Stress Relaxation In Films Research Articles

Структурные особенности, электронный и спиновый транспорт в эпитаксиальных тонких пленках иридатов

A technique for obtaining high-quality epitaxial films of iridate (SrIrO3) by high-frequency cathode sputtering on strontium titanate (SrTiO3) and neodymium gallate (NdGaO3) substrates has been developed. The structural features and the influence of mechanical stress relaxation in thin films caused by the difference in the lattice sizes of the film and the substrate have been studied using X-ray diffraction. From the analysis of diffraction data, it was found that lattice parameter deformation is observed in the film. This indicates the emergence of stresses due to the mismatch of the lattice parameters of the film and the substrate, which leads to non-uniform stress relaxation in the film. The electronic transport parameters of thin films and the excitation of spin current in SrIrO3/La0.7Sr0.3MnO3 heterostructures have been investigated.

A wrinkling-based method for investigating glassy polymer film relaxation as a function of film thickness and temperature.

We investigate the relaxation dynamics of thin polymer films at temperatures below the bulk glass transition Tg by first compressing polystyrene films supported on a polydimethylsiloxane substrate to create wrinkling patterns and then observing the slow relaxation of the wrinkled films back to their final equilibrium flat state by small angle light scattering. As with recent relaxation measurements on thin glassy films reported by Fakhraai and co-workers, we find the relaxation time of our wrinkled films to be strongly dependent on film thickness below an onset thickness on the order of 100 nm. By varying the temperature between room temperature and Tg (≈100 °C), we find that the relaxation time follows an Arrhenius-type temperature dependence to a good approximation at all film thicknesses investigated, where both the activation energy and the relaxation time pre-factor depend appreciably on film thickness. The wrinkling relaxation curves tend to cross at a common temperature somewhat below Tg, indicating an entropy-enthalpy compensation relation between the activation free energy parameters. This compensation effect has also been observed recently in simulated supported polymer films in the high temperature Arrhenius relaxation regime rather than the glassy state. In addition, we find that the film stress relaxation function, as well as the height of the wrinkle ridges, follows a stretched exponential time dependence and the short-time effective Young's modulus derived from our modeling decreases sigmoidally with increasing temperature-both characteristic features of glassy materials. The relatively facile nature of the wrinkling-based measurements in comparison to other film relaxation measurements makes our method attractive for practical materials development, as well as fundamental studies of glass formation.

Residual stress relaxation and stiffness in spin-coated polymer films: Characterization by ellipsometry and fluorescence

Ellipsometry and fluorescence are used via measurements of film thickness and pyrenyl dye emission spectral shape, respectively, to characterize residual stress relaxation in polystyrene (PS) films. In particular, fluorescence of pyrene-labeled PS (MPy-PS) films, with ∼1 mol% pyrene label, provides sensitivity to film stress relaxation and stiffness by the dependence of the ratio of the first to third vibronic peak intensities (I1/I3) of the pyrenyl dye to nanosecond-time-scale molecular caging. Both techniques show that residual stress relaxation occurs over a period of hours despite the PS films being 15–40 °C above the film glass transition temperature (Tg). Both techniques also show that film Tg is unaffected by stress relaxation, even when stress relaxation is accompanied by measurable changes in thickness. Fluorescence shows that stress relaxation time follows an Arrhenius temperature dependence with an activation energy of ∼110 kJ/mol, which is consistent with stress relaxation occurring by β-relaxation. Using a bilayer/fluorescence technique with bulk bilayer films, it was observed by I1/I3 measurements that a 30-nm-thick MPy-PS layer located at a glass substrate interface is stiffer than a 30-nm-thick MPy-PS layer located at a free surface. Over a 20–400 nm thickness range, fluorescence of MPy-PS films show a significant effect of substrate on molecular caging and hence stiffness, with stiffness increasing in the following order: free-standing films (no substrate) < films supported on a polydimethylsiloxane (soft) substrate < films supported on a glass (hard) substrate.

Cholesterol-Induced Buckling in Physisorbed Polymer-Tethered Lipid Monolayers

The influence of cholesterol concentration on the formation of buckling structures is studied in a physisorbed polymer-tethered lipid monolayer system using epifluorescence microscopy (EPI) and atomic force microscopy (AFM). The monolayer system, built using the Langmuir-Blodgett (LB) technique, consists of 3 mol % poly(ethylene glycol) (PEG) lipopolymers and various concentrations of the phospholipid, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and cholesterol (CHOL). In the absence of CHOL, AFM micrographs show only occasional buckling structures, which is caused by the presence of the lipopolymers in the monolayer. In contrast, a gradual increase of CHOL concentration in the range of 0–40 mol % leads to fascinating film stress relaxation phenomena in the form of enhanced membrane buckling. Buckling structures are moderately deficient in CHOL, but do not cause any notable phospholipid-lipopolymer phase separation. Our experiments demonstrate that membrane buckling in physisorbed polymer-tethered membranes can be controlled through CHOL-mediated adjustment of membrane elastic properties. They further show that CHOL may have a notable impact on molecular confinement in the presence of crowding agents, such as lipopolymers. Our results are significant, because they offer an intriguing prospective on the role of CHOL on the material properties in complex membrane architecture.

