Differential Imaging Microscopy of Physically Complex Surfaces Undergoing Atmospheric Corrosion

Frequently, optical observation of component materials is the only viable technique to evaluate degradation processes in-situ.  Unfortunately, due to the visually complex nature of many surfaces (e.g., scratches, etc.), environmental damage, particularly during the early stages of a corrosion process, can be difficult or impossible to resolve using traditional optical inspection techniques.  As a result, studies are limited to evaluating degradation well after initiation has taken place.  Thus, there is a need for a technique that allows the de-convolution of changes due to the degradation process from the background optical “noise” such that environmental degradation of a surface (corrosion, etc.) can be accurately observed at all stages of the degradation process.  Differential image processing is a technique through which subtle changes in a series of acquired images can be readily observed.  In this technique, a series of high resolution images of the surface are taken over time and quantitatively compared to one another through a mathematically based post-processing algorithm.   Minute changes in the surface morphology not readily resolvable using traditional image analysis techniques can then be readily quantified.  The capabilities of the Differential imaging technique will be demonstrated on a common microelectronic material - gold plated copper exposed to a sulfide containing atmosphere.  The utility of the differential imaging technique will be demonstrated by characterizing the life of numerous defects in the protective gold layer, illustrating how the processes of defect nucleation, growth, and subsequent stifling/death of a corrosion site can be monitored and related to the underlying material structure.