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

Sajal Sagar Singh,Prem Pal

doi:10.1186/2213-9621-1-1

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).

Highlights

The wafer manufacturing industries commonly produce the silicon wafers with three principle orientations namely {111}, {110} and {100}. Out of these three orientations, {100} silicon wafers are most widely employed for the fabrication of microelectromechanical systems (MEMS) and complementary metal oxide semiconductor (CMOS) devices
In the fabrication of MEMS, alkaline solution (e.g. potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), etc.) based silicon anisotropic etching is frequently used to make a wide range of microstructures in silicon wafers [1,2,3,4,5,6,7,8,9,10]
Why the intersecting {111} planes at the convex corner are vulnerable? The proposed undercut model answers it on the basis of the fact that the tangent plane on the intersecting {111} planes is {110} as shown in the Figure 2 and this plane exhibits high etch rate in pure KOH and TMAH solutions [7,10,11]

Summary

Introduction

The wafer manufacturing industries commonly produce the silicon wafers with three principle orientations namely {111}, {110} and {100}. A simple and robust model is proposed to explain the phenomenon of severe undercutting at the convex corner as well as no-undercutting at the concave corner in wet anisotropic etchants.

Results

Conclusion