Au Diffusion Research Articles

Growth of Intermetallics at Coated Silver Wire Bond Interfaces

Abstract The newly developed gold (Au) coated silver (Ag) wire exhibits axi-symmetrical free air ball (FAB) formation under atmosphere similar to Au wire. FAB of coated Ag showed better corrosion resistance than FAB of alloyed Ag, as Au dissolved along the periphery of Ag FAB forming into a solid-solution, Ag-Au alloy. Composition of the solid-solution Ag-Au alloy along the periphery ranges between 10 and 28wt% of Au. The diffusion depth of Au is 4μm from surface towards the center of FAB. For precise measurement of the diffusion length of Au into Ag, a solid thin layer is been parted using focus-ion beam (FIB). The solid thin layer is further thinned down to observe in Transmission Electron Microscope (TEM) and analyzed using line scan energy dispersive X-ray analysis (EDX) attached to it. Near the neck of FAB and wire, Au diffusion is high up to 58wt%. On thermal ageing at 150/175°C for 500/1000h, the coated Ag wire bonded to Al-0.5wt%Cu pad revealed no degradation on ball pull/shear. Bond interface revealed significant growth of silver aluminide (AgAl) to a thickness of 3μm for 1000h of ageing and the growth rate fits to square-root power law.

Advances in nanoscale characterization of refined nanoporous gold

The effect of Pt on the structural evolution of nanoporous gold (NPG) is complex and is controlled by processes including secondary dealloying of Ag, surface diffusion of Au, and Pt segregation. High-resolution studies are carried out to investigate the interplay of the inherent processes. Observations such as Pt segregation at nanoligament surfaces, core-shell structures of nanoligaments, and Ag retention inside individual nanoligaments are seen at unprecedented detail using high resolution characterization techniques, including Atom Probe Tomography (APT) and Electron Energy Loss Spectroscopy (EELS). Quantitative estimates of the surface area-to-volume ratios of nanoporous layers with different Pt content indicate their relative functionalities for surface area driven applications.

Silanol-rich ordered mesoporous silica modified thiol group for enhanced recovery performance of Au(III) in acidic leachate solution

Herein, we present a new approach to enhance the recovery performance of Au(III) using thiol-functionalized adsorbents in acidic leachate solution of urban mining. Ordered mesoporous silicas (OMS) with abundant surface silanol were obtained from mine tailings by solvent extraction. Overall, the surface silanol groups had the greatest effect on sulfur density and content under the conditions for functionalization with 3-mercaptopropyl-trimethoxysilane (3-MPTMS). Furthermore, silanol-rich OMS functionalized with thiol exhibited greater sulfur content and higher Au(III) adsorption capacity (231 mg/g) in a single-component solution. The adsorption rates follow the pseudo-second-order model, and the Au(III) adsorption isotherms fit well with the Langmuir adsorption model. These are attributed to the high sulfur content and its increased uniformity in silanol-rich OMS adsorbents, which increased the inward diffusion of Au(III) and effectively improved its accessibility to the activated sites. The adsorbents of Au(III) in acidic leachate solution achieved an adsorption efficiency of 213 mg/g. Moreover, they maintained the Au(III) selectivity of the functionalized thiol on surface silanol in acidic leachate solution, suggesting minimal non-selective metal ion adsorption. As a result of the regeneration, the adsorption efficiencies of Au(III) was maintained at over ca. 97.2%, even after five recovery cycles. These results demonstrate the potential application and recyclability of the developed adsorbent for Au(III) recovery in acidic leachate solution of urban mining and other prospective sources.

Diffusion and adsorption of Au and Pt adatoms on ideal and missing row reconstructed surfaces of Au(110): DFT and EAM calculations

Density-functional theory (DFT) and embedded atom method (EAM) are used to study the adsorption and diffusion on unreconstructed (1 × 1) and reconstructed (1 × 2), (1 × 3) and (1 × 4) surfaces for homogenous Au/Au(110) and heterogeneous Pt/Au(110) systems. Adsorption energies show a strong binding for Pt than Au on all considered geometries. Additionally, adsorption on the (111) microfacet of the (1 × 2), (1 × 3) and (1 × 4) geometries is investigated. Surprisingly the adsorption of Pt on the top hcp sites of the (111) microfacet of the (1 × 2), (1 × 3) and (1 × 4) geometries leads to the exchange phenomenon. The diffusion via hopping and metastable walk is investigated for both the homo and heterogeneous systems. The hopping diffusion barrier show a slight dependence on the reconstruction order and the diffusion is found to be more costly on the missing row reconstructed surface than the unreconstructed surfaces. The potential energy profile for the diffusion via metastable walk mechanism on the (1 × 2) missing row reconstructed geometry indicates that the metastable walk diffusion is more favorable to homogeneous system than to heterogeneous system.The results of this paper are important to understand the properties of metallic reconstructed surfaces.

