Interface quality and thermal stability of laser-deposited metal/MgO multilayers

Metal/MgO multilayers (metal of Fe, Ni80Nb20, and Ti) with bilayer periods in the range 1.2-3.0 nm have been prepared by pulsed laser deposition and characterized by both hard and soft-x-ray reflectometry. The interface roughness is found to be < or = 0.5 nm in all the samples and is nearly independent of the total number of deposited bilayers. The interface roughness, however, depends on the absolute thickness of the individual layers and increases from approximately 0.3 nm for a 3.0-nm period to approximately 0.5 nm for a bilayer period of 1.2 nm. The multilayers are found to be highly stable up to temperatures as high as 550 degrees C. The hard-x-ray reflectivity of the multilayers decreases for T > 300 degrees C, whereas the layered structure is stable up to 550 degrees C. The reflectivity in the water window region of soft x rays, lambda = 3.374 nm, was found to be 0.4% at an angle of incidence of approximately 54 degrees for multilayers with 60 bilayers at a period of approximately 2.1 nm.     

C/Si multilayer mirrors for the 25-30-nm wavelength region

We report a new material combination, C/Si, for normal-incidence multilayer mirrors in the wavelength region 25-30 nm. The multilayers, fabricated by ion-beam-sputtering deposition, were characterized by near-normal-incidence reflectance measurements by using a discharge source and a grazing-incidence monochromator. The highest measured near-normal-incidence reflectance was R = 23% (25.6 nm), R = 20% (28.3 nm), R = 25% (30.4 nm) at incident angles of 10 degrees , 12 degrees , and 4 degrees , respectively. The multilayers were also characterized by transmission electron microscopy, which revealed sharp layer interfaces and low interfacial roughness.     

Self-organization of CrSi2 nanoislands on Si(111) and growth of monocrystalline silicon with buried multilayers of CrSi2 nanocrystallites

The process of self-organization of CrSi2 nanosize islands on Si(111)7 x 7 surface has been investigated during reactive deposition (RDE) of Cr at 500 degrees C by methods of low energy diffraction (LEED) and differential reflectance spectroscopy (DRS). Morphology of grown samples has been studied by atomic force microscopy (AFM). DRS data have demonstrated the semiconductor nature of silicide islands from the beginning of Cr deposition at 500 degreesC. The optimal temperature (750 0C) and optimal Si thickness (50 nm) have been determined for silicon molecular epitaxy (MBE) growth atop CrSi2 nanosize islands. Monolithic silicon-silicide heterostructures with multilayers of CrSi2 nanosize crystallites have been grown.     

Ni80Fe20/Mo纳米多层膜的互扩散

针对Ni80Fe20/Mo纳米多层膜在实际应用中经常面临的热稳定性问题，研究了多层膜在343－683K温度范围内的互扩散行为，测定了其有效互扩散系数Dλ。小的激活焓来源于多层膜中存在的共格应变。当温度低于483K时，NiFe/Mo多层膜具有较强的抵制NiFe和Mo亚层间互扩散能力，这对NiFe/Mo多层膜今后的实际应用有重要意义。     

Aging and thermal stability of Mg/SiC and Mg/Y2O3 reflection multilayers in the 25-35 nm region

Reflection measurements in the 25-35 nm region were made for Mg/SiC and Mg/Y2O3 multilayers kept in a low-humidity atmosphere for 4 or 5 years. Aged Mg/SiC multilayers keep their reflectances, and the reflectance value at 31.2 nm is 0.44 at 10 degrees of the normal angle of incidence. Aged Mg/Y2O3 multilayers change reflectance as top layer materials, and the best value at 30.1 nm is 0.40 at 10 degrees. Reflection measurements are also made for Mg-based multilayers that are annealed from room temperature to 400 degrees C at 50 degrees C intervals. Both multilayers keep their reflectance at annealing temperatures of 200 degrees C. These results suggest that both Mg-based multilayers can be applied to practical optics.     

Supermirror hard-x-ray telescope

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.     

