Using a combination of copper (Cu) thermocompression bonding and silicon wafer thinning, a face-to-face silicon bi-layer layer stack is fabricated. The oxygen content in the bonded Cu layer is analyzed using secondary ion mass spectrometry (SIMS). Copper-covered wafers that are exposed to the air for 12 h and 12 days prior to bonding exhibit 0.08 at.% and 2.96 at.% of oxygen, respectively. However, prebonding forming gas anneal at 150°C for 15 min on 12-day-old Cu wafers successfully reduces the oxygen content in the bonded Cu layer to 0.52 at.%.
- Forming gas
- Thermocompression bonding
- Three-dimensional integrated circuits (3-D ICs)