Mechanical failure analysis of thin film transistor devices on steel and polyimide substrates for flexible display applications

The crack onset strain (COS) of 4-level thin film transistor (TFT) devices on both steel foils and thin polyimide (PI) films was investigated using tensile experiments carried out in situ in an optical microscope. Cracks initiated first within the SiO₂ insulator layer for both types of substrates. The COS was found to be equal to 1.15% and 0.24% for steel and PI, respectively. The influence of loading direction on failure of the TFT stack with anisotropic geometry was moreover found to be considerable, leading to recommendations for backplane design. The large difference in critical strain of the SiO₂ layer on the two substrates was analyzed using an energy release rate approach, and found to result from differences in layer/substrate mechanical contrast and in internal stress state. Based on this analysis a correlation between layer/substrate elastic contrast and tensile failure behavior was devised.     

Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays

The mechanical integrity of tin-doped indium oxide (ITO) thin films sputtered onto a high temperature aromatic polyester developed for flexible display applications was investigated by means of tensile experiments equipped with electrical measurement, and carried out in-situ in an optical microscope. Attention was paid to the influence of ITO thickness, composition and crystalline microstructure, internal stress, annealing, and polymer substrate. It was observed that process-induced internal stresses were systematically compressive, and that tensile cracks in the ITO always initiated at pin-hole defect sites. A transition from stable to unstable crack growth was detected when crack length was several 100 times coating thickness. The occurrence of such a transition, which corresponded to an increase in electrical resistance equal to approximately 10%, indicated that crack propagation controlled the loss of functional performance of the device. It was moreover found that an improved surface quality of the polymer substrate, such as that obtained with planarization hard coats, was a major factor to increase the cohesive properties of ITO thin films. It was also observed that the intrinsic crack onset strain followed classic fracture mechanics scaling, in inverse proportion to the square root of ITO thickness.     

Mechanical integrity of thin inorganic coatings on polymer substrates under quasi-static, thermal and fatigue loadings

The interplay between residual stress state, cohesive and adhesive properties of coatings on substrates is reviewed in this article. Attention is paid to thin inorganic coatings on polymers, characterized by a very high hygro-thermo-mechanical contrast between the brittle and stiff coating and the compliant and soft substrate. An approach to determine the intrinsic, thermal and hygroscopic contributions to the coating residual stress is detailed. The critical strain for coating failure, coating toughness and coating/substrate interface shear strength are derived from the analysis of progressive coating cracking under strain. Electro-fragmentation and electro-fatigue tests in situ in a microscope are described. These methods enable reproducing the thermo-mechanical loads present during processing and service life, hence identifying and modeling the critical conditions for failure. Several case studies relevant to food and pharmaceutical packaging, flexible electronics and thin film photovoltaic devices are discussed to illustrate the benefits and limits of the present methods and models.     

Control of the bias voltage in d.c. PVD processes on insulator substrates

In plasma-PVD processes, ion bombardment during the growth of thin films has a strong influence on films properties such as morphology, composition, structure, stress, electrical conductivity, and others. Therefore, an accurate control of substrate bias voltage is needed in order to deposit films with the desired properties. For insulator substrates, dc biasing the substrate holder is useless, since the surface shall not follow the applied bias but it will be at the non-controlled floating potential.In this work we present a simple method for the effective control of the substrate bias in dc PVD processes with insulator substrates, based on placing a metallic grid at a certain distance from the non-conductive surfaces to be coated. The desired negative bias is applied to this metallic grid which accelerates ions from the plasma and directs them to the surface to cover. This method has been successfully applied to the deposition of TiN coatings on glass and decorative ceramics by Cathodic Arc Deposition. The deposited films showed good adhesion and gold color, in contrast with the bad adhered and brownish films deposited without the grid. The dependencies on the color and on the mechanical properties of our TiN films deposited on insulating substrates with the value of the bias voltage applied to the substrate are similar to those typically reported in the literature when conductive substrates are used.     

