A simple and robust model to explain convex corner undercutting in wet bulk micromachining

Abstract

In this paper, a simple and robust model is presented to explain the main reason behind undercutting at convex corners and no-undercutting at concave corners. The etch rate of the tangent plane at convex corner and the role of dangling bond in etching process are utilized to explain the undercutting at convex corner and the no-undercutting at concave corner, respectively. The present model shows that {110} is the tangent plane at convex corner which exhibits higher etch rate than the neighboring {111} plane in all types of anisotropic etchants; consequently the undercutting occurs at convex corners. The absence of dangling bonds at concave corner prevents the undercutting there. Moreover, the same model explains the reason of very less undercutting when the etching is carried out in surfactant-added tetramethylammonium hydroxide (TMAH).