Intrinsic Stress Formation Research Articles

Influence of the Inert Gas Pressure on Intrinsic Stress in Diamond-Like Coating Deposited From Vacuum Arc Carbon Plasma

Within the framework of the model of the nonlocal thermoelastic peak of low-energy ion, the formation of intrinsic stress in a carbon coating deposited from the vacuum arc plasma in the argon atmosphere is theoretically studied. It is shown that the flow of particles bombarding the deposited coating contains, along with C+ ions, also Ar+ ions involved in the formation of intrinsic stress in the coating. The flux density of Ar+ ions resulting from ionization losses of C+ ions passing through the argon atmosphere is proportional to both the flux density of C+ ions and the density (pressure) of argon. Expressions are obtained for the intrinsic stress in the deposited carbon coating depending on the bias potential on the substrate and the argon pressure for the cases of both constant and pulsed potentials. The analysis of the obtained expressions shows that the intrinsic stress in the carbon coating decrease with increasing argon pressure.

Propagation of threading dislocations in heteroepitaxial diamond films with (111) orientation and their role in the formation of intrinsic stress

In the present study, systematic correlations were revealed between the propagation direction of threading dislocations, the off-axis growth conditions, and the stress state of heteroepitaxial diamond on Ir/YSZ/Si(111). Measurements of the strain tensor ε⃡ by X-ray diffraction and the subsequent calculation of the tensor of intrinsic stress σ⃡ showed stress-free samples as well as symmetric biaxial stress states for on-axis samples. Transmission electron microscopy (TEM) lamellas were prepared for plan-view studies along the [1¯1¯1¯] direction and for cross-section investigations along the [112¯] and [11¯0] zone axes. For samples grown on-axis with parameters which avoid the formation of intrinsic stress, the majority of dislocations have line vectors clearly aligned along [111]. A sudden change to conditions that promote stress formation is correlated with an abrupt bending of the dislocations away from [111]. This behaviour is in nice agreement with the predictions of a model that attributes formation of intrinsic stress to an effective climb of dislocations. Further growth experiments under off-axis conditions revealed the generation of stress states with pronounced in-plane anisotropy of several Gigapascal. Their formation is attributed to the combined action of two basic processes, i.e., the step flow driven dislocation tilting and the temperature dependent effective climb of dislocations. Again, our interpretation is supported by the dislocation propagation derived from TEM observations.

Multiple role of dislocations in the heteroepitaxial growth of diamond: A brief review

In the synthesis of epitaxial heterostructures with appreciable lattice misfit, dislocations represent a crucial linear defect. Since device quality is typically deteriorated by their presence, minimizing the dislocation density by optimized growth strategies is a key challenge for R&D. This especially holds also for the system diamond‐on‐iridium with its difference in lattice constants of more than 7%. As a consequence of this misfit, the initial dislocation density is very high. In this review we discuss different aspects of dislocations in heteroepitaxial diamond growth. They vary from mutual interaction mechanisms for an efficient reduction in density, over interaction with surface structures, their crucial role in the formation of intrinsic stress, their presumable role in formation of highly anisotropic stress and also their role as sink for the segregation of boron dopants. Finally, it is pointed out that the dislocation mediated stress formation may be used intentionally to strengthen devices by formation of compressively stressed surface layers. The present article is focused on recent studies from the authors’ research group.

Propagation and annihilation of threading dislocations during off-axis growth of heteroepitaxial diamond films

Threading dislocations in heteroepitaxial diamond films deposited on Ir/YSZ/Si(001) substrates with an off-axis angle of 4° towards [100] have been studied by cross-section transmission electron microscopy using the weak-beam dark-field (WBDF) technique for Burgers vector identification and large-angle convergent beam electron diffraction (LACBED) in order to facilitate a precise crystallographic determination of the local growth surface. Close to the diamond/iridium interface the films contain a high density of dislocations of both 90° and 45° types. Within the first micron of film growth their density decreases drastically by annihilation or mutual interaction resulting in a preference for 45° type dislocations. Vicinal growth surfaces cause a tilt of the dislocation line vector away from the crystallographic [001] axis in the step-flow direction. Tilting is influenced by gas phase impurities. With 100ppm nitrogen in the feed gas, step bunching produces a structured surface consisting of an alternating sequence of terraces with reduced off-axis angle crystallographically close to (001) and risers with increased off angle. The dislocations with lines close to [001] during terrace growth abruptly tilt by more than 20° at the transition to riser growth. The appearance of several discrete tilt angles during nitrogen free growth without pronounced step bunching is attributed to the corresponding Burgers vectors which cause either effective climb for the lower angles or effective glide for the higher ones. The present observations are of high relevance for improved strategies towards a further dislocation density reduction and an efficient control of intrinsic stress formation.