Mechanical properties and adhesion strength of TiN and Al coatings on HSS, steel, aluminium and copper characterized by four testing Methods

TiN and Al coatings on substrates of high-speed steel, steel, aluminium and copper have been used to study mechanical properties of coating systems, especially the adhesion of the coating. The quantities measured are internal stress of the coating, determined by X-ray diffraction, the critical load of the scratch test, the microhardness obtained by the indenter technique, and the interface fracture energy, determined by a three-point bend test developed recently by the authors. The fracture energy, G_c, is a measure for the adhesion strength of a coating system. The effect of bias voltage, sputter cleaning and contamination of the substrates on the adhesion strength and other mechanical properties are investigated with the four methods mentioned. Each of the testing methods reveal only specific aspects of the behaviour of the coating systems. The data obtained depend on bulk properties of the film and the substrate material and on properties of the interface. Variation of the bias voltage can change them in quite different ways. In addition, the inter-relations between the adhesion strength of the coating and the failure behaviour of the three-point bend test samples are discussed.     

Adhesion and degradation of hard coatings on poly (methyl methacrylate) substrates

The adhesion and time dependent crack growth behavior of polysiloxane based hard coatings on poly (methyl methacrylate) substrates were investigated. The adhesive fracture energies for different coatings were quantitatively characterized and varied between 1.4 J/m² and 22 J/m². Significant time dependent crack growth in various moist environments was observed and was consistent with a viscoelastic crack growth model. The effect of selected weathering treatments was also examined and resulted in a significant drop in coating adhesion. The coatings were analyzed using surface sensitive techniques; structural changes in the coatings resulting from various exposure doses were studied and mechanisms responsible for the observed degradation in adhesion were discussed.     

Micro-scratch analysis and mechanical properties of plasma-deposited silicon-based coatings on polymer substrates

Advanced optical applications require multifunctional coatings with specific mechanical properties, such as resistance to damage and good adhesion to different types of substrates, including polymers. In the present study we deposited amorphous hydrogenated silicon nitride (SiN_1.3) and oxide (SiO₂) films on polycarbonate and on silicon substrates by plasma enhanced chemical vapor deposition (PECVD), using a dual-mode microwave/radio frequency plasma system. The film adhesion was determined by the micro-scratch test. Depth-sensing indentation and substrate curvature measurements were used to evaluate the microhardness. Young's modulus and residual stresses of the films. The adhesion strength, represented by the critical load, L_c, when the film starts to delaminate, was determined as a function of the substrate material and the energy of bombarding ions. A direct correlation between the L_c values and the mechanical properties of the films was found. The formation of different crack patterns in the coatings during the scratch procedure is explained in terms of stress release mechanism depending on the mechanical properties of the film, the substrate and the interface region. In addition, different models applicable to the evaluation of the work of adhesion in the case of hard coatings on soft substrates are critically reviewed.     

Effect of ion cleaning pretreatment on interface microstructure, adhesive strength and tribological properties of GLC coatings on Al substrates

In this work, a series of ion cleaning procedures (bias and time) were performed on aluminum substrate surface prior to the deposition of graphite-like carbon (GLC) coatings. Special attention has been paid on the interface microstructure, coating/substrate bonding strength and tribological properties. It was found that ion cleaning critically influenced the adhesion and the wear resistance of GLC coatings. The optimization of ion cleaning pretreatment revealed that 400 V/30 min is the best ion cleaning conditions. HRTEM observations on the interfacial region showed that the oxide layer has been removed completely, a strong bonding diffusion interface formed. However, for the low energy ion cleaning (300 V/10 min), TEM observations on the interfacial region between the coating and the Al substrate showed that the oxide contamination still existed. The optimization of GLC layer thickness revealed that the GLC coating with 1 μm GLC layer exhibited the highest critical load and the lowest friction coefficient of 14.7 N and 0.065, respectively.     

