Intermetallic Diffusion Research Articles

IR‐VIS‐UV ellipsometry, XRD and AES investigation of In/Cu and In/Pd thin films

AbstractOptical and compositional properties of In, In/Pd and Pd/In/Pd thin films evaporated on Cu and SiO2 substrates in vacuum were investigated by means of X‐ray diffractometry, Auger electron spectroscopy and spectroscopic ellipsometry methods. Auger depth profile studies were performed in order to determine the composition of InCu and InPd structures. In both systems interdiffusion of metals was detected at room temperature. The XRD patterns indicated formation of CuIn2 and PdIn3 phases in the samples. Optical properties of the composite layers containing intermetallic phases were derived from ellipsometric quantities Ψ and Δ measured in the photon energy range 0.1‐6.0 eV at different angles of incidence using suitable multilayer models for the examined samples. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure and magnetic properties of electrodeposited cobalt films

Cobalt films were electrodeposited onto both iron and copper substrates from an aqueous solution containing a mixture of cobalt sulfate, boric acid, sodium citrate, and vanadyl sulfate. The structural, intermetallic diffusion and magnetic properties of the electrodeposited films were studied. Cobalt electrodeposition was carried out in a passively divided cell aided by addition of vanadyl sulfate to keep the counter electrode clean. The divided electrolytic cell with very negative current densities cause the electrodeposited Co to adopt a face-centered cubic (fcc) structure, which is more magnetically reversible than the hexagonally close-packed (hcp) structured Co. The coercive field is also significantly less in the fcc-electrodeposited cobalt than in the hcp. SEM images show dense, uniform Co films without any cracks or porosity. Beside the deposition current, thickness of the film was also found to affect the crystal orientation particularly on iron substrates. Diffusion of cobalt film into the iron substrate was studied under reduced environment and a fast process was observed.

The reflectivity of Mo/Ag/Au ohmic contacts on p-type GaN for flip-chip light-emitting diode (FCLED) applications

The Mo/Ag/Au contact for flip-chip light-emitting diode (FCLED) applications is examined on its contact resistance and light reflectance. A high reflectance of 90% is achieved in un-annealed contact, but a strong inter-diffusion of ohmic metals and GaN during the annealing process is found to result in poor reflectance (55% at the wavelength of 465 nm). The secondary ion mass spectrometry (SIMS) depth profiles indicate that a wide inter-diffusion region existed in the annealed contacts; thus the low reflectivity of the Mo/Ag/Au-annealed contacts can be attributed to the strong inter-diffusion of Au and Ag.

Structural analysis of In/Ag, In/Cu and In/Pd thin films on tungsten by ellipsometric, XRD and AES methods

Compositional, microstructural and optical properties of In/Cu, In/Ag and In/Pd thin films evaporated on W substrate in a vacuum were investigated by means of Auger electron spectroscopy, X-ray diffractometry, scanning electron microscopy, and spectroscopic ellipsometry methods. Thicknesses of deposited pure metal layers were adjusted to atomic concentration ratios In:Ag = 1:2, In:Cu = 2:1 and In:Pd = 3:1. Interdiffusion of metals and creation of intermetallic compounds AgIn 2, Ag 2In and CuIn 2were detected at room temperature. Phase transformation and changes in the surface morphology due to annealing of samples at 393 K for 60 min were revealed. Combined investigations indicated a layered structure of In/Ag films. A tendency of island-like structure formation was stronger for In/Cu and In/Pd films. The complex dielectric functions 〈 ϵ ˜ ( E ) 〉 = 〈 ϵ 1 ( E ) 〉 + i 〈 ϵ 2 ( E ) 〉 of In/Ag, In/Cu and In/Pd composite layers were determined from spectroellipsometric data.

Characterization of Pd–Ag membranes after exposure to hydrogen flux at high temperatures

Pd–Ag films were deposited by sequential electroless plating on porous stainless steel tubes. Two different intermediate layers of α-Fe 2O 3 and γ-Al 2O 3 oxides were employed as diffusion barriers to modify the stainless steel porous size and to avoid the intermetallic diffusion of Fe and Ni through the Pd–Ag layer. The membranes were tested in a gas permeation system to determine the hydrogen permeability and selectivity at temperatures between 673 and 923 K. After the high temperature treatments, they were characterized by XRD, XPS, SEM, AFM and EPMA. The Pd–Ag/PSS membranes had a good thermal resistance. After annealing at 923 K, the Auger depth profiles did not show the presence of Fe in the Pd–Ag/γ-Al 2O 3 membranes. However, iron was detected in the surface layer after long-term hydrogen permeation measurements at temperatures between 673 and 923 K, indicating that the γ-Al 2O 3 layer did not completely block the intermetallic diffusion under these severe conditions.

