Al Oxide Layer Research Articles

Modeling of optical and geometrical data of Al thin films on GaAs

In this paper, we show how optical excitation of surface plasmons (SPs) can be used to obtain optical and geometrical parameters of specific layers in multilayered thin film systems. The optimum coupling phenomenon between incoming p-polarized light and SPs appears as a minimum in the reflectance that is calculated using a standard matrix formalism. The sensitive dependence of the reflectance minimum on optical and geometrical parameters suggests that they can be determined accurately by fitting the measured attenuated total reflectance (ATR) to the matrix-calculated reflectance using the Simplex minimization method. The procedure is applied to the multilayered system: Prism/Air gap/Al-oxide/Al/GaAs. At fixed incident light wavelength, the fitting parameters are the Al-oxide optical constant and the thickness of the air gap, Al-oxide and Al layers. Fortran codes are implemented for the reflectance calculations and the fitting procedures. The results show that the theoretical reflectance fits well the measured ATR at 633-nm wavelength. Moreover, the modeled Al-oxide optical constant at this wavelength agrees well with the literature. However, the reflectance fits are less good at 590- and 458-nm wavelengths and their modeled Al-oxide optical constants show a dispersion effect in disagreement with the literature. The modeled geometrical parameters are consistent with the nominal values.

Plasma oxidation of the insulation layer in the magnetic tunneling junctions

Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as insulating layers were fabricated and the microstructure of the oxide layers was investigated with cross-sectional transmission electron microscopy (TEM). TEM analysis showed that as the Al-oxide thickness increased from 13 to 63 Å, the roughness rapidly increased and the magnetoresistive (MR) ratio also markedly dropped. The undulation of the Al-oxide with increasing thickness was proved to have resulted from the plasma oxidation process. In contrast, Ta-oxide layer remained flat regardless of the layer thickness. In comparison to the Al-oxide layer, partially oxidized Ta-layer junction exhibited poor MR characteristics. Growth mechanism for Al- and Ta-oxide layers microstructure is proposed in terms of oxidation kinetics and oxygen plasma characteristics.

Investigation of the effects of different annealing ambients on Ag/Al bilayers: Electrical properties and morphology

Ag (200 nm)/Al (8 nm) bilayer structures on SiO2 substrates were annealed at temperatures ranging from 300 °C–700 °C and for 30 min in He–H, Ar, or NH3 ambient. Upon annealing, Al segregates to the surface where it reacts with residual oxygen to form an Al-oxide passivation layer. Rutherford backscattering spectrometry and Auger spectroscopy showed the formation of an AlxOy diffusion barrier at the Ag/SiO2 interface. Apart from the surface passivation that resulted from annealing, the Ag/Al/SiO2 in different ambients, the adhesion of Ag to SiO2 was also improved. A strong correlation between the resistivity and the residual Al in the Ag film was observed. Resistivity values equivalent to that of the as-deposited sample were obtained only for temperatures at 700 °C or above. Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy was used to study the effect of the different ambients on the observed surface morphology. The SEM studies showed that the passivated surfaces were not uniform but have hillocks and holes randomly distributed. In contrast to Ag on SiO2, no agglomeration was observed in the Ag/Al/SiO2 system.

High-field sensitive magnetic tunneling junctions fabricated by inductively coupled plasma sputtering

High tunneling magnetoresistance ratios (TMR) up to 44.6% were observed at room temperature on magnetic tunnel junctions (MTJs) with Ta20/Ni 81Fe 1920/Co 90Fe 102/Al(1 nm)-oxide/Co 90Fe 102/Ni 81Fe 197/Fe 50Mn 5020/Ta5 (the thickness of other layers is in nm) structures fabricated by using an inductively coupled plasma sputtering system operating at a base pressure of 5×10 −10 Torr. High field sensitivity up to 6.1%/Oe has been achieved in these MTJs with dimensions of 120 μm×120 μm by using extremely soft ferromagnetic (FM) Ni 81Fe 19/Co 90Fe 10 electrode. Small ferromagnetic coupling field (11 Oe) between the top and bottom FM electrodes, high breakdown electrical field (1.2×10 7 V/cm), and high V h (340–414 mV) at which the TMR decreases to 50% of its initial value, were obtained as the surface roughness of the bottom FM electrode ( R ms <3 A ̊ ) and Al-oxide ( R ms <1 A ̊ ) layer have been improved.