Design of multilayer polymeric coatings for indentation resistance

Summary Indentation is often a mode of in-service loading for a thin coating deposited on a substrate. Under such a loading, the strong adherence of the coating to the substrate is a basic necessity for successful performance of the coating. In this study, we investigate the indentation behavior of thin single and multilayered polymeric coatings using the finite element method. The deformation patterns and the stress fields that are generated during indentation are obtained by employing constitutive models which accurately represent the elastic-viscoplastic and hyperelastic behavior of the glassy and rubbery states of the polymeric layers under investigation. Three types of loading conditions are considered: indentation to (1) a fixed depth; (2) a fixed work; and (3) a fixed force. For these loading conditions, we then investigate the mechanical performance of various composite coatings subjected to an overall thickness design constraint. The composite structure is altered via variation in individual layer material composition, layer thickness and layer arrangement. It is shown how the placement of different material layers in a multilayer coating can alter the flow pattern and hence the distribution of stress state and resulting failure. It is also shown that a soft rubber elastic layer acts to greatly minimize the interfacial shear stress at the substrate, thereby reducing the risk of delamination of the coating, but the presence of the rubber can also produce detrimental tensile stresses on the surface. We then demonstrate that the tensile stress state can be eliminated through manipulation of the rubber layer thickness, without increasing the interface shear stress. Through these examples, a framework for evaluation and design of multilayer coatings for indentation resistance is provided.     

Influence of steel composition and pre-treatment conditions on morphology and microstructure of TiO2 mesoporous layers produced by dip coating on steel substrates

Mesoporous titania films were prepared by template-assisted dip coating on AISI 430, 304 and 316Ti steel substrates that underwent different pre-treatment, namely grinding, grinding–calcination and grinding–etching. The influence of steel grade and pre-treatment conditions on the film morphology and microstructure was studied, and reasons for film degradation were elucidated. Among tested steels, 430 interfered with the coating process the most yielding films of poor integrity and with a strongly distorted pore structure. Steels 304 and 316Ti showed better resistance to the precursor solution, resulting in less fractured films and defined pore morphology. Grinding and subsequent calcination proved to be the best choice among tested pre-treatment methods which reflected in improved film quality. The results show that only a proper combination of steel grade and substrate pre-treatment affords high-quality titania coatings with defined porosity.     

Finite element analysis of contact induced adhesion failure in multilayer coatings with weak interfaces

Experimental work reveals that the Ag/ZnO interface in the multilayer solar control coatings is weakest and most likely to fail during contact. In this study, a cohesive zone model embedded in a finite element code was used to model delamination in multilayer stack consisting of ZnO/Ag/ZnO on glass during contact. It shows that delamination occurs at the upper ZnO/Ag interface during loading when penetration exceeds a critical value, while, the double pinned buckling delamination failure occurs at the lower Ag/ZnO interface during the unloading cycle. Furthermore, it reveals the model based on mechanism of lateral crack at interface yields reasonably accurate values of interfacial toughness when tensile stress induced delamination occurs during unloading.     

Mechanical properties of carbon-modified silicon oxide barrier films deposited by plasma enhanced chemical vapor deposition on polymer substrates

Cohesive and adhesive properties of silicon oxide barrier coatings deposited from an oxygen/hexamethyldisiloxane gas mixture by plasma enhanced chemical vapor deposition, with controlled incorporation of carbon on 12 μm thick polyethylene terephtalate films were investigated. The reactor was equipped with a 2.45 GHz slot antenna plasma source and a 13.56 MHz-biased substrate holder. The two plasma sources were operated separately or in a dual mode. It was found that no or negligible internal stresses were introduced in the silicon oxide coatings as long as the increase of energy experienced by the film was compensated by the densification of the oxide. For a range of process parameters and carbon content on the changes of the crack onset strain, adhesion, and cohesion were found to be similar. Generally a high crack onset strain or good adhesion and cohesion were measured for films with an increased carbon content, although this was obtained at the expense of the gas barrier performance. Promising approaches towards high-barrier thin films with good mechanical integrity are proposed, based on coatings with a gradient in the carbon content and in the mechanical properties, on nano-composite laminates, and on organo-silane treatments.     

