Film Stress Research Articles

Controlling Residual Stress and Suppression of Anomalous Grains in Aluminum Scandium Nitride Films Grown Directly on Silicon

Deposition of Aluminum Scandium Nitride (AlScN) films directly on Silicon at high Sc alloying levels, with controlled stress, and free of anomalous grains (AOGs) is demonstrated using reactive co-sputtering. The average stress and AOG formation consider the Sc alloying range between 20.3 and 36.6 atomic % and for film thicknesses from 400 nm to 1000 nm for 27.3% and 31.7% Sc alloying. The combination of a gradient seed layer and proper process gas mixture is shown to inhibit formation of AOGs in AlScN with up to 36% Sc alloying even when grown directly on Si. It is demonstrated that the total flow can be utilized to control the average film stress across the 20-36% doping range while the process gas mixture primarily impacts the density of AOG formation in the AlScN films. Bulk acoustic wave resonators fabricated from 380 nm and 485 nm thick low stress and AOG free Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.68</sub> Sc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.32</sub> N films grown directly on Si demonstrate high frequency operation of 3.6 and 4.8 GHz, high electromechanical coupling >20%, and quality factors in excess of 500. [2021-0252]

Influence of atomic incident kinetic energy on crystalline quality of epitaxial GaN thin films: A molecular dynamics study

The high-quality GaN thin films is of great significance for increasing the lifetime and performance of GaN devices. There are still few studies analyzing the effect of different incident energies on the crystalline quality of GaN thin films during epitaxy at the atomic scale. In this paper, the molecular dynamics (MD) method was adopted to simulate the epitaxial growth of GaN thin films under different particle incident energies. The crystal structure, surface morphology, dislocation evolution and internal stress of GaN films deposited under different growth energies were analyzed. The results showed that increasing the incident energy of the particles could help improve the epitaxial surface quality and make the atoms in the film more densely arranged. The tighter arrangement of atoms increased the nucleation height of dislocations in the film to a certain extent. In addition, as the thickness of the epitaxial films increased, the influence of the interfacial stress on the internal stress of the films gradually weakened.

Young’s modulus and ferroelectric property of BaTiO3 films formed by aerosol deposition in consideration of residual stress and film thickness

Film thickening by aerosol deposition (AD) is effective for the fabrication of self-sustained piezoelectric energy harvesting devices. Here we investigated the properties, microstructure, and residual stresses of BaTiO3 films deposited by AD at different film thicknesses. The Young’s modulus measured by the nanoindentation test showed no thickness dependence; however, it increased from approximately 130– 160 GPa with annealing. Ferroelectric hysteresis curves showed that the increase in film thickness facilitated polarization switching. The microstructure of the films showed no significant changes with the film thickness, while the results of X-ray diffraction and finite element analysis of thermal stress showed that the residual stress after annealing depended on the film thickness. The energy harvesting performance of the BaTiO3 films deposited by AD may increase owing to the residual stress, rather than the increase in the film thickness.

Compensation of the Stress Gradient in Physical Vapor Deposited Al1-xScxN Films for Microelectromechanical Systems with Low Out-of-Plane Bending.

Thin film through-thickness stress gradients produce out-of-plane bending in released microelectromechanical systems (MEMS) structures. We study the stress and stress gradient of Al0.68Sc0.32N thin films deposited directly on Si. We show that Al0.68Sc0.32N cantilever structures realized in films with low average film stress have significant out-of-plane bending when the Al1−xScxN material is deposited under constant sputtering conditions. We demonstrate a method where the total process gas flow is varied during the deposition to compensate for the native through-thickness stress gradient in sputtered Al1−xScxN thin films. This method is utilized to reduce the out-of-plane bending of 200 µm long, 500 nm thick Al0.68Sc0.32N MEMS cantilevers from greater than 128 µm to less than 3 µm.

