A fabrication process has been developed for making free‐standing gratings of silicon nitride. These structures are critical components of an atom interferometer, which uses four gratings as coherent beam splitters of atom waves. The quality of gratings necessary for an interferometer is considerably higher than is needed to demonstrate the diffraction of atoms. In particular, the gratings must be phase coherent over their entire area. This implies that the grating lines must be straight to the order of their linewidth over the full extent of the grating. In addition, the open fraction of the grating structure is more critical for an interferometer. In this paper the development of new fabrication techniques that were used to make free‐standing gratings with periods as small as 100 nm and support structure open fractions as high as 0.8 were reported on. To achieve depth‐to‐linewidth ratios greater than 4:1 with grating periods as small as 100 nm a selective, highly directional reactive ion etching (RIE) process has been developed.