Near-surface Stress Research Articles

Application of Low-k Liner for Stress and Capacitance Control in Cu-TSV

Through silicon via (TSV) is commonly fabricated by high aspect ratio deep silicon etching, lining with dielectric layer for electrical isolation and super-conformal filing with copper. It has been reported that Cu-TSV can exert thermo-mechanical stress on Si due to coefficient of thermal expansion (CTE) mismatch. This stress can result in undesired device mobility variation and structural reliability. In addition, TSV parasitic capacitance has the most predominant impact on the circuit operation. It is therefore imperative to reduce the TSV stress and capacitance. One solution is to use low-k dielectric as the liner since it has much lower elastic modulus and effective permittivity. In this work, low-k dielectric is successfully integrated in TSV as a liner. The implications on TSV stress, capacitance and leakage current are discussed. Due to its smaller elastic modulus (~7.2 GPa), the selected low-k carbon-doped oxide acts as a compliant layer to cushion the Cu-TSV stress on the Si compared with conventional oxide (75 GPa) by plasma enhanced chemical vapor deposition. This effectively reduces the near-surface compressive stress in Si by >25% compared with the conventional liner which is more rigid. Since the low-k liner has an effective dielectric constant of ~2.8, it is found that the integration of the low-k liner reduces the TSV capacitance by ~26% as compared with the conventional oxide liner. In summary, this work has provided evidence of the technical merits of a low-k material to mitigate the undesired Cu-TSV induced stress in the surrounding Si and the related parasitic capacitance.

Effect of Thermal Stresses on Carrier Mobility and Keep-Out Zone Around Through-Silicon Vias for 3-D Integration

Three-dimensional (3-D) integration with through-silicon vias (TSVs) has emerged as an effective solution to overcome the wiring limit imposed on device density and performance. However, thermal stresses induced in the TSV structures can affect the device performance by degrading carrier mobility and raise serious reliability concerns. In this paper, the effect of thermal stresses in TSV structures on carrier mobility and keep-out zone (KOZ) was investigated by focusing on the characteristics of the stresses near the surface where the electronic devices are located. The near-surface stresses were characterized by finite element analysis, and the stress effect on carrier mobility was evaluated by considering the piezoresistivity effect near the Si surface. In this paper, the elastic anisotropy of Si was taken into account to evaluate the effect on carrier mobility for both n- and p-channel MOSFET devices aligned along the [100] and [110] directions. The results showed a significant stress effect on carrier mobility, particularly for n-type Si with [100] device alignment and p-type Si with [110] device alignment. Based on these results, the dimension of the KOZ was estimated based on a criterion of 5% change in the carrier mobility. Finally, the effects due to stress interactions in a TSV array and plasticity in Cu vias on the KOZ were investigated. The effect of stress interaction was found to depend on the ratio of the pitch to diameter of the TSV array. When this ratio is less than 5, the stress interaction can increase the size of the KOZ. In contrast, the via material plasticity was found to be useful in reducing the stress level and hence the size of the KOZ.

Dynamic response of soil deposits to vertical SH waves for different rigidity depth-gradients

The response of soils to vertically propagating shear waves has hitherto been analytically studied by assuming a rigidity increase with depth according to some power-law. Solutions in terms of Bessel functions are well-known. However, the near-surface stress distribution due to the static foundation load or the presence of a stiff surface layer leads to a notable rigidity value at the surface that may decrease with depth. In the present paper, a three-parameter depth-function with bounded value at large depths, one that is capable of reproducing both positive and negative depth-gradients, is adopted. The analytical solution derived in terms of power series is used to study elastic-rock amplification characteristics and modal shapes. Approximate expressions for the fundamental natural frequency of an elastic layer are given, covering a wide range of the parameters involved. Using the transfer matrix approach for multi-layered ground and the solution for a linear depth-profile for the individual layers, the accuracy of discretization schemes is examined. Finally, the case of a 2D rigidity variation is investigated by means of a numerical code.