Research on the Adaptability of High-Performance Film for Full Recycling to the Curl-Up Film Collecting Method

Given the problem of the low tensile performance of the plastic film used in China, which brings about difficulties in curl-up film collecting, in this study, a contrast test was carried out on the tensile property of high-performance film for full recycling and the ordinary polyethylene film (PE film) that is used extensively in China. Test results showed that, within the service period, the elongation at break and tensile yield stress of the high-performance film were higher than those of ordinary polyethylene film, and, within the film-laying period of 0~30 days, the reduction scale of the elongation at break and tensile yield stress was higher than that within the film-laying period of 30~180 days. In this study, in order to obtain the lowest tensile performance of the film by curl-up film collecting, the operation principles of the curl-up film collectors were analyzed. The test on the force of curling up the film in the process of overcoming the force between the film and soil was analyzed. Test and analysis results showed that, for different sampling positions, film pick-up angles, and film types, the tensile stress on the film while pulling it up was within a range of 15.97~21.86 MPa. In order to verify the curling up effect of differently structured film collectors on different types of film with different thicknesses, a field test on film curl-up collecting was designed. A contrast test was carried out on two types of curl-up film collectors, 1JRM-2000 and 11SM-1.2, and the test results showed that the film recycling rate and working performance on the film laid in the same year by the film collector with a fixed film pick-up angle were higher than those for varying film pick-up angles. The curl-up film collector fixed with an automatic film-guiding mechanism is not affected by the velocity difference between the linear velocity of the film curl-up mechanism and the advancing velocity of the machine. The film recycling rate and working performance on the film laid in the same year by the 11SM-1.2 curl-up film collector can meet the operational requirements for collecting high-performance film with thicknesses of 0.008 mm and 0.01 mm. This research can provide a reference for simplifying the structure of residual plastic film collectors, increasing the film recycling rate, and reducing the cost.

Residual stress and tribological behavior of hydrogen-free Al-DLC films prepared by HiPIMS under different bias voltages

Hydrogen-free Al-DLC films were prepared by HiPIMS technology under different bias voltages through an AlC composite target. Surface and cross-sectional morphologies, chemical composition, microphase structure and bonding state, residual stress, hardness and elastic modulus, friction coefficient and wear resistance of Al-DLC films were investigated to study the influences of substrate bias voltage. Results show that bias voltage decreased film roughness (Ra) from 3.83 nm to 1.16 nm through ion bombardment. The increase of negative bias voltage improved film disorder and altered the content of sp3 bonds. However, the existence of sp3 bonds should not be responsible for residual stress. Local distortions generated by film disorder together with the mismatch between film and substrate give rise to residual compressive stress in Al-DLC films. Film hardness is in the range of 10 GPa to 17 GPa which is affected by the content of sp3 bonds and film disorder. The lowest wear rate of Al-DLC film is 6.07 × 10−8 mm3/Nm. Besides graphitization, friction can also generate more sp3 bonds at wear interface between Al-DLC film and the counterpart ball.

Substrate temperature dependence of GaN film deposited on sapphire substrate by high-density convergent plasma sputtering device

Substrate temperature dependence of GaN films deposited on a sapphire substrate was studied by the high-density convergent plasma sputtering device (CPSD). The crystal structure, surface morphology, and film stress of the GaN films were evaluated at 80–800 °C without changing plasma discharge conditions. The deposited GaN films tend to be preferentially oriented on the (0002) plane at all substrate temperature ranges. The in-plane φ-scans of x-ray diffraction measurements showed sixfold symmetric diffraction patterns of GaN(10−10) above 200 °C. At 800 °C, the film stress was down to one tenth compared with 80 °C and the full width at half maximum of the rocking curve at a GaN(0002) diffraction angle reached down to 1.1°. The GaN film deposition condition of 200 °C by CPSD suffices for the alignment of the twist angle of the c axis of GaN.

