Film Stress Research Articles

Co-Self-Assembled Monolayers Modified NiOx for Stable Inverted Perovskite Solar Cells.

[4-(3,6-dimethyl-9H-carbazol-9yl)butyl]phosphonic acid (Me-4PACz) self-assembled molecules (SAM) are an effective method to solve the problem of the buried interface of NiOx in inverted perovskite solar cells (PSCs). However, the Me-4PACz end group (carbazole core) cannot forcefully passivate defects at the bottom of the perovskite film. Here, a Co-SAM strategy is employed to modify the buried interface of PSCs. Me-4PACz is doped with phosphorylcholine chloride (PC) to form a Co-SAM to improve the monolayer coverage and reduce leakage current. The phosphate group and chloride ions (Cl-) in PC can inhibit NiOx surface defects. Meantime, the quaternary ammonium ions and Cl- in PC can fill organic cations and halogen vacancies in the perovskite film to enable defects passivation. Moreover, Co-SAM can promote the growth of perovskite crystals, collaboratively solve the problem of buried defects, suppress nonradiative recombination, accelerate carrier transmission, and relieve the residual stress of the perovskite film. Consequently, the Co-SAM modified devices show power conversion efficiencies as high as 25.09% as well as excellent device stability with 93% initial efficiency after 1000 h of operation under one-sun illumination. This work demonstrates the novel approach for enhancing the performance and stability of PSCs by modifying Co-SAM on NiOx.

Reduction of internal stress in InGaZnO (IGZO) thin film transistors by ultra-thin metal oxide layer

In this study, the modification of indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) with an ultrathin metal oxide layer successfully reduces internal film stress. The introduction of an Al2O3 metal oxide layer effectively diminishes defects within the film and minimizes distortion in the film densification process. A low-temperature annealing process at 200 °C was employed to mitigate thermal expansion differences between the film and the substrate, thus reducing internal stress within the device. Furthermore, we have discovered that this structural modification holds the potential to enhance mechanical stress resistance. This paper offers a novel perspective and an experimental foundation for the stress analysis of flexible TFTs.

A comparative study on the hardness, stress and surface free energy of HfN films grown by HIPIMS and direct-current magnetron sputtering

HfN films with rocksalt structure have been grown by high-power impulse magnetron sputtering (HIPIMS) with various duty cycles, in a comparison with direct-current magnetron sputtering (dcMS) process. Such two sputtering processes yield the quite close compositions. Increasing the duty cycle from 7.5 % to 9 % and 11 % in HIPIMS process, a compressive stress rises up from 1.5 to 1.9 and 2.1 GPa, which is much smaller than the value of 3.8 GPa from dcMS. The hardness of such low-stressed HfN films has been identified to be 25.5–25.9 GPa, which agrees well with the theoretical one of 22.5 GPa by the density functional theory without the consideration of stress. In addition, the surface free energy of 26.3–29.0 mJ/m2 has been determined in HfN films grown by HIPIMS, much lower than that of 31.5 mJ/m2 of the films grown by dcMS.

All inorganic CsPbI3 perovskite solar cells with reduced mobile ion concentration and film stress

All inorganic CsPbI3 perovskite solar cells with reduced mobile ion concentration and film stress

Study of the dependence of relative elongation on mechanical stress of polyvinyl chloride films

The dependence of relative elongation on mechanical stress in polyvinyl chloride samples of various widths and working lengths was experimentally studied. The purpose of the study was to study the dependence of deformation on mechanical stress of polyvinyl chloride films of various sizes (width and length). The scientific and practical significance of the work lies in the fact that the data can serve as the basis for future research. Polyvinyl chloride film with a thickness of 100 microns was chosen as the material under study. The film sheets were cut into strips 3, 5, 7, 10 mm wide using a special cutting device with adjustable sample cutting width and length. The working length of the samples was: 4, 5, 6, 7, 8 cm. The tests were carried out on a special tensile testing machine RU-50. This setup is designed to study the dependence of strain (relative elongation) ε on stress σ. Significant changes in the mechanical properties of the material were discovered depending on various geometric parameters. The data obtained are satisfactorily described by the authors' models. The value of this study lies in the fact that the results of the work, according to the authors, will serve as an incentive to improve the mechanical properties of the research object.