Micro-Raman spectroscopy and analysis of near-surface stresses in silicon around through-silicon vias for three-dimensional interconnects

Three-dimensional integration with through-silicon vias (TSVs) has emerged as an effective solution to overcome the wiring limit imposed on device density and performance. However, thermal stresses induced in TSV structures raise serious thermomechanical reliability concerns. In this paper, we analyze the near-surface stress distribution in a TSV structure based on a semi-analytic approach and finite element method, in comparison with micro-Raman measurements. In particular, the depth dependence of the stress distribution and the effect of elastic anisotropy of Si are illustrated to properly interpret the Raman data. The effects of the surface oxide layer and metal plasticity of the via material on the stress and Raman measurements are discussed. The near-surface stress characteristics revealed by the modeling and Raman measurements are important for design of TSV structures and device integration.

The role of frictional plasticity in the evolution of normal fault systems

Tectonic extension applied to the upper crust commonly results in the development of sub-parallel arrays of faults and fault belts. A computational model of an extending elastic-viscous-plastic crust is presented, which describes the evolution of stress, strain and displacement in the vicinity of an active normal fault. Within the upper crust discrete episodes of fault slip accommodate up to half the applied extension rate, with the balance accommodated by bands of frictional-plastic shearing representing young proto-faults. Repeated coseismic shear stress reductions in the near-fault vicinity impose a stress-failure shadow where the formation of new faults is prohibited. A near-surface compressional stress regime in the footwall causes the failure shadow to extend disproportionally into this fault block, thereby favoring the formation of new faults in the hanging wall. A continuous shallowing of fault dip is also documented and supports a scenario of migration of faulting into the hanging wall of existing faults, consistent with descriptions of the structural evolution in the western Gulf of Corinth, Greece.

Thermomechanical forcing of deep rock slope deformation: 1. Conceptual study of a simplified slope

[1] Thermo-elastic rock slope deformation is often considered to be of relatively minor importance and limited to shallow depths subject to seasonal warming and cooling. In this study, we demonstrate how thermomechanical (TM) effects can drive rock slope deformation at greater depths below the annual thermal active layer. Here in Part 1 of two companion papers, we present 2D numerical models of a simplified slope subject to annual surface temperature cycles. The slope geometry and discontinuity sets are loosely based on the Randa instability considered in detail in Part 2. Results show that near-surface thermo-elastic stresses can propagate to depths of 100 m and more as a result of topography and elasticity of the rock mass. Shear dislocation along discontinuities can have both a reversible component controlled by discontinuity compliance and, provided that the stress state is sufficiently close to the strength limit, an irreversible component (i.e., slip). Induced slip increments are followed by stress redistribution resulting in the propagation of slip fronts. Thus, deformation and progressive rock slope failure can be driven solely by thermomechanical forcing. The influence of TM-induced stress changes becomes stronger for increasing numbers of critically stressed discontinuities and is enhanced if failure of discontinuities involves slip-weakening. The net TM effect acts as a meso-scale fatigue process, involving incremental discontinuity slip and hysteresis driven by periodic loading.

A coherent look at stress

Molecular ligands are widely used to functionalize gold nanoparticles, but their influence on the particle structure has been difficult to probe. Coherent X-ray diffraction has now reached sufficient sensitivity to resolve adsorption-induced near-surface stress in a single nanocrystal.

Impact of Wide-Based Tires on the Near-Surface Pavement Stress States Based on Three-Dimensional Tire-Pavement Interaction Model

Impact of Wide-Based Tires on the Near-Surface Pavement Stress States Based on Three-Dimensional Tire-Pavement Interaction Model

Space Charge Induced Surface Stresses: Implications in Ceria and Other Ionic Solids

Volume changes associated with point defects in space charge layers can produce strains that substantially alter thermodynamic equilibrium near surfaces in ionic solids. For example, near-surface compressive stresses exceeding -10 GPa are predicted for ceria. The magnitude of this effect is consistent with anomalous lattice parameter increases that occur in ceria nanoparticles. These stresses should significantly alter defect concentrations and key transport properties in a wide range of materials (e.g., ceria electrolytes in fuel cells).

