Investigation of the influence of Ag reflective layer on the Correlated Color Temperature and the Angular Color Uniformity of LED with conformal phosphor coating

Even though reflection layer is quite commonly used in Lead-frame packaging structures for Low Power or Mid Power LEDs and COB structures, it can't be used for silicon-based packaging structures with cavity and TSVs for High Power LEDs. Since it is difficult to perform a TSV redistribution layer patterning in the cavity via photolithography technologies, fabricating a reflection layer on top of the cavity is impossible by traditional way. In this situation, a simple method using a conventional dispensing coating method is proposed to obtain a local coverage of photoresist coating in a Si cavity. An effective reflection layer can be achieved by photoresist coating, metal film deposition and Lift-off process. The influence of the reflective layer on Correlated Color Temperature (CCT) and Angular Color Uniformity of LEDs with conformal phosphor has been investigated numerically and experimentally. Using the same amount of phosphor, a structure with an Ag-coated reflective layer provides a lower CCT and a more uniform color distribution. This demonstrates that the application of a reflective layer can save phosphor consumption. The experimental data agrees well with the simulation results. The cavity size will also influence the color temperature and angular color uniformity. The simulation results indicate that CCT will be decreased and angular color uniformity will be improved when the cavity size is decreased. The reason is that the small size cavity fits well with the requirement of conformal coating, which is the thickness uniformity of the phosphor layer. The phosphor layer above chip is just 100 µm. That means, in order to get a conformal coating, the cavity size should be almost as small as the chip size. However the luminous efficiency will decrease when reducing the cavity length, due to the cavity side wall reflection influence. A cavity length of 3mm will be the optimized choice when balancing the luminous efficiency and angular color uniformity.