Superplastic Forming of Magnesium Alloys: Production of Microstructures, Superplastic Properties, Cavitation Behaviour

Superplastic forming (SPF) of magnesium alloys has received increasing attention in the recent past. The aim of this presentation is to review recent works dealing with SPF of Mg alloys with a three-fold objective: i. How to produce fine or ultra fine grained (UFG) microstructures ii. Are there specifities in superplastic deformation mechanisms iii. How SPF Mg alloys resist to cavitation Deformation mechanisms as well as damage variations in the superplastic regime will be preferentially discussed in relation with grain size, content in intermetallic particles and diffusion kinetics. For the sake of illustration, some results concerning the superplastic behaviour of UFG magnesium alloys produced by severe plastic deformation will be presented since such microstructures exhibit particularly attractive superplastic properties at quite low temperatures.

Preparation of thin palladium membranes on a porous support with rough surface

The integrity of thin composite palladium membranes is influenced by the surface roughness of the porous support. Supports with smooth surface and small pore size are expensive as they are composed of several layers with decreasing pore size which require multiple successive energy and time consuming sintering steps. In addition, smooth surfaces may cause poor membrane adhesion. It is therefore of interest to develop methods for preparation of thin defect-free palladium membranes over supports with rough surfaces. Porous stainless steel tubes coated by atmospheric plasma spraying with a porous layer of yttria-stabilized zirconia (YSZ) were used in this work as a support for palladium composite membranes. The YSZ layer served as a barrier against intermetallic diffusion between the palladium membrane and the metallic support. Three different techniques to create the membranes were compared: magnetron sputtering did not result in sufficiently dense films. Atmospheric plasma spraying produced relatively thick continuous films, but with some residual open porosity. Electroless plating gave the densest layers. Yet rather thick layers were required to limit the number of defects, which is associated with high cost and low hydrogen flux. Activation of the support surface by metal organic chemical vapor deposition of palladium instead of the conventional sensitization and activation pretreatment based on successive immersion in SnCl 2 and PdCl 2 solutions allowed to reduce the membrane thickness without compromising its integrity. The resulting membrane showed significantly higher permselectivity but at the same time decreased hydrogen permeability.

In-Situ Joining of Combustion Synthesized Ni3Al Intermetallic Compounds with AZ91D Mg Alloy

We focused on the surface reinforcement of ligth weight casting alloys with Ni3Al intermetallic compounds by in-situ combustion reaction to improve the surface properties of nonferrous casting components. In the present work, by setting the mixture of elemental Ni and Al powders in a casting mold, the powder mixture reacted to form Ni3Al intermetallic compound by SHS reaction ignited by the heat of molten AZ91D Mg alloy and simultaneously bonded with the Mg casting alloy. The AZ91D Mg alloy bonded with the Ni3Al intermetallic compound was sectioned and observed by optical microscopy and scanning electron microscopy(SEM). The chemical composition of intermetallic compounds and diffusion layer formed around bonding interface were identified by energy dispersive spectroscopy(EDS), X-ray diffraction analysis(XRD) and electron probe micro analyzer(EPMA). The main intermetallic compound was Ni3Al phase and a little Ni2Al3 intermetallic compound was formed from the Ni and Al powder mixtures. Residual pores were observed in the synthesized intermetallic compound. The AZ91D Mg alloy and Ni3Al intermetallic compound were bonded very soundly by the interdiffusion of Mg, Ni and Al elements, but some cracks were observed around the bonded interface on the interdiffusion layer. The diffusion length formed between AZ91D Mg alloy and Ni3Al was different depending on the diffusivity of Ni and Al elements into the molten Mg alloy. Ni was more deeply diffused into the Mg alloy than Al. The diffusion layer was about 200m thickness and various phases were formed by the interdiffusion of Mg, Ni and Al. From this challenge we have successfully produced a coating layer based on nickel aluminide on ligth weight Mg alloy using molten metal heat without any additional process. On the basis of the results obtained, it is concluded that near-net shaped nickel aluminide coating layer can be formed using this unique process.