Comparison of stress corrosion cracking susceptibilities of 308L and 309L cladding layers in high-temperature water with various dissolved oxygen concentrations

Oxide film properties and stress corrosion cracking (SCC) susceptibilities of 309L and 308L claddings are investigated in high-temperature water with various dissolved oxygen concentrations. The oxide films formed on both 308L and 309L in deaerated water exhibit the double-layered structure with the thicker continuous Cr-rich inner layer on 309L than on 308L. The outer and inner oxide layers on 309L are all Fe-rich in oxygenated water, exhibiting a nanocrystalline inner layer having local Cr-rich oxide on 309L, and a Fe-rich fine crystal sub-layer and a thin Cr-rich sub-layer in the inner layer on 308L. In both the deaerated and the oxygenated water environments, 308L shows better SCC resistance than 309L, as shown by the local SCC of 309L while no significant SCC of 308L in deaerated water, and more significant SCC of 309L than 308L in oxygenated water. A higher DO concentration favors the formation of a thicker and less protective nature oxide film, leading to a higher oxidation rate constant and the resultant higher SCC susceptibility. The Cr element dilution and less δ-ferrite content in 309L contribute to decreasing the protectiveness of the oxide film and increasing the SCC susceptibility along with hardening. The SCC susceptibilities of 309L and 308L are closely related to the oxide film properties in terms of DO concentration.

Electrode Drying Metrology Via Light Microscopy with Supporting Techniques

An important way to improve the performance of mass-produced lithium-ion batteries is to develop a better understanding of the manufacturing process’s effects on the electrode structure, and thereby its performance. Drying of the electrode slurry is a key step in that process. The parameters that affect drying electrodes have been mainly studied through ex-situ methods. There are far fewer examples of in-line metrologies of drying electrode properties, with notable exceptions including stress measurement by bending beam and dynamic mechanical analyzer, and ultrasonic measurements revealing the evolution of acoustic reflecting surfaces in the electrode (1). With so few in-line metrologies having been investigated, the possibilities are quite wide. However, it is striking that perhaps the most obvious of in-line metrologies has not been systematically investigated in the literature – that of simple visual observation of the drying surface (specifically with a microscope).Light microscopy can reveal a surprising amount of detail about the drying electrode. In Figure 1 micrographs of a drying electrode under two different lighting conditions – with ring lighting and with coaxial lighting – are shown. In the coaxial lighting condition, the surface of the drying solvent can be seen. In freshly coated electrode slurry, the surface of the drying solvent is uniform and unremarkable, then holes appear as the solvent evaporates eventually leading to a honeycomb-like structure. In the ring lit images, the active material particles themselves are visible directly, and their movement while drying can be tracked. Here, we will examine how microscopy supported by other techniques which yield averaged properties to complement microscopy’s local measurements, such as film stress, can elucidate the drying electrode more fully.1. Y. S. Zhang, N. E. Courtier, Z. Zhang, K. Liu, J. J. Bailey, A. M. Boyce, G. Richardson, P. R. Shearing, E. Kendrick and D. J. L. Brett, Advanced Energy Materials, 2102233 (2021). Figure 1

PECVD Silicon Nitride-Based Multilayers with Optimized Mechanical Properties

Silicon nitride (SiN) based films deposited by plasma-enhanced chemical vapor deposition (PECVD) have interesting optical, mechanical, and chemical properties. They are used for applications such as anti-reflective coatings and surface passivation layers in solar cells. Amorphous SiN-based films also are frequently used to create multilayer structures of alternatingly high-index and low-index films. This approach is very promising to fabricate narrow-bandwidth notch filters on photovoltaic cells to reflect on demand a wide variety of colors over the entire visible spectrum [1]. However, these multilayer structures need to be highly durable and mechanically stable since the lifecycle of photovoltaic cells in some applications [2] is very long, and the coated surfaces are very large. To satisfy these specific functional requirements, we need to adjust the films' optical and mechanical properties by changing the deposition parameters of a PECVD reactor. For instance, it is possible to strongly influence the chemical composition of amorphous SiN-based films, such as their stoichiometry, by tuning the gas flows [3]. This work investigates the effect of the deposition pressure, power, and source gas ratio on SiN-based monolayers during plasma deposition. We have deposited SiN and silicon oxynitride (SiON) films in an electron cyclotron resonance (ECR) PECVD reactor using a SiH4/N2/O2/Ar precursor mixture. We have measured the refractive index and the absorption of the films using a variable angle spectroscopic ellipsometry (VASE) to assess their optical quality. The mechanical properties of the films, such as residual stress and coefficient of thermal expansion, were measured ex-situ on a KLA-Tencor FLX-2320 film stress measurement system. The Young's modulus and hardness of the films were evaluated using nanoindentation. After studying the properties of monolayers, we have made numerical simulations and further characterizations to optimize the design of multilayer structures considering both the optical and mechanical properties. Through this discussion, we try to better understand the interactions taking place when amorphous SiN-based films with different structural properties and compositions are stacked on top of each other. Finally, we suggest methods to predict and control the residual stress in multilayer structures without affecting the optical properties.

