Study of Self-Limiting Etching Behavior in Wet Isotropic Etching of Silicon

This paper reports a self-limiting etching behavior of an isotropic silicon etchant (consisted of hydrofluoric, nitric and acetic acids) during the formation of a spherical cavity in a silicon wafer. We observe that the etch rate drops drastically when the etch-front diameter is approximately equal to 1.8 times the etch window size plus 400 µm. Such self-limiting behavior is due to the presence of gas bubbles (consisted of SiF4 and NO, etch by-products) which close the etch window and limit the mass transport of the etchant to the silicon surface. Because of that, the spherical cavity size has been observed to depend mostly on the size of the etch-mask opening, and to be independent of the etching time. A variety of mask materials and etching conditions have been investigated to obtain a large spherical cavity (2.5 mm in radius) with good circularity and smoothness for microelectromechanical systems (MEMS) applications.