Laser annealing

A UK company claims it has developed a more cost-effective technique for manufacturing flat screen displays that use less energy than conventional screens.

of Crawley, West Sussex has developed a powerful environmentally-friendly laser for producing active matrix organic light-emitting diode (AMOLED) screens.

These screens match the picture quality of their plasma equivalents but use much less energy. Until now their manufacturing technique has made them too expensive for most consumers.

Mike Mason, vice-president of technology at Powerlase, said the difference between an organic light- emitting diode (OLED) display and an AMOLED display is that each pixel in an AMOLED screen uses a number of small transistors — located next to each pixel within the display — to turn it on and off.

'To produce a good performance display with fast response time, the transistors must be made with high-quality silicon crystals,' said Mason.

Normally, silicon is coated on to the screen in a non-crystalline layer during the manufacturing process. To turn this into crystalline silicon it needs to be treated with a laser annealing process, which heats up the silicon then allows it to slowly cool to grow silicon crystals.

Mason said the company's new laser, the Starlase 400G, uses a solid-state crystal to produce laser light, designed to significantly reduce the cost of the annealing process.

'Previous processes relied on an excimer laser, which uses a chemical process involving chlorine and fluorine to produce laser light,' he said. 'These chemicals are environmentally-unfriendly and have to be replaced relatively frequently to keep the laser running. They are also harmful to the inside of the laser, so some components have to be replaced on a regular basis.' The maintenance schedule for the Starlase 400G is much less frequent than for excimer lasers.

It produces 400W of laser light at a wavelength of 532nm. 'The laser is pulsed and can produce double pulses,' said Mason.

This is an advantage, he said, because the annealing process involves rapid heating then controlled cooling of the non-crystalline silicon to grow long crystals. The cooling stage can be more controlled using a double rather than a single pulse. The length of the silicon crystals can therefore be controlled and the display performance is improved.

The laser is based on a neodymium doped rod pumped by several high-power laser diode bars. The rod and diode bars are housed in a water-cooled module and form the heart of the laser cavity. The cavity is modulated with an acousto-optic device to produce short pulses at high-repetition frequencies.  

'The laser can produce pulse durations of between 75 and 230 nanoseconds at pulse repetition frequencies between 10kHz and 20kHz,' said Mason.

It also contains a 'frequency doubling crystal' to convert the fundamental infrared wavelength of the material light to green, which interacts with silicon more effectively. 

'The interaction of materials with laser beams is always wavelength-dependent,' said Mason. 'it just so happens that silicon is more sensitive to green than infrared for this application.'

The overall goal of Powerlase is to make the production and the eventual end price of AMOLED screens cheaper. All indications, said Mason, show that this is possible.

Powerlase has estimated that the annual running costs for excimer laser annealing equipment is about £300,000. This equates to a process cost of approximately £5 a glass sheet.

Mason claimed the company's green laser annealing is predicted to be considerably cheaper than excimer laser annealing techniques.