Corrosion behavior of additively manufactured AISI 316L stainless steel under atmospheric conditions

AbstractThis study investigated the corrosion behavior of AISI 316L produced by direct energy deposition (DED). Microstructural and chemical analysis showed a homogeneous distribution of Si and Si–Mn inclusions of 0.5–1 µm and the Cr and Mo enrichment within interdendritic areas. Scanning Kelvin probe analysis of additively manufactured stainless steel highlighted a regular “striped‐like” surface potential feature with a potential gradient of 30 mV for a mean value of 0.320 ± 0.017 V versus standard hydrogen electrode. It can be related to the presence of the residual stress in the oxide film and the complex thermal history due to the fabrication process. A cyclic corrosion test simulating atmospheric conditions revealed the same corrosion properties for stainless steel fabricated by DED compared to cold rolled one. Various surface preparations of 316L were also exposed for corrosion tests. It was found that the “as‐received” and “brushed” surfaces exhibited poorer corrosion resistance due to the presence of an as‐build defective layer. However, prior passivation of brushed surface, machining, or mechanical grinding down to P1200 improve significantly the corrosion resistance.

Room temperature deposition of freestanding BaTiO3 films: temperature-induced irreversible structural and chemical relaxation

The room temperature aerosol deposition method is especially promising for the rapid deposition of ceramic thick films, making it interesting for functional components in energy, mobility, and telecommunications applications. Despite this, a number of challenges remain, such as an enhanced electrical conductivity and internal residual stresses in as-deposited films. In this work, a novel technique that integrates a sacrificial water-soluble buffer layer was used to fabricate freestanding ceramic thick films, which allows for direct observation of the film without influence of the substrate or prior thermal treatment. Here, the temperature-dependent chemical and structural relaxation phenomena in freestanding BaTiO3 films were directly investigated by characterizing the thermal expansion properties and temperature-dependent crystal structure as a function of oxygen partial pressure, where a clear nonlinear, hysteretic contraction was observed during heating, which is understood to be influenced by lattice defects. As such, aliovalent doping and atmosphere-dependent annealing experiments were used to demonstrate the influence of local chemical redistribution and oxygen vacancies on the thermal expansion, leading to insight into the origin of the high room temperature conductivity of as-deposited films as well as greater insight into the influence of the induced chemical, structural, and microstructural changes in room temperature deposited functional ceramic thick films.Graphical abstract

Variation of the lift force components of a settling particle caused by structural evolution of flowing foam

Wet foam fluid with a settling particle, as a special gas-liquid-solid three-phase flow, is increasingly found in related engineering fields. However, the detailed microscopic evolution of foam structure, which influences migration of the settling particle, has not been fully understood. In this study, the forces, exerted on the particle by foam film and bubble pressure in the flow direction, are defined as the lift force components Fnx and Fpx, respectively. The flow of the wet foam with a circular settling particle is studied by numerical method, and the distribution characteristics of the bubble-bubble and bubble-particle separations in the flowing foam are shown. More importantly, the influences of the microscopic structural evolution of foam film, such as the bubble-particle contact, the bubble-bubble and the bubble-particle separations, on the lift force components are in detail discussed. It is found that both the bubble-bubble and the bubble-particle separations can cause decreasing lift force and relaxing film stress, and the instantaneous bubble-particle contact can significantly change the lift force components. At the film scale, this study reveals the correlation between the particle movement and the microscopic structural evolution of foam fluid.