Effect of grain coalescence on dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates

The effect of grain coalescence on the dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates with low-temperature grown GaN and physical vapour deposition AlN nucleation layers is comparably investigated.

The strength of uncoated and coated ultra-thin flexible glass under cyclic load

<abstract> <p>Ultra-thin flexible glass with thicknesses of 100 µm or below is a substrate in the fields of optics, electronics, and semiconductors. Its brittleness is challenging in production processes like physical vapor deposition processes, especially in roll-to-roll production. In many cases, multiple geometric deformations take place and each step, like coating or cutting, influences the glass strength. By now, the relation between the strength of ultra-thin glass under quasi-static conditions and its strength under cyclic load has not been studied. Moreover, the effect of coatings has not been investigated. Both aspects are crucial to design reliable production processes. Therefore, the strength of ultra-thin glass under cyclic load was studied for uncoated and coated substrates. Two coating types were investigated: a single indium tin oxide film and a seven-layer antireflective layer stack. The coatings significantly influence the strength of the underlying glass in both test modes. The barrier properties, thin film stress, and the morphology/crystalline structure are identified as the major characteristics influencing the strength.</p> </abstract>

Advanced technologies of FBAR for tuning effective electromechanical coupling coefficient

The film bulk acoustic wave resonator (FBAR) is one of the most popular devices in the radio frequency field. Numerous researchers are simultaneously working to develop an effective electromechanical coupling coefficient (keff2) tuning technology, aiming to meet the diverse bandwidth requirements of the 5 G era. Based on a traditional FBAR process, this work prepares several different devices and then analyzes the four factors that influence keff2 from the perspective of process, material, and design. The pillar structure achieves the largest keff2 tuning range of 2.84% (33 MHz). The composite piezoelectric film can tune the overall resonant frequency in a wide range, and its keff2 tuning range is 1.75% (12.5 MHz). The area effect tunes the fs in a small range, ultimately achieving a keff2 tuning range of 1.16% (11 MHz). Film stress regulation achieves keff2 tuning range of 2.73% (30 MHz), but it has the greatest difficulty. The integration of various keff2 tunable methods has important guiding significance for the design of FBAR filters in the future.

Edge cracking behavior of Y2O3 films based on the stress intensity factor

Hastelloy C-276 alloy is the preferred substrate for YBa2Cu3O7-δ (YBCO) coated conductors prepared by Ion-beam-assisted deposition (IBAD) route, but its surface is too rough to meet application requirements. Y(CH3COO)3·4H2O precursor solution has a low flattening efficiency, and its film edges are prone to crack formations. In this paper, a model for the stress intensity factor was established to study the effect of the presence of cracks on film stress. The simulation results showed that increasing the coating thickness reduced the overall stress, but increased the stress concentration at the crack. The larger bottom layer defects also made it easier for cracks to be generated. Then, 10-m long Hastelloy C-276/Y2O3 strips were prepared by colloidal-solution deposition planarization (CSDP), with an average surface roughness Rq of 0.427 and only 10 coating layers. This improved the flattening efficiency by 40 %, and a CeO2 layer with a good biaxial texture was finally prepared.

Calculation and properties of thermal stress of monolayer film and solution of thermal stress of multilayer infrared antireflection coating by finite element analysis

