Silicon oxide memory


Generally SONOS is very similar to standard double polysilicon flash, but hypothetically offers higher quality storage.Flash requires the construction of a very high-performance insulating barrier on the gate leads of its transistors, often requiring as many as nine different steps, whereas the oxide layering in SONOS can be more easily produced on existing lines and more easily combined with CMOS logic.Additionally, traditional flash is less tolerant of oxide defects because a single shorting defect will discharge the entire polysilicon floating gate.This results in an energy barrier between the drain and the source, raising the threshold voltage V (the gate-source voltage necessary for current to flow through the transistor).If no current is seen the cell must be in the "trapped electrons" state, which is considered as "0" state.A SONOS memory cell is formed from a standard polysilicon N-channel MOSFET transistor with the addition of a small sliver of silicon nitride inserted inside the transistor's gate oxide.This is due to the smooth homogeneity of the Si film compared with polycrystalline film which has tiny irregularities.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.A further variant is "SHINOS" ("Silicon Hi-k Nitride Oxide Silicon").The needed voltages are normally about 2 V for the erased state, and around 4.5 V for the programmed state.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.Companies offering SONOS-based products include Global Foundries, Cypress Semiconductor, Macronix, Toshiba, United Microelectronics Corporation and Floadia .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.

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.The electrons can be removed again by applying a negative bias on the control gate.The sliver of nitride is non-conductive but contains a large number of charge trapping sites able to hold an electrostatic charge.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 "1".SONOS, short for "Silicon-Oxide-Nitride-Oxide-Silicon", is a type of non-volatile computer memory closely related to Flash RAM. It is distinguished from mainstream flash by the use of silicon nitride (Si) instead of polysilicon for the charge storage material.