Analysis of the diffusion processes in Al/Cr, Al/Fe and Cr/Fe multilayers using the MRI model

One of the methods for synthesis of intermetallic films consists of two steps: deposition of a multilayer film containing monometallic layers, and consequential annealing to induce a diffusion process and alloy formation. Though the diffusion coefficients are generally known for couples of common metals, they can considerably differ for films deposited by PVD due to their specific microstructure. Bimetallic multilayers Al/Cr, Al/Fe and Cr/Fe were analysed as the first step for formation of the complex metallic alloy Al₄(Cr,Fe). The films were deposited by triode sputtering with a total thickness of about 250 nm and consisted of 6 layers. Annealing was performed in vacuum at temperatures 240-650 °C.The depth distributions of elements in the films annealed at various temperatures were measured by Auger electron spectroscopy. Detailed analysis of the profile was conducted using the MRI model, which takes into account interface broadening of the measured profile due to three reasons: ion-induced atom mixing, roughness, and information depth of analysed electrons. Thus we reconstructed the true depth profile of the as-deposited samples and profiles of the annealed samples that allowed us to extract the diffusion coefficients.     

二元系纳米多层薄膜非线性互扩散研究

采用非均匀体系非线性动力学离散模型计算AB二元系纳米多层薄膜的非线性互扩散,研究了A、B原子间作用力之差VBB-VAA和有序能V对纳米多层薄膜浓度分布和界面结构的影响.对于V≤0的纳米多层薄膜,随着VSS-VAA由0减小到-0.05eV,扩散过程中多层薄膜浓度偏离对称分布,界面由宽化向平直转变,有序能V由0减小到-0.025eV,导致多层薄膜的扩散速度加快.当V>0时,扩散过程中多层薄膜出现相分离趋势导致的上坡扩散,初始宽化互混的界面变得平直,且随着VBB-VAA由0减小到-0.05V,多层薄膜扩散过程中形成的浓度起伏长大成为新的亚层.AB二元系纳米多层薄膜扩散过程中的浓度分布和界面结构演变与扩散非对称性系数m'、有序能V和初始时刻的浓度梯度有关.     

Effect of Cu surface segregation on properties of NiFe/FeMn bilayers

The films of NiFe/FeMn with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with Ta/Cu buffer is lower than that of the films with Ta buffer. The crystalline texture, surface roughness and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved X-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with Ta/Cu buffer layers is mainly caused by the diffusion of Cu atoms through NiFe layer, which stayed at the interface of NiFe/FeMn film or even intruded into FeMn layer. The present results indicate that Cu segregation through NiFe layer should be suppressed in order to improve the exchange bias field in giant magnetoresistance spin valves with Cu spacer.     

18O-tracer diffusion along nanoscaled Sc2O3/yttria stabilized zirconia (YSZ) multilayers: on the influence of strain

Abstract

The oxygen tracer diffusion coefficient describing transport along nano-/microscaled YSZ/Sc₂O₃ multilayers as a function of the thick­ness of the ion-conducting YSZ layers has been measured by isotope exchange depth profiling (IEDP), using secondary ion mass spec­trometry (SIMS). The multilayer samples were prepared by pulsed laser deposition (PLD) on (0001) Al₂O₃ single crystalline substrates. The values for the oxygen tracer diffusion coefficient were analyzed as a combination of contributions from bulk and interface contributions and compared with results from YSZ/Y₂O₃-multilayers with similar microstructure. Using the Nernst–Einstein equation as the relation between diffusivity and electrical conductivity we find very good agreement between conductivity and diffusion data, and we exclude substantial electronic conductivity in the multilayers. The effect of hetero-interface transport can be well explained by a simple interface strain model. As the multilayer samples consist of columnar film crystallites with a defined inter­face structure and texture, we also discuss the influence of this particular microstructure on the interfacial strain.     

