In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

The effect of film thickness on the failure strain of polymer-supported metal films

We perform uniaxial tensile tests on polyimide-supported copper films with a strong (1 1 1) fiber texture and with thicknesses varying from 50 nm to 1 μm. Films with thicknesses below 200 nm fail by intergranular fracture at elongations of only a few percent. Thicker films rupture by ductile transgranular fracture and local debonding from the substrate. The failure strain for transgranular fracture exhibits a maximum for film thicknesses around 500 nm. The transgranular failure mechanism is elucidated by performing finite element simulations that incorporate a cohesive zone along the film/substrate interface. As the film thickness increases from 200 to 500 nm, a decrease in the yield stress of the film makes it more difficult for the film to debond from the substrate, thus increasing the failure strain. As the thickness increases beyond 500 nm, however, the fraction of (1 0 0) grains in the (1 1 1)-textured films increases. On deformation, necking and debonding initiate at the (1 0 0) grains, leading to a reduction in the failure strain of the films.     

Texture transformations in Ag thin films

A thickness-dependent texture transformation during annealing of initially (1 1 1) fiber-textured face-centered-cubic metal thin films is phenomenologically well known: sufficiently thin films retain the (1 1 1) texture, while sufficiently thick films transform to a (1 0 0) fiber texture. This transformation has been explained based on minimization of strain and interface energies, but recent work calls into question the roles of both of these driving forces. A high-throughput experimental method for the study of this texture transformation has been developed and applied to thin silver films with and without Ti adhesion layers. More than 150 individual samples spanning a range of thicknesses and interface conditions were prepared in a single deposition run. The texture evolution of these samples was characterized using X-ray diffraction as a function of time and temperature during annealing. The transformation proceeds despite the fact that the stresses are too low according to the strain/interface energy model. For films with Ti adhesion layers, the transformation kinetics and extent of transformation depend on the film thickness in a surprising way with intermediate thickness films showing initially fast transformations and stable mixed textures, while thicker films show an incubation time but transform fully. The results are consistent with reduction in defect energy (e.g. dislocations or point defects) as the driving force for secondary grain growth in an environment in which only (1 0 0) recrystallization nuclei can form. The driving force increases with film thickness so the nonmonotonic variation in transformation rate implies that the density of (1 0 0) nuclei decreases with thickness.     

High-coercivity Nd–Fe–B thick films without heavy rare earth additions

The magnetic properties and microstructures of two Nd–Fe–B thick films with different Nd contents have been studied. The films were deposited in the amorphous state and were crystallized by post-deposition annealing. Both films show a strong 〈0 0 1〉 fibre texture out-of-plane. The film with the higher Nd content has a large room temperature coercivity of 2.7 T, while the one with the lower Nd content has a room temperature coercivity of only 0.7 T. The difference in coercivity may be explained by the fact that the film with the higher Nd content exhibits a continuous Nd-rich grain boundary phase, giving better isolation of the Nd₂Fe₁₄B grains with respect to magnetic exchange interactions. The extrusion of Nd-rich liquid to the top surface of the film with high Nd content during post-deposition annealing led to the formation of ripples in the Ta capping layer, indicating that the films are under compressive stress. This stress-induced flow of the Nd-rich material up through the film explains the excellent distribution of the Nd-rich grain boundary phase. Atom probe tomography has revealed the presence of Cu in the Nd-rich grain boundary phase, explaining the formation of the liquid phase at the relatively low temperature of 550 °C due to the eutectic reaction of Nd and Cu.     

The preparation of nanograin Ag–TCNQ thin films by vacuum evaporation with post-heat treatment

Ag–TCNQ organometallic complex with single phase and uniform grains in nanometer scale were prepared by vacuum evaporation and post-heat treatment. The grain size of the films was decreased by introducing the post-heat treatment process to about 50 nm. By applying a ramp voltage onto the film through STM probe tip in air at room temperature, the film will transfer from high impedence to low impedence at about 2.0 V. A writing dot of about 70 nm in diameter, corresponding to the low impedence state, was obtained after applying a pulse voltage of 5.0 V in amplitude and 5.0 ms in duration.     

Correlation between the microstructure studied by X-ray line profile analysis and the strength of high-pressure-torsion processed Nb and Ta

Niobium and tantalum are two body centered cubic metals with very different elastic anisotropy. The A_z = 2 × c₄₄/(c₁₁?c₁₂) constant for Nb and Ta is 0.51 and 1.58, respectively. The submicron grain-size state of the two refractory metals was produced by the method of high-pressure torsion with different pressure values of 2 and 4 GPa for Nb, and 4 and 8 GPa for Ta, and two different deformations of 0.25 and 1.5 rotations, respectively, with equivalent strains of up to ～40. The dislocation density and the grain size were determined by high-resolution diffraction peak-profile analysis. The beam size on the specimen surface was 0.2 × 1 mm, allowing the sub-structure along the radius of the specimen to be characterized. The strength of the two metals was correlated with the dislocation density and the grain size. It is found that, though the grain size is well below 100 nm, the role of dislocations in the flow stress of these two metals is significantly greater than that of the grain size.     

