Mechanism of indium tin oxide//indium tin oxide direct wafer bonding

We utilize indium tin oxide (ITO) as a material for direct bonding of Si wafers at low temperatures and without any pretreatment in order to create junctions of Si-ITO//ITO-Si. Transparent conductive oxides combine good conductivity with high optical transparency, rendering them ideal for joining III-V materials with Si based technology, e.g. to fabricate optoelectronic devices like light emitting diodes or solar cells. We found that bonded wafers did not display macroscopic defects and exhibit a bonding energy greater than 1 J/m2 determined by double cantilever beam test. This bonding strength is high enough to bear further front-of-line processing like backside grinding. The bonding interface was investigated by means of transmission electron microscopy to gain insight into the bonding mechanism. We observed that the bonding mechanism proceeds through grain growth of the ITO layers across the original bonding interface upon annealing above the crystallization temperature of ITO. To explore other factors which could influence the obtained wafer bonds, ITO layers were deposited on Si wafers and characterized before and after annealing by atomic force microscopy, scanning electron microscopy and X-Ray crystallography.