Size effects on the mechanical properties of thin polycrystalline metal films on substrates

An analysis of the effects of the thickness and grain size of polycrystalline thin films on substrates is presented with the objective of linking the film mismatch stress to the underlying characteristic size scales. The model is predicated on the notion that the relaxation of mismatch strain in the film is accommodated by the introduction of dislocation loops whose population, dimensions and interaction energies are controlled by the film thickness and microstructural dimensions. The model is capable of capturing the combined effects of these size scales by accounting for the interaction energies of the constrained dislocation structure, and provides quantitative predictions of the evolution of film stress during thermal excursions. The predictions of the analysis are compared with available experimental results for polycrystalline films of face-centered cubic materials on Si substrates. It is shown that the model correctly predicts the observed influence of film thickness and grain size on stress evolution during thermal excursions. Aspects of strain hardening in thin polycrystalline films with high dislocation densities are also discussed.     

A half-space Peierls–Nabarro model and the mobility of screw dislocations in a thin film

In this paper, a half-space Peierls–Nabarro (HSPN) model is proposed to re-examine the mobility of a screw dislocation along a thin film/substrate (half-space) interface. In this configuration, the screw dislocation is subjected to an image force due to the free surface, and we are concerned with the interaction between the dislocation and the free surface. Unlike the original Peierls–Nabarro (P–N) model, the HSPN model takes into account the effect of the image force, which leads to modifications on analytical expression of the Peierls barrier stress. The modified Peierls stress is a function of the thin film thickness, which allows us to accurately predict the mobility of a dislocation in the interface between the thin film and the substrate. Based on the proposed HSPN model, we have found that the Peierls stress of a surface screw dislocation may be about 5–15% less than that in bulk materials.     

Critical misfit of epitaxial growth metallic thin films

The critical misfit of epitaxial growth metallic thin films fc was thermodynamically considered. It is found that there exists a competition between the energy of the misfit dislocation of film and non-coherent interface energy of film-substrate. Equilibrium between these energies was present at a critical atomic misfit ft. When the atomic misfit is larger than the critical value, epitaxial growth does not occur. The critical misfit of the epitaxial growth thin films can be predicted. The results show that fe is proportional to the non-coherent interface energy of the film-substrate, and inversely proportional to the elastic modulus and the thickness of the film.     

In situ TEM observation of dislocation motion in thermally strained Al nanowires

Al thin films deposited epitaxially on (0001) -Al₂O₃ substrates, have been thinned cross-sectionally to form Al nanowires. The Al wires, consisting of two Σ3 twin variants, have been strained in situ by differential thermal expansion between the Al wires and the Al₂O₃ substrate during transmission electron microscopy heating. Dynamical observations show that maximum dislocation activity occurs in the first heating cycle up to 400°C, with decreasing activity during further cycles. The {111} Al || (0001) -Al₂O₃ interface acts as a source of dislocation half-loops. The motion of threading dislocations along the wires generates long trailing dislocation segments parallel to, and offset from, the {111} Al || (0001) -Al₂O₃ interface. Dislocation multiplication occurs by the reaction of half-loops and extended threading dislocation segments at the wire boundaries and substrate interface. The Σ3 twin grains bisecting the wires are observed to be stable during thermal cycling to 400°C, and their { } boundaries are weak pinning sites.     

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.     

工艺条件对溅射薄膜附着性的影响

溅射薄膜材料应用很广泛，其中薄膜的附着性是影响薄膜材料质量的重要因素，膜／基界面结合强度是制约薄膜材料实际应用的关键之一。本文综述了关于溅射薄膜附着性能的研究进展，总结了影响附着性的主要工艺因素，为改善薄膜质量提供参考依据。     

Engineering Design of High Quality GaN Epitaxy on Silicon Substrate Using Varying GaN/AlGaN Composite Buffer Structures

