Residual Stress In Thin Films Research Articles

Stretchable Substrates with 3D Wave Patterned Surface for Enhanced Mechanical Stability of Indium Tin Oxide Electrodes

AbstractFlexible electronic devices have promising applications in many future technologies such as wearables, implantables, robotics, and displays. Among the distinct types of mechanical flexibility, stretchability stands as a significant challenge. A particularly demanding objective is the realization of a high‐performance transparent electrode that endures stretching and can be mass produced, all while avoiding additional restrictions on device density. In this work, it is demonstrated that a 3D wave patterned surface provides a threefold improved strain performance of deposited indium tin oxide electrodes, in a statistical comparison of 3D wave patterned and planar surfaces, where indium tin oxide electrodes are stretched to electrical failure. Moreover, this platform alleviates residual thin film stress, allowing for easier handling of the substrates. This study demonstrates the feasibility of attaining stretchability for upcoming electronic devices using a scalable platform that incorporates high‐performance transparent electrode materials using only conventional materials and fabrication steps.

Computational tool for analyzing stress in thin films

A computational model has been developed to analyze calculations of residual stress in thin films determined from in-situ wafer curvature measurements. The model is based on several physical mechanisms that have been proposed to control the stress, including the effect of growth kinetics, grain growth and energetic particle bombardment. From a set of data, the program determines a set of kinetic parameters that can be used to understand the processes controlling stress. These parameters can be used to estimate the stress that will develop under different processing conditions in order to obtain a desired stress state. Several examples are described that illustrate the program's capabilities, including the effects of growth rate, growth temperature and particle energy (via chamber pressure in sputter deposition). Interested researchers may obtain the program and associated documentation to use the program to analyze their own data.

Nonlinear restructuring of patterned thin films by residual stress engineering into out-of-plane wavy-shaped electrostatic microactuators for high-performance radio-frequency switches

Electrostatic microelectromechanical (MEMS) switches are the basic building blocks for various radio-frequency (RF) transceivers. However, conventional cantilever-based designs of MEMS switches require a large actuation voltage, exhibit limited RF performance, and suffer from many performance tradeoffs due to their flat geometries restricted in two dimensions (2D). Here, by leveraging the residual stress in thin films, we report a novel development of three-dimensional (3D) wavy microstructures, which offer the potential to serve as high-performance RF switches. Relying on standard IC-compatible metallic materials, we devise a simple fabrication process to repeatedly manufacture out-of-plane wavy beams with controllable bending profiles and yields reaching 100%. We then demonstrate the utility of such metallic wavy beams as RF switches achieving both extremely low actuation voltage and improved RF performance owing to their unique geometry, which is tunable in three dimensions and exceeds the capabilities of current state-of-the-art flat-cantilever switches with 2D-restricted topology. As such, the wavy cantilever switch presented in this work actuates at voltages as low as 24 V while simultaneously exhibiting RF isolation and insertion loss of 20 dB and 0.75 dB, respectively, for frequencies up to 40 GHz. Wavy switch designs with 3D geometries break through the design limits set by traditional flat cantilevers and provide an additional degree of freedom or control knob in the switch design process, which could enable further optimization of switching networks used in current 5G and upcoming 6G communication scenarios.

Optical Interference Filters Combined with Thin Film Residual Stress Compensation for Image Contrast Enhancement

We propose two single-wavelength notch filters and one dual-wavelength (480 and 620 nm) notch filter to enhance image contrast. The stack structure of the notch filters was designed as (Ta2O5/SiO2)4Ta2O5 in Essential Macleod thin film simulation software. Dual-electron-beam evaporation with ion beam-assisted deposition was used to prepare optical interference filters with different center wavelengths. A multilayer notch filter with a center wavelength of 620 nm was deposited on the front surface of the glass, and then a notch filter with a center wavelength of 480 nm was coated on the rear surface of the same glass. The proposed dual-wavelength (480 and 620 nm) notch filter is a combination of two single-wavelength notch filters coated on a double-sided glass substrate to compensate for residual stress. The transmittance, residual stress, and surface roughness of the proposed notch filter were evaluated using different measuring instruments. The experimental results show that the residual stress of the dual-wavelength notch filter could be reduced to 10.8 MPa by using a double-sided coating technique. The root-mean-square (RMS) surface roughness of the notch filters was measured by using a Linnik microscopic interferometer. The RMS surface roughness was 1.80 for the 620 nm notch filter and 2.09 for the 480 nm notch filter. The image contrast obtained with the three different notch filters was measured using an optical microscope and a CMOS camera. The contrast value could be increased from 0.328 (without a filter) to 0.696 (dual-wavelength notch filter).

