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Growth of lattice mismatched films creates bending in the whole structure. There has been great interest in the study of these curvatures in epitaxially-grown materials. An analytical solution for the radius of curvature produced by stresses developed in growing lattice mismatched materials has been obtained. The analyses were based on beam bending theory and strain partitioning theory introduced by our group earlier. The expressions for radius of curvature were obtained for a two-layer heterostructure. The variation of the radius of curvature with the relative thicknesses, relative lattice constants, and relative elastic constants of the layers was determined. The model was verified by applying it to a symmetric tri-laminate structure. The above model can also be extended to determine the curvature for multi-layered heterostructures.
Stress/Strain fields associated with thin film buckling induced by compressive stresses or blistering due to the presence of gas bubbles underneath single crystal surfaces are difficult to measure owing to the microscale dimensions of these structures. In this work, we show that micro Scanning X-ray diffraction is a well suited technique for mapping the strain/stress tensor of these damaged structures.
The mechanixcal behavior of W/Cu multilayers with periods ranging from 24 down to 3 nm prepared by ion beam sputtering was analyzed using a method combining X-ray diffraction and tensile testing, and instrumented indentation. Cracks perpendicular to the tensile axis observed by optical microscopy were generated in the films under the largest applied tensile stresses. These cracks may appear in the multilayer while W layers are still in a compressive stress state. Elastic modulus and hardness values were extracted from nano-indentation data. Crack initiation and elastic constants were observed to depend on the period of these multilayers.
The wafer curvature technique was used to analyze stresses in fine-grained, 50 nm to 2 μm thick Au films on silicon substrates between room temperature and 500°C. The microstructural evolution was analyzed by scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In situ heating experiments inside a scanning electron microscope provided a comparison between the morphological development and the stress-temperature behavior of the film. Hillock formation was observed, but it can only partially account for the stress relaxation measured by the wafer curvature technique.
The propagation of crack fronts along a PET-glass interface is illustrated. The experimental set-up consists of an Asymmetric Double Cantilever Beam in an optical microscope. Image processing techniques used to isolate the crack fronts are discussed in some detail. The fronts are found to propagate inhomogeneously in space and time, in forward bursts that spread laterally along the front for some distance. In some cases the forward movement of a crack can be almost entirely due to the lateral movement of forward steps (analogous to “kinks”) along the crack front.
This paper presents two advanced acoustic methods for the determination of anisotropic elastic constants of deposited thin films. They are resonant-ultrasound spectroscopy with laser-Doppler interferometry (RUS/Laser method) and picosecond-laser ultrasound method. Deposited thin films usually exhibit elastic anisotropy between the film-growth direction and an in-plane direction, and they show five independent elastic constants denoted by C11,C33,C44,C66 and C13 when the x3 axis is set along the film-thickness direction. The former method determines four moduli except C44, the out-of-plane shear modulus, through free-vibration resonance frequencies of the film/substrate specimen. This method is applicable to thin films thicker than about 200 nm. The latter determines C33, the out-of-plane modulus, accurately bymeasuring the round-trip time of the longitudinal wave traveling along the film-thickness direction. This method is applicable to thin films thicker than about 20 nm. Thus, combination of these two methods allows us to discuss the elastic anisotropy of thin films. The results for Co/Pt superlattice thin film and copper thin film are presented.
Stress concentration at grain boundaries (GB), a phenomena arising from microstructural inhomogeneity, is an important factor in determining the mechanical properties of polycrystalline materials. In this study we use mesoscopic simulations to investigate characteristics of the deformation mechanism of grain-boundary diffusion creep (Coble creep) in a polycrystalline material. The stress distribution along the grain boundaries in a polycrystalline solid under externally applied stress is determined and the mechanism of how topological inhomogeneities introduce stress concentrations is investigated. Microstructures with inhomogeneities of various sizes and distributions are considered and their effect on the stress distribution and creep rate is quantified.
Polycrystalline lithium fluoride thin films have a number of existing and potential uses, but the optimization of their microstructure has not yet been addressed systematically. We have developed a means of measuring the porosity in LiF films, and a method for performing detailed electron-microscopical studies on this normally beam-sensitive material. These techniques have been applied to assess the structure of LiF films immediately after deposition from the vapor phase, and also after subsequent annealing.
Abstract. We propose novel stress metrology technique for measurement of local values stress tensor components in the coated wafers. New metrology is based on fiber-optic low coherence interferometry and can be applied to study stress not only in semicondiuctor wafers but in wide variety applications spanning from semiconductor to construction industry where measurements of plates covered by thin film encountered in flat panel displayes, solar cells, modern windows.