Carbon/Titanium multilayers as soft-x-ray mirrors for the water window

C/Ti multilayers with a period thickness of 2.1-2.7 nm were produced by electron-beam evaporation in ultrahigh vacuum as soft-x-ray mirrors in the water window (lambda = 2.3-4.4 nm). For smoothing the individual interfaces and thus enhancing the total reflectance, each layer was ion polished with an Ar(+) ion beam after deposition. For a multilayer of 85 bilayers, a reflectance of approximately 11% at an angle of incidence of 59 degrees (with respect to the surface normal) by use of s-polarized radiation at a wavelength of 2.77 nm was achieved.     

Structure and stability of 2.4 nm period amorphous Ni–Nb/C multilayers

Amorphous Ni–Nb/C multilayers with a period of 2.4 nm were prepared by pulsed laser ablation deposition. The as-deposited multilayers were found to have an interdiffused Ni_1/3Nb_1/3C_1/3 layer present at the two interfaces; Ni_1/2Nb_1/2/C and C/N_1/2Nb_1/2. The specular reflectivity and diffuse scattering studies show that the interface roughness is chemical and not morphological in origin. The structural studies performed using X-ray scattering techniques after a period of 2.5 years from deposition show that the behaviour does not change with time. These results indicate that the multilayered structure is temporally stable in spite of the strong composition dependent driving force for chemical homogenization. The structural stability against homogenization is due to the presence of an amorphous Ni_1/3Nb_1/3C_1/3 layer present at the interfaces which acts as a diffusion barrier.     

Supermodulus effect by grain-boundary wetting in nanostructured multilayers

The effect of thermal treatments on mechanical properties was systematically investigated in Ni/Mo multilayers with a constant modulation period(160 nm)prepared by magnetron sputtering deposition.A supermodulus effect was found in the annealed multilayers as compared to the as-deposited state.A large tensile stress development was observed in the multilayers.The evolution of grain-boundary(GB)wetting was observed at the interfaces of the multilayers,which results in an enhanced modulus based on the mechanism of GB-wetting-induced interfacial stress/strain.The GB wetting phenomenon was further supported by a thermodynamic calculation.The results not only bring clear evidence of the important role of interfacial structures in governing the elastic behavior of metallic multilayers,but also allow designing the multilayers with special properties through atomic diffusion and wetting at the interfaces based on the thermodynamic calculation.     

Effect of plating time on growth of nanocrystalline Ni–P from sulphate/glycine bath by electroless deposition method

Nanocrystalline nickel phosphorus (NC-Ni–P) deposits from sulphate/glycine bath using a simple electroless deposition process is demonstrated. In the present investigation, nanoporous alumina films are formed on the aluminium surface by anodization process followed by deposition of nickel onto the pores by electroless plating method. Anodic aluminium oxide surface was first sensitized and activated by using palladium chloride solution before immersing into the electroless nickel bath. Electroless nickel plating was carried out from the optimized bath by changing the deposition time from 20 to 1800 s at a constant temperature of 80 °C and a pH of 4·0. Surface morphology, elemental composition, structure and reflectance of the deposits have been analysed by using scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis, X-ray diffractometry and UV-visible spectroscopic studies, respectively. Electroless nickel deposits formed at an early stage produces dense uniform nanocrystals containing higher percentage of atomic phosphorus with cubic Ni (111) structure. As the deposition time increased, nanocrystalline sharp peak became amorphous and dimension of the crystal size varied from 54 to 72 nm.     

Survey of Ti-, B-, and Y-based soft x-ray-extreme ultraviolet multilayer mirrors for the 2- to 12-nm wavelength region

We have performed an experimental investigation of Ti-, B(4)C-, B-, and Y-based multilayer mirrors for the soft x-ray?extreme ultraviolet (XUV) wavelength region between 2.0 and 12.0 nm. Eleven different material pairs were studied: Ti/Ni, Ti/Co, Ti/Cu, Ti/W, B(4)C/Pd, B/Mo, Y/Pd, Y/Ag, Y/Mo, Y/Nb, and Y/C. The multilayers were sputter deposited and were characterized with a number of techniques, including low-angle x-ray diffraction and normal incidence XUV reflectometry. Among the Ti-based multilayers the best results were obtained with Ti/W, with peak reflectances up to 5.2% at 2.79 nm at 61° from normal incidence. The B(4)C/Pd and B/Mo multilayer mirrors had near-normal incidence (5°) peak reflectances of 11.5% at 8.46 nm and 9.4% at 6.67 nm, respectively, whereas a Y/Mo multilayer mirror had a maximum peak reflectance of 25.6% at 11.30 nm at the same angle. The factors limiting the peak reflectance of these different multilayer mirrors are discussed.     

