Film Fracture Research Articles

Flexible Patterning of Functional Microdot by Laser-Induced Dot Transfer

Laser-induced dot transfer (LIDT) is one of promising additive manufacturing techniques because it can realize flexible patterning of micron and submicron-sized dots even at atmospheric room-temperature conditions. In the LIDT process, a laser pulse is tightly focused onto a source film, leading to a transient melting of the film followed by sub-spot transfer using one-to-one microdot deposition with laser-illuminated area. Recently, we have developed a novel double-pulse LIDT process using the first pulse for preheating the source film and the second pulse for dot transfer. To investigate the double-pulse effect on microdot transfer of oxides, temporal high-temperature distributions of a thick indium tin oxide source film during double-pulse irradiation have been simulated using a Finite Element Method (FEM) approach. As a result, preheating by the first pulse decreased a difference in film temperature along both the film thickness and in-plane directions. This is effective to avoid film fracture, leading to high-quality patterning of a wide variety of microdots.

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 200nm Cu film on 23μm 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.

A variational model for fracture and debonding of thin films under in-plane loadings

We study fracture and debonding of a thin stiff film bonded to a rigid substrate through a thin compliant layer, introducing a two-dimensional variational fracture model in brittle elasticity. Fractures are naturally distinguished between transverse cracks in the film (curves in 2D) and debonded surfaces (2D planar regions). In order to study the mechanical response of such systems under increasing loads, we formulate a dimension-reduced, rate-independent, irreversible evolution law accounting for both transverse fracture and debonding. We propose a numerical implementation based on a regularized formulation of the fracture problem via a gradient damage functional, and provide an illustration of its capabilities exploring complex crack patterns, showing a qualitative comparison with geometrically involved real life examples. Moreover, we justify the underlying dimension-reduced model in the setting of scalar-valued displacement fields by a rigorous asymptotic analysis using Γ-convergence, starting from the three-dimensional variational fracture (free-discontinuity) problem under precise scaling hypotheses on material and geometric parameters.

A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings

The fracture toughness of thin ceramic films is an important material property that plays a role in determining the in-service mechanical performance and adhesion of this important class of engineering materials. Unfortunately, measurement of thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the upper portion of the pillar is almost fully relaxed, and when indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behaviour. Cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius and fracture toughness for a given material. The main novel aspect of this work is that neither crack geometries nor crack sizes need to be measured post test. In addition, the residual stress can be measured at the same time with toughness, by comparison of the indentation results obtained on the stress-free pillars and the as-deposited film. The method is tested on three different hard coatings created by physical vapour deposition, namely titanium nitride, chromium nitride and a CrAlN/Si3N4 nanocomposite. Results compare well to independently measured values of fracture toughness for the three brittle films. The technique offers several benefits over existing methods.

Study on surface chemistry and particle size of porous silicon prepared by electrochemical etching

In the present study, surface chemistry of porous silicon was tailored by thermal oxidation and thermal hydrosilylation method. Porous silicon nanoparticles were prepared by applying ultrasound to freestanding porous silicon films. The results showed a mesoporous silicon layer with an average pore diameter of 28.26nm, porosity of 70.7% and thickness of 4660nm. Porous silicon surface was modified and compared with native surface by Fourier transform infrared spectroscopy. Ultrasonic fracture of porous silicon films for 8h resulted in nanoparticles with average particle size of 149nm. Zeta potential measurements of nanoparticles at varying pH reveal presence of different surface chemistry of particles.

Finite Element Modelling of Stress-Induced Fracture in Ti-Si-N Films

Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood.

Effect of water uptake on the fracture behavior of low-k organosilicate glass

Water uptake in porous low-k dielectrics has become a significant challenge for both back-end-of-the-line integration and circuit reliability. This work examines the effects of water uptake on the fracture behavior of nanoporous low-k organosilicate glass. By using annealing dehydration and humidity conditioning, the roles of different water types and their concentrations are analyzed in detail. For as-deposited SiCOH films, annealing dehydration can enhance the resistance to crack occurrence, and these enhancements can be offset by higher humidity conditioning. It was found that the film-cracking threshold can be lowered by in-diffused water in the film as well as by water at the SiCOH/subtract interface. This occurs because the water decreases the film fracture energy and adhesion energy, respectively. By conditioning at high humidity, the variation of the film cracking threshold agrees well with the behavior of the film hardness and modulus of elasticity as a function of relative humidity. The crack morphologies of low-k porous films are also implicitly related to water uptake in the materials. Film cracking thresholds and crack morphologies of UV-cured low-k materials exhibit a weaker dependence on the water uptake, indicating a low degree of hydrophilicity of the SiCOH film after UV curing, which corroborates the previous results. Furthermore, by measuring the surface crack length, the material–fracture toughness can be found. The results demonstrate that neither annealing dehydration nor water uptake have significant effects on fracture toughness of as-deposited SiCOH, while for UV-cured SiCOH, annealing enhances the film-fracture toughness.