Strain-induced enhancement of coercivity in amorphous TbFeCo films

We report a strong size dependence of coercivity in amorphous ferrimagnetic TbFeCo films. The as-deposited film exhibited a low saturation magnetization (MS ∼ 100 emu/cc) and a high perpendicular anisotropy (KU ∼ 106 erg/cc). Hall-bar devices were fabricated for characterizing the magneto-transport behaviors. A significant increase in coercivity (up to ∼300%) was observed at room temperature as the width of Hall bar was reduced. The large coercivity enhancement was attributed to the relaxation of film stress. The effect of strain and dimensionality on the coercivity in TbFeCo makes it attractive for tunable coercivity and the magnetization reversal in future nanoscale devices.

Plastic Deformation in Profile-Coated Elliptical KB Mirrors

Profile coating has been successfully applied to produce elliptical Kirkpatrick-Baez (KB) mirrors using both cylindrical and flat Si substrates. Previously, focusing widths of 70 nm with 15-keV monochromatic and 80 nm with white beam were achieved using a flat Si substrate. Now, precision elliptical KB mirrors with sub-nm figure errors are produced with both Au and Pt coatings on flat substrates. Recent studies of bare Si-, Au-, and Pt-coated KB mirrors under prolonged synchrotron X-ray radiation and low-temperature vacuum annealing will be discussed in terms of film stress relaxation and Si plastic deformation.

Creep and stress relaxation in free-standing thin metal films controlled by coupled surface and grain boundary diffusion

An analytical solution is presented that interprets the effects of grain size, surface and grain boundary diffusivities, surface and grain boundary free energies, as well as grain boundary grooving on the creep rate in free-standing polycrystalline thin metal films. The Coble creep in the film plane is also taken into account; this has a significant effect on the creep rate of the film. The effects of diffusion ratio and free energy ratio between surface and grain boundary on film agglomeration are illustrated as well. A closed-form expression for stress relaxation in films under constrained strain conditions is derived from this solution. An exponential decay in stress is found as a function of the film microstructure. Results predicted by the solution are shown to be in agreement with the experiments.

Characterizing the viscoelastic properties of layer-by-layer carbon nanotube–polyelectrolytethin films

Many have sought to take advantage of carbon nanotubes for the fabrication ofmultifunctional thin films. However, before these carbon-nanotube-based filmscan be used for these intended functionalities, a complete understanding of themechanical properties is crucial. While most studies have focused on characterizingjust the thin film’s stress–strain performance, their time-dependent mechanicalproperties still need to be investigated. In this study, free-standing layer-by-layercarbon-nanotube-based thin films have been assembled, and the objective is to investigatetheir viscoelastic stress relaxation and creep response. First, the results fromstress relaxation tests are numerically fitted to two different models, namely thestandard linear solid (Zener) model and the linear increasing viscosity-modified Zener(LIV-Zener) model. The results of the fitting suggest that the LIV-Zener modelis better suited for describing thin film stress relaxation behavior, which alsomeans that these nanocomposites exhibit time-dependent viscosity. Second, theexperimental results from creep testing are also fitted to the LIV-Zener model. Theresults from both fittings are consistent and suggest that the LIV-Zener modelis more capable of describing carbon-nanotube-based thin film viscoelasticity.

High-resolution transmission electron microscopy of heteroepitaxial barium strontium titanate films grown on MgO substrates

Nanoscale (5–100 nm) ferroelectric heteroepitaxial Ba0.8Sr0.2TiO3 films on MgO(001) substrates were studied by methods of high-resolution transmission electron microscopy using Tecnai G2 and FEI Titan 80–300 microscopes. Misfit dislocations resulting from stress relaxation in films and insignificant local misorientation of film regions (block structure) were detected. The heterostructure was numerically simulated, which allowed the detection of the formation of oxygen (substrate)-titanium (film) bonds or the oxygen (substrate)-barium (film) bond at the film-substrate interface.

