Al Capping Layer Research Articles

Experimental studies of solid state surface wetting of tin (Sn) on aluminium (Al)

Under ultra high vacuum (UHV) conditions tin (Sn) forms a monoatomic wetting layer on aluminium (Al) surfaces if Sn-islands formed by a preceding deposition process are present. Previous experimental observations and Kinetic Monte Carlo (KMC) simulations suggest that wetting layer formation is governed by thermally activated surface diffusion and adsorption processes. This paper presents a systematic study of the wetting of the inner and outer interfaces of Al by Sn in sandwich systems consisting of a 400 nm Al-base layer, a 10 nm Sn interlayer and a 400 nm thick Al capping layer. The morphology and chemical composition of the sandwich systems is investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The wetting process is studied by scanning auger electron spectroscopy (AES) under UHV conditions. Depositing the Al-capping layer at different deposition rates allows for the assessment of the influence of the grain boundary density on the velocity of Sn-transport through the Al-capping layer. Studying the permeation speed of Sn through the capping layer at different temperatures shows that Sn penetrates the capping layer much more rapidly at elevated temperatures thus corroborating the involvement of thermally activated mechanisms in the transport process. # 2005 Elsevier B.V. All rights reserved.

Passivation of Al surfaces against oxidation by monoatomic Sn wetting layers

Tin (Sn) forms monoatomic wetting layers on aluminium (Al) interfaces under ultra-high vacuum (UHV) conditions. The wetting layer spreads over the Al vacuum interface or over Al grain boundaries via the emergence of Sn atoms from Sn islands. The islands can be generated by sputter-deposition of a 10 nm thick Sn film on a polycrystalline Al underlayer. If the Al/Sn bi-layer is covered by an Al capping layer, Sn will penetrate the capping layer along the grain boundaries and form a wetting layer on its surface. Al surfaces covered by the Sn wetting layer exposed to oxygen (O) are oxidized significantly slower compared to bare Al surfaces. The shape of the adsorption isotherms suggests that the oxidation process involves the formation of oxygen nuclei. Depositing the Al capping layer in the presence of oxygen leads to a striking effect in the optical appearance and the chemical composition of the capping layer: Light microscopy shows that, in the vicinity of Sn islands, the capping layer has a shiny metallic appearance while the residual areas have a dark colour. By secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM), it could be shown that the dark regions contain significantly more oxygen. This effect can be attributed to the suppression of Al oxidation during the growth of the Al capping layer by the presence of Sn in the vicinity of the Sn islands.

FMR studies in the TbFeCo/GdFeCoSi bilayers

Bilayers, TbFeCo/GdFeCoSi, made by sputtering on glass substrate with buffer and capping layers were studied by measuring the hysteresis loop and by ferromagnetic resonance (FMR). When the field H was applied along the film normal, a double H C hysteresis loop related to the two sublayers was observed. In ferromagnetic resonance measurements, a peculiar out-of-plane angular dependence of FMR spectrum was obtained. When scanning field H was 0–637 kA/m less than the anisotropy field of TbFeCo sublayer, two FMR peaks were observed. One peak was characteristic of uniaxial and unidirectional anisotropy. The anisotropy constants were obtained by fitting the data with the theory of FMR, and this peak was considered to be related to the low anisotropy GdFeCoSi layer. The second peak appeared only when the dc field H was orientated in a limited angular range around 180°. This peak was considered to be related to an uncoupled interfacial GdFeCoSi sublayer near Al capping layer. However, when H was scanned between 0–1114 kA/m, only one peak is observed due to magnetization reversal of TbFeCo layer with uniaxial anisotropy.

Effect of capping layer on interlayer coupling in synthetic spin valves

The magnetic and transport properties of high quality synthetic spin-valves with the structure of $\mathrm{Ta}∕\mathrm{Ni}\mathrm{Fe}∕\mathrm{Ir}\mathrm{Mn}∕\mathrm{Co}\mathrm{Fe}∕\mathrm{Ru}∕\mathrm{Co}\mathrm{Fe}∕\mathrm{NOL}∕\mathrm{Co}\mathrm{Fe}∕\mathrm{Cu}∕\mathrm{Co}\mathrm{Fe}∕\mathrm{CL}$ were studied by using magnetoresistance measurements. Here Ti, Hf, and Al are used as the capping layer. It is found that both the thickness and materials properties of the capping layers can affect the interlayer coupling field. The interlayer coupling field oscillates weakly with respect to the thickness of the Ti and Hf capping layers. Extremely strong ferromagnetic coupling has been observed when the thickness of the Al capping layer is in a certain range where resonant exchange coupling takes place. The strength of the interlayer coupling is inversely proportional to the square of the thickness of the spacer. It is a typical characteristic of quantum size effect.

