Abstract: During the formation of a spherical cavity in a substrate, self-limiting etching behavior of an isotropic etchant can be utilized when a tape is used as an etch mask. Such a 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 this silicon surface. Because of that, the spherical cavity size depends mostly on the size of the etch-mask opening, and is independent of the etching time. This self-limiting etching behavior precisely controls the dimension and uniformity of the spherical cavity.