Stress In Thin Films Research Articles

P‐126: Thin Film Stress Optimization for Bending Resistance Improvement of Flexible AMOLED Display

In order to improve the bending resistant of flexible AMOLED display, the stress of thin film should be well researched. In this paper, we analysis the defect of transversal line, which was very serious after module bending, metal crack was found in the process of TFT backplane. We solved the issue of metal crack by adjusting the stress of the backplane compound film. The bending resistant of the flexible screen was improved finally. We successfully fabricated a 5.46inch QHD flexible AMOLED display which passed one hundred thousand times bending test in a bending radius of 5mm.

Self-contained in-vacuum in situ thin film stress measurement tool.

A fully self-contained in-vacuum device for measuring thin film stress in situ is presented. The stress was measured by measuring the curvature of a cantilever on which the thin film was deposited. For this, a dual beam laser deflectometer was used. All optics and electronics needed to perform the measurement are placed inside a vacuum-compatible vessel with the form factor of the substrate holders of the deposition system used. The stand-alone nature of the setup allows the vessel to be moved inside a deposition system independently of optical or electronic feedthroughs while measuring continuously. A Mo/Si multilayer structure was analyzed to evaluate the performance of the setup. A radius of curvature resolution of 270 km was achieved. This allows small details of the stress development to be resolved, such as the interlayer formation between the layers and the amorphous-to-crystalline transition of the molybdenum which occurs at around 2 nm. The setup communicates with an external computer via a Wi-Fi connection. This wireless connection allows remote control over the acquisition and the live feedback of the measured stress. In principle, the vessel can act as a general metrology platform and add measurement capabilities to deposition setups with no modification to the deposition system.

Controlling Thin-Film Stress and Wrinkling during Perovskite Film Formation

Significant effort has focused on controlling the deposition of perovskite films to enable uniform films, enabling efficiencies to climb dramatically. However, little attention has been paid to the evolution of thin-film stresses during deposition and the consequent effect on film morphology. While a textured surface topology has potential benefits for light scattering, a smooth surface is desirable to enable the pinhole-free deposition of contact layers. We show that the highly textured morphology made by popular antisolvent conversion methods arises because of in-plane compressive stress experienced during the intermediate phase of film formation where the substrate constrains the film from expanding—leading to energy release in the form of wrinkling, resulting in trenches that can be hundreds of nanometers deep with periods of several micrometers. We demonstrate that the extent of wrinkling is correlated with the rate of film conversion and that ultrasmooth films are obtained by slowing the rate of fil...

Insulating and passivating plasma-enhanced atomic layer deposited aluminum oxide thin films for silicon solar cells

Plasma-enhanced atomic layer deposited (PEALD) aluminum oxide (Al2O3) thin film were investigated with respect to their application in silicon solar cells where not only excellent surface passivation but also excellent electrical insulation properties are required. In this study, the insulation properties of about 10 nm to almost 250 nm Al2O3 thin films, deposited at temperatures in the range of 100 °C to 350 °C and annealed in the range of 250 °C to 450 °C, were correlated to various thin film properties, most notably the thin film defect density, i. e. pinhole density, and the thin film stress. Al2O3 thin films provide sufficient electrical insulation after a post-deposition annealing step at 450 °C when deposited at 250 °C with thickness of at least 80 nm which can be attributed to a low defect density and low stress in the thin films, allowing a low leakage current density and a high breakdown voltage that was independent of the type of metals used for the electrode. Al2O3 thin films deposited and annealed at these temperatures enabled a very good surface passivation of silicon. These results indicate that Al2O3 thin films realized by PEALD are a promising candidate for silicon solar cells that rely on passivating as well as insulating thin films.

