Mechanical Stress Measurements Research Articles

Mechanical Properties and Residual Stress Measurements in Anodic Aluminium Oxide Structures Using Nanoindentation

Anodic alumina has been used widely for corrosion protection of aluminum surfaces or as a dielectric material in microelectronics applications. It exhibits a homogeneous morphology of parallel pores that can easily be controlled between 10 and 400 nm. It has been applied as a template for fabrication of the nanometer-scale composite. In this study, the mechanical properties of anodic aluminum oxide (AAO) structures are measured by the nanoindentation method. The nanoindentation technique is one of the most effective methods to measure the mechanical properties of nanostructures. Using the nanoindentation method, we investigated the residual stress and mechanical properties such as the indentation modulus and hardness of the AAO structure with different-sized nanoholes. As a result, we find that the “hole effect” that changes the mechanical properties is the hole size.

Microstructure, Phase Sequence and Superelasticity in Highly Oriented MBE-Grown NiTiCu Shape Memory Thin Films

Using MBE deposition highly oriented NiTiCu shape memory thin films were accomplished. The austenite-martensite transition in these films, single or two-stage transformation depending on the quantity of the Cu additions, occurs with comparable transition temperatures as known for sputter-deposited films. Mechanical stress measurements reveal a deformation recovery in the order of 400 MPa. Superelasticity spans to 1-1.5% strain as referred to spherical indentation data. The microstructure is remarkable in that the crystallites are oriented within {+-}5 along the film plane normal as observed by X-ray diffraction. The occurrence of this preferential order is deduced to the formation of a Ti-rich (Ti{sub 2}Ni) phase at the film-substrate interface providing a highly oriented initial crystallization template. The results described in this study outline the perspectives for tailoring the microstructure and transformation performance of shape memory thin films, especially in regard to technological applications. (orig.)

In situ measurements of thickness changes and mechanical stress upon gasochromic switching of thin MoOx films

Thin films of MoOx coated with platinum as a catalyst were prepared by dc magnetron sputtering from a molybdenum target in an oxygen and argon atmosphere. The films were colored and bleached by exposure to diluted hydrogen gas and air, respectively. In situ measurements of x-ray reflectance and substrate curvature were performed during switching cycles. Upon hydrogen exposure a remarkable thickness decrease of 11% and a density increase of 6.5% are observed by x-ray reflectance measurements. Corresponding changes in optical transmittance and mechanical stress up to several hundreds of MPa are also observed. These changes are not fully reversible as revealed by measurements of multiple switching cycles. Both the change of thickness and density, as well as the change of optical transmittance and mechanical stress can be explained by the existence of a mixed phase of edge and corner sharing MoO3 octahedra whereupon the first coloration the ratio between edge and corner sharing octahedra is changed.

Hydrogen-induced changes of mechanical stress and optical transmission in thin Pd films

Loading of palladium with hydrogen not only leads to pronounced changes of its optical and electrical properties, but is also accompanied by significant density changes leading to stress formation. Therefore, the loading has been investigated by in-situ mechanical stress and optical transmittance measurements. The exposure of palladium films to hydrogen results in a significant transmittance increase and mechanical stresses of several thousand MPa upon loading with hydrogen. The phase transition from the α-phase to the β-phase was observed in both stress and transmittance data. The influence of a thin oxide layer, which is formed in air and can be removed in a hydrogen gas, was observed. Hence, the stress and transmittance data are governed by a superposition of irreversible oxide removal and reversible loading with hydrogen.

A-Si:H alloy for stress sensor application

A novel device, based on amorphous silicon technology, suitable for mechanical stress measurement is presented. Due to the low temperature fabrication process, the sensor can be deposited directly on the mechanical part to be stressed without additional packaging and fixing procedures. The device is able to measure independently the bending force and the torsion force with good sensitivity and linearity. Moreover it can be packaged together with an integrated circuit to form a stand alone component, eventually including wireless input/output circuitry.

Raman-spectroscopic determination of inhomogeneous stress in submicron silicon devices

We present the deconvolution of measurements of inhomogeneous mechanical stress in silicon device structures obtained by UV micro-Raman spectroscopy. Due to the very small UV penetration depth of only 12 nm, averaging of stress over the depth is almost eliminated. Only the averaging of stress over the area of the laser spot remains. By deconvolution of the spectra and comparison with finite element simulations, it is now possible to extract information on mechanical stress from areas as small as 200 nm. Oppositely stressed regions in submicroscopic dimensions can be detected, which could not be detected in previous visible light measurements due to averaging.

A novel a-Si:H mechanical stress sensor

The piezoresistive behavior of a novel device for the mechanical stress measurement is presented. The sensor consists of n-type, p-type or micro-compensated amorphous silicon thin film with a sensitive area of 300×300 μm 2. It can measure the bending force and torsion force with good electrical properties, as good linearity of its response. Due to low temperature fabrication process the sensor element can be deposited directly on the stressed materials without additional packaging and fixing procedures. These are very interesting properties for its use in advanced robotic systems.

