Compressive Stress In Films Research Articles

Magnetic-field-induced spontaneous superlattice formation via spinodal decomposition in epitaxial strontium titanate thin films

Applying magnetic fields during growth of ceramic thin films creates a tipping point that spontaneously produces stress-based ferromagnets. While atoms in a normal crystal lattice are spaced closely together, researchers can now make ‘superlattices’ consisting of nanometer-wide strips of different crystals periodically stacked on top of each other. In semiconductors, thin films can be induced to form superlattices by tweaking the deposition conditions. Now, Naoki Wakiya from Shizuoka University in Japan and co-workers have replicated this feat for thin films of strontium titanate, an insulating oxide with magnetic and superconducting capabilities. The team used a pulsed laser to deposit strontium titanate onto a substrate and a strong magnetic field to stretch the usual crystal framework. The compressive stress in the new thin film initiates a transformation to a superlattice state and room-temperature ferroelectric behavior.

Structural and mechanical properties of Nb1 − xTixN thin films deposited by rf magnetron sputtering

Nb1−xTixNy thin films with different nitrogen contents (0.5<y<1.2) and Nb/Ti ratios (0≤x≤1) were investigated because of their potential to improve the lifetime of bearing components of orthopaedic implants. The films were deposited onto a medical-grade CoCrMo substrate by rf magnetron sputtering at 400°C under a (N2+Ar) atmosphere. The samples were characterised using Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), X-ray diffraction (XRD) at grazing incidence and Bragg–Brentano geometry, scanning and transmission electron microscopy (SEM, TEM), and nanoindentation to investigate the dependence of the phase configuration, microstructure, and mechanical behaviour on the nitrogen content and Nb/Ti ratio. The results demonstrated that the coatings develop mixed structures composed of fcc-cubic as the major phase and hexagonal (β, ε, and δ′) phases as the minor constituents. The formation of hexagonal phases is nitrogen-dependent: (β, ε) for N2/(N2+Ar)<20%, and (δ′) for ratios >40%. The cubic and hexagonal phases simultaneously grow during the nucleation and early stage of growth up to a thickness of approximately 0.6μm, but the hexagonal phase gradually decreases through the film thickness, thereby giving rise to single-phase deposits at the upper part of the coatings. The addition of Ti improved the crystallinity and favoured the formation of the cubic phase, whereas a solid solution fcc δ-Nb1−xTixN was observed for the entire range of (x). The dominant orientation of the crystallites was (111), while a significant contribution of (311) was detected at higher Nb/Ti. The determination of stressed and stress-free lattice parameters allowed correlating the relative reduction of the compressive residual stress, hardness, and modulus in the nitrogen-rich films to the formation of vacancies in the metal sublattice.

Large modification in insulator-metal transition of VO2 films grown on Al2O3 (001) by high energy ion irradiation in biased reactive sputtering

High energy ion irradiation in biased reactive sputtering enabled significant modification of insulator-metal transition (IMT) properties of VO2 films grown on Al2O3 (001). Even at a high biasing voltage with mean ion energy of around 325 eV induced by the rf substrate biasing power of 40 W, VO2 film revealed low IMT temperature (TIMT) at 309 K (36 °C) together with nearly two orders magnitude of resistance change. Raman measurements from −193 °C evidenced that the monoclinic VO2 lattice begins to transform to rutile-tetragonal lattice near room temperature. Raman spectra showed the in-plane compressive stress in biased VO2 films, which results in shortening of V–V distance along a-axis of monoclinic structure, aM-axis (cR-axis) and thus lowering the TIMT. In respect to that matter, significant effects in shortening the in-plane axis were observed through transmission electron microscopy observations. V2p3/2 spectra from XPS measurements suggested that high energy ion irradiation also induced oxygen vacancies and resulted for an early transition onset and rather broader transition properties. Earlier band gap closing against the temperature in VO2 film with higher biasing power was also probed by ultraviolet photoelectron spectroscopy. Present results with significant modification of IMT behavior of films deposited at high-energy ion irradiation with TIMT near the room temperature could be a newly and effective approach to both exploring mechanisms of IMT and further applications of this material, due to the fixed deposition conditions and rather thicker VO2 films.

Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

High-Performance Metal Hard Mask Process Using Fiber-Textured TiN Film for Cu Interconnects

One of the most challenging issues in the metal hard mask process for copper interconnects is controlling the residual stress in TiN mask. The relationship between the residual stress and the feature size of the trench was investigated using a mechanical simulation. It was found that the trench deformation at a specific pattern due to the residual compressive stress in TiN film resulted in void formation in Cu. To overcome this problem, the correlation between the residual stress and the film properties of TiN was investigated and a potential TiN mask in the metal hard mask process was studied. The residual stress in TiN film and the etching performance were correlated with the composition and the crystal structure of TiN. The grain growth was suppressed by reducing the energy of sputtered atoms during the deposition of TiN, resulting in low residual stress in TiN film with keeping the etching performance for TiN. By applying low stress TiN mask which had a fiber-textured structure, the trench deformation at a specific pattern could be suppressed and thus Cu filling was perfectly achieved. The metal hard mask process using TiN film which had a fiber-textured structure was demonstrated to be high performance.

High performance VO2 thin films growth by DC magnetron sputtering at low temperature for smart energy efficient window application

VO2 thin films were deposited on soda-lime glass substrates through DC-reactive magnetron sputtering at substrate bias voltage of −160 V and a low substrate temperature of 200 °C. The crystal structure, electrical and optical performances, evolution of vibrational modes, and surface morphology of the films were characterized via X-ray diffraction, four-point probe method, UV/VIS/NIR spectrophotometer, ellipsometry, Raman scattering measurements, and scanning electron microscopy, respectively. The obtained results show that the films have preferred orientation of VO2 (011) lattice. The change in film resistivity is nearly 1.5 orders of magnitude after heating the film from 25 °C to 80 °C. The phase transition temperature of the film is approximately 50 °C, which is lower than 68 °C of VO2 crystal. Compressive stress in the VO2 films maybe the real reason for this result. The maximum visible transmittance shows as higher as 40%, and the IR switching efficiency can reach 50% at 2500 nm. Raman band variation and the switching characteristics of optical constants also confirmed the good thermochromic properties of VO2 films.

Residual stress measurement in DLC films deposited by PBIID method using Raman microprobe spectroscopy

Abstract Diamond-like carbon (DLC) films were prepared and their residual stresses were measured nondestructively using Raman microprobe spectroscopy. The plasma-based ion implantation and deposition (PBIID) method was used to coat the DLC films on thin glass substrates using acetylene, a mixture of acetylene and toluene, or only toluene gas at 1.0 Pa. Peaks in the Raman spectra of the DLC films were assigned as the D′(disordered) or C–C bonding peaks at 1150 cm − 1 . The phonon deformation potentials ( a ′) of the films were estimated from data for the phonon deformation potentials for pure graphite and diamond and calculated using the sp 3 /sp 2 bonding ratio and the hydrogen content of the films. Thus, a relation was observed between the Raman shift of the G peak ( ω G ) and the residual stress ( σ c ) in each film. The Raman shifts ( ω 0 ) of the G peak for the films with no deformation were 1554, 1556, and 1562 cm − 1 for the films deposited using acetylene, a mixture gas and toluene gas. Moreover, only toluene had stress constants of − 0.378, − 0.384, and − 0.391 GPa/cm − 1 . The residual stresses constant in each film using (8.2 × 10 − 4 · a ′) − 1 ω 0 − 1 were estimated as − 0.379, − 0.384, and − 0.391 GPa/cm − 1 . The Raman shift of the D peak remained stationary as the compressive σ c in the films increased but changed when the deposition gas was varied. The distance the D peak moved from 1420 cm − 1 corresponded to that of the G peak from 1560 cm − 1 in the Raman spectra of the films in the stress-free state. In addition, the compressive residual stress in the DLC film had a major impact on the hardness.

Corrosion behavior and oxide microstructure of Zr-1Nb-xGe alloys corroded in 360 °C/18.6 MPa deionized water

The corrosion behavior and microstructures of the oxide film on Zr-1Nb-xGe alloys (x=0, 0.05, 0.1 and 0.2, wt%) corroded in deionized water at 360°C/18.6MPa were investigated. Results showed that the Zr-1Nb-0.05Ge alloy possessed much better corrosion resistance than the Zr-1Nb alloy. The Ge dissolved in the α-Zr matrix can retard the microstructural transformation of ZrO2 from the columnar grains to equiaxed grains and delay the process of the defects developing into micro-cracks in the oxide. The amorphous phase produced around the [Zr–Nb–Fe–Cr–O] SPPs can improve the corrosion resistance by relaxing the compressive stress in the oxide films. The sub-oxide layer identified to be cubic-ZrO may improve the corrosion resistance after corrosion transition.

