Intrinsic Compressive Stress Research Articles

A comparison between deuterium plasma induced blistering of tungsten surface and hillocks grown on tungsten thin film

Tungsten (W) film was prepared by magnetron sputtering, and the coarse crystalline W bulk was used as reference samples. All the samples were exposed to deuterium plasma with the exposure fluence of 1.67 × 1024 D/m2. The films were annealed in-situ at 723 K during deposition and hillocks were found on all film surfaces. Focused ion beam analysis of the subsurface structure of hillocks provides data support for the growth mechanism of hillocks. The formation of hillocks suggested that the underlying force is released the intrinsic compressive stress caused by thermal cycling during the magnetron sputtering. After deuterium plasma exposure, the W bulk showed obvious stepped high-dome deuterium blisters. The micrographs revealed the microscopic morphology characteristics of deuterium blisters and hillocks, and analyzed the similarities and differences between hillocks and surface deuterium blisters. By comparing the growth mechanism of hillocks on the W film with the surface blistering on the W bulk, the stress growth model of deuterium blisters on the surface of coarse crystalline W bulk is proposed. This work provides a new perspective for the transverse stress model of deuterium-induced surface blistering.

Electro-chemo-mechanical properties of anodic oxide (passive) films formed on Cu, Ni and Fe

Abstract Electro-chemo-mechanical properties of anodic oxide (passive) films formed on metals have been reviewed focusing on the results of stress variations caused by anodic oxidation of Cu, Ni, and Fe thin film electrodes in deaerated pH 8.4 borate buffer solution at 25 °C. The surface stress varies toward compressive direction due to adsorption of OH on Cu from aqueous solution as well as adsorption of oxygen on metals from gas phase. The stresses are generated with the growth of three-dimensional anodic oxide films on metals. The magnitude and sign (tensile or compressive) of the intrinsic film stress were determined by taking the residual stress of the substrate and the dielectrostriction into consideration. The tensile or compressive intrinsic film stress depends on p-type or n-type semiconductive properties of the anodic oxide films, which is explained in terms of the void formation or oxide formation in the metal side at the metal/film interface. Furthermore, the stress variation toward compressive direction during cathodic reduction of the anodic oxide films is explained in terms of the volume expansion due to the formation of intermediate species.

In-situ monitoring of stress evolution in high power impulse magnetron sputtering-deposited Ti-Al-N films: Effect of substrate bias and temperature

The origin of residual stress during film deposition is a topic of strong interest in the coating engineering community; it requires appropriate investigation tools to explain the different mechanisms acting on the evolution of stress concerning process conditions and material characteristics. In the present work, we used in-situ curvature stress probing combined with ex-situ techniques such as X-ray diffraction cos2α*sin2ѱ applied to the deposition of model Ti0.42Al0.58N coatings. The films were prepared at room temperature and 300 °C, and four substrate bias strategies were studied, namely two constant values of substrate bias (0 V and −75 V), and two systems with alternating high bias and no bias. At room temperature, the application of bias changed the stress from slightly tensile to compressive, while at 300 °C, the use of high bias increased the compressive intrinsic stress by 3 GPa along with hardness. Linking in-situ and ex-situ stress measurements permitted a direct assessment of thermal stress, which varied considerably with bias. The combined analysis revealed the presence of different contributors to stress generation, particularly at the grain boundaries. The relevance of each contributor to the final stress state was found to depend on the energetic growth conditions. Stress monitoring revealed significant differences between the growth of dense films on the porous ones compared to the opposite sequence, suggesting a straightforward pathway to stress mitigation. This is documented by films produced in the alternating −75/0/−75 V bias sequence that presented comparable mechanical properties as films deposited at a constant high (−75 V) bias, but with substantially lower residual stress.

Interface-dominated deformation mechanisms in Cr/CrN/Cr-DLC multilayer triggered by nanoindentation

The diamond-like carbon (DLC) film traditionally possesses low toughness owing to high intrinsic compressive stress. The incorporation of other elements into the DLC film provides an effective approach to improving toughness, but it is still limited and produces some detrimental effects. In this study, a new strategy of designing the Cr/CrN multilayers to serve as a functional transition layer and doping Cr into DLC film is attempted to simultaneously improve the strength and ductility of DLC film. Herein, a profound understanding of the deformation mechanisms of Cr/CrN/Cr-DLC multilayers with different bilayer periods (BPs) during the indentation process is investigated. The maximum strength and toughness values are achieved by the Cr/CrN/Cr-DLC multilayer with BP of 10 nm (P10). The cooperation of multiple strengthening mechanisms involving the diminishment of dislocation motion within individual layers and the formation of coherent twin boundaries (CTBs) and Lomer-Cottrell locks (L-C locks) contributed significantly to the high hardness of P10. The predominant toughening mechanism of P10 is the formation of a substantial number of high-density stacking faults (SFs) in CrN sublayers, which is attributed to that the pre-existed CTBs with numerous kinks in CrN can reduce the stacking fault energies (SFEs) for P10. Few deformation twins facilitated the deformation of P10. In contrast, a versatile deformation mechanism appears in Cr/CrN/Cr-DLC with BP of 100 nm (P100) involving dislocation motion, SFs, deformation twins, and, of particular note, amorphization. However, the released energy related to deformation in P100 is limited, leading to cracks in the Cr-DLC top layer. This multilayer structure can provide additional strengthening and toughening mechanisms to endow the DLC film with profound mechanical properties in harsh service conditions.