Diffusion of Au and its influence on the coercivity of [FePt/Au/FePt]2x thin films during annealing in different atmospheres

Influence of heat treatment atmosphere on magnetic properties of post-annealed [FePt(15 nm)/Au(7.5 nm)/FePt(15 nm)]2x thin films was investigated. Thin films were deposited by dc magnetron sputtering at room temperature onto SiO2(100 nm)/Si(100) substrates and post-annealed in a temperature range of 500 °C–800 °C for 30 s in flowing Ar and Ar + H2 (3 vol%) atmospheres. It was shown that hydrogen addition into annealing atmosphere allows to achieve high value of coercivity ~1671.1 kA/m after annealing at 600 °C. Such coercivity could be reached by heat treatment in pure Ar just after annealing at 700 °C. This effect could be related to acceleration of Au grain boundary diffusion into L10-FePt phase. Such acceleration could be possible due to two reasons: hydrogen incorporation into Au crystal lattice, leading to interatomic bounds break and point defects formation; and hydrogen segregation at most defective areas, leading to cohesive energy decrease and upraise of significant stresses. As a result, nonmagnetic grid in grain boundaries of ferromagnetic material was formed at temperature lower by 100 °C.

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode.

Solar cells based on methylammonium lead triiodide (MAPbI3) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long‐term stability of MAPbI3‐based solar cells has yet to be achieved. Besides the well‐known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current–voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI3. In addition to Au, many other metals have been used as electrodes in MAPbI3 solar cells. However, how the external metal impurities introduced by electrodes affect the long‐term stability of MAPbI3 solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI3 based on first‐principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI3 and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI3 while having low resistivities and suitable work functions for carrier extraction.

Characterization of interfacial morphology of low temperature, low pressure Au–Au thermocompression bonding

Au–Au thermocompression bonding is a versatile technique of high interest for a variety of applications. We have investigated Au–Au bonding using sputter deposited Au films under conditions of low temperature (150–250 °C) and low bonding pressure (∼3 MPa) for short times (15 min). The combination of low temperature and short times is important for applications involving both hermetic sealing and packaging scaling. The initial surface roughness of the Au film was in the 3–5 nm range with peak-to-valley heights of 20–30 nm and a lateral correlation length of ∼400 nm. For samples bonded at 150 °C, the void morphology at the bonded interface was related to the initial surface roughness. The void morphology was different when bonding at the higher temperatures: the void length (along the bonded interface) decreased significantly but the void height (perpendicular to the interface) increased. These results can be understood in terms of a combination of increased surface Au diffusivity and decreased yield stress and elastic modulus with increased bonding temperature.

Effect of Au diffusion in Au-coated (γ + α2) titanium aluminides

In order to understand the role of α2-Ti3Al precipitates during the oxidation of Au-coated γ-48-2-2-TiAl alloys, the γ-TiAl and α2-Ti3Al microstructures resulting from Au diffusion at 900 °C were investigated. The as-obtained coatings were characterized using scanning electron microscopy (SEM), combined energy and wavelength dispersive X-ray spectroscopies (EDS, WDS), electron backscatter diffraction analysis (EBSD), X-ray diffraction (XRD) and isothermal oxidation tests. Results showed that Au-coated-γ-TiAl alloys presented an upper aluminoformer TiAlAu2 layer and a lower non-protective TiAlAu layer, both with narrow homogeneity range, and with thicknesses that could be estimated with the use of the Prasad model. A more complex layer structure is obtained after Au diffusion in α2-Ti3Al alloys, since eight different phases were identified. In the lower layers, two new ternary tetragonal phases (Ti2AlAu and Ti3Al2Au) were found, and a partial diagram for the Al-Au-Ti 900 °C isotherm is proposed. Absence or depletion of Al was found in the upper layers and, in addition, a low oxidation resistance of the Au-coated-α2-Ti3Al alloy was reported.