High quality GaN epitaxy thin films have been desired for the energy-efficient,solid-state semiconductor illuminating devices.Silicon substrates offer high crystal quality,low wafer cost,large wafer size,and potential integration with the well-established silicon processing technologies.However,due to the large mismatch in lattice constants and thermal expansion coefficients,it is still challenging to grow high quality GaN on silicon substrates.In this study,high quality GaN epitaxy has been engineering designed to grow on Si(111)substrate using varying GaN/AlGaN composite buffer structures by an Axitron 200 metal-organic vapor phase epitaxy deposition system.A thin AlN seed layer of 25 nm was firstly grown at 720℃.AlGaN layer of different thickness was then grown at 1050℃with subsequent GaN thin film until the total thickness reached 500 nm.The thickness of the subsequent GaN thin film could be increased by reducing the AlGaN thickness in the composite buffer structures.The results have shown that the lower GaN/AlGaN thickness ratio would decrease the dislocation density and provide crack-free,mirror-like upper GaN crystal thin film.On the other hand,the GaN/AlGaN thickness ratio could be designed to be 2-6 to balance the processing cost and the thin film quality for engineering applications.The dislocation density has been about 2×10 9 cm-2.In addition,dislocation close loop was observed near the GaN/AlGaN interface.The annihilation mechanism could be depicted by the reduction in dislocation strain energy.     

Dislocation interactions in thin FCC metal films

High strength, high hardening rates, and strong Bauschinger-like effects in thin films have been attributed to constraints on dislocation motion and dislocation interactions. To understand these phenomena, dislocation interactions in (1 1 1) and (0 0 1) oriented single crystal FCC films were studied using dislocation dynamics simulations. Interactions on intersecting glide planes resulted in junction formation, annihilation, or attractive non-junction-forming configurations, while dislocations on parallel glide planes formed dipoles. The configurations adopted by interacting dislocations, and thus the strengths of the interactions, were found to be sensitive to the applied strain, film thickness, crystallographic orientation, and boundary conditions. Different interactions thus dominate film behavior in different ranges of film thickness and applied strain. Interactions are stronger on unloading than on loading. Interactions involving three or more dislocations are found to be different from pairwise interactions. The results suggest that simple analytical calculations are unlikely to describe film phenomena but that full 3-D simulations can be used to understand many features of thin film mechanical behavior.     

Some current topics in dislocation theory

Some topics of current interest in dislocation theory are discussed. Included are considerations of the elastic fields of dislocations with emphasis on nonlinear and anisotropic elastic effects and boundary conditions for computer simulations. Also discussed are core structures, extended dislocation arrays, boundaries and interfaces and dislocations in thin films.     

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.     

Phase-field simulations of thickness-dependent domain stability in PbTiO3 thin films

The phase-field approach is used to predict the effect of thickness on domain stability in ferroelectric thin films. The mechanism of strain relaxation and the critical thickness for dislocation formation from both the Matthews–Blakeslee and People–Bean models are employed. Thickness–strain domain stability diagrams are obtained for PbTiO₃ thin films for different strain relaxation models. The relative domain fractions as a function of film thickness are also calculated and compared with experimental measurements in PbTiO₃ thin films grown on SrTiO₃ and KTaO₃ substrates.     

Investigation on Mechanical Properties of Deformation TiNi Thin Films

The TiNi thin films were deposited onto copper substrates by magnetron sputtering. Tensile tests were carried out on CSS-44100 electron universal test-machine. X-ray diffraction profile Fourier analysis method and the Nano-Hardness Tester have been used to study the mechanical properties of deformation TiNi thin films. The mechanical properties of metals and alloys should be determined primarily by dislocation structure among these objects. The results showed that the dislocation density and the deformation storage energy density increased with the increasing elongation. Microhardness was calculated from the dislocation data. The results showed that the microhardness values were not having good agreement when comparing the calculated values with the measured values. The oxide layer on the surface and the precipitated phases of TiNi thin film affect the measured values of microhardness. The microhardness measured values were larger than the calculated values. The surface micrographs of the TiNi thin film were obtained using scanning electron microscopy (SEM). The experimental results showed that a series of parallel cracks grew in a concerted fashion across the thin film, and the cracks were equally spaced.     

