Intrinsic Stress Evolution Research Articles

Impact of Ge alloying on the early growth stages, microstructure and stress evolution of sputter-deposited Cu-Ge thin films

The effect of Ge alloying on the growth morphology and intrinsic stress evolution during deposition of Cu thin films is investigated using in situ and real time diagnostics, complemented by ex situ microstructural characterizations. Cu1-xGex alloys with x up to 0.15 were sputter-deposited at room temperature on Si substrates, while measuring simultaneously the substrate curvature evolution and surface differential reflectance spectroscopy during the early stages of growth. All films exhibit a typical compressive-tensile-compressive stress behavior, which underpins the distinct stages of a Volmer-Weber growth mode; however, it is revealed that the Ge content plays a decisive role on the nucleation and coalescence stages. This results in a grain size refinement and development of higher compressive stresses for Ge solute additions up to x = 0.07. At higher Ge contents, the strong chemical affinity between Cu and Ge atoms leads to a germanide compound (Cu5Ge) formation and even higher compressive stress. The stress evolutions are discussed considering the surfactant character of Ge atoms affecting Cu adatom mobility and growth morphology. This study highlights the possibility to tailor the early stages of growth and film microstructure (grain size, texture, surface roughness) by introducing solute concentrations of Ge in Cu films. We also determined for the elemental Cu film the percolation and film continuity thicknesses and discussed theses values with respect to those reported for other metallic films.

Intrinsic stress evolution during different growth stages of diamond film

The CVD diamond films at different growth stages were prepared on the same substrate by DC arc plasma jet CVD. The surface morphologies and residual stress of diamond films were characterized by scanning electron microscope and Raman spectroscopy. The intrinsic stress of diamond films were obtained by subtracting thermal stress from residual stress. The results showed that the intrinsic compress stress was 0.42GPa for the precoalescence stage due to the sp2 carbon inclusions in diamond islands. With the diamond islands growing up and even snapping together to form grain boundary at the coalescence stage the intrinsic compress stress was 0.06GPa. Eventually, the diamond islands coalesced into a continuous polycrystalline film, the intrinsic compress stress vanished and converted to tensile stress, the intrinsic tensile stress reached 1.41GPa due to the number of grain boundaries increasing. The evolution of intrinsic stress during diamond film initial growth was mainly attributed to the surface free energy of diamond islands changed to the free energy of grain boundaries which was consistent with the Nix-Clemens model.

Microstructure and intrinsic stress evolution during epitaxial film growth of an Ag0.93Al0.07 solid solution on Si(111); excessive planar faulting due to quantum confinement

The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ag0.93Al0.07 solid solution on a Si(111)-7×7 surface at 300 K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and real-time in-situ stress measurements. Up to a film thickness of 6 ± 2 nm, epitaxial Ag0.93Al0.07 film growth is characterized by the strikingly extensive formation of planar faults parallel to the film/substrate interface, while at larger thickness the film grows practically defect-free. As revealed by real-time in-situ stress measurements, the extensive formation of planar faults at the very initial stage of growth is not driven by the reduction of the system's elastic strain energy but is rather caused by a striking thickness-dependence of the stacking-fault energy owing to a quantum size effect of the ultrathin metal alloy film, resulting in a frequent succession of fcc and hcp stackings of close-packed layers during the initial stage of film growth. The extensive development of planar faults at the initial stage of film growth (<6 ± 2 nm) is associated with the occurrence of a high density of kinks and corners at thereby atomically rough surface ledges, which strongly enhances the downward transport of adatoms from higher to lower terraces (interlayer mass transport) by a reduction of the effective diffusion barrier at the edge of surface steps and by increasing the driving force for adatoms to attach to the surface ledges. As a result, the epitaxial Ag0.93Al0.07 film initially grows in a 2D layer-by-layer type of growth and thus establishes atomically smooth film surfaces. For the practically planar-fault-free growth at thicknesses beyond 6 ± 2 nm, interlayer mass transport becomes distinctively limited, thereby inducing a transition from 2D to 3D type of film growth.