High Resolution Electron Microscopy Observation of Different Al-oxide Layers in Magnetic Tunnel Junctions

Microstructure and morphology of Al-oxide layers in magnetic tunnel junctions (MTJs) were characterized using high resolution electron microscopy (HREM). The MTJs were fabricated using magnetron sputtering. The Al-oxide layers were prepared using four different methods; natural, thermal, and plasma oxidation techniques as well as sputtering from pure alumina. HREM results showed that sputtering conditions, surface roughness of the bottom electrode in the MTJs, and oxidation conditions for the Al-oxide are essential factors affecting the quality of the insulating layer. High sputtering power was required to deposit a relatively smooth layer of pure alumina. Al matrices were observed in Al-oxide with only 24 h natural oxidation time. It was found optimum to thermally oxidize an Al-oxide layer between 333 and 473 K for 48 h. It is more effective and useful to oxidize an Al layer at a slightly higher plasma power rather than using a longer oxidation time in plasma oxidation method.

Analysis of tunneling magnetoresistance test structures by low energy electron nanoscale-luminescence spectroscopy

The performance of state-of-the-art tunneling magnetoresistive (TMR) heads depends on the thickness of insulating layers less than a few nanometers thick that separate two magnetic films. We have used low energy electron nanoscale-luminescence (LEEN) spectroscopy to observe optical emission from TMR test structures with buried insulating oxides less than a few nanometers thick. TMR structures grown by metal evaporation consisted of a 0.8–3 nm Al oxide layer on a 2.4 nm CoFe alloy (84:16) on a multilayer metal-on-Si substrate, all capped with a 0.4 nm CoFe with Pt overlayer. LEEN studies with excitation energies ranging from 0.5 to 3 keV enabled emissions from the buried oxide layers versus the free surface to be distinguished. We used different compositions, thicknesses, and oxidation exposures to separate Al oxide from transition metal oxide emissions, as well as from the ambient-exposed Pt surface. A broad peak centered at 2.2–2.5 eV increased with O plasma exposure to Al films on the CoFe alloy. Emission from oxidized CoFe without Al consists of a broad emission centered at 2.5 eV. Common to all these spectra is emission at 1.8 eV, which energy-dependent LEEN demonstrates is due to the ambient-exposed Pt. Spectral changes versus oxygen exposure reveal the regime separating oxidation of the magnetic and nonmagnetic layers.

Local investigation of thin insulating barriers incorporated in magnetic tunnel junctions

In this work we study properties of very thin insulating Al oxide films, used as barriers in magnetic tunnel junctions. For the small barrier thicknesses required for technological applications (∼10 Å), the presence of pinholes (direct contact between the ferromagnetic metals through the barrier), or oxidation inhomogeneities, are the major factors for vanishing of the tunnel magnetoresistance effect. We have produced and characterized very thin, pinhole-free Al oxide layers, incorporated in magnetic tunnel junctions. The transport properties of the different barriers were analyzed by barrier impedance scanning microscopy and were correlated with the magnetotransport properties of the patterned microsized junctions.

Oxidation mechanism of the insulation layer in NiFe/Co/Al(Ta)-oxide /Co magnetic tunnel junctions

Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as the insulating layer were fabricated using metal mask and Inductively Coupled Plasma (ICP) sputtering system. To interpret growth mechanism of the insulating layer during plasma oxidation process, the microstructure of the oxide layers was investigated with cross-sectional TEM. TEM analysis showed Al-oxide had different microstructures depending on the thickness of the layer. At 13 Å, the Al-oxide layer was flat while the Al-oxide layer became progressively wavy with regular periodicity of 20 nm at increasing oxide thickness. Ta-oxide layer was partially oxidized under equal oxidizing conditions, but remained flat regardless of the thickness. Growth mechanism for the two different oxide layers is proposed in terms of oxidation kinetics and oxygen plasma.

Thermal stability of magnetic tunnel junctions studied by x-ray photoelectron spectroscopy

We have studied the evolution of chemical state of the metallic layers in NiFe/Al oxide/NiFe tunnel junction structures in as-deposited films and after postdeposition annealing. Both top and bottom NiFe layers in as-deposited films show significant Fe oxidation, but no Ni oxidation. This Fe is reduced in annealed samples, implying that oxygen migrates from the FeNi layers, possibly into the Al oxide layer. We also find that both top and bottom electrodes are significantly oxidized even in optimally annealed films.

Microfabrication and Magnetoelectric Properties of High-magnetoresistance Tunnel Junctions.

