We have proposed a stoichiometric and stress-reduced silicon nitride/silicon oxide/silicon nitride (N/O/N) triple-layer membrane sieves. The membrane sieves were designed to be ultra flat and thin, mechanically stress-reduced, and stable in their electrical and chemical properties. All insulating materials are deposited stoichiometrically by a low pressure chemical vapor deposition (LPCVD) system. The SiN/SiO/SiN membranes with a thickness of 0.4 mm have pores with diameters of 0.6-2 mm. The device is easily fabricated on a 6' silicon wafer with full compatibility with CMOS processes. To see the separation ability of blood plasma, an agarose gel matrix was attached to the sieves for improving the speed, which can be delayed by clogging. We could separate about 1 mL of plasma from 5 mL of human whole blood. We believes that our device can be applicable for the cell-based biosensors or analysis systems in analytical chemistry and bioseparation system.