More Reactive, Less Warming

As mentioned in Part 4 of Semiconductor Engineering’s series on fab sustainability, molecular fluorine is one alternative to PFCs or NF₃ for CVD chamber cleaning in the integrated circuit and flat panel display industries. It has a number of advantages relative to NF₃: an unstable, highly reactive molecule, F₂ breaks down easily and has no global warming potential. 

When NF₃ is used, atomic fluorine is still entirely responsible for the cleaning reaction. The nitrogen is not necessary, and so F₂ can serve as a direct replacement with minimal process changes. In fact, F₂ is more effective because it yields more atomic fluorine per unit of input gas. Since less energy is needed to break down the F₂ molecule, less energy is consumed in the cleaning process. 

On the other hand, F₂ is both more toxic and more reactive than NF₃, making it more difficult to handle and transport. Transport and cylinder handling are the riskiest operations for any gas. Since F₂ was introduced, on-site F₂ generation has established itself as a safe, cost-effective alternative.  

In a paper presented at the 2016 ASMC, Linde Electronics’ Jean-Charles Cigal explained that F₂ is generated by electrolysis of anhydrous HF dissolved in a molten potassium salt electrolyte. If power to the generator is cut, either in an emergency or because of extended idle time, fluorine generation ceases. (See Figure.) The byproducts of the reaction — hydrogen and fluorine gas — are collected at the anode and cathode, respectively. After dilution below the 4% explosive threshold, the hydrogen is routed to the fab scrubber, which removes any traces of HF before releasing the hydrogen to the atmosphere. Similarly, the F₂ is purified, compressed, and routed to the process equipment. 

Figure 1: Fluorine generation by electrolysis of HF. Image courtesy Linde Electronics

Paul Stockman, head of market development for Linde Electronics, said that both fab-scale and smaller equipment-scale generator units are available. Both minimize the amount of molecular fluorine present in the fab. For example, Stockman said, a large display customer using over 100 metric tons of fluorine per year would typically have less than 50 kg on-site at any given time, along with a few tons of liquid HF feedstock. 

As previously noted, greenhouse-friendly etching chemistries are still an open challenge for the industry. For chamber cleaning, though, F₂ appears to achieve good performance without the problems of either PFCs or NF₃. 

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