Metallic Multilayer Thin Films Research Articles

Dissolution of Mg(OH)2 by swift heavy ion irradiation in CoFe2O4/MgO/ZnFe2O4 multilayer thin films

In-situ system for metallic multilayer thin films used for magnetic tunnel junction (MTJ) required a multi-target system to avoid the formation of impurities which make these devices very costly. In this work, the study was performed to resolve this issue by using a single target system for multilayer formation and impurity phases were dissolved by swift heavy ion (SHI) irradiation. A pristine MgO/Si(100) and CoFe2O4/MgO/ZnFe2O4/Si(100) (CFMZF) multilayer thin film were prepared via radio frequency (RF) sputtering technique. These structures were irradiated by 75 MeV oxygen-ion (O-ion) fluence (5x1011, 1x1012, and 5x1012 ions/cm2) and 200 MeV silver-ion (Ag-ion) fluence (1x1012 ions/cm2) for the investigation of structural changes. The high-resolution X-ray diffraction (HRXRD) reveals that MgO and CFMZF thin films have impurity peaks, dissolved via O-ion and Ag-ion irradiation. Further HRXRD analysis confirmed that Ag-ion irradiation dissolves more peaks leaving one small intensity peak of (400) corresponding to MgO. The multilayer stack of CFMZF irradiated via Ag-ion shows that Mg(OH)2 phase were completely dissolved and appears only (311) less intense peak corresponding to the ferrite structure. The findings are also corroborated by cross-sectional field emission scanning electron microscopy of CFMZF multilayer thin film showing damaged layers with Ag-ion irradiation. Therefore, the SHI irradiation technique can be used to dissolve the impurity peaks and improve the interface of the multilayer stacks.

Interdiffusion in as-deposited Ni/Ti multilayer thin films analyzed by atom probe tomography

The nanoscale design of metallic multilayer thin films is one crucial factor that greatly influences the kinetics, often inducing unusual phase transformations. Metastable or amorphous phases may directly form in as-deposited films of certain thicknesses, which is common for Ni/Ti multilayers. Atom probe tomography and X-ray diffraction analyses are performed here to study the interdiffusion and structural changes as a function of the bilayer thickness in Ni/Ti multilayers. The films are deposited by DC magnetron sputtering with near 50:50 compositions. Multilayers with 5 nm bilayer thickness are found to be highly intermixed, with compositions up to ∼25 at.% for both diffuser metals, inducing amorphization reactions during deposition.

Atomic scale analysis of phase formation and diffusion kinetics in Ag/Al multilayer thin films

Thin films generally exhibit unusual kinetics leading to chemical reactions far from equilibrium conditions. Binary metallic multilayer thin films with miscible elements show some similar behaviors with respect to interdiffusion and phase formation mechanisms. Interfacial density, lattice defects, internal stresses, layer morphologies and deposition conditions strongly control the mass transport between the individual layers. In the present work, Ag/Al multilayer thin films are used as a simple model system, in which the effects of the sputtering power and the bilayer period thickness on the interdiffusion and film reactions are investigated. Multilayers deposited by DC magnetron sputtering undergo calorimetric and microstructural analyses. In particular, atom probe tomography is extensively used to provide quantitative information on concentration gradients, grain boundary segregations, and reaction mechanisms. The magnitude of interdiffusion was found to be inversely proportional to the period thickness for the films deposited under the same conditions, and was reduced using low sputtering power. Both the local segregation at grain boundaries as well as pronounced non-equilibrium supersaturation effects play crucial roles during the early stages of the film reactions. For multilayers with small periods of 10 nm supersaturation of the Al layers with Ag precedes the polymorphic nucleation and growth of the hcp γ-Ag2Al phase. In larger periods the γ phase formation is triggered at junctions between grain boundaries and layers interfaces, where the pathway to heterogeneous nucleation is local supersaturation. Other Ag-rich phases also form as intermediate phases due to asymmetric diffusion rates of parent phases in the γ phase during annealing.

