Finite Element Modeling for Wafer-to-Wafer Direct Bonding Behaviors and Alignment Prediction

For next-generation semiconductor devices, 3D IC package has attracted a lot of attention. Among technologies of the 3D IC package, a wafer-to-wafer (W2W) bonding has significant effect in decreasing 3D interconnect pitch scale. Nowadays, regardless of the type of semiconductor device, W2W bonding technique is widely applied. Unlike the fusion bonding where only dielectric layers exist on the boning surfaces, the bonding surface consists of dielectric layer and copper (Cu) pads in the case of hybrid copper bonding (HCB). In the HCB, the electrical interconnection between the upper and lower wafers can be realized by precise matching between the pads. Generally, the bonding quality of W2W HCB is evaluated by measuring the bonding alignment error which is the in-plane distance between corresponding Cu pads. Thus, if a sufficient level of the bonding alignment is not guaranteed, there would be limitations in the performance of devices. There are many factors affecting the bonding accuracy such as warpage of incoming wafers, surface treatment, bonding energy, bonder equipment and so on. In order to investigate the effects of each factor, much experimental efforts are required. Although it is valuable to optimize the bonding recipe via various experimental evaluations, it is very time consuming and costly. This is the reason why we propose a numerical simulation model describing the dynamics of W2W direct bonding and evaluating the bonding quality. The objective of this work is developing a numerical simulation model that can provide important guidelines to process designers. In this paper, we provide a multi-physics finite element (FE) model for the dynamics of W2W direct bonding. Only a bonding module of wafer bonder equipment is considered rather than the whole equipment. Various steps are sequentially mimicked such as chuck deformation, wafer chucking, pin striking, bonding propagation, and vacuum off. Bonding alignment errors are obtained by calculating in-plane distance between corresponding FE nodal points. From matching between experimental and numerical results, it is confirmed that the developed FE model can provide results that are qualitatively and quantitatively agree with the experimental data. Based on the developed FE model, we evaluate equipment-level solutions for good bonding alignment.