Surface effects on the wrinkles in a stiff thin film bonded to a compliant substrate

Surface effects are important to predict the mechanical behavior of nanostructures. In this paper, the wrinkling of a stiff thin film bonded to a compliant substrate is studied using an energy method accounting for surface elasticity and residual surface tension. The wavelength, critical buckling strain and amplitude are obtained analytically. These results provide valuable guide to the precise design and control of the wrinkling profile in many applications ranging from stretchable electronics to micro/nano scale surface patterning and precision metrology.     

Concomitant wrinkling and buckle-delamination of elastic thin films on compliant substrates

Compressing a thin elastic film attached to a thick compliant substrate can lead to buckling instability. Two commonly observed buckling modes, buckle-delamination and wrinkling, have each been analyzed separately in previous studies. Recent experiments have observed that the two modes can co-exist and co-evolve. In this paper, by analytical and finite element methods, we present a study on concomitant wrinkling and buckle-delamination for an elastic film on a highly compliant substrate. First, without delamination, we present an analytical solution for wrinkling that takes into account the effect of Poisson’s ratio of the substrate. In comparison with a nonlinear finite element analysis, an approximate formula is derived to estimate the normal traction at the interface and to predict initiation of wrinkle-induced delamination. Next, with a pre-existing delamination crack, the critical strain for the onset of buckling instability is predicted by finite element eigenvalue analysis. For an intermediate delamination size, a mixed buckling mode is predicted with the critical compressive strain lower than previous solutions for both wrinkling and buckle-delamination. Post-buckling analyses show a significant shear-lag effect with an effective load transfer length three orders of magnitude greater than the film thickness. Finally, concomitant wrinkling and buckle-delamination is simulated to illustrate the interaction between the two buckling modes, and the results are discussed in view of failure mechanisms and applications in thin film metrology.     

Viscoelastic wrinkling in compression-stressed metal film-polymer sublayer system

The laws of corrugation (wrinkling) that takes place in thin aluminum films on silicon substrates with styrene sublayers under the conditions of thermal treatment have been studied using atomic force microscopy techniques. Measurements of the amplitude and period (wavelength) of wrinkles revealed stages in the viscoelastic corrugation process at various temperatures. It is established that the evolution of wrinkles in the course of annealing is controlled by the periodic distribution of normal and tangential stresses at the film-sublayer interface.     

A buckling-based metrology for measuring the elastic moduli of polymeric thin films

As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).     

Instability-induced wrinkling in thermoelectric thin film/substrate structures for thermal protection systems in supersonic space shuttle applications

ABSTRACT

This paper examines the behavior of wrinkling instability of a thermoelectric thin film bonded to substrate. The critical temperature differences for wrinkling occurrence and buckling initiation are obtained. Damage growth following wrinkling is also determined. These critical temperatures can provide guidelines for the design of thermoelectric thin film devices. Numerical results show that the stability of thermoelectric thin film is affected by the electric current. The critical temperature differences become smaller when the electric current density in thermoelectric thin film is higher. Effect of the wavelength of wrinkling on the critical temperature differences of wrinkling occurrence is also identified.     

The fracture of brittle thin films on compliant substrates in flexible displays

One mechanical issue in flexible organic light emitting displays (OLED) is the fracture of extremely thin brittle conducting transparent oxide films deposited on thin flexible substrates. Understanding the behaviour of these films under flexed condition is essential for designer of flexible OLED. Controlled buckling experiments on the film and substrate have been designed to study the fracture of the films under both tension and compression. Fracture of the film is superficially similar in both tension and compression. However, under tension a channelling crack is formed, while under compression, the film delaminates, buckles and cracks in a tunnelling motion. The fracture toughness of the film and the delamination toughness have been estimated from these experiments. Design to maximise the flexibility of the device is discussed.     