Texture evolution and mechanical properties of ion-irradiated Au thin films

Microstructure control of thin films is of particular importance for improving the reliability of microdevices in terms of electromigration, fatigue damage and hillocking. High-energy ion bombardment has turned out to be an appropriate modification instrument as it leads to selective grain growth, resulting in single-crystal-like structures. The current work addresses the effect of 7 MeV Au⁺ and 1.5 MeV N⁺ irradiation at high fluences (up to 45 × 10¹⁶ ions cm^?2) on the microstructure and the mechanical properties of 500 nm Au thin films of small initial grain size (70–90 nm). The following microstructure changes were observed: selective grain growth, texture changes, sputtering, interfacial degradation, formation of geometrically necessary dislocations, and defect clusters. Hardening behavior was found to be a consequence of grain growth (Hall–Petch effect) and the formation of ion-induced defects.     

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m⁻². An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.     

A corrosion study of nanocrystalline copper thin films

Thin nanocrystalline, compact films, based on the copper–nitrogen system, up to 2.5 μm thickness and 3.5% nitrogen, were deposited by magnetron sputtering at different partial pressure ratios of N₂ and Ar, without formation of Cu_xN compounds, the nitrogen concentration influencing grain size (down to 30 nm) and film homogeneity. Electrochemical corrosion properties were investigated using polarization curves and electrochemical impedance spectroscopy in 0.5 M NaCl aqueous solution, and compared with pure bulk copper; morphology was examined by scanning electron microscopy. Significant variations in corrosion currents between samples were attributed to grain size and structural defects on the grain boundaries.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

In situ TEM study of microplasticity and Bauschinger effect in nanocrystalline metals

In situ transmission electron microscopy straining experiments with concurrent macroscopic stress–strain measurements were performed to study the effect of microstructural heterogeneity on the deformation behavior of nanocrystalline metal films. In microstructurally heterogeneous gold films (mean grain size d_m = 70 nm) comprising randomly oriented grains, dislocation activity is confined to relatively larger grains, with smaller grains deforming elastically, even at applied strains approaching 1.2%. This extended microplasticity leads to build-up of internal stresses, inducing a large Bauschinger effect during unloading. Microstructurally heterogeneous aluminum films (d_m = 140 nm) also show similar behavior. In contrast, microstructurally homogeneous aluminum films comprising mainly two grain families, both favorably oriented for dislocation glide, show limited microplastic deformation and minimal Bauschinger effect despite having a comparable mean grain size (d_m = 120 nm). A simple model is proposed to describe these observations. Overall, our results emphasize the need to consider both microstructural size and heterogeneity in modeling the mechanical behavior of nanocrystalline metals.     

Effect of the Zener–Hollomon parameter on the microstructures and mechanical properties of Cu subjected to plastic deformation

Pure Cu was deformed at different strain rates and temperatures, i.e. with different Zener–Hollomon parameters (Z) ranging within ln Z = 22–66, to investigate the effect of Z on its microstructures and mechanical properties. It was found that deformation twinning occurs when ln Z exceeds 30, and the number of twins increases at higher Z. The average twin/matrix lamellar thickness is independent of Z, being around 50 nm. Deformation-induced grain refinement is enhanced at higher Z, and the mean transverse grain size drops from 320 to 66 nm when ln Z increases from 22 to 66. The grain refinement is dominated by dislocation activities in low-Z processes, while deformation twinning plays a dominant role in high-Z deformation. An obvious increment in yield strength from 390 to 610 MPa was found in deformed Cu with increasing Z, owing to the significant grain refinement as well as the strengthening from nanoscale deformation twins.     

Fabrication and diffusion barrier properties of nanoscale Ta/Ta-N bi-layer

One of the most important processes in Cu metallization for ultra large scale integrated circuits (ULSI) is to fabricate better diffusion barrier. In this paper, Ta/Ta-N films were fabricated by dc magnetron reactive sputtering (DCMS) in N₂/Ar ambient, then Cu/Ta/Ta-N/Si multi-structures were prepared in suite. The thin-film samples were rapid thermal annealed (RTA) at variational temperatures in N₂ ambient. Alpha-Step IQ Profiler, four-point probe (FPP) sheet resistance measurer, atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD) and tape test were used to characterize the microstructure and diffusion properties of the thin-films. The results show that the nanoscale Ta/Ta-N thin-films have smooth surface, and the thermal stability and barrier performance are good. After 600 °C/300 s RTA, Ta (40 nm)/Ta-N (60 nm) thin-films can effectively block against Cu diffusion and keep good adhesion strength with Cu films. After higher temperature RTA process, Cu atoms penetrated through the barrier and reacted with silicon, the barrier fail.     

