Stress In Metal Films Research Articles

Stress Relaxation Related to Spontaneous Thin Film Buckling: Correlation between Finite Element Calculations and Micro Diffraction Analysis

Compressive residual stresses generated during thin film deposition may lead to undesirable film damage, such as delamination, buckling, and flaking, ultimately leading to the failure of the device employing the film. Understanding the residual stress generation and role in these damage mechanisms is necessary to preserve thin film integrity and optimize its functional properties. Thin shell theory has been used for decades to predict buckling but the results have not yet been correlated with experimental data since the techniques used to measure stress in metallic films were not able to do so at the required micron scale until recently. Micro scanning X-ray diffraction now enables the direct mapping of the local stress of metallic films. In this paper, finite element method based on thin shell theory and synchrotron X-ray micro diffraction have been used to determine stress maps of thin film buckling patterns. Calculations of the stress distribution in the metallic films have been performed taking into account the buckling geometry determined from optical measurements. Stress distributions over gold blisters and tungsten wrinkles obtained with the two techniques are in fair agreement and allow for the accurate determination of the stress relaxation profile from the bottom to the top of the buckling, validating the thin shell theory model.

Gold Layers on Elastomers near the Critical Stress Regime

AbstractSoft electrodes are essential components of soft robotics, tunable optics, microfluidics, flexible electronics, neuroprosthetics, and dielectric elastomeric transducers (DET). The two main paths employed to increase an electrode's compliance involve the manipulation of either its intrinsic material properties or its structural features, such as the introduction of wrinkles, which arise above the critical stress of metal films on elastomeric substrates. Herein, this study demonstrates that the interplay between functionalized oxygen‐plasma‐treated polydimethylsiloxane (PDMS) films and sputter‐deposited metal electrodes allows for conserving compressive stress within the electrode. Insulator–metal transition already occurs for 10 nm thin Au electrodes, and below this electrode thickness, atomic force microscopy nanoindentations with sub‐micrometer resolutions reveal no stiffening of the Au/PDMS heterostructure. These DETs exhibit reduced electrocreasing, which is a significant contributor to structural failure, while their enhanced dielectric breakdown field of up to 120 V µm–1 enables calculated strains above 10% — a crucial requirement for thin‐film DETs such as those used for artificial muscles.

Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

Computer simulation of cluster bombardment of a lead film on graphene

The problem of removal of a lead film from partially hydrogenated imperfect graphene by bombarding the target with a beam of Xe13 clusters having energies of 5–30 eV and an incident angle of 0° has been solved using the molecular dynamics method. Graphene is completely cleaned of lead at cluster energies of 10 and 15 eV. At higher beam energies, the lead can percolate through divacancies to the back side of graphene or, by virtue of plastic deformation, adhere to the front side. The separation of lead from the graphene film follows the cluster mechanism. The metal film torn off the graphene and acquired the cluster form has the lowest value of the horizontal mobility of the atoms among all of the systems under consideration and quite low values of vertical mobility. The separation of the lead film from the graphene can also result in a significant decrease in the value of certain components of the metal-film stress tensor and an increase in graphene roughness.

An experimental method for the determination of metal–polymer adhesion

A method is proposed in order to reduce significantly the influence of the coating shear stress and its cohesion failure on the normal adhesion stress of metallic films on polymers. Most methods used so far were not able to avoid the interference of the cohesion failure when the interface was delaminated in order to measure the adhesion strength. Our method has been proved to work and the results are more accurate than the standard pull-off method. The experiments consisted in delaminating copper films deposited by physical vapor deposition on four different kinds of polymeric substrates: polypropylene, polyamide 6, high impact polystyrene and polyethylene terephthalate. The polymer surfaces have also been modified by means of substrate sanding and flaming processes. Optical and scanning electron microscopies were used to study the morphology of both the removed copper film and the polymer surface after pulling-off. Adhesion work was measured by contact angle to determine the necessary energy to pull-off the metal–polymer interface.

