Intermetallic Void Formation in Cu-Sn Micro-Connects

The compatibility of new materials and their interfaces are key components in the pursuit of highly integrated and reliable systems. An extensive understanding is required of the behaviour and stability of the materials not only during device fabrication but over the entire functional lifetime of a device. A microstructural defect, and the focus of this thesis, that threatens the mechanical and electrical performance of 3D-intergrated systems is Cu-Sn intermetallic void formation. Results of this work has been separated as follows: (i) understanding the sporadic behaviour of void formation, (ii) understand the key parameters influencing voiding formation, (iii) examine the microstructural and chemical properties associated with void formation, (iv) present a void formation hypothesis and (v) discuss void reduction and detection methods for the microelectronic industry. Historically void formation has been sporadic and uncontrolled and led this author, in additional to several other authors, to question whether formation is only due to the Kirkendall effect (interdiffusion imbalance between two joined metals). The Cu electroplating process and the parameters used play a large role in effecting the voiding propensity. These parameters, including the additive chemistry and current density, influence the microstructural properties and chemical composition of the deposited film such as, grain structure, residual stresses, crystal defect density and trace impurities. A new intermetallic void formation hypothesis is proposed based on microstructural and chemical state of the Cu-Sn system. Intermetallic voids can be suppressed when the Cu electroplating process is well controlled, which requires careful observation of the electroplating parameters. This requires cooperation and understanding of the process between the semiconductor fabrication companies and the Cu process suppliers. However, it is difficult to control voiding completely, therefore non-destructive void detection methods need to be developed.