Electro-Fragmentation Analysis of Dielectric Thin Films on Flexible Polymer Substrates

Abstract

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.

23.1: Invited Paper: Models and Experiments of Mechanical Integrity for Flexible Displays

“…The above fragmentation method is accurate but time consuming. Two novel methods, electro-fragmentation and electro-fatigue were developed for automatic measurement of critical failure strain and damage under fatigue loading [9]. The key concept is to explore the development of damage with simultaneous observation in-situ in a microscope and monitoring of the electrical properties.…”

Channel cracking in inelastic film/substrate systems

“…While channel cracking in linearly-elastic systems has been studied extensively starting with the notable works of Gille (1985), Nakamura and Kamath (1992) and Beuth (1992), similar studies on inelastic systems are generally limited to linearly-elastic films on nonlinear substrates undergoing localized yielding, as is the case for thermal type loading (Beuth and Klingbeil, 1996;Ambrico and Begley, 2002). Beyond the usual concerns for structural integrity, channel cracking offers a viable means for determining fracture energy in elastic thin films (Hsueh and Yanaka, 2003;Andersons et al, 2008;Miller et al, 2009;Pinyol et al, 2009), generally with the aid of the steady-state analysis of Beuth (1992). Often this approach is also adopted for ductile films (Macionczyk and Bruckner, 1999;Alaca et al, 2002;Niu et al, 2007;Chen and Gan, 2007), although its validity is yet to be assessed.…”