(Invited) Residual Stress Evolution during Thin Film Growth: Kinetic Modeling and Monte Carlo Simulations

To reduce failure and enhance performance, it would be desirable to predict and control the residual stress in thin films. This requires a fundamental understanding of how the stress is related to the underlying processes controlling film growth. We describe the results of rate equation models and computer simulations focused on advancing this understanding. The rate equation models explore the competition between different stress-inducing mechanisms occurring simultaneously during growth. Tensile stress is induced by the interaction between islands as they merge to form new segments of grain boundary (island coalescence). At the same time, compressive stress is induced by diffusion of atoms into the grain boundary driven by the non-equilibrium conditions on the surface. The stress can be further modified by densification of the film due to grain growth. The model predictions for the dependence of the stress on growth rate, diffusivity and grain size are compared with measurements for systems with different types of microstructural evolution. To understand how the surface kinetics are related to the stress, we have also developed kinetic Monte Carlo simulations that include similar mechanisms for generating stress at the grain boundaries. This approach enables us to look at the stochastic effect of individual atom dynamics on the stress evolution. For instance, we can study how supersaturation on the surface due to the deposition flux acts as a driving force for compressive stress. Results for the evolution of stress at different growth rates, temperatures and grain sizes will be shown.

The Effect of Different Additives in the in-Situ Stress of Thin Co Films

Fe, Co and Ni (and their alloys) are widely used due to their interesting ferromagnetic, chemical, and physical properties. Electrodeposited thin films tend to develop sizable mechanical stresses as a result of the nucleation and growth process. Often, these stresses can approach or exceed the yield stress of the bulk material and can lead to loss of adhesion and the generation of bulk and surface defects. Models have been developed that treat steady-state stress as a dynamic competition between tensile and compressive stress generation mechanisms that are largely governed by atomic mobility, microstructure, and deposition rate. Although additives are often used to mitigate residual stress during thin film growth, the exact mechanisms by which these additives influence residual stress is an active area of research. In this talk, we examine the growth stress associated with electrodeposition of Co thin films onto (111)-textured Au cantilever electrodes from 0.5 M Na2SO4 containing 0.1 M CoSO4 and 0.5 M H3BO3. The stress was measured with an optical cantilever curvature technique that can be used during deposition to determine the real-time stress evolution. Deposition was also examined using an electrochemical quartz crystal microbalance (EQCM) in order to determine the current efficiency as a function of both overpotential and deposition time. We have measured the average biaxial film stress as a function of overpotential and observe a significant increase in tensile stress as the deposition potential is made more negative. These results are discussed in the context of stress generating models that appear in the literature. The growth stress was also examined from 0.5 M Na2SO4 + 0.1 M CoSO4 electrolyte with controlled additions of glycine and boric acid. Each additive was studied under a specific potential window, as determined by the observed influence the additives have on Co deposition. Glycine acts as a buffer, maintaining a stable H+ concentration. The buffering product glycinate (Gly-) complexes with Co2+, thereby shifting the deposition window. Baths with boric acid have an average current efficiency between 40 and 90%, produce deposits with high tensile stress which undergo little postdeposition relaxation. Current efficiency from glycine baths range between 20 and 70%. Deposits generally have lower tensile stress and exhibit additional post-deposition stress relaxation.

Molecular dynamics simulation of temperature effects on deposition of Cu film on Si by magnetron sputtering

The temperature effects on the growth of Cu thin film on Si (0 0 1) in the context of magnetron sputtering deposition were systematically studied using molecular dynamics (MD) method. To improve the comparability of simulation results at varying temperatures, the initial status data of incident Cu atoms used in all simulations were read from an identical file via LAMMPS-Python interface. In particular, crystalline microstructure, interface mixing and internal stress of Cu thin film deposited at different temperatures were investigated in detail. With raising the substrate temperature, the interspecies mixed volume and the proportion of face-centered cubic (fcc) structure in the deposited film both increased, while the internal compressive stress decreased. It was found that the fcc structure in the deposited Cu thin films was 〈1 1 1〉 oriented, which was reasonably explained by surface energy minimization and the selectivity of bombardment energy to the crystalline planes. The quantified analysis of interface mixing revealed that the diffusion of Cu atoms dominated the interface mixing, and the injection of incident Cu atoms resulted in the densification of phase near the film-substrate interface. More important, the distribution of atomic stress indicated that the compressive stress was mainly originated from the film-substrate interface, which might be attributed to the densification of interfacial phase at the initial stage of film deposition.

