Stress In Thin Films Research Articles

Stress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling.

The intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm).

Stress Formation During In - Ga Interdiffusion in Thin-Film CuIn1−xGaxSe2 Absorber Layers Leads to Stable Ga Gradients

To optimize the opto-electronic properties of compound semiconductors, a detailed understanding and control of compositional gradients forming during their synthesis is crucial. A common fabrication process for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) thin-film solar cells uses annealing at high temperatures, which---contrary to what could be expected from simple Fickian diffusion---results in the formation of steep and stable $\mathrm{Ga}$ gradients, deviating from the optimal $\mathrm{Ga}$ profile for high-efficiency CIGS absorbers. Here, we show that the formation of elastic stresses inside the material during the interdiffusion can have a profound effect on the final $\mathrm{Ga}$ distribution, resulting in persistent $\mathrm{Ga}$ gradients inside CIGS absorber layers. A comparison of numerical diffusion and stress-formation calculations with real-time synchroton-based energy-dispersive x-ray diffraction data acquired in-situ during selenization of CIGS thin films demonstrates that the model can reproduce the stagnation of $\mathrm{In}$-$\mathrm{Ga}$ interdiffusion. We discuss that a detailed understanding of the interplay between stress and diffusion processes in thin films may open alternative fabrication strategies for generating desired stable compositional gradients to improve the opto-electronic properties of compound semiconductors, such as chalcopyrite, kesterite, and perovskite solar cell absorbers for solar cells.

Structural and chemical network studies of thermal chemical vapour deposited ZnO nanostructured thin films

In this work, we have investigated the role of process temperature on microstructural, structural, vibrational and phonon behaviour of Zinc Oxide Nanostructured (ZnO-NS) thin films deposited via thermal chemical vapour deposition reactor (TCVD) on Si substrate. The various spectroscopic methods such as Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) and Raman Spectroscopy were used to evaluate its morphology, structural, vibrational and phononic behaviour. We have observed that the change in process temperature considerably modifies the inherent material properties of ZnO-NS thin films. The AFM result revealed that the density distribution of agglomerated ZnO-nanograins increases with increasing process temperature from 650° C to 750° C. XRD result revealed that the average crystallites size of ZnO-NS thin films was decreased from 22.35 to 12.47 nm with increasing process temperature. The lattice strain and stress in ZnO-NS thin films decrease with increasing process temperature. The FTIR spectra revealed the decreasing intensity of ZnO stretch with increasing process temperature indicates a change in defect distribution of nanostructure at the higher temperature. The Zn-O–H bond shows asymmetry towards the lower wavenumber region can be attributed to the presence of another chemical compound with similar bonding strength overlap with each other. The Raman spectrum confirmed that the FWHM of E1(TO), E2(high), A1(LO) and E1(LO) phonon were increased whereas FWHM of A1(TO) and SO phonon decreased with increasing process temperature. The phonon lifetime of various phonons was calculated using energy-time uncertainty relation and found that the phonon lifetime of E1(TO), E2(high), A1(LO) and E1(LO) phonon decreased with increasing process temperature.

Influence of tantalum's crystal phase growth on the microstructural, electrical and mechanical properties of sputter-deposited tantalum thin film layer

High melting point refractory tantalum (Ta) metal is frequently grown into thin film layer for many applications in biomedical implants, microelectronic devices and micro-level mechanical systems. Tantalum growth mechanism is still in debate and the inconsistent crystal phase outcome has puzzled many, though it is certain that the properties of the grown film are highly dependent on the formed crystal phase configuration. The microstructure, surface morphology, crystal orientation and residual stress of the sputter-deposited Ta thin films using direct current (DC) magnetron sputtering technique are studied at 0.4 Pa – 2 Pa of sputtering pressure, 100 W – 250 W of DC sputtering power on bare silicon and silicon dioxide substrate and 10 min – 50 min of sputtering duration. α-phase Ta is preferably grown at high sputtering time (high thickness) and high DC sputtering power (high growth rate). The sputtering pressure affects the thin film's microstructural porosity while the sputtering power controls the crystallisation's quality. Both sputtering pressure and power affect the generated argon plasma in the DC magnetron sputterer where α-phase Ta is preferably formed at high plasma. The presence of 3 μm silicon dioxide underlayer makes no difference compared to bare silicon substrate. Our study reveals that β-phase Ta is grown first irrespective of sputtering conditions and then transformed into α-phase Ta after reaching a certain thickness. The grown major α-phase content promotes small thin film sheet resistivity (58.9 μΩcm – 86.1 μΩcm) and is suspected to be the dominant factor that increases the compressive stress within the thin film layer and reduces the adhesion of Ta layer onto the substrate surface. The study has given a new insight in controlling the conductivity and adhesion level of Ta thin film based on the grown phase layer.

