In this paper, we present a novel process for the fabrication of thin diaphragms in lithium niobate (LiNbO3) wafers for various sensor applications. The process uses a thermal inversion layer as an etch stop. Thermal inversion is a process which forms a layer with a poling direction opposite to that of the bulk crystal. The thermal inversion layer can be used as an etch stop because the etch rate on the +Z face in hydrofluoric acid is much slower than that on the -Z face. The thickness of the thermal inversion layer determines the thickness of the final diaphragm; diaphragms with thicknesses of 6 and 30 μm have been fabricated in Z-cut and 128°-Y-cut wafers, respectively.