Effect of substrate on texture and mechanical properties of Mg–Cu–Zn thin films

In this work, thin films of Mg–Cu–Zn with 60 nm thicknesses have been deposited on the Si(100), Al, stainless steel, and Cu substrates using DC magnetron sputtering. FESEM images displayed uniformity of Mg–Cu–Zn particles on the different substrates. AFM micrograph revealed the roughness of thin film changes due to the different kinds of the substrates. XRD measurements showed the existence of strong Mg (002) reflections and weak Mg (101) peaks. Residual stress and adhesion force have been measured as the mechanical properties of the Mg–Cu–Zn thin films. The residual stresses of thin films which have been investigated by X-ray diffraction method revealed that the thin films sputtered on the Si and Cu substrates endure minimum and maximum stresses, respectively, during the deposition process. However, the force spectroscopy analysis indicated that the films grew on the Si and Cu experienced maximum and minimum adhesion force. The texture analysis has been done using XRD instrument to make pole figures of Mg (002) and Mg (101) reflections. ODFs have been calculated to evaluate the distribution of the orientations within the thin films. It was found that the texture and stress have an inverse relation, while the texture and the adhesion force of the Mg–Cu–Zn thin films have direct relation. A thin film that sustains the lowest residual stresses and highest adhesive force had the strongest {001} basal fiber texture.

Evaluation of residual stress and adhesion of Ti and TiN PVD films by laser spallation technique

The laser spallation technique was applied for measurement of residual stress and adhesion of thin films. Two films of different properties, ductile and soft Ti, and hard and brittle TiN, were studied. The films were produced on 304 steel substrate by PVD method. The residual stress value obtained by laser spallation technique LST were compared with stress value from X-ray diffraction method. Good agreement of stress values measured by both methods was attained. Additionally, the interface strength of the films was tested by laser adhesion spallation technique LASAT with use of VISAR system. It was shown that shock wave induced by a nanosecond laser pulse adequately determines properties of PVD thin films on metal substrate.

Tailoring stress in pyrolytic carbon for fabrication of nanomechanical string resonators

In order to achieve high resonance frequencies and quality factors of pyrolytic carbon MEMS string resonators the resonator material needs to have a large tensile stress. In this study, the influence of pyrolysis temperature, dwell time and ramping rate on the residual stress in thin pyrolytic carbon films is investigated with the bending plate method. The results show that the pyrolysis temperature is the most important parameter for tailoring the residual stress, with a transition from tensile stress at temperature below 800ºC to compressive stress at temperatures above 800ºC. Two kinds of photoresist: positive (AZ5214E) and negative (SU-8) and different pyrolysis conditions are used to fabricate pyrolytic carbon string resonators at variable pyrolysis conditions. The best performance is obtained for devices with a length of 400 μm fabricated at a pyrolysis temperature of 700ºC, ramping rate of 30ºC/min and 10 min dwell time corresponding to the conditions for maximum tensile stress in pyrolytic carbon thin films. The optimized pyrolytic carbon string resonators have resonant frequencies above 300 kHz and quality factors (Q) in the order of 104, which is suitable for their application as nanomechanical sensors.

Residual strain measurement of Cu2ZnSnS4 thin film using X-ray line broadening

ABSTRACTIn this study, the residual strain in Cu2ZnSnS4 (CZTS) photoabsorber for solar cell application was analyzed by X-ray line broadening. Metal precursor of CZTS was deposited on a glass substrate using the co-sputtering method with Cu, Zn, and Sn single targets at a substrate temperature of 100°C. Then, the metal precursor was sulfurized within a previously evacuated corning tube that at the present stage contained sulfur powder at an annealing temperature of 550°C for 10 min. The structural properties of a CZTS thin film were analyzed by performing X-ray diffraction. The main peaks, (112), (220), and (312) were clearly visible. The residual strain was determined by using a Williamson–Hall (W–H) plot, which is a very convenient tool for determining the residual strain of thin films. The slope of the graph indicates the residual strain of the CZTS thin film, and it is determined as 2.831 × 10− 4. Using the W–H plot, it was concluded that the residual stress of CZTS thin film is tensile.

