Measuring thick film stresses
One of the most common systems to measure stress in thin films is using the Flexus from Toho. In most cases, film thickness doesn’t exceed 1/100 of the substrate thickness. Sometimes, it seems to be desirable to measure the stress in much thicker films. Calculations for the stress measurements of thin films can be modified for applications with thicker films. To do this, correction factors are used which are based on the film/substrate thickness ratio.
Fig. 1: Stress distribution
One example of a thick film stress measurement is the stress in a silicon die attached to a package or a lead frame. The “film” is the silicon die itself and the “substrate” is the package or the lead frame. The film/substrate ratio in this case can be as high as 1/2. The stress distribution in the thick film and the substrate is schematically shown in Fig. 1. A film in compression is shown on a substrate (σ < 0) with an average compression in the film of σ0. The stress in the substrate ranges from a maximum tension of σm adjacent to the film, to a maximum compression on the backside.
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