A fabrication method for large-area, high-stress LPCVD \mathrm{Si_3N_4} membranes is presented. These devices can be used as mechanical resonators with very low dissipation, exploiting dissipation dilution. A phononic crystal pattern allows to work with a high-order localized mode, shielded from acoustic radiation in the substrate. The procedure is amenable to most research clean rooms, requiring conventional lithography techniques and wet etching in KOH for device undercut.
