Bert Braune

A new wafer level coating technique to reduce the color distribution of LEDs

Light emitting Diodes (LEDs) which use phosphor conversion might be a neat alternative to the more costly 3 chip RGB (red, green, blue) LEDs. Almost any color of phosphor conversion LEDs (pc-LEDs) can be adjusted by combining a light-emitting semiconductor chip and one or more phosphors. Depending on the ratio of unconverted and converted light, it is possible to verify both unsaturated and saturated colors. A very common type of phosphor conversion LED is composed of a reflector cavity, which contains a blue light emitting chip and is filled by a phosphor containing resin. At a fixed concentration, parameters like the thickness of the phosphor filled resin layer (conversion layer) above the chip and the wavelength influence the final color of the pc-LED. It is necessary to reduce the variation of the influencing parameters to be able to control the color and prevent yield losses in the production. A new phosphor coating technique developed at OSRAM OS makes it possible to precisely control the thickness of the conversion layer above the chip. A layer of a hard resin is applied on top of a wafer and afterwards its thickness is milled accurately to the desired value. With this new technique the color distribution can be reduced significantly compared to the common techniques.

New developments in LED lighting by novel phosphors

Recently new phosphors from various material classes have been developed for LED applications by Osram OS and partners. Excitation wavelengths of these phosphors range from below 400 nm to 470 nm, enabling the creation of purple and unsaturated LED colors and even the efficient conversion of near UV-radiation into white light. By addition of red and green phosphors to white LEDs, a warm white color impression can be achieved. These LEDs are suitable for all purposes of general lighting, where a high color rendering is required. An outlook to new applications with unsaturated and warm white LEDs will be given.

Highly efficient UV-based conversion LEDs for the generation of saturated colors with improved eye safety

Recently near UV conversion LEDs with an excitation wavelength of around 400nm have attracted increasing interest due to their high efficiency and output power. Favorably is also the great variety of efficient phosphors available for near UV excitation. For the generation of saturated colors this method is very efficient especially for dominant wavelengths from 480 nm to 570 nm. Direct light emission from InGaN chips show a strong temperature and current dependency increasing with the emission wavelength and may cause disturbing color shifts in applications. For the wavelength range between 530 nm to 570 nm the physical limitations of the InGaN system are reached and the efficiency of the InGaAlP system is not yet satisfying. Conversion LEDs based on near UV-light emitting chips provide solutions for both problems. But up to now a disadvantage of these LEDs is the residual near UV-light emission which could lead to severe eye damages over time. OSRAM OS developed a eye safe solution by avoiding the near UV-peak while maintaining the high luminous efficiency. A luminous efficiency of 28 lm/W for λdom of approx. 560 nm was demonstrated, a value more than ten times higher than the efficiency of green emitting InGaAlP diodes. For these LEDs no more restrictions because of eye safety regulations are expected.