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Laser System Manufacturers Vie To Open Up New Markets

Having carved out a niche in back-end processing of sapphire and SiC wafers over the past couple of years, some laser tool manufacturers are now targeting GaAs and InP wafers. Meanwhile, others believe that these material systems will remain the preserve of diamond scribe tools and saws.
Until a couple of years ago, scribe dicing with a diamond tool was the preferred method for dicing virtually all compound semiconductor devices. However, the traditional scribe tool is far from ideal when it comes to processing GaN-based devices grown on sapphire or silicon carbide substrates.

The problem is substrate hardness. The sharpness of diamond tips quickly degrades, which demands frequent replacement, resulting in variable scribe performance and reduced overall yields.

As a result of the problems associated with the diamond scribe tools, lasers have been able to penetrate the sapphire substrate market. Two US companies that have moved quickly to exploit the problems of diamond scribing are JP Sercel Associates (JPSA) and New Wave Research. The advantages are obvious, according to JPSA president Jeff Sercel: "With JPSA s method, the street width can be reduced to 20 µm (for 250 x 250 µm devices) for a typical 2 inch blue LED wafer on sapphire." This would give a 14% increase in die compared with diamond scribers that give 50 µm streets at the same device size.

"It is fair to say that laser scribing has become the new industry standard in cases where the wafer materials are very hard, such as silicon carbide and sapphire," added Sercel. "Most of the major players in the [sapphire LED] industry are either currently using or adopting laser die separation technology."

New Wave marketing vice-president May Su estimates that laser-based systems account for as much as 90% of the new capital equipment spending on scribers in Taiwanese sapphire-based LED manufacturing. She says that New Wave now has around 40 laser units installed worldwide.

According to Su, the company s latest system processes 350 x 350 µm die on 50 mm sapphire wafers at a rate of more than three wafers per hour (JPSA claims that up to eight wafers per hour are possible with its system). New Wave claims that its laser system has reduced LED-scribing costs by over half compared with diamond scribing, while simultaneously improving throughput.

Performance trade-offLaser processing of LED wafers does come with a trade-off in device performance, however. The extent of this trade-off is contentious.

While Sercel admits that laser processing does reduce LED output, he says that the laser systems which use a longer wavelength have a much more profound effect than the ultraviolet lasers in JPSA systems. "We have heard estimates of 10-20% light loss with longer-wavelength lasers. Our customers report far less light loss, 2-5% in production. At these levels it does not seem to be a problem."

Having installed its first six laser scribing systems in mid-2002, New Wave has enjoyed considerable success in the Taiwanese market. Su says that the greater emphasis on cost reduction at Taiwanese LED manufacturers made it the obvious entry point for laser scribers. New Wave appears to have also convinced one Japanese LED manufacturer - who typically put more emphasis on quality - that problems with the light-output quality of laser-scribed wafers are a thing of the past. New Wave installed its first laser-based sapphire scriber to a HB-LED manufacturer in Japan about a year ago, although Su admits business has been slow since.

Another common criticism of laser processing is the appearance of "burnt" wafers caused by the heating effect of the lasers. According to Sercel, the actual mechanism of the material removal relies on ablation rather than melting. This is due to the short wavelength of light and the short pulse durations used. "The result is a far less thermal process than directly vaporizing sapphire," he said. JPSA has a patent pending relating to the precise nature of this ablation mechanism.

Diamond tool manufacturers are also looking to partner laser-scribing companies by providing the breaking technology that is still needed. This is also the strategy being pursued by Dynatex, a diamond tool manufacturer, which previously specialized in the breaking aspect of the process. Leanne Schmidt, Dynatex s marketing manager, says that while laser cutting has undoubtedly had an impact on the sapphire-based LED market, there will always be a need for diamond scribers in the broader compound semiconductor sector. Schmidt told Compound Semiconductor the scribe-and-break technology provides a dry mechanical process avoiding water or heat damage. "Ultimately, throughput will determine the process technology," she said.
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