Sub-pixel image correlation: An alternative to SAM and dye penetrant for crack detection and mechanical stress localisation in semiconductor packages

Sub-pixel image correlation has been applied to the measurement of surface deformation, in particular to the measurement of thermally induced strain in microchips. A Fourier shift technique coupled with successive approximation is used to determine the sub-pixel component of the deformation. This reduces the effects of systematic errors on data associated with interpolation or curve-fitting present in other sub-pixel techniques. For all image correlation techniques, defocus errors can have serious effects on the accuracy of the technique. These errors are eliminated in this case through use of a set of images at different axial positions, where only the in focus information is used. This allows determination of the out-of-plane component of the displacement to within roughly 1/40 of the depth of focus (250 nm at ×10 magnification). An in-plane deformation/displacement uncertainty of roughly 2% of a pixel (<15 nm at ×10 magnification) is achievable on real-world samples.