Effect of substrate bias and magnetic annealing on soft magnetic properties of rf-sputtered Fe49Co49V2 films

Fe 49 Co 49 V 2 films were deposited by rf sputtering with and without substrate bias. All the as-deposited films deposited with or without substrate bias exhibit high coercivity of ∼200 Oe. The effect of magnetic annealing on soft magnetic properties was pronounced for films deposited with a substrate bias of 300 V. Excellent uniaxial anisotropy films with easy axis coercivity of 10 Oe were obtained for bias-sputtered and magnetic annealed films. X-ray diffraction examination and energy dispersive analysis of x-ray suggest that the improvement of soft magnetic properties by bias sputtering and magnetic annealing is associated with the relaxation of film stress and the recrystallization during magnetic annealing and variation of the composition of film by substrate bias. The dependence of coercivity on grain size D of these films appears to follow the 1/D law.

The corrosion behavior of sputter-deposited aluminum–tungsten alloys

Thin films on aluminum–tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al 80W 20–Al 67W 33 composition range. Passivity and corrosion behavior of amorphous Al–W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al–W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al 67W 33) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al–W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W +6) and mobile cation vacancies, which restrict the transport of Cl − through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al–W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed.

Irreversing thermal expansivity of materials coated with adhesive thin films detected by modulated-temperature dilatometry and differential thermal analysis

Usability of simultaneous modulated-temperature dilatometry (MT DIL) and modulated-temperature differential thermal analysis (MT DTA) for the quantitative description of degradation kinetics of wear-resistant film adhesion to the substrate as a function of temperature are presented in this work. The reversing thermal expansion coefficient of materials as a parameter sensitive to the relaxation of mechanical thermal stresses in films tested was used for this purpose. Sample experimental results allowing the process of film degradation to be described as a function of increasing temperature are quoted. Usefulness of this method considering the films deposited at diversified process parameters is also substantiated.

Excimer Laser Liftoff of Epitaxial Pb(Zr,Ti)O3 Thin Films and Heterostructures

AbstractEpitaxial (100) and (111)-oriented Pb(Zr,Ti)O3-based thin films and heterostructures have been transferred intact from their sapphire and MgO growth substrates to silicon and polymer substrates utilizing a novel laser liftoff process. The heterostructures, while on their growth substrate, were bonded to the receptor substrates using one of several bonding methods, including van der Waals bonding to an elastomer receptor, and transient liquid-phase Pd-In bonding to Si. A single 38 ns pulse from a KrF excimer laser (&lt;,= 248 nm) directed through the transparent growth substrate induced localized heating of the perovskite interfacial layer. At fluences corresponding to the onset of vaporization (&gt;300 mJ/cm2), the sapphire or MgO substrate was detached. Because of the short pulse length and the low thermal conductivity of Pb-based perovskite phases, heating of the top surface of the heterostructure was minimal, thus permitting film transfer to thermally-sensitive receptor substrates. X-ray rocking curves revealed slight broadening of the principal PLZT diffraction peaks (∼10–20%), suggesting local relaxation of film stress.

High quality YBa 2Cu 3O 7−x/SrTiO 3 bilayers for superconducting electric field effect transistor

We report the YBa 2Cu 3O 7−x/SrTiO 3 (YBCO/STO) bilayer structure to prevent the deterioration of superconductivity of YBCO channel layer having a grain-boundary junction in superconducting field effect transistor. Two types of method were used, such as the 20 nm-thick STO capping layer and the 100 nm-thick PBa 2Cu 3O 7−x buffer layer deposition. Transition temperature of 50 nm-thick ultrathin YBCO channel layer on STO bi-crystal substrate deposited by these method increased to 80 K owing to the sufficient oxidation and the relaxation of film stress in YBCO channel layer.

Film edge-induced stress in substrates and finite films

A numerical method developed originally for calculating the edge-induced relaxation of film stress in a semi-infinite thin film, and the corresponding stresses induced in the substrate, is extended to apply to stripes of finite width 2 l and thickness h. Values of relaxation are obtained by numerical integration for several values of the ratio R = Kl h ( K depends on the elastic constants of the film and substrate). Since numerical integration is necessary to evaluate the full solution for each value of R, an approximate expression for the relaxation is derived. This expression is accurate to within 6% for R = 1 and the accuracy increases rapidly as R increases; becoming practically zero for R > 3. Since all substrate stress components depend uniquely on the film relaxation, they can also be calculated using this expression. It is shown that earlier expressions used for the finite stripe give very large errors both in the stripe and in the substrate. A method to extend the use of this expression to calculate stresses in an array of stripes separated by valleys and in the substrate under the array is also described. The expression is compared with experimental results on GaAs stripes grown on Si substrates.