The deposition of infrared antireflection coatings on silicon lenses is a widely practiced technique that has been extensively investigated by researchers over an extended period. Thermal stress is essential to the adhesion between the film and the substrate. To measure the magnitude of thermal stresses, we use the mechanical analysis method to convert thermal stresses into shearing and peeling stresses. Expressions for these two stresses are then derived. Then, we analyzed the effect of the monolayer film's properties on the silicon substrate's stress. The results indicate that peeling stress is the main factor affecting the adhesion between the film and the substrate, and various approaches can be employed to successfully mitigate the peeling stress exerted on the substrate. The influence of the multilayer film's structure on the peeling stress experienced by the silicon substrate was investigated through the utilization of finite element analysis. In this study, three distinct infrared antireflection coatings were deposited on silicon substrates individually. Subsequently, these coated substrates underwent rigorous environmental testing. The findings from this testing confirmed the accuracy of the stress analysis results obtained by finite element analysis for the multilayer film. Our research has resulted in solutions for determining the magnitude of shearing and peeling stress. We have employed finite element analysis to identify a suitable structure for an antireflection coating that exhibits minimal peeling stress. This advancement has the potential to improve the effectiveness of design processes and the adaptability of infrared antireflection coatings in various environmental conditions.

Unravelling the effect of nitrogen on the morphological evolution of thin silver films on weakly-interacting substrates

We study the effect of nitrogen on the morphological evolution of thin silver (Ag) films deposited on weakly-interacting amorphous carbon (a-C) and silicon oxide (SiOx) surfaces. Films are synthesized at a deposition rate of 0.1nm·s-1 by direct current magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS), and electron-beam evaporation (EBE). We monitor growth in situ and in real time by measuring the evolution of film stress and optical properties, complemented by ex situ analyses of discontinuous-layer morphologies, film crystal structure, and film composition. We find that addition of molecular nitrogen (N2) to the plasmagenic gas (Ar) during DCMS and HiPIMS promotes a two-dimensional (2D) morphology. Concurrently, EBE-deposited films exhibit a significantly more pronounced three-dimensional morphological evolution, independently from the gas atmosphere composition. We argue that the 2D morphology in DCMS- and HiPIMS-grown films is enhanced due to incorporation of atomic nitrogen (N)—result of plasma-induced N2 dissociation—that hinders island reshaping during coalescence. This mechanism is not active during EBE due to the absence of energetic plasma electrons driving N2 dissociation. The overall results of the study show that accurate control of vapor-phase chemistry is of paramount importance when using gaseous species as agents for manipulating growth in weakly-interacting film-substrate systems.

Plasma Anneal As a Stress Control Method in Low Temperature Al2O3 ALD Coatings

The control of mechanical physical stress is an important requirement in a variety of applications, such as flexible electronics or thick optical coatings where film stress can cause delamination. Traditionally, in thermal ALD it has been difficult to control the stress. Al2O3 deposited at low temperature is consistently found to have a tensile stress level in the range of a 300 to 450 MPa ( refs 1-3). It is well established that In magnetron sputtering , film stress can be controlled straightforwardly using the Argon pressure during deposition, as this affects the level of bombardment of the growing film with energetic particles.Paranjpe et al ( ref 1) have demonstrated that the use of a plasma anneal leads to densification and transition of the stress from tensile to compressive. The process consists of a sequence of deposition of a thin layer ( e.g. 5 nm) by thermal ALD, followed by exposure to a low energy plasma. The atomic displacements caused by low energy ions from the plasma tend to lead to filling of nanoscopic voids and point defects, thus releasing tensile stress.In this presentation, we will review an in depth study of the effect of various process parameters on the ability of a capacitively coupled in-situ plasma anneal to control stress. The following parameters will be reviewed: Gas composition: Argon plasmas, Oxygen plasmas and Argon oxygen mixtures are compared. Gas Pressure: The effect of gas pressure on plasma characteristics and the resulting modification of the coating is studied RF Power: RF Power was varied between 0 and 300 W ( 1.32 W/cm2) Plasma duration : The duration of the plasma treatment was varied from 0 to 60 seconds.A process window was identified where the plasma anneal can be used as an effective, controllable tool to reduce stress in the coatings. This is illustrated in fig. 1, which shows the substrate warpage (“bow”) observed in 75 mm thick PET foils with a 50 nm Al2O3 coating deposited at 90 °C. It can be seen that the untreated coating (0 sec plasma) has a bow corresponding to a tensile stress of 370 MPa, in accordance with literature values. As the plasma anneal time increases, the stress value decreases. Applying a 60 sec plasma every 5 nm allows to deposit a virtually stress free coating. Simultaneously with the stress reduction, an increase of the optical index and a slight reduction of the thickness are observed, consistent with a densification of the coating.

