High accuracy dual side overlay with wet anisotropic etching for HAR MEMS.

H.W. van Zeijl, K. Best, P.M. Sarro
Delft University of Technology, NL

Keywords: dual side lithography, through wafer etch, overlay


Through wafer etching of silicon using anisotropic wet chemical etch ants like KOH is a key process in the fabrication of many types of MEMS devices. In this work, a dual side 2 mask process is used in combination with an advanced front to back side alignment system to characterize the precision of KOH trough wafer etch processes by means of electrical measurements. It was found that the shift in the front- to backwafer overlay (FTBO) is 2-3 m, determined by the crystallographic tilt of the wafers. However, the 3 sigma value of the FTBO is less than 350 nm. Through wafer deep reactive ion etching (DRIE) is much more accurate but less precise; the measured FTBO shift is less than 200 nm and the 3 FTBO is 50 nm. Wet etching is more precise compared with DRIE and when the crystal orientation of a batch of wafers is precisely known, high accuracy through wafer etch using anisotropic wet chemical etching is made possible in a high volume production environment.