Nanofracture mechanics : scanning force microscopy for the investigation of adhesion and corrosion at solid-solid interfaces

Fracture processes are crucially determined by structural features on the molecular/nanometer scale (cavities, occlusions, cracks, etc.) as well as on the atomic scale (e.g. interstitial, substitutional and vacancy defects). In this work, fracture mechanics experiments were performed with fabricated nanostructures, so-called nanopillars. Furthermore, material interfaces had been introduced into these nanopillars as weak links in order to act as well-defined breaking points. By exerting calibrated forces onto these nanostructures, the threshold force for fracture incidents can be determined and hence the adhesion strengths of the interfaces involved can be studied.
All such experiments were performed using a Scanning Force Microscope (SFM). Here, force and topography investigations, using a cantilever tip as a tool, reveal information about the fracture behavior of a particular interface as well as information regarding the mechanical strength. The SFM was used in the tapping (intermitted) or in the contact mode to fracture single nanopillars or an ensemble of them. For statistical examinations, an area of nanopillars was scanned with increased normal forces. Therefore, interfaces manufactured for microelectronic applications or micro-electro-mechanical systems (MEMS) can be studied by low forces applied to nanopillars exhibiting realistic interface dimensions.
Due to the small dimensions of the manufactured nanopillars, slow processes, such as the weakening of the interface by fatigue (also including heat cycling in devices) or by physico-chemical processes (e.g. by tribochemical processes or corrosion which may occur in a liquid environment) can be monitored on considerably shorter time scales and under easier to control conditions than with macroscopic specimens. Additionally, such fracture experiments performed with nanopillars designed to mimic macroscopic fracture experiments, in medium (characteristic cross section ~cm2) to large scale (> ~m2) engineering, are often less cost intensive compared to large, real-world samples in time consuming (~ many load/unload heat/cool cycles, extended exposure to ambient or corrosive fluids etc.) conventional fracture experiments.
Another important application comprises the study of a soft metal/polyimide interface, which is important for flexible microelectronic devices and flexible interconnect circuitry. Here, interface problems, specifically failure incidents after exposure to temperature cycling and/or mechanical load/unload cycles, have been associated with the occurrence of interfacial contamination, e.g. with residual water originating from the polyimide curing process. Hence, in a well-chosen model experiment under ultra-high vacuum (UHV) conditions, a precise amount of water was deposited on an in-situ produced polyimide sample which then was coated by a metal. Afterwards, the nanopillar structures were generated by Focused Ion Beam (FIB) milling.
This work established a radically new approach to perform fracture mechanics experiments down to the few nanometers, which provides a route towards a better understanding of fracture processes down to an atomic/molecular scale.