Ultra thermal stability of LED die-attach achieved by pressureless Ag stress-migration bonding at low temperature

Abstract

Using recently-developed Ag stress-migration bonding (SMB), a light-emitting diode (LED) die can be bonded on a glass substrate above 200°C in air without pressure, where both the die and substrate surfaces are coated with a few-microns-thick Ag layer. We investigate this emerging die-attach method and reveal the detailed bonding processes. The thermal residual stress in the Ag coating layer causes hillock formation on both the surfaces, and the extruded Ag grains grow to fill the gap between the contacted surfaces. A sound contact over the large bonded area can be achieved without applying any pressure. However, the LED dies exposed to a high working temperature may increase the risk of void formation in the bonding interface structure, leading to a crack or interface failure due to the bond strength degradation. In this study, we insert a barrier layer of a high melting-point metal like Pt or Ru under the Ag bond layer to prevent void diffusion from the Ag layer. The high bond strength achieved by Ag SMB is then unaffected by the heat exposure tests even at 440°C for 2h, assuring sufficiently long lifetime for the LED die-attach. Our pressureless Ag SMB method thus achieves excellent thermal stability, in addition to the ideal reflectivity and electric/heat conductivities of Ag, perfectly suitable for LED applications, as well as various other power devices of high-temperature wide-bandgap power semiconductors like GaN and SiC.