Control of stress and threading dislocation density in the thick GaN/AlN buffer layers grown on Si (111) substrates by low- temperature MBE

We report on successful growth by plasma-assisted molecular beam epitaxy on a Si(111) substrate crack-free GaN/AlN buffer layers with a thickness more than 1 μm. The layers fabricated at relatively low growth temperature of 780°C have at room temperature the residual compressive stress of -97 MPa. Intrinsic stress evolution during the GaN growth was monitored in situ with a multi-beam optical system. Strong dependence of a stress relaxation ratio in the growing layer vs growth temperature was observed. The best-quality crack-free layers with TDs density of ∼⃒109 cm-2 and roughly zero bowing were obtained in the sample with sharp 2D-GaN/2D-AlN interface.

Effects of oblique-angle deposition on intrinsic stress evolution during polycrystalline film growth

Precise control of the residual stress in polycrystalline films, which remains a central but difficult task in a variety of emerging technologies, requires synergistic manipulation of multiple processing parameters. In addition to the substrate temperature and the rate of deposition, the angle of incidence of the deposition flux is known to affect structure evolution processes during film formation. However, its role in influencing the evolution of the intrinsic stress has not been determined. In this work, we investigate the effects of oblique-angle deposition on intrinsic stress evolution in polycrystalline gold and nickel films, using in situ stress measurements as well as ex situ surface and microstructure characterization. We find that as the angle between the surface normal and the direction of the deposition flux increases, the thickness at which film coalescence occurs increases and the post-coalescence stress shifts toward the tensile direction. We suggest that the first trend is associated with the effects of shadowing on the nucleation density in the pre-coalescence regime, and we attribute the second trend to an increase in surface roughness and shadowing effects associated with the dome-shaped surfaces of individual grains. According to this view, oblique-angle deposition on a mesoscopically rough surface eliminates or reduces condensation at and near grain boundaries, and therefore lowers the rate of adatom–grain boundary attachment, resulting in reduction in the compressive component of the intrinsic stress. The stress evolution map built from this work suggests routes to achieve specific stress levels in polycrystalline films.

Intrinsic stress evolution during amorphous oxide film growth on Al surfaces

The intrinsic stress evolution during formation of ultrathin amorphous oxide films on Al(111) and Al(100) surfaces by thermal oxidation at room temperature was investigated in real-time by in-situ substrate curvature measurements and detailed atomic-scale microstructural analyses. During thickening of the oxide a considerable amount of growth stresses is generated in, remarkably even amorphous, ultrathin Al2O3 films. The surface orientation-dependent stress evolutions during O adsorption on the bare Al surfaces and during subsequent oxide-film growth can be interpreted as a result of (i) adsorption-induced surface stress changes and (ii) competing processes of free volume generation and structural relaxation, respectively.

Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the a-Ge substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO2 and a-Ge substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive → tensile → compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. It was concluded that (i) the microstructural development of metallic thin films is predominated by the surface diffusivity of the adatoms and (ii) the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity.

Impact of Surface Chemistry on Grain Boundary Induced Intrinsic Stress Evolution during Polycrystalline Thin Film Growth

First principles calculations were integrated with cohesive zone and growth chemistry models to demonstrate that adsorbed species can significantly alter stresses associated with grain boundary formation during polycrystalline film growth. Using diamond growth as an example, the results show that lower substrate temperatures increase the hydrogen content at the surface, which reduces tensile stress, widens the grain boundary separations, and permits additional atom insertions that can induce compressive stress. More generally, this work demonstrates that surface heteroatoms can lead to behavior which is not readily described by existing models of intrinsic stress evolution.

Intrinsic stress evolution in nanocrystalline diamond thin films with deposition temperature

The stress evolution in nanocrystalline diamond (NCD) films deposited at different temperatures (from 800to400°C) was investigated. Results showed that the intrinsic stress gradually changed from tensile to compressive with decreasing deposition temperature. Most importantly, the intrinsic stress can be tailored to zero by adjusting the deposition temperature, which is critical to many applications. It has been proven that more H as well as sp2 bonded carbon was incorporated into the grain boundaries, which was responsible for the evolution of stress and other mechanical properties with deposition temperature. Moreover, all the NCD films showed excellent mechanical properties.

