Reliability considerations in scaled SONOS nonvolatile memory devices

Abstract

The memory characteristics and reliability of a SONOS nonvolatile memory device are determined, to a large extent, by the charge transport and trapping properties of its storage nitride film. In this paper we investigate carrier trapping phenomenon in a scaled SONOS transistor with an effective gate oxide thickness of 94 and a nitride storage layer deposited with LPCVD at 680°C. Atomic Force Microscopy (AFM) has been employed to examine the surface quality of deposited nitride films. We have found that nitride film deposited at 680°C shows the lowest surface roughness, as compared to the films formed at 725C and 650°C. With a charge separation technique, we have demonstrated that the charge centroid is deep into the nitride layer at low injection levels and moves towards the center of the film as the injection level is increased. From our measurement, a trap density of 1.7×10 19 cm −3 in the nitride has been determined. Devices with nitride (2.1 Å RMS roughness) deposited at 680°C show less degradation in data retention with respect to erase/write cycling when compared to devices with nitride (2.9 Å RMS roughness) deposited at 725°C.