Strong single-crystalline Au films tested by a new synchrotron technique

Although it is well known that thin films exhibit mechanical properties very different from those of their bulk counterparts, knowledge of the underlying mechanisms is incomplete. Single-crystalline films have a favorable microstructure for investigating the scaling behavior of mechanical properties. We present a novel experimental route for preparing single-crystalline Au films on a compliant polyimide substrate. For such single-crystals, we have developed a synchrotron-based tensile testing technique to measure the isothermal stress–strain curves and average peak widths. The analysis of Laue diffraction patterns as well as a parallel transmission electron microscopy study give new insight in the initial and evolving microstructure of the films. Complex novel deformation mechanisms are found, including a transition of the dominant deformation mechanism from full to partial dislocations in films thinner than 160 nm. The scaling behavior is described in view of the coexistence of different deformation mechanisms where the nucleation stress for single dislocations very likely governs the behavior.     

Mechanism of induced nucleation of misfit dislocations in the Ge-on-Si(0 0 1) system and its role in the formation of the core structure of edge misfit dislocations

Ge-on-Si(0 0 1) films are grown by molecular beam epitaxy via a three-step epitaxial growth method (Ge/Ge seed/GeSi buffer/Si(0 0 1)). The dislocation structure of the Ge/GeSi buffer interface is studied by high-resolution electron microscopy. Misfit dislocations on the interface are edge dislocations and are aligned regularly with a period of 9–10 nm. A variety of atomic structures of the dislocation core is observed, known in the literature as dissociated or asymmetric Lomer edge dislocations. The assumption that atomic structures of various degrees of complexity are intermediate states in the formation of a perfect edge misfit dislocation in the course of plastic relaxation of a stressed film is justified. A model is proposed which explains the intermediate states in terms of statistical variation of the nucleation site of the complementary 60° dislocation which forms, together with the primary dislocation, a Lomer dislocation at the interface.     

Size-independent stresses in Al thin films thermally strained down to −100 °C

Size and temperature dependencies of thermal strains of {1 1 1} textured Al thin films were determined by in situ X-ray diffraction (XRD) in the temperature range of −100 to 350 °C. The experiments were performed on 50–2000 nm thick Al films sputter-deposited on oxidized silicon (1 0 0) substrates. The in-plane stresses were assessed by measuring the {3 3 1} lattice plane spacing at each temperature in steps of 25 °C during thermal cycling. At high temperatures, the films could only sustain small compressive stresses. The obtained stress–temperature evolutions show the well-known increase of flow stresses with decreasing film thickness for films thicker than 400 nm. However, for thinner films, the measured stress on cooling is independent of the film thickness. This lack of size effect is caused by the flow stresses in the thinnest films exceeding the maximum stress that can be applied to these samples using thermomechanical loading down to −100 °C. Thus, the measured stresses of ∼770 MPa in the thinnest film represent a lower limit for the actual flow stresses. The observed stresses are also discussed taking microstructural information and possible constraints on dislocation processes into account.     

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.     

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.     

Strain compensation by twinning in Au thin films: Experiment and model

Epitaxial Au thin films with thicknesses of 40–160 nm were grown on (1 0 0)-oriented single-crystal NaCl substrates at 300 °C by magnetron sputtering. Microstructural analyses revealed that all films possess orthogonal twin networks along the 〈0 1 1〉 directions. The experimentally observed relationship of an increase in twin density with decreasing film thickness is explained by kinematical and thermodynamical modeling. The developed twin model predicts a nanometer-sized width of the twins, in agreement with the experiment.     

Nanoindentation of single-crystalline gold thin films: Correlating hardness and the onset of plasticity

Understanding the mechanical properties of materials with external dimensions on the nanometer scale is crucial for the design and fabrication of nanoelectronics and nanosystems. Metal thin films exhibit a size-dependent hardening effect that scales inversely with the film thickness down to 200 nm. The thickness range below 200 nm is mostly unexplored and initial experiments indicate a change in the scaling law. Here, the mechanical properties of single-crystalline Au films are investigated in the thickness range from 31 to 858 nm by nanoindentation. Maximum shear stresses at the onset of plasticity are determined by the finite element method. While the hardness increases with decreasing film thickness, as expected from macroscopic experiments, the onset of plasticity shifts to lower shear stresses for thinner films. These observations are interpreted with respect to detailed observations of the microstructures of the films investigated.     