Heat treatment of thin carbon films and the effect on residual stress, modulus, thermal expansion and microstructure

Thin carbon films are used to hermetically seal and improve the performance of devices exposed to extreme conditions. Such films, which are deposited by chemical vapor deposition, develop residual thermal stresses due to a mismatch in the coefficient of thermal expansion between the film and substrate. Residual stresses reduce the adhesion of the film, and are a common cause of coating failure. This work investigates heat treatment as a potential technique to reduce residual stresses in thin carbon films. The magnitude of the residual stress has been challenging to measure due to the associated size scales and mechanical properties. In this study, experimental measurements of mechanical properties and residual stresses in thin carbon films are performed using nanoindentation and Raman spectroscopy. The results relate surface residual stresses to film thickness and heat treatment temperature. The approach presented in this study is a nondestructive and non-intrusive method for measuring residual surface stress and properties in thin films, and is ideal for small or curved-surface specimens such as optical fibers and other photonic devices.

Combinatorial Thin Film Synthesis of NiMnAl Magnetic Shape Memory Alloys Using MBE Technique

ABSTRACTComposition spreads close to the Heusler alloy Ni2MnAl were grown onto 4-inch wafer substrates using molecular beam epitaxy. Compositional variations of up to 10 at.% relative to each constituent enable a direct comparison of the chemical-structural relationship with respect to martensitic transformation and to magnetic ordering as well as an efficient identification of the emerging phase stability regions. In this study, we set the primary focus on the structural aspects of the transformation behavior as observed by X-ray microdiffraction in combination with a specially designed heating stage. Notably, cross-sectional HRTEM imaging of the respective composition areas reveals a laminated two-phase martensitic structure inside the single grains, identified as a sequence of 2M and 14M variants. Stress relief upon transformation as observed by mechanical stress measurements reaches to 400 MPa depending on the composition. Magnetization measurements so far indicate field-induced ordering to occur at low temperatures, here, below 50 K which is assumed to be closely related to a high degree of structural disorder on the Mn-Al sublattice. Single-crystal thin films were realized by means of an epitaxial relationship to MgO (001).

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation.

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Phase Formation and Structural Sequence of Highly-Oriented MBE-Grown NiTiCu Shape Memory Thin Films

We present a study demonstrating the capability for controlled shape memory thin film growth using molecular beam epitaxy. Here, NiTiCu alloy films were grown which are known to exhibit the martensitic transformation well above room temperature. Remarkably, the microstructure of these films was found to be very different compared to conventionally sputtered polycrystalline films: here, the crystallites are highly oriented within ±3 ◦ along the film plane normal. Moreover, a splitting of the martensite orientation is detected indicating the selection of two specific sets of martensite variants. Mechanical stress measurements reveal a high ratio of recoverable stress even for films below 500 nm thickness. These results open up the possibility to specifically modify the microstructure and crystallographic orientation of shape memory thin films and thus suggest promising characteristics, especially in regard to their superelastic behavior.

New CMOS-compatible mechanical shear stress sensor

A new approach to the measurement of mechanical stresses is presented. The new sensor design utilizes the fact that shear stresses in the silicon lattice generate an electric field perpendicular to an electric current. The sensor effect is characterized by a new piezo-bridge-coefficient, which relates the sensitivity of the sensor structure to its crystallographic orientation. The sensor is based on a CMOS-compatible structure. It offers the possibility to realize highly sensitive single-element stress sensors for use in MEMS or in smart force measurement strips, as well. An example of a signal conditioning circuit is shown. Special designs with improved sensitivity and low noise are presented. The response to parasitic magnetic fields is measured and strongly reduced. Furthermore, the temperature behavior was analyzed and finally optimized.

Mechanical stresses upon crystallization in phase change materials

Crystallization processes in different Te alloys, employed in phase change materials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase change film.

GMR and eddy current sensor in use of stress measurement

The use of high sensitive magnetic sensors like magnetoresistive sensors allows novel applications in non-destructive testing (NDT), i.e. monitoring the conditions of reinforced concrete constructions. Using novel magnetoresistive sensors, a stress measurement of prestressed steel bars in concrete is realizable. The giant magnetoresistive sensor (GMR sensor) and the eddy current sensor are introduced here for high sensitive stress measurement. The combination of the eddy current technique with the magnetoelastic stray field measurement basically improves the mechanical stress measurement.

Stress measurements on chrome-tanned leather

A sensitive method has been developed for the measurement of mechanical stresses in compliant planar samples originating from changes of their solvent content or temperature. The samples are mechanically conjoined with a silicon wafer. Thus, an alteration of the sample stress causes a spatial displacement of the wafer which is detected capacitively. The method is applied to the measurement of stresses in both acetone-dried and air-dried chrome-tanned leather which has been exposed to a stepwise modulation of ambient humidity. The stress development in the sample can be well characterized by a first-order exponential decay. The mean moisture content exhibits a similar time dependence. However, the relation between stress development and moisture content differs for swelling and shrinkage. This behavior is explained on the basis of a novel two-capillary model. Moreover, the response of the sample to the alteration of ambient humidity is found to be related to the structure of its collagen fiber network. The tight structure of air-dried leather with a poor isolation of fibrils yields much higher stresses than acetone-dried leather.