Wind Stress Curl and Coastal Upwelling in the Area of Monterey Bay Observed during AOSN-II

Abstract Aircraft measurements obtained during the 2003–04 Autonomous Ocean Sampling Network (AOSN-II) project were used to study the effect of small-scale variations of near-surface wind stress on coastal upwelling in the area of Monterey Bay. Using 5-km-long measurement segments at 35 m above the sea surface, wind stress and its curl were calculated with estimated accuracy of 0.02–0.03 N m−2 and 0.1–0.2 N m−2 per 100 kilometers, respectively. The spatial distribution of wind speed, wind stress, stress curl, and sea surface temperature were analyzed for four general wind conditions: northerly or southerly wind along the coastline, onshore flow, and offshore flow. Wind stress and speed maxima frequently were found to be noncollocated as bulk parameterizations imply owing to significant stability and nonhomogeneity effects at cold SST pools. The analyses revealed that complicated processes with different time scales (wind stress field variation, ocean response and upwelling, sea surface currents, and heating by solar radiation) affect the coastal sea surface temperature. It was found that the stress-curl-induced coastal upwelling only dominates in events during which positive curl extended systematically over a significant area (scales larger than 20 km). These events included cases with a northerly wind, which resulted in an expansion fan downstream from Point Año Nuevo (wind speed peaks greater than about 8–10 m s−1), and cases with an offshore/onshore flow, which are characterized by weak background upwelling due to Ekman transport. However, in general, observations show that cold pools of sea surface temperature in the central area of Monterey Bay were advected by ocean surface currents from strong upwelling regions. Aircraft vertical soundings taken in the bay area showed that dominant effects of the lee wave sheltering of coastal mountains resulted in weak atmospheric turbulence and affected the development of the atmospheric boundary layer. This effect causes low wind stress that limits upwelling, especially at the northern part of Monterey Bay. The sea surface temperature is generally warm in this part of the bay because of the shallow oceanic surface layer and solar heating of the upper ocean.

Analysis and Prediction of Residual Stresses in Nitrided Tool Steel

Plasma nitriding is a thermo-chemical process of high importance for engineering components, which through generation of near-surface compressive residual stresses significantly improves wear and fatigue resistance. A precise knowledge of the level and distribution of residual stresses that exist in surface engineered components is necessary for accurate prediction of a component’s fatigue resistance. However, measurement of residual stress is not always possible, especially in the case of industrial tools and dies. Therefore, other methods for residual stress evaluation and prediction are required by industry. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase by nitriding time and temperature. On the other hand, experimental data show that residual steel distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution.

3-D x-ray strain microscopy in two-phase composites at submicron length scales

Renewed interest in composite materials is driven by the fact that their mechanical properties can be superior to those of individual constituent phases. Interfaces between the phases and the properties of individual phases are the two key elements responsible for the unique micro-mechanisms of plastic deformation in composites. In this research summary we show how the depth-dependent residual strain distributed in the two phases and partitioned across the composite interfaces can be directly measured at submicron length-scales using x-ray microdiffraction and compared to a detailed simulation within the framework of micromechanical stress analysis. Interface strength is determined from the analysis of the so-called “slip zone” caused by the near-surface stress relaxation. Two examples are discussed: Mo-NiAl and AG-15 composites.