Relaxation behavior of intrinsic compressive stress in powder aerosol co-deposited films: Rethinking PAD films as nanomaterials

Pure TiO2 and TiO2/MgO co-deposited films were prepared by the powder aerosol deposition method (PAD). The extended Stoney's formula after Mézin was used to calculate the intrinsic compressive stress from the substrate deformation. Furthermore, crystallite size as well as microstrain were determined by X-ray diffraction and subsequent Rietveld refinement. The intrinsic stress in the TiO2 films already decreases at 100 °C and reaches a constant low level at 300 °C. The addition of MgO in the co-deposited film shifted this relaxation behavior by 50 °C to higher temperatures. Different possible explanations for this behavior are discussed, based on the microstructure and the nanocrystalline properties of the films.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Thermomechanical and Creep Behaviors of Multilayered Metallization Systems Developed for High‐Temperature Surface Acoustic Wave Sensors

AbstractThe structural stability and durability of a sensing device at high‐temperatures (HT) have important effects on its functionality and reliability. This becomes more critical in the devices that work based on the surface acoustic waves (SAW). In this study, the thermomechanical and creep behaviors of the metallization systems, RuAl, MoLa, and Mo suggested for the electrodes of HT‐SAW sensors are investigated. Several experiments are designed to obtain the state and the value of stress in the films deposited on a Si [100] substrate caused by the fabrication process and the operational conditions. Results show that after the film deposition, the intrinsic compressive stress in the MoLa film is significantly lower than the other films. However, after annealing the samples, the lowest residual stress is reached in the Mo. Linear stress variations during the thermal cycling of the annealed samples are obtained for all metallizations. To study the durability of the films at HT, the creep behavior of the samples is investigated by keeping them at a temperature of 600 °C for 10 h. The creep rate and its constitutive equation for the films are obtained. It is observed that the creep rate of MoLa and Mo films is much lower than RuAl.

Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films

Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux- and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy- and orientation-dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of ∼18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 °C.

Tunable band gap of diamond twin boundaries by strain engineering

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.

Effect of doping elements to hydrogen-free amorphous carbon coatings on structure and mechanical properties with special focus on crack resistance

The doping of amorphous carbon is one way of specifically modifying its properties. The present work is a contribution to understand the effect of dopant selection on structure, mechanical properties and crack resistance of hydrogen-free tetrahedral amorphous carbon (ta-C) coatings. For this purpose, nonmetallic (B, Si) and metallic (Mo, Fe, Cu) dopants were selected. The coatings were produced by the Laser-Arc technique and investigated by Raman spectroscopy, transmission electron microscopy, nanoindentation, bending, and scratch tests. It was found that doping generally affected the sp2 content and cluster size of the sp2 carbon phase, and thus hardness, Young's modulus, and intrinsic compressive stresses, but also the defect content and the crack resistance. The addition of metallic dopants led to an increase and clustering of the sp2-bonded carbon and, consequently, to a significant reduction in hardness and Young's modulus. This effect was much less pronounced when doping with non-metallic elements. The failure behavior changed toward higher ductility and crack resistance when metals were added. The crack resistance was found to be directly dependent on the H3/E2 value, with H3/E2 > 0.2 leading to a more brittle behavior with unstable crack propagation, while at H3/E2 < 0.2 the material was more ductile and exhibited stable crack propagation and thus higher crack resistance.

