Cool light leads to greener chips
June 30, 2006
A technique that could lead to cheap, environmentally-friendly microchips has been developed by UK researchers.
The team from University College London used low temperature, ultraviolet lamps to make silicon dioxide, a vital component of almost all modern chips.
At the moment, chip makers use energy intensive furnaces, heated to more than 1000C, to manufacture the material.
The new technique operates at room temperature and so requires less power and resources.
"This finding means that the industry's energy, and subsequent cost savings, could reduce the prices of electronic devices for consumers and, of course, create a positive environmental impact," said Professor Ian Boyd of UCL, one of the team behind the discovery.
Complex designs
Microchips are composed of complex electrical circuits made of a variety of silicon components, such as transistors.
A transistor is a basic electronic switch. Every chip needs large numbers of them, sometimes hundreds of millions to function. The more there are, the more calculations they can do.
These transistors are made of a combination of a conducting material, to channel the electrical charge through the device, and an insulator that inhibits the flow of electricity.
A common insulator is the oxidised form of silicon, silicon dioxide.
The compound can also be used to channel electrical charge in memory devices.
It is also often used as a "mask" that allow manufacturers to precisely pattern the chips with other elements to change the electrical properties of specific areas.
For example, phosphorous atoms are commonly added to parts of the silicon to increase conductivity.
This process, known as "doping", allows chip makers to change the electrical properties of specific areas of the chip to create precise pathways through which charge can flow.
These form the intricate circuits needed in modern devices.
Chip size
Silicon dioxide forms very slowly at room temperature.
In order to speed the process up chip-makers heat the silicon wafers, from which chips are cut, to between 900C and 1200C in the presence of oxygen.
This consumes a huge amount of energy.
Also, as the wafer is heated, chip components that have already been incorporated can warp and distort its structure.
This is a particular problem as researchers continue to chase Moore's Law, that says the number of transistors on a chip will double every couple of years.
As manufacturers try to squeeze smaller and smaller components on to chips they are packed closer together.
Heating the wafer with these densely packed chips can cause contamination of individual components as they become more fluid and "bleed" into one another.
A low temperature manufacturing process would overcome these problems and allow chip makers to continue to push the boundaries of chip size.
Cool temperature
The new technique uses a lamp that emits light from deep within the UV spectrum at a wavelength of 126 nanometres.
The UV lamp is about 30 cm long and looks like a common fluorescent tube. It is filled with argon gas that has a high voltage applied to it.
The emitted light causes oxygen molecules to break down into separate atoms.
This dissociation creates one atom with a lot of energy and one with much less.
The energetic atoms are the most useful for creating silicon dioxide.
"They're very aggressive, they're very keen to oxidise the silicon," explained Professor Boyd.
"You don't even need to heat the silicon," he said. "It oxidises at room temperature."
Future products
However, the silicon industry requires ultra pure materials to manufacture microchips,
According to Dr Douglas Paul, of the University of Cambridge semiconductor physics group, this may be the technique's biggest stumbling block.
"There have been many people who have shown similar results but all these techniques cannot be used for electronics because the defect densities are far too high," he said.
"By growing thermal oxides at high temperatures in present microelectronics manufacturing processes (from 700 to 1000 degrees C) most of the defects are all annealed out and you end up with an extremely low defect density".
Professor Boyd admits that prolonged exposure to other UV wavelengths produces defects, but points out that his technique uses a wavelength of light that has never been used before.
The next stage, he says, is to try the technique in clean room facilities, similar to those used in chip making plants, to prove the technology works on an industrial scale.
The team say they are already talking to companies about using the technique.
They believe that eventually it could be used not just in chip manufacturing but also to create circuits on other materials including cloth, for smart clothing, paper for electronic books or in plastic electronics.
"It opens the door to a whole array of technologies and possibilities," said Professor Boyd.
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