Flash memory won't last for ever. Vendors are finding it difficult to shrink the size of a flash memory cell, and it's performance in reading and writing data could be improved as well. The big advantage that flash has over other memory technologies, such as RAM, is that it is non-volatile; its contents remain stable when power is switched off. Any replacement semi-conductor technology has to have this characteristic.

Reducing flash size is a need typical of semi-conductor technologies. It means more capacity can be crammed into less space; the power need per unit component is lowered and the speed of the device is increased. In the ten years from 1990 to 2000 an average flash cell size decreased by a factor of thirty. Flash manufacturers think that it will be 2010 before current flash technology loses the ability to shrink cell size significantly further.

The post-flash era is generally supposed to commence in the second decade of this century. STMicroelectronics thinks that phase-change memory, or PCM, could be the post-flash technology. It was developed by Ovonyx in California, who licensed it from its original developers Energy Conversion Devices, Inc. and is already used in re-writable CDs, DVD-RAM and DVD+RW optical disk products.

The Ovonyx web site states: "The company's nonvolatile memory technology, Ovonyx OUM, offers significantly faster write and erase speeds and higher cycling endurance than conventional Flash memory. OUM also has the advantage of a simple fabrication process that permits the design of semiconductor chips with embedded nonvolatile memory using only a few additional mask steps."

The cycling endurance could be a key factor if flash memory-equipped devices have a long life. After a certain number or write cycles flash memory wears out. A longer cycle endurance will be valuable.

Ovonyx has a technical paper in PDF format about its technology. Its introduction states that it 'uses a reversible structural phase-change - from the amorphous phase to a crystalline phase - in a thin-film chalcogenide alloy material as the data storage mechanism.' A chalcogen is a bronze alloy with possible additions of oxygen and sulfur, or it may be a compound including any of the elements oxygen, sulphur, selenium, and tellurium.

It exists in two states, a crystal one and an amorphous one, and can be switched between the two reliably by the application of heat. Once switched it remains stable in that state until switched again - it is non-volatile. The amorphous state has high electrical resistance; the crystalline state has low resistance, thus providing the detection of binary ones and zeroes by a simple test for resistance level.

An OUM memory cell would include the chalcogen compound material, a tiny electrical heater, and a selection transistor used for read/write operations.

Ovonyx is licensing the OUM, Ovonic Unified Memory, technology and STMicroelectronics is one of its licensees. ST's chalcogen has tellurium as an important ingredient along with selenium and sulphur.

Manufacturing looks feasible as a standard CMOS process can be used with a few additional steps. ST has built an 8Mbit demonstrator chip and test results have been sufficiently good for ST to believe that PCM could become the non-volatile memory technology of choice once flash runs out of steam.

From 2010 onwards we might expect flash memory to be increasingly replaced by newer technology, such as PCM, that enables more compact and capacious memories with faster read and write times. It will make larger and faster devices possible that are less expensive per unit of capacity. Solid state storage will become increasingly important, useful and affordable and occupy a space between computer RAM and hard drives to provide exceptionally fast processing of data currently accessed from disk.

At present only the military and very high value businesses can afford to use solid state drives in this fashion. The promise is that in a little over ten years its use will become more widespread.

(ST is currently the number 3 supplier world-wide of NOR flash memory.)