Power Electronics

LED Lighting Roadmap Target of 8X Cost Drop by 2015

Paula Doe, SEMI Emerging and Adjacent Markets, says this year's roundtable of experts gathered by the U.S. Department of Energy to review its industry roadmap decided last year's ambitious goal of 10X cost reduction by 2020 in packaged LED die still won't be enough. They now think an 8X reduction by 2015 will be needed to create a solid state lighting industry that can compete with fluorescents.

Can the industry achieve this aggressive goal of reducing costs from about $18/klm now down to only $2.20/klm within five years? The industry is making good progress in everything from understanding the droop in efficiency at higher currents, to controlling uniformity of epitaxial deposition, to finding and fixing defects, to simplifying packaging by using fewer components. Industry experts will discuss this progress and next key issues at the major steps along the value chain at the HB-LED manufacturing technology program at SEMICON West, July 13 in San Francisco.

The ability to detect and classify defects on the wafer or substrate surface is turning out to be fundamental in improving yields in LED manufacturing. KLA-Tencor's Candela 8620 optical inspection tool can now map the submicron defects on just the front side of the transparent sapphire wafer with dark field sensitivity, so LED and substrate makers can see the pattern of micropits, microcracks and scratches on the surface for the first time.

Submicron pits are a direct cause of defective LEDs -- they fill with metal shorting the p/n junction and making an LED that doesn't light up -- and a direct indicator of uneven heating during epitaxial layer growth. Their pattern on the wafer map can suggest if the cause is wafer warp or a particle between the wafer and the heated pocket in the MOCVD reactor. Operators visually inspecting a few points across the wafer cannot catch these pits or these patterns.

Microcracks, meanwhile, come from the lattice mismatch between the GaN and the substrate. "It's just one atomic layer of sheared bonds, difficult to see, but becomes catastrophic reliability killers as the device heat cycles in the final application like an automotive headlight," notes Frank Burkeen, VP and GM, Candela Division, KLA-Tencor Corp. He notes that this new ability to see the microcrack defect patterns will likely be key to figuring out how to grow good nitride layers on silicon substrates.

While KLA was working with Philips Lumileds and others on a partially DOE-funded project to develop and apply this tool for micropits and microcracks, researchers also discovered that they could see other micro-defectivity on incoming sapphire substrates for the first time, which also turned out to directly impact deposition quality. Substrate makers have been able to make important strides in improving surface quality now that they can see the problem.

"Less than 5% of epi-ready LED substrates are now inspected with high sensitivity automated inspection," notes Burkeen. "Now that we have a more powerful toolkit, the more we use it, the more new things we see to work on." Improved yields of course translate directly into lower LED costs and open new applications for solid state lighting.

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