柔性电子器件的应用、结构、力学及展望

调控薄膜基底结构的表面不稳定性已被成功应用于制备可延展的新型电子设备中。然而,该类电子器件在工作中需要承受外部载荷作用,致使薄膜-基底结构界面处残余应力集中,容易诱发薄膜电子器件与基底脱粘与分层。该文将从理论分析和数值仿真角度,研究压电薄膜在柔性基底表面上的失稳特性。由于压电薄膜变形具有大位移小应变的特点,该文基于非线性Euler-Bernoulli梁理论与能量最小化原理,建立压电薄膜基底结构无屈曲、褶皱、局部屈曲及全脱层屈曲模式的理论分析模型;从能量角度定量分析了薄膜-基底结构4种模式相互演变的临界条件;通过数值仿真,验证了该文解析结的有效性,定量、定性的讨论了薄膜基底结构的材料、几何参数对4种模式演变的影响。研究结果表明：改变基底弹性模量、预应变、物理场强度和界面粘附系数能够调控压电薄膜基底结构的屈曲模式;通过调控温度变化量和电压的方式,能够实现对压电薄膜基底结构的失稳特性精细化调控。该文的研究结果将为提升薄膜基底型的电子器件的稳定性及优化设计提供理论支持。

     

Stress relief patterns of Co films deposited on circular silicone oil substrates

By using the expansive and mobile properties of silicone oil, circular liquid substrates with varied thicknesses naturally form on clean glass surfaces during deposition. Continuous cobalt (Co) films have been prepared on the circular liquid substrates by a direct current magnetron sputtering method, and the stress relief patterns of the Co films are investigated. The experiment shows that the Co film is susceptible to generating cracks due to thermal expansion of the silicone oil substrate during deposition. After deposition, subsequent cooling of the system creates a high compressive stress in the film, which is relieved by formation of various wrinkling morphologies. Based on the special growth mechanism and mechanical properties of the metal film deposited on a liquid substrate, the structural characteristics of the stress relief patterns and the underlying physical mechanisms are discussed in detail.     

Heating of polymer substrate by discharge plasma in radiofrequency magnetron sputtering deposition

The substrate used for the thin film deposition in a radiofrequency magnetron sputtering deposition system is heated by the deposition plasma. This may change drastically the surface properties of the polymer substrates. Deposition of titanium dioxide thin films on polymethyl methacrylate and polycarbonate substrates resulted in buckling of the substrate surfaces. This effect was evaluated by analysis of atomic force microscopy topography images of the deposited films. The amount of energy received by the substrate surface during the film deposition was determined by a thermal probe. Then, the results of the thermal probe measurements were used to compute the surface temperature of the polymer substrate. The computation revealed that the substrate surface temperature depends on the substrate thickness, discharge power and substrate holder temperature. For the case of the TiO₂ film depositions in the radiofrequency magnetron plasma, the computation indicated substrate surface temperature values under the polymer melting temperature. Therefore, the buckling of polymer substrate surface in the deposition plasma may not be regarded as a temperature driven surface instability, but more as an effect of argon ion bombardment.     

Anisotropic Nanoscale Wrinkling in Solid‐State Substrates

Pattern formation induced by wrinkling is a very common phenomenon exhibited in soft‐matter substrates. In all these systems, wrinkles develop in the presence of compressively stressed thin films lying on compliant substrates. Here, the controlled growth of self‐organized nanopatterns exploiting a wrinkling instability on a solid‐state substrate is demonstrated. Soda‐lime glasses are modified in the surface layers by a defocused ion beam, which triggers the formation of a compressively stressed surface layer deprived of alkali ions. When the substrate is heated up near its glass transition temperature, the wrinkling instability boosts the growth rate of the pattern by about two orders of magnitude. High‐aspect‐ratio anisotropic ripples bound by faceted ridges are thus formed, which represent an optimal template for guiding the growth of large‐area arrays of functional nanostructures. The engineering over large square centimeter areas of quasi‐1D arrays of Au nanostripe dimers endowed with tunable plasmonic response, strong optical dichroism, and high electrical conductivity is demonstrated. These peculiar functionalities allow these large‐area substrates to be exploited as active metamaterials in nanophotonics, biosensing, and optoelectronics.     