Influence of ultrasound and cathode rotation on formation of intrinsic stress in Ni films during electrodeposition

The influence of 25 kHz ultrasound and cathode rotation during electroplating of Ni films on Si wafers has been studied with respect to intrinsic stress formation. Current densities from 1·6 up to 28·3 A dm−2 were used in an additive free Ni sulphamate electrolyte. In general, more efficient agitation by either ultrasound or cathode rotation was found to reduce intrinsic stress towards compressive levels compared with conventional agitation with an electrolyte circulation pump. Furthermore, intrinsic stresses become less dependent on changes in current density. The latter effect is most pronounced for ultrasonic agitation. Structure analysis of samples deposited by ultrasonic agitation shows dense deposits with initially smaller grains at high ultrasonic effect. Locally increased temperature at the substrate surface could be an important effect of ultrasound agitation.

Growth of fullerene-like carbon nitride thin solid films by reactive magnetron sputtering; role of low-energy ion irradiation in determining microstructure and mechanical properties

Fullerene-like (FL) carbon nitride (CNx) films were deposited on Si (100) substrates by dc reactive, unbalanced, magnetron sputtering in a N2/Ar mixture from a high-purity pyrolythic graphite cathode in a dual-magnetron system with coupled magnetic fields. The N2 fraction in the discharge gas (0%–100%) and substrate bias (−25 V; −40 V) was varied, while the total pressure (0.4 Pa) and substrate temperature (450 °C) was kept constant. The coupled configuration of the magnetrons resulted in a reduced ion flux density, leading to a much lower average energy per incorporated particle, due to a less focused plasma as compared to a single magnetron. This enabled the evolution of a pronounced FL microstructure. The nitrogen concentration in the films saturated rapidly at 14–18 at. %, as determined by elastic recoil analysis, with a minor dependence on the discharge conditions. No correlations were detected between the photoelectron N1s core level spectra and the different microstructures, as observed by high-resolution electron microscopy. A variety of distinct FL structures were obtained, ranging from structures with elongated and aligned nitrogen-containing graphitic sheets to disordered structures, however, not exclusively linked to the total N concentration in the films. The microstructure evolution has rather to be seen as in equilibrium between the two competing processes of adsorption and desorption of nitrogen-containing species at the substrate. This balance is shifted by the energy and number of arriving species as well as by the substrate temperature. The most exceptional structure, for lower N2 fractions, consists of well-aligned, multi-layered circular features (nano-onions) with an inner diameter of approximately 0.7 nm and successive shells at a distance of ∼0.35 nm up to a diameter of 5 nm. It is shown that the intrinsic stress formation is closely linked with the evolution and accommodation of the heavily bent fullerene-like sheets. The FL CNx structures define the mechanical response of the films as revealed by nano-indentation. The material is highly elastic and fracture tough, and has reasonable hardness and elastic modulus values. On a nano-structured level, it is inferred the CNx stores deformation energy elastically by compression of the interplanar lattice spacing and buckling of the sheets, while crosslinks between sheets prevent gliding. Increasing the bias voltage from −25 to −40 V multiplies hardness and modulus values, while keeping their high ratio of up to 0.2, due to a higher degree of cross-linking.

Effect of plating parameters on the intrinsic stress in electroless nickel plating

With the increasing application of electroless nickel (EN) as a bumping or under-bumping metalization material, the microelectronic packaging engineers are required to have a deeper understanding and control of intrinsic stress in EN plating. The purpose of this work is to investigate the effect of some of the most important operating parameters on the formation of intrinsic stress during plating. The analysis of variances method was applied to obtain both single effects and compound effects of selected factors. Results indicate that high pH value and aged solution affect the intrinsic stress significantly, while surface roughness does not have much influence on the intrinsic stress for the range investigated. Aging at 190 °C for 170 h changes neither the Ni–P structure nor the intrinsic stress to a significant degree. Further investigation indicated that the intrinsic stress was probably related to the Ni–P microstructure as a result of the deposition rate. It was found that the higher the deposition rate, the higher the intrinsic stress.

