Low-Temperature and Pressureless Ag–Ag Direct Bonding for Light Emitting Diode Die-Attachment

Micrometer-sized Ag particles adsorb substantial oxygen above 200 °C. A dispersion of these particles in alcohol can be successfully used as a die-attach material by sintering the Ag particles to form a soft porous bonding layer. Since the low- temperature sintering of micrometer-sized Ag particles is unlikely to involve the nanometer size effect associated with melting, it was considered that the low-temperature sintering between Ag layers of micrometer thickness would occur in air. The contact surface of a light emitting diode die and a glass substrate were both coated with an Ag sputtered layer of 2 μm in thickness. The Ag-Ag direct bonding proceeded by sintering above 200 °C in air. The bonding strength of the Ag-Ag direct bonding exceeded that of the low-temperature sintering of micrometer-sized Ag particles, and the bonding strength depended on oxygen concentration in the sintering atmosphere. On the Ag sputtered surface of the glass substrate, a substantial morphology change was observed by heating the Ag sputtered layer in air. It was supposed that this morphology change was caused by abnormal grain growth of Ag in the Ag sputtered surface. It is concluded that low-temperature sintering of Ag materials is accelerated by the abnormal grain growth of Ag with oxygen adsorption near the Ag surface.