Measurement of the noibium/sapphire interface toughness via delamination

Niobium films were deposited on sapphire substrates using both physical vapor deposition (PVD) and ion beam assisted deposition (IBAD) at an ion energy of 1000 eV and an ion-to-atom arrival rate ratio of 0.4. The interface between the niobium film and the sapphire substrate was doped with up to 7.6 monolayers of silver. The films were patterned into fine lines using photolithography. During the photolithography process, curling and buckling were observed. The curling indicated a stress gradient in which the top of the film is tensile with respect to the bottom of the film, while buckling demonstrated that a portion of the film thickness must have been in compression. An analysis of the delamination showed that the critical energy release rate for the interface was on the order of 1 J m^–2, and that the compressive stress is of the order 1 GPa. The higher energy release rate of the IBAD samples confirmed that the stronger interface is due either to the orientation relationship between the ion beam textured niobium film and the (0001) sapphire surface or the interface mixing caused by ion bombardment.     

Grain size and film thickness effect on the thermal expansion coefficient of FCC metallic thin films

Thin films are used in wide range of applications in industry, such as solar cells and LEDs. When thin films are deposited on substrates, various stresses are generated due to the mechanical difference between the film and substrate. These stresses can cause defects, such as cracking and buckling. Therefore, knowledge of the mechanical properties is important for improving their reliability and stability. In this study, the thermal expansion coefficient of FCC metallic thin films, such as Ag and Cu, which have different grain sizes and thicknesses, were calculated using the thermal cycling method. As a result, thermal expansion coefficient increased with increasing grain size. However, the film thickness had no remarkable effect.     

Theoretical Study on the Bilayer Buckling Technique for Thin Film Metrology

Recently, a novel technique based on the wrinkling of a bilayer composite film resting on a compliant substrate was proposed to measure the elastic moduli of thin films. In this paper, this technique is studied via theoretical analysis and finite element simulations. We find that under an applied compressive strain, the composite system may exhibit various buckling modes, depending upon the applied compressive strain, geometric and material parameters of the system. The physical mechanisms underlying the occurrence of the two most typical buckling modes are analyzed from the viewpoint of energy. When the intermediate layer is much thicker than the top layer, the condition under which the bilayer buckling will occur prior to other modes is given. The results reported here may facilitate the design of the bilayer buckling technique for the thin film metrology.     

Numerical simulation of periodic cracking mechanism in microscopic surface films during roll bonding processes

The microscopic surface films existing on the top of metallic layers play an important role in the process of joining by plastic deformation. The bond formation during cold welding processes is basically associated with the fracturing of surface films to produce intimate metallic contacts. The present paper aims at providing a numerical model to describe the cracking pattern of brittle surface films bonded to the ductile substrates. A microscale finite element model is developed which takes into account the fracturing mechanisms of thin surface films in roll bonding processes. The presented model is calibrated by using the existing experimental data for an aluminum alloy covered by a thin layer of oxide film. The model is also validated against a well‐known analytical model for periodic cracking. The distribution of stresses within the fractured surface film demonstrates that the generated cracks in the surface film have essentially a periodic pattern. Moreover, it is shown that the crack spacing is highly dependent on the properties of the surface film. Finally, the obtained results for the roll bonding show that a crack density saturation takes place at the entry of the roll bite where a small surface expansion is applied to the rolled samples.     

Influences of argon plasma cleaning on the die-shear force of chip and copper/polyimide flexible substrate assembly using a non-conductive film

Abstract

A non-conductive film (NCF) and a thermal compression process were combined to assemble a chip onto a flexible substrate. To achieve direct bonding and to enhance the adhesion of a chip on the flexible substrate with NCF, argon plasma was used to clean copper electrodes over a flexible substrate. For flexible substrates which received argon plasma cleaning, low contact angles were obtained, and gold bumps directly contacted copper electrodes to form active input/output (I/O) paths, indicating the contaminants on the flexible substrate were removed and a clean bonding surface could be achieved. Neither delamination nor porosity was observed at the bonding interface between the NCF and copper electrodes. A sound bonding interface with satisfactory die-shear force was achieved. The argon plasma cleaning led to a significant improvement of the die-shear force of a chip bonded onto the flexible substrate using NCF.     

