Defect-Free Metallization of through-Glass Vias (TGV) with Engineered Geometry

Metallized through-glass vias (TGVs) have attracted interest in recent years due to their promise in enabling 2.5D and 3D electronic architectures and high-density MEMS devices. However, unlike metallization of PCB, metallization of TGV is challenging, and the electroplating processes are not well-established. This is mostly due to the form factor associated with TGVs - high length (L) and small diameter (Φ). During electrolytic process, for example electroplating of Copper inside the TGVs, such form factor often leads to severe transport limitation and subsequent formation of defects such as seams or voids in the samples. This has led to intense research in the past decade to develop special additives and processes to achieve defect-free filling of TGVs. It has been accepted that defect-free filling of TGVs is not possible without specialized additives in the solution. We will first demonstrate through simplified theoretical analysis and numerical modeling why that is the case when the geometry of the TGV has uniform cross-section. Even when the electroplating is carried out in in fully kinetic-controlled regime (Thiele modulus, μ ≤ 1), it will be clear from the analysis that a complete defect-free metallization is not achievable as long as the TGV cross-section is uniform. Next, we will report a novel metallization technique using an engineered via with an X-shape in the middle and operating in the kinetic-limited regime without the requirement of any specialized additives. This leads to Cu bridging of the vias at the waist, after which continued conformal plating ensures void-free filling. It would be emphasized that by combining these two effects – novel shape of TGV and kinetic controlled electroplating – void-free filling is achievable even in the absence of any additives. 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 talk 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.