The compressive stress transition in Al,V, Zr,Nb and W metal films sputtered at low working pressures

Investigation of internal stresses in thin sputtered films of Al, V, Zr, Nb and W extends the observation of compression at low working pressures that was originally detected with Cr (D. W. Hoffman and J. A. Thornton, Thin Solid Films, 40 (1977) 355–363) and probes the atomic mass dependence of the transition pressure that was subsequently detected with Ti, Ni, Mo and Ta (J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol., 14 (1977) 164–168). The observation of compressive stresses at sufficiently low sputtering pressures in all ten of the elements so far examined strongly supports the generality of this phenomenon. The five new metals under study also confirm the overall increase of the transition pressure with atomic mass indicated by the metals examined earlier. However, periodic deviations from the general trend are now evident among the group IVB, VB and VIB elements examined. The electrical conductivity and optical reflectance of the sputtered metal films exhibit abrupt changes in behavior near the transition pressure for compressive stresses. Above the transition pressure the conductivity and reflectance drop off rapidly; below the transition pressure these properties tend to level off at maximum values for the sputtered metal films. The interpretation of these observations in terms of a peening mechanism of energetic particle bombardment is discussed. Data are presented for films up to 0.3 μm thick sputtered onto glass substrates at a nominal deposition rate of 1 nm s^?1 over the pressure range 0.067–4.0 Pa of argon.     

Nanostructured Ni3Al layers and Ni3Al/Ni multilayers obtained by magnetron sputtering

Intermetallic Ni₃Al thin layers and Ni₃Al/Ni multilayers were deposited on a Si wafer by means of magnetron sputtering. The structure and morphology of the layers have been characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The polycrystalline films are textured in the (111) direction and have grain sizes below 20 nm. Superlattice reflections due to chemical order have been observed in the electron microscope. It is shown by x-ray diffraction that the multilayers grow coherently on the amorphous substrate.     

Structure and normal incidence soft-x-ray reflectivity of Ni-Nb/C amorphous multilayers

Multilayers for the water window region of the soft x rays have been prepared by pulsed laser ablation with amorphous Ni(50) Nb(50) and amorphous C. The structural characterization of the multilayers, period d = 2.41 nm, shows that the interfaces are sharp with a roughness of only 0.4 nm that is chemical, not morphological, in origin. The interface roughness was found to be uncorrelated in the direction normal to the plane of the film. The normal incidence soft-x-ray reflectivity of the multilayer at 4.85-nm wavelength is 0.06%, 1 order of magnitude lower than the theoretically predicted value. However, the resolution limit lambda/Dlambda of the multilayer was found to be 16.7, close to the theoretically predicted value.     

Design of grazing-incidence multilayer supermirrors for hard-x-ray reflectors

Extremely broadband grazing-incidence multilayers for hard-x-ray reflection can be obtained by a gradual change of the layer thicknesses down through the structure. Existing approaches for designing similar neutron optics, called supermirrors, are shown to provide respectable performance when applied to x-ray multilayers. However, none of these approaches consider the effects of imperfect layer interfaces and absorption in the overlying layers. Adaptations of neutron designs that take these effects into account are presented, and a thorough analysis of two specific applications (a single hard-x-ray reflector and a hard-x-ray telescope) shows that an improved performance can be obtained. A multilayer whose bilayer thicknesses are given by a power law expression is found to provide the best solution; however, it is only slightly better than some of the adapted neutron designs.     

Field deposition from metallic tips onto insulating substrates

The deposition of gold ions from atomic force microscope cantilever tips onto bulk insulating substrates with nearby surface electrodes is discussed. Numerical models of the potential distribution are used to estimate potential barriers for the desorption process. These models indicate deposition height thresholds of 7-10 nm with the tip 20-25 nm from the metallic electrode edge over a KBr surface but greater than 20 nm high for InP/GaAs/InP substrates with a two-dimensional electron gas (2DEG) as the back electrode. Experimental results for the deposition of gold clusters over KBr surfaces near metal electrodes in ultra-high vacuum (UHV) are presented and show promising agreement with calculations of the deposition threshold heights. Deposition of clusters over InP is discussed for comparison and indicates similar trends.     