Dielectric properties and structure of (1-x) BiFeO3 (BFO) - x Ba0.5Sr0.5TiO3 (BST) (x = 0 ∼1) solid solution thin films were investigated. All films were prepared at 600 oC on (111) oriented Pt / TiO2 / SiO2 / Si substrates by pulsed laser deposition (PLD) technique. Solid solution could be achieved in all composition ranges, evidenced by X-ray diffraction (XRD) and field emission scanning electric microscope (FE-SEM). The intermediate compositions (0.4 = x = 0.8) exhibited a distinct (111) oriented cubic perovskite structure, while rhombohedra symmetry was found in the x < 0.4 range. Dielectric constant and tunability of the (1-x) BFO – x BST films within this composition region (0.4 = x = 0.8) decreased from 1110 to 920 at 1 MHz, and increased from 28.34 % to 32.42 % at 200 kV/cm, respectively, while loss tangent remains constant. A systematic decrease in lattice parameter with BST addition reduced stress due to reduction of lattice parameter mismatch between film and the substrate. In that range, the improvement of the dielectric properties without a degradation of loss tangent is attributed to the presence of the stress relaxation, which was quantitatively confirmed by a surface profiler based on Stoney's equation.
This paper presents the results of nanoindentation experimental studies of Au thin films with different thicknesses. The effects of film thickness and microstructure on the hardnesses of electron-beam deposited Au films were studied in terms of Hall-Petch relationship. The effects of different thicknesses on indentation size effects (ISE) are explained within the framework of mechanism-based strain gradient (MSG) theory using the concept of microstructural length scale.
Based on the physical background, a new dislocation dynamics model fully incorporating the interaction among differential dislocation segments is developed to simulate 3D dislocation motion in crystals. As the numerical simulation results demonstrate, this new model completely solves the long-standing problem that simulation results are heavily dependent on dislocation-segment lengths in the classical dislocation dynamics theory. The proposed model is applied to simulate the effect of dislocations on the mechanical performance of thin films. The interactions among the dislocation loops, free surface and interfaces are rigorously computed by a decomposition method. This framework can be used to simulate how a surface loop evolves into two threading dislocations and to determine the critical thickness of thin films. Furthermore, the relationship between the film thickness and yield strength is established and compared with the conventional Hall-Petch relation.
To investigate the origin of the high strength of thin films, in-situ cross-sectional TEM deformation experiments have been performed on several metallic films attached to rigid substrates. Thermal cycles, comparable to those performed using laser reflectometry, were applied to thin foils inside the TEM and dislocation motion was recorded dynamically on video. These observations can be directly compared to the current models of dislocation hardening in thin films. As expected, the role of interfaces is crucial, but, depending on their nature, they can attract or repel dislocations. When the film/interface holds off dislocations, experimental values of film stress match those predicted by the Nix-Freund model. In contrast, the attracting case leads to higher stresses that are not explained by this model. Two possible hardening scenarios are explored here. The first one assumes that the dislocation/interface attraction reduces dislocation mobility and thus increases the yield stress of the film. The second one focuses on the lack of dislocation nucleation processes in the case of attracting interfaces, even though a few sources have been observed in-situ.
Reactive diffusion of the Ni/Si system has been studied by annealing nickel thin film on (100) silicon crystal. The measurement of the NiSi sheet resistance as a function of the annealing temperature and the type of annealing (Rapid Thermal Annealing and spike one) has been investigated. A kinetic model based on multiphase diffusion has been developed that fits experimental sheet resistance data. Residual stress in the thin film, measured by a curvature measurement technique, is correlated with the nature of the phases in the film. Finally the viscoplastic mechanical behavior of the Ni2Si and NiSi phases is analyzed in the case of low and fast thermal ramps.
The latest successful development of smart technologies, in particular, molecular-beam epitaxy technique and pulse-laser deposition method, made it possible to manufacture optoelectronic active elements based on semiconductor materials with sufficient mismatch of the lattice parameters. This problem is of special interest for preparing photosensitive devices with strained superlattices. The paper focuses on the analysis of charge carriers behavior in mechanically strained superlattices based on semiconductor materials from A2B6 and A4B6 (ZnSe, ZnTe and PbS) playing an important role in the optoelectronics design. Computational modeling is settled on the solution of one-dimensional Schroedinger equation.
Due to their large magnetic anisotropy perpendicular to the film plane, barium ferrite thick films (BaFe12O19, or BaM) with c-axis orientation are attractive candidates for microwave applications [1,2]. Barium ferrite thin films on silicon substrates without under layer have been deposited under various conditions by RF magnetron sputtering. The structure of the as-grown films is amorphous. External annealing in air has been done at 950°C for ten minutes to crystallize the films. C-axis oriented thin films with squareness of about 0.87 and coercivity of about 3.8KOe are obtained.