Spin-valve-type tunnel junctions with structure of Ta(5nm)/Ni79Fe21(3 nm)/Cu(20 nm)/Ni79Fe21(3 nm)/Ir22Mn78 (10 nm)/Co75Fe25(4 nm)/Al(0.80 nm)-oxide /Co75Fe25(4 nm)/Ni79Fe21(20 nm)/Ta(5 nm) were fabricated using a microfabrication technique. The optical lithography combined with Ar ion-beam etching and CF4 active etching was used to pattern the junction area with the size from 100x100 down to 3x3 μm2. A thinner barrier layer and a short plasma-oxidation time for Al-oxide layer were used in order to reduce the junction resistance and increase the TMR ratio. High TMR ratio of 69.1% at 4.2 K and 49.7% at room temperature were achieved. A spin-electron polarization tunneling model, based on magnon emission or absorption by the tunneling electrons during the tunnel process, was extended by defining an anisotropic wavelength cutoff energy of spin-wave in this work. Intrinsic magnetoelectric properties, such as the temperature dependence of TMR ratio and resistances from 4.2 to 300 K at 1.0 mV bias can be self-consistently evaluated using this extended model and a unique set of intrinsic parameters.

Surface Plasmons for Probing Optical Data of Multi- Layered Thin Films

In this paper, we show how optical excitation of surface plasmons (SPs) can be used to obtain optical and geometrical parameters of specific layers in multi-layered thin film systems. The optimum coupling phenomenon between incoming p-polarized light and SPs appears as a minimum in the reflectance that is calculated using a standard matrix formalism. The sensitive dependence of the reflectance minimum on optical and geometrical parameters suggests that they can be determined accurately by fitting the measured attenuated total reflectance (ATR) to the matrix-calculated reflectance using the Simplex minimization method. The procedure is applied to the multi-layered system: Prism / Air gap / Al-oxide / Al / GaAs. At fixed incident light wavelength, the fitting parameters are the Al-oxide optical constant and the thickness of the air gap, Al-oxide and Al layers. Fortran codes are implemented for the reflectance calculations and the fitting procedures. The results show that the theoretical reflectance fits well the measured ATR at 633 nm wavelength. Moreover, the modeled Al-oxide optical constant at this wavelength agrees well with the literature. However, the reflectance fits are less good at 590 nm and 458 nm wavelengths and their modeled Al-oxide optical constants show a dispersion effect in disagreement with the literature. The modeled geometrical parameters are consistent with the nominal values.

GaAs/AlGaAs quantum well intermixing using buried Al-oxide layer

Using buried wet-oxidised Al/sub x/O/sub j/ layers to enhance impurity free vacancy diffusion, the intermixing of GaAs/AlGaAs quantum wells has been achieved. A 70 /spl Aring/ thick GaAs quantum well shows a blueshift of 59 meV when the sample is annealed at 950/spl deg/C for 120s. By cathodoluminescence measurements, it is confirmed that the bandgap transition region is localised laterally within 1 /spl mu/m of the oxide/nonoxide interface.

In Situ Study of Titanium Film Growth On Different Substrates

AbstractThe chemical and microstructural properties of a surface have a strong influence on the growth mode and the morphology of a film evaporated onto this interface. Changes in the growth stress of thin titanium films, measured in situ by a cantilever beam technique, evaporated under UHV-conditions are used to monitor the chemical and microstructural properties of a substrate surface. The starting substrate film used in this study was a quasi single-crystalline TiO2-film (d=50 nm) prepared by reactive evaporation of titanium in an oxygen atmosphere and subsequent annealing (20 min, 400°C). The Ti-growth stress on this substrate is compressive up to monolayer coverage and tensile at higher film thickness, which is interpreted to indicate a strong interaction between TiO2 and the arriving Ti atoms at the interface during monolayer formation and strained (tensile) layer epitaxy at higher film thickness. In a second series of experiments the TiO2-film was covered with Al-overlayers of varying thickness. Due to oxygen interdiffusion from the TiO2-film an amorphous Al-oxide layer is formed at the interface eliminating the high degree of order of the substrate TiO2-film. On this amorphous substrate the stress vs. thickness curve of the Ti-film, in terms of our stress model, is interpreted to indicate island formation and growth of a polycrystalline Ti-film. At Al-layer thicknesses above about 3 nm the Al-interface becomes metallic. The structure of this Al-surface depends on the film thickness and substrate temperature during its deposition. During deposition of the first Ti-monolayer on metallic Al a large incremental tensile stress (up to 45 GPa) is measured. The magnitude of this tensile stress is closely related to the surface microstructure of the Al substrate. The surface roughness deduced from the tensile interface stress is compared with the surface roughness measured by AFM.For comparison, analogous experiments were made with Al2O3/Al substrate bilayers. The results of these experiments qualitatively agree with those on the TiO2/Al-substrate. The general shape of the stress vs. thickness curve is comparable, however quantitative differences are interpreted to be due to differences in the structure and/or chemical composition of the substrate Al-film.

