By Applied Microengineering Ltd.
The AWB systems offer the option of in-situ radical-activation, enabling low-temperature bonding. Radicals from oxygen or nitrogen plasma are used for effective surface activation. The plasma is confined within a ring that surrounds the wafers, consisting of three mesh electrodes – the two outers are at ground potential and the inner carries the electrical excitation. The discharge is struck between this and the electrodes to either side. Only the free radicals are carried past the mesh under their own momentum, unlike in energised plasma, and the wafer surfaces are then activated without any risk of excessive etching. To aid the initial bonding, radical-activation is immediately followed by exposure to water vapour in order to create hydrogen bonding between the surfaces. This significantly increases the initial bond strength that is then improved further during low temperature 200–300°C annealing. Why in-situ? Performing the sequential activation, alignment and bonding procedures in-situ, within the same chamber and therefore under controlled conditions, avoids any risk of unwanted reactions or contamination of the bonding surfaces and results in a more controllable and reproducible process. Why use radicals? Activation using direct exposure to a plasma can result in damage to sensitive devices on the wafer, due to energetic ions. In addition, plasma activation has a narrow process window with too long an exposure leading to surface roughening. The use of radicals avoids such potential problems. Benefits of in–situ 'RAD' activation process: - Only requires low temperature annealing to achieve high bond strength (up to 2 J/m2) - Produces hydrophilic surfaces that attract hydrogen bonding - More stable, reliable and uniform bonds - Reduced handling of wafers - higher yields due to in-situ process - Wider process window for activation and subsequent bonding - No change in AFM topographical roughness after activation Applications: - Layer transfer - production of SOI wafers - MEMS fabrication – sensors, actuators - Hybrid bonding – Direct bonding together with Cu-Cu bonding in advanced 3D wafer stacking