Effect of multiaxial stress in thin films on the ferromagnetic resonance mode characteristics

The effects of multiaxial stress-induced anisotropy on the ferromagnetic resonance modes have been investigated. Equivalent stress quantities (σ1, σ2) are introduced. σ1 and σ2 induce out-of-plane and in-plane magnetic anisotropies, respectively; the corresponding anisotropy fields Hσ1 and Hσ2 are defined. Analytical expressions are derived for the resonance relations, the frequency (Δf) and field (ΔH) linewidths, the imaginary part of magnetic susceptibility (χ′′), and mode intensities (IH and If) for different configurations of the applied magnetic field and magnetization. Saturated and unsaturated states have been considered. Interesting features are observed for the multiaxial stress case and compared to the uniaxial stress one. The differences between the two cases are highlighted. It will be shown how these mode characteristics can be used to detect and measure the multiaxial stress in thin films.

Figure correction of borosilicate glass substrates by nanosecond UV excimer laser irradiation.

The mechanical stress in thin films can have a deleterious effect on the quality of optical components by deforming the underlying substrate. In addition, the substrate might be deformed by gravity or stress induced by mounting. We suggest a method to compensate the substrate deformation by laser generated tensile stresses in the backside of the substrate. We show results for irradiation of the borosilicate glass Schott D263M with an ArF excimer laser. We measured the integrated stress in dependence of the laser fluence and corrected a sample for the deformation by a chromium coating. We show that also antibiaxial plane stress components can be induced. For precise corrections a scheme for stabilization of the generated surface stresses still needs to be developed.

Influence of Ni-Doping in ZnO Thin Films Coated on Porous Silicon Substrates and ZnO|PS Based Hetero-Junction Diodes

Ni2+-doped ZnO thin films were prepared for various Ni concentration on the porous silicon substrates. The residual stress in the ZnO thin film is relaxed with increase in the concentration of Ni. FESEM images show the growth of pillar-like nanostructures over the entire porous silicon substrates. The variation of resistivity due to UV illumination was observed for the Ni-doped ZnO thin films. Ideality factor value is less for the ZnO:Ni|PS hetero-junction diode than ZnO|PS hetero-junction, Ni doping in ZnO improves the rectifying behavior.

WITHDRAWN: Comparative study of structure, optical properties and residual stress of boron carbide thin films deposited by RF-PECVD and PLD technique

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Quantifying Strain via Buckling Instabilities in Surface-Modified Polymer Brushes

A compressive strain applied to bilayer films (e.g. thin film adhered to a thick substrate) can lead to buckled or wrinkled morphologies, which has many important applications in stretchable electronics, anti-counterfeit technology, and high-precision micro and nano-metrology. A number of buckling-based metrology methods have been developed to quantify the residual stress and viscoelastic properties of polymer thin films. However, in some systems (e.g. solvent-induced swelling or thermal strain), the compressive strain is unknown or difficult to measure. We present a quantitative method of measuring the compressive strain of wrinkled polymer films and coatings with knowledge of the "skin" thickness, wrinkle wavelength, and wrinkle amplitude. The derived analytical expression is validated with a well-studied model system, e.g., stiff, thin film (PS) bonded to a thick, compliant substrate (PDMS). After validation, we use our expression to quantify the applied swelling strain of previously reported wrinkled poly(styrene-alt-maleic anhydride) brush surfaces. Finally, the applied strain is used to rationalize the observed persistence length of aligned wrinkles created during atomic force microscopy (AFM) lithography and subsequent solvent exposure.