Thick BaM films with c-axis orientation are difficult to achieve with one single deposition. Multilayer technique looks promising to grow thick films . The external annealing process is difficult to incorporate with the multilayer procedure. An in-situ annealing procedure has been developed to obtain films, which can be used as the basic component for future multilayer deposition. Barium ferrites are first magnetron sputtered on bare silicon substrates in Ar + O2 atmosphere at substrate temperature of 500-600°C, the deposition pressure was kept about 0.008 torr. After the deposition, the temperature of the substrate is immediately increased to about 860°C for ten minutes in 140 torr of argon (80%) and oxygen (20%) mixture of gas, which was introduced into the chamber without breaking the vacuum. With the in-situ process, c-axis oriented thin films of 0.88 squareness and coercivity value of about 4.3KOe are obtained.
Both annealing methods seem to have the similar effect on the perpendicular squareness and coercivity at various film thicknesses. The average value of the saturation magnetization Ms obtained from the in-situ annealing using multilayer technique is higher than that of the external one. We have grown films up to 1.0 micron thickness using the multilayer technique, in which three layers of 0.3 μm thickness each are deposited until the final thickness is reached. After the deposition of each layer, it was in-situ annealed before starting the deposition of the next layer. With the multilayer technique, coercivity of about 3.5 KOe and average value of the saturation magnetization Ms of about 4.0 K Gauss is obtained.
Epitaxial Mo(110)/Ni(111) superlattices were grown on (112 0) single-crystal sapphiresubstrates, by ion beam sputtering (IBS) and thermal evaporation (TE), in order to investigate the role of deposited energy on the interfacial mixing process observed in Mo sublayers. To separate intermixing and growth stress contributions, a careful and detailed characterization of the stress/strain state of both samples was performed by X-ray Diffraction (XRD). Non-equal biaxial coherency stresses are observed in both samples. For the IBS specimen, an additional source of stress, of hydrostatic type, due to growth-induced point defects, is present, resulting in a triaxial stress state. The use of ion irradiation to achieve a controlled stress relaxation can provide additional data and, as shown elsewhere, allows to obtain the stress-free lattice parameter a0 solely linked to chemical effects. For the TE sample, a standard biaxial analysis gives a0. In both samples, the a0 value is lower than the bulk lattice parameter, due to the presence of intermixed Mo(Ni) layers. However, the intermixing is larger in the sputtered Mo sublayers than in the thermal evaporated ones, putting forward the prime role of energy and/or momentum transfer occurring during energetic bombardment.
Decreasing the circuit dimensions is driving the need for low-k materials with a lower dielectric constant to reduce RC delay, crosstalk, and power consumption. In case of spin-on organosilicate low-k films, the incorporation of a porogen is regarded as the only foreseeable route to decrease dielectric constant of 2.2 or below by changing a packing density. In this study, MTMS-BTMSE copolymers that had superior mechanical properties than MSSQ were blended with decomposable polymers as pore generators. While adding up to 40 wt % porogen into MTMS:BTMSE=100:50 matrix, optical, electrical, and mechanical properties were measured and the pore structure was also characterized by PALS. The result confirmed that there existed a tradeoff in attaining the low dielectric constant and desirable mechanical strength, and no more pores than necessary to achieve the dielectric objective should be incorporated. When the dielectric constant was fixed to approximately 2.3 by controlling BTMSE and porogen contents simultaneously, the thermo-mechanical properties of the porous films were also investigated for the comparison purpose. Under the same dielectric constant, the increase in BTMSE and porogen contents led to improvement in modulus measured by the nanoindentation technique but deterioration of adhesion strength obtained by the modified edge lift-off test.
Mechanical and thermal properties of silicon nitride films deposited by different plasma process type have been studied. The initial mechanical stress, thickness and hydrogen content have been evaluated respectively by wafer curvature measurements, ellipsometry and Fourier Transform Infra Red spectrometry. These nitrides presented as-deposited stress values ranging from compressive to tensile. High temperature Rapid Thermal Anneal (RTA) at 1100°C or longer thermal treatments at medium temperature, from 700°C to 850°C were carried out on these materials. The evolution of their properties along the different anneals have been measured and compared to the behaviour of high temperature thermal nitride. One can observe that these stoechiometric plasma nitrides have shifted to an equilibrium tensile stress around 1100-1200 Mpa when submitted to the RTA, independently of their initial stress values. Results are interpreted in terms of H desorption and Si-N bond formation. Chemical reaction Si2-N-H + 2 N-H → 2 Si-N + NH3 appears to be the best candidate to figure out the phenomena.
Stresses in thin films are routinely measured by the so-called substrate curvature technique. These experiments are usually carried out in air or under a protective gas atmosphere. In this contribution we describe a new set-up capable of performing substrate curvature measurements under ultra-high vacuum conditions. The advantages are the absence of possible artifacts due to gas/film interactions, better control of gas composition, and the possibility to measure chemical effects on mechanical properties in a controlled way. We present first results that indicate an unexpected sensitivity even of polycrystalline Cu films to the gas environment.