A 3-bit multilevel cell programming method in nitride memory devices

Nitride Read Only Memory (NROM) device is difficult to achieve eight-level cell operation using the conventional channel hot electron injection programming method due to low reliability. In this work, a high density eight-level cell programming method is proposed in a 90 nm NROM device. This new method first uses a double-side band-to-band tunneling hot hole injection (BBHHI) mechanism for a uniform erasing along the channel. A new initial state with threshold voltage of -1 V is obtained. After that, the multi-level cell operations were performed by applying a low voltage pulse agitated substrate hot electron injection programming and a single-side BBHHI erasing at the drain or source side. This new operating mode provides the double memory window over the conventional multi-level cell programming method. An eight-level 3-bit/cell storage is achieved with acceptable cycling endurance and data retention characteristics.