Non-Traditional Cell Geometry for Improved Copper Plating Uniformity

Abstract

Many factors contribute to uniform electrochemical processing in industrial applications. The electrochemical cell provides the foundation for the process and as such an effective geometry is critical to obtain desired properties and to maximize process efficiency. A non-traditional cell geometry has been engineered to provide a uniform boundary layer thickness to dampen uneven localized current distributions and promote consequent plating uniformity for full size printed circuit board panels. This tank has been extensively characterized for thickness distribution under direct current plating conditions and compared with commercial plating cell geometries that utilize both educator flow and/or air sparging agitation. Compared to the conventional cells, the variation of copper deposit thickness across the panel in the non-traditional is significantly less, and meets the requirement of the printed circuit board industry. Cell geometry, panel features and operating parameters have been modeled and show good agreement with experimental data. The results of this study suggest that through hole patterns on a printed circuit board increase the non-uniformity of the boundary layer thickness. In the manufacture of printed circuit boards (PCBs), copper electrodeposition plays an important role in the final product's reliability and performance. Deposits that are non-uniform may induce reliability issues and ultimately hamper device performance. Furthermore, copper deposits with columnar grain structures are prone to cracking and may result in device failure. As such, implementation of a robust process that provides high levels of uniformity and the desired grain structure promotes higher throughput and higher quality products, decreasing manufacturing costs.Processing conditions such as agitation mechanism, electrolyte composition and electric field parameters control the properties of copper deposits as well as governing the uniformity of metallization over the PCB surface. Conventionally, much emphasis has been placed on electrolyte composition, specifically use of organic additives (in parts-per-million (ppm) concentrations), as a means to help control copper deposit uniformity and improve throw/fill within the micron size features. However, regardless of the electrolyte composition used, optimization of the plating cell geometry and agitation mechanism may greatly enhance the uniformity achieved in the copper electrodeposition process.In traditional plating cells, air sparging, and more recently eductor flow, is used as the primary agitation mechanism. Other agitation schemes (lateral oscillation, knife-edge oscillation and vibration of the panel) have been added in attempts to achieve a time averaged boundary layer thickness, which yields uniform current distribution and consequently uniform plating thicknesses. Another technology for controlling solution flow in a plating cell uses a paddle to create a turbulent wave of electrolyte that moves back and forth across the surface of the workpiece, 1 in an attempt to cre...

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