The influence of mechanical constraint imposed by device geometry upon the switching response of a ferroelectric thin film memory capacitor is investigated. The memory capacitor was represented by two-dimensional ferroelectric islands of different aspect ratio, mechanically constrained by surrounding materials. Its ferroelectric non-linear behaviour was modeled by a crystal plasticity constitutive law and calculated using the finite element method. The switching response of the device, in terms of remnant charge storage, was determined as a function of geometry and constraint. The switching response under applied in-plane tensile stress and hydrostatic pressure was also studied experimentally. Our results showed that (1) the capacitor’s aspect ratio could significantly affect the clamping behaviour and thus the remnant polarization, (2) it was possible to maximise the switching charge through the optimisation of the device geometry, and (3) it is possible to find a critical switching stress at zero electric field and a critical coercive field at zero residual stress.

The role of mechanical constraint upon the switching response of a ferroelectric thin film memory capacitor is explored. The memory capacitor is represented by a two dimensional ferroelectric island whose non-linear behaviour is modelled by a crystal plasticity constitutive law within the finite element method. The switching response of the device, in terms of remnant charge storage, is determined as a function of geometry and constraint. Various types of constraint on the ferroelectric capacitor are considered, including the presence of a silicon dioxide passivation layer, a silicon substrate and metallic electrodes. The effect of the relative resistance to 90 degree switching and 180 degree switching is also explored in a tetragonal ferroelectric device. Throughout the study, the finite element calculations are compared with the behaviour of a material element subjected to various degrees of mechanical constraint.

The performance of one and two-dimensional ferroelectric memory capacitors characterized by the hysteresis of applied electric field versus the surface charge density is investigated using the finite element method. Sensitivity of the electrical hysteresis of the 2D capacitor constrained by compliant layers to selected geometrical variables is explored. The remnant polarization and the coercive filed are compared with those of a free-standing film and a film fully constrained by the substrate. The aspect ratio can significantly influence the clamping behavior and thus the remnant polarization of the capacitor.

The finite element method is used to investigate the performance of a ferroelectric random access memory as a function of its geometry. Performance is characterised by the charge versus electric field relation, and the sensitivity of performance to geometry is explored. The primary geometric variables are the dimensions of a prismatic two-dimensional (2D) island of ferroelectric material, and the edge inclination angle caused by the etching process along the sides of the island. The performance of the two-dimensional ferroelectric device is compared to those of an unsupported ferroelectric thin film and of a ferroelectric film bonded to a substrate.

We applied uniform in-plane tensile stress on PZTN ferroelectric thin films of 130 nm in thickness, using a four-point bending test rig and measured the corresponding changes in polarization and leakage current. An initial polarization hysteresis loop, which is different from the repeatable one after repetitive loadings, was observed. In the range of the tensile stress we applied (up to 150 MPa), the remnant polarization decreased linearly in reference to the repeatable hysteresis at a rate of similar to 0.002 mu C cm(-2) MPa-1 which is more than two orders smaller than that of bulk PZT materials. The corresponding coercive voltage increased very little (< 0.02V), but the leakage current at small voltages showed a sudden increase during the loading process, changing from an ohmic behaviour (n = 1) when the stress is < 53 MPa to a sub-linear one (n = 0.4) for I similar to V-n when the stress is > 109MPa.

Surface and subsurface responses during frictionless indentation of elasto-plastic solids are investigated. Cases of monotonic and repeated loading are considered. It is shown that the role of plasticity parameters on indentation behavior cannot be well described in terms of surface response alone. It must be tied with subsurface response. In particular, the difference between the materials exhibiting isotropic hardening and that exhibiting kinematic hardening is less apparent when the force-displacement response and the deformed surface from the two are compared. Yet, it is quite apparent when subsurface response such as plastic strain, residual stress and plastic zone dimension are compared. An attempt has been made to characterize such surface and subsurface responses, for different plastic behavior, and to compare them with estimations obtained from analytical solutions.

Simulations of spherical indentation of a substrate coated with a protective layer have been performed in the elastic deformation range. They provide key responses that can be used to examine the load resistance of a coated substrate. The key responses result in diagrams indicating whether yielding or fracture occurs first. In this kind of diagram, the ratio of coating fracture strength to substrate yield is related to the contact radius in indentation. As it is difficult to determine the contact radius experimentally, the latter has been found numerically and has also been approximated analytically. The analytical approximation plays an important role as it identifies the general loading condition for various sphere radii. This approximation can be extended to predicting the unloading curve which allows the elastic properties of a coated substrate to be determined.

In metal forming, failure of coated tools often indicate problems related to sliding contact, fretting, and substrate plasticity. To date, failure prediction of tools subjected to sliding and fretting contact has been largely based on elastic analysis if not empirical. As an effort to understand the issue and specifically, to investigate the role of plasticity, a finite element-based analysis of plane strain contact of brittle coating is performed. The problem consists of cyclically loading a rounded-edge punch in normal and tangential directions to a coated substrate. The results are presented for the maximum principal stress in the coating indicative of brittle failure and the evolution of plastic deformation in the substrate. These quantities are analyzed as functions of substrate's plasticity parameters, coating's elastic modulus, friction, and loading parameters. A combined isotropic-kinematic hardening plasticity law is considered. The prediction of the tool's service life based on the above failure conditions is also discussed.

A bifurcation analysis was carried out for a class of layered materials whose constitutive behavior were described by the model of Christoffersen and Jensen, 1996. The problem was solved in uniaxial plane strain compression for compressible materials without volumetric plastic strain. The elastic-plastic response of the constituent materials is described by J(2)-deformation theory and J(2)-flow theory. Results indicate that bifurcation stresses vary smoothly with wavelength and are sensitive, to the moduli ratio between constituent materials and the volume fractions. The results show that there is a transition in the bifurcation mode from an Euler type elastic bifurcation of a slender column made of a composite material to a surface bifurcation mode in the plastic regime for a wide plate. Furthermore, effects of residual stress were also incorporated and studied. (C) 2004 Elsevier SAS. All rights reserved.