Al-Al thermocompression bonding for wafer-level MEMS packaging

The Al-Al thermocompression bonding is studied on test structures suitable for wafer level packaging of MEMS devices. Si wafers with protruding frame structures have been bonded to planar Si wafers all covered with a 1 μm sputtered Al film. The varied bonding process variables were temperature (400 °C-550 °C), bonding force (18-36 kN) and frame widths (100 μm, 200 μm, rounded or sharp corners). The delamination caused by dicing and pull tests is systematically studied. It is concluded that bonding is incomplete at 400 °C, with a low dicing yield. The quality of the bonding is increased by increasing bonding temperature and force as expected. The fractured surfaces and the bonding strength have been studied in detail. The test structures showed an average strength of 20-50 MPa for bonding at or above 450 °C. The current study indicates that strong Al-Al thermocompression bonds can be achieved at or above 450 °C for a typical MEMS bond frame.