Improved mechanical models and Ic estimation for the whole life cycle of high temperature superconducting coated conductors

To evaluate the effect of mechanical behaviors on the critical current (Ic) of superconducting conductors, this paper develops an improved modeling method at all the composite-layer, conductor, and cable levels, which provides a means to analyze the stress and strain for the whole manufacture process. First, a sandwich model based on composite shells was developed to calculate the residual thermal stress of coated conductors during the fabrication process. Then, the uniaxial tensile model was used to verify the loading method for applying the residual thermal stress to subsequent models as initial conditions. With the pre-stress loaded, mechanical behaviors of coated conductors under bending and torsion were analyzed. Further, an exponential function model was adopted to describe the relationship between the normalized critical current and longitudinal strain. Lastly, a series of numerical models were built to analyze the bending behavior of CORC (conductor on round core) cables. Combined with tests of a sample cable, the strain-dependent critical current predicted the Ic degradation under different bending radii. Calculation results show that the sandwich model is hundreds of times faster than other applicable finite element (FE) models. The loading method provides the initial stress for any geometry and software to precisely analyze the stress of coated conductors, especially the rare earth-barium-copper-oxide (ReBCO) layer with intrinsic compressive stress. The electromechanical behaviors of bending and torsion are in good agreement with experimental data, and the exponential mode obtained from conductor performance can estimate Ic degradation of the CORC cable within a relative error of 1.46 % in the reversible region.

Unravelling the ion-energy-dependent structure evolution and its implications for the elastic properties of (V,Al)N thin films

Ion irradiation-induced changes in the structure and mechanical properties of metastable cubic (V,Al)N deposited by reactive high power pulsed magnetron sputtering are systematically investigated by correlating experiments and theory in the ion kinetic energy (Ek) range from 4 to 154 eV. Increasing Ek results in film densification and the evolution from a columnar (111) oriented structure at Ek ≤ 24 eV to a fine-grained structure with (100) preferred orientation for Ek ≥ 104 eV. Furthermore, the compressive intrinsic stress increases by 336 % to -4.8 GPa as Ek is increased from 4 to 104 eV. Higher ion kinetic energy causes stress relaxation to -2.7 GPa at 154 eV. These ion irradiation-induced changes in the thin film stress state are in good agreement with density functional theory simulations. Furthermore, the measured elastic moduli of (V,Al)N thin films exhibit no significant dependence on Ek. The apparent independence of the elastic modulus on Ek can be rationalized by considering the concurrent and balancing effects of bombardment-induced formation of Frenkel pairs (causing a decrease in elastic modulus) and evolution of compressive intrinsic stress (causing an increase in elastic modulus). Hence, the evolution of the film stresses and mechanical properties can be understood based on the complex interplay of ion irradiation-induced defect generation and annihilation.

Microstructure and properties of TiAlCrN ceramic coatings deposited by hybrid HiPIMS/DC magnetron co-sputtering

TiAlCrN ceramic coatings were prepared utilizing a hybrid deposition technique consisting of High Power Impulse Magnetron Sputtering (HiPIMS) and Direct Current Magnetron Sputtering (DCMS). The chemical composition, phase structure, morphologies, mechanical and tribological properties of such coatings were systematically investigated. Results indicated that the content of Ti element increased monotonically from 0 at.% to 22 at.% with increasing of Ti target power. The TiAlCrN ceramic coatings presented a competitive growth tendency between (111) and (200) crystal plane through the energetic ion bombardment. Higher Ti target power resulted in stronger compressive intrinsic stress, which significantly suppressed the precipitation of hcp-AlN phase. With enhancing ion bombardment, diffusion energy and nucleation rate of adatoms on the growing surface increased, which caused a denser structure and ultra-smooth surface. The hardness and toughness also varied as a function of Ti target power, with the maximum hardness of 28.3 GPa under a Ti target power of 5 kW. Positive correlation between the adhesion strength (i.e., the critical load of the scratch test) and H3/E2 ratio was discovered indicating a strong dependence of adhesion properties on toughness for the TiAlCrN ceramic coatings in this study, which agreed well with the literatures. As for the tribological behavior, the lowest wear rate of 8.9 × 10-17 m3N-1m-1 was obtained for the TiAlCrN ceramic coating deposited at a Ti target power of 5 kW.

Optoelectronic properties of ZnO thin films grown by radio frequency magnetron sputtering

In the present work, Zinc oxide (ZnO) thin films with suitable optoelectronic properties required for application as transparent electrodes have been grown successfully on glass and silicon substrates by radio frequency magnetron sputtering technique at room temperature. A systematic study of the effect of film thickness on optical, electrical, and structural properties of the films was carried out by spectrophotometer, four-point probe, X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM). It is observed that the film growth rate increases with increasing film thickness. The obtained ZnO films not only have an average transmittance greater than 90% in the visible region but also have low resistivity (ρ = 4 × 10− 2 Ω cm). All the deposited films are polycrystalline with a wurtzite structure and highly textured along the c-axis perpendicular to the substrate surface. As the film thickness increases, the intrinsic compressive stress decreases.