Lamellar growth in sputter-deposited aluminum–tin–aluminum sandwich layers

In recent experiments, the formation of lamellar features within an Al film was observed when deposited on a thin Sn layer. The present study investigates the influence of the deposition rate of the Al film on the lamella thickness to gain insight into the mechanism of lamella formation.Al–Sn–Al sandwich layers were prepared in HV by magnetron sputtering at 180 °C substrate temperature on natively oxidized Si wafers. Al and Sn were sequentially sputtered from a 100-mm diameter DC magnetron target. The samples consisted of an Al base layer (400 nm), a Sn interlayer (10 nm) and an Al capping layer (400 nm). The base layer and the Sn interlayer were deposited at a rate of 0.4 and 0.1 nm s−1, respectively. The deposition rate for the capping layer was varied between 0.1 and 0.8 nm s−1.Cross-sectional transmission electron microscopy (X-TEM) shows that, with increasing deposition rate of the capping layer, the lamella thickness slightly decreases. This is in contradiction to a previously formulated hypothesis of lamella growth which predicted an increase of lamella thickness with increasing deposition rate. On the other hand, contact mode atomic force microscopy (AFM) as well as X-TEM show a significant decrease of the Al grain size in the polycrystalline capping layer with increasing deposition rate. If the resulting higher grain boundary density is taken into account, the experimentally observed lamella thicknesses can reasonably be described by a model combining the diffusion of Sn along the Al grain boundaries and the oxidation of Sn on the growth surface.

Enhanced magnetoresistance ratio and improved thermal stability of specular spin valve multilayer with capping layer

Si/Ta/Ni–Fe/Mn–Ir/Co–Fe/NOL/Co–Fe/Cu/Co–Fe/Cu/capping bottom specular spin valves were prepared by using the DC magnetron sputtering method. We studied the effect of capping layers on the magnetic and thermal properties of specular spin valves. We can obtain a high magneto resistance ratio, high exchange bias field, and low interlayer coupling field by using Ta and Al capping layers. The thermal aging experiment showed that the Ta capping layer was more stable than the Al capping layer. We can obtain low interlayer coupling filed about 1 kA/m by using Al 2O 3 oxide capping layer.

Solid-state wetting and spreading of tin (Sn) on aluminium interfaces

The deposition of tin (Sn) layers on polycrystalline aluminium surfaces exhibits pronounced islanding. The detection of a continuous Sn coverage (“wetting layer”) between the islands indicates a Stranski–Krastanov growth mode. If the Al–Sn-bilayer system is covered by an Al-capping layer the surface morphology of this overlayer is significantly different depending on whether the surface of the underlying Al–Sn-bilayer was oxidized or not. The present study investigates the islanding mechanism and the dependence of the morphology of the Al-overlayer on the oxidation state of the underlying surface by combined SEM, atomic force microscopy and scanning Auger microscopic (SAM) investigations. The islanding mechanism as well as the overlayer morphology are related to the wetting of the Al-surface by Sn in the absence of oxygen. This was proved by in situ sputter cleaning of the sample surface in the SAM chamber. For the Al–Sn-bilayer it was possible to completely remove the wetting layer by the sputter cleaning process. A clean, oxygen free Al-surface was recovered at room temperature by Sn after several minutes as observed by spatially resolved SAM measurements. Sn emerges from the Sn islands and spreads over the Al-surface uniformly. In the case of an Al–Sn–Al sandwich structure Sn permeated to the surface of the topmost Al-layer when the Al-overlayer was deposited on an unoxidized Al–Sn-bilayer. For Al-deposition on an oxidized Al–Sn-bilayer no Sn transport to the overlayer surface was detected at all. These results indicate that metallic Sn is wetting every type of interface of polycrystalline Al (grain boundaries as well as the solid/vacuum interface) and also acts as a surfactant which changes the morphology of the Al-overlayer. Oxygen acts as a potent inhibitor of Sn wetting and spreading.

