo and Si layers are determined. However, these parameters do not evolve significantly as a function of the Mo thickness. It is observed by diffuse scattering at 1.33 nm that the lateral correlation length of the roughness strongly decreases when the Mo thickness goes from 2 to 3 nm. This is due to the development of Mo crystallites parallel to the multilayer surface.     

On the interplay of internal/external stress and thermal stability of Mo/Si multilayers

Mo/Si multilayer (ML) systems were deposited on Si(100) substrate by DC magnetron sputtering. The MLs were annealed at temperatures up to 440 °C under high-vacuum conditions, both with and without the influence of external mechanical stress, and characterized before and after thermal treatment by means of X-ray reflectometry, wide-angle X-ray scattering and optical microscopy. Two ML configurations were compared, one composed of pure Mo and Si layers and another with additional B₄C and C interlayers at the Mo/Si interfaces, respectively. The external mechanical stress applied caused bending of the substrate and adherent ML, with an accompanied internal stress of approximately 60 GPa. An important outcome of the investigation was that dedicated release bending of MLs can reduce/compensate the influences of the internal stressed states. Thermal stability could be increased for both ML systems during sample annealing. For ML samples with additional B₄C and C layers at the Mo/Si interfaces, the influence of external stress was more significant compared to that for pure Mo/Si MLs. This indicates that the additional layers mainly act as diffusion barriers and additionally as stress-relaxing buffers. PACS 68.60.Dv; 68.65.Ac; 42.79.Bh     

Interface roughness, surface roughness and soft X-ray reflectivity of Mo/Si multilayers with different layer number

A series of Mo/Si multilayers with the same periodic length and different periodic number were prepared by magnetron sputtering, whose top layers were respectively Mo layer and Si layer. Periodic length and interface roughness of Mo/Si multilayers were determined by small angle X-ray diffraction (SAXRD).Surface roughness change curve of Mo/Si multilayer with increasing layer number was studied by atomic force microscope (AFM). Soft X-ray reflectivity of Mo/Si multilayers was measured in National Synchrotron Radiation Laboratory (NSRL). Theoretical and experimental results show that the soft X-ray reflectivity of Mo/Si multilayer is mainly determined by periodic number and interface roughness, surface roughness has little effect on reflectivity.     

Multilayer La/B4C mirrors in the spectral region near 6.7 nm

The results from measuring reflection factors are presented for multilayer La/B₄/C mirrors with barrier layers of carbon and scandium. It is shown that a higher reflection factor is obtained by depositing a layer of carbon onto a surface of boron carbide. This effect is caused by diminishing absorption in the structure and the reduced width of transition regions.     

La/B4C small period multilayer interferential mirror for the analysis of boron

An La/B₄C multilayer interferential mirror with small period d (4.8 nm) was produced by diode sputtering for the detection of the boron K emission by wavelength‐dispersive x‐ray spectrometry at a large Bragg angle (close to 45°). The structure of the mirror was characterized by grazing incidence x‐ray reflectometry and its performance at the energy of the boron K emission (183 eV) was evaluated by means of polarized synchrotron radiation. Spectrometric measurements showed that the La/B₄C mirror improved the detection limit of boron using by a factor of 2 with respect to similar Mo/B₄C mirrors and by a factor of 4 with respect to a lead stearate crystal. Copyright © 2005 John Wiley & Sons, Ltd.     

Multilayer X-ray mirrors based on La/B<Subscript>4</Subscript>C and La/B<Subscript>9</Subscript>C

Multilayer X-ray mirrors that are based on La/B₄C and La/B₉C and intended for the reflection of X-ray radiation in the spectral region near λ = 6.7 nm are prepared and studied. Reflection coefficients at a level of 40–60% are achieved for mirrors with various periods. The difference in the interlayer roughnesses reconstructed from the data measured in the hard and soft X-ray spectral regions is explained using a structural model with an asymmetric permittivity profile in a mirror period. A proximate technique is developed to estimate the permittivity profile in a multilayer-structure period using reflectometry data. The effect of antidiffusion Sn, Cr, and Mo barriers on the reflection coefficient of multilayer La/B₄C structures is studied experimentally     

An etched multilayer as a dispersive element in a curved‐crystal spectrometer: implementation and performance

Etched multilayers obtained by forming a laminar grating pattern within interferential multilayer mirrors are used in the soft X‐ray range to improve the spectral resolution of wavelength dispersive spectrometers equipped with periodic multilayers. We describe the fabrication process of such an etched multilayer dispersive element, its characterization through reflectivity measurement and simulations, and its implementation in a high‐resolution Johann‐type spectrometer. The specially designed patterning of a Mo/B₄C multilayer is found fruitful in the range of the C K emission as the diffraction pattern narrows by a factor 4 with respect to the non‐etched structure. This dispersive element with an improved spectral resolution was successfully implemented for electronic structure study with an improved spectral resolution by X‐ray emission spectroscopy. As first results, we present the distinction between the chemical states of carbon atoms in various compounds, such as graphite, SiC and B₄C, by the different shape of their C K emission band. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermal stability and diffusion processes in Mo x Si y /Si multilayers studied with high-resolution RBS

Mo _x Si _y /Si multilayers with a period thickness of ∼7.5 nm and bilayers Mo _x Si _y /Si have been fabricated by e⁻-beam evaporation in UHV at a deposition temperature of 150°C [1]. The composition of the as-deposited layer systems and changes in the composition after baking the samples have been studied with high-resolution RBS. For a multilayer with a mixing ratioy/x≃2, no interdiffusion is observed up to a baking temperature of 830°C. For samples with a mixing ratioy/x≃1, diffusion is observed up to a baking temperature of 630°C, resulting in a mixing ratio close toy/x≃2. This mixing ratio remains almost stable up to ∼830°C, and considerable interdiffusion is only observed in those systems where regions with a mixing ratio smaller than 2 still exist. Possible reasons for the high thermal stability of the samples are the lack of a concentration gradient for Si in the system and/or the crystallization of MoSi₂.