Thermomechanical analysis of thin films on temperature-dependent elastomeric substrates in flexible heterogeneous electronics

Thermomechanical analysis is presented to study the basic temperature effects on elastomeric substrate of flexible electronics. Strains of a films-on-substrate structure related with three key temperatures are given based on the interfacial continuum model. An improved strain model is given and compared with other two models. The role of the temperature-dependent effects is highlighted and adopted to design a flexible inorganic/organic heterogeneous structure subject to little thermal action. The sensitivity analysis of three key temperatures is investigated, by which proper selection of technological parameter for poly(dimethylsiloxane) fabrication may be determined to eliminate the variation of stress of the interface in circumstances with temperature varying severely. This work contributes to systemic reliability and compatibility, structural design and thermal management of flexible electronics.     

Tribological property and drilling application of Ti-C:H and Cr-C:H coatings on high-speed steel substrates

This study investigates the tribological properties and cutting performance of Ti-DLC and Cr-DLC doped metal coatings. The tribological properties of the coatings are evaluated by testing coated disks against an AISI 1045 steel counterbody under dry conditions using an oscillating friction wear tester, and then measuring the subsequent wear depth on the coated disk, the wear width on the steel counterbody, and the friction coefficient. The cutting performance of the coatings is evaluated by using coated high-speed drills to machine stainless steel workpieces, and then measuring the resulting flank wear and hole surface roughness. The results of the wear tests show that the Ti-C:H and Ti-C:H/TiC/TiCN/TiN coatings possess excellent tribological properties, including low coefficients of friction, low wear depths, and low wear widths. Regarding the machining tests, the Ti-C:H/TiC/TiCN/TiN coating has the lowest flank wear and yields the highest hole surface quality under both dry and cutting fluid drilling conditions. The single Ti-C:H coating has excellent tribological properties, but demonstrates a relatively poorer performance in the drilling of stainless steel. Finally, the Cr-DLC coatings all exhibit a poor cutting performance under dry cutting conditions.     

Mechanical properties of co‐extruded wear resistant powder metallurgical layers on steel substrates

Materials with high resistance against abrasive wear are of interest for many tool applications. For economical reasons, thick coatings of several millimetres are requested. The cladding of these materials to low alloyed substrates is commonly performed using hot isostatic pressing, being a cost intensive process in particular for long products. Thus, a novel manufacturing route via direct hot extrusion of encapsulated bulk steel bars and presintered tool steel powders was recently developed. In this manner, wear resistant claddings of PM tool steels and wear resistant MMC on steel substrates could be processed. Heating to process temperature leads to presintering of the powder and only a weak bonding between the steel substrate and the powdery layer. However, after direct hot extrusion at 1150 °C an interface free of macroscopic defects is formed between both materials. The quality and strength of this bond zone was investigated by micro tensile, 4 point bending and shear tests for different materials combinations. For high strength substrate materials, failure always occurs in the brittle wear resistant layer and not at the interface. These results are in agreement with microstructural investigations, exhibiting a pore‐ and defect‐free interface dominated by interdiffusion processes.     

Crystallography of magnetron sputtered TiN coatings on steel substrates

Structure formation processes in TiN coatings deposited by reactive CFUBMS on steel substrates have been investigated by X-ray diffraction experiments in symmetric Bragg-Brentano (B-B) and grazing incidence asymmetric Bragg diffraction (GIABD) modes and by SEM. The results show that the deposits with thicknesses of 500 and 4000 nm are built-up of polycrystalline stoichiometric TiN, in addition to which, some negligible amount of Ti-O and Ti-N-O phases have also been observed predominantly at their surfaces. In the thinner 500 nm films only columnar crystallites with {1 1 1}, {2 0 0} and {2 2 0} crystallographic planes parallel to the surface were formed. The share of the micro-volumes belonging to the 〈1 1 1〉 out-of-plane texture component varied between 70% and 80% depending on the target current (I_d) used (4 or 8 A in the present experiments). During the more advanced stages of growth the 〈1 1 1〉 texture weakens and new texture components appear; the process being more pronounced when the application has been performed at higher I_d values. The obtained crystallographic texture results for the thinner films and their changes during the more advanced stages of the coatings formation are discussed with particular consideration of the crystallography of the TiN lattice and the anisotropy of its elastic parameters. Based on a precise estimation of the interplanar distances, d_{〈u v w〉}, corresponding to the main texture components of the investigated films in the direction along the surface macro-normal, it has been revealed that the elastic strain, ε_{〈u v w〉}, caused by the compressive residual macro-stresses acting parallel to the film surfaces and the corresponding elastic stored energy, U_{〈u v w〉}, values obey the following relationships: ε_{〈1 1 1〉}>ε_{〈2 2 0〉}> ε_{〈2 0 0〉} and U_{〈1 1 1〉}>U_{〈2 2 0〉}>U_{〈2 0 0〉}, respectively. The observed ε_{〈u v w〉} and U_{〈u v w〉} anisotropy is found to be more pronounced in the thinner coatings and is such that, at more advanced stages of growth, it would be expected to favour the transition from 〈1 1 1〉 to 〈2 0 0〉 out-of-plane preferred orientation. However, the experimental results do not confirm this expectation, which points out that the texture-formation at these stages is not governed solely by the minimization of the stored elastic energy, but is a rather complicated process depending on a larger number of factors, some of which are discussed in the paper.     