Investigation of Intermixing/Implantation in Pulsed Laser Deposited Metallic Cr/Ni80Fe20 and Ni80Fe20/Polypyrrole Multilayer Thin Films

Cr/Ni80Fe20 and Ni80Fe20/Polypyrrole multilayer thin films were prepared by pulsed laser deposition in vacuum and in Argon background gas at different pressures. Intermixing/implantation at their interfaces were investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analysis. Surface morphology (step height and roughness) of these samples were carried out by atomic force microscopy (AFM) and magnetic property (saturation magnetic flux) measurement by hysteresis loop tracer. The magnetic and surface morphology characterisations were used to calculate the interfacial mixing and/or implantation depth which have been found to agree with direct measured values. A correlation of these intermixing depths with Monte Carlo simulations of ion bombardment in the deposition process, using Stopping range of ion in matter (SRIM) incorporated with Transport of ion in matter (TRIM) gave the energy of the most implanted ionic species in a very simple way. Under high vacuum pressure, intermixing/implantation depth at the interfaces was high and occurs due to the presence of energetic species in the ablated plasma plume. With Argon gas background of increasing pressure, a reduction in energetic species energy is followed with a decrease in intermixing/implantation depth and the scattering of the ablated species leads to a decrease in the growth rate of Cr. An Argon background gas pressure of 20 mTorr gave the kinetic energy of the most energetic Cr species as 150 eV (from simulation) with 0.61 nm intermixing/implantation depth, while the vacuum pressure of 1.3 × 10− 7Torr gave the most energetic Cr energy as 3.2 keV with 2.38 nm depth of intermixing. Vacuum pressure 5 × 10− 7Torr with deposition of Permalloy (Py) Ni80Fe20 into polypyrrole gave measured intermixing/implantation depths of 6.81 nm with Ni80Fe20 energy of about 800 eV. The surface of the Py/polypyrrole (Ppy) was very rough at vacuum pressure 5 × 10− 3 Torr deposition as confirmed by the atomic force microscope measurement and STEM image than under background gas pressure 1 × 10− 3 Torr deposition. This is helpful for stoichiometric preparation of metallic multilayer thin films with well defined individual sharp interfaces and surface.

Strain rate sensitivity and activation volume of Cu/Ni metallic multilayer thin films measured via micropillar compression

Micropillar compression testing with repeated jumps in strain rate is used to circumvent inherent difficulties associated with nanoindentation and tensile testing of free-standing films. Application to sputtered 21 nm/21 nm Cu/Ni multilayer thin films with a cube-on-cube texture reveals an average strain rate sensitivity (m = 0.014) and activation volume (V = 17 b3), comparable to nanocrystalline face-centered cubic metals. Yet, m increases by ∼50% and V decreases by 70% with increasing strain, opposite to trends reported for nanotwinned Cu. The large, strain-dependent shifts in m and V are dependent on the underlying misfit dislocation structure of Cu/Ni interfaces.

Estimates of interfacial properties in Cu/Ni multilayer thin films using hardness data

Modeling the defect structure and mechanical properties of metallic multilayer thin films requires estimates of dislocation parameters such as interfacial line energy and barrier strength, as well as resistance to confined layer slip. A method is presented to estimate these parameters using experimental measurements of hardness vs. individual layer thickness h . Application to Cu/Ni multilayers suggests that the interfacial line energy increases dramatically with h (5 ⩽ h ⩽ 50 nm), far in excess of values assumed in analytic and dislocation dynamics models.