Study of residual stress in reactively sputtered epitaxial Si-doped GaN films

Si-doped GaN films were grown epitaxially on c-sapphire by rf magnetron reactive co-sputtering of GaAs and Si at various partial pressures of N2 in Ar–N2 growth atmosphere. Energy dispersive x-ray spectroscopy revealed ∼2 at.% Si in all the films, but the N/Ga ratio decreased substantially as N2 percentage was reduced from 100% to 10%. The lattice parameters (a and c) were obtained independently to determine the in-plane and out-of-plane components of strain in films, which were analyzed to deduce the hydrostatic and biaxial strain contributions. High resolution x-ray diffraction revealed the dominant presence of edge dislocations (∼1012 cm−2) in the films grown at 30%–100% N2, which decreases and becomes comparable to the density of screw dislocations (∼1011 cm−2) at lower N2 percentages. The films grown at 75%–100% N2 displayed compressive biaxial stress along with large hydrostatic strain and micro-strain due to the incorporation of nitrogen at interstitial locations and grain boundaries. Both hydrostatic strain and micro-strain decrease in the films grown below 75% N2, in which, a reversal of biaxial stress to tensile is seen due to the prevalence of in-plane tensile stress generated during coalescence. The decrease of tensile stress in films grown below 30% N2, accompanied by increase in micro-strain and hydrostatic strain are ascribed to Ar incorporation and voided morphology. The presence of Si does not have a significant influence on residual stress of films and appears to be masked by the dominant growth-related intrinsic effects.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Inhomogeneous intrinsic stress in sputtering-deposited platinum films and its effect on the formation of helical structures

Recently, we reported an approach to form helical nanostructures, namely nanocoils, through the spontaneous bending deformation of fiber-shaped Pt films. In that study, the inhomogeneous intrinsic stress developed during Pt sputtering was proposed to be the origin of the driving force for deformation. Unfortunately, the existence of inhomogeneous intrinsic stress in the Pt films was not verified experimentally. Moreover, the intrinsic stress, which affects the bending deformation and determines the shape of the nanocoils, was not quantitatively measured. Thus, the current study investigates the intrinsic stress in Pt films deposited by two magnetron sputtering systems by measuring the film forces; moreover, it discusses the effect of stress inhomogeneity on the formation of Pt nanocoils. An inhomogeneous intrinsic stress distribution along the thickness direction was confirmed, and the tensile and compressive stresses were found to reach hundreds of megapascals. The Pt films deposited by the two sputtering systems exhibited completely different stress distributions. Regardless of this, the Pt film with inhomogeneous intrinsic stress eventually led to the formation of Pt nanocoils, whereas that with a uniform stress did not. It was therefore verified that inhomogeneous intrinsic stress was the origin of the driving force for the formation of a helical structure. Finally, it was found that the bending radius could be estimated by measuring the intrinsic stress. The results of this study will provide a perspective on the application of intrinsic stress to helical-structure manufacturing.

Self-Rolled-Up Aluminum Nitride-Based 3D Architectures Enabled by Record-High Differential Stress.

Aluminum nitride (AlN) continues to kindle considerable interest in various microelectromechanical system (MEMS)-related fields because of its superior optical, mechanical, thermal, and piezoelectric properties. In this study, we use magnetron sputtering to tailor intrinsic stress in AlN thin films from highly compressive (-1200 MPa) to highly tensile (+700 MPa), with a differential stress of 1900 MPa. By monolithically combining the compressive and tensile ultrathin AlN bilayer membranes (20-60 nm) during deposition, perfectly curved three-dimensional (3D) architectures are spontaneously formed upon dry-releasing from the substrate via a 3D MEMS approach: the complementary metal-oxide-semiconductor (CMOS)-compatible strain-induced self-rolled-up membrane (S-RuM) method. The thermal stability of the AlN 3D architectures is examined, and the curvature of S-RuM microtubes and helical structures as a function of the cumulative membrane thickness and stress are characterized experimentally and simulated using a finite-element physiomechanic method. By combining AlN with various materials such as metal (Cu) and silicon nitride (SiNx), AlN-based hybrid S-RuM microtubes with diameters as small as ∼6 μm are demonstrated with a near-unity yield (∼99%). Compared with other stressed thin films for S-RuMs, including PECVD SiNx, magnetron-sputtered AlN-based S-RuMs show better structural controllability and versatility, probably due to the high Young's modulus and stress uniformity. This work establishes the sputtered AlN thin film as a superior stress-configurable S-RuM shell material for high-performance applications in miniaturizing and integrating electronic components beyond those based on other materials such as SiNx. In addition, for the first time, a single-crystal Al1-xScxN/AlN bilayer grown by molecular beam epitaxy is successfully rolled-up with the diameter varying from ∼9 to 14 μm, paving the way for 3D tubular Al1-xScxN piezoelectric devices.