Structural characterization and lifetime stability of Mo/Y extreme-ultraviolet multilayer mirrors

We have observed a dramatic dependence of the extreme ultraviolet (EUV) reflectivity of Mo/Y multilayers on the oxygen content of yttrium. This is explained as being due to a change in the microstructure and an increase in roughness of the yttrium layers and not just to an increase in absorption owing to the amount of oxygen within the yttrium layers. We found that the best reflectivity of 38.4% was achieved with an oxygen content of 25%, which was reduced to 32.6% and 29.6% for multilayers manufactured from oxygen-free yttrium and 39%-oxygen yttrium, respectively. These results highlight the importance of including experimentally determined optical constants as well as interface roughness in multilayer calculations. In addition, the lifetime stability of Mo/Y multilayers with different capping layers was monitored for 1 year. The molybdenum- and palladium-capped samples exhibited low surface roughness and approximately 4% relative reflectivity loss in 1 year. The relative reflectivity loss of the yttrium-capped sample (yttrium with 39% oxygen) was approximately 8%. However, the reflectivity loss in all three capping layers occurred within the first 100 days after the deposition, and the reflectivity remained stable afterward.     

The role of energetic ions in the development of multilayered X-ray reflection optics: An overview

Multilayer systems at nanometre scale have been applied as spectroscopic elements for wavelengths >1 nm and optical reflection systems in the short-wavelength region (<100 nm). Main requirements are a high optical contrast for the multilayer components in the wavelength region of application and a density profile with sharp interfaces between the components. Two sources are responsible for interface roughness, the development of surface roughness during deposition of the multilayer components and intermixing of these components at the interfaces. Etching a freshly deposited layer by low-energy ions has demonstrated to be successful in reducing the surface roughness of W on C, Ni on C and Si on Mo. Implantation of low-energy N⁺ ions into Si has been applied to reduce the chemical reactivity at the interface with Ni. Formation of a carbide interlayer by implantation of Si with C⁺ ions successfully prevented intermixing of the Mo on Si interfaces, improving the thermal stability of Mo/Si multilayers. Finally, a combination of implantation and annealing was applied to form Si/SiC multilayers.     

Nonlinear interdiffusion in binary nanometer-scale multilayers submitted to thermal annealing

The concentration profile in binary A-B nanometer-scale multilayers submitted to thermal annealing was calculated based on the Martin's kinetic discrete model for one-dimension nonlinear interdiffusion by a diffusion asymmetry coefficient m′ and an ordering energy V between A and B atoms. With decreasing the diffusion asymmetry coefficient m′ from 0 to − 6, the concentration profile of the multilayers deviated from symmetrical distribution, and their interfaces became sharp and shifted towards the side of the sublayer with lower pair interaction energy. The difference of diffusion coefficient of A and B atoms caused by the diffusion asymmetry coefficient m′ led to the difference of net fluxes of A and B atoms in the multilayers. When the ordering energy V changed from − 0.001 eV to − 0.05 eV, change in the concentration profile and interface structure was same for the multilayers with a given diffusion asymmetry coefficient m′, but the calculated diffusion time decreased correspondently. The lower ordering energy V makes the A and B atoms aggregating more easily during the interdiffusion. It is found that the nonlinear interdiffusion of a series of binary nanometer-scale multilayers can be characterized by the diffusion asymmetry coefficient m′ and the ordering energy V, to explore the solid state reaction between the sublayers of nanometer-scale multilayers.     

Kinetic Monte Carlo simulation of {1 1 1}-oriented SiC film with chemical vapor deposition

A three-dimensional atomic-scale kinetic Monte Carlo (KMC) model of {1 1 1}-oriented SiC film deposited by chemical vapor deposition is established in this paper. The growth process of {1 1 1}-oriented atomic-scale SiC film is simulated. The model includes two parts: the first is kinetic process of chemical reaction and the second is deposition and diffusion of substrate surface. In this model, the relationship between temperature and growth rate, surface roughness and relative density and the relationship between growth rate and surface roughness and relative density are studied. The result indicates that the growth of film has three stages including formation of little islets, mergence and expanding of islets and dynamic balance between islets. With increase of substrate temperature, deposition rate, surface roughness and height of film all increase. With increase of deposition rate, surface roughness increases while relative density decreases.     

