Non-Volatile Memory from Carbon

So far, many problems have inhibited development of carbon based memory devices. Not any more, as IBM and the EMPA have solved those problems and come up with the possible use of oxygenated amorphous carbon for non-volatile memory applications. The new non-volatile memory is based on a Redox reaction that takes place in thin films of oxygenated amorphous carbon known as a-COx. The film is a process of PVD or Physical Vapor Deposition.

EMPA, the Swiss Electron Microscopy Center and IBM, Zurich, have published the details of their research. The latest release about their work discusses the results of device measurements. IBM is now a holder of a patent in this area.

Earlier research in this field has shown carbon and carbon nanotubes to possess some potential for NV memory application. However, development in the direction of products did not proceed because of lack of reproducibility, processing difficulties and limited write/erase endurance.

Amorphous carbon, because of its high electrical resistance, has not been receiving much attention. People have been studying the electrical properties of other allotropes of carbon. They have been focusing on carbon-based electronics as a challenge to silicon or as its follow-on.

However, the high electrical resistance of amorphous carbon is of immense importance as far as memory applications are concerned. The latest research on the use of oxygenated amorphous carbon for NV memory application has the added advantage of being able to use the conventional silicon-compatible process of thin-film deposition.

Manufacturers fabricate memory devices on a 500nm thick thermal film of silicon dioxide, which forms on a substrate of silicon wafer. A tungsten film forms the bottom electrode and it has circular pores delineating its active contact area. The pores are etched in the 35nm thick silicon dioxide film overlaying the tungsten and the pore diameters range from 100nm up to 4µm.

In the next step, manufacturers use a graphite carbon target in oxygen for physical vapor deposition of the a-COx active material into the pores, which then makes contact with the bottom electrode. A platinum top electrode metal deposition finally completes this planar sandwich construction. However, before the deposition of the COx, any native oxide is removed from the surface of the tungsten electrode by sputter cleaning. This is an important step, as it ensures non-contamination and non-compromise of the part of the Redox action involving the tungsten and Cox interface.

The next stage necessary is the forming step for bringing the memory device to its normal operating state. For this, a triangular shaped pulse of positive polarity is applied to the bottom electrode. As the applied voltage nears the forming voltage Vf of around 4-5V, a function of the thickness, there is an abrupt increase in the current flow through the cell. This switches the cell from its virgin state to an LRS or low-resistance state, which is also called its SET state. A sequence of 1µs-wide triangular pulses may also be used for forming the a-COx cells.

The device can be brought back to its HRS or high-resistance state or RESET state by applying a 10ns pulse of negative polarity to the bottom electrode. This does not require the use of the built-in current limiting resistor.