Interplay of electrical and mechanical behaviors in dielectrics

Dielectric films are widely integrated into advanced devices for both their functional and mechanical properties. In terms of functional properties, dielectrics are essentially expected to prevent electrical leakages, sustain large electric fields, limit capacitive coupling… In terms of mechanical properties, dielectrics contribute to the stiffness of device architecture but, like all ceramics, they are also preferential loci for crack nucleation/propagation. Consequently, understanding the interplay between the mechanical and electrical behaviors of dielectric films is essential to predict the lifetime of functional devices.<br/>Nanoindentation is a powerful technique to probe the mechanical behavior of small-scale systems. When combined to electrical measurements, electrical-nanoindentation can be used to assess this mechanical-electrical interplay.<br/>In the first part of this presentation, we will show how a mechanical stress can modify the electrical conduction mechanism in an ultra-low-k dielectric (nanoporous SiOCH film deposited on a silicon substrate). Experimentally, electrical leakage was monitored in-situ during nanoindentation with a Berkovich indenter. These experiments reveal a counterintuitive electrical conduction drop under high mechanical stresses. This phenomenon is reproduced numerically (by FEM analysis) by correcting the Poole-Frenkel conduction law with a strain-dependent factor, and described analytically in terms of space-charge build-up induced by the trapping of holes at the mechanically-generated defects. A threshold strain is identified as the keystone linking this strain-dependent conduction to the current line distribution within the dielectric.<br/>In the second part, we show how an electrical stress can degrade the mechanical properties of dielectrics. Experiments were carried out on a wide panel of dielectric systems: high-k and low-k dielectrics, thin and thick films, with or without bottom electrodes… Thanks to the fine in-situ coupling of mechanical and electrical monitoring, a mechanical collapse of the film is observed after the application of an electrical stress. Various origins of this collapse were first ruled-out (electrostatic force, local heating…), before it was finally demonstrated that the injection of electrical charges drives this mechanical collapse. Numerical modelling (FEM analysis) is used to discuss which film property is actually modified: stiffness or hardness. This phenomenon appears to be universal, since it is observed on all kinds of dielectric systems assessed in this study.