Influence of hydrogen absorption on stress changes in thin catalytic metal films dedicated for sensors application

Abstract In the paper results of investigation of the influence of low concentration hydrogen on stress changes in thin catalytic metal films were discussed. The concentration of H2 was altered from 6 ppm to 1% of hydrogen (6N) in nitrogen (6N). Silicon beams covered with palladium or platinum films of various thicknesses were anchored at one end and their deflection at the other end was measured by atomic force microscope. Stress changes were determined by application of modified Stoney formula and compared with results of computer modelling. The influence of stress caused by hydrogen absorption on the alteration of output characteristics of AIII-N based hydrogen sensors was excluded. The time dependence of stress in metallic films for various hydrogen concentrations indicated dissociation limited mechanism of hydrogen absorption.

The Relationship between Residual Stress and Resistivity of Au/NiCr/Ta Multi-Layered Metallic Films by Magnetron Sputtering

Au/NiCr/Ta multi-layered metallic films were deposited on Al2O3 substrate by magnetron sputtering at different substrate temperature. The effect of substrate temperature on magnetron sputtering Au/NiCr/Ta films in crystal orientation, residual stress and resistivity was investigated. The all magnetron sputtering films were highly textured with dominant Au-(111) orientation or a mixture of Au-(111) and Au-(200) orientation. The residual stress in magnetron sputtering films at different substrate temperature was tensile stress with 155MPa-400MPa. A smallest resistivity of 3.6µΩ.cm was obtained for Au/NiCr/Ta multi-layered metallic films at substrate temperature 180°C. The experiment results reveal that the resistivity increased with the increase of the residual stress of metallic films.

Thermomechanical Stresses in Copper Films at Elevated Temperature

Thermomechanical stresses in metallic films are a root cause for material fatigue which limits the lifetime of electronic devices. Since the yield stress of metals is temperature dependent, plastic deformations during thermal cycling are increased at elevated temperature. This effect reduces the reliability of electronic parts. In order to investigate this problem, a 20 μm thick copper film was deposited on a silicon wafer. After annealing at 400°C, the sample was exposed to thermal cycles in the temperature range between room temperature and 600°C. The different values for the coefficient of thermal expansion of copper and silicon lead to a curvature of the sample. The wafer curvature was measured by a multilaser beam method. On the basis of the experimental results, a new theoretical model was developed that describes the stress evolution in the film during thermal cycling. In this investigation, the relation between wafer curvature and film stress is calculated by analogy to a model by Freund which is an improvement to the well-known Stoney formula. In addition to the elastic response, the new model considers plasticity of the copper film as well as temperature dependence of creep. It is demonstrated that the model can describe the experiment well and thus thermomechanical stress in copper films.

Apparatus for measuring local stress of metallic films, using an array of parallel laser beams during rapid thermal processing

The novel apparatus described here was developed to investigate the thermo-mechanical behavior of metallic films on a substrate by acquiring the wafer curvature. It comprises an optical module producing and measuring an array of parallel laser beams, a high resolution scanning stage, a rapid thermal processing (RTP) chamber and several accessorial gas control modules. Unlike most traditional systems which only calculate the average wafer curvature, this system has the capability to measure the curvature locally in 30 ms. Consequently, the real-time development of biaxial stress involved in thin films can be fully captured during any thermal treatments such as temperature cycling or annealing processes. In addition, the multiple parallel laser beam technique cancels electrical, vibrational and other random noise sources that would otherwise make an in situ measurement very difficult. Furthermore, other advanced features such as the in situ acid treatment and active cooling extend the experimental conditions to provide new insights into thin film properties and material behavior.