Review Article: Stress in thin films and coatings: Current status, challenges, and prospects

The issue of stress in thin films and functional coatings is a persistent problem in materials science and technology that has congregated many efforts, both from experimental and fundamental points of view, to get a better understanding on how to deal with, how to tailor, and how to manage stress in many areas of applications. With the miniaturization of device components, the quest for increasingly complex film architectures and multiphase systems and the continuous demands for enhanced performance, there is a need toward the reliable assessment of stress on a submicron scale from spatially resolved techniques. Also, the stress evolution during film and coating synthesis using physical vapor deposition (PVD), chemical vapor deposition, plasma enhanced chemical vapor deposition (PECVD), and related processes is the result of many interrelated factors and competing stress sources so that the task to provide a unified picture and a comprehensive model from the vast amount of stress data remains very challenging. This article summarizes the recent advances, challenges, and prospects of both fundamental and applied aspects of stress in thin films and engineering coatings and systems, based on recent achievements presented during the 2016 Stress Workshop entitled “Stress Evolution in Thin Films and Coatings: from Fundamental Understanding to Control.” Evaluation methods, implying wafer curvature, x-ray diffraction, or focused ion beam removal techniques, are reviewed. Selected examples of stress evolution in elemental and alloyed systems, graded layers, and multilayer-stacks as well as amorphous films deposited using a variety of PVD and PECVD techniques are highlighted. Based on mechanisms uncovered by in situ and real-time diagnostics, a kinetic model is outlined that is capable of reproducing the dependence of intrinsic (growth) stress on the grain size, growth rate, and deposited energy. The problems and solutions related to stress in the context of optical coatings, inorganic coatings on plastic substrates, and tribological coatings for aerospace applications are critically examined. This review also suggests strategies to mitigate excessive stress levels from novel coating synthesis perspectives to microstructural design approaches, including the ability to empower crack-based fabrication processes, pathways leading to stress relaxation and compensation, as well as management of the film and coating growth conditions with respect to energetic ion bombardment. Future opportunities and challenges for stress engineering and stress modeling are considered and outlined.

Residual Stress in Cr99Al1 Polycrystalline Thin Films

Residual Stress in Cr₉₉Al₁ Polycrystalline Thin Films

Effect of residual stress in Pb (Zr0.52Ti0.48) O3 – BiFeO3 multilayer cantilever structure

This paper discussed about the integration issues of Pb (Zr0.52Ti0.48) O3 – BiFeO3 (PZT - BFO) multilayer thin film deposited on silicon substrate for possible application in future micro-electro-mechanical system (MEMS) devices. The PZT - BFO multilayer thin film is deposited on silicon wafer by sol-gel technique. The multilayer film is annealed at 650 °C in air for 60 min. The deposited multilayer film is found to be polycrystalline in nature. The PZT-BFO multilayer exhibited room temperature multiferroic properties (remnant polarization of 37 mC/ cm 2 and remnant magnetization of 3.1 emu / cm 3 ). To fabricate the PZT - BFO multiferroic cantilever structures, a two-mask process flow is developed. Etch rates of the PZT - BFO multilayer (180 nm/ min), ZrO2 buffer layer (35 nm/ min) and SiO2 layer (350 nm/ min) are optimized in CHF3 plasma. The multiferroic cantilever structures are released by isotropic etching of silicon using SF6 plasma. Bending and cracks are observed in the released cantilever structures due to the generation of residual stress in the multilayer thin film. Effect of residual stress on the PZT - BFO cantilever structure is also verified by simulation.

Spectroscopic Analysis of Film Stress Mechanism in PECVD Silicon Nitride

We investigated the stoichiometry of silicon nitride film deposited by plasma-enhanced chemical vapor deposition (PECVD). Lowering the NH3/SiH4 gas flow ratio resulted in slightly Si-rich film due to the lack of reactive nitrogen in the plasma. It may reduce the film stress, but the refractive index of Si-rich nitride film can be increased by 50% at its maximum. The aforementioned stress adjustment methods are made practically useful by performing multiple experiments to achieve immediate stress release. It suffers from figures of merit of the PECVD-deposited silicon nitride film. Thin film stress was measured both before and after deposition, and the stoichiometry of deposited film on wafer was measured by Fourier transform-infrared (FT-IR) spectroscopy, while plasma chemistry and its conditions were monitored in real time during the deposition process. The purpose of this study was to correlate the optically measured film stress with the stoichiometry of the deposited film and plasma chemistry. Nitrogen-rich plasma processes tend to have higher tensile stress, and FT-IR measurements are in accordance with the measured mechanical stresses of the wafers.