Numerical simulation of interface crack in thin films

The de-adhesion of a thin film from a rigid substrate is studied. It is assumed, that a periodic array of micro-cracks exists along the film/substrate interface. During formation, the film expands, while being constrained by the substrate. This phenomenon leads to development of compressive stresses. Then buckling may occur and cause crack growth either along the interface or in the film towards the free surface. A finite element model has been developed, which simulates film buckling and subsequent interfacial crack growth, based on film/substrate adhesive constitutive relations. These relations have been motivated by atomistic calculations on bimaterial failure. The model does not require any facture criterion. Interfacial work of separation has a significant effect on damage growth ahead of the crack tip, along the interface. Also, a critical remote compressive stress exists, at which damage progresses without further loading of the film. The relation between the critical compressive stress for extensive damage and the interfacial work of separation can be used in combination with experiments for the quantitative characterization of the film/substrate interface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Moisture‐Responsive Wrinkling Surfaces with Tunable Dynamics

The wrinkle dynamics (such as reversibility and stability) of human skin are affected by the external stimuli, as well as the skin's structure and mechanical properties. Inspired by these tunable responses, three types of moisture‐responsive wrinkle dynamics are achieved, for the first time, through a single film–substrate system. These dynamics include: (1) completely reversible wrinkles formation; (2) irreversible wrinkles formation I: the initially formed wrinkles can be permanently erased and never reappear; and (3) irreversible wrinkles formation II: once the wrinkles form, they can no longer be erased. The key to success is to control the stiffness and thickness ratios of the film and the substrate, and tailor the crosslink degree/gradient of the film to allow for moisture‐dependent changes of modulus and swelling degree. These unique responsive dynamics motivate the invention of a series of optical devices triggered by moisture, including anticounterfeit tabs, encryption devices, water indicators, light diffusors, and antiglare films. This study also paves the road for further understanding of the skin wrinkling dynamics and manipulation.     

Solvent-induced local deformation zones in polyimide films adhered to a rigid substrate

Thin films of a polyimide (Ciba-Geigy XU293) adhered to rigid substrates developed special type of local deformation zones when immersed in xylene. These deformation zones were initiated because of the presence of the combination of xylene and the equi-biaxial tension resulting from the drying process. Their microstructure was strongly influenced by the local bonding to the substrate. In the bonded regions, the zones resembled a narrow trench containing deformed but unvoided material. In the unbonded regions, complicated fibrillar structure was observed in the much wider zones. Although they retained a smooth surface on the face in contact with the substrate in the bonded regions, the deformation zones released a significant amount of stress. The stress release was measured by a photo-elastic method and was found to be in excellent agreement with that calculated from the local thickness change assuming simple elasticity. Although not cracked, there were lines of weakness; after long soaking times the films cracked and delaminated along the defects.     

High optical quality ZnO films grown on graphite substrate for transferable optoelectronics devices by ultrasonic spray pyrolysis

ZnO thin films were deposited on graphite substrates by ultrasonic spray pyrolysis method. The effects of substrate temperature and film thickness on the crystalline structure, morphology, and optical properties of the as-grown ZnO films were investigated systemically. Results illustrated that dense ZnO films with hexagonal wurtzite structure were uniformly distributed on the substrate. Strong near-band edge ultraviolet (UV) emission peaks were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO films could be achieved via this easy process under optimal conditions. The successful growth of polycrystalline ZnO films on graphite offers the significant opportunity to be readily transferred onto any rigid or flexible foreign substrates, since the graphite substrates consist of weakly bonded layer structure.