Solid state diffusion of Sn in polycyrstalline Al films

The formation of a wetting layer of Sn and the Stranski–Krastanov growth of Sn on a polycrystalline Al film surface has been proved recently. In the present experiments the influence of Sn on the structure evolution of Al films has been analyzed in detail. The films were prepared in HV by magnetron sputtering at 180 °C substrate temperature on Si wafers covered by the native oxide. Al and Sn were sequentially sputtered from a 100-mm diameter DC magnetron target. The samples consisted of an Al-base layer (400 nm), a Sn interlayer (10 nm) and an Al-capping layer (400 nm) and were investigated by analytical and high resolution X-TEM, AFM and scanning AES. If the Sn-interlayer is oxidized, no effect on the growth of the Al-capping layer is observable. If, on the other hand, the Sn-interlayer is prevented from oxidation grain growth is promoted in the Al-capping layer as confirmed by X-TEM and AFM. The capping layer contains intragranular line-like features roughly parallel to the substrate plane. Microanalysis shows the presence of Sn in the capping layer and indicates that Sn is associated with these linear features. The promotion of grain growth in the capping layer can be attributed to the permanent transport of Sn to the growth front via grain boundaries which was confirmed by dynamic scanning AES-measurements. The linear intragranular feature formation is explained by the oxidation of Sn on the free surface due to the residual oxygen present in the background gas.

Modification of the perpendicular magnetic anisotropy of ultrathin Co films due to the presence of overlayers

In order to understand the effect of interfacial strain on perpendicular magnetic anisotropy (PMA) of ultrathin Co films, ex situ Brillouin light scattering measurements were performed for various overlayer materials. The samples were prepared by molecular beam epitaxy and Cu, Al, and Au were used as overlayer materials. We observed a decrease in PMA of the Co with the Cu and Al overlayers. In addition, we found an unexpected result that the PMA is significantly reduced when an Au overlayer of 35 Å is covered by an extra Al capping layer. The amount of this reduction depends on the thickness of the Al layer. Our results lead us to speculate that misfit strain at the interface between the Al and the Au can be propagated through the Au layer to affect the magnetic properties of Co.

Ultrathin layer alkaline earth metals as stable electron-injecting electrodes for polymer light emitting diodes

Polymer light emitting diodes in which the cathode comprises an ultrathin layer of alkaline earth metal show much better operating lifetime, especially at high temperatures, in comparison with devices fabricated with cathodes from the same metals with thickness greater than 100 Å. The operating lifetime is dependent on the specific low work function metal used. Among the alkali and alkaline earth metals, devices with ∼30 Å Ba as cathode show the best half life during continuous operation. Using an ultrathin Ba cathode with an Al or Ag capping layer and poly[2-(3,7-dimethyloctyloxy)-5-methoxy-1,4-phenylenevinylene] as luminescent polymer, the half life reaches 400 h at 85 °C and exceeds 10 000 h at room temperature when operated at a current density of 8.3 mA/cm2 with a luminance of 100–140 cd/cm2.

Reduction of ion beam induced and atmospheric ageing of porous silicon using Al and caps

Ion beam analysis data are presented on both freshly prepared and `aged' porous silicon layers of widely varying (10 - 85%) porosity. Both evaporated Al and PECVD surface capping layers are shown to suppress some of the rapid ion beam induced changes in chemical composition that have been reported during analysis of uncapped layers. Specifically, these capping layers were successful in eliminating carbon and oxygen accumulation within the material but they only slightly reduced hydrogen loss under ion bombardment. Evaporated aluminium surface capping layers were partially successful in suppressing long term ambient oxygen pick-up.

Influence of Al capping layers on growth, topography, and magnetic properties on MnBi thin films

Bi/Mn bilayers have been deposited on fused-quartz substrates at room temperature and annealed at elevated temperatures of 300–380 °C. To investigate the influence of capping layers on MnBi crystallite size and magnetic properties, some of the Bi/Mn bilayers were protected by Al and SiOx layers. The coercive fields of the resulting MnBi films without a protective layer reach values of up to 1.25 T. In case of depositing an Al capping layers prior to annealing, the coercive fields are decreasing strongly showing coercive fields in the range of 0.6 T. In contrast, using SiOx as a capping layer, the Kerr hysteresis loops show a nonlinearity near the coercive field indicating an inhomogeneous film. The change in the coercive field is explained by the influence of capping layers on the MnBi crystallite size during annealing.