Hybrid bonding examples and materials. (a) Cu/BCB hybrid bonding, where BCB-BCB is bonded at lower temperature (~250C), followed by raising the temperature above 350C for Cu-Cu bonding. (b) and (c) Cu/PECVD oxide hybrid bonding, where pre-bonding is done at room temperature and the permanent bond is formed during the post-bond anneal at above 350C. A ~4 nm thick CuO x crystalline layer is present before the post-bonding anneal.