APT analyses of deuterium-loaded Fe/V multi-layered films

Interaction of hydrogen with metallic multi-layered thin films remains as a hot topic in recent days. Detailed knowledge on such chemically modulated systems is required if they are desired for application in hydrogen energy system as storage media. In this study, the deuterium concentration profile of Fe/V multi-layer was investigated by atom probe tomography (APT) at 60 and 30 K. It is firstly shown that deuterium-loaded sample can easily react with oxygen at the Pd capping layer on Fe/V and therefore, it is highly desired to avoid any oxygen exposure after D 2 loading before APT analysis. The analysis temperature also has an impact on D concentration profile. The result taken at 60 K shows clear traces of surface segregation of D atoms towards analysis surface. The observed diffusion profile of D allows us to estimate an apparent diffusion coefficient D. The calculated D at 60 K is in the order of 10 −17 cm 2/s, deviating 6 orders of magnitude from an extrapolated value. This was interpreted with alloying, D-trapping at defects and effects of the large extension to which the extrapolation was done. A D concentration profile taken at 30 K shows no segregation anymore and a homogeneous distribution at c D=0.05(2) D/Me, which is in good accordance with that measured in the corresponding pressure–composition isotherm.

Effect of dislocation source length on yield strength of nanostructured metallic multilayer thin films

There is great interest in metallic multilayer thin films with the layer thickness on the order of nanometers due to their high hardness and strength. The yield strength deviates from Hall–Petch and Orowan predictions with layer thickness as the controlling factor, and other factors are in need to explain this phenomenon. In this work, the effect of dislocation source length on yield strength of multilayer system in the nanoscale regime is investigated using a three dimensional cellular automaton dislocation model. Different initial dislocation source lengths are assumed in the model for the metallic multilayer thin films with different layer thicknesses. The multilayer system studied here is composed of alternating A and B components with FCC structure, cube-on-cube orientation relationship, and the same layer thickness. The results show that the initial dislocation source length can provide an explanation for the above deviations.

Analytical electron microscopy study of growth mechanism for smoothing of metallic multilayer thin films

We have used scanning transmission electron microscopy and electron energy-loss spectroscopy techniques to study the nature of interfacial interactions in a Ta∕CuOx stack that give rise to a smooth surface morphology, which can be utilized for seeding thin magnetic multilayer devices. Our measurements reveal that the interfacial smoothing is mainly due to the preferential reaction of Ta with O at the Ta∕CuOx interface assisted by grain boundary diffusion of oxygen which thereby acts to smooth out the surface roughness created by the large crystalline grains of Cu.

Dislocation-based modeling of the mechanical behavior of epitaxial metallic multilayer thin films

A 3D dislocation cellular automaton model is employed to simulate yield and hardening in nanostructured metallic multilayer thin films during in-plane, biaxial tensile loading. The films consist of two types of alternating, single-crystalline FCC layers with (0 0 1) epitaxy, a mismatch in stress-free lattice parameter, but no elastic modulus mismatch. The simulations monitor the operation of interfacial and threading sources with lengths greater than the individual layer thickness. At larger layer thickness, strength increases with decreasing layer thickness, due to slip confinement to individual layers. At sufficiently small layer thickness, slip confinement is not possible, even during initial stages of plastic deformation. Consequently, strength is not controlled by layer thickness but rather by source length, as well as coherency stress and interfacial barrier strength. Here, strength may increase, decrease, or reach a plateau depending on how source length and barrier strength vary with layer thickness.

Some aspects of atom probe specimen preparation and analysis of thin film materials

Some of the factors in the preparation of atom probe specimens of metallic multilayer thin films have been investigated. A series of Ti/Nb multilayer films were sputtered deposited on n-doped Si [0 0 1] substrates with either 5 or 0.05 Ω cm resistivity. Each wafer was pre-fabricated into a series of 5 μm×5 μm× ≈80 μm island posts by photolithography and reactive ion etching. Once the film was grown on the wafer, a Si post was mounted to either a tungsten or stainless steel fine tip needle that was mechanically crimped to a Cu tube for handling. The specimen was then loaded into a Focus Ion Beam instrument where a sacrificial Pt cap was in situ deposited onto the surface of the film and subsequently annularly ion milled into the appropriate geometry. The Pt cap was found to be an effective method in reducing Ga ion damage and implantation into the film during milling. The multilayers deposited on the high resistivity Si exhibited uncontrolled field evaporation which lead to high mass tails in the mass spectra, a reduction in the mass resolution, high background noise, propensity for “flash-failure”, and a variation in the apparent layer thickness as the experiment elapsed in time. The multilayers deposited on lower resistivity Si did not suffer from these artifacts.

