Phosphor coating technique with slurry method in application of white LED
White light was realized through combining a cerium-doped yttrium aluminum garnet (YAG:Ce3+) phosphor with a gallium nitride (GaN)-based blue LED. In order to coat the high quality yellow phosphor layer on a GaN chip, the slurry coating technique was focused on because of the speed and coating uniformity. The slurry method is based on a phosphor suspension in a water-soluble photoresist, which is consisted of polyvinyl alcohol (PVA) and ammonium dichromate (ADC). And self-exposure of LED chip was employed as an optimized exposing way to make the photoresist crosslinking. The properties of the phosphor layer in terms of profile, light intensity and uniformity and spectrum were investigated. To improve the qualities of phosphor layer some additives were introduced such as polyvinyl acetate (PVAc) for improving adhesion strength, dispersants and defoamer for uniformity. The thickness of phosphor layer was controlled by altering the time of exposure under UV or blue light. The phosphor pattern with clear margins according to the mask and conformal shape were achieved in different substrates either glasses or LED chips. The white light output of high intensity and good uniformity due to the optimized thickness of phosphor layer were achieved. A defect of photoresist is that the ADC would absorb parts of blue light emitted from LED chip and quench light output, especially in the high concentration, some approaches can be employed to diminish and avoid this defect, such as reduce the ADC's concentration, and find new photosensitizer to replace the ADC.
This paper was published in SPIE Proceedings Vol. 6841