Defect-free metallization of through-glass vias with engineered geometry in additive-free electrolyte

Defect-free metallization of through-glass vias (TGVs) with copper using an additive-free electrolyte is presented in this communication. Engineered vias with an X-shape in the middle were electroplated in the kinetic-limited regime (Thiele modulus, mu. <= 1) to achieve pinching of the vias at the waist, after which continued conformal plating ensured void-free filling. Thiele modulus analysis and an 1D steady-state model predicted that the deposition front moves uniformly within the via when plating is carried out in the kinetic-limited regime. It was hypothesized that engineering the via geometry to an X-shape could enable pinching in the middle. However, neither of the two conditions are enough on their own to achieve defect-free filling. Combining these two allows conformal movement of the plating front and pinching in the middle, which results in complete filling of the via. While additive-based bath is required and will continue to be used to optimize between plating speed, low stress and roughness of plated copper, the objective of this article is to demonstrate the advantage of engineered vias in which even an additive-free bath could provide defect-free metallization. At very low current densities, an anomalous increase in thickness in the middle compared to the top of the via was observed. This cannot be explained by a simple 1D model and is an interesting next step in understanding additive-free electroplating.