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.     

Mechanical spectroscopy of thin layers of PPV polymer on Si substrates

The vibrating reed technique with electro“static” excitation and optical detection has been applied to investigate thin layers of poly-phenylene-vinylene, deposited by spin coating onto microfabricated Si cantilevers, during temperature cycling programs between 90 and 540 K at a rate of 1 K/min. From the vibration frequencies the Young’s modulus of the film can be estimated to be about 10 MPa at room temperature in the precursor phase (if prepared from a solution in toluene), which increases by conversion to the conjugate bonded polymer to about 50 MPa. The temperature dependence of internal friction reveals the processes of γ relaxations (crankshaft motion of side branches in the precursor) and β-relaxation (movements of a few monomer blocks in the polymer chain), as well as peaks indicating the structural transformations during conversion, and possibly a glass transition in the amorphous precursor phase. After conversion only the β-relaxation persists.     

The characteristics of transparent conducting Al-doped zinc oxide thin films deposited on polymer substrates

Al-doped zinc oxide (AZO) transparent, conductive thin films were deposited on inexpensive polyethylene terephthalate substrates, using radio frequency (rf) magnetron sputtering, with an AZO ceramic target (the Al₂O₃ content is approximately 2 wt%). This paper presents an effective method for the optimization of the parameters for the deposition process for AZO thin films with multiple performance characteristics, using the Taguchi method, combined with grey relational analysis. Using the Taguchi quality design concept, an L₉ orthogonal array was chosen for the experiments. The effects of various process parameters (rf power, substrate-to-target distance, substrate temperature and deposition time) on the electrical, structural, morphological and optical properties of AZO films were investigated. In the confirmation runs, using grey relational analysis, the electrical resistivity of the AZO films was found to have decreased from 5.0?×?10^?3 to 1.6?×?10^?3?Ω-cm and the optical transmittance was found to have increased from 74.39 to 79.40%. The results demonstrate that the Taguchi method combined with grey relational analysis is an economical way to obtain the multiple performance characteristics of AZO films with the fewest experimental data. Additionally, by applying an Al buffer layer, of thickness 10?nm, the results show that the electrical resistivity was 3.1?×?10^?4?Ω-cm and the average optical transmittance, in the visible part of the spectrum, was approximately 79.12%.     

Mechanical properties of ZnO thin films deposited on polyester substrates used in flexible device applications

ZnO is growing in importance as a functional film in flexible devices because of the wide range of electrical properties that can be achieved through appropriate doping and the relative abundance of Zn. We have deposited ZnO films with various thicknesses by sputtering on polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) flexible substrates and measured their mechanical properties using compression and scratch tests coupled with in-situ optical microscopy. The cracking of ZnO, during compression, is thickness dependent and at lower thicknesses the films sputtered on PEN exhibit the highest crack onset strains, around 2%. During scratch testing, two major scratch failure mechanisms are observed, analyzed and discussed. It is also found that scratch resistance of ZnO is thickness dependent for both PET and PEN. At high scratch loads a secondary failure mechanism due to impregnation of film debris into the polymer substrates is observed.     

Quality improvement of organic thin films deposited on vibrating substrates

Most of the Organic Light-Emitting Diodes (OLEDs) have a multilayered structure composed of functional organic layers sandwiched between two electrodes. Thin films of small molecules are generally deposited by thermal evaporation onto glass or other rigid or flexible substrates. The interface state between two organic layers in OLED device depends on the surface morphology of the layers and affects deeply the OLED performance. The morphology of organic thin films depends mostly on substrate temperature and deposition rate. Generally, the control of the substrate temperature allows improving the quality of the deposited films. For organic compounds substrate temperature cannot be increased too much due to their poor thermal stability. However, studies in inorganic thin films indicate that it is possible to modify the morphology of a film by using substrate vibration without increasing the substrate temperature. In this work, the effect of the resonance vibration of glass and silicon substrates during thermal deposition in high vacuum environment of tris(8-quinolinolate)aluminum(III) (Alq₃) and N,N′-Bis(naphthalene-2-yl)-N,N′-bis(phenyl)-benzidine (β-NPB) organic thin films with different deposition rates was investigated. The vibration used was in the range of hundreds of Hz and the substrates were kept at room temperature during the process. The nucleation and subsequent growth of the organic films on the substrates have been studied by atomic force microscopy technique. For Alq₃ and β-NPB films grown with 0.1 nm/s as deposition rate and using a frequency of 100 Hz with oscillation amplitude of some micrometers, the results indicate a reduction of cluster density and a roughness decreasing. Moreover, OLEDs fabricated with organic films deposited under these conditions improved their power efficiency, driven at 4 mA/cm², passing from 0.11 lm/W to 0.24 lm/W with an increase in their luminance of about 352 cd/m² corresponding to an increase of about 250% in the luminance with respect to the same OLEDs fabricated in the same way and with the same conditions without substrate vibration.     