Formation process of high reflective Ni∕Ag∕Au Ohmic contact for GaN flip-chip light-emitting diodes

The combinations of Ni, Ag, and Au which form both Ohmic and reflective multilayer contacts for flip-chip light-emitting diode applications are examined. A strong interdiffusion of Ohmic metals and GaN during the annealing process is found to result in poor reflectance (63% at the wavelength of 465nm). The authors propose a two step metallization method, the Ni∕Ag double layer is first deposited/annealed followed by the deposition of Au, to improve the correspondent reflectivity (as high as 92%). This method is different from the conventional one step Ni∕Ag∕Au formation method in which these three metals are deposited together and then annealed. The secondary ion mass spectrometry depth profiles indicate that a wide interdiffusion region existed only in the conventional one step formation samples; thus the low reflectivity of Ni∕Ag∕Au-annealed contacts can be attributed to the strong interdiffusion of Ohmic metal into GaN. Their proposed two step metallization method avoids the strong interdiffusion and improves the reflectivity effectively.

Characterization of In/Pd and Pd/In/Pd thin films by ellipsometric, XRD and AES methods

In/Pd and Pd/In/Pd thin films were prepared by thermal evaporation on the SiO2 substrate in a vacuum. The structural and optical properties of the films were investigated by means of X-ray diffractometry (XRD), Auger electron spectroscopy (AES) and spectroscopic ellipsometry (SE). Auger depth profile studies were performed in order to determine the composition of elements in the Pd-In systems. Interdiffusion of metals was detected at room temperature. Optical properties of Pd-In composite layers formed due to the interdiffusion were derived from ellipsometric quantities Ψ and Δ measured in the photon energy range 0.75–6.50eV at different angles of incidence. The effective optical spectra show absorption peaks dependent on the composition of nonuniform films. The XRD patterns indicated formation of Pd1−xInx intermetallic phases in the samples.

Concept for diamond 3-D integrated UV sensor

In this investigation we present a novel technology for three-dimensional integration of diamond sensors and actuators. Based on a previously developed technology for the integration of a 3-D CMOS camera stack, in this attempt nano-crystalline diamond layers on silicon are used at the sensor level. The processing steps involve silicon thinning, through-silicon etching and metallization, as well as a soldering process using inter-metallic diffusion. Strong emphasis lies on a low thermal budget process, making the technology suitable for backend processing. The concept is demonstrated, however active CMOS chips are not yet implemented. First electrical measurements of the through-connected UV sensors are presented.

Synthesis of composite Pd-porous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier

Intermetallic diffusion of support elements (Fe, Cr and Ni) into the membrane layer during high temperature hydrogen separation applications is a key factor affecting the overall performance of Pd and Pd/Ag membranes supported on porous sintered metals. Pd/Ag membranes supported on porous stainless steel (PSS) supports were prepared by the bi-metal multi-layer (BMML) deposition technique, which involved the formation of a porous Pd–Ag composite layer by consecutive deposition of Pd and Ag layers with no intermediate surface activation. While the conventional electroless plating involved intermediate drying and activating steps between Pd and Ag depositions, BMML deposition provided continuous plating with alternating Pd and Ag baths. The synthesis of these membranes involved a finishing step of the application of a gas tight Pd layer upon the BMML giving the membrane high hydrogen selectivity and long-term durability due to the fact that the BMML formed an extremely effective intermetallic diffusion barrier. Several membranes prepared by this technique have been stable under hydrogen permeation conditions for over 500 h at temperatures exceeding 500 °C. He flux data and SEI micrographs, prior to application of the Pd top coat, showed that the BMML formed a graded support, without significantly changing the total resistance of the support. In addition, heat treatment studies coupled with the X-ray phase identification analysis showed the formation of a Pd/Ag alloy phase, which might further improve the hydrogen permeability.

Elastodynamic wave propagation in graded materials: Simulations, experiments, phenomena, and applications

A two-dimensional numerical simulation model for the elastodynamic wave propagation in two linear elastic, isotropic, joint half-spaces is presented. The border between the two half-spaced is graded in a way, that the values of the elastic properties and the densities vary smoothly (sinusoidally) from the values of one continuum to the values of the other continuum within a transition zone of a defined thickness. It is demonstrated, that a graded layer leads to a frequency and wavelength dependent refraction and reflection behavior of elastodynamic waves. Numerical results show that wavelengths which are long compared with the transition layer thickness are dominantly reflected whereas short waves are dominantly transmitted, a phenomena which does not occur in the case of an infinitely thin transition layer. Furthermore the frequency dependent reflection and transmission behavior of elastodynamic waves is verified experimentally. There the interface between two vapor deposited films is graded due to intermetallic diffusion effects. These graded microstructures are analyzed with a short-pulse-laser-acoustic set-up. The corresponding frequencies of the elastodynamic waves which are filtered with these functionally graded microstructures are in the range of 0.5 THz.