Measurements of hydrate film fracture under conditions simulating the rise of hydrated gas bubbles in deep water

In view of gas hydrate formation in the containment of deep water oil/gas blowout, the evolution of methane hydrate films on gas bubbles rising naturally in deep water was simulated and observed by visual microscopy. The results reveal that hydrate films on gas bubbles formed at conditions corresponding to deeper depths (higher pressure) rose intact for longer distances and showed more pronounced fractures on the film. Secondary hydrate growth happened along the cracks of the hydrate film as the hydrated bubble rose upward. The fractured hydrate films were more likely to close with secondary growth when formed at subcoolings higher than 3.0K. The strength of methane hydrate film formed on the surface of a suspended gas bubble was examined by rapidly decreasing the external pressure of the hydrate film soon after its formation. The maximum tension that the hydrate film could withstand without fracturing as well as the tensile strength were evaluated for hydrate films formed at 277.2K and different subcoolings. It was found that the tensile strength of hydrate film increased with increasing subcooling at the formation conditions.

Assessment of mechanical properties of metallic thin-films through micro-beam testing

Microelectronic industry is driven by the continuous miniaturization process conducing to the introduction of materials with better performance. These materials are subjected to stresses mainly due to thermal mismatch, microstructural changes or process integration which can be in the origin of mechanical reliability issues. To study these phenomena and even electromigration a good mechanical characterization of the materials is needed. This work aims at developing tests to assess fracture and elastoplastic behavior of thin Cu films. The tests developed are based on the deflection of microbeams (micromachined using a focused ion beam) using a nanoindenter. Different test geometries for microbeams have been evaluated and quantitative data have been obtained combining experimental results with analytical or numerical models, depending on the property under study. Microbeam response shows a strong dependence on the orientation of the grains close to the fixed end. Grain orientation has been measured by electron backscatter diffraction and the plastic behavior has been modeled by the finite element method using an in-house crystal plasticity subroutine. The effect of film thickness on fracture energy has been determined from tests of notched beams.

Fracture and solid particle erosion of micro-crystalline, nano-crystalline and boron-doped diamond films

Chemical vapor deposition (CVD) diamond film has been considered as a good candidate protective coating under hostile and abrasive conditions due to its outstanding erosion behavior. In the present study, the conventional micro-crystalline diamond (MCD) film, nano-crystalline diamond (NCD) film and the boron-doped diamond (BDD) film are respectively fabricated by the hot filament CVD (HFCVD) method. Solid particle erosion tests are conducted on diamond films in an air–sand erosion rig under different impact velocities (v) and angles (α) with angular SiC sands as the erodents, and WC–Co and SiC materials are adopted as comparisons. The field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, Rockwell hardness tester and surface profilometer are respectively used to characterize diamond films before and after tests, exhibiting that compared with MCD and NCD films, the BDD film performs relatively higher fracture strength and better adhesion. Typical solid impact erosion stages of the different diamond films are observed. The solid particle erosion behavior of diamond films has close relationships to their fracture strength and adhesion. As a result, when the impact angle α ≤30°, the steady-state erosion rate of the BDD film is lower than those of MCD and NCD ones. More importantly, under all the impact angles and velocities, much quicker film delamination and removal can be noticed on the MCD and NCD films, especially the NCD one. By contrast with WC–Co and SiC materials, diamond films present much better erosion resistance and higher velocity exponents, which should be attributed to different impact mechanisms caused by the different hardness of eroded samples.

Thermo-compression of biodegradable thermoplastic corn starch films containing chitin and chitosan

Films based on thermoplastic corn starch (TPS) and chitosan/chitin were obtained by melt-mixing and thermo-compression. Chitosan and chitin incorporation to TPS matrix induced some structural modifications due mainly to the interactions between starch hydroxyl and chitosan/chitin amino groups. Crystallinity degree of TPS films was increased with biopolymers incorporation. Enthalpy melting values for TPS–chitosan/chitin films resulted lower than those corresponding to TPS control ones. Films had homogeneous and smooth surfaces, without pores and cracks and no glycerol migration was evidenced by Scanning Electronic Microscopy. Films fracture surfaces were uniform without the presence of unmelting starch granules neither chitosan/chitin agglomerates. Films with chitosan/chitin presented higher color, UV absorption capacity and opacity than TPS films. Addition of 10 g chitosan or chitin/100 g starch decreased 35 and 56% water vapor permeability, respectively. Biopolymers addition to TPS increased tensile strength and elastic modulus, and decreased elongation at break. Starch and glycerol-rich domains where evidenced in TPS matrixes by Dynamic Mechanical Analysis. Finally, TPS–chitosan films reduced Staphylococcus aureus and Escherichia coli growth in the contact zone.

