The SiN membranes were deposited by using high temperature LPCVD system. The SiN films deposited over 1000°C showed the suitable properties for X-ray mask, such as well-controlled tensile stress of 5 × 107 Pa, high optical transmittance over 95% and low impurity concentrations. The high optical transmittance of the SiN films deposited over 1000°C was related to the high N/Si. The X-ray masks fabricated by using the SiN membranes deposited at 1000°C showed the high optical transmittance of about 92% and X-ray durability. The pattern position displacement induced by SR irradiation was simulated using FEM calculation with qualitative agreement.