Wafer Level Fusion and Hybrid Bonding: Impact of Critical Process Parameters on Bond Quality

Hybrid bonding, with wafer level bonding to form oxide-oxide bonds and Cu-Cu bonds is a promising technology for 3D integrated circuits. In this paper, the impact of various process parameters on the wafer-level bond quality of fusion (direct) and hybrid bonding have been evaluated. Investigated process parameters include: TEOS based oxide (Tetra-Ethyl-Ortho-Silicate) process deposition temperature, oxide densification annealing conditions, plasma conditions for wafer surface activation, chemical mechanical polishing (CMP) process conditions and post-bonding annealing conditions. In addition to these process parameters, design parameter such as Cu pad density is also critical. For fusion/hybrid bonding, surface properties are very critical. This includes surface roughness, bow and warpage, flatness and edge roll-off. Thus, all these surface properties were continuously monitored during the wafers fabrication. Bond quality was characterized by scanning acoustic microscopy (SAM) images. Bonding energy was calculated using Maszara model, where the crack length is measured from the SAM micrographs. Target objective of this study is to determine optimum process conditions in order to achieve eventual bonding energy > 1J/m ² , which is a pre-requisite for 3D multi-wafer stacking without any delamination.