Nanoindentation fracture and fatigue characterization of PECVD silicon nitride films subjected to rapid thermal annealing

In this work, the fracture and fatigue behaviors of PECVD silicon nitride films deposited on silicon substrates after rapid thermal annealing were characterized. Specimens were fabricated using different fabrication parameters and post-deposition annealing temperatures and duration. Indentation methods were primarily used for qualitatively examining the effect of process conditions on the resultant mechanical properties. The experimental results indicated that the residual stress, fracture toughness, interfacial strength, and fatigue crack propagation, strongly depend on the processing conditions, such as deposition temperatures, chamber pressures, temperature, and duration of annealing. Specifically, preliminary results indicated that the specimen deposited at a lower temperature and a lower pressure exhibited much less residual tensile stress and better interface strength. Meanwhile, the study of rapid thermal annealing duration on the film stress demonstrated that the evolution of the residual stress was mainly controlled by the intrinsic stress generation dynamics and stress relaxation. It was also found that rapid thermal annealing could enhance the interfacial strength but the high tensile stress generated could actually reduce the equivalent toughness and lead to structural reliability concerns. In sum, the characterization results should provide useful information for correlating mechanical reliability and processing parameters for future structural design optimization and for improving the structural integrity of PECVD silicon nitride films for MEMS and IC fabrication.

Gas Permeation, Mechanical Behavior and Cytocompatibility of Ultrathin Pure and Doped Diamond-Like Carbon and Silicon Oxide Films

Protective ultra-thin barrier films gather increasing economic interest for controlling permeation and diffusion from the biological surrounding in implanted sensor and electronic devices in future medicine. Thus, the aim of this work was a benchmarking of the mechanical oxygen permeation barrier, cytocompatibility, and microbiological properties of inorganic ~25 nm thin films, deposited by vacuum deposition techniques on 50 µm thin polyetheretherketone (PEEK) foils. Plasma-activated chemical vapor deposition (direct deposition from an ion source) was applied to deposit pure and nitrogen doped diamond-like carbon films, while physical vapor deposition (magnetron sputtering in pulsed DC mode) was used for the formation of silicon as well as titanium doped diamond-like carbon films. Silicon oxide films were deposited by radio frequency magnetron sputtering. The results indicate a strong influence of nanoporosity on the oxygen transmission rate for all coating types, while the low content of microporosity (particulates, etc.) is shown to be of lesser importance. Due to the low thickness of the foil substrates, being easily bent, the toughness as a measure of tendency to film fracture together with the elasticity index of the thin films influence the oxygen barrier. All investigated coatings are non-pyrogenic, cause no cytotoxic effects and do not influence bacterial growth.

Adherence Energy of Asphalt Thin Films Measured by Force-Displacement Atomic Force Microscopy

AbstractViscoelastic materials exhibit rate- and temperature-dependant behavior in terms of stress-strain response and fracture. As a representative of this class of materials, rate-dependant fracture should be expected for the bituminous asphalt binders used in the construction of pavement. Preliminary results are reported regarding studies using atomic force microscopy (AFM) to determine rate- and temperature-dependent adhesive fracture in bitumen thin films. This AFM technique involves creating then fracturing a microadhesive contact joint between a bitumen thin film and a glass microbead tip affixed to a cantilever by application of a direct tensile force to the contact. The mechanical work required to fracture this contact is measured as a function of temperature and separation rate. Results of this study suggest that adherence fracture energy of bitumen films is rate and temperature dependant based on measures of energy dissipation at and below the temperature of 25°C. These materials also exhibit a...