Kinetic Monte Carlo simulation of Cu thin film growth

A three-dimensional kinetic Monte Carlo technique has been developed for simulating the growth of thin Cu films. The model involves incident atom attachment, surface diffusion of the atoms on the growing surface and atom detachment from the growing surface. A significant improvement in calculation of activation barriers for the surface atom diffusion on the growing film was made. The related effects caused by surface atom diffusion were taken into account. The results showed that there exist a transition temperature T_t at a certain deposition rate. When the substrate temperature approaches T_t, the growing surface becomes smoother and the relative density of the films increases. The surface roughness minimizes and the relative density saturates at T_t. The surface roughness increases with increased substrate temperature when the temperature is higher than T_t. T_t is a function of the deposition rate. The influence of the deposition rate on the surface roughness is dependent on the substrate temperature. The simulation results also showed that the relative density decreases with increasing deposition rate and average thickness of the film.     

Monte Carlo Study of the Interface Effects on the Magnetic Properties of the Magnetic/Nonmagnetic (M/NM) Layers

A Monte Carlo study of the thermal variation of multilayers magnetization is carried out. We find a number of characteristics behaviors of the interface magnetization with its roughness. The spontaneous magnetization and magnetic susceptibility are calculated. It is found that multilayers magnetization decreases and phase transition temperature from ferromagnetic to paramagnetic varies with increasing interface thickness and concentration of nonmagnetic atoms in interface. Moreover, the susceptibility exhibits peaks at the transition temperature and it is interface morphology dependent. The dependence to the spin-exchange coupling is reported.     

Grenzflächen‐optimierte Mo/Si Multischichten als Reflektoren für den EUV Spektralbereich. Interface‐optimized Mo/Si multilayers as reflectors for the EUV spectral range

EUV lithography is the most promising technique for the fabrication of semiconductor structures below 50 nm. This requires the use of reflecting multilayers as optical elements. These multilayers must have reflectances as high as possible since it determines the efficiency of the technique and therefore the throughput of a future chip fab. In this work we present investigations on the interface quality of Mo/Si multilayers which are prepared by magnetron sputter deposition. Starting from the two‐component Mo/Si system, that has mainly been optimized with respect to interface roughness, we show that interface interdiffusion can also be reduced by the introduction of tiny barrier layers. In pure Mo/Si multilayers particularly a low Ar sputter gas pressure is important to get smooth layers, whereas the interdiffusion can be reduced by the deposition of C and B₄C barrier layers on the individual interfaces. As result of our work, we have prepared Mo/Si multilayers with outstanding high reflectances: R_EUV = 70.1 % (λ = 13.3 nm, α = 1.5°), R_EUV = 71.4 % (λ = 12.5 nm, α = 22.5°).     

Extreme-ultraviolet Mo/Si multilayer mirrors deposited by radio-frequency-magnetron sputtering

Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 130-135-? spectral region have been deposited by rf magnetron sputtering. A new and quick technique was used to calibrate the deposition rates. Characterization by transmission electron microscopy (TEM) and grazing-incidence x-ray diffraction (XRD) indicated good thickness control of the deposition process and low interface roughness. However, the TEM and, indirectly, the XRD reflectance measurements, indicated that the interfaces are asymmetric. A brief review discussing the origin of the modulation of the Bragg peak intensities in the XRD reflectance is given. An analytical formula was derived for periodic multilayers that describes the effect of asymmetric interfaces on the amplitude of the Bragg peak modulation. Theoretically, in XRD reflectance measurements, any asymmetry in the interdiffusion of the Mo-Si interfaces results in a decrease of the usual amplitude modulation of the Bragg peaks. Extreme-ultraviolet (XUV) reflectance measurements were also made with synchrotron radiation on a new high-resolution reflectometer. The near-normal-incidence peak reflectances measured at λ = 134 ? were ~ 59% for the best multilayer mirrors. Good fits to both XRD and XUV reflectance measurements have been obtained with a model that allows for interface asymmetry.     

