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.