This paper describes a fabrication method of a circular diaphragm using boron etching stop method. It will be applied to acoustic transducers such as microphones or microspeakers and so on. The sensitivity is expected to be increased with the circular diaphragm through the simulation results to compare with a general rectangular diaphragm. The borondoped layer which is doped with solid source is sufficient for achieving an etching stop in 20 wt% TMAH, and the thickness is about $7.4{mu}m$. The diameter of the circular silicon nitride diaphragm was measured to be 2 mm with $1{mu}m$ thickness. The fabrication of piezoelectric acoustic devices was completed.