Optimization of a BEOL Aluminum Deposition Process Enabling Wafer Level Al-Al Thermo-Compression Bonding

The main challenges for Al-Al wafer bonding are the fast oxidation and the high roughness of the Al surface. This paper describes an optimized Al sputter-deposition process reducing the surface roughness to values below 2 nm. Based on this, a wafer level Al-Al thermo-compression bonding process is presented, where a surface treatment and the subsequent bonding are both performed in a high vacuum cluster. The patterned wafers were bonded with temperatures between 300 and 500 degrees C for 1 h using a bonding force of 60 kN. Scanning acoustic microscopy and transmission electron microscopy studies revealed a reliable bonding, an accurate alignment and a high uniformity for 200 mm wafers. The electrical characterization of contact chains with Al bonding pad sizes of 20x20 mu m(2) showed resistances lower than 50 m Omega per contact, which might indicate areas of oxide-free bonding.