By Frédéric Raynal, Chief Technology Officer, aveni, S.A.
There’s a famous quote that says you never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete. I pondered this quote after reading an excellent article (1) that appeared recently in the Journal of the Electrochemical Society. The article posits that boric acid has multiple functions in cobalt electroplating and superfill, but says nothing about its dangers.
However, there is a revolution underway. Cobalt is replacing copper, with major device makers now using it in metal plugs in their 10 nm and below technologies. This significant shift is underway because cobalt’s resistance and electro migration characteristics are more adapted/efficient/suitable at this node.
While Boric acid is an effective antifungal, antibacterial and insecticide, a full understanding of its role is lacking any real consensus in the scientific world. In fact:
- The lowest pKa of boric acid is 9.28, which is far from the classical pH=4 used in the processes. Even if higher concentrations of the compound induce lower pKa, it never really goes under pH=5 in the bulk for buffer effects.
- Metal complexes of nickel borate are known but their stability constant is so low that they are virtually undetectable on the surface. Moreover, no publication has demonstrated the existence of cobalt borate complexes.
- Cobalt crystallography induced by boric acid, and its impact on conductivity, has never been thoroughly explored at the nanoscale dimension.
- No publication has found that boric acid alone has a bottom-up effect.
Chemical suppliers and foundries around the world have conducted their own research into the use of boric acid, as has the European Union regulations organization REACH, whose mandate is to identify and rule on substances of very high concern (SVHCs). In July 2015, boric acid and borate salts were added to SVHCs and were recommended for inclusion in the authorization list. According to the authorities, these substances could damage fertility and unborn children.
I believe it’s only a matter of time before these substances are banned altogether, and moved to Annex XIV where chromates or phthalates are currently listed. Until then, factories with concentrations higher than 15-20 g/l of boric acid must be extremely careful while working with these substances and the disposal of their chemical waste closely monitored.
Our strategy at aveni is simply to avoid the use of boric acid altogether. Our formulation:
- Decreases the pH to 2-2,5 range to avoid oxides and hydroxides formation and to yield very low resistivity.
- Adds a natural product that acts as a buffer at the aforementioned pH range.
- Uses chlorides instead of sulfates to avoid protons and water reductions (2).
- Allows us to make the trench fill AND the overburden in one step, in one tool, with one formulation.
Most important, our solution is free of boric acid.
In conclusion, while boric acid has its advantages in cobalt plating, its use is waning due to a myriad of safety concerns. aveni’s solution is positioned—to paraphrase Buckminster Fuller—to make the existing technology obsolete.
1) Matthew A. Rigsby, Tighe A. Spurlin, and Jonathan D. Reid, Journal of The Electrochemical Society, 2020 167 112507
2) N. Zech, D. Landolt, Electrochimica Acta 45 (2000) 3461–3471