Impact of Wide-Based Tires on the Near-Surface Pavement Stress States Based on Three-Dimensional Tire-Pavement Interaction Model

ABSTRACT The effects of truck tire types on near-surface pavement responses were evaluated via finite element anyalysis. First, two wide-based truck radial tires (425/65R22.5 and 445/50R22.5) were modeled based on the tire geometries and specifications from the tire manufactures. Accordingly, tire-pavement interaction models were developed. These models were then calibrated to make sure models can be used for further evaluation purpose. A study on how truck tire types affect near-surface respones were investigated based on calibrated tire-pavement contact models. The results indicated that the Super Single (SS) (425/65R22.5) tire produced the worst damage to the pavement in terms of top-down cracking and AC rutting in Asphalt Concrete (AC) layers, while New Generation Wide-Based (NGWB) tire (445/50R22.50) induced approximately the same damage as the standard dual tire assembly (11R22.5) evaluated in this study.

Evaluation of Near-Surface Stress States in Asphalt Concrete Pavement

A sophisticated three-dimensional (3-D) tire model was placed directly on a three-layer asphalt concrete (AC) pavement system to form a 3-D tire–pavement contact model. The model was then verified with measured contact stresses and was used to investigate near-surface stress states in the AC layer. When compared with the traditional uniform vertical loading model, the 3-D tire–pavement contact model produced stress states not only higher in magnitude but also more variable in distribution. The 3-D tire–pavement contact model produced much higher principal tensile stress and maximum shear stress near the tire edge; the increased stress was a possible explanation for instability rutting and associated top-down cracking. A critical shear plane was developed for the top 50 mm of the AC layer by using a p-q diagram, and the tire–pavement contact model produced a much higher shear yield percentage.

Mechanisms of Residual Stress Relaxation and Redistribution in Welded High-Strength Steel Specimens under Mechanical Loading

In this article, the relaxation of surface and near-surface welding residual stress fields in S690QL under static and cyclic loadings has been studied. The influence of local mechanical properties in the weld metal, heat-affected zone and in the base metal upon relaxation and redistribution of residual stress were considered. Welded specimens were loaded statically and cyclically. Under static loading, the variation of the residual stress field under tension and compression was investigated and it was observed that, by increasing the load, residual stress relaxes continuously. However, the relaxation under compression sets in with delay, since the tensile residual stresses should be first overcome. Under cyclic loading, with or without mean stress, the behaviour of the residual stresses depends on the initial residual stress, local yield strength and maximum applied load. If the von Mises stress, which is a function of residual stress, load and mean stress, exceeds the local yield strength, plastic deformation and thus a relaxation occurs. In the case of relaxation, the first load cycle is of importance, in which a considerable amount of the initial residual stress field is relaxed. After the first relaxation, no significant variation of the residual stress field in the fatigue crack free phase could be observed. When no residual stress relaxation in the first load cycle occurred, the number of load cycles did not change the residual stress field. The specimens were loaded until failure or maximum 2×106 cycles.

Implementation of a Nonlinear Subfilter Turbulence Stress Model for Large-Eddy Simulation in the Advanced Research WRF Model

Abstract Two formulations of a nonlinear turbulence subfilter-scale (SFS) stress model were implemented into the Advanced Research Weather Research and Forecasting model (ARW-WRF) version 3.0 for improved large-eddy simulation performance. The new models were evaluated against the WRF model’s standard Smagorinsky and 1.5-order turbulence kinetic energy (TKE) linear eddy-viscosity SFS stress models in simulations of geostrophically forced, neutral boundary layer flow over both flat terrain and a shallow, symmetric transverse ridge. Comparisons of simulation results with similarity profiles indicate that the nonlinear models significantly improve agreement with the expected profiles near the surface, reducing the overprediction of near-surface stress characteristic of linear eddy-viscosity models with no near-wall damping. Comparisons of simulations conducted using different mesh sizes indicate that the nonlinear model simulations at coarser resolutions agree more closely with the higher-resolution results than corresponding lower-resolution simulations using the standard WRF SFS stress models. The nonlinear models produced flows featuring a broader range of eddy sizes, with less spectral power at lower frequencies and more spectral power at higher frequencies. In simulated flow over the transverse ridge, distributions of flow separation and reversal near the surface simulated at higher resolution were likewise better depicted in coarser-resolution simulations using the nonlinear models.

