On the role of indium underlays in the prevention of thermal grooving in thin gold films

Thin gold films are potentially important for metallizations in microelectronic devices because of the high activation energy of gold for electrotransport. A high activation energy ensures a longer lifetime of microelectronic devices compared with those in which aluminum metallizations are used. When electromigration is no longer the principal failure mechanism, other failure mechanisms, caused by d.c. stressing, might become important. One possibility is grain boundary grooving. Preliminary studies have shown that grain boundary grooving in thin gold films is prevented by inserting an indium underlay between the gold film and the substrate. The objective of this work was to investigate the mechanisms for the prevention of grain boundary grooving in In/Au composite films by comparing the microstructural evolution of pure gold films with In/Au composite films during isothermal annealing. Microstructures were characterized in terms of grain size, grain size distribution, preferred orientation and surface morphology utilizing transmission electron microscopy (TEM), cross-sectional TEM, scanning electron microscopy and X-ray diffraction. The chemical reactions and the distributions of the phases were monitored by selected area diffraction in TEM, and by Auger electron spectroscopy sputter profiling.It was found that the principal mechanisms that inhibit grain boundary grooving in IN/Au composite films area as follows.

• 1.(1) Indium underlays modify the microstructure of gold films by randomizing the orientation of the grains, refining the grain size, narrowing the grain size distribution and roughening the surface of the gold films.
• 2.(2) Indium is redistributed on gold films and forms In₂O₃ on the free surface and within the film during air annealing.
• 3.(3) The In₂O₃ on the surface “caps” the surface of gold films and limits mass transport during annealing.
• 4.(4) The In₂O₃ within the gold film, presumably residing at grain boundaries, impedes grain growth by pinning the grain boundary migration.

     

Microstructural features, texture and strengthening mechanisms of nanostructured AA6063 alloy processed by powder metallurgy

Nanostructured AA6063 (NS-Al) powder with an average grain size of ∼100 nm was synthesized by high-energy attrition milling of gas-atomized AA6063 powder followed by hot extrusion. The microstructural features of the consolidated specimen were studied by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques and compared with those of coarse-grained AA6063 (CG-Al) produced by hot powder extrusion of gas-atomized powder (without using mechanical milling). The consolidated NS-Al alloy consisted of elongated ultrafine grains (aspect ratio of ∼2.9) and equiaxed nanostructured grains. A high fraction (∼78%) of high-angle grain boundaries with average misorientation angle of 33° was noticed. Microtexture evaluation by plotting pole-figures and orientation distribution function (ODF) analysis showed Copper and P texture components for both the consolidated Al alloys. Tensile test at room temperature and microhardness measurement revealed that a significant improvement in the strength of AA6063 alloy is obtained through refinement of the grain structure. The strengthening mechanisms are discussed based on the dislocation-based models. The role of high-angle and low-angle grain boundaries on the strengthening mechanisms is discussed.     

Dynamics of grain boundaries under applied mechanical stress

Recent results of experimental research into stress-induced grain boundary migration in aluminum bicrystals are reported. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for planar symmetrical 〈100〉 tilt boundaries. This coupling proved to be the typical migration mode of any 〈100〉 tilt boundary, no matter whether low- or high-angle, low Σ CSL coincidence or non-coincidence boundary. The measured ratios of normal boundary motion to the tangential displacement of grains are in an excellent agreement with theoretical predictions. The migration-shear coupling is also observed for asymmetrical 〈100〉 boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress-induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.     

Microstructural features of cadmium telluride photovoltaic thin film devices

In order to study the microstructure of cadmium telluride (CdTe) photovoltaic thin film solar cells, manufactured by an in-line manufacturing process, Scanning Electron Microscopy characterization (SEM) and X-ray diffraction (XRD) characterization were performed. SEM measurement showed that no substantial changes in the grain structure of CdTe layers occurred during the Cadmium Chloride (CdCl₂) treatment. No change in the cubic CdTe lattice parameter “a” was observed for the CdCl₂ treated sample. It is inferred that the primary effect of the CdCl₂ treatment in the devices studied is the passivation of grain boundaries and bulk defects. XRD studies show a loss of preferred orientation (as determined from the peak ratios) of planes during the copper compound treatment indicating recrystallization of the grains due to the Cu treatment. Also the Cu treated sample showed decrease in value of the lattice parameter “a”.     