Degradation mechanism of AlGaN/GaN HEMTs during high temperature operation stress

The degradation behavior and its physical mechanism of AlGaN/GaN high electron mobility transistors (HEMTs) during high temperature operation (HTO) stress were investigated in this paper. The results show that the gate leakage current of AlGaN/GaN HEMTs after 1000 h HTO stress is two orders of magnitude larger than that of the fresh ones. The maximum transconductance of the AlGaN/GaN HEMTs obviously decreases from 0.31 to 0.21 s after 1000 h HTO stress, which indicates the gate degraded seriously. The mechanism for the gate degradation could be attributed to the Au diffusion that degrades the barrier. Moreover, the output characteristics are also deteriorated seriously, and the maximum variation of drain-to-source current is up to 119 mA under the conditions of gate-to-source voltage of 0 V and drain-to-source voltage of 5 V. The reason for the deterioration of output characteristics could be attributed to two mechanisms of gate degradation and structural damage such as pit-like, crack-like damages. The results may be useful in the design and application of AlGaN/GaN HEMTs.

Ion beam heating of kinetically constrained nanomaterials

The gallium ion beam heating on electron transparent transmission electron microscopy (TEM) samples of Au/Ni bilayer films supported by SiO2 substrates was studied by in-situ TEM combined with energy dispersive X-ray spectroscopy. Brief Ga+ ion beam irradiation during sample transfer inside the focused ion beam instrument was found to induce dewetting of bilayer films. The observed morphological changes of the metal films are complemented by considerable Au diffusion through the underlying polycrystalline Ni film and adsorption at the Ni/substrate interface. In-situ heating experiments confirm that alterations of the metal bilayer films caused by ion beam irradiation are consistent with thermal annealing at 400 °C for several minutes in the absence of any ion bombardment. Ion beam damage effects equivalent to prolonged heating may pose considerable limitations to ion beam microscopy of samples with reduced dimensions. Ex-situ lift-out procedures of electron transparent samples in the absence of any ion beam irradiation lead to successful conservation of sample morphologies.

Influence of thermal annealing and radiation enhanced diffusion processes on surface plasmon resonance of gold implanted dielectric matrices

Gold nanoparticles (AuNPs) embedded in fused silica and sapphire dielectric matrices were synthesized by Au ion implantation. Systematic investigations were carried out to study the influence of implantation dose, post annealing temperature, swift heavy ion (SHI) irradiation and radiation enhanced diffusion (RED). Rutherford Backscattering Spectrometry (RBS) measurements were carried out to quantify concentration and depth profile of Au present in the host matrices. X-ray diffraction (XRD) was employed to characterize AuNPs formation. As-implanted and post-annealed films were irradiated using 100MeV Ag ions to investigate the effect of electronic energy deposition on size and shape of NPs, which is estimated indirectly by the peak shape analysis of surface plasmon resonance (SPR). The effect of volume fraction of Au and their redistribution is also reported. A strong absorption in near infra red region is also noticed and understood by the formation of percolated NPs in dielectric matrices. It is quite clear from these results that the effect of RED assisted Oswald ripening is much more pronounced than the conventional Oswald ripening for the growth of NPs in the case of silica host matrices. However for sapphire matrices, it seems that growth of NPs already completed during implantation and it may be attributed to the high diffusivity of Au in sapphire matrices during implantation process.

Thermal stability and small-signal characteristics of AlGaN/GaN HEMTs with gate insertion metal layer for millimeter-wave applications

The effect that insertion gate metals have on GaN millimeter-wave devices undergoing a postmetallization annealing (PMA) process was investigated. It was found that the PMA process increases the gate resistance (Rg), which is responsible for a decrease in the maximum oscillation frequency (fmax). The resistance was examined as a function of line patterns containing various gate metal stacks, including Ni/Au and Ni/Mo/Au, before and after annealing from a low temperature to 550 °C. The metal stack with an Mo insertion layer effectively suppressed Au diffusion into GaN and reduced the increase in the gate metal resistance. For the fabricated AlGaN/GaN-on-Si high-electron-mobility transistors with a Ni/Mo/Au gate, stable gate reliability, improved current collapse characteristics, and small-signal characteristics were also achieved compared to those of the Ni/Au gate.