(Invited) Thermodynamics of Deposition Flux Dependent Intrinsic Film Stress

The growth of polycrystalline films at temperatures above ~0.2 of the melting temperature is accompanied by compressive stress development after film closure. Mysteriously, a significant part of this stress has a reversible nature: it disappears when the deposition is stopped and re-emerges upon resumption. Chason [1] even showed the corresponding inverse behavior during electrochemical etching All these observations clearly point towards a steady-state, flux (etching rate) dependent equilibrium phenomenon, which should, therefore, be treated properly by thermodynamics. It has been suggested that the variation of the surface chemical potential upon starting/stopping of the deposition may cause adatoms to diffuse in/out of the grain boundaries leading to the development/relaxation of the intrinsic compressive film stress. However, film growth involves a myriad of atomic processes such that the mystery is not yet solved and new mechanisms and ideas are still published frequently. Here we present an analytical derivation, in which we address, for the first time, the reason why additional atoms want to get incorporated in the grain boundaries. This background delivers the required driving force independently of the precise way the atoms get incorporated. In this sense we are not proposing a new model, but delivering the basis for all different models proposed. To derive the driving force, we calculate the variation of the chemical potential of the surface, the grain boundaries, and the film. Surprisingly, using pure thermodynamic arguments, our results fully agree with the magnitude of the reversible compressive stress: the tremendous stress levels observed in the experiments can indeed be explained by the flux induced density variations in the extremely dilute adatom gas on the surface! [1] Shin and Chason, Phys. Rev. Lett. 103, 056102 (2009) E-mail:rost@physics.leidenuniv.nl Web: www.physics.leidenuniv.nl/rost

Carbon ions irradiation induced modifications in structural and electrical resistivity characteristics of ZrN thin films

Zirconium nitride (ZrN) thin films are irradiated with 800keV energetic carbon (C) ions in a 5UDH-Pelletron accelerator and the ions irradiation induced effects are investigated. The films are irradiated at various C ions fluences, ranging from 1013 to 1015ions/cm2. The scanning electron microscopy study of the films indicates the development of zirconium (Zr) nanoparticles at ions irradiated region. X-ray diffraction (XRD) patterns of C ions irradiated films also show the formation of (100) and (002) oriented nanocrystalline metallic Zr phases. The irradiated films spectra depict a shift in ZrN peaks towards higher 2θ values, exhibiting that C ions bombardment induces compressive stress in the irradiated films. The appearance of C related peaks in Fourier transform infrared (FTIR) spectra confirms the incorporation of C atoms into ZrN film. Compressive stress has been calculated from the IR peak shift which indicates that higher ion dose (≥5×1014ions/cm2) produce lower compressive stress relative to the lower ions fluences. Effect of ion dose on the film resistivity is also reported.

Stress relaxation in dual ion beam sputtered Nb2O5 and SiO2 thin films: application in a Fabry–Pérot filter array with 3D nanoimprinted cavities

Miniaturized spectrometers can be implemented using Fabry–Pérot (FP) filter arrays. Such filters are defined by two parallel mirrors with a resonance cavity in between. For high optical quality, ion beam sputtered distributed Bragg reflectors (DBRs), with alternating high and low refractive index material pairs, can be used as the FP mirrors; while 3D nanoimprint technology provides an efficient way of implementing multiple organic FP cavities of different heights in a single step. However, the high residual stress in ion beam sputtered films results in poor adhesion between the DBR films and the organic polymer cavities, causing debonding of the DBR. Therefore, the residual stress of the ion beam sputtered films forming the DBRs must be reduced. Niobium pentoxide (Nb2O5) and silicon dioxide (SiO2) are used as the DBR materials in this work due to their high index contrast, resulting in high reflectivity for only a few alternating pairs. Stress relaxation in ion beam sputtered Nb2O5 and SiO2 films is achieved in this work by deposition under simultaneous high energy ion bombardment (oxygen and argon gas mixture) from a second ion source. Using this technique, the film density and hence compressive film stress for both Nb2O5 and SiO2 films is reduced without introducing any additional optical absorption in the films. FP filter arrays fabricated with stress reduced Nb2O5 and SiO2 as DBR films exhibit high optical and mechanical performance, with good adhesion between the films and the polymer cavity.

Radiation-induced modification of polymer surfaces

Modification of the surfaces of polymer films of polyimide, polyethylene terephthalate and polyetheretherketone upon γ irradiation and ion implantation with Ni+, Ag+, Au+, Fe+ and B+ ions at an energy of 30–100 keV in the dose range of 1 × 1015–1.5 × 1017 cm−2 is studied by the atomic-force microscopy. The processes and mechanisms of radiation-defect formation in polymers beyond the track of the implanted ions is considered. The formation of cone-shaped structures with a height of up to 80 nm and a base diameter of up to 400 nm randomly arranged on the surface of the polyimide, polyetheretherketone and polyethylene terephthalate films is observed under γ irradiation and low-dose ion implantation. These structures are observed not only in the field of ion deceleration but also beyond the region of high-energy action at a distance of up to 40 μm. Their formation is due to relaxation of the local elastic compressive stress in the polymer film.