Impact of sintering temperatures on conduction behaviors of ZnO nanoparticles- and MnO-doped SnO2-based thick film varistors obtained by screen printing

In this study, ZnO nanoparticles and MnO-doped SnO2-based thick film varistors (TFVs) were fabricated using screen printing technique. The sintering temperature had significant impact on the SZM-based TFVs, especially in terms of grain growth, even at a low sintering temperature of 1100[Formula: see text]C. The strong solid-state reaction during sintering may be attributed to the large surface area of the 20 nm ZnO nanoparticles that promoted strong surface reaction even at low sintering temperatures. Moreover, the X-ray diffraction lattice constant and full wave at half maximum data indicated that the sintering process also improved the grain crystallinity with the decrease in intrinsic compressive stress. The sintering temperatures also significantly influenced the electrical properties of the SZM-based TFVs with a significant decrease in the breakdown field from 360 V/mm (sample at 1100[Formula: see text]C) to 158 V/mm (sample at 1250[Formula: see text]C). The grain boundary resistance ([Formula: see text] also experienced a dramatic drop from 266.4 k[Formula: see text] (sample at 1100[Formula: see text]C) to 89.46 k[Formula: see text] (sample at 1250[Formula: see text]C). The sample sintered at 1200[Formula: see text]C exhibited superior electrical behaviors with a nonlinear exponent of 61 and leakage current of 115 [Formula: see text]A. Furthermore, it achieved high permittivity and low dissipation factor at the low frequency range. The conduction behaviors of [Formula: see text] ions with activation energy of approximately 0.6 eV was dominated by decreasing [Formula: see text] and [Formula: see text] defects (around 0.4 eV) with increasing sintering temperature. Therefore, the sintering process can be applied to control the conduction behaviors of SZM-based TFVs doped with ZnO nanoparticle powder and achieve improved structural and electrical properties with good nonlinear behaviors.

Mechanical Properties of Hydrogen Free Diamond-Like Carbon Thin Films Deposited by High Power Impulse Magnetron Sputtering with Ne

Hydrogen-free diamond-like carbon (DLC) thin films are attractive for a wide range of industrial applications. One of the challenges related to the use of hard DLC lies in the high intrinsic compressive stresses that limit the film adhesion. Here, we report on the mechanical and tribological properties of DLC films deposited by High Power Impulse Magnetron Sputtering (HiPIMS) with Ne as the process gas. In contrast to standard magnetron sputtering as well as standard Ar-based HiPIMS process, the Ne-HiPIMS lead to dense DLC films with increased mass density (up to 2.65 g/cm3) and a hardness of 23 GPa when deposited on steel with a Cr + CrN adhesion interlayer. Tribological testing by the pin-on-disk method revealed a friction coefficient of 0.22 against steel and a wear rate of 2 × 10−17 m3/Nm. The wear rate is about an order of magnitude lower than that of the films deposited using Ar. The differences in the film properties are attributed to an enhanced C ionization in the Ne-HiPIMS discharge.

Wrinkling patterns of tantalum films on modulus-gradient compliant substrates

Wrinkling patterns are ubiquitous in nature and they are beneficial for a wide range of practical applications. Here we report on the wrinkling patterns in tantalum films sputter deposited on modulus-gradient compliant polydimethylsiloxane substrates. It is found that the sputtering process generates intrinsic and thermal compressive stresses, which are relieved by formation of locally ordered (herringbone) but overall disordered (labyrinth) wrinkling. The wrinkle sizes are strongly dependent on the substrate stiffness and film thickness. The formation mechanisms and evolutional behaviors of the wrinkling patterns have been discussed and analyzed in detail based on the continuum elasticity theory. Some potential applications of gradient wrinkling including surface hydrophobicity and measurement of substrate modulus are also presented in this paper.

Buckling and postbuckling of etching-induced wiggling in a bilayer structure with intrinsic compressive stress

In this study, we investigate buckling and postbuckling of etching-induced wiggling in a bilayer structure consisting of mask and masked layers. To show effects of explicit modeling of etching process, two mask–masked ridge models with and without etching (Models w/E and w/oE) are analyzed using finite element analysis. The etching process is explicitly introduced via step-by-step eigenvalue buckling analysis. Although Model w/oE predicts a constant value of the critical wavelength of wiggling regardless of the change in ridge width, Model w/E predicts a shorter wavelength depending on the decrease in ridge width and the increase in intrinsic compressive stress in the mask layer. In postbuckling analysis, Model w/oE predicts a monotonic increase in the wiggling amplitude with the constant wavelength, whereas Model w/E predicts saturation of the wiggling amplitude owing to the decreasing wavelength. In the explicit modeling of etching process, the wiggling behavior shows completely opposite tendencies. Dimensional analysis is performed to obtain empirical equations, which are compared with an experiment.

Analysis of wiggling buckling induced by etching process for bilayer structure under intrinsic compressive stress

Analysis of wiggling buckling induced by etching process for bilayer structure under intrinsic compressive stress

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries.

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.