SONOS
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| Non-Volatile |
SONOS, short for Semiconductor-Oxide-Nitride-Oxide-Semiconductor, is a type of high-performance non-volatile memory, or NVRAM. It is similar to the widely used Flash RAM, but offers lower power usage and a somewhat longer lifetime. SONOS is being developed as one of a number of potential Flash replacements, and is currently used in Cypress Semiconductor's PSoC line of products.
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[edit] History
SONOS was first invented in the 1960s, but the semiconductor fabrication methodologies needed to develop it practically have not existed until recently.
[edit] Description
Generally SONOS is very similar to Flash, but is, in theory at least, much easier to produce. Flash requires the construction of a very high-performance insulating barrier on the base 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.
One of the advantages of SONOS is that it can be built at a smaller scale than Flash, which has a definite "lower limit" to the size of the insulator layer, around 7 to 12 nm, one that means it will be difficult for Flash devices to scale smaller than about 45 nm linewidths.
SONOS "cells" consist of a standard NMOS transistor with an additional layering of insulators on the gate (the transistor's "switch"). The layering consists of an oxide layer approximately 2 nm thick, a silicon nitride layer about 5 nm, and a second oxide layer with a thickness between 5 and 10 nm.
[edit] Operation
When the gate is biased positively, electrons from the emitter-collector circuit "above" the layer 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 Vt (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 gate.
[edit] Construction
A memory is constructed by fabricating a grid of SONOS transistors along with a small amount of control circuitry. After storing or erasing the cell, the controller can measure the state of the cell by passing a small voltage across the emitter-collector pair; if current flows the cell must be in the "no trapped electrons" state, which is considered to mean "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.
[edit] Advantages
Additionally, the voltage needed to bias the gate during writing is much smaller than in Flash. In order to write Flash, power is first built up in a separate device known as a charge pump, which multiplies the input voltage to between 9 V to 20 V. This process takes some time, meaning that writing to a Flash cell is much slower than reading, often between 100 and 1000 times slower. The pulse of high power also degrades the cells slightly, meaning that Flash devices can only be written to between 10,000 and 100,000 times, depending on the type. SONOS devices require much lower write power, typically 5 to 8 V, and does not degrade in the same way. SONOS does suffer from an unrelated problem, however, where electrons become strongly trapped in the ONO later and cannot be removed again. Over long usage this can eventually lead to enough trapped electrons to permanently set the cell to the "1" state, similar to the problems in Flash. However, in SONOS this requires on the order of a 100,000,000 write cycles, 1000 to 10,000 times better than Flash.
[edit] Research
Philips is one of the groups working on SONOS devices[1], and have produced small 26-bit demonstrators with excellent lifetimes at a 120 nm linewidth. It is not clear if this research is ongoing, however, given the rapid advances in Flash technology that have led to very large gains in areal density. Other groups are also working on SONOS for more specialized tasks, notably military and space systems due to its excellent radiation hardness[2].
