Integrated Digital Image Correlation for Multi-Beam Optical Stress Sensor

In a wide variety of technology, thin films are of substantial importance to obtain high performance and reliability. Internal stresses in thin films may cause a loss of functionality. In order to control the internal stresses, first an accurate stress measurement is required. Multi beam optical stress sensor (MOSS) is an extremely reliable and sensitive system for in-situ real-time stress measurements. By measuring the curvature of a thin film substrate system, the film stress can be deduced by applying Stoney’s equation without knowledge of thin film material properties.

Traditionally, a two-step method is used to calculate the curvature. First, the centroids of an array of parallel laser beams are fitted with Gaussian profiles. Secondly, the shifts of the centroids are converted in a curvature using a least square method. In this paper, integrated digital image correlation(IDIC) is performed to directly calculate the curvature without making assumptions about the shape of the laser.

Analytical formulas, describing the laser beam profile including deformations, are derived to obtain virtual images, mimicking the experiment, to optimize both methods and compare the methods with each other. With these virtual images, it is proven that IDIC outperforms the Gaussian method.