By Applied Microengineering Ltd.
Any two flat, highly polished, clean surfaces will stick together if they are brought into contact. The bond is of the Van der Waal's or hydrogen type and is of low strength, but can be significantly improved by post-bond thermal treatment. This process has been successfully exploited for MEMS fabrication, using Silicon-Silicon bonding with either plain or oxidised wafers. Although the initial bonding is normally carried out at room temperature, because of the thermal annealing treatment the technique is also referred to as silicon fusion bonding. It should be noted that the surface roughness of wafers is critical for successful direct bonding, with roughness <0.5nm required. The high temperature (typically 1000°C) that has been required to achieve acceptable bond strength has been a limitation with regard to some applications for silicon direct bonding. However, developments in wafer surface preparation (e.g. using plasma or AML's radical-activation) have demonstrated that high temperatures are not necessarily needed. This is important in MEMS design as it enables stress-free bonds to be achieved at temperatures that are compatible with metallisation layers and opens up a much wider range of applications for silicon direct bonding. To reduce the risk of voids, AML wafers bonders feature a central pin used to produce a deliberate bow in the upper wafer such that the first point of contact between the wafers is at the centre. This ensures that when the bond forms it does so starting at the centre, spreading outwards towards the wafer edges and avoiding trapped air at the interface. This reduces voids in the bonding and leads to higher quality and yield bonds. Advantages of Direct Bonding: - No interlayer required - Wafers have matching thermal expansion and therefore minimal stress in bonded wafers (if same material wafers used) - High temperature stability of bonded wafers - CMOS compatible, direct bonded wafers can be used for subsequent IC processing Disadvantages: - Surface roughness requirements for direct bonding are critical, being <5nm - Highly sensitive to surface contamination and defects