A Comparison of Low-Temperature Deposition for SiO2 in PECVD Using SiH4 and TEOS

We have developed a method for depositing SiO2 as an insulating layer at temperatures below 100°C using PECVD. This method has potential applications in the development of flexible displays and organic transistors. We have previously reported on TSV via-hole formation by the Bosch process and TEOS-SiO2 deposition on sidewall by PECVD at 150°C [1]. In this study, we performed PECVD using a power supply frequency of 400 kHz. The deposition temperature was set at 100°C. We compared the deposition rates and the film stress of SiO2 films deposited using SiH4 and TEOS at different RF power densities.Figure 1 shows the relationship between 400 kHz power and the deposition rates. In this power density range, the deposition rate of SiH4 was almost constant at approximately 52 nm/min regardless of power density. On the other hand, the rate using TEOS tended to decrease as the power density increased above 0.28 W/cm2.Figure 2 shows the film stress of each film immediately after deposition and after few days of atmospheric exposure. The minus side of the stress indicates compressive stress. The stress of SiH4 was -50 to -70 MPa immediately after deposition; however, after few days of atmospheric exposure, the stress shifted by approximately -50 MPa at all power densities. On the other hand, in the case of TEOS, the stress did not change at approximately -120 MPa at a power density of 0.18 W/cm2 or higher immediately after deposition; however, after few days of exposure, the amount of change to the compressive stress side became smaller as the power density increased. It has been reported that the change in stress to the compressive side is influenced by moisture in the atmosphere [2]. The small change in stress is considered to mean that the film is denser, and that moisture is less likely to enter the film. The deposition rate is considered to decrease during the process of film densification.In summary, it is thought that by increasing the power density with TEOS-SiO2, a dense film is formed even at a low temperature of 100°C.

A Theoretical Analysis for Arbitrary Residual Stress of Thin Film/Substrate System With Nonnegligible Film Thickness

Abstract Stoney formula is widely used in advanced devices to estimate the residual stress of thin film/substrate system by measuring surface curvature. Many hypotheses including that thin film thickness is ignored are required, thus bringing significant error in characterizing the inhomogeneous residual stress distribution. In this article, arbitrary residual stresses on thin film/substrate structures with nonnegligible film thickness are modeled and characterized. We introduce nonuniform misfit strain and establish the governing equations including mismatched strain, displacements, and interfacial stresses based on the basic elastic theory. The parameterization method and the method of constant variation are used in the process of equation decoupling. The expressions between displacements, surface curvatures, and misfit strain are determined through decoupling calculations. By substituting misfit strain, residual stresses are expressed by parametric equation related to surface curvature. It further indicates that there is a “non-local” part between the film stress and curvature at the same point. Compared to neglecting the film thickness, the proposed method eliminate relative errors up to 58.3%, which is of great significance for stress measurement of thin films and substrates.

(Invited) Fundamentals of Thin Film Growth and Etching: How Small Atomic Processes Govern Macroscopic Effects