Fe/V多层膜中亚稳结构Fe的形成及其磁学性能

采用电子束蒸镀的方法,通过改变多层膜的周期结构,成功地制备出具有不同晶格常数的bcc亚稳结构铁相的Fe/V多层膜,并研究了亚稳结构铁相形成对其磁性影响的规律.实验结果表明,多层膜中Fe与V层均由纳米晶粒组成.Fe层厚度小于2nm时,受多层膜界面自由能作用,Fe与V相互准外延生长,多层膜由点阵常数一致的体心立方相组成,其点阵常数随样品V/Fe层厚度比的增大而增加.多层膜平均原子磁矩随铁或钒层厚度的改变发生明显变化:当钒层厚度固定为6nm时,铁原子磁矩随铁层厚度的增加逐渐下降,在2nm处出现极小值后又随铁层增厚而回升;对于铁层厚度固定为1.6nm的样品,磁矩在钒层厚度为3nm时出现极大值.     

Nanosized Ni/SiO2 catalyst prepared by homogeneous precipitation in wet silica gel

A Ni/SiO2 catalyst was prepared by homogeneous precipitation of nickel hydroxide in a sol-gel-derived wet silica gel. The preparation process consists of two successive steps: gelation of silica in the presence of nickel nitrate and urea at 50 degrees C, followed by aging at higher temperature, typically at 80 degrees C, to decompose the urea. The decomposition of urea increases the pH of the solution in the wet gel, leading to the concurrence of structural rearrangement of silica gel and deposition of nickel species. As a result, the structure of the silica changes from a ramified polymeric network into particle aggregates that entrap the nickel cations in the particles. The resulting Ni/SiO2 contains large mesopores that have high thermal stability up to 1000 degrees C and highly dispersed Ni metal particles with typical crystallite size of 4 nm even at high Ni content at 20 wt%.     

Separate cobalt-copper interface smoothening under the action of low-energy argon ion bombardment

Multilayer film structures of the types “buffer gold layer (20 nm)-copper (x nm)-cobalt (y nm)-protective gold layer (10 nm)” and “buffer gold layer (20 nm)-cobalt (y nm)-copper (x nm)-protective gold layer (10 nm)” with x+y≈6 nm obtained by ion beam deposition in vacuum were studied. It was established that the smoothest surface is obtained for a structure of the second type in which the cobalt layer prior to copper deposition was additionally irradiated with a beam of argon ions at an energy below the threshold energy of sputtering in the common vacuum deposition cycle. Additional irradiation with a mixture of argon and helium ions under analogous conditions leads to the formation of a less smooth structure. It is concluded that the cobalt-copper interface is smoothened as a result of multiple collisions of the low-energy argon ion with cobalt atoms during the former ion stopping within two to three of uppermost atomic layers of the target.     

Anomaly in Fabrication Processes for Large-Scale Array Detectors of Superconducting Tunnel Junctions

The STJ array detectors with an effective detection area of 4 mm², which consist of 100 Nb/Al-AlOx/Al/Nb junctions with a size of 200×200 μm, have been fabricated. In order to improve the reproducibility of the STJ array fabrication, we investigated a correlation between the junction surface structures and the leakage currents. It has been found that the junctions near the fringe of the array detectors have a step of about 5 nm at the middle of the array detector, of which leakage currents are considerably larger than 1 μA. The step structure was formed after the etching of the bottom Nb layer for complete separation of Nb/Al/AlOx/Al/Nb/Si. In case of the sputtered Nb/Al/Nb/Si multilayers without 1 nm-thick tunneling barrier, no stepped surface was observed even after the bottom Nb layer etching. Therefore, it is apparent that the 5 nm step structure is a cause of the large leakage currents. We solved the step-fringe problem by a kind of extra patterning along the fringe of the array or lift-off patterning of the Nb/Al multilayers. It is concluded that the number of the junctions with the step structure depends on a slight difference in film deposition or etching conditions.