ICMCTF2012 Session F6-1: Coatings for Compliant Substrates

Interfaces are the critical feature governing performance of compliant substrate and multilayer polymer-metal thin film structures where differing properties between adjacent films can induce strong interlaminar normal and shear stresses and catastrophic failure. However, it is experimentally difficult to measure properties in these systems at small scales and theoretically difficult to isolate contributions from the films, interfaces, and substrates. Analysis is further complicated with the onset of substrate yielding. As a result our understanding of deformation and fracture behavior in these systems is limited. This has motivated a study of buckle driven delamination of compliant substrate and polymer-metal thin film systems that combined experiments and cohesive zone simulations. One set of experiments employed compressively stressed tungsten films that varied in thickness on commercial and high purity PMMA substrates. A second set of experiments employed spin coating PMMA films with thicknesses ranging from 10nm to 650nm onto copper coated silicon substrates followed with a sputter deposited overlayer of highly stressed tungsten. In both sets of experiments, the high film stresses triggered spontaneous delamination and buckling along the PMMA-tungsten interface accompanied by intense deformation in the PMMA substrates and PMMA layers. Of particular interest, the intensity of deformation varied markedly between each system studied and from model elastic behavior. Cohesive zone simulations that included substrate compliance showed a marked increase in fracture energies over rigid elastic solutions for thick films and compliant substrates. The fracture energies did approach rigid elastic solutions for the thinnest films tested. In this presentation we will use tests and simulations to show that film compliance provides a lower bound to behavior for all but the thinnest samples while constrained yielding accounts for the pronounced differences in behavior between samples. This work was supported by Sandia National Laboratories under USDOE Contract DE-AC04 94AL85000.

The move towards functional devices on flexible substrates puts severe demands on the mechanical properties of the coatings from which they are constructed. For instance the use of brittle transparent conducting oxide electrodes in displays and organic light emitting diodes results in failure due to fracture when a flexible substrate is bent to a small enough radius of curvature. In many cases, the materials used in such devices are not available in the bulk form and the only practical technique to measure mechanical data is nanoindentation. Whereas it is possible to make good measurements of coating properties on stiff substrates such as silicon using nanoindentation there are serious issues with the reliability of data obtained from coatings on compliant substrates such as the PET used for plastic electronics. This presentation will highlight the effect of coating architecture and thickness on the indentation response of thin films used in microelectronic devices, displays and OLEDS on stiff and compliant substrates. A simple model to assess the contribution of the coating layers and substrate to the measured contact modulus will be introduced and the results illustrated for single and multilayer coatings on silicon, glass and PET.

The mechanical properties of interfaces and more precisely the adhesion are of great importance to understand the reliability of Organic Thin Film Transistor (OTFT) on compliant substrate. Since these devices are flexible and intended for different fields of application like sensors and displays, they will undergo a lot of mechanical stress during their useful life. Many adhesion test techniques have been developed to measure adhesion energy of thin films but they are hard to implement in the case of submicronic organic thin film deposited on flexible substrate. Recently, the feasibility and repeatability of the scratch test technique as a tool for testing the adhesion and the damage behaviour of ultra-thin film on polymeric substrate have been demonstrated [1]. However, direct comparison of critical load between samples was not straightforward since different failure mechanisms were induced. In the present work, we have investigated the way to obtain more quantitative data. We have also performed mechanical ageing tests on specimens and proved that the scratch test technique is sensitive enough to monitor the degradation of the interface properties.

[1] G. Covarel, B. Bensaid, X. Boddaert, S. Giljean, P. Benaben, P. Louis, Surf. and Coat. Technol , 2011 in press.

Electroactive polymer (EAP) actuators are very promising candidates for the ambitious aim of developing soft actuator systems for artificial muscles, haptic devices and vibration-less actuators. Dielectric elastomer transducers benefit of important advantages compared to other electro-mechanical actuators such as high energy density and large and noise-free deformation capability. Today's EAP devices usually work at high voltage (> 1000 V), which preclude their use in or close to the human body, as such high voltages cause obvious safety problems. The electrode material also presents a challenge, since clean and fast processes suited to miniaturize EAP devices are still missing. To solve these drawbacks, we developed a new fabrication process aiming at reducing the dielectric layer thickness down to <20 μm and to increase the efficiency using highly conductive electrode materials deposited by magnetron sputtering. In this work we show that thin metallic films deposited my magnetron sputtering can be prepared in such a way that they are able to maintain high electrical conductivity at more than 10% stretching. These highly compliant films are characterized by X-Ray Diffraction, electrical conductivity measurements and Atomic Force Microscopy. The film properties and their implications for new EAPs will be discussed.