In order not to have our children inherit a huge problem in the future, we quickly have to realize the energy transition to renewable sources, which requires also energy storage devices and, thus, Li ion batteries. Although significant progress has been made during recent years regarding battery capacitance, cycle life time, deterioration, and fast charging capabilities, any further improvement becomes increasingly more difficult.The reason for this is manifested in the top-down approach, which quickly delivers results, and thus products, by screening the parameter space and evaluating available options. At this point, however, further improvements often require a deep, fundamental understanding of all processes, and a change to a bottom-up approach is highly recommended, if not even essential.Using Scanning Tunneling Microscopy movies, I will show in my talk how a small change in adatom formation energy and adatom diffusion barrier leads to a huge morphology change of the growing film. After providing the insights to the well-known homoepitaxial growth modes, I will introduce one specific atomic barrier that leads to an instability and triggers 3D growth. The 2D analogous of this barrier at a step edge, leads to dendritic island shapes, and the combination of both, the 2D and 3D instability, leads to dendrites as observed also in the batteries. With this in mind, I will show the dualism between adatoms and vacancies, which naturally explains also the equivalent, inverse growth modes during etching or dissolution including their instabilities.From here we will switch gears and focus on polycrystalline film aspects, and, thus, the grain boundary network. I will show, again on the atomic scale, how the grain boundary energy determines the surface structure from extremely rough to flat and smooth. Quite surprisingly, I will further demonstrate how a tiny variation in the dilute adatom/vacancy equilibrium-pressure on the surface, during growth or dissolution, can setup pressures variations in the grain boundary network that exceed the pressure of the deepest point of all our world seas: the Mariana Trench.These enormous compressive (and tensile) stresses are to a large extent reversible. However, due to the differences in diffusion constants in combination with the huge reservoir of grain boundary volume in the film, complications arise on the atomic scale at the triple point/lines, where the grain boundaries penetrate the surface. Mass accumulation or depletion leads to hillocks and grooves. Moreover, the remaining accumulated intrinsic film stress can also trigger whisker growth, which, on the basis of the involved atomic processes, clearly has to be distinguished from dendritic growth.Although fundamentally still not understood, whisker growth leads to many failures, like short circuits in electronic devices and batteries. Their penetrating forces can be enormous, pushing through even hard oxide layers and films (and the SEI).As many of the described effects play a major and critical role in Li ion batteries, and while I realize that I am adding seemingly hopeless, additional complexity, I will finally provide some basic strategies and concepts that we learned on how to tune and influence the previously mentioned effects efficiently on the atomic scale by using additives.

Process-induced residual stress in a single carbon fiber semicrystalline polypropylene thin film

During the processing of semicrystalline thermoplastic composites, various models for the prediction of residual stress have been proposed but experimental validation is limited. In this study, a modeling framework is developed to predict the evolution of residual stress in a single carbon fiber embedded in a semicrystalline polypropylene thin film subjected to non-isothermal cooling. Validation is based on in-situ measurement of axial fiber strain after processing using micro-Raman Spectroscopy. A material model for polypropylene (50% equilibrium crystallinity) is presented that incorporates the effects of non-isothermal cooling on crystallinity-dependent resin shrinkage and crystallinity-dependent resin modulus from the amorphous polymer melt to room temperature. The evolution of residual stress is calculated using a finite element (FE) model with a user-developed subroutine incorporating the resin material models. Single-fiber polypropylene films are fabricated using a range of fiber pretension levels to prevent fiber waviness during cooling and to induce a wide range of axial strain levels in the fiber for model validation. The fiber axial strain was measured over the length of the fiber, from the free edge into the bulk, capturing the ineffective length region where strain builds up and plateaus. A good correlation has been obtained between the model results and the in-situ measurements of axial compression strain in the fiber. A key finding was the importance of including temperature-dependent resin modulus and that the contribution of crystallinity shrinkage to residual strain in the carbon fiber is relatively low. The model developed in this study is also compared to residual stress models in the literature which shows the importance of including the effects of crystallization and cooling rate on temperature and crystallinity-dependent modulus and resin shrinkage for accurate predictions.

Giant Electromechanical Effect in Piezoelectric Nanomembranes Based on Hf0.5Zr0.5O2.

Since ultrathin ferroelectric HfO2 films can be conformally grown by atomic layer deposition even on complex three-dimensional structures, new horizons in the development of next-generation piezoelectric devices are opened. However, hafnium oxide has a significant drawback for piezoelectric applications: its piezoelectric coefficients are much smaller than those of classical materials currently used in piezoelectric devices. Therefore, new approaches to the development of high-performance piezoelectric devices based on exploiting the unique properties of HfO2 are of paramount importance. In this work, a giant electromechanical effect in miniature piezoelectric membrane devices based on a 10 nm-thick ferroelectric Hf0.5Zr0.5O2 (HZO) film is experimentally demonstrated. Compared to the pure piezoelectric effect in the HZO film, the gain of the electromechanical response in membrane devices reaches 25 times. Numerical simulations confirm that this effect stems from the asymmetric shape of the membranes and can be further improved by designing the device geometry. Furthermore, according to first-principles calculations, an additional opportunity to improve the piezoelectric coefficient, and hence, the device efficiency is provided by the engineering of the mechanical stress in the HZO film. The proposed approach enables the development of new promising piezoelectric devices including miniature reflectors, nanoactuators, and nanoswitches.