Model for intrinsic stress formation in amorphous thin films.

Metallic amorphous thin films evaporated on a substrate can be characterized by different growth regimes in dependence of the film thickness concerning surface morphology and intrinsic film stresses, independent of the details of the applied material systems. Here, a model is presented to link the surface topography and characteristic surface measures with the observed film stresses. This allows quantitative prediction of stresses in dependence of film preparation parameters for a tailored film production.

Structural and mechanical properties of amorphous Zr–based alloy thin films

AbstractThe evolution of surface structures of coevaporated and sputtered amorphous Zr65Al7.5Cu27.5films with varying deposition conditions is investigated primarily with scanning tunneling microscopy (STM). While vapor deposited thin films reveal pronounced structure formation, depending on parameters, such as substrate temperature, film composition (variation of the Al versus the Cu content) and the angle of incidence, comparable sputtered films hardly show any structure formation. With the help of a numerical analysis of the STM data, surface diffusion, self–shadowing and energy transfer in the case of sputtering can be identified as the main structure forming mechanisms. Presuming these atomic processes, it is possible to model the main experimentally observed features of amorphous thin film growth by the use of stochastic continuum growth equations, which are numerically solved. Additionally, the connection to intrinsic stress formation during film growth is discussed.

Internal interfaces and intrinsic stress in thin amorphous Cu-Ti and Co-Tb films

A combined investigation of intrinsic stress formation by in situ substrate curvature measurements and of surface morphology evolution by scanning tunneling microscopy during the growth of amorphous Cu-Ti and Co-Tb films is reported. Intrinsic tensile stress and surface morphology are clearly correlated: all films that show intrinsic tensile stress formation in the late growth stages exhibit a cluster-like surface morphology and vice versa. Both the magnitude of the intrinsic tensile stress and the cluster size at the surface depend systematically on the reduced substrate temperature during film preparation. This dependence fits Hoffman’s model for tensile stress formation in thin films. Thus, the observed surface clusters are probably the top domes of growth columns, and the atomic mismatch at the column boundaries gives rise to tensile stress formation.

Surface Roughening, Columnar Growth and Intrinsic Stress Formation in Amorphous CuTi Films

AbstractThe growth of amorphous CuTi films, prepared by electron beam evaporation, is investigated by Scanning Tunneling Microscopy (STM), Small Angle Neutron Scattering (SANS) and in situ measurements of intrinsic mechanical stresses (ISM). In early growth stages the films develop compressive stresses and, with increasing film thickness, a crossover to tensile stresses. In the same thickness range the STM investigations show a change in the growth mode. Our experiments suggest a transition from planar growth with statistical surface roughening to columnar growth.

Macroscopic intrinsic stress formation in amorphous CuTi films

AbstractDuring the growth of amorphous CuTi films prepared under UHV conditions onto quartz substrates, macroscopic intrinsic stresses are generated. The intrinsic stresses are measured in situ as a function of the film thickness for a wide range of substrate temperatures and film compositions. Depending on the preparation conditions, compressive stresses during the early growth stages and thickness‐independent tensile stresses at higher thicknesses are observed. Films with Cu‐content above 50 at% deposited at room temperature do not generate any detectable intrinsic stresses. The results are discussed in terms of a model for the growth of amorphous binary alloy films published earlier.

Effects of argon pressure and substrate temperature on the structure and properties of sputtered copper films

The influence of substrate temperature (20 °– 450 °C) and sputter gas pressure (0.5–100 Pa) on the evolution of coating morphology during growth of magnetron deposited copper films has been investigated for film thicknesses up to 10 μm. Classification of films according to structure and surface topography yields a structure zone model with substrate temperature and argon gas deposition pressure dependence which is in basic agreement with a general model proposed by Thornton. The structural variations can be accounted for in terms of the interaction of various coating atom condensation processes and effects associated with increased water vapor partial pressure and the formation of intrinsic stress in the films.