Using Silver Alloys in the Reflection Layer of LEDs

Because silver offers excellent reflective properties across the visible and near infra-red light spectrum. it is an ideal candidate material for applications that require reflective coatings. A thin layer of no more than 100 nm is sufficient to maximize its reflectivity. Thicker layers do not increase reflectivity and can add to the cost of manufacturing.

Silver Use in LEDs

One application for silver is in light-emitting diodes (LED), a growing technology and market for general illumination.  Silver is used as the backside reflective layer on the LED chip.  Its purpose is to reflect stray light back in the intended forward direction.  Silver can also be used to coat the surfaces of the submount and reflector in which the LED chip is placed.  In order for LEDs to be cost competitive with alternative forms of lighting, it is important that all generated photons be steered in the appropriate direction. 

Advantage of Silver Alloys

With all of its excellent properties, there is a downside to pure silver. Although it does not oxidize easily, it is susceptible to environmental degradation through such factors as high humidity and high temperature exposure, viz., 85ºC + 85% relative humidity.  (The degradation work was a co-development with Tell-Nexx, a division of Applied Materials. See following Chart.) This can result in the formation of sulfide tarnish. Therefore, pure silver is not the ideal choice of material for use in LED reflective layers, due to its potential loss of reflectivity from tarnishing.

                            Chart of Laboratory Data: Ag Alloys Resistance to Environmental Degradation. 
                                 Research by Materion and TEL-NEXX, Div of Applied Materials

Fortunately, Materion offers several silver alloys that have demonstrated superior resistance to environmental degradation compared to pure silver. These alloys include our trademarked Blu-X™ and Blu-S™.  These special alloys were developed for the optical media market, but their tarnish-resistant properties can be applied to LEDs as well.   For more information contact Dr. David VanHeerden, Applications Engineer, David.VanHeerden@materion.com.