Modeling the Development and Relaxation of Stresses in Films

The mechanics for the development and relaxation of stresses in films is briefly reviewed. The discussion initially focuses on how stresses are distributed within a film and substrate, and on the resultant deformations that these stresses induce. Although the particular case of an elastically homogeneous system with a uniform stress within the film is addressed in detail, the modifications that would result from a nonhomogeneous system, or a nonuniform stress, are also outlined. A brief discussion of stress relaxation in epitaxial systems and the glide of dislocations in thin films then follows. In polycrystalline films, diffusion of atoms as well as the motion of dislocations can contribute to the relaxation of stresses. It is possible to use constitutive models for the different relaxation processes to predict the development of stresses during thermal cycling. A comparison of these predictions with experimental observations allows general conclusions about the dominant relaxation mechanisms that operate in particular thin-film systems to be made. An additional mechanism of stress relief

Modeling the Development and Relaxation of Stresses in Films

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical ...Read More

The relationship between deposition conditions, the beta to alpha phase transformation, and stress relaxation in tantalum thin films

We demonstrate that the high temperature polymorphic tantalum phase transition from the tetragonal beta phase to the cubic alpha phase causes a large decrease in the resistance of thin films and a complete stress relaxation in films that were intrinsically compressively stressed. 100 nm beta tantalum thin films with intrinsic stresses of 2.0×1010 dynes/cm2 (tensile) to −2.3×1010 dynes/cm2 (compressive) were deposited onto thermally oxidized (100) silicon wafers by evaporation or dc magnetron sputtering with argon. In situ stress and resistance at temperature were measured at 10 °C/min up to 850 °C in purified helium. Upon heating, the main stress mechanisms were elastic deformation at low temperature, plastic deformation at moderate temperatures and stress relief because of the beta-to-alpha phase transition at high temperatures. The temperature ranges over which the elastic and plastic deformation and the beta-to-alpha phase transition occurred varied with deposition pressure and substrate biasing. Incomplete compressive stress relaxation at high temperatures was observed if the film was initially deposited in the alpha phase or if the beta phase did not completely transform into alpha by 800 °C due to substrate biasing during the deposition. We conclude that the main stress relief mechanism for tantalum films with intrinsic compressive stresses to completely relax their stress is the beta-to-alpha phase transition, while for intrinsically tensile films, this transformation has a much smaller effect on the stress.

Formation of (1120) ZnO Films by Controlling the Selftexture and the Relaxation of Film Stress

ABSTRACTZnO film has a tendency to grow parallel to the &lt;0001&gt; axis. Therefore, it is difficult to obtain epitaxial or oriented films with any other orientation. Nevertheless, we obtained excellent epitaxial (1120) ZnO films on R-cut, (0112), sapphire by controlling the self-texture. Epitaxial (1120) ZnO on R-cut sapphire exhibits an anisotropic misfit at the interface. To clarify the growth mechanism, the stress relaxation was calculated by measuring the curvature of the substrate and the lattice spacings. The change in stress calculated by these two methods differs from each other. We discuss this discrepancy in terms of the growth mechanism.

Hole formation in thin aluminium films under controlled variation of strain and temperature

This paper reports a systematic study of local stress relaxation phenomena in aluminium films deposited by evaporation and sputtering in the thickness range 0.25–2.2 μm, with special emphasis on relaxation by hole formation. Tensile film stresses were generated during the cooling phase of an annealing cycle as well as by micromechanical bending of the substrate in a heating stage mounted in situ in a scanning electron microscope. Hence the relaxation phenomena could be observed as they happened, and characterized with respect to number, size, shape and critical formation temperature. Critical formation stresses were estimated. Relaxation by hole formation was observed only in evaporated and sputtered films of submicron thickness; thicker films relaxed by grain collapse. Hole formation occurred only at elevated temperature, in evaporated films during the first heating cycle, in sputtered films not until the second cycle. The holes were rather extended and richly branched out in the thinnest films, but displayed more rounded shapes in the thickest of the submicron films. A mechanism for the hole formation phenomenon, based on creep cavitation and interfacial slip, is suggested. The results show for the first time that hole formation does not require that the aluminium film be coated with a passivation layer, nor does it require the aluminium film to be shaped into narrow stripes. Tensile stress alone gives rise to holes. In a special study it was shown that the original, thin aluminium surface oxide is extraordinarily pliable. During film stress relaxation by hole formation the oxide film remains intact and is in fact suspended like a drumhead over the holes, some of which are several micrometres in size. The oxide film does not break until it is punctured by violent external action.