Determination of thermal stress distribution in a model microelectronic device encapsulated with alumina filled epoxy resin using fluorescence spectroscopy

A method for measuring residual and applied stresses in particulate polymer systems, which utilizes the piezo-spectroscopic effect of the optical fluorescence of filled particles, is presented. Fluorescence piezo-spectroscopy (PS) is non-destructive and provides microscopic lateral resolution upon using an optical microprobe system. Epoxy resin filled with α-alumina particles is used in this study. Stress values are obtained by frequency shift measurement of the characteristic optical fluorescence lines produced by Cr3+ impurities in alumina. The relationships between peak frequency shift of these lines and stress are derived by using a 4-point-bending test. The peak frequency shift shows linear correlation with tensile stress, while a non-linear relation between peak frequency shift and stresses is found in the compressive stress region. To a first order approximation, residual stresses were calculated by the frequency shift divided by the linear correlation coefficient in the tensile stress region. As an application of the PS method, we determined micron order residual stress distributions in a model plastic encapsulated silicon substrate for microelectronic devices and compared the stress data with those calculated using a two-dimensional finite element analysis of the device. The experimental results were in good agreement with the tensile stress components that have been obtained by theoretical calculation. Therefore, PS techniques can be used to measure residual stresses in polymer compounds utilizing the information obtained from the fluorescence lines of a dispersed ceramic powder.