Thermomechanical Stresses in Copper Films at Elevated Temperature

Thermomechanical stresses in metallic films are a root cause for material fatigue which limits the lifetime of electronic devices. Since the yield stress of metals is temperature dependent, plastic deformations during thermal cycling are increased at elevated temperature. This effect reduces the reliability of electronic parts. In order to investigate this problem, a 20μm thick copper film was deposited on a silicon wafer. After annealing at 400°C, the sample was exposed to thermal cycles in the temperature range between room temperature and 600°C. The different values for the CTE of copper and silicon lead to a curvature of the sample. The wafer curvature was measured by a multi-laser beam method. On the basis of the experimental results, a new theoretical model was developed, which describes the stress evolution in the film during thermal cycling. In this investigation, the relation between wafer curvature and film stress is calculated by analogy to a model by Freund [1] which is an improvement to the well known Stoney formula. In addition to the elastic response, the new model considers plasticity of the copper film as well as temperature dependence of creep. It is demonstrated that the model can well describe the experiment and thus thermomechanical stress in copper films.

Measuring residual stress in glasses and ceramics using instrumented indentation

Instrumented indentation has yielded mixed results when used to measure surface residual stresses in metal films. Relative to metals, many glasses and ceramics have a low modulus-to-yield strength (E/σy) ratio. The advantage of this characteristic for measuring residual stress using instrumented indentation is demonstrated by a series of comparative spherical and conical tip finite element simulations. Two cases are considered: (i) a material with E/σy = 24—similar to glass and (ii) a material with E/σy = 120—similar to metal films. In both cases, compressive residual stress shifts the simulated load–displacement response toward increasing hardness, irrespective of tip geometry. This shift is shown to be entirely due to pile up for the “metal” case, but primarily due to the direct influence of the residual stress for the “glass” case. Hardness changes and load–displacement curve shifts are explained by using the spherical cavity model. Supporting experimental results on stressed glasses are provided.

Impact of Gate Metal-Induced Stress on Performance Modulation in Gate-Last Metal–Oxide–Semiconductor Field-Effect Transistors

The effect of gate metal film stress on the performance of gate-last metal–oxide–semiconductor field-effect transistors (MOSFETs) with a W/TiN/SiO2 gate stack was investigated. The channel strain induced by the electrode film was controlled by the W film stress. The W film stress was determined by the deposition method and film thickness. The compressive and tensile stresses of the electrode film enhance nFET and pFET performances, respectively. The enhancements of the performances are introduced not only by the stress of the film on the channel but also by the stress of the film on the side in the groove of the gate-last MOSEFTs. Metal electrode stress can be a further controlling factor in the performance of scaled devices.

Fabrication of Sharp Tetrahedral Probes with Platinum Coating

Metal-deposited probes with a 13 nm tip radius for conducting scanning probe microscopy (SPM) are obtained by sputter-coating a silicon cantilever with platinum on a single side. By optimizing the metal layered structure and the film stress, the silicon cantilever, whose spring constant and resonant frequency are 2 N/m and 70 kHz, respectively, is freestanding without bending. Besides the cantilever bending, curling of the probe apex was monitored and minimized in the series of the experiments conducted. The relatively soft probe apex was another part of the cantilever chip that was deformed by the metal film stress because the basic silicon tetrahedral probe was thin and inclined against the normal direction of the cantilever. In the final process, aluminum was deposited as a reflex coating on the back side of the silicon cantilever by vacuum evaporation because the light-reflectance was twice as high as that for platinum coating. The sharp probe apex, good conductivity of precious-metals and high reflection contribute to high-resolution conduction SPM measurements.

Intrinsic residual stresses in metal films synthesized by energetic particle deposition

Residual stresses are intrinsic to all physical vapor deposited coatings. Bombardment with energetic particles (ions or neutrals) has a strong influence on both the sign and the magnitude of intrinsic stresses. For example, stresses can change from large tensile to large compressive with increasing particle bombardment. We develop a model for the evolution of tensile residual stress with increasing bombardment based on the hypothesis that the tensile stresses originate due to the attractive interatomic forces between the coalescing crystallites. The model predictions are shown to compare favorably with experimental results.

