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Wafer-level package overturns flip-chip LEDs

Metal pads on the surface of a photodielectric resin minimize sacrifice of quantum well layer for contact with n-type GaN.

Korean researchers have developed a method for wafer-level packaging GaN LEDs that they say will lead to brighter, easier-to-produce, LED components.

Gwangju Institute of Science and Technology and the Korea Photonics Technology Institute claim their approach has several advantages over flip-chip LED manufacturing processes popular today.

The devices in the team s forthcoming IEEE Electron Device Letters paper retain larger active layer areas than LEDs produced by rival methods.

The wafer-level packaged (WLP) LEDs consequently produce more light, and are less susceptible to drop-off in efficiency at higher driving current densities than other nominally equivalent-size products.

“The key idea of this technology is the rearranged large bonding pad,” said Gwangju Institute of Science and Technology's Gun Young Jung.

As in both conventional and flip-chip LED designs, a portion of the WLP LEDs p-GaN and quantum well layers must be etched away. This allows metal to be deposited, forming an electrical contact with the n-GaN layer, but the Korean researchers minimize this contact area.

These contacts are connected to larger electrodes on the LEDs outer surface through the photodielectric resin used to passivate and encapsulate the semiconductor layers. This also avoids the need for sub-mounts or package frames used in other approaches.

“These LEDs, with large bonding pads, can be directly mounted on metal PCBs without wire-bonding, thus reducing the packaging thickness,” Gun Young Jung told compoundsemiconductor.net.

Other than reducing size, the ability to produce an LED package with exclusively wafer-level processing steps also shortens the manufacturing process.

The Korean team s LEDs otherwise emit light through the sapphire substrate and have reflective indium-tin-oxide deposited on top of the final p-type GaN layer, similar to flip-chip LEDs.

The 460 nm emitter demonstrated 75 mW optical power, 6.2 percent wall-plug efficiency and 3.2 V forward voltage at 350 mA.

Gun Young Jung says that many companies in Korea are interested in the approach, with a technology transfer deal already in discussion. He foresees no problems in scaling up the approach to manufacturing volume.

The team now plans to change from depositing chromium, nickel and gold alloy bonding pads to electroplating higher thermal conductivity materials. They will also examine conformal coating phosphors and epoxy encapsulants onto the WLP LEDs to provide complete replacements for today's surface-mount products.

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