Constrained diffusional creep in UHV-produced copper thin films

The thermomechanical behavior of metallic thin films on stiff substrates is relevant for thin-film devices, but its mechanisms are not fully understood. In this investigation, the mechanical properties of pure Cu and Cu–1 at.% Al films on diffusion-barrier coated Si substrates were studied with the wafer-curvature technique. In ultra-pure films, which were sputtered and annealed under ultra-high vacuum conditions, characteristic stress relaxation at high temperatures was measured, which could be clearly attributed to diffusional creep. Good quantitative agreement with a recent model of diffusional creep constrained by a substrate was obtained. These features were absent in the Cu–Al alloy films, in which Al surface segregation and oxidation had produced a “self-passivating” effect, and in films produced in less clean environments. Based on these results, we propose a model of thin-film deformation based on dislocation glide and constrained diffusional creep.     

Bauschinger and size effects in thin-film plasticity

We present an experimental investigation of the effects of surface passivation, film thickness and grain size on the plastic behavior of freestanding Cu thin films. The stress–strain curves of the films are measured using the plane–strain bulge test. Films with a passivation layer on one or both surfaces have an offset yield stress that increases significantly with decreasing film thickness; the yield stress of unpassivated films, by contrast, is relatively independent of film thickness and increases mainly as a result of grain-size strengthening. The stress–strain curves of passivated films show an unusual Bauschinger effect on unloading. This effect is not observed for unpassivated films. Our experimental results suggest that passivation layers prevent dislocations from exiting the films and that they block slip bands at the film–passivation interface. The back stresses associated with these blocked slip bands increase the resistance to forward plastic flow on loading and cause reverse plastic flow on unloading. The effect of the back stresses increases with decreasing film thickness and leads to the observed strengthening of the passivated films. The constraint of a passivating layer on dislocation motion and hence on plastic flow cannot be described by classical plasticity theories, but can be modeled with some strain–gradient plasticity theories. We evaluate the suitability of the strain–gradient plasticity theory developed by Fleck and Hutchinson to describe our experimental results in a continuum framework. Comparison between experimental results and calculations yields very good agreement for the effect of film thickness, but the strain–gradient plasticity model fails to describe the Bauschinger effect observed in passivated films.     

In situ observations of crack arrest and bridging by nanoscale twins in copper thin films

In situ tensile experiments in a transmission electron microscope revealed that micro-cracks in ultrafine grained, free-standing, thin copper foils containing nanoscale twins initiated in matrix domains separated by the twins and then arrested at twin boundaries as twin boundary sliding proceeded. The adjacent microcracks eventually coalesced through shear failure of the bridging twins. To investigate the atomic mechanism of this rarely seen nanoscale crack bridging behavior, molecular dynamics simulations were performed to show that during crack propagation twin boundaries are impinged upon by numerous dislocations from the plastically deforming matrix. These dislocations react at the interface and evolve into substantially impenetrable dislocation walls that strongly confine crack nucleation and resist crack propagation, leading to the experimentally observed crack bridging behavior. The present results raise an approach to significantly toughening polycrystalline thin films by incorporating nanoscale twin structures into individual grains that serve as crack bridging ligaments.     

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.     

微波等离子体刻恂对WC—Co硬质合金基体金刚石薄膜附着力的影响

金刚石薄膜的附着力是影响CVD金刚石涂层刀具切削性能的关键因素,本文采用EACVD法在硬质合金（YG6）基体上沉积金刚石涂层;用Ar-H2微波等离子WC－C0衬底进行刻蚀处理,以改变基体表面与金刚石涂层间的界面结构,提高金刚石涂层的附着力;采用压痕法评估涂层附着力,借助SEM等观察刻蚀预处理方法对膜基界面的影响,并对此进行分析和讨论。     

Recent progress in thin film epitaxy across the misfit scale (2011 Acta Gold Medal Paper)

