Saturday, January 31, 2009

Graphene to replace Silicon in IC fabrication

Silicon has been long used as the base to fabricate Integrated Circuits(ICs). But with the fabrication industry strictly following the Moore's law that packing density of chips doubles every 18 months, it won't be long before Silicon technology saturates. The signs have been seen and thus the fabrication industry is keenly searching an alternative as in near future we won't be able to increase the packing density of circuits based on silicon as it achieves saturation. One of the solution could be to use Graphene instead.

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The requirement of any material to be used as substrate in IC fabrication is to have metallic, semiconductor and insulating properties either in free state and/or in compound state of that very material. The semiconductor nature helps in controlling the action of transistors(the building blocks of chips). The metallic property is needed to have high speed movement of carriers. The insulator quality is needed to isolate the transistors and also differentiate the on and off state resistance of transistors that need to have high variance.

The graphene sheet, rolled so as to form tubes, can act as metallic or semiconducting in nature based on the angle of rolling. The metallic nature of these tubes has long been appreciated by the fabrication industry. They have remarkably high electron mobilty at room temperature, with reported values in excess of 15,000 cm2V−1s−1. Also the mobility of holes and electrons are almost equal. The conductivity remains constant for a high range of temperatures. The semiconducting nature of graphene can be exploited by rolling the sheet at certain angle or by bending the tubes to an angle. This bending of graphene tubes decreases the conductivity at the bends and thus can be made to act as semicontuctor after some calculations. The only problem scientists were facing with graphene as a substitute of silicon was the insulating property of graphene which was very poor.

But , scientists in Manchester University have shown that graphene can be easily modified to act as an insulator by adding hydrogen atoms to its surface. The new material , called graphane, is made by exposing a graphene sheet to ionized hydrogen gas for some two hours. Until now, graphene was used to make transistors with other materials. But this process can't be used in IC fabrication due to the time and cost that comes with such processes. Commercially, this would fail. The need is of one material to be used as the base of the IC which can be then modified to act as conductor, semiconductor or insulator. Now, with this major successful experiment, graphene can be used as a subsitute of silicon. Also, this technology being new, would last for long time in the fabrication industry before it achieves its saturation. That means, it would be 10-20 years at least before we would start hunting for new material to substitute graphene for IC fabrication.

Thus, I see graphene substituting silicon in the ICs in the near future so as to fulfil the rising demand of higher speed of the market and to follow the Moore's law of doubling of packing density every 18 months.

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