Growth Stress of Thin Ti-, Al- and TiAlx-Films Deposited Under UHV-conditions and its Dependence on Substrate Temperature

AbstractThe growth stress of metal films was measured continuously both during as well as after their deposition under UHV-conditions with a cantilever beam technique. The metal films were deposited onto 10 rim thick alumina substrate films prepared by reactive evaporation of Al in an oxygen atmosphere. The substrate temperature for the metal deposition was varied from -20°C to 500°C.The growth stress of both titanium and aluminum films deposited at room temperature and above is characteristic of island growth and the formation of a polycrystalline film. The film stress built up in these films decreases with increasing substrate temperature. Below RT the growth stress of titanium films indicates a transition from island growth to layer by layer growth due to a reduced adatom mobility. The temperature range in which this transition in the growth mode occurs is strongly affected by impurities in the Ti-evaporation source material and gas ambient.In the last part of this paper we present results of experiments in which the above metals were evaporated simultaneously from separate sources to form alloy films with TiAl3-stoichiometry. Sudden changes in the incremental film stress are tentatively attributed to segregation and phase formation phenomena.

Mechanical properties of Al and Al alloys and their relationship to interconnection reliability

Pure Al, Al-3 wt %Si, and Al-15 at. % C films are characterized macroscopically in terms of yield stress and hardness and microscopically in terms of a grain structure to investigate the effects of Si and carbon additives to modify mechanical properties of the Al matrix. A theory is presented to estimate the yield stress of metal films. The experimental method is based on laser interferometer measurements of the varied substrate curvature involved with thermal treatment of a film–substrate composite. It is shown that carbon addition increases yield stress and hardness of Al, whereas Si addition decreases these properties. Carbon addition suppresses grain growth by 450 °C annealing as small as about 0.04 μm. Reliability tests on Al and Al-alloy interconnects revealed that increased yield stress improves resistance against failures due to the stress at high temperatures and high current densities, but degrades resistance against cycled thermal shock. The double layered Al–C/Al structure was highly resistant against both thermal and current stress.

X-ray diffraction determination of the effect of various passivations on stress in metal films and patterned lines

Direct x-ray diffraction determination of elastic strain and stress in aluminum and aluminum-silicon films and patterned lines has been used to investigate the effect of various passivations. Passivation over uniform metal films has very little effect. Passivation over patterned metal results in substantial triaxial tensile stress. Contrary to the conventional wisdom, high compressive stress in the passivation does not result in additional tensile stress in the metal. The deleterious effects of highly compressive silicon nitride on metal is probably due to the effect of excess hydrogen in the silicon nitride.

Stress relaxation during thermal cycling in metal/polyimide layered films

The stress relaxation behavior during thermal cycling of metal/quartz and metal/polyimide/quartz layered structures has been investigated using a cantilever bending beam technique. The metals chosen for this study include Cu and Cr, two materials with contrasting mechanical and interfacial bonding properties. A finite element analysis, as well as an analytical calculation, has been carried out to deduce the stress distribution in the layered structures. Results indicate that the extent of stress relaxation strongly depends on the intrinsic mechanical property of the metal film with significantly higher stresses residing in the Cr film than in the Cu film. The interfacial polyimide layer has been found to serve as an effective buffering layer to reduce the residual stress in metal films. Observations from transmission electron microscopy suggest that the stress is partially released through the deformation of the polyimide near the metal/polyimide interface.

Internal stresses in metallic films deposited by cylindrical magnetron sputtering

Previous studies of the internal stresses in thin films deposited at low T/T m using cylindrical post-cathode magnetron sputtering sources have identifieda transition working pressure for each coating material, below which the internal film stresses are compressive and above which they are tensile. New stress versus working pressure measurements for gadolinium, rhodium, chromium and three nickel-based alloys substantiate the generality of the stress-pressure dependence. The occurrence of compressive stresses correlates with the presence of entrapped argon in the deposits; however, the estimated magnitudes of the corresponding compressive strains remain invariant over large changes in the argon content. It is concluded that the mere presence of argon in the films does not cause compression. When materials sputtered from both post-cathode and hollow-cathode magnetron sources were deposited onto specially shielded substrates, large compressive stresses occurred only where geometric factors indicated enhanced bombardment by energetic argon reflected from the cathode. Atomic peening (forward sputtering and recoil damage to the depositing films) is suggested as a cause of the compressive stresses in the films sputtered at low working pressures.