Investigation of oxidation process on plasma treated thin Al-films by GIXR and GIXRD

The sensitivity of grazing incidence X-ray reflectometry (GIXR) for amorphous as well as crystalline layers and for layer formations in the nm-range combined with information about structural states by grazing incidence X-ray diffractometry (GIXRD) enable these methods to study interfaces and surface layers. Thin aluminium layers (30–60 nm) were deposited on Si by thermal evaporation. These Al-layers were plasma oxidized for 5 h with interruptions every hour. The plasma treatments were carried out in a 2.45 GHz microwave discharge (SLAN-slot antenna) with different plasma types (Ar, O2) at low temperatures. In this way a thin oxide layer additional to the native alumina on the top was formed. The properties of the oxide layers such as thickness, roughness and density were studied by GIXR. The chemical composition was investigated by GIXRD and XPS. In addition AFM was used for surface topography observations. All these methods were applied before and after the plasma treatment. The properties of the thin Al-oxide layers were found to depend on the type of plasma but in all cases plasma leads to a smoothing of the films as well as an increasing of the thickness of the upper alumina layer.

Effect of in situ plasma oxidation of TiN diffusion barrier for AlSiCu/TiN/Ti metallization structure of integrated circuits

An oxygen plasma treatment of the TiN diffusion barrier prior to Al deposition of an AlSiCu/TiN/Ti structure is studied and compared to an air break process in order to determine the reactions occurring at the Al/TiN interface as well as the mechanisms responsible for the improved barrier performance using an ex situ and an in situ oxygen plasma, respectively. Various experimental techniques such as grazing angle x-ray diffraction, elastic recoil detection and field emission scanning electron microscopy were used. The ternary and quaternary phase diagrams were used to predict the phases formed. It is first observed that an oxygen plasma increases the oxygen concentration at the Al/TiN interface as well as in the TiN barrier. It is conjectured that first the oxygen reacts with Ti in order to form Ti oxides in the TiN grain boundaries and on the TiN surface. The Al film then reacts with this Ti oxide layer in order to form an Al2O3 layer upon sintering which in turn acts as an additional diffusion barrier at the Al/TiN interface. It is also determined that the Al2O3 phase formation is increased after an oxygen plasma treatment of the TiN diffusion barrier. This Al oxide layer as well as the Ti oxides in the TiN grain boundaries could reduce the Al and Ti intermixing as observed by the reduction in the formation of TiAl3. The reduced Al diffusion in the barrier would then stop Si outdiffusion from the substrate. In addition, the penetration of Si would also be blocked at the Al/TiN interface by the Al2O3 layer. The decreased Si penetration in the upper layers in addition to the lower TiAl3 formation are the mechanisms proposed to explain the decreased formation of Ti7Al5Si12 after an in situ oxygen plasma treatment of the TiN diffusion barrier. It is concluded that an oxygen plasma-treated TiN barrier is more efficient than an air-exposed TiN one due to the lower degree of Ti7Al5Si12 formation which also explains the reduced risks of junction spiking. The advantage of the in situ oxygen plasma treatment is therefore related to better process control in addition to higher throughput.

High-performance 1.3-μm InAsP strained-layer quantum-well ACIS (Al-oxide confined inner stripe) lasers

We have demonstrated high-performance 1.3-/spl mu/m InAsP strained-layer quantum-well lasers with Al-oxide confined inner stripe (ACIS) structure. The oxidized layers are consisted of strain-compensated AlAs-InP-AlInAs (6 nm/2 ML/3 nm) superlattice (SAS) layers and grown on InP substrate using gas-source molecular beam epitaxy (GS-MBE). For the structural optimization of the ACIS laser, the dependence of quantum efficiency, threshold current and lateral mode characteristics on the initial ridge width and the current aperture width have been investigated. A very low-threshold current of 2.0 mA, a high differential quantum efficiency of 67% and stable single lateral mode operation of over 40 mW were obtained for the optimized ACIS lasers. The excellent characteristics obtained in the ACIS lasers are attributed to both the current and the optical confinements by the Al-oxide layer. In addition, we confirmed high uniformity of the threshold current and the oxidized width. This device is very promising candidate for low-cost access networks and optical interconnects.