In situ stress measurements during pulsed laser deposition of BaTiO3 and SrTiO3 atomic layers on Pt(0 0 1)

We apply the cantilever deflection technique to measure stress in nm thin BaTiO3 and SrTiO3 films during pulsed laser deposition on a Pt(001) single crystal cantilever substrate. We find a compressive film stress of −4.2GPa for BaTiO3 on Pt(001) (misfit = −2.3%), whereas the deposition of SrTiO3 (misfit = +0.4%) induces a tensile stress of +1.5GPa. The stress measurements are augmented by in situ low energy electron diffraction experiments which indicate an epitaxial order of the films. We apply continuum elasticity to calculate film stress. We conclude that sign and magnitude of the measured stress are due to the epitaxial misfit between film and substrate, which is −2.3% and +0.4% for BaTiO3 and SrTiO3, respectively.We identify that in addition to misfit also the oxygen partial pressure during PLD film growth influences film stress. PLD growth in an oxygen-free environment leads to factor of two increased tensile stress in SrTiO3 on Pt(001) as compared to growth at pO2=10−4mbar. The role of film stoichiometry for film stress is discussed.

Wrinkling and folding of thin films by viscous stress.

We examine the buckling of a thin elastic film floating on a viscous liquid layer which is itself supported on a prestretched rubber sheet. Releasing the prestretch in the rubber induces a viscous stress in the liquid, which in turn induces a compressive stress in the elastic film, leading to buckling. Unlike many previous studies on wrinkling of floating films, the buckling process in the present study is dominated by viscous effects whereas gravitational effects are negligible. An approximate shear lag model predicts the evolution of the stress profile in the unbuckled film that depends on three parameters: the rate at which the prestretch is released, the thickness of the liquid layer, and the length of the elastic film. A linear perturbation analysis is developed to predict the wavelength of wrinkles. Numerical simulations are conducted to predict nonlinear evolution of the wrinkle wavelength and amplitude. Experiments using elastic polymer films and viscous polymer liquids show trends that are qualitatively consistent with the predictions although quantitatively, the experimentally-observed wrinkle wavelengths are longer than predicted. Although this article is focused only on small-strain wrinkling behavior, we show that application of large nominal strains (on the order of 100%) leads to sharply localized folds. Thus this approach may be useful for developing buckled features with high aspect ratio on surfaces.

Hillock formation by surface drift-diffusion driven by the gradient of elastic dipole interaction energy under compressive stresses in bi-crystalline thin films

We investigated surface drift diffusion induced grain boundary (GB) grooving and ridge (hillock) formation and growth, under the combined actions of the capillary forces and applied uniaxial compressive stresses, in bi-crystal thin films with dynamical computer simulations. In the present theory, the generalized driving force for the stress induced surface drift diffusion includes not only the usual gradient of the elastic strain energy density, but also the elastic dipole tensor interaction energy. During the morphological evolution of GB ridge formation and growth, triple junction (TJ) displacement and its velocity are continuously tracked down in order to resolve precisely the crossover time and depth at which velocity sign inversion takes place. An incubation time for the onset of the ridge growth stage coupled to the GB-TJ displacement velocity inversion is defined and its dependence on the stress is investigated. This analysis implies that the ridge growth stage is not controlled by Ziegler’s ‘maximum entropy production principle’ but rather Prigogine’s ‘minimum entropy production hypothesis’ for the stationary non-equilibrium states in complex systems, which are exposed to external applied body forces and surface tractions. Keywords: Surfaces and interfaces; Grain boundary grooving; Non-equilibrium thermodynamics; Surface/grain boundary diffusion; Compressive stresses; Thin films DOI: 10.17350/HJSE19030000010 Full Text:

Microstructural Development and Possible Whiskering Behavior of Thin Sn Films Electrodeposited on Cu(Zn) Substrates

The aging behavior at room temperature of thin Sn films, electrodeposited on top of Cu(Zn) substrates containing 15 wt.% and 36 wt.% Zn, was investigated, using focused ion beam microscopy and x-ray diffraction analysis to evaluate the microstructural and (residual) stress development in the specimens. For comparison, parallel experiments and similar analyses were performed with Sn films electrodeposited on pure Cu substrates. Whereas Sn whiskering was observed for the Sn films deposited on the Cu substrates, such whiskering was not observed for the Sn films deposited on the Cu(Zn) substrates. It was found that alloying the Cu substrates with Zn strongly slows down the formation rate of the intermetallic compound Cu6Sn5 at the Sn/Cu(alloy) interface. The Sn films on the Cu(Zn) substrates remained whisker free for the entire time of investigation even though an overall compressive state of stress has developed after several weeks of aging. It was concluded that a homogeneous, compressive stress in the Sn film does not lead to whisker formation: the presence of negative stress gradients is essential for Sn whisker growth.

