Plastic fantastic

While sputtered hard disk drives are at the heart of most computers, a variant of the technology is being developed to coat large areas of thin plastic. Large pull-out screens for mobile communications devices - long predicted, including by publications such as this - should at last become reality, as should large area photovoltaic cells capable of competing on cost grounds with conventionally generated electric power. The crucial technology is the remote plasma generation machines pioneered by Plasma Quest in Hook, Hampshire. These use coils excited at 13.5MHz to generate high-density argon plasmas in chambers separate from work areas, and then use magnetic fields to direct plasmas at targets that sputter coatings on to substrates. Their big advantage over conventional sputtering machines, which have permanent magnets beneath the target to enhance ionisation, is that they use the entire target area fairly evenly, instead of eroding a ring-shaped ‘race track’. They also avoid problems associated with target poisoning in localised areas when undertaking reactive sputtering, where the target material reacts with nitrogen or oxygen in the plasma to deposit nitrides and oxides. The man focus of Plasma Quest founder Professor Michael Thwaites, who calls his patented process HiTUS (High Target Utilisation Sputtering), is contract research and selling advanced machines to academia. His customer list includes the universities of Manchester, Sheffield, Loughborough and York, as well as institutions in Singapore and China. His most recent breakthrough is his ability to be able to sputter high quality films with zero internal stress on to fairly large areas of thin plastic. Thwaites can lay down indium tin oxide on to PEN or PET for flexible displays, achieving coatings with resistivities of 20 ohms per square. “You have to be able to do this at low temperatures,” he points out. He can also deposit “gold on PET, which sticks to it, for flat panel displays or flexible interconnects”. So far, the deposits he has been putting down on to plastic are single layers, but one of his company’s capabilities is being able to handle up to six different materials by having separate targets on a carousel. The power inputs and fields can be changed for each material. But, as he points out, “people want large areas”, and this has led to the development of machines in which, instead of magnetically steering the beam on to a flat target surface, it is made to surround a cylindrical target bar. The side arm plasma source is itself annular in cross section, while the target neither intersects nor blocks the plasma generation region, so that the plasma is continuous along and around the length of the target. The target sputters material uniformly with a 360 degree axial coverage, making it suitable for use in web coating or other continuous feed applications. Along one side of the company’s workshop is a coating unit that is in the process of being converted into an in-line linear coating system, equipped with five of these bar target sputtering units. This could be used to coat trays of wafers passing beneath. However, Thwaites has received a DTI Award to produce silicon solar cells on flexible substrates and hopes eventually to use the linear system machine to carry this out. If successful, it will transform the production of silicon photovoltaic cells from expensive devices dependent on wafers of crystalline silicon into something that comes on a roll and can be spread on a roof by semi-skilled technicians. Although efficiencies are unlikely to be as high as from crystalline devices, the cost per watt will be greatly reduced and should compete with electricity from conventional sources. Applied to displays, it could reduce them to sheets of plastic, with a chip in one corner. The newspapers in the Harry Potter films, with their animated graphics, could be available for ‘muggle’ purchasers within the next few years. Pointers * Systems utilise targets fully, and can put down films of metal, oxides and nitrides at high rates, stress free, and very evenly * It is possible to lay down sputtered coatings on thin layers of plastic for flexible displays and low-cost solar photovoltaic cells * Developments are underway to undertake such processes on continuous webs of material