The effect of thickness on the steady-state creep properties of freestanding aluminum nano-films

To clarify the effects of film thickness on the creep properties of nano-films we conducted tensile creep experiments on freestanding aluminum films with thickness values in the range ～100–800 nm at room temperature. The nano-films showed typical creep behavior comprising transient, steady-state, and accelerated creep stages. The steady-state creep exponents of the 100–800 nm thick specimens were 0.84–2.7 in the stress range 30–120 MPa, which are close to the value for diffusion creep (1). Creep deformation clearly shows a thickness effect: the steady-state creep rate increases as the thickness decreases from 800 to 400 nm, shows a peak in the range 400–200 nm, and then decreases in the 200–100 nm thickness range. The creep experiments under a small stress of 1 MPa show a negative strain rate, indicating the presence of a driving force to reduce the surface area due to surface tension. The explanation for the thickness effect is as follows. Since the ratio of surface and grain boundary area to volume increases with decreasing thickness, diffusion creep along these paths is enhanced, resulting in an increase in the creep rate. As the thickness decreases to 200–100 nm, however, the surface tension effect to reduce the surface area becomes dominant, decreasing the creep rate. In addition, the creep rate of the nano-films is about two or three orders of magnitude smaller than that of the bulk material dominated by the dislocation creep mechanism.     

Epitaxial Ni–Mn–Ga/MgO(1 0 0) thin films ranging in thickness from 10 to 100 nm

Thin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L2₁ austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization.     

Characterization of nickel-based microlattice materials with structural hierarchy from the nanometer to the millimeter scale

Novel nickel-based microlattice materials with structural hierarchy spanning three different length scales (nm, μm, mm) are characterized microstructurally and mechanically. These materials are produced by plating a sacrificial template obtained by self-propagating photopolymer waveguide prototyping. Ni–P films with a thickness of 120 nm to 3 μm are deposited by electroless plating, whereas thicker films (5–26 μm) are obtained by subsequent electrodeposition of a pure Ni layer. This results in cellular materials spanning three orders of magnitude in relative density, from 0.01% to 8.5%. The thin electroless Ni–P films have ultra-fine grain size (7 nm) and a yield strength of ～2.5 GPa, whereas the thicker electrodeposited Ni films exhibit a much broader distribution with average grain size of 116 nm and strong (1 0 0) texture in the plating direction, resulting in a yield strength of ～1 GPa. Uniaxial compression experiments reveal two distinct mechanical responses. At ultra-low densities (<0.1%), these lattices exhibit nearly full recovery after strains up to more than 50%, and damping coefficients an order of magnitude larger than for conventional Ni foams. At higher densities (0.1–10%), the compression behavior is fully plastic, similar to traditional cellular metals. A simple mechanical analysis reveals that the transition occurs when the thickness-to-diameter ratio of the truss elements is of the order of the yield strain of the material, in agreement with experimental observations. Optical and electron imaging of deformed lattices show that the deformation largely localizes around the nodes. In the ultra-light regime, the microlattice materials are stiffer and stronger than any existing alternative.     

Development of an alkali-metal-free bath for electroless deposition of Co-W-P capping layers for copper interconnections

A bath containing alkali-metal-free chemicals was developed for electroless deposition of Co-W-P thin films on a copper substrate and an optimisation of bath compositions was made. Ammonium cobalt sulphate, ammonium tungstate and ammonium hypophosphite were used as the precursors of cobalt, tungsten and phosphorus, respectively. Dimethylamine borane and ammonium citrate were used as reducing and complexing agents, respectively. It was found that the cobalt content, film thickness and grain size increased with increase in cobalt ion concentration in the bath. Tungsten in the films increased from 1 to 6 at.% when its concentration in the bath was increased from 0.001 to 0.009 M. A variation of phosphorus content from 2 to 12 at.% was made by increasing its concentration from 0.01 to 0.05 M. It was found that the deposition rate decreased with increasing citrate ion concentration. Amorphous films were obtained when the combined amount of phosphorus and tungsten exceeded 12 at.% in the films. The crystalline film had small spherical crystallites with diameter less than 40 nm.     

The manufacture and properties of polyaniline nano-films prepared through vapor-phase polymerization

We succeeded in the chemical preparation of nano-level thick polyaniline (PANI) emeraldine salt films on plastic substrate by an in situ vapor-phase deposition (VDP) polymerization method under ambient conditions, using a self-assembly method which is unprecedented. Homogeneous conductive PANI thin films were uniformly fabricated at nano-level thickness (20–100 nm), but their morphologies could grow as polycrystalline grains of a highly ordered structure, depending on the deposition conditions. The grain size was also controlled between 30 and 100 μm depending on the deposition time/temperature. The surface resistance of PANI films was enhanced up to 10⁴ Ω/square with crystallization and light transmittance was increased up to 90% in the case of a film less than 30 nm thick. A typical spectrum for the oxidized PANI, the emeraldine salts form, showing π–π^* transition and a polaron lattice were observed by UV–visible/IR and infrared /Raman spectroscopy.     

Formation of nanostructures in commercial-purity titanium via cryorolling

Microstructure evolution in commercial-purity titanium during plane-strain multipass rolling to a true thickness strain of 2.66 at 77 and 293 K was quantified. Deformation at both temperatures was accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the duration of the twinning stage. Rolling to a true thickness strain of 2.66 resulted in the formation of a microstructure with a grain/subgrain size of ～80 nm at 77 K or ～200 nm at 293 K. The contribution of various mechanisms to the strength of titanium following rolling at 77 and 293 K was analyzed quantitatively.