A review of crystallographic textures in chemical vapor-deposited diamond films

Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the〈100〉direction to that along the 〈111〉direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond.     

Imaging and microanalysis of liquid phase sintered silicon-based ceramic microstructures

This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si₃N₄ or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure.     

Microstructure of ZnO films synthesized on MgAl2O4 from low-temperature aqueous solution: growth and post-annealing

The microstructure of ZnO films synthesized from low-temperature (90 °C) aqueous solution on (111) MgAl₂O₄ single crystal substrates was characterized by X-ray diffraction, high-resolution scanning electron microscopy, conventional and high-resolution transmission electron microscopy. To examine the thermally activated microstructural evolution of the ZnO, both as-deposited and annealed films were characterized. The ZnO films were confirmed to have a ZnO $ [10\bar{1}0](0001)\left\| {{\text{MgAl}}_{ 2} {\text{O}}_{4} [011](1\bar{1}1)} \right. $ orientation relationship, with Zn polarity normal to the surface. Despite their highly oriented nature, the ZnO films have a columnar grain structure with low-angle (<2.5°) grain boundaries. In addition to lattice dislocations forming low-angle grain boundaries, threading dislocations were observed, emanating from the interface with the substrate. In annealed films, thermally generated voids were observed and appeared to preferentially form at grain boundaries and dislocations. Based on these characterization results, mechanisms are proposed for film growth and microstructural evolution. Finally, the diffusion coefficient of vacancies via dislocations at grain boundaries in the produced ZnO films was estimated.     

Dual orientation and variant selection during diffusional transformation of austenite to allotriomorphic ferrite

The crystallographic relationship between austenite and grain boundary nucleated allotriomorphic ferrite has been investigated using electron back-scattered diffraction with a view to establishing a mechanism of variant selection. It is possible in some circumstances for the ferrite to adopt a favoured orientation relationship with both of the austenite grains with which it is in contact. However, the theoretical probability for the development of such a dual orientation has in previous work been shown to be very small, although experiments indicate otherwise. In this work, we have discovered experimentally that the probability of dual orientations is significantly increased when adjacent austenite grains are connected by special high-angle boundaries. Crystallographic calculations validate these observations and lead to the conclusion that simultaneous lattice matching between ferrite and its parent austenite grains is more likely in the presence of certain kinds of microscopic texture in the austenite. The phenomenon of dual orientation provides a criterion for crystallographic variant selection during diffusional transformation.     

Correlation between grain boundary misorientation and M23C6 precipitation behaviors in a wrought Ni-based superalloy

The correlation between the grain boundary misorientation and the precipitation behaviors of intergranular M₂₃C₆ carbides in a wrought Ni–Cr–W superalloy was investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that the grain boundaries with a misorientation angle less than 20°, as well as all coincidence site lattice (CSL) boundaries, are immune to precipitation of the M₂₃C₆ carbides; in contrast, the random high-angle grain boundaries with a misorientation angle of 20°–40° provide preferential precipitation sites of the M₂₃C₆ carbides at the random high-angle grain boundaries with a higher misorientation angle of 55°–60°/[2 2 3] turn to retard precipitation of M₂₃C₆ carbides owing to their nature like the Σ3 grain boundaries and retard the precipitation of M₂₃C₆ carbides. The low-angle and certain random grain boundary segments induced by twins were found to interrupt the precipitation of the M₂₃C₆ carbides along the high-angle grain boundaries.     

Changes in misorientations of grain boundaries in titanium during deformation

The changes in grain boundary misorientations during plastic deformation of titanium were studied by means of the EBSD technique. The misorientation of all types of the grain boundaries including low- and high-angle boundaries, coincident site lattice (twin) and arbitrary boundaries, deformation-induced boundaries and the boundaries of the original grains was found to change during deformation. It was shown that the deformation may result in either increasing or lowering of the boundaries misorientation and different segments of the same grain boundary may develop principally differently. The most significant changes of the boundary misorientation were found to be associated with the boundary junctions. The change in misorientations during deformation was discussed in terms of the interaction of a boundary with dislocations and/or with other boundaries.     

