Design and Simulation of Symmetric Wafer-to-Wafer Bonding Compesating a Gravity Effect

In this study, a process optimization methodology was proposed to minimize the alignment error between contact pads during the direct wafer-to-wafer bonding process. The reason of occurring alignment error in the conventional wafer-to- wafer bonding process is that there is a deformation difference between upper wafer and lower wafer during the bonding process. This deformation difference is occurred by asymmetric pressure head for initiating the bond, wafer anisotropy, and gravitational influence during bonding. In this study, a FEA (Finite Element Analysis) model which can simulate the WtW bonding for optimization of bonding process was developed. In order to define the bonding force acting between the wafers during bonding, characteristics of wafer surface were analyzed by molecular dynamics analysis using SiCN surface model that is plasma-treated with oxide. The bonding force with respect to the distance obtained by analysis were implemented as subroutine and applied to the wafer bonding simulation model in ABAQUS, which is commercial FEA software. The simulation model was verified through comparing bonding propagation distance and alignment error measurement results. We conducted the process optimization to minimize the alignment error using our FEA model. As a result of optimization, wafer-to-wafer bonding process that has an alignment error about tens nm level was proposed.