Synthesis and Intrinsic Stress of Ti-Containing Carbon Nitride Nanocomposite Films

Titanium containing carbon nitride (CNx) films are prepared by radio frequency magnetron sputtering method. The evolution of intrinsic stress within the nanocomposite films is monitored during growth by using an in situ bending-plate method. The effect of Ti-containing concentration on intrinsic stress is investigated. XRD data shows that TiN nanocrystals are synthesized to embed into the CNx matrix. The film intrinsic stress depends obviously on the content of Ti. We suggest that grain boundary effect is responsible to the intrinsic stress variation as a function of the content of Ti.

Growth stresses and cracking in GaN films on (111) Si grown by metal-organic chemical-vapor deposition. I. AlN buffer layers

Intrinsic stress evolution during the growth of GaN by metal-organic chemical-vapor deposition on (111) Si, using an AlN buffer layer, was monitored in situ with a multiple-beam optical stress sensor. Data show that stress evolution takes place in two stages: an initial compressive regime up to about 100nm in thickness followed by a transition to a constant tensile stress, ∼0.3GPa, in films up to 1μm thick. Correlation of the stress evolution with surface morphological evolution by sequential atomic force microscopy images clearly shows that the incremental stress remains compressive in spite of grain coalescence, which is generally considered to be the dominant source of tensile stress in GaN films on sapphire. Rather, the most dominant feature accompanying the transition in stress from compressive to tensile, which takes place after grain coalescence, is an increase in the lateral size of individual islands. It is shown that this incremental tensile stress accompanied by an increase in lateral grain size can be accounted for by the annihilation of free volume associated with the grain boundaries. On samples cooled to room temperature, surface cracks mainly on the (1010) planes are observed to have channeled in films thicker than 250nm. Analysis of cracking using the theory of brittle fracture, using the measured growth stress profile and value for the critical thickness, yields a thermal-expansion mismatch stress off 1.1GPa for GaN films deposited at 1100°C and cooled to room temperature.

Residual stress and microstructural evolution in thin film materials for a micro solid oxide fuel cell (SOFC).

ABSTRACTThe stability of multilayered membrane structures is a major challenge in the development of microfabricated solid oxide fuel cells (SOFC). The work presented here explores residual stress in sputter-deposited yttria stabilized zirconia (YSZ) thin films (5nm – 1000nm thickness) as a function of deposition pressure and substrate temperature. The results indicate variations in intrinsic stress from ∼0.5GPa compressive to mildly tensile (∼50 MPa). Microstructure is characterized by x-ray diffraction (XRD). The evolution of intrinsic stress with temperature is investigated by thermally cycling YSZ films deposited on silicon wafers. Observed changes of 100s of MPa in the intrinsic stress component of the film serve as indicators of possible changes in microstructure. Such changes in microstructure are subsequently characterized using x-ray diffraction of as-deposited and annealed films. Correlations with relevant mechanisms and models of residual stress evolution are discussed. Finally, use of such residual stress data in the fabrication and design of mechanically stable multilayered membranes for micro SOFC devices is discussed.

Intrinsic stress evolution in laser deposited thin films

In situ stress measurements were performed on polycrystalline Permalloy and Ag thin films laser deposited in ultrahigh vacuum (UHV) and at different Ar gas pressures. In UHV, when the kinetic energy of the particles is high (about 100 eV), in the initial growth stage the stress is dominated by the surface energy and intermixing effects. With increasing deposition time, capillary-induced compressive growth stress is observed. Additionally, the film stress is strongly influenced by the growth mode (island growth or layer-by-layer growth). In the case of Volmer–Weber growth, island zipping generates tensile stress, as soon as island impingement and coalescence occurs. In the late stages, compressive stress due to shot-peening and implantation dominates the measurements, similar as in sputtered films. The depth of influence of the impinging particles is determined to be about 3 nm. With increasing Ar pressure (or at low laser fluence) the impinging particles are slowed down and implantation or intermixing effects are diminished. This is accompanied by changes in the film morphology and texture. At high Ar pressures a compressive-to-tensile transition occurs and the laser deposited films become more comparable to evaporated samples with an open structure. These results can be understood by a combination of stress formation and relaxation effects below the film surface.