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.     

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

Uniaxial compression of fcc Au nanopillars on an MgO substrate: The effects of prestraining and annealing

The size-dependent strength of face-centered cubic (fcc) metals, as revealed by uniaxial compression of nanopillars, suggests that plasticity is dislocation source-controlled, with fewer sources in smaller pillars producing a “smaller is stronger” effect. To further investigate this phenomenon we have studied the effects of prestraining and annealing on the deformation properties of [0 0 1] Au nanopillars. By making pillars from an epitaxial film of [0 0 1] Au on [0 0 1] MgO, using focused ion beam machining, we are able to create both puck-shaped pillars that can be stably prestrained and pillars with a high aspect ratio, which can be tested in uniaxial compression. We find that prestraining dramatically reduces the flow strength of nanopillars while annealing restores the strength to the pristine levels. These unusual effects are not seen in bulk fcc metals, which behave in the opposite way. We discuss their possible causes in terms of dislocation densities using transmission electron microscopy.     

Effects of temperature on structure and mobility of the 〈1 0 0〉 edge dislocation in body-centred cubic iron

Dislocation segments with Burgers vector b = 〈1 0 0〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2〈1 1 1〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈1 0 0〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈1 1 1〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈1 0 0〉 dislocation. It is concluded that the response of the 〈1 0 0〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈1 1 1〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.     

Structural changes during the reaction of Ni thin films with (1 0 0) silicon substrates

Ultrathin films of nickel deposited onto (1 0 0) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO₂ on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5 nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15 nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi₂ with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.     

Deformation mechanisms in a Ru–Ni–Al ternary B2 intermetallic alloy

Deformation mechanisms in a B2 Al50Ni5Ru45 alloy have been studied in compression over the temperature range 298–1323 K. The alloy exhibited a low temperature sensitivity of the flow stress over the temperature range 298–973 K. The strain rate sensitivity below 973 K was relatively low, similar to binary RuAl-based alloys. Dislocation analyses after room temperature compression indicate the presence of 〈1 0 0〉 and 〈1 1 0〉 dislocations on {1 1 0} planes, with the 〈1 0 0〉 dislocations present with slightly higher densities. Compression creep tests at stress levels between 300 MPa and 500 MPa revealed exceptional creep strength in the temperature range investigated. The predominant dislocation substructure after creep deformation consisted of uniformly distributed, cusped 〈1 0 0〉-type screw dislocations on {1 1 0} planes. The deformation behavior and creep mechanisms are discussed in comparison with other high melting temperature B2 intermetallics.     

Misfit dislocations of anisotropic magnetoresistant Nd0.45Sr0.55MnO3 thin films grown on SrTiO3 (1 1 0) substrates

Nd_0.45Sr_0.55MnO₃ is an A-type antiferromagnetic manganite showing obvious angular-dependent magnetoresistance, which can be tuned by misfit strain. The misfit strain relaxation of Nd_0.45Sr_0.55MnO₃ thin films is of both fundamental and technical importance. In this paper, microstructures of epitaxial Nd_0.45Sr_0.55MnO₃ thin films grown on SrTiO₃ (1 1 0) substrates by pulsed laser deposition were investigated by means of (scanning) transmission electron microscopy. The Nd_0.45Sr_0.55MnO₃ thin films exhibit a two-layered structure: a continuous perovskite layer epitaxial grown on the substrate followed by epitaxially grown columnar nanostructures. An approximately periodic array of misfit dislocations is found along the interface with line directions of both 〈1 1 1〉 and [0 0 1]. High-resolution (scanning) transmission electron microscopy reveals that all the misfit dislocations possess a〈1 1 0〉-type Burgers vectors. A formation mechanism based on gliding or climbing of the dislocations is proposed to elucidate this novel misfit dislocation configuration. These misfit dislocations have complex effects on the strain relaxation and microstructure of the films, and thus their influence needs further consideration for heteroepitaxial perovskite thin film systems, especially for films grown on substrates with low-symmetry surfaces such as SrTiO₃ (1 1 0) and (1 1 1), which are attracting attention for their potentially new functions.     

