Current thinking: PFAS alternatives for semiconductors

The semiconductor industry has long been reliant on PFAS but that could be set to change thanks to an innovative partnership between a US manufacturer and the University of Massachusetts, as Kerry Hebden finds out

Known for their extreme persistence in the environment, a trait which has led to them becoming synonymous with the word “forever”, PFAS (per- and polyfluorinated alkyl substances) represent a group of nearly 15,000 synthetic chemicals.

They come mired in controversy, however. That’s because scientific research suggests that repeated exposure to certain PFAS may lead to adverse health outcomes in humans, including reproductive and developmental affects, and increased risk of some cancers. In Europe alone, according to a report submitted to the European Chemicals Agency, between €52bn-€84bn (US$54.8bn-US$85.5bn) is spent each year on health costs following on from exposure to PFAS. Denmark, Germany, the Netherlands, Norway, and Sweden, the co-authors of the report, are proposing around 10,000 PFAS should be restricted.¹

PFAS longevity is down to its main ingredients, carbon and fluorine atoms. The atomic bond between these two is one of the strongest, and as a result, the chemicals do not degrade easily. Ironically, it is this feature that has led them to be used in hundreds of everyday products − from nonstick cookware and water-repellent clothing, to carpets, toiletries, and food packaging. Their enduring nature means that most PFAS are easily transported and can travel for thousands of kilometres from their point of release − they have even been found in an ice core from the Arctic Svalbard wilderness.²

Despite mounting evidence to justify a restriction, or ban on PFAS, not everyone agrees. One group opposed to restrictions is the semiconductor industry, which says that the unique soluble characteristics of fluorinated materials make them indispensable in the manufacture of many electrical components. Along with being hydrophobic, they are also oleophobic (oil-repellent) and can prevent intermixing between layers in a complex system such as an anti-reflection coating. The addition of small quantities of fluorinated materials also enables patterning capabilities that are otherwise not possible, leading to superior device performance.

Such is their usefulness that replacements are hard to find, and some firms, including Intel, have formed working groups to stave off potential restrictions in the US. Chemours, which together with DuPont, and Corteva, has agreed to settle claims upwards of US$1bn that PFAS contaminated US public water systems, also openly advocates for their use.³ Chemours has vowed to continue making fluoropolymers, a specific class of PFAS, as it says there are no alternatives that possess all their unique properties while delivering the same high level of performance, and to ban them would have a devastating impact on jobs, supply chains, and the economy.

But one firm who saw the change coming, and took steps to do something about it, is Transene. Based in Danvers, Massachusetts, US, Transene has been developing and manufacturing materials for the electronics industry, and many other sectors, since 1965. They supply a large range of products including wet chemical etchants, photoresist developers, and photomask chemicals, all which traditionally contain PFAS.

Already familiar with changing trends in chemicals, when talk turned to the reduction, or elimination of PFAS, the firm started to investigate safer alternatives to get ahead of the curve.

Their search brought them to the Toxics Use Reduction Institute (TURI) based at University of Massachusetts at Lowell (UMass Lowell). TURI paired Transene up with Ramaswamy Nagarajan, a professor in plastics engineering at UMass, whose team were awarded a grant to fund a research project to help the firm transition away from PFAS.

“We’ve had to overhaul the way we work before, such as when perfluorooctyl sulfonate (PFOS) was banned in the early 2000s,” Christuk said. “It was a very effective surfactant and used in everything. But we couldn’t find a singular alternative to replace it, so we ended up with a number of different surfactants – it was a huge undertaking to change it all.”

Christuk said this time around the firm wanted to be ahead of the game, and more importantly didn’t want a substitute that itself was going to be a problem either now or down the road. “The collaboration with UMass was very beneficial as we’re a small family-owned business that doesn’t have the extra staff or time to devote to the research that professor Ramaswamy and his team could achieve,” he said.

Ramaswamy and team focused on replacing etching solutions commonly used in the semiconductor industry. These are predominantly aqueous-based − up to 75% for tetra methyl ammonium hydroxide (TMAH), and chrome etchants − and highly polar in nature, such as PAN etchant which is a mixture of phosphoric acid, acetic acid, and nitric acid. “Some etchings are really, really harsh, and our replacements had to work just as well, but not be as toxic,” explained Ramaswamy.

The strict criteria that needed to be met for the replacements included better spreading capability, low-foaming characteristics, enhanced surface release of gases, and the prevention of unwanted impurities. The team assessed four types of typical etching solutions to base their alternatives on, and tested the replacements to make sure they had the same properties as the PFAS. “We wanted the alternatives to be drop-in replacements, so we used chemicals that are readily available,” Ramaswamy said. TURI’s Pollution Prevention Options Analysis System (P2OASys) tool was then used to evaluate the safety of the new surfactants.