By Applied Microengineering Ltd.
Thermo-compression bonding is joining of two surfaces via the welding of a layer of soft metals on each surface. The most common metal for MEMS applications is gold, together with a suitable adhesion layer. Moderate temperatures (~300°C) and high contact pressures are required and the process can readily be carried out by AML's AWB wafer bonders. The technique offers the benefit of very low outgassing and is therefore attractive for the sealing of evacuated cavities. Cu-Cu Bonding, and removal of surface oxide Copper-copper bonding is a thermocompression bonding process that has traditionally relied on high contact force to break through the oxide layers on the copper, to achieve the direct copper-copper contact that is required to produce low-resistance ohmic contacts. This technique is attractive for 3D integration. However, the required force can be considerably reduced by an in-situ chemistry step to remove the oxide, using either forming gas or formic acid vapour. The AWB systems offer an option for the delivery of a saturated vapour pressure of formic acid vapour into the bond chamber - pumping down to high vacuum immediately after this step, and then aligning and contacting the wafers, ensures that the wafers are bonded without any opportunity for the copper oxide to regrow. The use of this in-situ chemistry step to remove the surface oxide enables bonding at lower temperatures and forces than would otherwise be the case. Advantages of thermo-compression bonding: - Maintains integrity of structures in substrates - Good thermal coupling between the substrates - Simple, pre-bonding metallisation process Disadvantages: - Requires very good surface topography, for intimate contact and hermetic sealing - Requires high bonding forces - Lateral feedthroughs for electrical interconnections cannot be used Applications: - MEMS - 3D stacking - WLP