Effect of the residual near-surface stresses of the working surface on the contact resistance of rolls

The results of contact resistance tests of roll steel samples with various levels of near-surface residual compressive stresses are analyzed. A relation between these stresses and the contact resistance is found. Recommendations are made for the formation of rational near-surface residual stresses in the active surfaces of rolls in order to increase their resistance.

Calculating Reynolds Stresses from ADCP Measurements in the Presence of Surface Gravity Waves Using the Cospectra-Fit Method

Abstract Recently, the velocity observations of acoustic Doppler current profilers (ADCPs) have been successfully used to estimate turbulent Reynolds stresses in estuaries and tidal channels. However, the presence of surface gravity waves can significantly bias stress estimates, limiting application of the technique in the coastal ocean. This work describes a new approach to estimate Reynolds stresses from ADCP velocities obtained in the presence of waves. The method fits an established semiempirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves to estimate the stress. Applied to ADCP observations made in weakly stratified waters and variable significant wave heights, estimated near-bottom and near-surface stresses using this method compared well with independent estimates of the boundary stresses in contrast to previous methods. Additionally, the vertical structure of tidal stress estimated using the new approach matched that inferred from a linear momentum balance at stress levels below the estimated stress uncertainties. Because the method makes an estimate of the horizontal turbulent length scales present as part of the model fit, these results can also enable a direct correction for the mean bias errors resulting from instrument tilt, if these scales are long relative to the beam separation.

The present-day stress field in Egypt

The present-day stress field has been investigated by the analysis of the directions of maximum horizontal stress (o 1 ) inferred from earthquake focal mechanisms and borehole breakouts in Egypt. The results indicate that strike-slip and normal faulting movements characterize the majority of the earthquake focal mechanisms; only a few events are of reverse faulting type. The analysis of 35 mechanisms suggests that the present-day stress field in Southern Egypt is dominated by a strike-slip stress regime (SS) and it is mainly transtensional (NS: normal faulting with strike-slip component) in Northern Egypt. The orientation of P-axes reflects that the maximum horizontal stress (o 1) in Southern Egypt is uniform and aligned to nearly E-W direction while in Northern Egypt it is aligned with an even mix of NW-SE and nearly E-W compression. Along the Gulf of Aqaba, the southern part of the Dead Sea Fault (DSF), the focal mechanism solutions indicate that the maximum horizontal stress is presently oriented NW-SE, corresponding to a strike-slip mechanism in concert with geological evidence. More detailed investigations have been performed for the Gulf of Suez. We compare our results to the near-surface stress measurements from borehole breakouts to see if there is a change in orientations with depth. Shallow stress directions derived from borehole breakouts are not consistent with the deep stress directions derived from earthquakes focal mechanisms. About 73% of 30 borehole breakouts measurements indicate NW-SE alignment of the maximum horizontal stress and 27% are ENE-WSW. The direction of o 1 inferred from the focal mechanism solutions is changing from NE-SW to ENE-WSW. Therefore, at least in this area, the stress direction is not constant throughout the crust.

Near-surface failure of layered viscoelastic materials

Within the framework of a piecewise homogeneous body model, with the use of exact equations of the geometrically nonlinear theory of viscoelastic bodies, the distribution of near-surface self-balanced normal stresses in a body consisting of a viscoelastic half-plane, an elastic locally curved bond layer, and a viscoelastic covering layer is investigated. A method for solving the problem considered by employing the Laplace and Fourier transformations is developed. Numerical results for the self-balanced normal stresses caused by a local curving (imperfection) of an elastic bond layer upon tension and compression of the body mentioned along the free face plane are presented and analyzed. The viscoelasticity of the materials is described by the Rabotnov fractional-exponential operators. A macroscopic failure criterion is proposed, and the validity of this criterion is examined.