Maskless etching of three-dimensional silicon structures in KOH

A novel micromechanical technique using maskless etching of three-dimensional anisotropically etched silicon structures is investigated. The structures investigated are convex prismatic edges included by {100} and {111} planes and the convex corners of a rectangular mesa, both formed by a previous masked anisotropic etching in KOH. Experimental results verify that the cutting planes developed at the mesa edges are {311} planes that make the contour change obviously. Analytical relations have been found to predict the evolution of the contour and the undercutting rate at the convex edges and corners based on the fact that the cutting planes at the convex edge are {311} planes while the cutting planes at the convex corner are {411}. The relations have been confirmed by experiments and the etching rate of {311} planes is found for various KOH concentrations. As an example of application, a symmetric cantilever beam-mass structure with beams horizontally located at the central plane of the wafer has been fabricated for a differential capacitive accelerometer with minimized cross-axis sensitivity.