Radical activation is a process used for Low Temperature direct bonding.
Various material combinations can be used, most commonly Si, SiO2 and III-Vs but also quartz, glass and sapphire. The process is performed at room temperature and high bond strength can be achieved after annealing at temperatures as low as 200°C. This allows bonding of materials with different CTEs.
Most commonly used plasmas are Oxygen and Nitrogen.
This is an in-situ surface treatment, using neutral radicals that diffuse from plasma. The plasma is contained within a ring of mesh electrodes that surrounds the wafers. After the activation the wafers are aligned and bonded, all in one process without exposure to atmosphere.
Real view of RAD ring with N2 plasma inside the AWB chamber.
The benefits of using only neutral radicals are very apparent – there is no risk of damaging the devices or roughening the surface as with direct, energised plasma. Although the process is very gentle, it still allows cleaning of hydrocarbons and increases the hydrophilicity of the surfaces which encourages hydrogen bonding.
Because the surface treatment is performed in-situ, there is no opportunity for the wafers to adsorb any atmospheric contaminants between activation and bonding. This results in improved process control and reproducibility from bond to bond.
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