Preparation and characterization of composite palladium membranes on sinter-metal supports with a ceramic barrier against intermetallic diffusion

Palladium composite membranes on sinter-metal supports were prepared by electroless plating. Zirconia (ZrO 2), yttria-stabilized zirconia (YSZ), and titania (TiO 2) were applied as porous barriers between the palladium membrane and the sinter-metal support to prevent intermetallic diffusion. The ceramic layers were coated by magnetron sputtering (MS-ZrO 2, thickness ∼2 μm), atmospheric plasma spraying (APS-YSZ, thickness ∼10–70 μm), and wet powder spraying (WPS-TiO 2, thickness ∼40–60 μm), respectively. They differ considerably in terms of thickness, pore size, surface roughness, and open porosity. The barriers and the final membranes were characterized by light-optical microscopy (LM), scanning electron microscopy (SEM), capillary flow porometry, in terms of the adhesion of the membrane layer, its gas-tightness, and by hydrogen permeation measurements. Moreover, the barrier function against intermetallic diffusion between the palladium and the metals from the support was demonstrated by annealing of membrane samples in pure hydrogen at 600 °C for up to 23 days. APS-YSZ was identified as the most promising barrier, followed by WPS-TiO 2 and MS-ZrO 2. Palladium membranes on APS-YSZ have to be significantly thicker (∼15–20 μm) than membranes on WPS-TiO 2 due to the extremely rough surface of the APS-YSZ layer. In contrast, the WPS-TiO 2 surface is smooth and uniform which allows the plating of very thin (∼8–10 μm) and selective palladium membranes. However, worse adhesion of the palladium membrane to the TiO 2 surface and indications of a change of the composition of the TiO 2 layer in intimate contact with the palladium membrane after exposure to hydrogen at high temperature give rise to doubt about their long-term stability. Pd/MS-ZrO 2 membranes showed intermetallic diffusion at locations where direct contact between the palladium and the sinter-metal support persisted. Among the three membrane types, Pd/WPS-TiO 2 demonstrated the best hydrogen permeance, i.e. 0.154 m N 3 m − 2 h − 1 P a − 0.5 , as well as the best H 2/N 2-permselectivity, ∼800, at 500 °C. However, the highest hydrogen permeability, as determined from the permeance and the nominal thickness of the palladium membrane, was reached by Pd/APS-YSZ ( 1.6 m N 3 μ m m − 2 h − 1 P a − 0.5 at 500 °C).

Intermetallic compound formation and diffusion path evolution in eutectic tin‐lead flip chip solder bumps after aging

PurposeIn the flip‐chip technology (FCT) used in current microelectronic packages, a Ni‐based under‐bump metallurgy (UBM) is widely used due to its slow reaction rate with Sn. In this study, solders joints of eutectic Pb‐Sn with a Ni UBM were employed to investigate the intermetallic compound (IMC) formation after aging at 150°C for various periods of time.Design/methodology/approachThe compositions and elemental re‐distribution in the IMC formed due to the interfacial reaction between the Ni/Cu UBM and eutectic Sn‐Pb solders were evaluated with an electron probe microanalyzer. The interfacial morphologies were revealed with the aid of a field‐emission scanning electron microscope through a special etching technique.FindingsAt the centre of the chip side, two IMCs were found between the solder and Ni metallization. The scalloped‐like IMC was determined to be (Cu, Ni)6Sn5, while the nodule‐like IMC was (Ni,Cu)3Sn4. However, at the edge of the chip side, three IMCs were revealed. The scalloped‐like IMC was (Cu1−y,Niy)6Sn5, the nodule‐like IMC was (Ni1−x,Cux)3Sn4, and the layer‐type IMC was (Cu1−z,Niz)3Sn.Originality/valueOn the basis of the elemental distributions from the quantitative analysis of the IMC and the related phase transitions during the IMC formation, two distinct diffusion paths are proposed to illustrate the interfacial reaction and phase transformation between IMCs and solder in Sn‐Pb joints aged at 150°C. These diffusion paths demonstrated two kinds of phase equilibrium, including (Cu1−z,Niz)3Sn/(Cu1−y,Niy)6Sn5/solder and (Ni1−x,Cux)3Sn4/(Cu1−yNiy)6Sn5/solder.