Evolution of intrinsic stress in nanocrystalline-diamond film deposited by continuous H[sup +] ion bombardment

An in situ bending-plate method is used to measure intrinsic stress during the growth process of nanocrystalline-diamond film. Nanometer-sized diamond film is prepared by continuous H+ ion bombardment under different energies induced by applying a negative-bias voltage at the substrate relative to the grounded-vacuum chamber using the microwave-plasma chemical-vapor-deposition (MWPCVD) method. The effects of substrate-bias voltage, temperature, total pressure, and CH4 concentration on intrinsic stress during the film-growth process are investigated. The results indicate that high bias voltage and high substrate temperature are beneficial when synthesizing smooth nanocrystalline-diamond film. This film usually relates to high intrinsic compressive stress. This stress can be attributed to the high secondary-nucleation rate and grain-boundary density. The evolution of intrinsic stress presents a complicated process at different pressures and CH4 concentrations. The compressive stress of the film can be controlled by modifying the grain size at various deposition parameters.

Intrinsic stress evolution in aluminum nitride thin films and the influence of multistep processing

Curvature measurements were used to investigate intrinsic stresses in AlN thin films grown by molecular beam epitaxy. In situ stress evolution experiments show that tensile stress is generated at the time where individual islands coalesce into a continuous film, and for some time after coalescence. Compressive stress is also generated both before and after coalescence. Introducing an intermediate annealing stage appears to reduce tensile stress evolution after coalescence, ultimately increasing the final intrinsic compressive stress. The maximum reduction in the tensile intrinsic stress is obtained when the sample is annealed at roughly the time where the islands coalesce into a continuous film.

Real Time Optical Method of Stress Measurements in Thin Films

Techniques generally used to measure stress in thin films cannot give sufficient information to be correlate to film properties. An in situ stress measurement method is described in this paper. This optical technique, based on the cantilever one, allows to follow up the evolution of intrinsic stress with film thickness. It provides complementary information which can be correlate with film microstructure. Moreover, after deposition, this technique determines the Young's modulus and thermal expansion coefficient of the film by measuring the thermal stress during cooling.

OPTICAL MEASUREMENT OF INTRINSIC STRESS DURING DEPOSITION: APPLICATION TO THE DETERMINATION OF THE SUBSTRATE NATURE INFLUENCE ON STRESSES IN CHROMIUM FILMS

Application of an in situ stress measurement technique in thin films is the purpose of this paper. This technique, based on the cantilever method, allows to avoid the low thermal stress contribution to the measured stress and thus to follow the evolution of intrinsic stress with film thickness. Experimental investigations have been carried out on chromium films deposited by magnetron sputtering. The influence of substrate nature on stress for three different argon pressures is presented. Stress evolution as a function of deposited thickness is different according to the gas pressure and to the substrate nature. Nevertheless, for a given pressure, the evolution of intrinsic stress along the film thickness presents the same profile for the two studied substrates : one conclude that stress magnitude mainly depends on substrate roughness.

Evolution of Intrinsic Stress During Nucleation and Growth of Polycrystalline Tungsten Films by Chemical Vapor Deposition.

AbstractWe present in situ measurements of the intrinsic (growth) stress during nucleation and growth of thin tungsten films by chemical vapor deposition. It is shown that interfering stress sources, as recrystallization or plasic flow, do not contribute to the intrinsic stress in these films. This makes W-CVD a model system for the experimental study of the relation between the evolution of microstructure and the development ofgrowth stress.