Bridging Fidelities to Predict Nanoindentation Tip Radii Using Interpretable Deep Learning Models.

As the need for miniaturized structural and functional materials has increased, the need for precise materials characterizaton has also expanded. Nanoindentation is a popular method that can be used to measure material mechanical behavior which enables high-throughput experiments and, in some cases, can also provide images of the indented area through scanning. Both indenting and scanning can cause tip wear that can influence the measurements. Therefore, precise characterization of tip radii is needed to improve data evaluation. A data fusion method is introduced which uses finite element simulations and experimental data to estimate the tip radius in situ in a meaningful way using an interpretable multi-fidelity deep learning approach. By interpreting the machine learning models, it is shown that the approaches are able to accurately capture physical indentation phenomena.

Materials Engineering for Flexible Metallic Thin Film Applications.

More and more flexible, bendable, and stretchable sensors and displays are becoming a reality. While complex engineering and fabrication methods exist to manufacture flexible thin film systems, materials engineering through advanced metallic thin film deposition methods can also be utilized to create robust and long-lasting flexible devices. In this review, materials engineering concepts as well as electro-mechanical testing aspects will be discussed for metallic films. Through the use of residual stress, film thickness, or microstructure tailoring, all controlled by the film deposition parameters, long-lasting flexible film systems in terms of increased fracture or deformation strains, electrical or mechanical reliability, can be generated. These topics, as well as concrete examples, will be discussed. One objective of this work is to provide a toolbox with sustainable and scalable methods to create robust metal thin films for flexible, bendable, and stretchable applications.

The transverse and longitudinal elastic constants of pulp fibers in paper sheets.

Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests can only be performed along the axis of the fiber and are associated with problems of enforcing restraints. Alternative indirect approaches, such as micro-mechanical modeling, can help but yield results that are not fully decoupled from the model assumptions. Here, we compare these methods with nanoindentation as a method to extract elastic material constants of the individual fibers. We show that both the longitudinal and the transverse elastic modulus can be determined, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper such that the entire industrial process history is captured. The obtained longitudinal modulus is comparable to traditional methods for larger indents but with a strongly increased scatter as the size of the indentation is decreased further.

Microstructural Effects on the Interfacial Adhesion of Nanometer-Thick Cu Films on Glass Substrates: Implications for Microelectronic Devices.

Improving the interface stability for nanosized thin films on brittle substrates is crucial for technological applications such as microelectronics because the so-called brittle-ductile interfaces limit their overall reliability. By tuning the thin film properties, interface adhesion can be improved because of extrinsic toughening mechanisms during delamination. In this work, the influence of the film microstructure on interface adhesion was studied on a model brittle-ductile interface consisting of nanosized Cu films on brittle glass substrates. Therefore, 110 nm thin Cu films were deposited on glass substrates using magnetron sputtering. While film thickness, residual stresses, and texture of the Cu films were maintained comparable in the sputtering processes, the film microstructure was varied during deposition and via isothermal annealing, resulting in four different Cu films with bimodal grain size distributions. The interface adhesion of each Cu film was then determined using stressed Mo overlayers, which triggered Cu film delaminations in the shape of straight, spontaneous buckles. The mixed-mode adhesion energy for each film ranged from 2.35 J/m2 for the films with larger grains to 4.90 J/m2 for the films with the highest amount of nanosized grains. This surprising result could be clarified using an additional study of the buckles using focused ion beam cutting and quantification via confocal laser scanning microscopy to decouple and quantify the amount of elastic and plastic deformation stored in the buckled thin film. It could be shown that the films with smaller grains exhibit the possibility of absorbing a higher amount of energy during delamination, which explains their higher adhesion energy.

Highly ductile amorphous oxide at room temperature and high strain rate.

Oxide glasses are an integral part of the modern world, but their usefulness can be limited by their characteristic brittleness at room temperature. We show that amorphous aluminum oxide can permanently deform without fracture at room temperature and high strain rate by a viscous creep mechanism. These thin-films can reach flow stress at room temperature and can flow plastically up to a total elongation of 100%, provided that the material is dense and free of geometrical flaws. Our study demonstrates a much higher ductility for an amorphous oxide at low temperature than previous observations. This discovery may facilitate the realization of damage-tolerant glass materials that contribute in new ways, with the potential to improve the mechanical resistance and reliability of applications such as electronic devices and batteries.

Exceptional fracture resistance of ultrathin metallic glass films due to an intrinsic size effect.

