Highly Reliable and Highly Reflective Ag Metallizing of a Sapphire Surface of an LED Die

Previously, we reported the introduction of the low-temperature, pressure-less Ag direct bonding method, which has a high bond strength. In spite of this, when Ag was deposited directly on the sapphire surface of a GaN-based light-emitting diode die as a reflective layer, there were still some remaining problems with unreliable adhesion. Hence, this paper focuses on improving adhesion of Ag to sapphire by modifying the properties of Ag. Specifically, doping oxide into an Ag layer is investigated. As a result, excellent improvements in adhesion, and particularly in thermal shock resistance, are achieved. A similar effect is observed with various oxides. Additionally, oxide-doped Ag films have a high reflectivity that is almost equal to pure Ag. It is proposed that the adhesion improvement is caused by the formation a pseudotransition layer at the interface between the sapphire substrate and the Ag layer; oxide and Ag coexist in the pseudotransition layer.