Nanometric multilayers: A new approach for joining TiAl

A novel intermetallic alloy diffusion bonding procedure is being developed. The innovative aspect relies on the use of sputtered nanometallic multilayers made up of the elements present in the bulk intermetallics to enhance the bonding mechanisms. For this purpose a deep knowledge of the multilayer thin films is required, focusing on thermal phase stability and grain size evolution. γ-TiAl was selected for this study and Ti/Al multilayer thin films with nanometric period ( Λ = 4 nm) were deposited onto Ti–(45–49)Al–(3–2)Nb–2Cr (at.%) substrates by d.c. magnetron sputtering. The as-deposited titanium and aluminium nanolayers with crystallite sizes of 5–50 nm evolve toward the equilibrium γ-TiAl structure after heat treatment for at least up to 600 °C at a 10 °C min −1 heating rate. Whatever the heating temperature and holding time, between 600 and 1000 °C, the γ-TiAl phase is stable. During the thermal cycle the growth of the nanometric grains is promoted, this effect being more pronounced as the temperature and holding time increase. Consequently, the hardness decreases from 11.9 GPa (600 °C, 1 h) to 6.5 GPa (1000 °C, 3 h). This study allowed the thermal treatment required to join γ-TiAl parts to be established.

Fabrication of n- and p-type doped CuFeSe2 thin films achieved by selenization of metal precursors

CuFeSe2 thin films have been grown by selenization of a CuFe alloy precursor. The CuFe alloy has been obtained by successive evaporation of thin Cu/Fe/Cu… layers on glass substrates heated at 550 °C. After deposition the metal alloy precursor is polycrystalline and there is an interdiffusion of metals all along the thickness. The atomic ratio Cu/Fe was varied from 0.8 to 1.15. The metal alloy precursor is selenized in a vacuum chamber; after heating at 550 °C it is exposed to selenium vapour (evaporation rate 3–4 nm s−1 for 45 min).At the end of the process only CuFeSe2 diffraction peaks are visible in the x-ray diagram. The films exhibit a (112) preferential orientation. The scanning electron microscope (SEM) visualization of the films shows that films are composed of well-faceted grains. The Cu-rich films are slightly porous while the Fe-rich are more homogeneous.The SEM study shows that the thickness of the grains is of the same order of magnitude as that of the CuFeSe2 films.Electrical measurements have shown that Cu-rich films are n-type with a room temperature resistivity of (5–9) × 102 Ω cm, while Fe-rich films are p-type with a room temperature resistivity of (3–5) × 101 Ω cm. It is shown that the electrical properties of the films are controlled by the grain boundary scattering effect.

Characterization of In/Pd and Pd/In/Pd thin films by ellipsometric, XRD and AES methods

Abstract In/Pd and Pd/In/Pd thin films were prepared by thermal evaporation on the SiO 2 substrate in a vacuum. The structural and optical properties of the films were investigated by means of X-ray diffractometry (XRD), Auger electron spectroscopy (AES) and spectroscopic ellipsometry (SE). Auger depth profile studies were performed in order to determine the composition of elements in the Pd-In systems. Interdiffusion of metals was detected at room temperature. Optical properties of Pd-In composite layers formed due to the interdiffusion were derived from ellipsometric quantities Ψ and Δ measured in the photon energy range 0.75–6.50 eV at different angles of incidence. The effective optical spectra show absorption peaks dependent on the composition of nonuniform films. The XRD patterns indicated formation of Pd 1− x In x intermetallic phases in the samples.

Reflow and Electrical Characteristics of Nanoscale Solder

Soldering nanojoints: Nanowires with a diameter range of 30–200 nm were patterned with solder segments, and the reflow of the solder on these wires was investigated (see Figure). Low-resistance ohmic contacts were observed across solder joints after reflow. Corrosion, oxidation, and intermetallic diffusion play a crucial role in solder reflow and joint formation at the nanoscale.

Different oxides used as diffusion barriers in composite hydrogen permeable membranes

Composite Pd–Ag membranes supported on modified porous stainless steel substrates were prepared by sequential electroless plating. To improve the stability of the Pd–Ag alloy, two different intermediate layers of α-Fe 2O 3 and γ-Al 2O 3 oxides were employed as diffusion barriers. A new activation method was also investigated which consisted in the impregnation of a layer of Pd supported γ-Al 2O 3 to generate palladium seeds. The membranes were studied by XRD, Auger depth profiling, SEM and EPMA. They were tested in a gas permeation system to determine the hydrogen permeability and selectivity. The Pd–Ag/PSS membranes had a good thermal resistance. The membranes prepared with alumina diffusion barriers exhibited lower ideal selectivity than the Pd–Ag/α-Fe 2O 3/PSS. However, the former membranes had two times higher hydrogen permeability. The Auger depth profiles did not show the presence of Fe in the Pd–Ag film, indicating that the γ-Al 2O 3 layer effectively blocked the intermetallic diffusion.