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.

Influence of deposition parameters on the residual stresses of WC-Wo sputtered thin films

AbstractControl and manipulation of residual stresses in thin films is a key for attaining coatings with high mechanical and tribological performance. It is therefore imperative to have reliable residual stress measurements methods to further understand the dynamics involved. The sin2ψ method of X-ray diffraction was used to investigate the residual stresses on the tungsten carbide cobalt thin films deposited on a mild steel surface to understand the how the deposition parameters influence the generation of residual stresses within the substrate surface. X-ray spectra of the surface revealed an amorphous phase of the thin film therefore the stress measured was of the substrate surface and the effects of sputtering parameters on residual stress were analysed. Compressive stresses were identified within all samples studied. The results reveal that as the sputtering parameters are varied, the residual stresses also change. Optimum deposition parameters in terms of residual stresses were suggested.

Compression and decompression of structural tantalum films exposed to buffered hydrofluoric acid

As a refractory metal with a high melting temperature and large coefficient of thermal expansion, tantalum (Ta) is of great interest as a structural material in MEMS. One of the major issues with any freestanding structure, however, is the control of residual stress. Here, we observe that suspended fixed–fixed beams made from Ta films under tension buckle after a buffered hydrofluoric (BHF) acid release step, implying a large change of stress towards compression. We find that the change in uniaxial stress is proportional to BHF exposure time and reaches −1 GPa after 150 min. Although there are many sources of residual stress in metal thin films, we demonstrate definitively that hydrogen (H) injection due to BHF is the major cause of this change. We show further that the residual stress can be largely recovered by degassing at 500 °C in a high vacuum environment.

The influence of substrate bias and sputtering pressure on the deposited aluminium nitride for magnetoelectric sensors

A residual stress of Aluminium nitride (AlN) thin films has been a problem in the magnetoelectric (ME) composites that are used in the AC magnetic field sensors. The present work aims to optimize the deposition process of AlN in order to fabricate a nearly-zero stress of AlN thin films as well as ME composites without losing the microstructural and piezoelectric properties. The influences of RF bias power and sputtering pressure on the residual stress, microstructure, and piezoelectric response have been investigated. Two different stacks are deposited on Si/SiO2 substrates: Ta/Pt/AlN and Ta/FeCoSiB/Ta/Pt/AlN. Pulsed DC reactive sputter depositions have been performed to deposit AlN films. With increasing the substrate bias, stress of the AlN films and the stacks with the magneto strictive layer are augmented. A variation of the sputtering pressure is a promising way to fabricate nearly zero stress of the AlN films and the stacks without the magneto strictive layer. A transition from tensile to compressive stress has been observed at the low sputtering pressure. Sputtering pressure also affects the stress of AlN films and the stacks with the magneto strictive layer. FWHM of AlN (0002) peaks are nearly constant within the ranges of the substrate bias. By reducing the sputtering pressure, FWHM is broadened due to lower ionization degree associated with AlN formation and greater number of micro-arcs. However, the magnitude of e31,f is increased due to a lower residual stress at the low sputtering pressure.

Using Digital Image Correlation on SEM Images of Strain Field after Ion Beam Milling for the Residual Stress Measurement of Thin Films.

The residual stress of thin films during the deposition process can cause the components to have unpredictable deformation and damage, which could affect the service life and reliability of the microsystems. Developing an accurate and reliable method for measuring the residual stress of thin films at the micrometer and nanometer scale is a great challenge. To analyze the residual stress regarding factors such as the mechanical anisotropy and preferred orientation of the materials, information related to the in-depth lattice strain function is required when calculating the depth profiles of the residual strain. For depth-resolved measurements of residual stress, it is strategically advantageous to develop a measurement procedure that is microstructurally independent. Here, by performing an incremental focused ion beam (FIB) ring-core drilling experiment with various depth steps, the digital image correlation (DIC) of the specimen images was obtained. The feasibility of DIC to FIB images was evaluated after the translation test, and an appropriate procedure for reliable results was established. Furthermore, the condition of the film in the function of residual stress was assessed and compared to elucidate the applicability of this technology.

