AlAl thermocompression bonding for wafer-level MEMS sealing

• AlAl thermocompression bonding was demonstrated at a temperature as low as 400 °C. High dicing yield was achieved by applying a high bond force of 60 kN.
• The possibility of reducing the bond force to 18 kN was demonstrated. The reduction in bond force required an increase in the bonding temperature to 450 °C.
• Cohesive fracture in the bulk silicon below the Al layers was observed to a large extent, indicating good adhesion between Si and Al, and strong AlAl bonded interfaces.

Al–Al thermocompression bonding has been studied using test structures relevant for wafer level sealing of MEMS devices. Si wafers with protruding frame structures were bonded to planar Si wafers, both covered with a sputtered Al film of 1 μm thickness. The varied bonding process variables were the bonding temperature (400, 450 and 550 °C) and the bonding force (18, 36 and 60 kN). Frame widths 100 μm, 200 μm, with rounded or sharp frame corners were used. After bonding, laminates were diced into single chips and pull tested. The effect of process and design parameters was studied systematically with respect to dicing yield, bond strength and resulting fractured surfaces. The test structures showed an average strength of 20–50 MPa for bonding at or above 450 °C for all three bonding forces or bonding at 400 °C with 60 kN bond force. The current study indicates that strong AlAl thermocompression bonds can be achieved either at or above 450 °C bonding temperature for low (18 kN) and medium (36 kN) bond force or by high bond force (60 kN) at 400 °C. The results show that an increased bond force is required to compensate for a reduced bonding temperature for AlAl thermocompression bonding in the studied temperature regime.