Energy barriers of single-adatoms diffusion on unreconstructed and reconstructed (110) surfaces

The present paper is aimed mainly to investigate theoretically the diffusion of Ag, Cu, Au and Pt adatoms on the (1 × 1) unreconstructed geometry for Ag, Cu and Pt (110), and reconstructed geometries ((1 × 2), (1 × 3) and (1 × 4)) for Pt and Au (110) surfaces. We consider the single adatom diffusion when additional atoms are deposited in adjacent row. For this study, we have used the molecular statics simulations combined with the embedded atom method. For several systems, we have calculated the activation barriers for hopping mechanism. For the diffusion on the unreconstructed surfaces, the trends for the activation barriers are the same for all considered systems except for Cu/Ag (110) system, where the activation barrier do not change. Further, our results indicate that additional atoms lead to a small decreasing of activation barriers for diffusion on reconstructed surfaces for some systems, while for other systems; the activation barrier remains practically unchanged.

Gold diffusion into and out of quartz-hosted fluid inclusions during re-equilibration experiments at 600–800 °C and 2 kbar

Fluid inclusions were synthesized in quartz and subsequently re-equilibrated in fluids of different compositions to investigate whether the Au content of fluid inclusions can be modified via Au diffusion through the host quartz. In one experiment fluid inclusions containing 5wt% KCl were synthesized in platinum capsules at 2kbar, 800°C and then re-equilibrated at the same conditions for 4days in a fluid containing 20.1wt% NaCl, 3.5wt% HCl and ~100ppm Cs enclosed in a gold capsule. LA-ICP-MS analyses of 5–10 fluid inclusions before and after the re-equilibration experiment demonstrate that the inclusions gained up to 570ppm Au and up to 0.23wt% Na during the re-equilibration process. Their unchanged high K-content and the absence of Cs within the inclusions proves that they remained physically closed during the re-equilibration, and thus that Na and Au were gained by diffusion through the quartz lattice. In a similar experiment conducted at 600°C the fluid inclusions gained up to 140ppm Au within 14days. Reverse experiments were conducted by synthesizing Au-bearing fluid inclusions in gold capsules and then re-equilibrating them in platinum capsules, which caused the inclusions to lose up to several hundred ppm Au. In both type of experiments Au diffusion appears to have been driven by chemical potential gradients of Au between the fluid inclusions and the external fluid. Additional experiments were performed to test whether Au is able to diffuse into fluid inclusions in response to loss of H+, or indirectly as a consequence of diffusional Cu-gain and corresponding formation of Cu-rich sulfide daughter crystals. However no evidence for Au uptake via such mechanisms was found.If Au concentrations in natural, quartz-hosted fluid inclusions from magmatic-hydrothermal ore deposits are affected, then they likely lost Au rather than gained it. At temperatures below 400°C diffusional loss or gain of Au is considered unlikely in magmatic-hydrothermal systems, but potentially was significant in samples from orogenic Au gold deposits.

Thermal stability of a Schottky diode fabricated with transfer-free deposition of multilayer graphene on n-GaN by solid-phase reactions

Multilayer graphene (MLG)/n-GaN Schottky diodes were fabricated by transfer-free deposition of MLG on n-GaN by solid-phase reactions with cobalt as a catalyst. The thermal stability of the diodes was determined from the current–voltage (I–V) characteristics after annealing the diodes in vacuum at 200–500 °C, at intervals of 100 °C. The diode characteristics evaluated using a thermionic emission model and Cheung’s function using I–V data revealed that the Schottky barrier diode (SBD) fabricated with MLG as a Schottky contact on n-GaN showed better thermal stability than the conventional Ni/n-GaN SBD. The prevention of Au diffusion to n-GaN with MLG as a diffusion barrier layer and the unaffected interface reactions between n-GaN and MLG are possible reasons for the improved thermal stability, enabling potential application of this new diode in high-power and high-temperature operations.

Self-similarity in the structure of coarsened nanoporous gold

Nanoporous gold (np-Au) samples of ligament sizes 68.6, 248.6, 462.9, and 710.9nm are prepared by heat treatment, and representative volumes are reconstructed by focused ion beam (FIB) tomography. The increase in relative density by thermal coarsening is not pronounced. We analyze the distribution of ligament size, surface-to-volume ratio, and scaled connectivity density for coarsened np-Au, revealing that np-Au coarsens in a self-similar way. The measured activation energy for thermal coarsening supports that it is accomplished by surface diffusion of Au.