Механизмы возникновения напряжений в тонких пленках и покрытиях

The paper considers current conceptions of generation of mechanical stresses in epitaxial, polycrystalline and amorphous films during their growth and under different external actions. The mechanism of stress generation in geteroepitaxial films due to misfit in crystal lattices of the film and substrate is described. The relation between arising of the misfit stress in heterostructures and variation of their growth mode is shown. The mechanisms of generation of compressive and tensile stresses in polycrystalline films caused by nucleation and coalescence of islands at the beginning of their growth are considered. Different aspects of evolution of intrinsic stresses in continuous films are discussed in dependence of their deposition conditions, chemical composition, microstructure and mechanical properties. Special attention is given to consideration of generation mechanisms of intrinsic stresses in thin films concerned with formation of pint defects, incorporation of impurities and phase transformations during deposition. Factors leading to arising extrinsic stresses in thin films during their storage and operation are described in details.

Dislocation Plasticity and Detwinning Under Thermal Stresses in Nanotwinned Ag Thin Films

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic signature in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic - ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the signature thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

磁控共溅射制备Si掺杂Al薄膜的应力研究

磁控共溅射制备Si掺杂Al薄膜的应力研究

Nano-scale residual stress depth profiling in Cu/W nano-multilayers as a function of magnetron sputtering pressure

Residual stresses in thin films and multi-layered coatings fabricated by physical vapour deposition largely affect their mechanical and thermal reliability during operation in numerous fields of applications. By changing the argon working pressure in between each multilayer planar DC magnetron sputter deposition step, it is possible to control the residual stress distribution within coatings. A combination of FIB-DIC ring-core strain analysis, synchrotron XRD analysis based on the sin2(Ψ) method and micro-cantilever deflection analysis is used to reconstruct the in-plane stress state of multilayer coatings at different deposition pressures, with a residual stress depth profile resolution of 50 nm. A clear transition from compressive to tensile residual stresses is observed with an increase of working pressure, with pronounced stress peaks near the substrate-coating interface. These peak stresses resolved by FIB-DIC ring-core analysis exceed the average XRD stress measurements significantly, thus providing a reasonable explanation for multilayer failure. Experimental results are presented and comprehensively discussed in the context of deposition conditions for different thin film applications.

Electrochemical Mechanics of Metal Thin Films: Charge‐Induced Reversible Surface Stress for Actuation

AbstractThe intrinsic charge‐induced surface stress of Ni thin films during electrochemical reactions with an alkaline electrolyte is measured in situ. Surface stresses induced by H absorption/desorption, α‐Ni(OH)2 formation, capacitive double‐layer charging, the α‐ to β‐Ni(OH)2 transformation, and β‐Ni(OH)2/β‐NiOOH redox reactions are identified, and each provided additive contributions to the overall stress state. Surface stresses are magnified in high‐surface‐area nanoporous Ni because local stress‐relaxation mechanisms are restricted when compared to a smooth Ni film. Ni film reversible tensile/compressive surface stresses correlate with anodic/cathodic potential scanning but with an opposite trend to that of a less reactive Au film. Surface stresses in the Ni films are up to 40 times that of Au films and suggest the possibility of using controlled surface‐stress generation for electrochemical actuation.

Tuning Stress in Cu Thin Films by Developing Highly (111)-Oriented Nanotwinned Structure

We have examined the effect of different bath temperatures on residual stress of both the random-oriented Cu films and the highly (111)-oriented nanotwinned Cu films by synchrotron radiation x-ray measurements. The bath temperature varied from 15°C to 40°C. The results indicate that the average residual stress in the highly (111)-oriented nanotwinned films is higher than that in the randomly oriented Cu films. However, the stress in the highly (111)-oriented Cu decreases with increasing bath temperature. The average residual stress can be reduced from 253 MPa electroplated at 15°C to 95 MPa under a bath temperature of 35°C. We could successfully tune and measure residual stress of the Cu thin films. The films with low residual stress prevent warpage from occurring on the substrate and lower the processing failure in copper direct bonding and other processes that need alignment.