Diamond devices are forever, almost

4 min read

Tom Shelley monitors progress in the commercial development of micro-fabricated devices based on the ultimate material

Extreme hardness, thermal conductivity five times that of copper, chemical resistance, high temperature performance and biocompatibility make diamond the ultimate material for many uses. And applications abound in power electronics, biotechnology, aerospace and medicine. The main drawback is cost but polycrystalline synthetic diamond (PCD) is much cheaper than single crystals or the natural variety, and it can be used to make high power micro relays, novel sensors suitable for the human body and cutting tools with a tip radius of 3nm. A team at the University of Ulm in Germany has devoted itself to the development of PCD-based commercial products and has a long list of working prototypes. Its one commercial product, the ‘Diamaze’ range of surgical scalpels (left), has spun off into production through collaboration with Daimler Chrysler. This particular breakthrough is finding great favour with surgeons because the Diamaze scalpels have the virtue of being exceptionally sharp and, unlike metal scalpels, they do not lose their sharpness in use. The manufacturing technique is to grow the diamond film on silicon to a thickness of 30 to 35µm and then to use plasma polishing to produce a very fine tip radius. Like all the devices being developed at Ulm, it is manufactured using many of the ideas and techniques developed and proven in conventional semiconductor manufacture. Incidentally, the narrowest blade made so far is 0.12mm wide and is listed in the Guinness Book of World Records. Another medical application, according to Aleksandar Aleksov, a PhD student in the university’s Department of Electron Devices and Circuits, is to use diamond for spot heater arrays in catheters used for non-invasive heart surgery instead of the usual metal bipolar electrodes. Diamond doped with up to 1% boron conducts electricity quite well, and can be driven fairly hard per unit volume as a heating element. This makes it eminently suitable for use in micro ejectors which may be used to remove unwanted deposits. It also shows a fairly high variation of resistance with temperature, making it suitable for use as a temperature sensor – essential for precise control of such devices – and, unlike most materials, it is highly biocompatible. At the recent Hanover Fair, Aleksov demonstrated to Eureka a 0.5 x 0.5mm diamond heater that, he says, dissipated 400W just beneath the surface of a liquid. In this way, he claims it is possible to generate a fluid pressure of several MPa, sufficient for surface treatments. Similarly, diamond heaters and nozzle plates could be used in a bubblejet print head, making it acid cleanable and exceptionally long lived (opposite top). One spin-off vision is of high density chemical reactor arrays, with perhaps 1,000 x 1,000 chambers, capable of manufacturing long DNA sequences. Diamond is the ideal material because it will withstand corrosive chemicals and cavitation. The combination of strength, high temperature performance, electrical conductivity and very high thermal conductivity also makes diamond the ideal material for truly microscopic power relays. These employ diamond cantilevers with the cantilever arm moved downwards by electrostatic force between the arm and a pad beneath most of its length. Current passes when the cantilever makes contact with an additional pad beneath its far end. One intended application is for transmit/receive switches in phased array radar systems, where beams are moved around by directing signals at different times to fixed antenna elements, instead of mechanically moving a large dish. This technology employs two power transistors for each antenna element. The proposed mechanical relay, on the other hand, can be made with a very low capacitance, ensuring better isolation in the ‘off’ condition, while ‘on’ resistance is down to 200mohm. Power handling is up to 50MW/cm2 before diamond starts to sublimate. And because diamond sublimates when it gets hot, as opposed to bending or melting, there is no danger of overloaded relays sticking or becoming plastically deformed. Phased arrays have been likened to insect eyes. Current interests focus on very precisely directed systems for air traffic control; stealthier radars in military systems without stray emissions that could be picked up by enemies; and low-cost versions for anti collision systems in cars. Diamond is better In the experimental devices being made at Ulm, the cantilevers are 400µm to 1.5mm long, and 200 to 500µm across. Switching time from ‘off’ to ‘on’ is typically around 175µs. Eureka’s October 1998 issue revealed that large arrays of micron-sized mechanical relays etched out of gold had been researched and patented as data storage devices at the Cavendish Laboratory in Cambridge. Diamond devices should be able to perform the same function, but under more adverse environmental conditions. This is certainly the case with experimental micromachined diamond accelerometers analogous to those made in silicon. Doped diamond free-standing beams, like those made of silicon, change their resistances in response to deflection. However, the Ulm team has fabricated a diamond bridge accelerometer that has been tested at Bosch. Simulations show that the configuration used is able to withstand 7,000g. An industrial partner is now interested in developing pressure sensors incorporating diamond as opposed to silicon beams for service at 450 to 600°C. The Ulm group has also developed diamond-based Schottky diodes for ultraviolet sensing. Diamond can be used as a basis for diode and transistor-based electronics exactly analogous to those based on silicon but with a higher temperature capability and a potentially higher performance. Tetrahedral Amorphous Carbon (a kind of diamond)-based power transistors developed at the Engineering Department in Cambridge were described in Eureka’s August 1993 issue but have not been taken further. Aleksov speaks admiringly of the Cambridge work but says: “To make diamond transistors is easy, but device design is hard because physics is not yet properly understood.” The latest approach, apparently, is to base circuitry on single crystalline diamond wafers grown on Iridium substrates, a technique demonstrated at the University of Augsburg, because it has the same lattice size. And standard 4 x 4 x 1mm single crystal diamonds made by a high temperature, high pressure route are commercially available from Sumitomo. (More information: Diamaze, Aleksandar Alexov and R Mueller)