An optical fluorescence method is introduced for determining the localized stresses in passivated aluminum lines on sapphire substrates containing a thin epitaxial ruby film at the AI/AI2O3 interface. The method is based on the piezo-spectroscopic properties of the ruby film, which acts as an in situ sensor. By focusing a laser beam through the sapphire substrate and onto the bottom of an aluminum line, the fluorescence from the ruby sensor can be excited and collected through an optical microscope. The frequency shift of the fluorescence lines is proportional to the stress in the aluminum line. To illustrate our observations two sets of measurements are presented: the spatial variation perpendicular to an interconnect line; and the temperature dependence on heating upto the deposition temperature of the SiN used to passivate the interconnects.
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Wen, Q., Ma, Q. & Clarke, D.R. In-Situ Fluorescence Strain Sensing of the Stress in Interconnects. MRS Online Proceedings Library 356, 591–596 (1994). https://doi.org/10.1557/PROC-356-591