MICROCHIPS made of synthesised diamond, instead of silicon, have come a step closer, opening up the possibility of building rugged chips that will work in environments that would fry silicon electronics.
Space scientists have long had their eye on diamond as a replacement for silicon in circuits that have to operate at high temperatures and in high solar radiation environments – such as chips in communications satellites. Synthetic diamond shows promise because its melting point and thermal conductivity allow it to be used in temperatures far higher than silicon – and it has 30 times the resilience to the kind of chip-destroying electrostatic discharges by a spacecraft moving through an electric field.
“Because of its melting point and thermal conductivity synthetic diamonds can be used in places where silicon chips would fry”
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But until now, engineers have been stymied by the contamination of synthetic diamond by hydrogen. Now Patrick Reichart, a physicist at the University of Melbourne, Australia, and colleagues from Germany have found a way to overcome this (Science, vol 306, p 1537).
Hydrogen impurities wreck synthetic diamond’s electrical performance by reducing the “mobility” of electrons, severely limiting how fast a chip’s transistors can switch on and off.
The hydrogen comes from the way synthetic diamonds are made in a process called chemical vapour deposition (CVD). In this, a stream of hydrogen and methane gas is used to deposit carbon on a substrate under very high temperatures and pressure – in a process designed to mimic the forces that make natural diamonds. But the result is not a continuous crystal, like gem diamonds, but a “polycrystalline” layer made up of diamond nanocrystals containing some of the carrier hydrogen.
Until now, no one knew how much hydrogen ended up in the synthetic lattice, or where it was. This makes it impossible to predict a particular synthetic diamond chip’s electronic properties. But by making clever use of a particle accelerator, Reichart’s team has revealed the location and extent of hydrogen contamination. Knowing where the hydrogen is, chip designers now design around the problems, Reichart says.
His trick was to use a proton accelerator to perform some elegant “nuclear billiards”: when protons hit hydrogen nuclei, more protons, the protons shoot out at a characteristic 90-degree angle.
Their set-up used a high-energy beam focused to a width of less than a micrometre – narrow enough to home in on the fissures between diamond nanocrystals. Protons are deflected by both carbon and hydrogen atoms, but only collisions with another hydrogen nucleus cause the incident proton and the hydrogen nucleus to fly off at a 90-degree angle to each other. It’s called proton-proton elastic scattering. Detectors positioned to capture these 90-degree protons allow the amount and location of hydrogen in the sample to be worked out.
The team found that the hydrogen nestles in the fissures between diamond nanocrystals, rather than in the bulk of the crystal. The hydrogen ions are thought to bond to free electrons on the edges of the crystals. Knowing this, physicists will be able to work out how best to “dope” diamond with positive and negative impurities to make chips that perform far better in extreme environments than any other devices to date.