Review Paper: Residual Stresses in Deposited Thin-Film Material Layers for Micro- and Nano-Systems Manufacturing.

This review paper covers a topic of significant importance in micro- and nano-systems development and manufacturing, specifically the residual stresses in deposited thin-film material layers and methods to control or mitigate their impact on device behavior. A residual stress is defined as the presence of a state of stress in a thin-film material layer without any externally applied forces wherein the residual stress can be compressive or tensile. While many material properties of deposited thin-film layers are dependent on the specific processing conditions, the residual stress often exhibits the most variability. It is not uncommon for residual stresses in deposited thin-film layers to vary over extremely large ranges of values (100% percent or more) and even exhibit changes in the sign of the stress state. Residual stresses in deposited layers are known to be highly dependent on a number of factors including: processing conditions used during the deposition; type of material system (thin-films and substrate materials); and other processing steps performed after the thin-film layer has been deposited, particularly those involving exposure to elevated temperatures. The origins of residual stress can involve a number of complex and interrelated factors. As a consequence, there is still no generally applicable theory to predict residual stresses in thin-films. Hence, device designers usually do not have sufficient information about the residual stresses values when they perform the device design. Obviously, this is a far less than ideal situation. The impact of this is micro- and nano-systems device development takes longer, is considerably more expensive, and presents higher risk levels. The outline of this paper is as follows: a discussion of the origins of residual stresses in deposited thin-film layers is given, followed by an example demonstrating the impact on device behavior. This is followed by a review of thin-film deposition methods outlining the process parameters known to affect the resultant residual stress in the deposited layers. Then, a review of the reported methods used to measure residual stresses in thin-films are described. A review of some of the literature to illustrate the level of variations in residual stresses depending on processing conditions is then provided. Methods which can be used to control the stresses and mitigate the impact of residual stresses in micro- and nano-systems device design and fabrication are then covered, followed by some recent development of interest.

(Invited) Understanding Residual Stress in Thin Films through a Kinetic Model

Stress in thin films is a critical issue that affects the performance and lifetime of coatings. Multiple parameters have been shown to affect the stress evolution, including the growth rate, temperature, grain size and gas pressure. In addition, the deposition method (e.g., evaporation, electrodeposition, sputtering) is known to have an influence. To develop a unified picture of the processes controlling stress, a kinetic model has been developed that includes the role of different processing parameters and the microstructural evolution. Results are shown from applying the model to numerous published results for transition metals deposited by different methods. Trends in the fitting parameters are discussed in terms of the values expected from the physical mechanisms included in the model.

67‐3: Predicting the Impact Resistance of Flexible Display Panels Based on Mo Thin‐Film Residual Stress

In the present work we developed advanced system for not only diagnosing but also predicting the impact resistance of flexile display panel by evaluating residual stress of Mo thin film with non‐destructive X‐ray diffraction. The impact resistance property of flexible display was dominantly affected by the residual stress of Mo thin film. When Mo thin film has the compressive residual stress, the panel has a tolerable impact resistance. In contrast, the panel has a poor impact resistance with tensile residual stressed Mo thin film. Therefore, precisely controlled the Mo thin film residual stress can be able to design robust flexible display panels.

