In-Situ Fluorescence Strain Sensing of the Stress in Interconnects
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ABSTRACT 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/A12 0 3 interface. The method is based on the piezospectroscopic 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. INTRODUCTION
As stress in interconnects provides a driving force for both stress voiding during fabrication and long-term cavitation due to electromigration, a detailed knowledge of the stresses is a pre-requisite for reliability modelling. However, the measurement of the stress in an interconnect, and variations along it, pose a formidable experimental
challenge since high spatial resolution is necessary. Recent developments in X-ray diffraction have enabled the direct determination of the three principal strain components in passivated metal lines [1] but, due to practical limitations, the X-ray method lacks spatial resolution and provides information about the stresses averaged over a large number of nominally identical lines. In this contribution, we introduce a novel sample design, utilizing an in-situ strain sensor, to obtain the stress in an interconnect with a spatial resolution of approximately 2-5 microns. The basis of the method is that an interconnect under stress induces strains in the underlying substrate that are proportional to the interconnect stress. The distribution of strains in the substrate is spatially complex and depends in detail on both the geometrical features and the constitutive mechanical behavior of the interconnect (2]. Nevertheless, the strain in the substrate immediately beneath the center of the interconnect is only weakly dependent of position and is directly proportional to the hydrostatic stress in the interconnect [2]. Thus, by probing the strain in the substrate at the metal/substrate interface, the interconnect stress can be determined. The sample design and experimental configuration used is shown schematically in figure 1. The substrate is a c-axis single-crystal sapphire chosen for its transparency so that the underside of a metal line can be accessed optically. On the top surface of the sapphire substrate, an epitaxial thin ruby film was formed using one of a several 591
Mat. Res. Soc. Symp. Proc. Vol. 356 ©1995 Materials Research Society
Thin Ruby Layer
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Laser Probe Microscope Lens
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where x and (yy are the stresses caused by the interconnect in the interconnect coordinates. In our experiments, the fr
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