Reducing the climate impact of the semiconductors industry
Solvaclean® offers semiconductor manufacturers an alternative cleaning gas
Semiconductors are everywhere. From electronics to household appliances to cars, it is estimated that every person in the world uses 200 to 300 semiconductors every day without knowing it. What the vast majority of consumers are also unaware of is that manufacturing them is a complex process that involves potent greenhouse gases.
Semiconductors are produced as wafers, in batches of several hundreds of units, in industrial ovens known as Chemical Vapor Deposition tools. They’re made by alternatively piling up extremely thin layers of conductive and dielectric materials. The dielectric layers are silicon, and they must be absolutely particle-free. Due to the minute scale at which they are produced, the smallest imperfection can have a drastic effect on their behavior as a semiconducting material. The only way to do this is to produce them in a closed system where the air has been replaced by gas. Each deposition process is followed by a cleaning phase, in which unwanted silicon oxide particles are burned with plasma, so the oven can be used to make another batch of perfectly pure wafers under identical tool conditions.
The problem is that this cleaning process is traditionally carried out using perfluorocarbons (better known as PFCs), such as hexafluoroethane (C2F6) or tetrafluoromethane (CF4), which are highly potent greenhouse gases. Since the 1980s, these gases have been commonly replaced by the more efficient nitrogen trifluoride (NF3). Unfortunately, the problem of our planet’s climate remains: in fact, as a greenhouse gas, NF3 is 17,000 times stronger than carbon dioxide!
Greenhouse effect-free, pure fluorine
The key in all of these cleaning gases is the fluorine radical. As it reacts with silicon oxide, it’s the inevitable active element for the efficient cleaning of silicon wafers. So why not resort to pure fluorine for an even more potent cleaning gas?
That’s exactly what Solvay is offering with Solvaclean®, which has the active ingredient of fluorine in its gaseous form, F2. “In plasma conditions, F2 decomposes into two fluorine ions, so there is no by-product to speak of,” explains Michael Pittroff, Global Marketing F2 & Electronic Gases, at Solvay’s Special Chem. “And fluorine isn’t a greenhouse gas: in the atmosphere, it transforms into hydrogen fluoride by reacting with the moisture in the air.”
The fluorine in Solvaclean® generates no by-product to speak of, and it has no greenhouse effect.
There is one thing to know about F2, though: it’s highly toxic and combustive. To circumvent this issue, Solvaclean® contains only 20-30% fluorine in order to remain below the combustion threshold – the rest being inert gases, argon and nitrogen. “This is a necessary precaution, otherwise the slightest leak would cause instant combustion,” continues Michael. “That being said, semiconductor manufacturing is carried out in completely closed systems designed to handle explosive gases, so it can be used safely.”
Initially tested in the 2000s, but only commercially available since 2019, Solvaclean® is a new solution in a market where changes take time. The semiconductors industry is a high-investment one, with each CVD tool costing between €3M and €9M, and manufacturing plants having several hundred of them, any new process requires thorough testing and vetting. Also, since F2 is a highly aggressive gas, operators tend to worry it might cause damage to their expensive equipment. As for the sustainability benefit of switching to a non-greenhouse gas, so far, it hasn’t been a priority in this highly competitive industry.
An officially labeled a sustainable Solar Impulse Efficient solution
As awareness increases over the dangers of NF3, of which about 700 tons are released in the atmosphere every year with quantities growing fast, this industry mindset will undoubtedly change. What’s more, in February 2021, Solvaclean® received the “Efficient Solution” label from the Solar Impulse Foundation, which should contribute to boosting its adoption.
And there are of course many other environmental added benefits: fluorine, the most reactive gas in nature, requires much lower dissociation energy than more stable gases, meaning much smaller quantities of electricity are needed to heat the plasma for the cleaning process. Furthermore, plants in Europe are equipped with plasma burners to destroy NF3 and avoid releasing it into the atmosphere; with F2, this step is no longer necessary, resulting in additional energy savings. And lastly, as less gas byproduct is generated with F2, less water is needed to wash out the waste gases from the oven, adding water savings to energy reductions.
With cleaning representing about 60% of a CVD oven’s tool time and plants functioning 24/7, making even small modifications to the cleaning process can lead to major changes. Like all high-stakes, high-competition sectors, the semiconductors industry, which is fully global with manufacturers spread out across Eastern Asia, the USA and Europe, is slow-changing, also because of its sheer technical complexity. “But Solvaclean® is in the last kilometer of this marathon race for adoption,” sums up Michael. “And the climate benefits it provides will ultimately tip the balance.”