Owing to their high optical and mechanical quality suspended silicon nitride thin films are widely used for photonics and sensing applications. We discuss the fabrication and characterization of highly reflective one- dimensional subwavelength gratings patterned on commercial high-tensile stress Si3N4 membranes. Their non- invasive structural characterization using Atomic Force Microscopy provides detailed information on both the grating transverse profile and the deflection of the films after etching, which are compared with optical measurements and mechanical simulations, respectively. We then apply these ultrathin, low-loss optical components to optical spatial differentiation and demonstrate high quality first- and second-order spatial differentiation of the transverse profile of a Gaussian beam.