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Cree embeds phosphors at LED die level

The chip maker says that blue-emitting GaN die can be converted to broad wavelength white emitters by material deposited in etched trenches.

Cree has invented a method that makes packaged white LEDs smaller and easier to produce by spinning phosphor material directly into its SiC substrates.

The Durham, North Carolina, company s US patent application 20080272386, published in November, covers trenches etched directly into substrates using laser ablation or sawing.

Semiconductor nanocrystals or phosphors can then be filled into the trenches, potentially forming a convex bridge in-between, using a spin-on process.

These fillings can then convert the blue light emitted from GaN LEDs formed on the SiC into broader wavelength white light.

Etching trenches into the substrate is an advance from existing methods where an LED is placed in a cup-shaped submount and encapsulated in light conversion material.

“Such techniques may limit the minimum size of a broad spectrum LED,” writes inventor Peter Andrews in the application.

Just as the submount may be shaped and made reflective in order to help light escape from the LED as efficiently as possible, so Cree s trenches can also boost light extraction.

As well as conventional LED phosphors Andrews points out that using semiconductor nanocrystals such as CdSe in the trenches offers a high degree of control over emission wavelengths.

Covering its options for applying such novel materials, Cree suggests that inkjet or screen printing, or an airbrush application system could be used instead of spinning them on.

Outside the additional materials for converting the wavelength of light, and trenches to improve light extraction, the LED die retain a standard arrangement of electrical contacts.

Consequently the broad spectrum emitters produced with this method can go on to be made into an LED package or the die can be used directly from production.

“LEDs may be mounted directly on a circuit board or substrate with or without further encapsulation of the device,” Andrews writes.

Also in November, Cree announced record efficiency of 161 lm/W for a white power LED using 1 mm x 1 mm die. Commenting to compoundsemiconductor.net the company simply credited “advances in chip and packaging technology” as responsible for its record.

Cree also says that it will soon be moving into manufacturing with the advances seen in its record LED. “We continually bring innovations into our production processes, and anticipate some elements of this device will be available within the next year,” a spokesperson said.

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