This paper discusses recent progress in thin film epitaxy across the misfit scale through the paradigm of domain matching epitaxy (DME). This epitaxy across the misfit scale is critical for integrating multifunctionality on a chip and creating smart structures for next-generation solid-state devices. There are three sources of strains that are cumulative at the growth temperature, and the relaxation process starts during the growth process. Upon cooling, unrelaxed lattice, thermal and defect strains give rise to net residual strains. In large misfit (ε ? 10%) systems, where lattice misfit strain is predominant, it can be relaxed completely, and then only thermal and defect strains remain upon cooling. In low misfit systems, all three sources contribute to the residual strain upon cooling, as result of incomplete lattice relaxation. The predominant strain relaxation mechanism in thin films is by nucleation of dislocations at the free surface, as the nucleation energy in the bulk is considerably higher. At the free surface, the activation barrier for dislocation nucleation is considerably lower at the steps. Since the step formation energy is lower under a compressive stress compared with tensile stress, it reduces nucleation energy under compressive stress and lowers the critical thickness compared with tensile stresses in thin films. Once the dislocation nucleates, it propagates or glides to the interface to relieve the strain. However, if lattice frictional stress in the film is high, most dislocations may not reach the interface, depending upon the growth temperature and rate. Thus, these two key steps, dislocation nucleation and propagation, play a critical role in the thin film relaxation process. Once the dislocations reach the interface, the atomic structure of the dislocation at the heterointerfaces determines its electronic properties, specifically trapping and recombination characteristics. It is found that the atomic structure of the dislocation is determined by the interplay between strain and chemical free energies. Thus, the dislocations (representing missing or extra planes) play a critical role in the relaxation of thin film heterostructures. This paper focuses on epitaxy across the misfit scale, based upon matching of integral multiples of lattice planes. If the misfit falls between the integral multiples, it is accommodated by the principle of domain variation, where domains alternate to accommodate the misfit. Details of epitaxy from low misfit (～4%) in Ge/Si) to large misfit (～22%) in TiN/Si are shown. In III-nitride/sapphire and II-oxide/sapphire systems, this paper deals with polar orientations, where misfit is uniform in the basal plane, and non-polar orientations, where misfit varies over an order of magnitude in the film plane. It is shown that the DME paradigm is key to the integration of thin film heterostructures across the misfit scale and other complex systems such as vanadium oxide and PZT systems on Si(1 0 0) substrates for the integration of functionalities on a computer chip. Finally, it is shown that the formation of epitaxial and self-assembled nanodots on Si(1 0 0) provides a critical advance, with tremendous implications for information and data storage and related nanomagnetics applications.     

自支撑GaN厚膜制备及其光学性能研究

采用镀Ti插入层在氢化物外延设备中制备了高质量自支撑GaN厚膜。X射线衍射测试发现(0002)峰摇摆曲线的半高宽为260 arcsec;5 K下样品带边发光峰的半高宽为3 meV,室温下样品的带边发光峰也只有20 meV,并且在室温的PL谱中观察不到黄光带;扫描电子显微镜观察显示,腐蚀后的自支撑GaN厚膜表面有位错延伸形成的六角坑,并估算出样品位错密度约为2.1×l07 cm-2。这些结果说明镀Ti插入层有助于提高GaN外延层的晶体质量。通过Raman和低温荧光分析,可以看出自支撑GaN厚膜表面应力已经完全释放。研究了不同温度下样品的荧光特性,证明得到的无应力自支撑GaN厚膜具有很好的晶体质量和光学质量     

Control of strain relaxation in tensile and compressive oxide thin films

Tensile and compressive solid-solution thin films based on LaAlO₃ and CaZrO₃ compositions were grown on perovskite oxide substrates using pulsed laser deposition to study growth mode transitions and strain relaxation. A buried layer of SrRuO₃ between the thin film and the SrTiO₃ substrate was also introduced to provide an auxiliary embedded strain gauge, which helps identify the critical conditions for the onset of catastrophic strain relaxation events – cracking and dislocation cascades. The results are compared with theoretical predictions to provide guidelines on some general deposition conditions that may be used to obtain smooth, crystalline and defect-free thin films of interest to perovskite-based heterostructures.