Thermal Stability of Self-Supported Metallic Multilayered Thin Films

ABSTRACTThe morphological stability and strength retention following elevated temperature exposure or thermal cycling will be crucial in exploiting the extremely high strengths of nanolayered materials in advanced engineering applications. The effects of elevated temperature (≤800°C) vacuum annealing on the morphological stability and mechanical properties of sputter deposited Cu-Nb multilayers with 75 nm bilayer period are reported here. Even after 800°C/ 1 hour anneal, the continuity of the layered structure is maintained and the bilayer periods are unchanged. The in-plane grain sizes in both Cu and Nb coarsened but were anchored at grooved boundaries preventing further growth. For a constant bilayer period, the effect of increasing the in-plane grain size on the multilayer hardness is found to be insignificant. After annealing, the layers are observed to be offset by shear along a vertical plane at the triple point junctions that have equilibrium groove angles aligned in a zig-zag pattern. A new mechanism is proposed for the evolution of this “anchored” structure that is resistant to further morphological instability.

The power-law magnetic-field-dependence of the giant magnetoresistance ratio in thin film

The temperature- and magnetic-field-dependences of the resistance in giant magnetoresistance (GMR) (LCMO) thin films have been studied. It is found that the magnetoresistance ratio can be expressed as near the ferromagnetic transition temperature when H<3 T. The temperature-dependence of is determined by A(T) and the exponent below 60 K and above 80 K. Under certain conditions, this kind of power-law magnetic-field-dependence of in LCMO thin film is similar to that in metallic magnetic multilayer or grain-type GMR thin films.

Growth and structure of polycrystalline Cr/Au multilayered thin films

Metallic multilayered thin films have recently been investigated due to their new magnetic and transport properties. The interest here is focussed on the characterization of the interfaces between the layers. The analysis of growth and structure of polycrystalline Cr/Au multilayers is accomplished by two complementary techniques: in situ ultrahigh vacuum scanning tunnelling microscopy and ex situ transmission electron microscopy. The combination of these powerful methods provides detailed information about structural characteristics such as crystallite size, surface roughness and crystallographic orientation. Moreover, conclusions can be drawn on the atomic arrangement and growth mechanism at the Cr-Au interface. The results are supported by semiempirical and theoretical expectations.

High performance metallic multilayer thin films on modified polypropylene with calcium carbonate fillers

Metallic multilayer thin films have been successfully plated on modified polypropylene with finely dispersed calcium carbonate (CaCO 3) fillers. The peel strength of plated thin films was found to increase, mainly owing to the effect of the addition of filler rather than to the pretreatment parameters. Polypropylene moulded with 20% powdered CaCO 3 filler shows the best peel strength, reaching a maximum value 3.14 kgf cm -1, which is higher than previously reported for such metal platings. A scanning electron microscope was used to study the surface morphology of etched specimens with different filler ratios to elucidate the relationship between microstructure and peel adhesion. The mechanical bonding between metal and plastic was determined by the etched holes, whose distribution, width and depth on the surface influence the peeling strength of plated films. The ultimate peel strength was found to be attained before the surface became overetched. The advantages and wide functional applications of the newly developed multilayer metal coatings on modified polypropylene are also discussed.

Elastic Moduli of Metallic Multilayered Thin Films Ag/Cu, Al/Cu and Co/Cu Measured by Brillouin Scattering Method

AbstractThe Rayleigh wave velocity of metallic superlattices Ag/Cu, Al/Cu and Co/Cu deposited by ion beam sputtering were measured as a function of layering period Λ using Brillouin scattering method. The VR-Λ relationship can be well interpreted using a phenomenological model consisting of three elastically different layers, i.e. two original and an intermediate layer. We have assumed in this model that the elastic modulus and the thickness of the intermediate layer are independent on the layering period.