A self-healing artificial muscle was realized by fitting the electrode membrane with the self-healing actuating membrane with a folded structure

As a part of biomimetic gelatinous polymer actuator (BGPA), hydrogel artificial muscle has the advantages of extreme flexibility, low driving voltage and controllable driving direction. However, such artificial muscles do not have self-healing properties and it is difficult to continue using them if they break, which considerably reduces their lifespan. In this paper, we propose a hydrogel artificial muscle with self-healing capability by gluing a membrane of electrodes with a pleated structure to a self-healing actuator layer. The crosslinking reaction between polyacrylic acid molecular chains and carboxylated chitosan (CLC) molecular chains was utilized to fabricat e self-healing actuator layers, while multi-walled carbon nanotubes and chitosan were employed for electrode films. The impact of CLC doping content on the self-healing properties, mechanical properties, electrical response output force properties, and electrochemical properties of self-healing artificial muscles was investigated. Experimental results demonstrated that the output force density of the self-healing artificial muscle could reach 14.7 mN g−1 with an addition of 0.2 g CLC; even after fracture-self-healing, the maximum output force density of the artificial muscle still remained above 90%, and the maximum stretching stress of the actuator film maintained a range from 91% to 99%, showcasing exceptional self-healing performance.

Mechanism of interstitial atoms on oxidation and fracture of niobium‑silicon alloys

To investigate the effect of interstitial atoms on high-temperature oxidation behavior and fracture room-temperature fracture toughness of niobium‑silicon alloys. The Nb-16Si-24Ti-xC-yB (x = 1, 2; y = 5, 10) alloys were prepared by arc melting. Results show that the addition of C and B elements shifts solidification path to hypereutectic direction and promotes the generation of coarse silicide and lamellar Nb5Si3-Nbss eutectic structure. TiB2 appears in 1C10B and 2C10B alloy as primary phase. The KQ value of 1C5B alloy reaches 12.30 MPa·m1/2 and increased by 95.5% compared to Nb-16Si-24Ti alloy. The surfaces of alloys do not generate stable oxide layer which could defense oxidation effectively. However, the increase of silicide and the obstructive effect of interstitial atoms on inward diffusion of O atoms improves the oxidation resistance of 1C10B and 2C10B alloy. The existence of interstitial atoms reduces the internal stress of oxide film and improves the integrity of oxide layers.

A phase field fracture model for ultra-thin micro-/nano-films with surface effects

Surface effects usually remarkably affect the mechanical response of ultra-thin micro-/nano-structures. However, the mechanisms of surface effects on the fracture characteristics of ultra-thin films are still not fully understood. To this end, this paper develops a modeling framework to investigate the fracture of ultra-thin films at microscales or below. Such a framework couples the Gurtin–Murdoch theory with a phase-field fracture model, in which the former is adopted to introduce the surface effects, i.e., the surface residual stress and surface elasticity of a thin film, and the latter is able to model crack evolution without requiring predefined crack paths or any criteria. Furthermore, a novel crack driving force is introduced, which encompasses the tensile components of both bulk elastic energy and surface elastic energy. Several numerical examples including the biaxial tension test as well as the single-edge notched tension/shear test are performed. The simulation results indicate that the surface strain energy plays a major role in the total elastic strain energy of an ultra-thin film when its thickness is at a micro level, thus demonstrating the significance of surface effects. Moreover, the mode-I fracture test shows that the surface elasticity and surface residual stress have a remarkable influence on the displacement at failure, while for the mode-II fracture test, the surface residual stress significantly influences the fracture characteristics such as the crack path and failure displacement. The developed model paves the way for revealing the fracture mechanisms of ultra-thin micro-/nano-films and conducting their safety assessment.