Process Control for Advanced Packaging Metallization

Abstract

Despite advances in optical interconnect, metal interconnect remains the dominant technology, however the type and diversity of metal structures is rapidly changing. Wire-bonding cannot meet required interconnect density and is being replaced with electrodeposition-based flip chip technology. Modern packaging scheme required high diversity of interconnects within the same device: RDL, mircobump, pillar, TSV, tall (mega) pillar, etc. using sequential deposition of different metals such as Cu, Ni, SnAg/Sn, Au. Diverse challenging requirement leads to evolution of chemistries and process control approaches. Many 20th century chemistries were limited to 1 organic additive for Cu process. Today, up to 4 individual additives are used to dial the process. While most of semiconductor front end processing enjoys the luxury of single-use chemistry, it is not yet considered a viable option for electrodeposition. The same electrolyte can be used for processing over 1000 parts in replenishment mode. Maintaining performance under these conditions requires close monitoring of component breakdown and contaminants accumulated in the processes. There is also a real possibility for disproportional trends for individual components of replenishment package. It is no longer enough to monitor just main 3–6 components of the bath. Successful high volume metallization for advanced packaging requires comprehensive metrology/process control. Presentation will provide specific examples of metrology and process control strategy for various metallization solution with emphasize on breakdown products and contaminants. There is no single technology which can cover diversity of metrology needs. CVS is a versatile technology which can be used not only for control of main additives but also to monitor breakdown products of accelerator, suppressor, leveler, Cu(I) contaminants, H2O2 contaminant and even leached photoresist. In case of Ni chemistry, it is useful to monitor breakdown products of sulfamate. UV-Vis spectroscopy shines at monitoring of leached photoresists, Fe(III), breakdown products of sulfamate as well as control of many main components: Cu, Ni, Co, Au, additives. ICP is the best for ultra low level of metal contaminants which can harms deposition process, as well as low level of some target components, like Tl in Au. Novel approach is developed to track additive turnover process based on ICP results. HPLC is valuable to characterize multiple breakdown products of organic additives, while ion chromatography helps to track transformation of ions, such as oxidation of sulfite to sulfate in Au solution. Surface tension aids monitoring of surfactants and their degradation. It is important to differentiate between R&D study and robust “fit-to-purpose” practical process control. While Ion Chromatography approach would be more appropriate for academic study and perhaps process development, simple robust methods like titration, specific gravity are preferable for industrial process control. Similarly, HPLC is a great scientific tool but CVS, spectroscopy and titration are better fit and safer alternative for in-fab operation. Presentation will provide specific examples and comparative analysis of different analytical methods.