The effect of RF plasma power on remote plasma sputtered AZO thin films

Aluminium-doped ZnO (AZO) thin films were deposited by remote plasma sputtering of a ZnO:Al2O3 98:2 wt% ceramic target in a pulsed DC configuration. The target power was kept constant at 445 W and the RF plasma power was varied between 0.5 and 2.5 kW. The as-deposited AZO thin films exhibited an optimum resistivity of 6.35 × 10−4 Ω·cm and optical transmittance of 92% at a RF plasma power of 1.5 kW. The thin film microstructure, chemical composition, and residual stress were investigated using SEM, RBS, XPS and XRD. Accurate determination of the chemical composition and correct interpretation of GIXRD data for AZO thin films are a particular focus of this work. The AZO layer thickness was 500–700 nm and Al content in the range of 2.3–3.0 at.%, determined by RBS. The AZO thin films exhibited a strong (002) preferential orientation and grain sizes between 70 and 110 nm. The (103) peak intensity enhancement in GIXRD is proven to be a result of the strong (002) preferential orientation and GIXRD geometrical configuration rather than a change in the crystallite orientation at the surface. XPS depth profiles show preferential sputtering of O and Al using a 500 eV Ar+ beam, which can be reduced, but not eradicated using an 8 keV Ar150+ beam. The preferential sputtering can be successfully modelled using the simulation software TRIDYN. A plasma power of 1.5 kW corresponds to a highly ionised plasma and various microstructural and compositional factors have all contributed to the optimum low resistivity occurring at this plasma power. The grain size exhibits a maximum in the 1.25–1.5 kW range and there is improved (002) orientation, minimising grain boundary scattering. The highest carrier concentration and mobility was observed at the plasma power of 1.5 kW which may be associated with the maximum in the aluminium doping concentration (3.0 at.%). The lowest residual stress is also observed at 1.5 kW.

Crystal orientation and residual stress in TiN film by synchrotron radiation

Abstract The residual stress in a TiN film, the thickness of which is less than 1 µm, deposited by arc ion plating was measured using the SPring-8 synchrotron radiation facility installed at Japan Synchrotron Radiation Research Institute (JASRI). Films thicker than 300 nm have a {111} texture, whereas those with a thickness of 100 nm have a relatively random orientation. Instead of the conventional laboratory X-ray equipment, ultrabright synchrotron radiation can be used to precisely measure the residual stress in thin films with a thickness of 100 to 800 nm. The residual stress in thicker TiN films is about 7 GPa in compression and it is not affected by the film thickness. The slightly small compressive stress in a film of 100 nm in thickness may be due to the different crystal orientations in the film.

Optical and photoluminescence studies of precursor stabilised Aluminium-Gallium Zinc oxide thin films

A low cost, versatile, and cost-effective ultrasonic spray pyrolysis approach was used to generate aluminium and gallium doped Zinc Oxide thin films on stretchable corning glass substrates at 400 °C temperature. Ammonium acetate is employed as a stabilizing ingredient in the precursor solution. According to X-ray diffraction patterns, high quality polycrystalline films on glass substrates grew successfully. As Ga doping levels rose, photoluminescence spectra showed an increase in exciton peak emission. As time passes, photoluminescence spectra show a strong emission peak centred at 3.24 eV that is progressively pushed toward higher wavelength. When Aluminum and Gallium were added into the Zinc Oxide lattice, the crystalline size was reduced and the thin film residual stress was raised. In the visible area, all films were extremely clear, with an average transparency of 80%. The optical energy band gap increased from 3.12 eV to 3.3 eV when the doping concentration increased.

Residual Film Stresses in Perovskite Solar Cells: Origins, Effects, and Mitigation Strategies.

Metal halide perovskites are an emerging class of materials that are promising for low-cost and high-quality next-generation optoelectronic devices. Despite this potential, perovskites suffer from poor thermomechanical and chemical stability that must be overcome before the technology is commercially viable. Key sources of the instabilities in perovskites are ion migration and defects that can be tied to high residual stresses accumulated in the perovskite thin films during processing. This Mini-Review serves as a general overview of residual thin-film stresses, specifically in perovskite solar cells. A brief introduction to the origin of residual stresses in thin films is followed by the effects of these stresses in perovskite films specifically. Mitigation strategies for these stresses are then highlighted, followed by potential avenues of further exploration of residual stresses in perovskite films.