Nanoindentation-induced interfacial fracture of ZnO thin films deposited on Si(111) substrates by atomic layer deposition

The structural and nanomechanical characteristics of ZnO thin films grown on Si(1 1 1) substrates by using the ALD process are investigated by combining XRD, AFM and Berkovich nanoindentation techniques. Results indicated that the ZnO/Si(1 1 1) thin films also exhibited clear evidences of fracture behaviors-induced pop-in phenomenon in the load–displacement curve during nanoindentation. Moreover, based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated. • ZnO thin films are deposited on Si(1 1 1) substrates using atomic layer deposition. • The interfacial fracture behaviors for ZnO thin films are characterized by Berkovich nanoindentation. • The fracture toughness of ZnO/Si(1 1 1) thin films is about 3.1 MPa m 1/2 . In this study, the structural and nanomechanical characteristics of ZnO thin films are investigated by mans of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The ZnO thin films are deposited on Si(1 1 1) substrates by using atomic layer deposition (ALD). The interfacial fracture behaviors for ALD-derived ZnO thin films are characterized by Berkovich nanoindentation, and the morphologies of indentations are revealed by using scanning electron microscopy (SEM). During nanoindentation, the interfacial fracture of ZnO thin films is significantly observed as the indentation load reaches 50 mN and, the corresponding loading segment in the load–displacement curve displays an obvious discontinuity event. Based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated.

Origins of saccharin-induced stress reduction based on observed fracture behavior of electrodeposited Ni films

This research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films where saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate-based electro chemical baths, where it is empirically known that mmol/l concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size up to a few nanometer scales. Some previous observations and several theoretical models suggest that saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which demonstrated island size effects on stress evolution, and enabled a well-defined notch geometry along one of the island boundaries to facilitate fracture experiments. In situ uniaxial tensile testing under in a scanning electron microscope was then used to obtain the fracture strength of such specimens. This technique provided real time recording of microscopic deformation during uniaxial tensile loading. The observed relationships among residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture.

Guided fracture of films on soft substrates to create micro/nano-feature arrays with controlled periodicity.

While the formation of cracks is often stochastic and considered undesirable, controlled fracture would enable rapid and low cost manufacture of micro/nanostructures. Here, we report a propagation-controlled technique to guide fracture of thin films supported on soft substrates to create crack arrays with highly controlled periodicity. Precision crack patterns are obtained by the use of strategically positioned stress-focusing V-notch features under conditions of slow application of strain to a degree where the notch features and intrinsic crack spacing match. This simple but robust approach provides a variety of precisely spaced crack arrays on both flat and curved surfaces. The general principles are applicable to a wide variety of multi-layered materials systems because the method does not require the careful control of defects associated with initiation-controlled approaches. There are also no intrinsic limitations on the area over which such patterning can be performed opening the way for large area micro/nano-manufacturing.

Passive film-induced stress and mechanical properties of α-Ti in methanol solution

Passive films play an important role in corrosion and stress corrosion cracking of titanium. Correlation between passive film-induced stress and film mechanical properties on α-Ti in methanol solution with varying water content was investigated. Film-induced stress in the methanol solution was measured by the flow stress differential method, and thin film fracture properties were determined by nanoindentation. For small amounts of water (⩽0.15%), both film-induced tensile stress and film fracture load were higher. However, the film-induced tensile stress and fracture load decreased significantly in more dilute solutions. Thus, water reduces Ti SCC susceptibility in methanol solution.

Assessment of stress relaxation experiments on diamond coatings analyzed by digital image correlation and micro-Raman spectroscopy

The analysis and control of residual stresses are important for understanding the fracture and delamination behavior of coatings and thin films. In this work, the stress relaxation in microcrystalline diamond coatings after focused ion beam (FIB) milling was assessed by micro-Raman spectroscopy and digital image correlation. Using the FIB, the so called H-Bar geometry was milled in the diamond coating. The cutting introduced a strain relief in the coating perpendicular to the longer sides of the bar. The relaxation strains were recorded from high resolution scanning electron microscopy images of the H-Bar before and after FIB cutting. Using finite element modeling of the milled geometry, the residual stress level in the coating is evaluated from the relaxation strains. Furthermore, μ-Raman spectroscopy was used to assess the biaxial residual stress level in the coating before and after FIB cutting. The μ-Raman signal inside the bar showed strong incidence for stress relaxation and confirmed to some extent the applicability of the FIB–DIC method.

A variational approach to the fracture of brittle thin films subject to out-of-plane loading

We address the problem of fracture in homogenous linear elastic thin films using a variational model. We restrict our attention to quasi-static problems assuming that kinetic effects are minimal. We focus on out-of-plane displacement of the film and investigate the effect of bending on fracture. Our analysis is based on a two-dimensional model where the thickness of the film does not need to be resolved. We derive this model through a formal asymptotic analysis. We present numerical simulations in a highly idealized setting for the purpose of verification, as well as more realistic micro-indentation experiments.