Metallic glasses typically fail in a brittle manner through shear band propagation but can exhibit significant ductility when the sample size is reduced below a few hundreds of nanometers. To date the size effect was mainly demonstrated for free-standing samples and the role of extrinsic setup parameters on the observed behavior is still under debate. Therefore, in the present work we investigated the mechanical properties of polymer-supported sputtered amorphous Pd82Si18 thin films with various thicknesses. We show that the films exhibit brittle fracture for thicknesses far below 100 nm. A pronounced size effect resulting in extended crack-free deformation up to 6% strain was observed only in films as thin as 7 nm - a thickness which is lower than the typical shear band thickness. This size effect results in exceptional cyclic reliability of ultrathin metallic glass films which can sustain cyclic strains of 3% up to at least 30,000 cycles without any indication of fatigue damage or electrical conductivity degradation. Since the enhancement of mechanical properties is observed at ambient conditions using inexpensive substrates and an industrially scalable sputter deposition technique, a new research avenue for utilization of ultrathin metallic glasses in microelectronics, flexible electronics or nanoelectromechanical devices is opened up.

Substrate-Influenced Thermo-Mechanical Fatigue of Copper Metallizations: Limits of Stoney's Equation.

Rapid progress in the reduction of substrate thickness for silicon-based microelectronics leads to a significant reduction of the device bending stiffness and the need to address its implication for the thermo-mechanical fatigue behavior of metallization layers. Results on 5 µm thick Cu films reveal a strong substrate thickness-dependent microstructural evolution. Substrates with hs = 323 and 220 µm showed that the Cu microstructure exhibits accelerated grain growth and surface roughening. Moreover, curvature-strain data indicates that Stoney's simplified curvature-stress relation is not valid for thin substrates with regard to the expected strains, but can be addressed using more sophisticated plate bending theories.

Deformation behavior of Re alloyed Mo thin films on flexible substrates: In situ fragmentation analysis supported by first-principles calculations.

A major obstacle in the utilization of Mo thin films in flexible electronics is their brittle fracture behavior. Within this study, alloying with Re is explored as a potential strategy to improve the resistance to fracture. The sputter-deposited Mo1-xRex films (with 0 ≤ x ≤ 0.31) were characterized in terms of structural and mechanical properties, residual stresses as well as electrical resistivity. Their deformation behavior was assessed by straining 50 nm thin films on polyimide substrates in uniaxial tension, while monitoring crack initiation and propagation in situ by optical microscopy and electrical resistance measurements. A significant toughness enhancement occurs with increasing Re content for all body-centered cubic solid solution films (x ≤ 0.23). However, at higher Re concentrations (x > 0.23) the positive effect of Re is inhibited due to the formation of dual-phase films with the additional close packed A15 Mo3Re phase. The mechanisms responsible for the observed toughness behavior are discussed based on experimental observations and electronic structure calculations. Re gives rise to both increased plasticity and bond strengthening in these Mo-Re solid solutions.

Electromigration in Gold Films on Flexible Polyimide Substrates as a Self-healing Mechanism.

The study of electromigration (EM) in metallisations for flexible thin film systems has not been a major concern due to low applied current densities in today's flexible electronic devices. However, the trend towards smaller and more powerful devices demands increasing current densities for future applications, making EM a reliability matter. This work investigates EM in 50 nm Au thin films with a 10 nm Cr adhesion layer on a flexible polyimide substrate at high current densities. Results indicate that EM does occur and could be used as a self-healing mechanism for flexible electronics.

Measuring electro-mechanical properties of thin films on polymer substrates.

In order to advance flexible electronic technologies it is important to study the electrical properties of thin metal films on polymer substrates under mechanical load. At the same time, the observation of film deformation and fracture as well as the stresses that are present in the films during straining are also crucial to investigate. To address both the electromechanical and deformation behavior of metal films supported by polymer substrates, in-situ 4 point probe resistance measurements were performed with in-situ atomic force microscopy imaging of the film surface during straining. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of localized thinning and crack formation. Furthermore, in-situ synchrotron tensile tests provide information about the stresses in the film and show the yield stress where the deformation initiates and the relaxation of the film during imaging. A thin 200 nm Cu film on 23 µm thick PET substrate will be used to illustrate the combined techniques. The combination of electrical measurements, surface imaging, and stress measurements allow for a better understanding of electromechanical behavior needed for the improvement and future success of flexible electronic devices.

Improved electro-mechanical performance of gold films on polyimide without adhesion layers.

Thin metal films on polymer substrates are of interest for flexible electronic applications and often utilize a thin interlayer to improve adhesion of metal films on flexible substrates. This work investigates the effect of a 10 nm Cr interlayer on the electro-mechanical properties of 50 nm Au films on polyimide substrates. Ex situ and in situ fragmentation experiments reveal the Cr interlayer causes brittle electro-mechanical behaviour, and thin Au films without an interlayer can support strains up to 15% without significantly degrading electrical conductivity.

The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model.

Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50-200 nm thick), bonded to polyimide directly or via a 10 nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50 nm than in the 200 nm copper films.