Influence of growth rate on the structural and morphological properties of TiN, ZrN and TiN/ZrN multilayers

The effect of the deposition rate on the structural and morphological properties of TiN and ZrN single layers and TiN/ZrN multilayers deposited by radiofrequency reactive magnetron sputtering has been studied. The total pressure was kept constant and the growth rate variation was obtained by small difference of nitrogen concentration in the fed gas. The decreasing deposition rate results in a structural change in the thin films from (111) orientation to (100) one. As consequence the surface morphology becomes smoother. Films roughness is strongly related with texture and it decreases with an increase in the (100) X-ray diffraction line intensity. In order to achieve a clear interpretation of our experimental results, the ratio between the N⁺ ions of the plasma and the atoms number reaching the substrate was considered. At high deposition rate with respect to the N⁺ concentration, the chemical potential of transition metal on (100) growth surface is higher than (111) one favouring the (111) orientation of the films. On the contrary, when the growth rate is low with respect to the nitrogen concentration, the chemical potential of transition metal on (111) growth surface is higher than the (100) one leading to a preferential growth in the (100) direction.     

Effects of ion-assisted growth on the layer definition in Cr/Sc multilayers

Nano-structural evolution of layer morphology and interfacial roughness in Cr/Sc metal multilayers grown with ion assistance during magnetron sputter deposition has been investigated by high resolution transmission electron microscopy and hard X-ray reflectivity. Calculations based on a binary collision model predict an ion-assisted growth window for optimized Cr/Sc multilayer interface sharpness, within the ion energy range of 21 eV to 37 eV and an ion flux of ∼ 10 ions per deposited atom. Multilayers with nominal modulation periods in the range of 1.6 nm to 10.2 nm, grown with these conditions, exhibit a well-defined layer structure with an improved flattening and abruptness of the interfaces. It is shown that multilayers with a modulation period smaller than 3.4 nm have clear benefit from the reduced intermixing obtained by utilizing a two-stage ion energy modulation for each individual layer. The amorphization of Sc and Cr layers, below certain thicknesses, is found to be independent of the low energy ion-assistance. It is also shown that the Cr/Sc multilayers, containing periods less than ∼ 2 nm are ‘self healing’ i.e. they re-gain abrupt interfaces and flat layers after morphological disturbances during ion assisted growth. In comparison, multilayers grown without ion-assistance exhibited severe roughness and layer distortions.     

Analyse de multicouches W/C par spectroscopie d'électrons Auger

Multilayered structures made of alternate carbon and tungsten films a few nanometres in thickness were analysed by Auger electron spectroscopy. The films were prepared by electron gun evaporation in a high vacuum. To obviate the lack of resolution of conventional depth profiling by sputter removal of very thin films, a special device was used which permitted the analysis to be performed during continuous deposition of the layers at room temperature.The amorphous carbon layers are free of contaminants; the shape of the carbon Auger lines reveals the occurrence of chemical bonding at both the C-W and the W-C interfaces. The tungsten layers contain oxygen at a concentration which varies with the deposition rate under a given pressure in the vacuum bell-jar.Comparison between computer simulation and experimental curves of the variation in Auger amplitude with film thickness shows that the migration of adatoms during the growth process allows the formation of continuous and uniform layers with a mean thickness in the nanometre range. As there is no evidence for interdiffusion between the two materials, it is concluded that the variation in electron density occurs locally over a depth comparable with the interatomic distance at both interfaces. It is thus expected that the optical performance of the W/C multilayers in the soft X-ray range will be essentially limited by interface roughness.     