Stress tuning in sputter-grown Cu and W films for Cu/W nanomultilayer design

Controlling growth stresses during thin film fabrication is of paramount importance to solve reliability issues during operation of functional thin films in harsh environments. A combination of different methods for thin-film stress determination, such as in situ wafer curvature and ex situ x-ray diffraction, is usually required to reveal and tailor growth stresses in thin film systems, as well as to extract interface stress contributions in multilayered coatings. In this article, the tuning of intrinsic growth stresses in thin films of Cu and W, as grown by magnetron sputtering, was performed by varying the Ar pressure and gun power during thin-film deposition. The average growth stress in Cu and W thin films could be tuned between tensile and compressive. Next, the thus obtained knowledge on stress engineering of Cu and W single layers was applied to investigate the corresponding intrinsic stresses in Cu/W nanomultilayer coatings, for which interface stress was found to play an important role.

In-vitro evaluation of the evaporation retardation by Meibomian lipids in homogeneous and non-homogeneous evaporation

HypothesisAn important function of the Tear Film Lipid Layer (TFLL) is the retardation of evaporation. We propose two micro-scaled systems to quantify the influence of the TFLL in evaporation for single patients, which may contribute as an improvement on the diagnosis of Meibomian Gland Dysfunctions (MGD). ExperimentsMeibum was extracted from 10 patients with hypersecretory MGD and 9 healthy controls. The lipids were placed over water, and the evaporation was determined in the case of homogeneous evaporation over a surface (pendant drop), and the case where the evaporation depends on a pinned triple contact line (meniscus). FindingsFor the homogeneous case, the presence of Meibum reduced evaporation in 30%, although there was no significant difference between controls and MGD patients. However, evaporation induced by menisci was 25 % higher in MGD patients.Our results contribute to the evidence of the inhibition of evaporation by Meibum. Our study also suggests that the evaporation induced by contact points may be a more relevant model to measure differences in evaporation due to the composition of Meibum. This model may also have connotations in the occurrence of internal stresses in the tear film, inducing its instability.

Stress-balancing in piezoelectric adjustable x-ray optics

Next-generation x-ray observatories require lightweight, high throughput optics that maintain a <0.5 arcsec resolution to probe the physics of black holes and gain understanding of the early universe. One potential type of x-ray mirror consists of a 400-μm thick curved Corning EAGLE XG® glass substrate with a Cr/Ir x-ray mirror coating deposited on the front (concave) side and an array of radio frequency sputtered Pb0.995 ( Zr0.52Ti0.48)0.99Nb0.01O3 piezoelectric thin film actuators on the back (convex) side to enable correction of figure errors. A stress-balancing process was developed to correct the figure distortion arising from thin film stresses in the actuator layers. Compressively stressed SiO2 films were deposited on the convex side of the mirror to balance the tensile integrated stress of the actuator array while also matching the film thickness distribution. Finite-element methods were used to assess the impact of film thickness distributions on the convex and concave substrate surfaces. The resulting models show peak-to-valley figure errors of 105 nm, well within the 1-μm peak-to-valley dynamic range of the piezoelectric adjusters. In contrast, when stress compensation was done with an iridium mirror film deposited on the front side, the mismatched thickness distribution results in peak-to-valley figure errors over 3 μm.