Material Union
Molecular contact between two polished wafers creates a rigid interface without the addition of intermediate adhesive layers. When applied to silicon-on-insulator or micromechanical stacks, fusion bonding relies on surface hydration and subsequent high-temperature annealing to convert hydrogen bonds into covalent siloxane links. The process fails if surface roughness exceeds several nanometers or if particulate contamination prevents uniform initial contact.
Interface Quality
Uniformity across the whole wafer diameter ensures that individual die cavities maintain pressure integrity over time. Optical inspection via infrared transmission reveals voids where the fusion bonding process was unsuccessful or where trapped air resisted the closing force. These voids act as stress concentrators and sites for potential future delamination.
Higher annealing temperatures generally improve the bond strength until the dopant profiles or metal features begin to degrade.
Process Requirement
Activation of the surface usually involves oxygen plasma or chemical cleaning to maximize the density of hydroxyl groups. Cleanliness remains the most significant driver of yield for fusion bonding within high-volume production lines. Automatic alignment systems must place the wafers with micron-level precision to ensure electrical or fluidic channels coincide.
Pressure must be applied consistently from the center outward to avoid trapping large pockets of air during the sequence.
Long-Term Reliability
Bond strength values often approach the fracture toughness of the bulk silicon itself. Because no organic materials are present, fusion bonding offers excellent stability against outgassing and chemical attack. Thermal cycling poses less risk when the expansion coefficients of the two joined wafers match perfectly.
Performance in the field is verified through shear testing of sample dies from each lot.
