Crack on a chip test method for thin freestanding films

Abstract

Fracture mechanics has been applied for more than two decades to various configurations of cracks in films on substrate. Fracture toughness data are indeed needed for the design and integrity assessment of many coatings and microelectronics devices. Nevertheless, it is sometimes complicated to deconvolute the constraint exerted by the substrate on the cracking process especially in the presence of viscoelastic or plastic dissipation. Here, a new on chip test method has been developed to determine the fracture toughness of freestanding submicron films. Beside the advantage of avoiding the constraint induced by the substrate, freestanding films allow, if sufficiently thin, direct observation of the fracture mechanisms by transmission electron microscopy. The design of this new nano(micro)-testing consists of two long actuator beams undergoing large internal stress. A specimen is attached to these two actuators, incorporating a notch produced by lithography. Two types of geometries are addressed, one being a double cantilever beam type configuration, while the other is a center cracked panel. Both actuators and specimen are deposited on a sacrificial layer. The etching of the sacrificial layer induces the release of the test structure, with the actuators then contracting and pulling on the test specimen. A crack is initiated from the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Analytical equations that describe the stress intensity factor as a function of the geometrical characteristics of the test structures are worked out to guide the dimensional analysis. Extensive finite element analysis provides the full parameter variations necessary to quantify the fracture toughness from experimental data and to capture the process of initiation, unstable cracking and arrest followed by possible further stable propagation. For the sake of a proof of concept, ∼50 and ∼100 nm-thick silicon nitride films produced by low pressure chemical vapor deposition were tested, leading to a mean fracture toughness equal to ∼2 MPam.