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.     

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.     

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.     

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.     

Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites

Ni_51.4Mn_28.3Ga_20.3 thin films deposited on alumina ceramics have been studied by X-ray powder diffraction and dynamic mechanical analysis. Substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25–200 °C and 0.1–5 μm, respectively. The coefficient of thermal expansion (CTE) of the film in the paramagnetic cubic phase has been determined to be equal to (15 ± 1) × 10⁻⁶ K⁻¹ for all the films, in agreement with the CTE of bulk material. The thickness dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally induced tensile stress in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversible, thermally induced change in shape of the Ni–Mn–Ga/alumina cantilever actuator.     

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.     

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.     

Influence of heat treatment and gamma-rays irradiation on the structural and optical characterizations of nano-crystalline cobalt phthalocyanine thin films

Thin films of nano-crystalline cobalt phthalocyanine (CoPc) were prepared by thermal evaporation technique under high vacuum at room temperature and were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The prepared films were divided into three groups for study; the first was the as deposited films, the second was heat treated under vacuum for 1 h at 630 K, and the third group was irradiated in gamma cell type ⁶⁰Co source in air at room temperature with total absorbed dose of 150 kGy. The optical properties for the three groups were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 200 to 2500 nm. The optical constants, refractive index, n, and absorption index, k, were calculated and found to be independent of film thickness in the measured film thickness range. The dispersion energy, E_d the oscillator energy, E₀ and the high frequency dielectric constant, ?_∞ of the three groups were obtained. The energy band model was applied and the types of the optical transitions responsible for optical absorption were found to be indirect allowed transition. The onset and optical energy gaps were calculated. Discussion of the obtained results and their comparison with the previously published data were also given.     

Efficient electrochromic nickel oxide thin films by electrodeposition

Nickel oxide (NiO) thin films were prepared by electrodeposition technique onto the fluorine doped tin oxide (FTO) coated glass substrates in one step deposition at 20, 30, 40 and 50 min deposition times respectively. The effect of film thickness (thereby microstructural changes) on their structural, morphological, optical and electrochromic properties was investigated. The mass change with potential and cyclic voltammogram was recorded in the range from +0.3 to ?0.8 V versus Ag/AgCl. One step deposition of polycrystalline cubic phase NiO was confirmed from X-ray diffraction study. Optical absorption study revealed direct band gap energy of 3.2 eV. The optical transmittance of the film decreased with increase in film thickness. A uniform granular and porous morphology of the films deposited for 20 min was observed. The film becomes more compact and devoid of pores when deposition time was increased to 30 min. Thereafter severe cracks are observed. All the films exhibit anodic electrochromism in OH^? containing electrolyte (0.1 M KOH). The maximum coloration efficiency of 107 cm²/C and electrochemical stability of up to 10⁴ colour/bleach cycles were observed for the films deposited for 20 min (film thickness of 104 nm).     

Tensile strength of gold nanointerconnects without the influence of strain gradients

Gold is an ideal material for wires and electrodes on the nanoscale because of its corrosion resistance and biocompatibility. For system integration the mechanical properties of gold nanowires are highly important. In this paper, a study on the deformation behavior of parallel line arrays of gold on polyimide substrates is presented. Arrays of 45–60 nm nanolines were produced by extreme ultraviolet interference lithography, evaporation of gold and a subsequent lift-off process. Tensile testing of these samples was performed by using an in situ synchrotron-based technique, which allows for the fast acquisition of diffraction peaks. The samples show yield stresses of about 400 MPa. This is comparable to thin films of similar thickness, but significantly lower than previously reported results on single electrodeposited wires deformed in bending. The differences are explained in terms of strain gradient plasticity theory.     

Mechanical deterioration of polyethylene greenhouses covering under arid conditions

One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-μm thick polyethylene.Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force–distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.     

Plasma polymer multilayers of organosilicones and their optical properties controlled by RF power

Hydrogenated amorphous carbon–silicon alloys were prepared from tetravinylsilane monomer using plasma-enhanced chemical vapor deposition at different powers (10–70 W). The optical properties of the plasma polymer films were analyzed by spectroscopic ellipsometry in the range 250–830 nm. The refractive index of pp-TVS films can be controlled by RF power, which for a wavelength of 633 nm ranged from 1.69 (10 W) to 2.08 (70 W). Alloys of selected optical properties were used to construct layered films with sharp interfaces. We could confirm that individual layers retain their optical properties in layered structures if the layer thickness is between 1 μm and 25 nm. Nanolayered and gradual films of controlled optical properties can be developed for nanolayered composites and optical devices.     

Anisothermal solid-state reactions of Ni/Al nanometric multilayers

The structural evolution of Ni/Al multilayer thin films with temperature was studied by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD). Thin films with nanometric Ni and Al alternated layers were deposited by d.c. magnetron sputtering. In our experiments, we used a bilayer thickness of 5, 14 and 30 nm and a total film thickness ranging from 2 to 2.7 μm. The XRD patterns of the as-deposited sample revealed only peaks of Al and Ni. DSC experiments were performed on freestanding films, from room temperature to 700 °C at 10 and 40 °C/min. Two exothermic reactions were detected in the DSC curves of the film with a 30 nm bilayer thickness, with peak temperatures at 230 and 330 °C. The films with 5 and 14 nm bilayer thickness presented only one exothermic peak at 190 and 250 °C, respectively. To identify the intermetallic reaction products, DSC samples were examined by XRD. NiAl formation corresponds to one single DSC peak, for films with short bilayer thicknesses (5 and 14 nm). The films with 30 nm bilayer thickness were heated at 250 °C (T = T_{1st peak}), 300 °C (T_{1st peak} < T < T_{2nd peak}), 450 °C (T > T_{2nd peak}) and 700 °C. The XRD results indicated that at 250 °C the phase formed was NiAl₃, whilst NiAl₃ and Ni₂Al₃ phases were identified at 300 °C. For the 450 °C sample, only NiAl was detected. Further heating to 700 °C promotes the growth of NiAl grains.     

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.     

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.     

Evidence of a highly compressed nanolayer at the epitaxial silicon carbide interface with silicon

Through a novel methodology for evaluating layer-by-layer residual stresses in epitaxial silicon carbide films with resolution down to 10 nm, we indicate the existence of a highly compressed interfacial nanolayer between the films and their silicon substrates. This layer is consistently present underneath all types of silicon carbide films examined herein, regardless of the extent of residual tensile stress measured in the full thickness of the films, which varies from 300 up to 1300 MPa. We link this nanolayer to the carbonization step of the film growth process and we discuss in detail the implications in terms of fracture behaviour by bulge testing of micromachined membranes.     

Growth of nanocomposite films: From dynamic roughening to dynamic smoothening

This paper reports several new findings on the breakdown of dynamic roughening in thin film growth. With increasing energy flux of concurrent ion impingement during pulsed DC sputtering, a transition from dynamic roughening to dynamic smoothening is observed in the growth behavior of TiC/a-C nanocomposite films. The nanocomposite films show a negative growth exponent and ultra-smoothness (RMS roughness ～0.2 nm at a film thickness of 1.5 μm). Based on high-resolution cross-sectional transmission electron microscopy observations we conclude that during growth an amorphous front layer of 2 nm covers the nanocomposite film and suppresses the influence of nanocrystallites on the roughness evolution of the nanocomposite films. We were able to predict the evolution of surface roughness based on a linear equation of surface growth which contains two diffusivity parameters that control the atomic mobility along the growing outer surface. The model is in good agreement with atomic force microscopy measurements of roughness evolution.     

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.     

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.     

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.