聚碳酸酯表面铬酸预处理对涂层附着性能影响

聚碳酸酯材料表面呈惰性,不利于涂料的完全润湿和界面化学键的结合.为增强聚碳酸酯材料表面的涂层流平性能和附着性能,本文利用铬酸湿化学方法对聚碳酸酯透明材料进行表面预处理,测试预处理前后聚碳酸酯的表面能、元素组成、表面形貌以及应力-溶剂银纹,研究预处理对聚碳酸酯表面性能的影响.结果表明,铬酸的强氧化作用使聚碳酸酯表面产生了羰基、磺酸基及羧基3种含氧极性基团,增加了基材表面粗糙度和润湿性能.通过拉开法和涂层/基材界面扫描电镜两种方法比较了铬酸预处理对耐磨涂层附着性能的影响,研究发现,受涂层与PC之间界面共价键及机械锚接作用力,预处理后涂层附着性能明显增强.涂层附着力与铬酸处理时间密切相关,而较长时间处理会降低PC表面抗应力-溶剂银纹性能,实验表明在不损失材料抗银纹性能前提下,铬酸预处理5 min可以显著提高涂层附着力.     

Thermal strain in indium thin films deposited on GaAs substrates

Thermal strains in indium thin films deposited on GaAs substrates, which were introduced into the films upon cooling from 293 to 25 K, were measured using the conventional X-ray diffraction technique. Since the thermal expansion coefficients and the elastic constants of indium have strong dependences on the crystal orientation, we studied intensively the crystal orientation dependence of the strains perpendicular to the film surface. The strains measured for films with 60 nm thickness agreed well with those calculated based on a biaxial strain model using the difference in thermal expansions between the indium and the GaAs. The present strain analysis indicated that the anisotropy in the thermal expansion of indium is the prinary cause of different strains measured in grains with various crystal orientations. For the thicker films the strains were observed to deviate from the theoretical values owing to strain relaxation upon cooling.     

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.     

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.     

Growth and structure of TiCxNy coatings chemically vapour deposited on graphite substrates

TiCxNy coatings were grown on graphite substrates in a computer-controlled, hot-wall chemical vapour deposition (CVD) reactor, using gas mixtures of TiCl4–CH4–N2–H2 at a total pressure of 10.7 kPa (80 torr) and at a temperature of 1400 K. Growth rate, composition, morphology and crystallographic texture of the TiCxNy coatings were investigated as a function of the CH4/CH4+N2 ratio in the range 0–1 at a constant CH4+N2 flow rate of 370 standard cubic centimeters per minute (sccm). The C/C+N ratio and growth rate of the TiCxNy coatings increased with increasing CH4/CH4+N2 ratio in the gas phase. The compositions of the coatings with C/C+N ratios in the range 0–1 were found to be between the thermodynamic and the kinetic predictions. Morphology and preferred orientation of the coatings were observed to be strongly affected by the CH4/CH4+N2 ratio in the gas phase.     

Characterization of transparent conducting oxide thin films deposited on ceramic substrates

In this work, we investigate the optical and electrical properties of various transparent conductive oxide (TCO) thin films deposited on insulating ceramics for emerging optoelectronic applications. Thin films investigated include indium tin oxide (ITO), ruthenium oxide (RuO₂), and iridium oxide (IrO₂) on Al₂O₃ ceramic substrates. The conducting films have been deposited by various techniques including RF magnetron sputtering and low-cost spray pyrolysis. The morphological characteristics of the films were carried out using high magnification optical microscopy and atomic force microscopy (AFM). Optical and electrical characterization was carried out by optical absorbance/transmittance, van der Pauw, current-voltage (I-V), and Hall effect measurements. The results are presented in this paper.     

Ductility of thin copper films on rough polymer substrates

Electronic components in modern flexible electronics are connected by interconnects, which typically have the form of metal films resting on polymer substrates. This paper firstly studies experimentally the ductility of Cu films deposited on polyimide substrate with roughened surface (due to sandblasting) and finds that, upon tensile loading along the direction of film surface, the density of surface cracks in the film decreases with increasing surface roughness. The method of finite elements is subsequently employed to study the distribution of tensile stresses in the film and their influence on film cracking (initiation and propagation). It is demonstrated that a rough (curved) interface can reduce the tensile stresses along the film surface so as to restrain channel cracking of the film. Finally, the cohesive zone model is used to study the initiation and spreading of damage in the film and interfacial debonding of the curved interface. Both the interfacial damage and interface crack length are reduced as a result of interface roughening.     

Growth and structure of TiC coatings chemically vapour deposited on graphite substrates

TiC coatings were grown on graphite substrates by the chemical vapour deposition technique, using gas mixtures of CH₄-TiCl₄-H₂ at a total pressure of 10.7 kPa and at temperatures of 1400 and 1425 K. The growth rate and structure of the TiC coatings were investigated as a function of CH₄ and H₂ concentrations. The deposition rate of TiC increased with increasing CH₄ flow rate, but did not change with H₂ flow rate. This behaviour was explained by a mass transport theory. Thermodynamic analyses based on minimization of Gibbs' free energy predicted carbon codeposition with TiC. X-ray diffraction and Auger electron spectroscopy (AES) studies and microstructural observations, however, suggested that free carbon did not form. Textural analyses indicated that the growth of TiC coatings was initiated as randomly oriented crystallites, and as the thickness of the coatings increased, preferentially oriented columnar grains developed. The textures of TiC coatings with the same thickness changed from the 110 orientation to the 100 orientation with decreasing H₂ flow rate for a constant CH₄ flow rate. The CH₄ concentration also greatly influenced the preferred orientation of the coatings.     