Mo/Si multilayers for EUV lithography by ion beam sputter deposition

Mo/Si multilayers for applications in extreme ultraviolet (EUV) lithography have been prepared on Si wafer substrates using ion beam deposition. The multilayers were characterised by transmission electron microscopy, secondary ion mass spectroscopy, atomic force microscopy, photoelectron spectroscopy, X-ray reflectometry at grazing incidence, and EUV-reflectivity measurements at nearly normal incidence. The surface and the interfaces of the multilayers are rather smooth with only small roughness. The material properties of the layers are characterised by some intermixing and silicide formation at the Mo-Si interfaces and a polycrystalline grain structure of the Mo layers, which is in agreement with prior studies. Appearance of multilayer diffraction spots, well-resolved Kiessig fringes and other diffraction evidence indicate very good coherence of the wave fields and in this manner a good reproducibility of the multilayer period of 6.7 nm. Normal incidence peak reflectivities of 64-65% in the EUV spectral range were routinely obtained at 13.4 nm wavelength. This reflectivity value and the formation of an EUV standing wave field are confirmed using photoelectron spectroscopy, and an application for defect particle analysis is proposed. The obtained results are discussed in comparison to literature data of multilayers prepared by other deposition techniques and considering new attempts of interface engineering.     

Modeling and visualization of polycrystalline thin film growth

A novel polycrystalline thin film growth simulator, FACET, has been developed. FACET is a multi-scale model with two major components: an atomic level one-dimensional kinetic lattice Monte Carlo (1D KLMC) model and a real time feature scale two-dimensional facet nucleation and growth model.

The 1D KLMC model has been developed to calculate inter-facet diffusion rates. By inputting the diffusion activation energies, the model will calculate the inter-facet atomic flux between {1 0 0}, {1 1 0}, and {1 1 1} facets of FCC materials at any temperature. The results of the 1D KLMC model have been verified by comparison with a full three-dimensional kinetic lattice Monte Carlo (3D KLMC) model.

The feature scale polycrystalline thin film nucleation and growth model is based on describing grains in terms of two-dimensional faceted surfaces and grain boundaries. The profile of the nuclei are described by crystallographically appropriate facets. The position and orientation of the nuclei can be randomly selected or preferred textures can be created. Growth rates are determined from different deposition fluxes and surface diffusion effects. Quantitative microstructural characterization tools, including roughness analysis, average grain size analysis, and orientation distribution analysis, were incorporated into the model, which allows the users to design, conduct and analyze the virtual experiments within one integrated graphical user interface. Users can also visualize the nucleation and growth process of the film and obtain the final film microstructure. The effects of thickness, temperature, and deposition flux on thin film microstructures have been studied by FACET.     

On slippage induced by surface diffusion

The mechanism of surface diffusion is taken at the basis of the phenomenon of slippage of the contact line of a liquid film. With the aid of the condition of continuity of the traction vectors at the solid-liquid interface, we obtained an evolution equation for the velocity of the fluid particles at the wall which shows a marked resemblance with Millikan's equation for the slippage coefficient of gases and reduces, in the limit of small surface diffusivity, to the classical Stokes-Einstein model. The influence of surface roughness is explicitely taken into account and, among other results, cases of absence of slip caused by the attachment of the liquid film to the solid surface and of slippage solely induced by surface roughness are found. Finally, the effect of the surface deformation upon the surface velocity of the fluid particles is examined in some detail.     

A computer simulation of nucleation and growth of thin films

A three-dimensional kinetic Monte Carlo technique has been developed for simulating the nucleation and growth of thin films. The model involves incident atom attachment, surface diffusion of the atoms on the growing surface and atom detachment from the growing surface. Related effects caused by atom diffusion were taken into account. A significant improvement in calculation of activation energy for the atom diffusion was made based on a reasonable assumption of interaction potential between atoms. Trace files were created during the simulation and snapshots showing the morphology of the nucleation and growth of the thin films were taken by computer graph technique. The results showed that the density of the nucleus decreases and the size of island nucleation increases with increasing the substrate temperature and decreasing the deposition rate. At the meantime, a transition from two-dimension to three-dimension nucleation was observed. There exist three critical temperatures at a certain deposition rate: T_n at which the nucleation rate reaches maximum, T_r at which the surface roughness minimizes and T_d at which the relative film density saturates. The three critical temperatures are functions of the deposition rate. The nucleation rate is close to constant under lower temperatures while it increases with deposition rate at higher temperatures. The film surface roughness depends on the density of island nucleation, it increases with temperature at lower temperatures and decreases at higher temperatures. The relative film density decreases with increasing the deposition rate.     

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.