Electrochemical study of hydroxyapatite coatings on stainless steel substrates

Hydroxyapatite coatings were synthesized electrochemically onto stainless steel. In this study, the composition and morphology of the coatings changed with the deposition and sintering conditions. The electrolyte was kept close to the composition of simulated body fluid with an adjusted pH of 8.0. Deposition temperature affected the purity of the deposits with higher temperatures (65 °C) giving better coatings. The sintering techniques were also shown to affect the deposits, with x-ray diffraction patterns showing well-defined peaks for hydroxyapatite when sintering under vacuum conditions. Coating density and corrosion resistance was improved when applying a double-layer coating technique versus a single-layer. Grain sizes were 30 to 40 nm even after sintering of these coatings in air. The formed coatings were characterized by powder x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and x-ray photoelectron spectroscopy.     

Washcoating of low surface area cerium oxide on complex geometry substrates

ABSTRACT

An acid-free formulation based on water, glycerol, and polyvinyl alcohol (PVA) was studied to disperse and stabilize, via steric-like interaction, low surface area cerium oxide powders. A dispersion route that implies a milling process and a proper ratio among the components was experienced. PVA was used as viscosity modulator, to enhance viscosity and system stability. Newtonian fluids, suitable for foams dip-coating, were obtained in the application shear range. Foams with different porosities (20, 30, and 40 pore per inch density) were coated. It was found that withdrawal velocity did not affect deposition: a constancy in coating load was obtained once rheology was fixed, while multiple dipping was effective to enhance load. An optimal flash drying temperature (350°C) was identified to consolidate the coated layer and to decompose the organic additives. Good loads, homogeneous coverage, and no pore clogging were obtained after calcination at 900°C. Even though acceptable weight losses were obtained, further investigations need to be accomplished to understand data scattering after adhesion tests.     

Wet chemical deposited ITO coatings on flexible substrates for organic photodiodes

ITO (tin doped indium oxide) coatings were produced by gravure printing process on PET and PEN foils. The printing paste consists of ITO nanoparticles, which are dispersed in a solvent and mixed with a binder. By modification of the printing paste, the sheet resistance (R/sq) of the ITO coatings after hardening under UV-irradiation at low temperatures (< 130 °C) could be decreased to 1 kΩ/sq. R/sq could be further reduced down to 0.5 kΩ/sq by heat treatment under forming gas atmosphere (N₂/H₂), the transmission of the ITO coated foils still being more than 80% in the visible range. The application of these ITO films as a bottom electrode in organic photodiodes (OPDs) is shown, and the current density-voltage characteristics of the OPDs are presented.     

Electrochemical synthesis of calcium carbonate coatings on stainless steel substrates

Calcite CaCO₃ has been electrocrystallized on stainless steel substrates by the galvanostatic cathodic reduction of aqueous calcium bicarbonate solutions. The deposition is controlled by pH changes occurring close to the cathode due to electrogeneration of base. The deposit morphology varies from facetted rhombs observed at low (1-20 mA cm⁻²) current densities to corner-rounded particles observed at high (40 mA cm⁻²) current densities.