Plasma -Induced Metal Oxide Reduction for Improved Intermetallic Contact Performance in Packaging Applications

The AI (Artificial Intelligence) revolution has forced the electronics industry to focus on interconnect and packaging solutions that can produce smaller devices, optimized power management, and higher levels of I/O density, in 2D, 2.5D and 3D device packaging formats. Thermocompression bonding (TCB) is a common method used to form flip-chip interconnects between dies and substrates. This bonding process requires high precision and superior material quality to ensure that mechanical reliability, thermal management, and electrical conductivity are optimized. 

For advanced packaging applications, copper pillar-based flip-chip interconnects offer significant advantages but are prone to oxidation at high temperatures (>220°C), leading to the formation of defects in the pillars and loss of adhesion. Solder fluxes and formic acid vapor, even mechanical methods, such as micro-scrubbing, have been extensively used to remove copper oxide prior to TCB. Another approach taken is the application of protective metal coatings on copper, such as Electroless Nickel Immersion Gold (ENIG), Immersion Tin (ImSn), Immersion Silver (ImAg), and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), that are less susceptible to oxidation. However, these methods can be costly and add more processing steps and complexity to the process.

Recent advancements of plasma technology have shown that plasma-induced metal oxide removal is a promising alternative to conventional methods. In particular, metal oxide removal in plasmas generated under ambient pressure conditions, not requiring the use of a vacuum chamber, is a dry, acid-free, and fast process that specifically removes oxide layers without inducing roughness to the treated metal.

In this talk, the results of using forming gas (N2/H2) plasma for the reduction of Cu, Ag, Sn and Ni, will be presented. The process involves the use of a large area plasma jet operating under atmospheric pressure conditions, in an oxygen-free environment. The fully automated plasma system is equipped with a conveyor belt to allow loading of parts of various formats: substrates, fully assembled printed circuit boards (PCBs), leadframes, etc. After the plasma exposure, the treated surfaces enter a cooling zone which prevents immediate reoxidation after the oxide reduction process.

Results from the elemental surface analysis of the oxidized metals pre- and post- plasma processing will be presented. For instance, X-ray photoelectron spectroscopy (XPS) data from CuOx surfaces that were collected immediately after plasma exposure showed a 47% increase of the metallic phase Cu(0). Aside from the oxide layer removal, results from the analysis of the copper surfaces revealed a decrease of oxygen and carbon atomic concentrations, an indication that the plasma was effective in removing other types of surface contaminants, and not just the native oxide layers.

Similar trends were observed in our studies focusing on the removal of Sn, Ni and Ag oxides. Results from these metals will also be shown, demonstrating the effect of plasma processing parameters on the oxide reduction of the above listed metals. 

Overall, pre-treating surfaces with plasma can enable reliable bonds at significantly lower compression forces, which is crucial for reducing stress on semiconductor dies. A plasma cleaned, oxide-free surface creates a more robust metallurgical bond, forming a stronger and more reliable intermetallic compound (IMC) layer at the joint interfaces. This scalable, cost-effective approach has the potential to boost interconnect reliability in high-performance electronics, paving the way for next-generation packaging solutions.