H and Au diffusion in high mobility a-InGaZnO thin-film transistors via low temperature KrF excimer laser annealing

We report the fabrication of high mobility amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) irradiated by a single shot of a 248 nm KrF excimer laser. Very high mobilities (μ) of up to 43.5 cm2/V s were obtained after the low temperature excimer laser annealing (ELA) process. ELA induces high temperatures primarily in the upper layers and maintains very low temperatures of less than 50 °C in the substrate region. Scanning Transmission Electron micrographs show no laser induced damage and clear interfaces after the laser irradiation. In addition, several characterization studies were performed to determine the μ improvement mechanism. The analysis of Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy suggests incorporation of H mainly from the hybrid passivation layer into the channel. Moreover, Energy-dispersive X-ray Spectroscopy results show that Au diffused into the channel after ELA. Both KrF ELA-induced H and Au diffusion contributed to the higher μ. These results demonstrate that ELA can greatly enhance the electrical properties of a-IGZO TFTs for promising applications in large area, transparent, and flexible electronics.

Improvement of mechanical performances and characteristics of bulk Bi-2212 materials exposed to Au diffusion and stabilization of durable tetragonal phase by Au

This study centers sensitively on the determination of optimum diffusion annealing temperature (650–850°C) for the electrical, superconducting, crystal structure, and especially mechanical characteristics of Au surface-layered Bi-2212 superconducting materials with the aid of dc electrical resistivity, powder X-ray diffraction and microhardness measurements. The experimental results show that all the characteristic properties (normal state resistivity, critical transition temperatures, degree of broadening, phase fractions, lattice cell parameters, elastic modulus, yield strength, fracture toughness, brittleness index and flexural strength parameters) improve considerably with the increment in the annealing temperature up to 800°C as a consequence of the decreased local structural distortions, lattice strains, disorders, defects and grain boundary interaction problems in the Cu-O2 consecutively stacked layers. After the critical annealing temperature value of 800°C, the parameters immediately recrudesce towards their global minimum points. Similarly, the highest Bi-2223 phase fraction and c-axis length are observed at the 800°C annealing temperature due to the best crystallinity and crystal plane alignments. Additionally, the optimum value strengthens the mechanical durability and ideal flexural strength as a result of the stabilization of durable tetragonal phase. Thus, the presence of Au impurities increases the critical stress value so that the crack-producing flaws and cracks propagation divert or slow down rapidly. On the other hand, the excess temperature value such as 850°C leads to the deleterious effect on the mechanical performances of Au surface-layered Bi-2212 compound because of the increased residual porosity and omnipresent flaws (stress raisers and crack initiation sites). Further, it is at least equally important that the crack propagation or dislocation movement more proceeds through the transgranular regions instead of along the intergranular regions with increasing temperature up to the optimum value beyond which the limited number of operable slip systems enhances noticeably and the intergranular fracture becomes more dominant.

Diffusion of single Au, Ag and Cu atoms inside Si(111)-(7 × 7) half unit cells: A comparative study

The diffusion behaviors of single Au, Ag and Cu atoms on Si(111)-(7×7) half unit cells have been investigated via combining scanning tunneling microscopy and first-principles calculations. Despite the similar adsorption sites between both half unit cells among these elements, the diffusion dynamics show obvious differences between Ag and the other two. Although obvious asymmetry has been found in the diffusion behaviors of Au and Cu atoms in two half unit cells of Si(111)-(7×7), the asymmetry behaves in a way different from that of Ag atoms and no dual-time character has been observed for the diffusions of Au and Cu in both half unit cells. Theoretical calculations suggest a different potential energy profile caused by the stronger hybridization between d states of Au (Cu) and Si states make the concept of basin useless for the diffusion of Au and Cu atoms inside the half unit cells of Si(111)-(7×7).

Spirals and helices by asymmetric active surface growth.

We show that spiral and helical Au nanowires can be directly grown via the active surface growth mechanism. The formation of spiral nanowires as opposed to straight nanowires is not triggered by the presence of a particular reactant, but controlled by the ratio of reactant concentrations. We propose that the asymmetric blocking of the Au-substrate interface induces imbalanced growth of the nanowire, causing it to curve. Blocking a single corner of the active interface leads to spiral nanowires whereas blocking two corners leads to helical nanowires. Spiral and helical nanowires become more frequent when the diffusion of Au is the limiting factor, as the reactant ratio falls below a critical value. The transition from helices to spirals and finally to nearly straight nanowires indicates a gradual loss of the blocked sites, hence supporting the asymmetric blocking mechanism.