The relationship between the fracture toughness and grain boundary characteristics in hot-dip galvanized zinc coatings

This publication presents an experimental study on the relation between the grain boundary (GB) characteristics and the intergranular cracking resistance in a hot dip zinc coating. The cracking was studied using in situ tensile tests in a scanning electron microscope on small tensile samples of a hot dip galvanized steel sheet. In situ testing offered a series of advantages like monitoring the kinematical evolution of cracking without unloading, or making micrographs and OIM imaging on the same area of the tensile sample. The grain boundaries were classified into random and special boundaries (respectively Low angle boundaries and Coincidence site lattice—CSL boundaries). These special boundaries which account for 3.5% of the whole boundaries clearly show better cracking resistance than the random boundaries. The only special boundaries which present cracking failure are in an orientation with their normal direction close to the tensile direction, i.e. submitted to a maximum effective stress. The grain boundaries characteristics are obtained from EBSD individual orientation measurements.     

Role of the crystallinity of ZnO films in the electrical properties of bottom-gate thin film transistors

The strong correlation between the microstructural characteristics of ZnO channel layers grown at various temperatures by radio-frequency magnetron sputtering and the electrical performances of resulting bottom-gate thin film transistors (TFTs) was reported. Transmission electron microscopy revealed that increasing growth temperature enhanced degree of c-axis preferred orientation and enlarged width of columns in the ZnO films. The ZnO channel layers grown at 250 and 350 °C exhibited TFT saturation behavior. However, growing them at ≥ 350 °C produced small grains in the junctions of ZnO/SiO₂ interface and grain boundaries, which led to hump behavior in TFT transfer curve caused by formation of additional boundaries.     

Structure of special grain boundaries in SiAlON ceramics

The microstructure of hot-pressed samples of the 15 R polytype phase in the Si-Al-O-N system was studied by means of TEM. Emphasis was put on studies of high angle grain boundaries. In this material high angle boundaries of arbitrary orientation usually possess a vitreous grain boundary phase. However, special grain boundaries were found, which were free of any vitreous grain boundary phase. From the orientation relation of the adjacent grains Σ=1 (coherent reflection twin boundary), Σ=7 and Σ=13 boundaries were found. For their explanation a coincidence site lattice (CSL) model was developed for (0001) twist boundaries. By assuming an exponential form for the potential of atomic interaction, the calculation of minimum grain boundary energies for special twist angles was in accordance with the experimental observations.     

Thickness-dependent properties of indium-doped ZnO films

The thickness dependence of microstructural, electrical and optical properties of transparent conducting indium-doped ZnO films was studied. The density of trap states due to chemisorbed oxygen at the grain boundaries was found to depend on the orientation of grains which, in turn, depended on the film thickness. The thickness dependence of the electrical parameters of oxygen-annealed films was explained on the basis of changes in the trap state density. The changes in the electrical parameters of vacuum-annealed films were attributed to a decrease in grain size. The shifts in optical gap on vacuum and oxygen annealing were attributed to changes in carrier concentration, considering two simultaneous effects on band gap widening and narrowing.     

Anneal induced recrystallization of CdTe films electrodeposited on stainless steel foil: The effect of CdCl<Subscript>2</Subscript>

Electrodeposited CdTe films were treated with a saturated solution of CdCl₂, and later annealed in air at various temperatures and time durations in order to investigate the influence of post deposition CdCl₂ annealing treatments on the structure of the films. The XRD results showed that the CdCl₂ treatment has a noticeable influence on the stress, grain growth and the re-crystallization of CdTe. The value of activation energy for 20% re-crystallization in CdTe was estimated as 1.17 ± 0.4 eV and 0.99 ± 0.1 eV respectively for untreated and CdCl₂ treated CdTe films. In the early stages of annealing the re-crystallization is dominated by random orientation of the grains followed by a second phase in which once again the crystallites tend to orient in a particular direction. The Lattice constant (a) increases upon annealing and reaches a maximum and on further annealing for a long time it decreases and attains a value less than that of the powder sample.This work is part of the Ph.D. thesis of J.P. Enriquez at CIE-UNAM.     

