Part 2: "Epi shift"
“Epi shift” is the term given to the X, Y translational displacement of alignment targets and device patterns, which is a function of epitaxy thickness. This displacement is often quite large, roughly 0.4um to 0.9um for every 1um of epitaxy grown.
This phenomenon is most prevalent in the <111> orientation crystal plane and is well described in this article in Solid-State-Technology magazine
In this particular case, what looked like an overlay problem, turned into an epitaxy reactor characterization study since epitaxy thickness variation induced a variation in translation standard deviation due to the relationship of dx to dz. Of course, to measure the mean value of epi shift you would need to uncover a target at the substrate/epitaxy interface, buried beneath the thick epitaxy. To recover such a target has proved to be very difficult, so SEM cross sections are typically used to determine this value. Next month, we will discuss a novel solution that can be used to quantify both the mean “epi shift” and the epitaxy thickness variation.
For more discussion about “epi shift “and metrology solutions, please
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