The bulge test provides a convenient way to measure the mechanical properties, including fracture strength, of thin ceramic membranes. Thin (≈ 3000Å) films of non-stoichiometric SiNx, nominally defect-free and containing flaws introduced byfocused ion-beam machining, were examined. The use of such membranes in masks for projection electron-beam lithography systems makes it important to determine their ultimate strength and reliability. The mean fracture strength of the defect-free membranes was found to be 1.79±0.04GPa, and the Weibull modulus was estimated to be 49±15, giving a safe operating stress of 1.5GPa. The results of fracturing the samples containing flaws indicated that holes smaller than 15x0.5μm had no effect on the fracture strength of the membranes. By estimating the stress concentration factor for the slots, it was determined that the critical fracture stress was 16±3GPa.