In etching of mono silicon, why does the etching process around the corners differ from the etching process around the flat edges? What brings about this difference and is there some way that MEMS engineers could quantify the process around the corners?
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$\begingroup$ Are you asking about anisotropic wet etching or about reactive ion etching? Also, by "around the corners", do you mean perpendicular to the face of the wafer? $\endgroup$– Michael DM DrydenCommented Dec 12, 2014 at 15:12
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$\begingroup$ anisotropic wet etching and yes perpendicular to the face of the wafer, for example in a <100> wafer, the selectivity of the etching rates across different faces makes it etch along a <111> face, but around a square mask, something different is supposed to happen according to my professor. $\endgroup$– Ghosal_CCommented Dec 12, 2014 at 15:22
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$\begingroup$ Related to your previous question: chemistry.stackexchange.com/questions/19618/… $\endgroup$– Geoff HutchisonCommented Dec 12, 2014 at 20:16
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1 Answer
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What happens is that convex (outside) corners get severely undercut in wet etching. On a <100> cut wafer, the edges leading up to the corner reveal <111> planes which are resistant to etching, but right at the corner, where the two planes intersect, etching reveals a <110> plane, which is etched much more quickly. The difference in etch rates has to do with the number of "dangling" bonds—faces with fewer Si-Si bonds at the surface etch faster. The reason why concave corners don't undercut, despite having the same <111> plane intersection is that there are no dangling bonds in an inside corner so it doesn't etch any faster than the <111> planes.
It should also be noted that adding a small amount of surfactant to the etchant can dramatically decrease undercutting by adsorption to the corners.
This (fortunately open-access) article explains the phenomenon quite nicely.
answered Dec 12, 2014 at 17:54