A MANUFACTURING process that could slash the price of blue lasers almost a thousandfold has been developed by Japanese electronics company Sharp. The development should kick-start the manufacture of affordable players for a new generation of high-capacity DVDs.
The new discs will hold five times as much video data as today鈥檚 DVDs. But the players that will be needed to read them will have to use blue laser diodes, which until now have cost around $1000 each because they can only be made in an expensive batch process. But engineers at Sharp鈥檚 UK lab in Oxford revealed last week that they have been able to make blue lasers in a process similar to the one now used to make red lasers, which should make them far cheaper.
All LED lasers comprise several thin layers of semiconductor material. A central junction converts electricity into light when electrons from an electron-rich area on one side lose energy by falling into vacancies, known as holes, flowing from an electron-depleted area on the other side.
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Red or infrared lasers are manufactured using a technique called molecular beam epitaxy (MBE) in which ions of elements such as gallium and arsenic are fired in precise beams onto a semiconductor substrate in a very high vacuum to produce layers of metal arsenide. MBE allows the thickness of the layers to be very finely controlled, guaranteeing a high yield of working lasers.
But blue lasers have to be made of metal nitrides, rather than arsenides. Until now the only way to make these devices has involved depositing aluminium, indium and gallium on a sapphire substrate, in the presence of ammonia gas to provide nitrogen for the nitrides. This requires a different technique called metal organic chemical vapour deposition (MOCVD) in which the metal atoms are combined with large organic gas molecules and passed over a hot sapphire wafer in a chamber containing ammonia at atmospheric pressure.
The hot wafer breaks up the organic molecules, releasing and depositing the aluminium, indium and gallium nitride on the surface, layer by layer. But the thickness of the layers in the devices produced is not nearly as easy to control as it is with MBE, since they simply condense out of a gas rather than being precisely beamed in. The resulting unevenness means that lasers produced this way are inefficient, and require a much heavier current than red lasers. So they wear out much sooner than CD and DVD lasers, which have a lifetime of about 10,000 hours.
The large amount of ammonia used in the MOCVD process also causes problems, because hydrogen produced when the ammonia breaks down contaminates the lasers and stops them conducting. To prevent this, the finished devices are baked to expel the hydrogen. This leads to thermal cracking, and further reduces their lifetime.
But after 10 years鈥� research, Sharp researchers have found a way to use MBE to make blue lasers. The trick, says Jonathan Heffernan, a member of the team, is to alter the temperature of the sapphire substrate in very precisely defined steps as each layer is deposited. In total 10 different temperature steps are needed, between 650 掳C and 1000 掳C, each one maintained to within 10 掳C. Getting the temperature slightly wrong in just one of these steps stops the laser working.
But the stepped MBE process needs only 1 per cent of the amount of ammonia required for MOCVD, which means there is no need to bake the material, avoiding the risk of cracking the layers. So if the temperature is properly controlled, the yield is high 鈥� and the lasers are cheap.
The new process should drastically cut the cost of blue lasers. The infrared laser diodes in CD players now cost just under $1, while the red optical lasers for DVD players sell for just under $2. An affordable blue laser will open the door to machines that will play DVDs with a capacity of more than 20 gigabytes, compared with the 4.7 gigabytes of today鈥檚 discs.
Two industry groups are developing rival standards for these new-generation DVDs. Dutch company Philips, a member of one of the groups, has welcomed the new technique. 鈥淭his breakthrough will enable efficient mass production of blue-laser diodes and paves the way for our next-generation optical discs,鈥� says a spokesman.