Buckling of an elastic plate due to surface-attached thin films with intrinsic stresses

We analyze the buckling of a thin elastic plate due to intrinsic stresses in thin films attached to the surfaces of the plate. In the case of cylindrical buckling, it is shown that for a plate with clamped edges compressive intrinsic film stresses can cause flexural buckling of the plate, while tensile intrinsic film stresses cannot. For a plate with free edges, film intrinsic stresses, compressive or tensile, cannot cause buckling.

Structural and Optical Properties of Sol-gel Processed ZnCdMgO Nanostructured Films as Transparent Conductor

In the present work, we report the structural and optical properties of sol-gel synthesized Zn0.9(Cd1-xMgx)0.1O (0 ≤ x ≤ 1.0) nanostructured films investigated by using the X-ray diffraction, scanning electron microscopy, atomic force microscopy, electrical resistivity, optical absorption and photoluminescence spectroscopic techniques. The X-ray diffraction study has revealed the hexagonal wurtzite crystal structure having favorable c-axis orientation for the increase in Mg concentration. The stress-strain calculation reveals the compressive stresses in Cd rich films whereas Mg rich films experience the tensile stress. Reduction of grain size and surface roughness has been observed with the increase in Mg concentration with more spherical grains. The AFM and SEM micrographs reveal the smooth surface morphology of the synthesized films. The magnesium rich films show high transmission in the visible and NIR region but show decrease in it with the increase in Cd concentration. The band gap increases from 3.19 to 3.40 eV with increase in Mg content. The photoluminescence measurement reveals the decrease in the defects and increase in band gap with the increase in Mg content in the films. The electrical resistivity has been found to be increased from 0.3×10 2 to 169.4×10 2 Ω-cm with increase in Mg concentration. The present study reports that the compositions x=0.4 and x=0.6 have the optimized combination of optical transmittance and better electrical resistivity values in this system for their possible applications as transparent conductors. The present results provide important data for TCOs processing with an optimized content of metal dopants for better transparency and conductivity. Copyright © 2014 VBRI press.

Effect of zinc induced compressive stresses on different properties of copper oxide thin films

Thin films of pure and zinc doped copper oxide have been grown on Si (1 0 0) substrate using pulsed laser deposition at varying concentrations of zinc. The effects of the doping concentration on structural, surface and optical properties of all thin films have been investigated. X-ray diffraction shows the presence of monoclinic CuO phase and compressive stresses in all thin films. The Raman spectra show an up-shift in the Raman peak position for doped thin films. Micrographs show smooth surface morphology, free from micron sized laser generated particulates. Spectroscopic el- lipsometry has been used to study the optical constants (w, D ,n , k,e1, e2). Optical bandgap energies have been calculated and found to be dependent on stresses in the films. Consequently, Zn doping induced stresses have strong effects on microstructure and optical properties of thin films.

Stress Modulation and Ferroelectric Properties of Nanograined PbTiO3 Thick Films on the Different Substrates Fabricated by Aerosol Deposition

Nanograined PbTiO3 (PT) thick films were deposited on Si, yttria‐stabilized zirconia (YSZ), and Ni substrates using an aerosol deposition (AD) method at room temperature. The AD PT thick films on each different substrate were annealed at 500°C and 700°C for 1 h to increase the crystallinity. The stresses in the PT film were modulated by controlling the difference in the coefficient of thermal expansion (CTE) between the films and substrates during the thermal annealing process. The morphology of the AD PT films was examined from the polycrystalline dense structure (thickness ~8 μm), and the changes in the crystallographic phase, in‐plane stresses, and ferroelectric properties in annealed films were investigated. In‐plane stress analysis showed that the PT films annealed at 500°C and 700°C on each substrate exhibited compressive stress. Owing to the effects of compressive stress in the PT film, the film showed less tetragonality (c/a ratio) and enhanced ferroelectric behaviors. The change in the polarization–electric field (P–E) hysteresis loop of the PT films was explained by the stress induced from CTE mismatch between the films and substrates.