Understanding residual stress in thin films: Analyzing wafer curvature measurements for Ag, Cu, Ni, Fe, Ti, and Cr with a kinetic model

An analytical model for the evolution of residual stress in polycrystalline thin films is used to analyze numerous previously reported wafer curvature measurements obtained for a variety of materials and processing conditions. The model, which has been described in previous publications, considers stress-generating mechanisms that occur at the grain boundary as it forms between adjacent grains and stress due to the subsurface grain growth in layers that have already been deposited. Current work extends the model to include different types of microstructural evolutions. A set of parameters for each dataset is obtained by non-linear least square fitting. Model parameters that are not expected to depend on the processing conditions are constrained to have a common value when fitting the multiple datasets for each material. The dependence of the fitting parameters on the material and process conditions is evaluated and compared with the physical mechanisms implemented in the model.

On the PZT/Si unimorph cantilever design for the signal-to-noise ratio enhancement of piezoelectric MEMS microphone

This study presents the design, fabrication, and testing of a novel piezoelectric micro-electro-mechanical-systems microphone with the partially removed lead zirconate titanate (PZT) unimorph structure for signal-to-noise ratio (SNR) enhancement. To demonstrate the feasibility of the presented concept, the unimorph cantilever array with the patterned (triangular shape) PZT film on top of a rectangular Si layer is simulated, fabricated, and tested. In comparison, the rectangular and triangular unimorph cantilever arrays with fully covered PZT film are also investigated. The proposed and reference cantilever arrays have the same foot print for fair comparison. Simulations show the proposed design has the highest output electrical energy which indicates the proposed design could successfully enhance the SNR. Moreover, acoustic measurements in the standard anechoic box show the proposed Si cantilever with patterned PZT film design could significantlyimprove the SNR. As compared to the reference microphone with triangular PZT/Si unimorph cantilever, the proposed design shows a 9.8 dB enhancement in SNR. Finally, the influences of initial structure deflections (due to the thin film residual stresses) on the low frequency responses for proposed and reference microphone designs are also investigated.

Large-Area and Ultrathin MEMS Mirror Using Silicon Micro Rim.

A large-area and ultrathin MEMS (microelectromechanical system) mirror can provide efficient light-coupling, a large scanning area, and high energy efficiency for actuation. However, the ultrathin mirror is significantly vulnerable to diverse film deformation due to residual thin film stresses, so that high flatness of the mirror is hardly achieved. Here, we report a MEMS mirror of large-area and ultrathin membrane with high flatness by using the silicon rim microstructure (SRM). The ultrathin MEMS mirror with SRM (SRM-mirror) consists of aluminum (Al) deposited silicon nitride membrane, bimorph actuator, and the SRM. The SRM is simply fabricated underneath the silicon nitride membrane, and thus effectively inhibits the tensile stress relaxation of the membrane. As a result, the membrane has high flatness of 10.6 m−1 film curvature at minimum without any deformation. The electrothermal actuation of the SRM-mirror shows large tilting angles from 15° to −45° depending on the applied DC voltage of 0~4 VDC, preserving high flatness of the tilting membrane. This stable and statically actuated SRM-mirror spurs diverse micro-optic applications such as optical sensing, beam alignment, or optical switching.

Ceramic buckling for determining the residual stress in thin films

A new technique for determination of residual stress in thin films and coatings has been presented. The method consists of focused ion beam milling to create a lamella of thin film, followed by analysis of stress driven buckling profile of undercut lamella (beam) to extract residual stress. The residual stresses in crystalline TiN and Al2O3 films, produced by reactive magnetron sputtering and thermal oxidation, respectively, have been successfully determined by this new technique and validated by conventional X-ray diffraction and photoluminescence piezospectroscopy techniques, respectively. This new technique successfully measures and tracks the evolution of residual stress in as-deposited and different thermally cycled amorphous SiAlN films induced by thermal mismatch or relieved via mechanical twinning in interlayer, where diffraction methods are not applicable, thereby evaluating the thermal cycling performance of amorphous SiAlN coatings for protection of Ti at high temperature.

