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Copper laser boosts blue diode yield

10 Feb 2003

A copper-vapour laser specialist says its scribing system improves the yield of blue laser diodes from semiconductor wafers.

Production of blue laser diodes looks set for a massive increase with the advent of next-generation DVD equipment, but one problem with the current manufacturing process is the low yield from semiconductor wafers based on a sapphire substrate. These wafers are currently scribed and then cleaved with a diamond tool, but this method renders useless at least a third of the devices fabricated.

The Japanese company Nichia, which until recently has had a stranglehold on the blue laser diode market, uses a sapphire substrate in its devices.

But according to Oxford Lasers' founder Colin Webb, the problem is that sapphire crystals cleave awkwardly because of their hexagonal structure. "If the scribing groove is rather shallow - as it is typically with diamond tools - then the cleave may not follow the scribed line over its complete length. Areas of the wafer containing active devices may be fractured during the breaking process," he explained.

The new laser scribing system, which is based around a frequency-doubled copper source operating at 255 nm, makes deeper cuts than the diamond technique. "With the deep laser cuts, the cleave is much more predictable and accurate breaking of the wafer is possible," said Webb.

Device yields with the laser scriber have not yet been released, although Webb says that a collaborative study with Strathclyde University will shortly publish its findings to this end.

The system uses a pulsed laser, typically emitting around 1 W average power with a pulse energy of 0.17 mJ and a 6 kHz repetition rate. Although other lasers, such as excimer sources and frequency-quadrupled Nd:YAGs operate at a similar wavelength Webb says that the copper source has some key advantages.

"It terms of output wavelength, there is no great difference, but excimers tend to produce high pulse energies at low repetition rates with beams of low optical quality. The doubled copper vapour laser produces near diffraction-limited quality beams that can create a trench with a very small kerf width and a V-shaped groove which makes cleaving more predictable."

Webb told Optics.org that the scribing system has been designed for production-line deployment, and that trials with semiconductor device manufacturers based in the far-East are underway. He adds that Oxford Lasers offers an in-house service for scribing or cutting sapphire to both industrial and academic customers.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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