Comparison between the effects of CdCl2 heat treatment on CdTe films prepared by RF magnetron sputtering and close spaced sublimation methods

CdTe films were prepared on Fluorine-doped tin oxide substrate by RF magnetron sputtering and close spaced sublimation (CSS) methods, respectively. These CdTe films were then treated with a wet CdCl₂ heat process at different temperatures. The structural and optical properties of CdTe films were investigated by X-ray diffraction, scanning electron microscope and UV–Visible spectrophotometer. The results reveal that both types of CdTe films have a better crystalline and larger grain size after CdCl₂ heat treatment. However, the (422) peak has a more preferential orientation than (511) peak after CdCl₂ activation for CdTe films prepared by sputtering method, while these two peaks almost have the same intensity for CSS-prepared CdTe films. The transmittance of CdTe films prepared by CSS is apparently lower than sputtered CdTe films. Correspondingly, the efficiency of solar cells with CSS-prepared CdTe is 7.3, 2.6 % of sputtered CdTe films.     

The effectiveness of coincidence site lattice criteria in predicting creep cavitation resistance

The coincidence site lattice (CSL) concept is often used in microstructural characterization by researchers studying grain boundary engineering as a method for improving the performance of polycrystalline materials. It is assumed that a high degree of shared lattice sites in the boundary between two grains will result in improved mechanical properties. For practical application of the CSL concept to experimental results, a maximum deviation from ideal CSL orientation relationships must be defined to distinguish potential CSL boundaries from random boundaries that are not likely to exhibit “special” properties. Several different maximum deviation criteria have been proposed in the literature. In this study, four of these criteria are investigated for their effectiveness in predicting the creep cavitation resistance of boundaries of different CSL character in three model alloys: pure Cu, Cu-Bi, and Cu-Sb. Bi and Sb strongly segregate to Cu grain boundaries and are detrimental to creep life. The experimental observations are compared to simulation results for a non-textured polycrystal. It is observed that only boundaries related to cubic annealing twins (Σ3 and Σ9) exhibit special resistance to creep cavitation, that boundaries with Σ > 3 are affected by the presence of segregants, and that the fraction of non-Σ(3,9) boundaries tracks closely with what would be expected from a random polycrystal. It is shown that more restrictive criteria result in more reliable characterization of the fraction of cavitation-resistant boundaries only because they exclude more non-Σ(3,9) boundaries from the analysis.     

Oxygen diffusion and defect mechanism in c-axis textured thin films of YBa2Cu3O7−x by resistivity measurements

Isothermal resistivity measurements have been carried out to study the dynamics of oxygen out- and in-diffusion in thin films of YBa₂Cu₃O_7−x in the temperature range 648–773 K. The activation energies for the out- and in-diffusion were determined to be 1·36 and 0·7 eV respectively. We have modelled the resistance-time plots for the oxygen in-diffusion using an equation for one dimensional diffusion into a plane. The microstructural defects such as low angle grain boundaries associated with the c-axis oriented grains are believed to provide the required diffusion paths in thin films of YBCO.     

Wetting transition of grain boundaries in the Sn-rich part of the Sn–Bi phase diagram

The microstructural evolution of tin-rich Sn–Bi alloys after the grain boundary wetting phase transition in the (liquid + β-Sn) two-phase region of the Sn–Bi phase diagram was investigated. Three Sn–Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 °C for 24 h. The micrographs of Sn–Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures.     

Electron microscopic study of sputter-deposited Ir films

The microstructural development of iridium (Ir) film deposited on isotropic graphite by a sputtering method was investigated using a transmission electron microscope. A columnar structure was developed in which the diameters of columnar grains were distributed from 3 to 50 nm at the substrate temperature of 330 K. Directions of grains were almost perpendicular to the substrate surface, and grain boundaries were wavy. Grains forming the columnar structure indicated different orientations of a growth direction, though growth orientations of grains showed the weak preferred orientation of a [111] direction. Non coincident related boundaries, as well as low angle grain boundaries and twin boundaries tend to be observed more frequently than coincidence lattice related boundaries.