Theoretical Study on Synchronous Characterization of Surface and Interfacial Mechanical Properties of Thin-Film/Substrate Systems with Residual Stress Based on Pressure Blister Test Technique.

In this study, based on the pressure blister test technique, a theoretical study on the synchronous characterization of surface and interfacial mechanical properties of thin-film/substrate systems with residual stress was presented, where the problem of axisymmetric deformation of a blistering film with initial stress was analytically solved and its closed-form solution was presented. The expressions to determine Poisson’s ratios, Young’s modulus, and residual stress of surface thin films were derived; the work done by the applied external load and the elastic energy stored in the blistering thin film were analyzed in detail and their expressions were derived; and the interfacial adhesion energy released per unit delamination area of thin-film/substrate (i.e., energy release rate) was finally presented. The synchronous characterization technique presented here has theoretically made a big step forward, due to the consideration for the residual stress in surface thin films.

基底温度和离子源能量对薄膜应力的影响

基底温度和离子源能量对薄膜应力的影响

A case study of thin film stress evolution at a dissimilar material interface via molecular dynamics simulations

Evolution of deformation and stress in growing thin films has been studied in this work using computational simulations that resolve matter at atomic length and time scales. For the surface layers of films laying on the substrate of a dissimilar material, the stress distribution analysis around defects becomes more challenging. Herein, spatial and temporal distribution of deformation and associated stress evolution are presented for different thin film formation events including (1) sub-monolayer growth during an early film nucleation stage and (2) coalescence of adjacent monolayer “islands.” Validity of the stress computed via local computations of the virial expression for stress in a system of interacting particles was checked by comparing to results obtained from considerations of local atomic deformation in conjunction with existing expressions for epitaxial thin film growth stress. For the geometries studied here, where a monolayer of film with a highly characterized linear defect, as in the case of a stacking fault, was simulated for coalescence, fairly good agreement was found. This result demonstrates that, for similar defects at the surface layer, with sufficient sub-ensemble averaging of the standard virial expression for stress, semiquantitative spatial stress distribution information can be obtained from atomic scale simulations. Using our validated stress computation method, we reveal significant stress localization during thin film growth processes, leading to pronounced differences in maximum and minimum stress observed over very small spatial extent (of order multiple GPa over 3–6 nm distances). One prominent mechanism of stress localization revealed here is coalescence between adjacent growing islands. For geometries explored here, stress manifesting during coalescence is highly localized.

Si micro-cantilever sensor chips for space-resolved stress measurements in physical and plasma-enhanced chemical vapour deposition

The control of extrinsic and intrinsic mechanical stresses in thin films is crucial. Stresses can limit the film performance e.g. by stress-induced delamination or undesired bending of film/substrate combinations; however, stresses can also be used to obtain functionality. Thus, understanding of stress-inducing mechanisms, correlations of stress with film synthesis parameters and controlling the sign and amplitude of stresses in thin films is important and a facile and reliable stress measurement method is necessary. Here, a stress measurement chip is presented which is based on the measurement of the residual overall film stress by a film-substrate combination curvature-based measurement technique. The novel Si-based cantilever sensor chip can measure residual stress in films from a few nanometers thickness up to several microns. Moreover, the sensor chips are applicable for determining the coefficient of thermal expansion, and for examining the film thickness homogeneity over large areas in a deposition system. They can be applied in physical vapor deposition and plasma-enhanced chemical vapor deposition processes with different geometrical and process-related boundary conditions. Exemplary results which were obtained with the sensor chips are discussed to demonstrate their easy applicability, accuracy, versatility, reliability, the thickness dependence of the residual stress and the homogeneity of SiOx films as well as the residual stress and the thermal expansion values of Al-Cr-N films.

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.