Laser-based Determination of Decohesion and Fracture Strength of Interfaces and Solids by Nonlinear Stress Pulses

Abstract

To determine the mechanical strength of films, coatings and solids the loss of adhesion or fracture must be investigated. The failure of interfaces and bulk materials is a dominant issue in microelectronics with multilayer systems and micro-electro-mechanical-system (MEMS), nano-electro-mechanical-system (NEMS) and sensor devices, based essentially on single-crystal silicon as basic material, which is considered here in detail. A versatile tool that is quite often used for strength analysis is the scratch tester, where a diamond stylus is drawn across the coated surface under increasing load to determine the critical load, or the stylus is used as an indenter. These methods are versatile, but besides being influenced by the properties of the system itself they also depend on several test parameters, such as scratching velocity and stylus properties, which affect the critical load. Owing to the complexity of the failure processes involved, in connection with strongly inhomogeneous deformation fields, it is generally very difficult to extract quantitative values of the cohesion or fracture strength (Lacombe, 2006). In fact, the most widely used testing methods, such as peel, pull, scratch, blister, indentation and beam-bending tests, usually involve plastic deformations, which are difficult to analyze (Wei & Hutchinson, 1998). Some of these quasi-static methods do not reach the strength limit of the strongest material systems or require intricate sample preparation. In this review the contact-free measurement of the strength of interfaces and bulk materials will be discussed employing pulsed lasers to launch strongly nonlinear stress pulses. In fact, quantitative information on the failure strength of materials can be obtained by laser-based excitation and detection techniques. Normally a nanosecond laser pulse is used to excite either bulk or surfaces acoustic stress waves, which develop shocks during propagation. In these laser-controlled pump-probe setups a continuous-wave (cw) laser probe is used to measure the transient surface displacement or surface velocity, providing the information on the elastic stresses achieved. The laser techniques are contact-free and normally need no artificial seed crack to induce failure because stresses of 5-10 GPa can be attained in the elastic shock pulses generated. For studying interfacial strengths usually bulk acoustic waves are used. With a laser pulse a one-dimensional (1D) compressive longitudinal wave packet is launched in a thin metal film covering the back side of the substrate. The critical failure stress of the film/substrate