SONOSRAM- Silicon-Oxide-Nitride-Oxide-Silicon RAM

SONOS, short for "Silicon-Oxide-Nitride-Oxide-Silicon", is a type of non-volatile computer memory closely related to Flash RAM. It distinguished from mainstream Flash by the use of silicon nitride (Si3N4) instead of polysilicon for the charge storage material. A further variant is "SHINOS" ("Silicon Hi-k Nitride Oxide Silicon"). SONOS promises lower programming voltages and higher program/erase cycle endurance than polysilicon-based Flash, and is an area of active research and development effort. Companies offering SONOS-based products include Chartered Semiconductor Manufacturing, Cypress Semiconductor, Macronix, Toshiba, and United Microelectronics Corporation.

Description:

A SONOS memory cell is formed from a standard polysilicon NMOS transistor with the addition of a small sliver of silicon nitride inserted inside the transistor's gate oxide. The sliver of nitride is non-conductive but contains a large number of charge trapping sites able to hold an electrostatic charge. The nitride layer is electrically isolated from the surrounding transistor, although charges stored on the nitride directly affect the conductivity of the underlying transistor channel. The oxide/nitride sandwich typically consists of a 2 nm thick oxide lower layer, a 5 nm thick silicon nitride middle layer, and a 5—10 nm oxide upper layer.

When the polysilicon control gate is biased positively, electrons from the transistor source and drain regions tunnel through the oxide layer and get trapped in the silicon nitride. This results in an energy barrier between the drain and the source, raising the threshold voltage V_t (the gate-source voltage necessary for current to flow through the transistor). The electrons can be removed again by applying a negative bias on the control gate.

A SONOS memory array is constructed by fabricating a grid of SONOS transistors which are connected by horizontal and vertical control lines (wordlines and bitlines) to peripheral circuitry such as address decoders and sense amplifiers. After storing or erasing the cell, the controller can measure the state of the cell by passing a small voltage across the source-drain nodes; if current flows the cell must be in the "no trapped electrons" state, which is considered a logical "0". If no current is seen the cell is in the "1" state. The needed voltages are normally about 2 V for the erased state, and around 4.5 V for the programmed state.