Highly Oriented NiTiCu Shape Memory Thin Films Grown by Molecular Beam Epitaxy

ABSTRACTWe present a study demonstrating the capability for controlled shape memory thin film growth using molecular beam epitaxy. Here, NiTiCu alloy films were grown which are known to exhibit the martensitic transformation well above room temperature. Remarkably, the microstructure of these films was found to be very different compared to conventionally sputtered polycrystalline films: here, the crystallites are highly oriented within 3° along the film plane normal. Moreover, a splitting of the martensite orientation is detected indicating the selection of only two specific martensite variants. Mechanical stress measurements reveal a high ratio of recoverable stress even for films below 500 nm thickness. These results open up the possibility for tailoring microstructure and crystallographic orientation of shape memory thin films and thus suggest promising characteristics, especially in regard to their superelastic behavior.

In situ measurement of mechanical stress in polycrystalline zinc-oxide thin films prepared by magnetron sputtering

The generation of stress in magnetron-sputtered undoped and aluminum-doped zinc-oxide (ZnO:Al) films has been measured by an in situ technique. Radio-frequency magnetron sputtering from ceramic targets in an argon atmosphere was used for film deposition. The stress was measured by optically detecting the deflection of a thinned silicon cantilever on which the films were deposited. This in situ stress measurement setup was tested by investigating the sputtering pressure dependence of stress in molybdenum films. For zinc-oxide films it was found that by increasing the argon-sputtering pressure during deposition the intrinsic compressive stress decreased by about a factor of 2, both for undoped and doped zinc-oxide films. The undoped films exhibit significantly higher compressive stresses (1–2.5 GPa) compared to the aluminum-doped films (0.7–1.5 GPa). This fact was ascribed to a surfactant-like effect of the dopant aluminum, which improved the crystallinity of the doped films. In contrast to magnetron-sputtered metallic films (like molybdenum) the zinc-oxide films did not show a transition from compressive to tensile stress when the argon-sputtering pressure was increased. It is assumed that this behavior of the zinc-oxide films is due to an additional incorporation of oxygen into the films. By adjusting the oxygen partial pressure during the deposition at a fixed argon pressure, a minimum of the compressive stress in ZnO: Al films was found which coincided with a minimum of the film resistivity. This observation stresses the role of even small changes in deposition parameters on the mechanical and electrical properties of zinc-oxide films. The possibilities of the in situ stress measurement are demonstrated during the deposition of zinc-oxide multilayers.

Origin of MeV ion irradiation-induced stress changes in SiO2

The 4 MeV Xe ion irradiation of a thin thermally grown SiO2 film on a Si substrate leads to four different effects in which each manifests itself by a characteristic change in the mechanical stress state of the film: densification, ascribed to a beam-induced structural change in the silica network; stress relaxation by radiation-enhanced plastic flow; anisotropic expansion and stress generation; and transient stress relaxation ascribed to the annealing of point defects. Using sensitive wafer-curvature measurements, in situ measurements of the in-plane mechanical stress were made during and after ion irradiation at various temperatures in the range from 95 to 575 K, in order to study the magnitude of these effects, the mechanism behind them, as well as their interplay. It is found that the structural transformation leads to a state with an equilibrium density that is 1.7%–3.2% higher than the initial state, depending on the irradiation temperature. Due to the constraint imposed by the substrate, this transformation causes a tensile in-plane stress in the oxide film. This stress is relaxed by plastic flow, leading to densification of the film. The anisotropic strain-generation rate decreases linearly with temperature from (2.5±0.4)×10−17 cm2/ion at 95 K to (−0.9±0.7)×10−17 cm2/ion at 575 K. The spectrum of irradiation-induced point defects, measured from the stress change after the ion beam was switched off, peaks below 0.23 eV and extends up to 0.80 eV. All four irradiation-induced effects can be described using a thermal spike model.

Structural Phase Transformation Observed with In-Situ Mechanical Stress Measurement during the Growth of Amorphous Fe<sub>100-x</sub>Zr<sub>x</sub> Films

Structural Phase Transformation Observed with In-Situ Mechanical Stress Measurement during the Growth of Amorphous Fe<sub>100-x</sub>Zr<sub>x</sub> Films

Structural Phase Transformation Observed with In-Situ Mechanical Stress Measurement during the Growth of Amorphous Fe<sub>100-x</sub>Zr<sub>x</sub> Films

Structural Phase Transformation Observed with In-Situ Mechanical Stress Measurement during the Growth of Amorphous Fe<sub>100-x</sub>Zr<sub>x</sub> Films