High‐Accuracy Thin‐Film Stress Sensor Exploiting Integrated Digital Image Correlation

At present, a multibeam optical stress sensor (MOSS) is the preferred instrument for measuring thin‐film stress in situ and in real time. The thin‐film stress‐induced curvature is measured using the reflection from the substrate of an array of parallel laser beams, while the curvature is converted to the thin‐film stress using Stoney's equation. To accurately track the displacements on the detector, each laser spot is fitted with a Gaussian intensity profile, a method that requires a few, narrow, isolated laser spots and, consequently, most of the detector area remains unused. To increase the accuracy, this work proposes an alternative method named the speckle stress sensor (SSS). It is based on (i) integrated digital image correlation (IDIC), known for its high displacement field accuracy for non‐optimal brightness patterns if an accurate description of the actual kinematics is incorporated, and (ii) a suitable pattern of a large number of overlapping spots, created with a simple LED and perforated plate. First, it is shown that IDIC is a more robust and more accurate method, even for analysing images with isolated laser spots. Next, a simple SSS LED set‐up has been developed that generates high‐quality images with overlapping spots, resulting in at least a twofold increase in stress sensor accuracy. Suggestions for further improvements are also proposed.