Low-Temperature Rough-Surface Wafer Bonding with AlN/AlN Via Oxygen Plasma Activation

We demonstrate breaking through the main obstacle to achieve perfect wafer bonding: the requirement of the surface needs to be very flat and smooth. Strictly, the roughness must be less than 0.5 Å. We use sintered aluminum nitride as an example. AlN is a ceramic material with excellent dielectric and thermal properties and is usually used in the fields of microelectronics and energy. Despite CMP polishing, the high roughness (rms> 10 nm) of AlN wafers due to voids formed in sintering cannot meet the surface requirements for direct wafer bonding (rms < 5 Å). We present that bonding an AlN to another AlN with a high roughness surface can be achieved by design bonding processing. First, we used oxygen plasma to activate the hydrophobic rough surface to become a hydrophilic surface to introduce strong capillary action. Then, both surfaces of the bonded AlN/AlN pair were reacted with bonding species (OH-). The reaction was strengthened via electron transfer caused by clamping the bonded AlN/Al pair. Finally, low-temperature annealing (<150°C) was performed on the bonded AlN/AlN pair to synthesize Al2O3 at the bonding interface. After annealing at 150°C for 4 hours, the bonding interface of the AlN bonding pair was observed with scanning electron microscopy (SEM), as shown in Figure 1. There is a transition layer between the bonding surfaces, in which the nitrogen concentration decreases and the oxygen concentration increases significantly. Alumina layer is formed by hydrolysis and dehydration, which acts as a bridge to firmly connect the two AlN wafers.