Contrasting damage characteristics in direct incidence and surface plasmon mediated single-shot laser ablation of aluminium films

Thin, oxidised Al films grown on one face of fused silica prisms are exposed, under ambient conditions, to single shots from an excimer laser operating at wavelength 248 nm. Preliminary characterisation of the films using attenuated total reflection yields optical and thickness data for the Al and Al oxide layers; this step facilitates the subsequent, accurate tuning of the excimer laser pulse to the surface plasmon resonance at the Al/(oxide)/air interface and the calculation of the fluence actually absorbed by the thin film system. Ablation damage is characterised using scanning electron, and atomic force microscopy. When the laser pulse is incident, through the prism, on the sample at less than critical angle, the damage features are molten in nature with small islands of sub-micrometer dimension much in evidence; a mechanism of film melt-through and subsequent blow-off due to the build up of vapour pressure at the substrate/film interface is appropriate. By contrast, when the optical input is surface plasmon mediated, predominately mechanical damage results with the film fragmenting into large flakes of dimensions on the order of 10 μm. It is suggested that the ability of surface plasmons to transport energy leads to enhanced, preferential absorption of energy at defect sites causing stress throughout the film which exceeds the ultimate tensile stress for the film; this in turn leads to film break-up before melting can onset.

Chemical alteration of thin alumina films on aluminum during hydrogen-atom exposures

Hot-rolled and partially oxidized Al foil surfaces were examined before and after H-atom exposures using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) to monitor the changes which occur at the sample surfaces. The near-surface region of the native oxide of the hot-rolled Al foil contains primarily C and O. Cl and Ca contamination also are present at the outermost atomic layer according to ISS. H-atom exposures reduce the amount of carbonates and hydrocarbons but causes an enrichment of sulfur in the outermost layer of the foil. The O-to-Al ratio is reduced, and the AlO x species initially present is converted to Al 2O 3. After sputtering the Al foil to remove the Cl, C, and Ca contaminants, the Al was exposed to oxygen at 10 −7 Torr for 10 min. The near-surface region of this re-oxidized surface contains both Al metal and Al 2O 3. Exposing this partially oxidized surface to H-atoms produces a chemically-induced driving force which causes subsurface O to migrate toward the surface of the foil. This results in an increase of the Al 2O 3 concentration at the sample surface. These results suggest that the thickness of Al oxide layers may be controlled using H-atom exposures. Furthermore, the amounts of carbon contamination can be decreased and possibly eliminated without reducing the alumina.

Segregation of Cu on Etched and Non-Etched Al(Cu) Surface

AbstractIn this paper, we have studied the segregation phenomenon of Cu on the surfaces of patterned lines, dry-etched films and non-etched films, by using X-ray photoelectron spectroscopy and lower energy Rutherford Backscattering Spectrometry. Significant enrichment of Cu is found on the sidewall of the lines. Annealing at 350°C and above cause the disappearance of this enrichment. Origin and evolution of this Cu enrichment have been investigated on films taken out from different steps of the etching process. It has been found that most of the Cu products induced by the plasma etching are CuCl and CuCl2 and they are removed mostly from the top Al oxide layer by the strip process. On the interface area between Al and the native oxide, considerable quantities of etched induced Cu are retained. This Cu is identified to be mainly metallic Cu. Different from the mechanism explained above, thermal annealing can also cause Cu segregation. We have found that Cu atoms diffuse into the native Al oxide where they form Cu2O.

アルミニウム合金のジンケイト前処理におよぼす試料表面調整の効果

Surface pretreatment effects of Al and Al-Mg alloy on Zn deposition in the zincate process were evaluated using SEM and XPS. Al surface dissolution in a concentrated NaOH solution followed by zincate treatment produced rapid Zn particles nucleation with uniform size due to Al surface oxide film thinning. The anodic oxidation pretreatment of Al used jointly with electropolishing followed by zincate treatment also resulted in deposition of uniform Zn particles. Anodic oxidation pretreatment without electropolishing, however, resulted in nonuniform Zn deposition indicating that defects in the oxide film could be active sites for Zn nucleation. Al-Mg alloys require both alkaline and acid cleaning in surface pretreatment because Al dissolves in concentrated alkaline solution, and Mg dissolves in acid solution. Magnesium or its enriched compounds on the surface during zincate process in an alkaline zincate solution inhibits Al dissolution and Zn deposition. XPS showed that Zn2+ remains in the Al oxide layer, which partially charged after Zn stripping in a HNO3 solution. The role of the first zincate process in double zincate treatment is discussed based on these results.