Issue 48

Y. Sun et alii, Frattura ed Integrità Strutturale, 48 (2019) 648-665; DOI: 10.3221/IGF-ESIS.48.62 649 I NTRODUCTION n the past two decades, film/substrate structures have been widely used in many high-tech fields including microelectronic systems, sensors, magnetic recording media, optoelectronic devices and microstructure fabrication. As two different materials, the film and the substrate have different physical and chemical properties. When they are simultaneously subjected to mechanical force, chemical treatment or thermal stress, mismatched deformation between them inevitably occurs. In order to balance the system, some stress modes will form in the films. The stress modes are strongly dependent on the stress properties, the mechanical properties of the films and substrates as well as the interfacial interaction properties between the films and substrates. Under sustained stretching stress, the films will fracture into various crack patterns including straight [1-5], circular [6-8], ridge [9], spiral [6,10-12], radial [7,8,13] and networked cracks [2]. The dimension and morphology of the cracks can be tuned by altering the film thickness and substrate properties. In the case of continuous compressive stress, the films will form various buckling modes such as wrinkles [6,7,14-21], folds [22-24], creases [23,25] and telephone cord buckles [9,26,27]. The stress magnitude, the film thickness, the substrate elasticity and the adhesion performance between the films and the substrates can determine the wavelength, amplitude and morphology of the buckling. For a film/substrate structure, it will form wrinkles when the following four conditions hold: 1) the substrate is thicker than the film, 2) the substrate is softer than the film, 3) the interfacial adhesion is strong and the film tightly adheres to the substrate, 4) the compressive stress of the film is beyond the critical wrinkling stress. In an equi-biaxial stress state, the film tends to spontaneously form labyrinth or herringbone patterns [3,28-30]. Because these wrinkle patterns are irregular and uncontrollable, they are usually considered undesirable results. The ordered wrinkle patterns can be fabricated by using some effective tuning methods to locally alter the equi-biaxial stress state. The stress tuning methods include lithographic pre-patterning of substrate [14,31-33], uniaxial pre-stretch releasing of substrate [34,35], solvent assist [17,36-40], contact- line mechanics [41-43], axial compression [44,45], point contact deformation [6,7,16,19,20,22,46-52], simultaneous and sequential release of biaxial pre-stretching [53,54], template adhesion [55-59], constrained edge effect [60,61], selective adhesion to substrate [62], introducing of surface defect [1-4,6-8,36,63], pre-wrinkle directing [64], etc . For example, the homogeneous sinusoidal wrinkle patterns can be fabricated in a stiff film/ elastic substrate by releasing the uniaxial pre- strain of the substrate [34]. The localized wrinkling patterns such as radial wrinkles can be fabricated in a polymer film floating on the surface of water by using the method of the point contact deformation [16]. The characteristic parameters can be tuned by controlling the film thickness and stress magnitude. The controllable wrinkles have vast applications in microfluidic channels [34,65], optical gratings [66], flexible electronic devices [67,68], fabricating ordered microstructures [69,70], improving surface adhesion performance [71], culturing viruses [72], measuring film’s Young modulus [73,74] and quantifying residual stress [75]. Therefore, the understanding of the formation and evolution mechanisms of homogeneous global and localized wrinkling patterns has become a hot topic for researchers. Among various film materials, the metal films are widely used in the film/substrate systems to fabricate microstructures due to the relatively simple manufacturing process. For the metal materials such as gold, silver and copper, the chemical and physical properties are relatively stable at high temperature, and the films are generally formed by vacuum evaporation and sputtering deposition. Due to the thermal expansion mismatch between the films and substrates, a high level of residual stress will be stored in the films after deposition. Once the residual stress is beyond the critical wrinkling stress of the films, the global wrinkle patterns will form in the films, which causes the localized wrinkle patterns to be rarely observed in experiments. Previous studies have shown that surface defects such as boundaries, cracks and dusts in the films and the substrates can locally alter equi-biaxial stress state in films and thus alter the local wrinkling morphology [1,3,14]. This implies that the localized wrinkles can be fabricated in the metal films by introducing regularly arranged defects into the films or the substrates. Here, we review some recent progress on the formation and evolution mechanisms of the localized wrinkle patterns in the metal films deposited on the elastic and liquid substrates induced by constrained edges and cracks. The in- depth studies of these controllable wrinkle and crack patterns are not only important for understanding the micro-failure mechanics of film structures and devices, but also for further guiding the improvement of the mechanical stability and lifetime of the film structures. This review is outlined as follows. In section 2, we briefly review the formation mechanisms of the wrinkles and cracks. In section 3, we describe some typically localized wrinkling patterns in the metal films/elastic substrates and their morphological evolution behavior. In section 4, we introduce some localized wrinkling patterns in the metal films/liquid substrates and discuss their evolution mechanisms. Finally, this review is concluded with some suggestions about the future studies of the localized wrinkles. I