Inter- and intragranular plasticity mechanisms in ultrafine-grained Al thin films: An in situ TEM study

The nature of the elementary deformation mechanisms in small-grained metals has been the subject of numerous recent studies. In the submicron range, mechanisms other than intragranular dislocation mechanisms, such as grain boundary (GB)-based mechanisms, are active and can explain the deviations from the Hall–Petch law. Here, we report observations performed during in situ transmission electron microscopy (TEM) tensile tests on initially dislocation-free Al thin films with a mean grain size around 250 nm prepared by microfabrication techniques. Intergranular plasticity is activated at the onset of plasticity. It consists of the motion of dislocations in the GB plane irrespective of the GB character. Surface imperfections, such as GB grooves, are supposed to trigger intergranular plasticity. At larger deformations, the motion of the intergranular dislocations leads to GB sliding and eventually cavitation. In the meantime, GB stress-assisted migration and dislocation emission inside the grain from GB sources have also been observed. The observation of these different mechanisms during the deformation provides an important insight into the understanding of the mechanical properties of metallic thin films.     

Study on the indentation size effect in CaF2: Dislocation structure and hardness

In this study nanoindentations have been performed on a cleaved surface of a CaF₂ single crystal and the dislocation structure has been investigated by the etch pit technique using atomic force microscopy. The deformation during indentation is first purely elastic until dislocations are created observable in a pop-in in the load displacement data, as well as in a dislocation rosette around the indentation. After pop-in a relatively high hardness is observed, which gradually decreases, until at 3 μm a nearly constant hardness is found. By using sequential polishing, etching and imaging, the dislocation structure underneath indentations with indentation depths of 300 nm and 110 nm (load: 5 mN, 1 mN) is quantified. The dislocation density and radial distribution of dislocation density depend on the indentation depth, where a smaller indentation depth leads to a higher dislocation density, which is in qualitative agreement with the observed increase in hardness.     

Morphological changes of Au nanoparticles in Au–polystyrene nanocomposites: A combined spectroscopy,atomic force microscopy and X-ray scattering study

Au nanoparticles sputter deposited on polystyrene-coated Si and fused quartz substrates have been studied using optical spectroscopy, atomic force microscopy and X-ray reflectivity. Under the same deposition conditions, both spectroscopy and atomic force microscopy indicate clearly that the nanoparticles undergo a shape transition from near-spherical to progressively ellipsoidal as the polystyrene film becomes thinner than 4R_g, R_g being the radius of gyration of the polymer. There is a gradual increase in the in-plane ellipticity a/b, a(b) = semi-major (semi-minor) axis, with decrease in polystyrene film thickness from 230 to 20 nm, where b remains almost invariant for a particular deposition time while the semi-major axis increases in dimension. Electron density profiles along the depth of the films, extracted from X-ray reflectivity data, show that the Au nanoparticles sit on the top of the polystyrene film with c, the third ellipsoid axis having a dimension of about 3.0 nm irrespective of film thickness or deposition times used (7, 10, 12 and 15 s).     

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.     

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.     

Ultra-fast sulphur grain boundary segregation during hot deformation of nickel

Sulphur grain boundary segregation during hot-compression of nickel (5.4 wt. ppm S) is monitored using Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy. The deformation conditions (temperature/deformation rate) investigated are: 550 °C/0, 550 °C/3 × 10^?5 s^?1, 550 °C/3 × 10^?4 s^?1 and 450 °C/3 × 10^?5 s^?1. It is shown that plastic deformation accelerates the kinetics of sulphur grain boundary segregation by a factor of ～10³ to a few 10⁵, depending on the deformation conditions. Very high levels of segregation (～0.8 monolayer of sulphur) are obtained after very low deformation (～5%). In addition a linear dependence of the segregation level with time and deformation is demonstrated. The segregation kinetics during plastic deformation is proportional to the deformation rate and almost independent of temperature. Several metallurgical mechanisms are discussed and confronted with the experimental results: dislocations dragging, pipe diffusion, dislocation collection/diffusion and acceleration by excess vacancies. It appears that the models developed in this work on the basis of the two latter mechanisms (dislocation collection/diffusion and acceleration by excess vacancies) predict the experimental data correctly.