Modeling of elastoplastic behavior of freestanding square thin films under bulge testing

Bulge testing is an experimental technique applied to the characterization of thin films, with which the mechanical properties are obtained employing numerical and analytical approaches. For rectangular and circular thin films, classical methods have been well adopted to calculate the stresses and strains at the central point without the dependency of its properties. It is due to that the equilibrium conditions are dependent on the curvatures generated at the maximum deflection point. Therefore, the kinematics of the bulged surface can be represented by a spherical cap and cylindrical shapes in these films. For square thin films, the deflected surface presents a complex displacement field that implies the development of analytical models for the estimation of these. In this sense, equations to represent the equibiaxial stress state (central point) for freestanding square thin films are developed and applied in this study. The curvatures are determined using a reviewed nonlinear deflection field. Virtual experiments were driven by applying finite element analysis to validate the stress and strain models through parametric analysis. In order to impose the residual stress in thin films, two approaches of modeling for the simulations are described. In order to validate the applicability of the proposed equations, different cases were presented for the linear elastic analysis, and two examples were developed to characterize elastoplastic behavior (bilinear and nonlinear) from load–deflection curves. The obtained results show that the developed models for square thin films are in good agreement with the finite element simulations since these predicted the stress and strain states with good accuracy.

Nonparametric estimation of SiC film residual stress from the wafer surface profile

Thin film residual stress is proportional to substrate curvature change after film deposition, based on Stoney’s equation. Curvature is approximately equal to the second derivative of substrate deflection. It is common to apply Stoney’s equation locally, where the residual stress at a selected point and direction is estimated from the local substrate curvature. The locally weighted least squares regression method (LowLSR) is adapted for estimating the substrate curvature in the radial directions across the wafer from the corresponding deflection profiles measured by a profilometer. LowLSR is implemented in the R package npregderiv developed by the authors. The changing film thickness profiles in the radial directions are estimated using the local linear estimator and plugged into Stoney’s equation. Thus, this research is the first attempt of using nonparametric statistical methods to estimate the residual stress in SiC films with non-uniform thickness.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

CMOS-MEMS capacitive tactile sensor with vertically integrated sensing electrode array for sensitivity enhancement

This study presents the design, fabrication, and testing of a novel capacitive type CMOS-MEMS tactile sensor with vertically integrated sensing structures for enhancing sensitivity, and discretized sensing array design for inhibiting residual stress warpage. The proposed type tactile sensor utilizes polymer buffer above the sensor and polymer fill-in between the vertically integrated sensing structures as force transmission layers. This enables effective force loading onto the sensor, and more importantly, simultaneous deformation of the two vertically integrated sensing structures during force loading, thereby guaranteeing an increase in sensitivity. The enhancement in sensitivity of the proposed tactile sensor was demonstrated to be achieved under the same footprint area. To mitigate warpage issues that may occur due to thin film residual stresses, the sensor was discretized into arrays of separate sensing units. Sensing units are electrically connected in a parallel fashion to ensure maximum capacitance signal. As compared to a reference tactile sensor with conventional sensing electrodes, the proposed design showed a near 1.3-fold enhancement in sensitivity. Measurements also indicate the 5 × 5 array design could tolerate the influence of residual stress. Additionally, effect of operating temperature on initial capacitance (temperature coefficient of offset, TCO) and capacitance change signal during force loadings (temperature coefficient of sensitivity, TCS) were investigated for both the reference and proposed type tactile sensors.

The early stages in the nucleation process and residual stress of electrodeposited [formula omitted] on Si(111)

In this work we investigated the early stages in the nucleation process and the residual stress of thin films of CoxFe100−x grown directly on n-Si(111) substrates. The films were prepared potentiostatically at the composition of x=70 and x=90 atomic %. The morphology and growth were investigated by scanning electron microscopy at deposition time ranging from 0.5 s to 5 s. Our findings reveal for both the compositions hemispherical nuclei growing from hemispherical centers. The crystallographic characterization and residual stress analysis by X-ray diffraction reveal structural phase transition from a body-centered cubic (bcc) to face-centered cubic (fcc) structure by increasing the amount of Co in the alloy and a compressive regime of strain in the deposits, respectively.