Cross Correlation Method for Images Alignment: Application to 4 Buckets Calculation in Thermoreflectance

Correlation methods are generally used in mechanics to study the deformation in materials. These techniques permit the determination of a moving field by comparing the digital photos before and after deformation. In this article, we have applied correlation techniques to superposed images after displacement due to thermal effect during thermoreflectance measurements on an insulated gate bipolar transistor (IGBT) power module using “4 Buckets” technique at magnification x50. The “4 Buckets” technique uses a simple calculation on light intensities reflected by a component biased with sinusoidal excitation to obtain the variation of reflectivity for each pixel in a picture when the images are superposed perfectly. Then using a calibration, it is possible to obtain a thermal image. We have applied correlation technique to “4 Buckets” images before calculation and studied their effects on thermal images and calibration coefficients. Preliminary results indicate that cross correlation is the best method to superpose each image to a reference and make calculation in order to eliminate any artifacts due to thermal expansion in thermoreflectance measurements.