Self-assembly of pillars modified with vapor deposited polymer coatings

In this paper, we demonstrate that thin layers of polymer coatings can be used to self-assemble pillars into stable microstructures. Polymer coatings are deposited onto elastomeric pillars using solventless initiated chemical vapor deposition and capillary forces are used to collapse the coated pillars into microstructures. The location of pillar collapse can be controlled by patterning regions of hydrophilicity and hydrophobicity. Poly(hydroxyethyl methacrylate) and poly(methacrylic acid) coatings stabilize the self-assembled microstructures by providing an adhesive force through solvent bonding. These solvent bonds allow the response of the microstructures to be tuned by varying the thickness of the polymer coating and the solubility parameter of the solvent. The coating process described in this paper is substrate-independent and therefore can be applied to pillars composed of any material.     

Investigation of structural properties of ITO thin films deposited on different substrates

The influence of the substrate nature on the structure and morphology of ITO thin films grown by thermal evaporation in vacuum is investigated. The as-prepared metal films with Sn/In molar ratio of 0.1 were subsequently annealed for 2 h at 723 K in air (to obtain tin doped indium oxide), then annealed in vacuum at 523 K, followed by UV irradiation (to reduce the electrical resistivity). Irrespective of substrate nature, XRD data evidence a (222) preferential orientation in films. Substrate nature, annealing in vacuum and UV irradiation influence the structure, morphology, optical, electrical and surface wetting properties of the films' surface.     

Electro-fragmentation analysis of dielectric thin films on flexible polymer substrates

An electro-fragmentation method was developed as a fast alternative to the time consuming fragmentation test carried out in situ in a microscope, to investigate the failure of dielectric inorganic coatings on polymer substrates. An ultrathin conductive layer was used to probe the onset of tensile failure in the dielectric coating through changes of its electrical resistance. A careful selection of the conductive layer has been carried out to avoid artifacts resulting for instance from a change of the cohesive properties (e.g. internal stress state) of the investigated structures. Au layers were found to be too ductile, contrary to Al-Ti layers that were too brittle, which invalidated the use of both materials to probe the failure of the dielectric coatings. In contrast, for structures on high-temperature polymer substrates, a 10 nm thick amorphous graphite (a-G) layer was found to accurately reproduce the cracking of the coating. The Young's modulus and coefficient of thermal expansion of the a-G layer are low enough not to impact the internal strain, hence the crack onset strain of the dielectric coating. The a-G layer is also sufficiently brittle, and its cohesive failure and resulting increase of electrical resistance is triggered by the failure of the dielectric coating. The a-G electro-fragmentation method is presently limited to polymers substrates with a glass-transition temperature higher than 100 °C.     

Thermal dewetting of thin Au films deposited onto line-patterned substrates

Au films are deposited onto line-patterned substrates, and the dewetting of the line-shaped films is studied. The results show that the line-patterned substrates can clearly lead to both, a decrease and an increase in the resulting particle size and particle spacing when compared to dewetting occurring on a flat substrate. The size and the spacing of the dewetted Au particles scale with the feature size of the line-shaped films in a Rayleigh-like way, but this scaling law ceases when the ratio of line width to film thickness reaches or exceeds a critical value.     

Elasto-plastic properties of gold thin films deposited onto polymeric substrates

This work examines mechanical properties of 50–300 nm gold thin films deposited onto micrometer-thick flexible polymer substrates by means of tensile testing of the film–substrate system and modeling. The film properties are extracted from mechanical testing of the film–substrate system and modeling of the bimaterial. Unlike materials in bulk geometry, the film elastic modulus and yield strength present an important dependence with film thickness, with modulus and yield strength of about 520 and 30 GPa, respectively, for the thinner films and decreasing toward the bulk value as the film thickness increases. The relation between grain size, film thickness, and yield strength is examined. Finite element analysis provides further insight into the stress distribution in the film–substrate system. L. Llanes—MS student at ITM, Merida, Mexico.     

Structure and optical properties of thin titanium films deposited on different substrates

Thin films of titanium were deposited on different substrates at room temperature. Measurements were made of the optical constants and of the transmittance of titanium films evaporated on to fused quartz. Films of titanium 10 to 40 nm thick were found to have quite uniform transmittance throughout the visible spectrum. Because titanium getters strongly during its evaporation, pure and compact titanium films can only be produced by fast evaporation under extremely good vacuum conditions. All films prepared for optical measurements, for X-ray and for scanning electron microscopy studies were, therefore, deposited at a pressure 10^–4 Pa and with deposition rate 4 nm sec^–1. The measurements were made using a Beckman double-beam spectrophotometer UV 5230, Siemens D 500 X-ray diffractometer, and SEMCO nanolab 7 scanning electron microscopy.